,transcriber's ,note ,when italics were used in the original book1 the corresponding text has been surrounded by underscores except in the case of single letter variables used in the ,notes section1 where the italics were not represented4 ,mixed fractions have been displayed with a hyphen between whole number and fraction for clarity4 ,superscripted characters are preceded by and when more than one character is superscripted1 they are surrounded by 4 ,this book uses some unusual characters1 such as those representing the constellation ,aries 77 and ,libra 774 ,these characters may fail to display correctly if the font you are using does not support them4 ,some corrections have been made to the printed text4 ,these are listed in a second transcriber's note at the end of the text4 ,illustration3 ,,mary ,,somerville ,j4 ,,cooper ,sc4 ,,on ,,the ,,connexion ,,of ,,the ,,physical ,,sciences4 ,,by ,,mary ,,somerville1 ,,authoress ,,of #8,,mechanism ,,of ,,the ,,heavens1' ,,and #8,,physical ,,geography4' --------------------- 8,no natural phenomenon can be adequately studied in itself alone--but1 to be understood1 it must be considered as it stands connected with all ,nature40--,,bacon4 --------------------- ,ninth ,edition1 completely ,revised4 ,,london3 ,,john ,,murray1 ,,albemarle ,,street4 #aheh4 ,the right of ,translation is reserved4 ,,london3 ,,printed ,,by ,w4 ,,clowes ,,and ,,sons1 ,,duke ,,street1 ,,stamford ,,street1 ,,and ,,charing ,,cross4 ,this ,book is ,dedicated ,,to ,,her ,,dear ,,children1 ,,by ,,their ,,affectionate ,,mother1 ,,mary ,,somerville4 ,florence1 ,nov4 #a1 #aheh4 ,,contents4 ,,introduction ,page #a ,,section ,i4 ,attraction of a ,sphere--,form of ,celestial ,bodies--,terrestrial ,gravitation retains the ,moon in her ,orbit--,the ,heavenly ,bodies move in ,conic ,sections--,gravitation ,proportional to ,mass--,gravitation of the ,particles of ,matter--,figure of the ,planets--,how it affects the ,motions of their ,satellites--,rotation and ,translation impressed by the same ,impulse--,motion of the ,sun and ,solar ,system #d ,,section ,,ii4 ,elliptical ,motion--,mean and ,true ,motion--,equinoctial--,ecliptic--,equin oxes--,mean and ,true ,longitude--,equation of ,centre--,inclination of the ,orbits of ,planets--,celestial ,latitude--,nodes--,elements of an ,orbit--,undisturbed or ,elliptical ,orbits--,great ,inclination of the ,orbits of the ,new ,planets--,universal ,gravitation the ,cause of ,perturbations in the ,motions of the ,heavenly ,bodies--,problem of the ,three ,bodies--,stability of ,solar ,system depends upon the ,primitive ,momentum of the ,bodies #h ,,section ,,iii4 ,perturbations1 ,periodic and ,secular--,disturbing ,action equivalent to three ,partial ,forces--,tangential ,force the cause of the ,periodic ,inequalities in ,longitude1 and ,secular ,inequalities in the ,form and ,position of the ,orbit in its own ,plane--,radial ,force the cause of ,variations in the ,planet's ,distance from the ,sun--,it combines with the ,tangential ,force to produce the ,secular ,variations in the ,form and ,position of the ,orbit in its own ,plane--,perpendicular ,force the cause of ,periodic ,perturbations in ,latitude1 and ,secular ,variations in the ,position of the ,orbit with regard to the ,plane of the ,ecliptic--,mean ,motion and ,major ,axis ,invariable--,stability of ,system--,effects of a ,resisting ,medium--,invariable ,plane of the ,solar ,system and of the ,universe--,great ,inequality of ,jupiter and ,saturn #ac ,,section ,,iv4 ,theory of ,jupiter's ,satellites--,effects of the ,figure of ,jupiter upon his ,satellites--,position of their ,orbits--,singular ,laws among the ,motions of the first ,three ,satellites--,eclipses of the ,satellites--,velocity of ,light--,aberration--,ethereal ,medium--,satellites of ,saturn and ,uranus #bg ,,section ,v4 ,lunar ,theory--,periodic ,perturbations of the ,moon--,equation of ,centre--,evection--,variation--,annual ,equation--,direct and ,indirect ,action of ,planets--,the ,moon's ,action on the ,earth disturbs her own ,motion--,excentricity and ,inclination of ,lunar ,orbit invariable--,acceleration--,secular ,variation in ,nodes and ,perigee--,motion of ,nodes and ,perigee inseparably connected with the ,acceleration--,nutation of ,lunar ,orbit--,form and ,internal ,structure of the ,earth determined from it--,lunar1 ,solar1 and ,planetary ,eclipses--,occultations and ,lunar ,distances--,mean ,distance of the ,sun from the ,earth obtained from ,lunar ,theory--,absolute ,distances of the ,planets1 how found #cd ,,section ,,vi4 ,form of the ,earth and ,planets--,figure of a ,homogeneous ,spheroid in ,rotation--,figure of a ,spheroid of variable ,density--,figure of the ,earth1 supposing it to be an ,ellipsoid of ,revolution--,mensuration of a ,degree of the ,meridian--,compression and ,size of the ,earth from ,degrees of ,meridian--,figure of ,earth from the ,pendulum #dd ,,section ,,vii4 ,parallax--,lunar ,parallax found from ,direct ,observation--,solar ,parallax deduced from the ,transit of ,venus--,distance of the ,sun from the ,earth--,annual ,parallax--,distance of the ,fixed ,stars #eb ,,section ,,viii4 ,masses of ,planets that have no ,satellites determined from their ,perturbations--,masses of the others obtained from the ,motions of their ,satellites--,masses of the ,sun1 the ,earth1 of ,jupiter and of the ,jovial ,system--,mass of the ,moon--,real ,diameters of ,planets1 how obtained--,size of ,sun1 ,densities of the ,heavenly ,bodies--,formation of ,astronomical ,tables--,requisite ,data and ,means of obtaining them #ee ,,section ,,ix4 ,rotation of the ,sun and ,planets--,saturn's ,rings--,periods of the ,rotation of the ,moon and other ,satellites equal to the ,periods of their ,revolutions--,form of ,lunar ,spheroid--,libration1 ,aspect1 and ,constitution of the ,moon--,rotation of ,jupiter's ,satellites #fe ,,section ,x4 ,rotation of the ,earth invariable--,decrease in the ,earth's mean ,temperature--,earth originally in a state of ,fusion--,length of ,day constant--,decrease of ,temperature ascribed by ,sir ,john ,herschel to the variation in the ,excentricity of the ,terrestrial ,orbit--,difference in the ,temperature of the two ,hemispheres erroneously ascribed to the ,excess in the ,length of ,spring and ,summer in the ,southern ,hemisphere2 attributed by ,sir ,charles ,lyell to the ,operation of existing ,causes--,three principal ,axes of ,rotation--,position of the ,axis of ,rotation on the ,surface of the ,earth invariable--,ocean not sufficient to restore the ,equilibrium of the ,earth if deranged--,its ,density and mean ,depth--,internal ,structure of the ,earth #ga ,,section ,,xi4 ,precession and ,nutation--,their ,effects on the ,apparent ,places of the ,fixed ,stars #gi ,,section ,,xii4 ,mean and ,apparent ,sidereal ,time--,mean and ,apparent ,solar ,time--,equation of ,time--,english and ,french ,subdivisions of ,time--,leap ,year--,christian ,era--,equinoctial ,time--,remarkable ,eras depending upon the ,position of the ,solar ,perigee--,inequality of the ,lengths of the ,seasons in the two ,hemispheres--,application of ,astronomy to ,chronology--,english and ,french ,standards of ,weights and ,measures #hc ,,section ,,xiii4 ,tides--,forces that produce them--,origin and ,course of ,tidal ,wave--,its ,speed--,three kinds of ,oscillations in the ,ocean--,the ,semidiurnal ,tides--,equinoctial ,tides--,effects of the ,declination of the ,sun and ,moon--,theory insufficient without ,observation--,direction of the ,tidal ,wave--,height of ,tides--,mass of ,moon obtained from her ,action on the ,tides--,interference of ,undulations--,impossibility of a ,universal ,inundation--,currents #ia ,,section ,,xiv4 ,molecular ,forces--,permanency of the ultimate ,particles of ,matter--,interstices--,mossotti's ,theory--,rankin's ,theory of ,molecular ,vortices--,gases reduced to ,liquids by ,pressure--,gravitation of ,particles--,cohesion--,crystallization- -,cleavage--,isomorphism--,minuteness of the ,particles--,height of ,atmosphere--,chemical ,affinity--,definite ,proportions and ,relative ,weights of ,atoms--,faraday's ,discovery with regard to ,affinity--,capillary ,attraction #ajb ,,section ,,xv4 ,analysis of the ,atmosphere--,its pressure--,law of ,decrease in ,density--,law of ,decrease in ,temperature--,measurement of ,heights by the ,barometer--,extent of the ,atmosphere--,barometrical ,variations--,oscillations--,trade-,wind s--,cloud-,ring--,monsoons--,rotation of ,winds--,laws of ,hurricanes #aag ,,section ,,xvi4 ,sound--,propagation of ,sound illustrated by a ,field of ,standing ,corn--,nature of ,waves--,propagation of ,sound through the ,atmosphere--,intensity--,noises--,a ,musical ,sound--,quality--,pitch--,extent of ,human ,hearing--,velocity of ,sound in ,air1 ,water1 and ,solids--,causes of the ,obstruction of ,sound--,law of its ,intensity--,reflection of ,sound--,echoes--,thunder--,refraction of ,sound--,interference of ,sounds #abi ,,section ,,xvii4 ,vibration of ,musical ,strings--,harmonic ,sounds--,nodes--,vibration of ,air in ,wind-,instruments--,vibration of ,solids--,vibrating ,plates--,bells--,harmony--,sounding ,boards--,forced ,vibrations--,resonance--,speaking ,machines #adj ,,section ,,xviii4 ,refraction--,astronomical ,refraction and its ,laws--,formation of ,tables of ,refraction--,terrestrial ,refraction--,its ,quantity--,instances of ,extraordinary ,refraction--,reflection--,instances of ,extraordinary ,reflection--,loss of ,light by the ,absorbing ,power of the ,atmosphere--,apparent ,magnitude of ,sun and ,moon in the ,horizon #aec ,,section ,,xix4 ,constitution of ,light according to ,sir ,isaac ,newton--,absorption of ,light--,colours of ,bodies--,constitution of ,light according to ,sir ,david ,brewster--,new ,colours--,fraunhofer's ,dark ,lines--,dispersion of ,light--,the ,achromatic ,telescope--,homogeneous ,light--,accidental and ,complementary ,colours--,m4 ,plateau's ,experiments and ,theory of ,accidental ,colours #aei ,,section ,,xx4 ,interference of ,light--,undulatory ,theory of ,light--,propagation of ,light--,newton's ,rings--,measurement of the ,length of the ,waves of ,light1 and of the ,frequency of the ,vibrations of ,ether for each ,colour--,newton's ,scale of ,colours--,diffraction of ,light--,sir ,john ,herschel's ,theory of the ,absorption of ,light--,refraction and ,reflection of ,light #afg ,,section ,,xxi4 ,polarization of ,light--,defined--,polarization by ,refraction--,properties of the ,tourmaline--,double ,refraction--,all doubly ,refracted ,light is ,polarized--,properties of ,iceland ,spar--,tourmaline absorbs one of the two ,refracted ,rays--,undulations of ,natural ,light--,undulations of ,polarized ,light--,the ,optic ,axes of ,crystals--,m4 ,fresnel's ,discoveries on the ,rays passing along the ,optic ,axis--,polarization by ,reflection #agi ,,section ,,xxii4 ,phenomena exhibited by the ,passage of ,polarized ,light through ,mica and ,sulphate of ,lime--,the ,coloured ,images produced by ,polarized ,light passing through ,crystals having one and two ,optic ,axes--,circular ,polarization--,elliptical ,polarization--,discoveries of ,,mm4 ,biot1 ,fresnel1 and ,professor ,airy--,coloured ,images produced by the ,interference of ,polarized ,rays--,fluorescence #ahf ,,section ,,xxiii4 ,objections to the ,undulatory ,theory1 from a difference in the ,action of ,sound and ,light under the same circumstances1 removed--,the ,dispersion of ,light according to the ,undulatory ,theory--,arago's final proof that the ,undulatory ,theory is the ,law of ,nature #aii ,,section ,,xxiv4 ,chemical or ,photographic ,rays of ,solar ,spectrum--,scheele1 ,ritter1 and ,wollaston's ,discoveries--,wedgwood's and ,sir ,humphry ,davy's ,photographic ,pictures--,the ,calotype--,the ,daguerreotype--,the ,chromatype--,the ,cyanotype--,collodion--,sir ,john ,herschel's ,discoveries in the ,chemical ,spectrum--,m4 ,becquerel's ,discoveries of ,inactive ,lines in ditto--,thermic ,spectrum--,phosphoric ,spectrum--,electrical ,properties--,parathermic ,rays--,moser and ,hunt's ,experiments--,general ,structure and antagonist ,properties of ,solar ,spectrum--,defracted ,spectrum #bjc ,,section ,,xxv4 ,size and ,constitution of the ,sun--,the ,solar ,spots--,intensity of the ,sun's ,light and ,heat--,the ,sun's ,atmosphere--,his influence on the ,planets--,atmospheres of the ,planets--,the ,moon has none--,lunar heat--,the ,differential ,telescope--,temperature of ,space--,internal ,heat of the ,earth--,zone of constant ,temperature--,increase of ,heat with the ,depth--,central ,heat--,volcanic ,action--,quantity of ,heat received from the ,sun--,isogeothermal ,lines--,line of ,perpetual ,congelation--,climate--,isothermal ,lines--,same quantity of ,heat annually received and radiated by the ,earth #bbd ,,section ,,xxvi4 ,influence of ,temperature on ,vegetation--,vegetation varies with the ,latitude and ,height above the ,sea--,geographical ,distribution of ,land ,plants--,distribution of ,marine ,plants--,corallines1 ,shell-fish1 ,reptiles1 ,insects1 ,birds1 and ,quadrupeds--,varieties of ,mankind1 yet identity of ,species #bdh ,,section ,,xxvii4 ,terrestrial ,heat--,radiation--,transmission--,mello ni's experiments--,heat in ,solar ,spectrum--,polarization of ,heat--,nature of ,heat--,absorptions--,dew--,rain--,combu stion--,expansion--,compensation ,pendulum--,transmission through ,crystals--,propagation--,dynamic ,theory of ,heat--,mechanical equivalent of ,heat--,latent ,heat is the ,force of ,expansion--,steam--,work performed by ,heat--,conservation of ,force--,mechanical ,power in the ,tides--,dynamical ,power of ,light--,analogy between ,light1 ,heat1 and ,sound #beg ,,section ,,xxviii4 ,common or ,static ,electricity1 or ,electricity of ,tension--,a ,dual ,power--,methods of exciting it--,attraction and ,repulsion--,conduction--,electrics and ,non-electrics--,induction--,dielectrics --,tension--,law of the ,electric ,force--,distribution--,laws of ,distribution--,heat of ,electricity--,electrical ,light and its ,spectrum--,velocity--,atmospheric ,electricity--,its cause--,electric ,clouds--,violent effects of ,lightning--,back ,stroke--,electric ,glow--,phosphorescence #bhb ,,section ,,xxix4 ,voltaic ,electricity--,the ,voltaic ,battery--,intensity--,quantity--,static ,electricity1 and ,electricity in ,motion--,luminous ,effects--,mr4 ,grove on the ,electric ,arc and ,light--,decomposition of ,water--,formation of ,crystals by ,voltaic ,electricity--,photo-galvanic ,engraving--,conduction--,heat of ,voltaic ,electricity--,electric ,fish #big ,,section ,,xxx4 ,discovery of ,electro-magnetism--,deflection of the ,magnetic ,needle by a ,current of ,electricity--,direction of the ,force--,rotatory ,motion by ,electricity--,rotation of a ,wire and a ,magnet--,rotation of a ,magnet about its ,axis--,of ,mercury and ,water--,electro-,magnetic ,cylinder or ,helix--,suspension of a ,needle in a ,helix--,electro-,magnetic ,induction--,temporary ,magnets--,the ,galvanometer #cab ,,section ,,xxxi4 ,electro-,dynamics--,reciprocal ,action of ,electric ,currents--,identity of ,electro-,dynamic ,cylinders and ,magnets--,differences between the ,action of ,voltaic ,electricity and ,electricity of ,tension--,effects of a ,voltaic ,current--,ampre's ,theory--,dr4 ,faraday's ,experiment of ,electrifying and ,magnetising a ,ray of ,light #caf ,,section ,,xxxii4 ,magneto-,electricity--,volta-,electric ,induction--,magneto-,electric ,induction--,identity in the ,action of ,electricity and ,magnetism--,description of a ,magneto-,electric ,apparatus and its ,effects--,identity of ,magnetism and ,electricity--,the ,submarine ,telegraph #cbb ,,section ,,xxxiii4 ,electricity produced by ,rotation--,direction of the ,currents--,electricity from the ,rotation of a ,magnet--,m4 ,arago's ,experiment explained--,rotation of a ,plate of ,iron between the ,poles of a ,magnet--,relation of ,substances to ,magnets of three ,kinds--,thermo-,electricity #ccj ,,section ,,xxxiv4 ,magnetism a ,dual ,power--,antithetic ,character of ,paramagnetism and ,diamagnetism--,the ,earth ,paramagnetic--,properties of ,paramagnetic ,bodies--,polarity--,induction--,lines of ,magnetic ,force--,currents of ,electricity induced by them--,proved to be ,closed ,curves--,analogy and ,identity of ,electricity and ,magnetism--,terrestrial ,magnetism--,mean ,values of the ,three ,magnetic ,elements--,their ,variations in ,double ,progression proved to consist of ,two ,superposed ,variations--,discovery of the ,periodicity of the ,magnetic ,storms--,the ,decennial ,period of the ,magnetic ,elements the same with that of the ,solar ,spots--,magnetism of the ,atmosphere--,diamagnetism--,action of ,electro-,magnetism on ,paramagnetic1 ,diamagnetic ,bodies1 and on ,copper1 very different--,proof of ,diamagnetic ,polarity and ,induction--,magnecrystallic ,action--,effects of ,compression1 ,heat1 and ,cleavage on ,magnetic ,bodies--,mutual ,dependence of ,light1 ,heat1 ,electricity1 c4 c4--,the ,conservation of ,force and the ,permanency of ,matter ,primary ,laws of ,nature--,definition of ,gravity not according to that ,law--,gravity only the ,residual ,force of a ,universal ,power--,magnetism of the ,ethereal ,medium #cce ,,section ,,xxxv4 ,ethereal ,medium--,comets--,do not disturb the ,solar ,system--,their ,orbits and ,disturbances--,m4 ,faye's ,comet probably the same with ,lexel's--,periods of other three known--,acceleration in the mean ,motions of ,encke's and ,biela's ,comets--,the ,shock of a ,comet--,disturbing ,action of the ,earth and ,planets on ,encke's and ,biela's ,comets--,velocity of ,comets--,the ,comet of #abfd--,the great ,comet of #acdc--,physical ,constitution--,shine by borrowed ,light--,estimation of their ,number #ceh ,,section ,,xxxvi4 ,the ,fixed ,stars--,their ,number--,the ,milky ,way--,double ,stars--,binary ,systems--,their ,orbits and ,periodic ,times--,colours of the ,stars--,stars that have vanished--,variable ,stars--,variation in ,sun's ,light--,parallax and ,distances of the ,fixed ,stars--,masses of the ,stars--,comparative ,light of the ,stars--,proper ,motions of the ,stars--,apparent ,motions of the ,stars--,motion and ,velocity of the ,sun and ,solar ,system--,the ,nebul--,their ,number--,catalogue of them--,consist of ,two ,classes--,diffuse ,nebul--,definitely formed ,nebul--,globular ,clusters--,splendour of ,milky ,way--,distribution of the ,nebul--,the ,magellanic ,clouds--,nebul round ,args--,constitution of ,nebul1 and the ,forces that maintain them--,meteorites and ,shooting ,stars #chd ,,section ,,xxxvii4 ,diffusion of ,matter through ,space--,gravitation--,its ,velocity--,simplicity of its ,laws--,gravitation independent of the ,magnitude and ,distances of the ,bodies--,not impeded by the intervention of any ,substance--,its ,intensity invariable--,general ,laws--,recapitulation and ,conclusion #dbd ,,notes #dbi ,,index #dgi ,,the ,,connection ,,of ,,the ,,physical ,,sciences4 ,,introduction4 ,,science1 regarded as the pursuit of truth1 must ever afford occupation of consummate interest1 and subject of elevated meditation4 ,the contemplation of the works of creation elevates the mind to the admiration of whatever is great and noble2 accomplishing the object of all study1 which1 in the eloquent language of ,sir ,james ,mackintosh1 8is to inspire the love of truth1 of wisdom1 of beauty--especially of goodness1 the highest beauty--and of that supreme and eternal ,mind1 which contains all truth and wisdom1 all beauty and goodness4 ,by the love or delightful contemplation and pursuit of these transcendent aims1 for their own sake only1 the mind of man is raised from low and perishable objects1 and prepared for those high destinies which are appointed for all those who are capable of them40 ,astronomy affords the most extensive example of the connection of the physical sciences4 ,in it are combined the sciences of number and quantity1 of rest and motion4 ,in it we perceive the operation of a force which is mixed up with everything that exists in the heavens or on earth2 which pervades every atom1 rules the motions of animate and inanimate beings1 and is as sensible in the descent of a rain-drop as in the falls of ,niagara2 in the weight of the air1 as in the periods of the moon4 ,gravitation not only binds satellites to their planet1 and planets to the sun1 but it connects sun with sun throughout the wide extent of creation1 and is the cause of the disturbances1 as well as of the order of nature2 since every tremor it excites in any one planet is immediately transmitted to the farthest limits of the system1 in oscillations which correspond in their periods with the cause producing them1 like sympathetic notes in music1 or vibrations from the deep tones of an organ4 ,the heavens afford the most sublime subject of study which can be derived from science4 ,the magnitude and splendour of the objects1 the inconceivable rapidity with which they move1 and the enormous distances between them1 impress the mind with some notion of the energy that maintains them in their motions1 with a durability to which we can see no limit4 ,equally conspicuous is the goodness of the great ,first ,cause1 in having endowed man with faculties1 by which he can not only appreciate the magnificence of ,his works1 but trace1 with precision1 the operation of ,his laws1 use the globe he inhabits as a base wherewith to measure the magnitude and distance of the sun and planets1 and make the diameter 7,note #a7 of the earth's orbit the first step of a scale by which he may ascend to the starry firmament4 ,such pursuits1 while they ennoble the mind1 at the same time inculcate humility1 by showing that there is a barrier which no energy1 mental or physical1 can ever enable us to pass3 that1 however profoundly we may penetrate the depths of space1 there still remain innumerable systems1 compared with which1 those apparently so vast must dwindle into insignificance1 or even become invisible2 and that not only man1 but the globe he inhabits--nay1 the whole system of which it forms so small a part--might be annihilated1 and its extinction be unperceived in the immensity of creation4 ,a complete acquaintance with physical astronomy can be attained by those only who are well versed in the higher branches of mathematical and mechanical science 7,n4 #b71 and they alone can appreciate the extreme beauty of the results1 and of the means by which these results are obtained4 ,it is nevertheless true1 that a sufficient skill in analysis 7,n4 #c7 to follow the general outline--to see the mutual dependence of the different parts of the system1 and to comprehend by what means the most extraordinary conclusions have been arrived at1--is within the reach of many who shrink from the task1 appalled by difficulties1 not more formidable than those incident to the study of the elements of every branch of knowledge4 ,there is a wide distinction between the degree of mathematical acquirement necessary for making discoveries1 and that which is requisite for understanding what others have done4 ,our knowledge of external objects is founded upon experience1 which furnishes facts2 the comparison of these facts establishes relations1 from which the belief that like causes will produce like effects leads to general laws4 ,thus1 experience teaches that bodies fall at the surface of the earth with an accelerated velocity1 and with a force proportional to their masses4 ,by comparison1 ,newton proved that the force which occasions the fall of bodies at the earth's surface is identical with that which retains the moon in her orbit2 and he concluded1 that1 as the moon is kept in her orbit by the attraction of the earth1 so the planets might be retained in their orbits by the attraction of the sun4 ,by such steps he was led to the discovery of one of those powers with which the ,creator has ordained that matter should reciprocally act upon matter4 ,physical astronomy is the science which compares and identifies the laws of motion observed on earth with the motions that take place in the heavens3 and which traces1 by an uninterrupted chain of deduction from the great principle that governs the universe1 the revolutions and rotations of the planets1 and the oscillations 7,n4 #d7 of the fluids at their surfaces2 and which estimates the changes the system has hitherto undergone1 or may hereafter experience--changes which require millions of years for their accomplishment4 ,the accumulated efforts of astronomers1 from the earliest dawn of civilization1 have been necessary to establish the mechanical theory of astronomy4 ,the courses of the planets have been observed for ages1 with a degree of perseverance that is astonishing1 if we consider the imperfection and even the want of instruments4 ,the real motions of the earth have been separated from the apparent motions of the planets2 the laws of the planetary revolutions have been discovered2 and the discovery of these laws has led to the knowledge of the gravitation 7,n4 #e7 of matter4 ,on the other hand1 descending from the principle of gravitation1 every motion in the solar system has been so completely explained1 that the laws of any astronomical phenomena that may hereafter occur are already determined4 ,,section ,i4 ,attraction of a ,sphere--,form of ,celestial ,bodies--,terrestrial ,gravitation retains the ,moon in her ,orbit--,the ,heavenly ,bodies move in ,conic ,sections--,gravitation ,proportional to ,mass--,gravitation of the ,particles of ,matter--,figure of the ,planets--,how it affects the ,motions of their ,satellites--,rotation and ,translation impressed by the same ,impulse--,motion of the ,sun and ,solar ,system4 ,,it has been proved by ,newton1 that a particle of matter 7,n4 #f7 placed without the surface of a hollow sphere 7,n4 #g7 is attracted by it in the same manner as if the mass of the hollow sphere1 or the whole matter it contains1 were collected into one dense particle in its centre4 ,the same is therefore true of a solid sphere1 which may be supposed to consist of an infinite number of concentric hollow spheres 7,n4 #h74 ,this1 however1 is not the case with a spheroid 7,n4 #i72 but the celestial bodies are so nearly spherical1 and at such remote distances from one another1 that they attract and are attracted as if each were condensed into a single particle situate in its centre of gravity 7,n4 #aj7--;a circumstance which greatly facilitates the investigation of their motions4 ,newton has shown that the force which retains the moon in her orbit is the same with that which causes heavy substances to fall at the surface of the earth4 ,if the earth were a sphere1 and at rest1 a body would be equally attracted1 that is1 it would have the same weight at every point of its surface1 because the surface of a sphere is everywhere equally distant from its centre4 ,but1 as our planet is flattened at the poles 7,n4 #aa71 and bulges at the equator1 the weight of the same body gradually decreases from the poles1 where it is greatest1 to the equator1 where it is least4 ,there is1 however1 a certain mean 7,n4 #ab7 latitude 7,n4 #ac71 or part of the earth intermediate between the pole and the equator1 where the attraction of the earth on bodies at its surface is the same as if it were a sphere2 and experience shows that bodies there fall through #afjfig feet in a second4 ,the mean distance 7,n4 #ad7 of the moon from the earth is about sixty times the mean radius 7,n4 #ae7 of the earth4 ,when the number #afjfig is diminished in the ratio 7,n4 #af7 of #a to #cfjj1 which is the square of the moon's distance 7,n4 #ag7 from the earth's centre1 estimated in terrestrial radii1 it is found to be exactly the space the moon would fall through in the first second of her descent to the earth1 were she not prevented by the centrifugal force 7,n4 #ah7 arising from the velocity with which she moves in her orbit4 ,the moon is thus retained in her orbit by a force having the same origin1 and regulated by the same law1 with that which causes a stone to fall at the earth's surface4 ,the earth may1 therefore1 be regarded as the centre of a force which extends to the moon2 and1 as experience shows that the action and reaction of matter are equal and contrary 7,n4 #ai71 the moon must attract the earth with an equal and contrary force4 ,newton also ascertained that a body projected 7,n4 #bj7 in space 7,n4 #ba7 will move in a conic section 7,n4 #bb71 if attracted by a force proceeding from a fixed point1 with an intensity inversely as the square of the distance 7,n4 #bc72 but that any deviation from that law will cause it to move in a curve of a different nature4 ,kepler found1 by direct observation1 that the planets describe ellipses 7,n4 #bd71 or oval paths1 round the sun4 ,later observations show that comets also move in conic sections4 ,it consequently follows that the sun attracts all the planets and comets inversely as the square of their distances from its centre2 the sun1 therefore1 is the centre of a force extending indefinitely in space1 and including all the bodies of the system in its action4 ,kepler also deduced from observation that the squares of the periodic times 7,n4 #be7 of the planets1 or the times of their revolutions round the sun1 are proportional to the cubes of their mean distances from its centre 7,n4 #bf74 ,hence the intensity of gravitation of all the bodies towards the sun is the same at equal distances4 ,consequently1 gravitation is proportional to the masses 7,n4 #bg72 for1 if the planets and comets were at equal distances from the sun1 and left to the effects of gravity1 they would arrive at his surface at the same time 7,n4 #bh74 ,the satellites also gravitate to their primaries 7,n4 #bi7 according to the same law that their primaries do to the sun4 ,thus1 by the law of action and reaction1 each body is itself the centre of an attractive force extending indefinitely in space1 causing all the mutual disturbances which render the celestial motions so complicated1 and their investigation so difficult4 ,the gravitation of matter directed to a centre1 and attracting directly as the mass and inversely as the square of the distance1 does not belong to it when considered in mass only2 particle acts on particle according to the same law when at sensible distances from each other4 ,if the sun acted on the centre of the earth1 without attracting each of its particles1 the tides would be very much greater than they now are1 and would also1 in other respects1 be very different4 ,the gravitation of the earth to the sun results from the gravitation of all its particles1 which1 in their turn1 attract the sun in the ratio of their respective masses4 ,there is a reciprocal action likewise between the earth and every particle at its surface4 ,the earth and a feather mutually attract each other in the proportion of the mass of the earth to the mass of the feather4 ,were this not the case1 and were any portion of the earth1 however small1 to attract another portion1 and not be itself attracted1 the centre of gravity of the earth would be moved in space by this action1 which is impossible4 ,the forms of the planets result from the reciprocal attraction of their component particles4 ,a detached fluid mass1 if at rest1 would assume the form of a sphere1 from the reciprocal attraction of its particles4 ,but if the mass revolve about an axis1 it becomes flattened at the poles and bulges at the equator 7,n4 #aa71 in consequence of the centrifugal force arising from the velocity of rotation 7,n4 #cj72 for the centrifugal force diminishes the gravity of the particles at the equator1 and equilibrium can only exist where these two forces are balanced by an increase of gravity4 ,therefore1 as the attractive force is the same on all particles at equal distances from the centre of a sphere1 the equatorial particles would recede from the centre1 till their increase in number balance the centrifugal force by their attraction4 ,consequently1 the sphere would become an oblate or flattened spheroid1 and a fluid1 partially or entirely covering a solid1 as the ocean and atmosphere cover the earth1 must assume that form in order to remain in equilibrio4 ,the surface of the sea is1 therefore1 spheroidal1 and the surface of the earth only deviates from that figure where it rises above or sinks below the level of the sea4 ,but the deviation is so small1 that it is unimportant when compared with the magnitude of the earth2 for the mighty chain of the ,andes1 and the yet more lofty ,himalaya1 bear about the same proportion to the earth that a grain of sand does to a globe three feet in diameter4 ,such is the form of the earth and planets4 ,the compression 7,n4 #ca7 or flattening at their poles is1 however1 so small1 that even ,jupiter1 whose rotation is the most rapid1 and therefore the most elliptical of the planets1 may1 from his great distance1 be regarded as spherical4 ,although the planets attract each other as if they were spheres1 on account of their distances1 yet the satellites 7,n4 #cb7 are near enough to be sensibly affected in their motions by the forms of their primaries4 ,the moon1 for example1 is so near the earth1 that the reciprocal attraction between each of her particles1 and each of the particles in the prominent mass at the terrestrial equator1 occasions considerable disturbances in the motions of both bodies2 for the action of the moon on the matter at the earth's equator produces a nutation 7,n4 #cc7 in the axis 7,n4 #cd7 of rotation1 and the reaction of that matter on the moon is the cause of a corresponding nutation in the lunar orbit 7,n4 #ce74 ,if a sphere at rest in space receive an impulse passing through its centre of gravity1 all its parts will move with an equal velocity in a straight line2 but1 if the impulse does not pass through the centre of gravity1 its particles1 having unequal velocities1 will have a rotatory or revolving motion1 at the same time that it is translated 7,n4 #cf7 in space4 ,these motions are independent of one another2 so that a contrary impulse1 passing through its centre of gravity1 will impede its progress1 without interfering with its rotation4 ,the sun rotates about an axis1 and modern observations show that an impulse in a contrary direction has not been given to his centre of gravity1 for he moves in space1 accompanied by all those bodies which compose the solar system--a circumstance which in no way interferes with their relative motions2 for1 in consequence of the principle that force is proportional to velocity 7,n4 #cg71 the reciprocal attractions of a system remain the same whether its centre of gravity be at rest1 or moving uniformly in space4 ,it is computed that1 had the earth received its motion from a single impulse1 that impulse must have passed through a point about twenty-five miles from its centre4 ,since the motions of rotation and translation of the planets are independent of each other1 though probably communicated by the same impulse1 they form separate subjects of investigation4 ,,section ,,ii4 ,elliptical ,motion--,mean and ,true ,motion--,equinoctial--,ecliptic--,equin oxes--,mean and ,true ,longitude--,equation of ,centre--,inclination of the ,orbits of ,planets--,celestial ,latitude--,nodes--,elements of an ,orbit--,undisturbed or ,elliptical ,orbits--,great ,inclination of the ,orbits of the ,new ,planets--,universal ,gravitation the ,cause of ,perturbations in the ,motions of the ,heavenly ,bodies--,problem of the ,three ,bodies--,stability of ,solar ,system depends upon the ,primitive ,momentum of the ,bodies4 ,a ,,planet moves in its elliptical orbit with a velocity varying every instant1 in consequence of two forces1 one tending to the centre of the sun1 and the other in the direction of a tangent 7,n4 #ch7 to its orbit1 arising from the primitive impulse given at the time when it was launched into space4 ,should the force in the tangent cease1 the planet would fall to the sun by its gravity4 ,were the sun not to attract it1 the planet would fly off in the tangent4 ,thus1 when the planet is at the point of its orbit farthest from the sun1 his action overcomes the planet's velocity1 and brings it towards him with such an accelerated motion1 that at last it overcomes the sun's attraction1 and1 shooting past him1 gradually decreases in velocity until it arrives at the most distant point1 where the sun's attraction again prevails 7,n4 #ci74 ,in this motion the radii vectores 7,n4 #dj71 or imaginary lines joining the centres of the sun and the planets1 pass over equal areas or spaces in equal times 7,n4 #da74 ,the mean distance of a planet from the sun is equal to half the major axis 7,n4 #db7 of its orbit3 if1 therefore1 the planet described a circle 7,n4 #dc7 round the sun at its mean distance1 the motion would be uniform1 and the periodic time unaltered1 because the planet would arrive at the extremities of the major axis at the same instant1 and would have the same velocity1 whether it moved in the circular or elliptical orbit1 since the curves coincide in these points4 ,but in every other part the elliptical1 or true motion 7,n4 #dd71 would either be faster or slower than the circular or mean motion 7,n4 #de74 ,as it is necessary to have some fixed point in the heavens from whence to estimate these motions1 the vernal equinox 7,n4 #df7 at a given epoch has been chosen4 ,the equinoctial1 which is a great circle traced in the starry heavens by the imaginary extension of the plane of the terrestrial equator1 is intersected by the ecliptic1 or apparent path of the sun1 in two points diametrically opposite to one another1 called the vernal and autumnal equinoxes4 ,the vernal equinox is the point through which the sun passes in going from the southern to the northern hemisphere2 and the autumnal1 that in which he crosses from the northern to the southern4 ,the mean or circular motion of a body1 estimated from the vernal equinox1 is its mean longitude2 and its elliptical1 or true motion1 reckoned from that point1 is its true longitude 7,n4 #dg73 both being estimated from west to east1 the direction in which the bodies move4 ,the difference between the two is called the equation of the centre 7,n4 #dh72 which consequently vanishes at the apsides 7,n4 #di71 or extremities of the major axis1 and is at its maximum ninety degrees 7,n4 #ej7 distant from these points1 or in quadratures 7,n4 #ea71 where it measures the excentricity 7,n4 #eb7 of the orbit2 so that the place of the planet in its elliptical orbit is obtained by adding or subtracting the equation of the centre to or from its mean longitude4 ,the orbits of the principal planets have a very small obliquity or inclination 7,n4 #ec7 to the plane of the ecliptic in which the earth moves2 and1 on that account1 astronomers refer their motions to this plane at a given epoch as a known and fixed position4 ,the angular distance of a planet from the plane of the ecliptic is its latitude 7,n4 #ed71 which is south or north according as the planet is south or north of that plane4 ,when the planet is in the plane of the ecliptic1 its latitude is zero2 it is then said to be in its nodes 7,n4 #ee74 ,the ascending node is that point in the ecliptic through which the planet passes in going from the southern to the northern hemisphere4 ,the descending node is a corresponding point in the plane of the ecliptic diametrically opposite to the other1 through which the planet descends in going from the northern to the southern hemisphere4 ,the longitude and latitude of a planet cannot be obtained by direct observation1 but are deduced from observations made at the surface of the earth by a very simple computation4 ,these two quantities1 however1 will not give the place of a planet in space4 ,its distance from the sun 7,n4 #ef7 must also be known2 and1 for the complete determination of its elliptical motion1 the nature and position of its orbit must be ascertained by observation4 ,this depends upon seven quantities1 called the elements of the orbit 7,n4 #eg74 ,these are1 the length of the major axis1 and the excentricity1 which determine the form of the orbit2 the longitude of the planet when at its least distance from the sun1 called the longitude of the perihelion2 the inclination of the orbit to the plane of the ecliptic1 and the longitude of its ascending node3 these give the position of the orbit in space2 but the periodic time1 and the longitude of the planet at a given instant1 called the longitude of the epoch1 are necessary for finding the place of the body in its orbit at all times4 ,a perfect knowledge of these seven elements is requisite for ascertaining all the circumstances of undisturbed elliptical motion4 ,by such means it is found that the paths of the planets1 when their mutual disturbances are omitted1 are ellipses nearly approaching to circles1 whose planes1 slightly inclined to the ecliptic1 cut it in straight lines1 passing through the centre of the sun 7,n4 #eh74 ,the orbits of the recently-discovered planets deviate more from the ecliptic than those of the ancient planets3 that of ,pallas1 for instance1 has an inclination of #cd #db #bih to it2 on which account it is more difficult to determine their motions4 ,were the planets attracted by the sun only1 they would always move in ellipses1 invariable in form and position2 and because his action is proportional to his mass1 which is much larger than that of all the planets put together1 the elliptical is the nearest approximation to their true motions4 ,the true motions of the planets are extremely complicated1 in consequence of their mutual attraction1 so that they do not move in any known or symmetrical curve1 but in paths now approaching to1 now receding from1 the elliptical form2 and their radii vectores do not describe areas or spaces exactly proportional to the time1 so that the areas become a test of disturbing forces4 ,to determine the motion of each body1 when disturbed by all the rest1 is beyond the power of analysis4 ,it is therefore necessary to estimate the disturbing action of one planet at a time1 whence the celebrated problem of the three bodies1 originally applied to the moon1 the earth1 and the sun--namely1 the masses being given of three bodies projected from three given points1 with velocities given both in quantity and direction2 and supposing the bodies to gravitate to one another with forces that are directly as their masses1 and inversely as the squares of the distances1 to find the lines described by these bodies1 and their positions at any given instant2 or1 in other words1 to determine the path of a celestial body when attracted by a second body1 and disturbed in its motion round the second body by a third--a problem equally applicable to planets1 satellites1 and comets4 ,by this problem the motions of translation of the celestial bodies are determined4 ,it is an extremely difficult one1 and would be infinitely more so if the disturbing action were not very small when compared with the central force2 that is1 if the action of the planets on one another were not very small when compared with that of the sun4 ,as the disturbing influence of each body may be found separately1 it is assumed that the action of the whole system1 in disturbing any one planet1 is equal to the sum of all the particular disturbances it experiences1 on the general mechanical principle1 that the sum of any number of small oscillations is nearly equal to their simultaneous and joint effect4 ,on account of the reciprocal action of matter1 the stability of the system depends upon the intensity of the primitive momentum 7,n4 #ei7 of the planets1 and the ratio of their masses to that of the sun2 for the nature of the conic sections in which the celestial bodies move depends upon the velocity with which they were first propelled in space4 ,had that velocity been such as to make the planets move in orbits of unstable equilibrium 7,n4 #fj71 their mutual attractions might have changed them into parabolas1 or even hyperbolas 7,n4 #bb72 so that the earth and planets might1 ages ago1 have been sweeping far from our sun through the abyss of space4 ,but as the orbits differ very little from circles1 the momentum of the planets1 when projected1 must have been exactly sufficient to ensure the permanency and stability of the system4 ,besides1 the mass of the sun is vastly greater than that of any planet2 and as their inequalities bear the same ratio to their elliptical motions that their masses do to that of the sun1 their mutual disturbances only increase or diminish the excentricities of their orbits by very minute quantities2 consequently the magnitude of the sun's mass is the principal cause of the stability of the system4 ,there is not in the physical world a more splendid example of the adaptation of means to the accomplishment of an end than is exhibited in the nice adjustment of these forces1 at once the cause of the variety and of the order of ,nature4 ,,section ,,iii4 ,perturbations1 ,periodic and ,secular--,disturbing ,action equivalent to three ,partial ,forces--,tangential ,force the cause of the ,periodic ,inequalities in ,longitude1 and ,secular ,inequalities in the ,form and ,position of the ,orbit in its own ,plane--,radial ,force the cause of ,variations in the ,planet's ,distance from the ,sun--,it combines with the ,tangential ,force to produce the ,secular ,variations in the ,form and ,position of the ,orbit in its own ,plane--,perpendicular ,force the cause of ,periodic ,perturbations in ,latitude1 and ,secular ,variations in the ,position of the ,orbit with regard to the ,plane of the ,ecliptic--,mean ,motion and ,major ,axis ,invariable--,stability of ,system--,effects of a ,resisting ,medium--,invariable ,plane of the ,solar ,system and of the ,universe--,great ,inequality of ,jupiter and ,saturn4 ,,the planets are subject to disturbances of two kinds1 both resulting from the constant operation of their reciprocal attraction3 one kind1 depending upon their positions with regard to each other1 begins from zero1 increases to a maximum1 decreases1 and becomes zero again1 when the planets return to the same relative positions4 ,in consequence of these1 the disturbed planet is sometimes drawn away from the sun1 sometimes brought nearer to him3 sometimes it is accelerated in its motion1 and sometimes retarded4 ,at one time it is drawn above the plane of its orbit1 at another time below it1 according to the position of the disturbing body4 ,all such changes1 being accomplished in short periods1 some in a few months1 others in years1 or in hundreds of years1 are denominated periodic inequalities4 ,the inequalities of the other kind1 though occasioned likewise by the disturbing energy of the planets1 are entirely independent of their relative positions4 ,they depend upon the relative positions of the orbits alone1 whose forms and places in space are altered by very minute quantities1 in immense periods of time1 and are therefore called secular inequalities4 ,the periodical perturbations are compensated when the bodies return to the same relative positions with regard to one another and to the sun3 the secular inequalities are compensated when the orbits return to the same positions relatively to one another and to the plane of the ecliptic4 ,planetary motion1 including both these kinds of disturbance1 may be represented by a body revolving in an ellipse1 and making small and transient deviations1 now on one side of its path1 and now on the other1 whilst the ellipse itself is slowly1 but perpetually1 changing both in form and position4 ,the periodic inequalities are merely transient deviations of a planet from its path1 the most remarkable of which only lasts about #iah years2 but1 in consequence of the secular disturbances1 the apsides1 or extremities of the major axes of all the orbits1 have a direct but variable motion in space1 excepting those of the orbit of ,venus1 which are retrograde 7,n4 #fa71 and the lines of the nodes move with a variable velocity in a contrary direction4 ,besides these1 the inclination and excentricity of every orbit are in a state of perpetual but slow change4 ,these effects result from the disturbing action of all the planets on each4 ,but1 as it is only necessary to estimate the disturbing influence of one body at a time1 what follows may convey some idea of the manner in which one planet disturbs the elliptical motion of another4 ,suppose two planets moving in ellipses round the sun2 if one of them attracted the other and the sun with equal intensity1 and in parallel directions 7,n4 #fb71 it would have no effect in disturbing the elliptical motion4 ,the inequality of this attraction is the sole cause of perturbation1 and the difference between the disturbing planet's action on the sun and on the disturbed planet constitutes the disturbing force1 which consequently varies in intensity and direction with every change in the relative positions of the three bodies4 ,although both the sun and planet are under the influence of the disturbing force1 the motion of the disturbed planet is referred to the centre of the sun as a fixed point1 for convenience4 ,the whole force 7,n4 #fc7 which disturbs a planet is equivalent to three partial forces4 ,one of these acts on the disturbed planet1 in the direction of a tangent to its orbit1 and is called the tangential force3 it occasions secular inequalities in the form and position of the orbit in its own plane1 and is the sole cause of the periodical perturbations in the planet's longitude4 ,another acts upon the same body in the direction of its radius vector1 that is1 in the line joining the centres of the sun and planet1 and is called the radial force3 it produces periodical changes in the distance of the planet from the sun1 and affects the form and position of the orbit in its own plane4 ,the third1 which may be called the perpendicular force1 acts at right angles to the plane of the orbit1 occasions the periodic inequalities in the planet's latitude1 and affects the position of the orbit with regard to the plane of the ecliptic4 ,it has been observed1 that the radius vector of a planet1 moving in a perfectly elliptical orbit1 passes over equal spaces or areas in equal times2 a circumstance which is independent of the law of the force1 and would be the same whether it varied inversely as the square of the distance1 or not1 provided only that it be directed to the centre of the sun4 ,hence the tangential force1 not being directed to the centre1 occasions an unequable description of areas1 or1 what is the same thing1 it disturbs the motion of the planet in longitude4 ,the tangential force sometimes accelerates the planet's motion1 sometimes retards it1 and occasionally has no effect at all4 ,were the orbits of both planets circular1 a complete compensation would take place at each revolution of the two planets1 because the arcs in which the accelerations and retardations take place would be symmetrical on each side of the disturbing force4 ,for it is clear1 that if the motion be accelerated through a certain space1 and then retarded through as much1 the motion at the end of the time will be the same as if no change had taken place4 ,but1 as the orbits of the planets are ellipses1 this symmetry does not hold3 for1 as the planet moves unequably in its orbit1 it is in some positions more directly1 and for a longer time1 under the influence of the disturbing force than in others4 ,and1 although multitudes of variations do compensate each other in short periods1 there are others1 depending on peculiar relations among the periodic times of the planets1 which do not compensate each other till after one1 or even till after many revolutions of both bodies4 ,a periodical inequality of this kind in the motions of ,jupiter and ,saturn has a period of no less than #iah years4 ,the radial force1 or that part of the disturbing force which acts in the direction of the line joining the centres of the sun and disturbed planet1 has no effect on the areas1 but is the cause of periodical changes of small extent in the distance of the planet from the sun4 ,it has already been shown1 that the force producing perfectly elliptical motion varies inversely as the square of the distance1 and that a force following any other law would cause the body to move in a curve of a very different kind4 ,now1 the radial disturbing force varies directly as the distance2 and1 as it sometimes combines with and increases the intensity of the sun's attraction for the disturbed body1 and at other times opposes and consequently diminishes it1 in both cases it causes the sun's attraction to deviate from the exact law of gravity1 and the whole action of this compound central force on the disturbed body is either greater or less than what is requisite for perfectly elliptical motion4 ,when greater1 the curvature of the disturbed planet's path1 on leaving its perihelion 7,n4 #fd71 or point nearest the sun1 is greater than it would be in the ellipse1 which brings the planet to its aphelion 7,n4 #fe71 or point farthest from the sun1 before it has passed through #ahj1 as it would do if undisturbed4 ,so that in this case the apsides1 or extremities of the major axis1 advance in space4 ,when the central force is less than the law of gravity requires1 the curvature of the planet's path is less than the curvature of the ellipse4 ,so that the planet1 on leaving its perihelion1 would pass through more than #ahj before arriving at its aphelion1 which causes the apsides to recede in space 7,n4 #ff74 ,cases both of advance and recess occur during a revolution of the two planets2 but those in which the apsides advance preponderate4 ,this1 however1 is not the full amount of the motion of the apsides2 part arises also from the tangential force 7,n4 #fc71 which alternately accelerates and retards the velocity of the disturbed planet4 ,an increase in the planet's tangential velocity diminishes the curvature of its orbit1 and is equivalent to a decrease of central force4 ,on the contrary1 a decrease of the tangential velocity1 which increases the curvature of the orbit1 is equivalent to an increase of central force4 ,these fluctuations1 owing to the tangential force1 occasion an alternate recess and advance of the apsides1 after the manner already explained 7,n4 #ff74 ,an uncompensated portion of the direct motion1 arising from this cause1 conspires with that already impressed by the radial force1 and in some cases even nearly doubles the direct motion of these points4 ,the motion of the apsides may be represented by supposing a planet to move in an ellipse1 while the ellipse itself is slowly revolving about the sun in the same plane 7,n4 #fg74 ,this motion of the major axis1 which is direct in all the orbits except that of the planet ,venus1 is irregular1 and so slow that it requires more than #aji1hcj years for the major axis of the earth's orbit to accomplish a sidereal revolution 7,n4 #fh71 that is1 to return to the same stars2 and #bj1ihd years to complete its tropical revolution 7,n4 #fi71 or to return to the same equinox4 ,the difference between these two periods arises from a retrograde motion in the equinoctial point1 which meets the advancing axis before it has completed its revolution with regard to the stars4 ,the major axis of ,jupiter's orbit requires no less than #bjj1faj years to perform its sidereal revolution1 and #bb1gdh years to accomplish its tropical revolution from the disturbing action of ,saturn alone4 ,a variation in the excentricity of the disturbed planet's orbit is an immediate consequence of the deviation from elliptical curvature1 caused by the action of the disturbing force4 ,when the path of the body1 in proceeding from its perihelion to its aphelion1 is more curved than it ought to be from the effect of the disturbing forces1 it falls within the elliptical orbit1 the excentricity is diminished1 and the orbit becomes more nearly circular2 when that curvature is less than it ought to be1 the path of the planet falls without its elliptical orbit 7,n4 #ff71 and the excentricity is increased2 during these changes1 the length of the major axis is not altered1 the orbit only bulges out1 or becomes more flat 7,n4 #gj74 ,thus the variation in the excentricity arises from the same cause that occasions the motion of the apsides 7,n4 #fg74 ,there is an inseparable connection between these two elements3 they vary simultaneously1 and have the same period2 so that1 whilst the major axis revolves in an immense period of time1 the excentricity increases and decreases by very small quantities1 and at length returns to its original magnitude at each revolution of the apsides4 ,the terrestrial excentricity is decreasing at the rate of about #dj miles annually2 and1 if it were to decrease equably1 it would be #ci1hfa years before the earth's orbit became a circle4 ,the mutual action of ,jupiter and ,saturn occasions variations in the excentricity of both orbits1 the greatest excentricity of ,jupiter's orbit corresponding to the least of ,saturn's4 ,the period in which these vicissitudes are accomplished is #gj1dad years1 estimating the action of these two planets alone2 but1 if the action of all the planets were estimated1 the cycle would extend to millions of years4 ,that part of the disturbing force is now to be considered which acts perpendicularly to the plane of the orbit1 causing periodic perturbations in latitude1 secular variations in the inclination of the orbit1 and a retrograde motion to its nodes on the true plane of the ecliptic 7,n4 #ga74 ,this force tends to pull the disturbed body above1 or push 7,n4 #gb7 it below1 the plane of its orbit1 according to the relative positions of the two planets with regard to the sun1 considered to be fixed4 ,by this action1 it sometimes makes the plane of the orbit of the disturbed body tend to coincide with the plane of the ecliptic1 and sometimes increases its inclination to that plane4 ,in consequence of which1 its nodes alternately recede or advance on the ecliptic 7,n4 #gc74 ,when the disturbing planet is in the line of the disturbed planet's nodes 7,n4 #gd71 it neither affects these points1 the latitude1 nor the inclination1 because both planets are then in the same plane4 ,when it is at right angles to the line of the nodes1 and the orbit symmetrical on each side of the disturbing force1 the average motion of these points1 after a revolution of the disturbed body1 is retrograde1 and comparatively rapid3 but1 when the disturbing planet is so situated that the orbit of the disturbed planet is not symmetrical on each side of the disturbing force1 which is most frequently the case1 every possible variety of action takes place4 ,consequently1 the nodes are perpetually advancing or receding with unequal velocity2 but1 as a compensation is not effected1 their motion is1 on the whole1 retrograde4 ,with regard to the variations in the inclination1 it is clear1 that1 when the orbit is symmetrical on each side of the disturbing force1 all its variations are compensated after a revolution of the disturbed body1 and are merely periodical perturbations in the planet's latitude2 and no secular change is induced in the inclination of the orbit4 ,when1 on the contrary1 that orbit is not symmetrical on each side of the disturbing force1 although many of the variations in latitude are transient or periodical1 still1 after a complete revolution of the disturbed body1 a portion remains uncompensated1 which forms a secular change in the inclination of the orbit to the plane of the ecliptic4 ,it is true1 part of this secular change in the inclination is compensated by the revolution of the disturbing body1 whose motion has not hitherto been taken into the account1 so that perturbation compensates perturbation2 but still a comparatively permanent change is effected in the inclination1 which is not compensated till the nodes have accomplished a complete revolution4 ,the changes in the inclination are extremely minute 7,n4 #ge71 compared with the motion of the nodes1 and there is the same kind of inseparable connection between their secular changes that there is between the variation of the excentricity and the motion of the major axis4 ,the nodes and inclinations vary simultaneously2 their periods are the same1 and very great4 ,the nodes of ,jupiter's orbit1 from the action of ,saturn alone1 require #cf1bfa years to accomplish even a tropical revolution4 ,in what precedes1 the influence of only one disturbing body has been considered2 but1 when the action and reaction of the whole system are taken into account1 every planet is acted upon1 and does itself act1 in this manner1 on all the others2 and the joint effect keeps the inclinations and excentricities in a state of perpetual variation4 ,it makes the major axes of all the orbits continually revolve1 and causes1 on an average1 a retrograde motion of the nodes of each orbit upon every other4 ,the ecliptic 7,n4 #ga7 itself is in motion from the mutual action of the earth and planets1 so that the whole is a compound phenomenon of great complexity1 extending through unknown ages4 ,at the present time the inclinations of all the orbits are decreasing1 but so slowly1 that the inclination of ,jupiter's orbit is only about six minutes less than it was in the age of ,ptolemy4 ,but1 in the midst of all these vicissitudes1 the length of the major axes and the mean motions of the planets remain permanently independent of secular changes4 ,they are so connected by ,kepler's law1 of the squares of the periodic times being proportional to the cubes of the mean distances of the planets from the sun1 that one cannot vary without affecting the other4 ,and it is proved1 that any variations which do take place are transient1 and depend only on the relative positions of the bodies4 ,it is true that1 according to theory1 the radial disturbing force should permanently alter the dimensions of all the orbits1 and the periodic times of all the planets1 to a certain degree4 ,for example1 the masses of all the planets revolving within the orbit of any one1 such as ,mars1 by adding to the interior mass1 increase the attracting force of the sun1 which1 therefore1 must contract the dimensions of the orbit of that planet1 and diminish its periodic time2 whilst the planets exterior to ,mars's orbit must have the contrary effect4 ,but the mass of the whole of the planets and satellites taken together is so small1 when compared with that of the sun1 that these effects are quite insensible1 and could only have been discovered by theory4 ,and1 as it is certain that the length of the major axes and the mean motions are not permanently changed by any other power whatever1 it may be concluded that they are invariable4 ,with the exception of these two elements1 it appears that all the bodies are in motion1 and every orbit in a state of perpetual change4 ,minute as these changes are1 they might be supposed to accumulate in the course of ages1 sufficiently to derange the whole order of nature1 to alter the relative positions of the planets1 to put an end to the vicissitudes of the seasons1 and to bring about collisions which would involve our whole system1 now so harmonious1 in chaotic confusion4 ,it is natural to inquire1 what proof exists that nature will be preserved from such a catastrophe8 ,nothing can be known from observation1 since the existence of the human race has occupied comparatively but a point in duration1 while these vicissitudes embrace myriads of ages4 ,the proof is simple and conclusive4 ,all the variations of the solar system1 secular as well as periodic1 are expressed analytically by the sines and cosines of circular arcs 7,n4 #gf71 which increase with the time2 and1 as a sine or cosine can never exceed the radius1 but must oscillate between zero and unity1 however much the time may increase1 it follows that when the variations have accumulated to a maximum by slow changes1 in however long a time1 they decrease1 by the same slow degrees1 till they arrive at their smallest value1 again to begin a new course2 thus for ever oscillating about a mean value4 ,this circumstance1 however1 would be insufficient1 were it not for the small excentricities of the planetary orbits1 their minute inclinations to the plane of the ecliptic1 and the revolutions of all the bodies1 as well planets as satellites1 in the same direction4 ,these secure the perpetual stability of the solar system 7,n4 #gg74 ,however1 at the time that the stability was proved by ,la ,grange and ,la ,place1 the telescopic planets between ,mars and ,jupiter had not been discovered2 but ,la ,grange1 having investigated the subject under a very general point of view1 showed that1 if a planetary system be composed of very unequal masses1 the whole of the larger would maintain an unalterable stability with regard to the form and position of their orbits1 while the orbits of the lesser might undergo unlimited changes4 ,m4 ,le ,verrier has applied this to the solar system1 and has found that the orbits of all the larger planets will for ever maintain an unalterable stability in form and position2 for1 though liable to mutations of very long periods1 they return again exactly to what they originally were1 oscillating between very narrow limits2 but he found a zone of instability between the orbit of ,mars1 and twice the mean distance of the earth from the sun1#a or between #ae and #bjj2 therefore the position and form of the orbits of such of the telescopic planets as revolve within that zone will be subject to unlimited variations4 ,but the orbits of those more remote from the sun than ,flora1 or beyond #bbj1 will be stable1 so that their excentricities and inclinations must always have been1 and will always remain1 very great1 since they must have depended upon the primitive conditions that prevailed when these planetary atoms were launched into space4 ,the #east of these small bodies1 which was discovered1 and the elements of its orbit determined1 by ,m4 ,valz1 at ,nimes1 has a mean distance of #ahh2 so it revolves within the zone of instability4 ,it has a shorter periodic time than any of those previously discovered1 and a greater excentricity1 with the exception of ,nysa4 ,its orbit cuts that of ,mars1 and comes nearer to the earth than the orbits of either ,mars or ,venus1 a circumstance which would be favourable for correcting the parallax of the sun1 or confirming its accuracy4 ,the telescopic planets1 numerous as they are1 have no influence on the motions of the larger planets1 for ,jupiter has a diameter of #ij1gcd miles1 while that of ,pallas1 his nearest neighbour1 is only #ig miles1 little more than the distance from ,london to ,bath4 ,the diameter of ,mars1 on the other side of the small planets1 is #dedf miles1 and that of the earth #gibe-a_/b miles1 so that the telescopic group are too minute to disturb the others4 ,m4 ,le ,verrier found another zone of instability between ,venus and the sun1 on the border of which ,mercury is revolving1 the inclination of whose orbit to the plane of the ecliptic is about #g1 which is more than that of any of the large planets4 ,neptune's orbit is1 no doubt1 as stable as that of any other of the large planets1 as the inclination is very small1 but he will have periodical variations of very long duration from the reciprocal attraction between him and ,uranus1 one especially of an enormous duration1 similar to those of ,jupiter and ,saturn1 and1 like them1 depending on the time of his revolution round the sun1 being nearly twice as long as that of ,saturn4 ,mr4 ,adams has computed that ,neptune produces a periodical perturbation in the motion of ,uranus1 whose duration is about #fhjj years4 ,the equilibrium of the system1 however1 would be deranged if the planets moved in a resisting medium 7,n4 #gh7 sufficiently dense to diminish their tangential velocity1 for then both the excentricities and the major axes of the orbits would vary with the time1 so that the stability of the system would be ultimately destroyed4 ,the existence of an ethereal medium is now proved2 and1 although it is so extremely rare that hitherto its effects on the motions of the planets have been altogether insensible1 there can be no doubt that1 in the immensity of time1 it will modify the forms of the planetary orbits1 and may at last even cause the destruction of our system1 which in itself contains no principle of decay1 unless a rotatory motion from west to east has been given to this medium by the bodies of the solar system1 which have all been revolving about the sun in that direction for unknown ages4 ,this rotation1 which seems to be highly probable1 may even have been coeval with its creation4 ,such a vortex would have no effect on bodies moving with it1 but it would influence the motions of those revolving in a contrary direction4 ,it is possible that the disturbances experienced by comets1 which have already revealed the existence of this medium1 may also1 in time1 disclose its rotatory motion4 ,the form and position of the planetary orbits1 and the motion of the bodies in the same direction1 together with the periodicity of the terms in which the inequalities are expressed1 assure us that the variations of the system are confined within very narrow limits1 and that1 although we do not know the extent of the limits1 nor the period of that grand cycle which probably embraces millions of years1 yet they never will exceed what is requisite for the stability and harmony of the whole2 for the preservation of which every circumstance is so beautifully and wonderfully adapted4 ,the plane of the ecliptic itself1 though assumed to be fixed at a given epoch for the convenience of astronomical computation1 is subject to a minute secular variation of #deg1 occasioned by the reciprocal action of the planets4 ,but1 as this is also periodical1 and cannot exceed #b #db1 the terrestrial equator1 which is inclined to it at an angle#b of #bc #bg #bhbi1 will never coincide with the plane of the ecliptic3 so there never can be perpetual spring 7,n4 #gi74 ,the rotation of the earth is uniform2 therefore day and night1 summer and winter1 will continue their vicissitudes while the system endures1 or is undisturbed by foreign causes4 ,yonder starry sphere ,of planets and of fix'd1 in all her wheels1 ,resembles nearest mazes intricate1 ,eccentric1 intervolved1 yet regular1 ,then most1 when most irregular they seem4 ,the stability of our system was established by ,la ,grange3 8a discovery10 says ,professor ,playfair1 8that must render the name for ever memorable in science1 and revered by those who delight in the contemplation of whatever is excellent and sublime40 ,after ,newton's discovery of the mechanical laws of the elliptical orbits of the planets1 that of their periodical inequalities1 by ,la ,grange1 is1 without doubt1 the noblest truth in the mechanism of the heavens2 and1 in respect of the doctrine of final causes1 it may be regarded as the greatest of all4 ,notwithstanding the permanency of our system1 the secular variations in the planetary orbits would have been extremely embarrassing to astronomers when it became necessary to compare observations separated by long periods4 ,the difficulty was in part obviated1 and the principle for accomplishing it established1 by ,la ,place1 and has since been extended by ,m4 ,poinsot4 ,it appears that there exists an invariable plane 7,n4 #hj71 passing through the centre of gravity of the system1 about which the whole oscillates within very narrow limits1 and that this plane will always remain parallel to itself1 whatever changes time may induce in the orbits of the planets1 in the plane of the ecliptic1 or even in the law of gravitation2 provided only that our system remains unconnected with any other4 ,the position of the plane is determined by this property--that1 if each particle in the system be multiplied by the area described upon this plane in a given time1 by the projection of its radius vector about the common centre of gravity of the whole1 the sum of all these products will be a maximum 7,n4 #ha74 ,la ,place found that the plane in question is inclined to the ecliptic at an angle of nearly #a #cd #ae1 and that1 in passing through the sun1 and about midway between the orbits of ,jupiter and ,saturn1 it may be regarded as the equator of the solar system1 dividing it into two parts1 which balance one another in all their motions4 ,this plane of greatest inertia1 by no means peculiar to the solar system1 but existing in every system of bodies submitted to their mutual attractions only1 always maintains a fixed position1 whence the oscillations of the system may be estimated through unlimited time4 ,future astronomers will know1 from its immutability or variation1 whether the sun and his attendants are connected or not with the other systems of the universe4 ,should there be no link between them1 it may be inferred1 from the rotation of the sun1 that the centre of gravity 7,n4 #hb7 of the system situate within his mass describes a straight line in this invariable plane or great equator of the solar system1 which1 unaffected by the changes of time1 will maintain its stability through endless ages4 ,but1 if the fixed stars1 comets1 or any unknown and unseen bodies1 affect our sun and planets1 the nodes of this plane will slowly recede on the plane of that immense orbit which the sun may describe about some most distant centre1 in a period which it transcends the power of man to determine4 ,there is every reason to believe that this is the case2 for it is more than probable that1 remote as the fixed stars are1 they in some degree influence our system1 and that even the invariability of this plane is relative1 only appearing fixed to creatures incapable of estimating its minute and slow changes during the small extent of time and space granted to the human race4 8,the development of such changes10 as ,m4 ,poinsot justly observes1 8is similar to an enormous curve1 of which we see so small an arc that we imagine it to be a straight line40 ,if we raise our views to the whole extent of the universe1 and consider the stars1 together with the sun1 to be wandering bodies1 revolving about the common centre of creation1 we may then recognise in the equatorial plane passing through the centre of gravity of the universe the only instance of absolute and eternal repose4 ,all the periodic and secular inequalities deduced from the law of gravitation are so perfectly confirmed by observation1 that analysis has become one of the most certain means of discovering the planetary irregularities1 either when they are too small1 or too long in their periods1 to be detected by other methods4 ,jupiter and ,saturn1 however1 exhibit inequalities which for a long time seemed discordant with that law4 ,all observations1 from those of the ,chinese and ,arabs down to the present day1 prove that for ages the mean motions of ,jupiter and ,saturn have been affected by a great inequality of a very long period1 forming an apparent anomaly in the theory of the planets4 ,it was long known by observation that five times the mean motion of ,saturn is nearly equal to twice that of ,jupiter2 a relation which the sagacity of ,la ,place perceived to be the cause of a periodic irregularity in the mean motion of each of these planets1 which completes its period in nearly #iah years1 the one being retarded while the other is accelerated2 but both the magnitude and period of these quantities vary1 in consequence of the secular variations in the elements of the orbits4 ,suppose the two planets to be on the same side of the sun1 and all three in the same straight line1 they are then said to be in conjunction 7,n4 #hc74 ,now1 if they begin to move at the same time1 one making exactly five revolutions in its orbit while the other only accomplishes two1 it is clear that ,saturn1 the slow-moving body1 will only have got through a part of its orbit during the time that ,jupiter has made one whole revolution and part of another1 before they be again in conjunction4 ,it is found that during this time their mutual action is such as to produce a great many perturbations which compensate each other1 but that there still remains a portion outstanding1 owing to the length of time during which the forces act in the same manner2 and1 if the conjunction always happened in the same point of the orbit1 this uncompensated inequality in the mean motion would go on increasing till the periodic times and forms of the orbits were completely and permanently changed3 a case that would actually take place if ,jupiter accomplished exactly five revolutions in the time ,saturn performed two4 ,these revolutions are1 however1 not exactly commensurable2 the points in which the conjunctions take place are in advance each time as much as #hcg2 so that the conjunctions do not happen exactly in the same points of the orbits till after a period of #hej years2 and1 in consequence of this small advance1 the planets are brought into such relative positions1 that the inequality1 which seemed to threaten the stability of the system1 is completely compensated1 and the bodies1 having returned to the same relative positions with regard to one another and the sun1 begin a new course4 ,the secular variations in the elements of the orbit increase the period of the inequality to #iah years 7,n4 #hd74 ,as any perturbation which affects the mean motion affects also the major axis1 the disturbing forces tend to diminish the major axis of ,jupiter's orbit1 and increase that of ,saturn's1 during one half of the period1 and the contrary during the other half4 ,this inequality is strictly periodical1 since it depends upon the configuration 7,n4 #he7 of the two planets2 and theory is confirmed by observation1 which shows that1 in the course of twenty centuries1 ,jupiter's mean motion has been accelerated by about #c #bc1 and ,saturn's retarded by #e #ac4 ,several instances of perturbations of this kind occur in the solar system4 ,one1 in the mean motions of the ,earth and ,venus1 only amounting to a few seconds1 has been recently worked out with immense labour by ,professor ,airy4 ,it accomplishes its changes in #bdj years1 and arises from the circumstance of thirteen times the periodic time of ,venus being nearly equal to eight times that of the ,earth4 ,small as it is1 it is sensible in the motions of the ,earth4 ,it might be imagined that the reciprocal action of such planets as have satellites would be different from the influence of those that have none4 ,but the distances of the satellites from their primaries are incomparably less than the distances of the planets from the sun1 and from one another4 ,so that the system of a planet and its satellites moves nearly as if all these bodies were united in their common centre of gravity4 ,the action of the sun1 however1 in some degree disturbs the motion of the satellites about their primary4 ,,section ,,iv4 ,theory of ,jupiter's ,satellites--,effects of the ,figure of ,jupiter upon his ,satellites--,position of their ,orbits--,singular ,laws among the ,motions of the first ,three ,satellites--,eclipses of the ,satellites--,velocity of ,light--,aberration--,ethereal ,medium--,satellites of ,saturn and ,uranus4 ,,the changes which take place in the planetary system are exhibited on a smaller scale by ,jupiter and his satellites2 and1 as the period requisite for the development of the inequalities of these moons only extends to a few centuries1 it may be regarded as an epitome of that grand cycle which will not be accomplished by the planets in myriads of ages4 ,the revolutions of the satellites about ,jupiter are precisely similar to those of the planets about the sun2 it is true they are disturbed by the sun1 but his distance is so great1 that their motions are nearly the same as if they were not under his influence4 ,the satellites1 like the planets1 were probably projected in elliptical orbits3 but1 as the masses of the satellites are nearly #ajj1jjj times less than that of ,jupiter2 and as the compression of ,jupiter's spheroid is so great1 in consequence of his rapid rotation1 that his equatorial diameter exceeds his polar diameter by no less than #fjjj miles2 the immense quantity of prominent matter at his equator must soon have given the circular form observed in the orbits of the first and second satellites1 which its superior attraction will always maintain4 ,the third and fourth satellites1 being farther removed from its influence1 revolve in orbits with a very small excentricity4 ,and1 although the first two sensibly move in circles1 their orbits acquire a small ellipticity1 from the disturbances they experience 7,n4 #hf74 ,it has been stated1 that the attraction of a sphere on an exterior body is the same as if its mass were united in one particle in its centre of gravity1 and therefore inversely as the square of the distance4 ,in a spheroid1 however1 there is an additional force arising from the bulging mass at its equator1 which1 not following the exact law of gravity1 acts as a disturbing force4 ,one effect of this disturbing force in the spheroid of ,jupiter is to occasion a direct motion in the greater axes of the orbits of all his satellites1 which is more rapid the nearer the satellite is to the planet1 and very much greater than that part of their motion which arises from the disturbing action of the sun4 ,the same cause occasions the orbits of the satellites to remain nearly in the plane of ,jupiter's equator 7,n4 #hg71 on account of which the satellites are always seen nearly in the same line 7,n4 #hh72 and the powerful action of that quantity of prominent matter is the reason why the motions of the nodes of these small bodies are so much more rapid than those of the planet4 ,the nodes of the fourth satellite accomplish a tropical revolution in #eca years1 while those of ,jupiter's orbit require no less than #cf1bfa years2--a proof of the reciprocal attraction between each particle of ,jupiter's equator and of the satellites4 ,in fact1 if the satellites moved exactly in the plane of ,jupiter's equator1 they would not be pulled out of that plane1 because his attraction would be equal on both sides of it4 ,but1 as their orbits have a small inclination to the plane of the planet's equator1 there is a want of symmetry1 and the action of the protuberant matter tends to make the nodes regress by pulling the satellites above or below the planes of their orbits2 an action which is so great on the interior satellites1 that the motions of their nodes are nearly the same as if no other disturbing force existed4 ,the orbits of the satellites do not retain a permanent inclination1 either to the plane of ,jupiter's equator1 or to that of his orbit1 but to certain planes passing between the two1 and through their intersection4 ,these have a greater inclination to his equator the farther the satellite is removed1 owing to the influence of ,jupiter's compression2 and they have a slow motion corresponding to secular variations in the planes of ,jupiter's orbit and equator4 ,the satellites are not only subject to periodic and secular inequalities from their mutual attraction1 similar to those which affect the motions and orbits of the planets1 but also to others peculiar to themselves4 ,of the periodic inequalities arising from their mutual attraction the most remarkable take place in the angular motions 7,n4 #hi7 of the three nearest to ,jupiter1 the second of which receives from the first a perturbation similar to that which it produces in the third2 and it experiences from the third a perturbation similar to that which it communicates to the first4 ,in the eclipses these two inequalities are combined into one1 whose period is #dcgfei days4 ,the variations peculiar to the satellites arise from the secular inequalities occasioned by the action of the planets in the form and position of ,jupiter's orbit1 and from the displacement of his equator4 ,it is obvious that whatever alters the relative positions of the sun1 ,jupiter1 and his satellites1 must occasion a change in the directions and intensities of the forces1 which will affect the motions and orbits of the satellites4 ,for this reason the secular variations in the excentricity of ,jupiter's orbit occasion secular inequalities in the mean motions of the satellites1 and in the motions of the nodes and apsides of their orbits4 ,the displacement of the orbit of ,jupiter1 and the variation in the position of his equator1 also affect these small bodies 7,n4 #ij74 ,the plane of ,jupiter's equator is inclined to the plane of his orbit at an angle of #c #e #cj1 so that the action of the sun and of the satellites themselves produces a nutation and precession 7,n4 #ia7 in his equator1 precisely similar to that which takes place in the rotation of the earth1 from the action of the sun and moon4 ,hence the protuberant matter at ,jupiter's equator is continually changing its position with regard to the satellites1 and produces corresponding mutations in their motions4 ,and1 as the cause must be proportional to the effect1 these inequalities afford the means1 not only of ascertaining the compression of ,jupiter's spheroid1 but they prove that his mass is not homogeneous4 ,although the apparent diameters of the satellites are too small to be measured1 yet their perturbations give the values of their masses with considerable accuracy--a striking proof of the power of analysis4 ,a singular law obtains among the mean motions and mean longitudes of the first three satellites4 ,it appears from observation that the mean motion of the first satellite1 plus twice that of the third1 is equal to three times that of the second2 and that the mean longitude of the first satellite1 minus three times that of the second1 plus twice that of the third1 is always equal to two right angles4 ,it is proved by theory1 that1 if these relations had only been approximate when the satellites were first launched into space1 their mutual attractions would have established and maintained them1 notwithstanding the secular inequalities to which they are liable4 ,they extend to the synodic motions 7,n4 #ib7 of the satellites2 consequently they affect their eclipses1 and have a very great influence on their whole theory4 ,the satellites move so nearly in the plane of ,jupiter's equator1 which has a very small inclination to his orbit1 that the first three are eclipsed at each revolution by the shadow of the planet1 which is much larger than the shadow of the moon3 the fourth satellite is not eclipsed so frequently as the others4 ,the eclipses take place close to the disc of ,jupiter when he is near opposition 7,n4 #ic72 but at times his shadow is so projected with regard to the earth1 that the third and fourth satellites vanish and reappear on the same side of the disc 7,n4 #id74 ,these eclipses are in all respects similar to those of the moon3 but1 occasionally1 the satellites eclipse ,jupiter1 sometimes passing like obscure spots across his surface1 resembling annular eclipses of the sun1 and sometimes like a bright spot traversing one of his dark belts4 ,before opposition1 the shadow of the satellite1 like a round black spot1 precedes its passage over the disc of the planet2 and1 after opposition1 the shadow follows the satellite4 ,in consequence of the relations already mentioned in the mean motions and mean longitudes of the first three satellites1 they never can be all eclipsed at the same time3 for1 when the second and third are in one direction1 the first is in the opposite direction2 consequently1 when the first is eclipsed1 the other two must be between the sun and ,jupiter4 ,the instant of the beginning or end of an eclipse of a satellite marks the same instant of absolute time to all the inhabitants of the earth2 therefore1 the time of these eclipses observed by a traveller1 when compared with the time of the eclipse computed for ,greenwich1 or any other fixed meridian 7,n4 #ie71 gives the difference of the meridians in time1 and1 consequently1 the longitude of the place of observation4 ,the longitude is determined with extreme precision whenever it is possible to convey the time instantaneously by means of electricity from one place to another1 since it obviates the errors of clocks and chronometers4 ,the eclipses of ,jupiter's satellites have been the means of a discovery which1 though not so immediately applicable to the wants of man1 unfolds one of the properties of light--that medium without whose cheering influence all the beauties of the creation would have been to us a blank4 ,it is observed1 that those eclipses of the first satellite which happen when ,jupiter is near conjunction 7,n4 #if71 are later by #af #bff than those which take place when the planet is in opposition4 ,as ,jupiter is nearer to us when in opposition by the whole breadth of the earth's orbit than when in conjunction1 this circumstance is to be attributed to the time employed by the rays of light in crossing the earth's orbit1 a distance of about #aij1jjj1jjj of miles2 whence it is estimated that light travels at the rate of #aib1jjj miles in one second4 ,such is its velocity1 that the earth1 moving at the rate of nineteen miles in a second1 would take two months to pass through a distance which a ray of light would dart over in eight minutes4 ,the subsequent discovery of the aberration of light has fully confirmed this astonishing result4 ,objects appear to be situate in the direction of the rays which proceed from them4 ,were light propagated instantaneously1 every object1 whether at rest or in motion1 would appear in the direction of these rays2 but1 as light takes some time to travel1 we see ,jupiter in conjunction1 by means of rays that left him #afm #bfs#f before2 but1 during that time1 we have changed our position1 in consequence of the motion of the earth in its orbit3 we therefore refer ,jupiter to a place in which he is not4 ,his true position is in the diagonal 7,n4 #ig7 of the parallelogram1 whose sides are in the ratio of the velocity of light to the velocity of the earth in its orbit1 which is as #aib1jjj to #ai1 or nearly as #aj1jjj to #a4 ,in consequence of the aberration of light1 the heavenly bodies seem to be in places in which they are not4 ,in fact1 if the earth were at rest1 rays from a star would pass along the axis of a telescope directed to it2 but1 if the earth were to begin to move in its orbit with its usual velocity1 these rays would strike against the side of the tube2 it would1 therefore1 be necessary to incline the telescope a little1 in order to see the star4 ,the angle contained between the axis of the telescope and a line drawn to the true place of the star is its aberration1 which varies in quantity and direction in different parts of the earth's orbit2 but1 as it is only #bjdha1 it is insensible in ordinary cases 7,n4 #ih74 ,the velocity of light deduced from the observed aberration of the fixed stars perfectly corresponds with that given by the eclipses of the first satellite4 ,the same result1 obtained from sources so different1 leaves not a doubt of its truth4 ,many such beautiful coincidences1 derived from circumstances apparently the most unpromising and dissimilar1 occur in physical astronomy1 and prove connections which we might otherwise be unable to trace4 ,the identity of the velocity of light1 at the distance of ,jupiter1 and on the earth's surface1 shows that its velocity is uniform2 and as light consists in the vibrations of an elastic medium or ether filling space1 the uniformity of its velocity shows that the density of the medium throughout the whole extent of the solar system must be proportional to its elasticity 7,n4 #ii74 ,among the fortunate conjectures which have been confirmed by subsequent experience1 that of ,bacon is not the least remarkable1 8,it produces in me10 says the restorer of true philosophy1 8a doubt whether the face of the serene and starry heavens be seen at the instant it really exists1 or not till some time later3 and whether there be not1 with respect to the heavenly bodies1 a true time and an apparent time1 no less than a true place and an apparent place1 as astronomers say1 on account of parallax4 ,for it seems incredible that the species or rays of the celestial bodies can pass through the immense interval between them and us in an instant1 or that they do not even require some considerable portion of time40 ,great discoveries generally lead to a variety of conclusions3 the aberration of light affords a direct proof of the motion of the earth in its orbit2 and its rotation is proved by the theory of falling bodies1 since the centrifugal force it induces retards the oscillations of the pendulum 7,n4 #ajj7 in going from the pole to the equator4 ,thus a high degree of scientific knowledge has been requisite to dispel the errors of the senses 7,n4 #bcg74 ,the little that is known of the theories of the satellites of ,saturn and ,uranus is1 in all respects1 similar to that of ,jupiter4 ,saturn is accompanied by eight satellites4 ,the seventh is about the size of ,mars1 and the eighth was simultaneously discovered by ,mr4 ,bond in ,america1 and that distinguished astronomer ,mr4 ,lassell1 of ,liverpool4 ,the orbits of the two last have a sensible inclination to the plane of the ring2 but the great compression of ,saturn occasions the other satellites to move nearly in the plane of his equator4 ,so many circumstances must concur to render the two interior satellites visible1 that they have very rarely been seen4 ,they move exactly at the edge of the ring1 and their orbits never deviate from its plane4 ,in #aghi ,sir ,william ,herschel saw them like beads1 threading the slender line of light which the ring is reduced to when seen edgewise from the earth4 ,and for a short time he perceived them advancing off it at each end1 when turning round in their orbits4 ,the eclipses of the exterior satellites only take place when the ring is in this position4 ,mr4 ,lassell1 with a powerful telescope1 made by himself1 has seen ,iapetus1 the nearest of the two1 on several occasions1 even when the opening of the ring was very wide1 which made it extremely difficult to see so minute an object4 ,of the situation of the equator of ,uranus we know nothing1 nor of his compression2 but the orbits of his satellites are nearly perpendicular to the plane of the ecliptic2 and1 by analogy1 they ought to be in the plane of his equator4 ,uranus is so remote that he has more the appearance of a planetary nebula than a planet1 which renders it extremely difficult to distinguish the satellites at all2 and quite hopeless without such a telescope as is rarely to be met with even in observatories4 ,sir ,william ,herschel discovered the two that are farthest from the planet1 and ascertained their approximate periods1 which his son afterwards determined to be #ac;d #aa;h #gm #abs#f and #h;d #af;h #efm #bhs#f respectively4 ,the orbits of both seem to have an inclination of about #ajab to the plane of the ecliptic4 ,the two interior satellites are so faint and small1 and so near the edge of the planet1 that they can with difficulty be seen even under the most favourable circumstances3 however1 ,mr4 ,lassell has ascertained that the more distant of the two revolves about ,uranus in #d days1 and that nearest to the planet in #b-a_/b days1 and from a long and minute examination he is convinced that the system only consists of four satellites4 ,soon after ,neptune was seen ,mr4 ,lassell discovered the only satellite known to belong to that planet4 ,the satellites of ,uranus and ,neptune1 the two planets on the remotest verge of the solar system1 offer the singular and only instance of a revolution from east to west1 while all the planets and all the other satellites revolve from west to east4 ,retrograde motion is occasionally met with in the comets and double stars4 ,,section ,v4 ,lunar ,theory--,periodic ,perturbations of the ,moon--,equation of ,centre--,evection--,variation--,annual ,equation--,direct and ,indirect ,action of ,planets--,the ,moon's ,action on the ,earth disturbs her own ,motion--,excentricity and ,inclination of ,lunar ,orbit invariable--,acceleration--,secular ,variation in ,nodes and ,perigee--,motion of ,nodes and ,perigee inseparably connected with the ,acceleration--,nutation of ,lunar ,orbit--,form and ,internal ,structure of the ,earth determined from it--,lunar1 ,solar1 and ,planetary ,eclipses--,occultations and ,lunar ,distances--,mean ,distance of the ,sun from the ,earth obtained from ,lunar ,theory--,absolute ,distances of the ,planets1 how found4 ,,our constant companion1 the moon1 next claims our attention4 ,several circumstances concur to render her motions the most interesting1 and at the same time the most difficult to investigate1 of all the bodies of our system4 ,in the solar system1 planet troubles planet2 but1 in the lunar theory1 the sun is the great disturbing cause1 his vast distance being compensated by his enormous magnitude1 so that the motions of the moon are more irregular than those of the planets2 and1 on account of the great ellipticity of her orbit1 and the size of the sun1 the approximations to her motions are tedious and difficult1 beyond what those unaccustomed to such investigations could imagine4 ,the average distance of the moon from the centre of the earth is only #bch1gic miles1 so that her motion among the stars is perceptible in a few hours4 ,she completes a circuit of the heavens in #bg;d #g;h #dcm #aas#e1 moving in an orbit whose excentricity is about #ab1ihe miles4 ,the moon is about four hundred times nearer to the earth than the sun4 ,the proximity of the moon to the earth keeps them together4 ,for so great is the attraction of the sun1 that1 if the moon were farther from the earth1 she would leave it altogether1 and would revolve as an independent planet about the sun4 ,the disturbing action 7,n4 #aja7 of the sun on the moon is equivalent to three forces4 ,the first1 acting in the direction of the line joining the moon and earth1 increases or diminishes her gravity to the earth4 ,the second1 acting in the direction of a tangent to her orbit1 disturbs her motion in longitude4 ,and the third1 acting perpendicularly to the plane of her orbit1 disturbs her motion in latitude2 that is1 it brings her nearer to1 or removes her farther from1 the plane of the ecliptic than she would otherwise be4 ,the periodic perturbations in the moon1 arising from these forces1 are perfectly similar to the periodic perturbations of the planets4 ,but they are much greater and more numerous2 because the sun is so large1 that many inequalities which are quite insensible in the motions of the planets1 are of great magnitude in those of the moon4 ,among the innumerable periodic inequalities to which the moon's motion in longitude is liable1 the most remarkable are1 the ,equation of the ,centre1 which is the difference between the moon's mean and true longitude1 the ,evection1 the ,variation1 and the ,annual ,equation4 ,the disturbing force which acts in the line joining the moon and earth produces the ,evection3 it diminishes the excentricity of the lunar orbit in conjunction and opposition1 thereby making it more circular1 and augments it in quadrature1 which consequently renders it more elliptical4 ,the period of this inequality is less than thirty-two days4 ,were the increase and diminution always the same1 the ,evection would only depend upon the distance of the moon from the sun2 but its absolute value also varies with her distance from the perigee 7,n4 #ajb7 of her orbit4 ,ancient astronomers1 who observed the moon solely with a view to the prediction of eclipses1 which can only happen in conjunction and opposition1 where the excentricity is diminished by the ,evection1 assigned too small a value to the ellipticity of her orbit 7,n4 #ajc74 ,the ,evection was discovered by ,ptolemy from observation1 about ,a4,d4 #adj4 ,the ,variation produced by the tangential disturbing force1 which is at its maximum when the moon is #de distant from the sun1 vanishes when that distance amounts to a quadrant1 and also when the moon is in conjunction and opposition2 consequently1 that inequality never could have been discovered from the eclipses3 its period is half a lunar month 7,n4 #ajd74 ,the ,annual ,equation depends upon the sun's distance from the earth3 it arises from the moon's motion being accelerated when that of the earth is retarded1 and vice vers--for1 when the earth is in its perihelion1 the lunar orbit is enlarged by the action of the sun2 therefore1 the moon requires more time to perform her revolution4 ,but1 as the earth approaches its aphelion1 the moon's orbit contracts1 and less time is necessary to accomplish her motion--its period1 consequently1 depends upon the time of the year4 ,in the eclipses the ,annual ,equation combines with the ,equation of the ,centre of the terrestrial orbit1 so that ancient astronomers imagined the earth's orbit to have a greater excentricity than modern astronomers assign to it4 ,the planets disturb the motion of the moon both directly and indirectly2 their action on the earth alters its relative position with regard to the sun and moon1 and occasions inequalities in the moon's motion1 which are more considerable than those arising from their direct action2 for the same reason the moon1 by disturbing the earth1 indirectly disturbs her own motion4 ,neither the excentricity of the lunar orbit1 nor its mean inclination to the plane of the ecliptic1 have experienced any changes from secular inequalities2 for1 although the mean action of the sun on the moon depends upon the inclination of the lunar orbit to the ecliptic1 and the position of the ecliptic is subject to a secular inequality1 yet analysis shows that it does not occasion a secular variation in the inclination of the lunar orbit1 because the action of the sun constantly brings the moon's orbit to the same inclination to the ecliptic4 ,the mean motion1 the nodes1 and the perigee1 however1 are subject to very remarkable variations4 ,from the eclipse observed at ,babylon1 on the #aith of ,march1 seven hundred and twenty-one years before the ,christian era1 the place of the moon is known from that of the sun at the instant of opposition 7,n4 #hc71 whence her mean longitude may be found4 ,but the comparison of this mean longitude with another mean longitude1 computed back for the instant of the eclipse from modern observations1 shows that the moon performs her revolution round the earth more rapidly and in a shorter time now than she did formerly1 and that the acceleration in her mean motion has been increasing from age to age as the square of the time 7,n4 #aje74 ,all ancient and intermediate eclipses confirm this result4 ,as the mean motions of the planets have no secular inequalities1 this seemed to be an unaccountable anomaly4 ,it was at one time attributed to the resistance of an ethereal medium pervading space1 and at another to the successive transmission of the gravitating force4 ,but1 as ,la ,place proved that neither of these causes1 even if they exist1 have any influence on the motions of the lunar perigee 7,n4 #ajb7 or nodes1 they could not affect the mean motion2 a variation in the mean motion from such causes being inseparably connected with variations in the motions of the perigee and nodes4 ,that great mathematician1 in studying the theory of ,jupiter's satellites1 perceived that the secular variation in the elements of ,jupiter's orbit1 from the action of the planets1 occasions corresponding changes in the motions of the satellites1 which led him to suspect that the acceleration in the mean motion of the moon might be connected with the secular variation in the excentricity of the terrestrial orbit4 ,analysis has shown that he assigned the true cause of the acceleration4 ,it is proved that the greater the excentricity of the terrestrial orbit1 the greater is the disturbing action of the sun on the moon4 ,now1 as the excentricity has been decreasing for ages1 the effect of the sun in disturbing the moon has been diminishing during that time4 ,consequently the attraction of the earth has had a more and more powerful effect on the moon1 and has been continually diminishing the size of the lunar orbit4 ,so that the moon's velocity has been gradually augmenting for many centuries to balance the increase of the earth's attraction4 ,this secular increase in the moon's velocity is called the ,acceleration1 a name peculiarly appropriate at present1 and which will continue to be so for a vast number of ages2 because1 as long as the earth's excentricity diminishes1 the moon's mean motion will be accelerated2 but when the excentricity has passed its minimum1 and begins to increase1 the mean motion will be retarded from age to age4 ,the secular acceleration is now about #aai1 but its effect on the moon's place increases as the square of the time 7,n4 #ajf74 ,it is remarkable that the action of the planets1 thus reflected by the sun to the moon1 is much more sensible than their direct action either on the earth or moon4 ,the secular diminution in the excentricity1 which has not altered the equation of the centre of the sun by eight minutes since the earliest recorded eclipses1 has produced a variation of about #a #dh in the moon's longitude1 and of #g #ab in her mean anomaly 7,n4 #ajg74 ,the action of the sun occasions a rapid but variable motion in the nodes and perigee of the lunar orbit4 ,though the nodes recede during the greater part of the moon's revolution1 and advance during the smaller1 they perform their sidereal revolution in #fgic;d #i;h #bcm #is#c1 or about #ah-f_/aj years2 and the perigee accomplishes a revolution1 called of the moon's apsides1 in #cbcb;d #ac;h #dhm #bis#f1 or a little more than nine years1 notwithstanding its motion is sometimes retrograde and sometimes direct3 but such is the difference between the disturbing energy of the sun and that of all the planets put together1 that it requires no less than #aji1hcj years for the greater axis of the terrestrial orbit to do the same1 moving at the rate of #aah annually4 ,the form of the earth has no sensible effect either on the lunar nodes or apsides4 ,it is evident that the same secular variation which changes the sun's distance from the earth1 and occasions the acceleration in the moon's mean motion1 must affect the nodes and perigee4 ,it consequently appears1 from theory as well as observation1 that both these elements are subject to a secular inequality1 arising from the variation in the excentricity of the earth's orbit1 which connects them with the ,acceleration1 so that both are retarded when the mean motion is anticipated4 ,the secular variations in these three elements are in the ratio of the numbers #c1 #jgce1 and #a2 whence the three motions of the moon1 with regard to the sun1 to her perigee1 and to her nodes1 are continually accelerated1 and their secular equations are as the numbers #a1 #dgjb1 and #jfab4 ,a comparison of ancient eclipses observed by the ,arabs1 ,greeks1 and ,chaldeans1 imperfect as they are1 with modern observations1 confirms these results of analysis4 ,future ages will develop these great inequalities1 which at some most distant period will amount to many circumferences 7,n4 #ajh74 ,they are1 indeed1 periodic2 but who shall tell their period8 ,millions of years must elapse before that great cycle is accomplished4 ,the moon is so near1 that the excess of matter at the earth's equator occasions periodic variations in her longitude1 and also that remarkable inequality in her latitude1 already mentioned as a nutation in the lunar orbit1 which diminishes its inclination to the ecliptic when the moon's ascending node coincides with the equinox of spring1 and augments it when that node coincides with the equinox of autumn4 ,as the cause must be proportional to the effect1 a comparison of these inequalities1 computed from theory1 with the same given by observation1 shows that the compression of the terrestrial spheroid1 or the ratio of the difference between the polar and the equatorial diameters1 to the diameter of the equator1 is #a_/cjeje4 ,it is proved analytically1 that1 if a fluid mass of homogeneous matter1 whose particles attract each other inversely as the squares of the distance1 were to revolve about an axis as the earth does1 it would assume the form of a spheroid whose compression is #a_/bcj4 ,since that is not the case1 the earth cannot be homogeneous1 but must decrease in density from its centre to its circumference4 ,thus the moon's eclipses show the earth to be round2 and her inequalities not only determine the form1 but even the internal structure of our planet2 results of analysis which could not have been anticipated4 ,similar inequalities in the motions of ,jupiter's satellites prove that his mass is not homogeneous1 and that his compression is #a_/ach4 ,his equatorial diameter exceeds his polar diameter by about #fjjj miles4 ,the phases 7,n4 #aji7 of the moon1 which vary from a slender silvery crescent soon after conjunction1 to a complete circular disc of light in opposition1 decrease by the same degrees till the moon is again enveloped in the morning beams of the sun4 ,these changes regulate the returns of the eclipses4 ,those of the sun can only happen in conjunction1 when the moon1 coming between the earth and the sun1 intercepts his light4 ,those of the moon are occasioned by the earth intervening between the sun and moon when in opposition4 ,as the earth is opaque and nearly spherical1 it throws a conical shadow on the side of the moon opposite to the sun1 the axis of which passes through the centres of the sun and earth 7,n4 #aaj74 ,the length of the shadow terminates at the point where the apparent diameters 7,n4 #aaa7 of the sun and earth would be the same4 ,when the moon is in opposition1 and at her mean distance1 the diameter of the sun would be seen from her centre under an angle of #aiaha4 ,that of the earth would appear under an angle of #fijhc4 ,so that the length of the shadow is at least three times and a half greater than the distance of the moon from the earth1 and the breadth of the shadow1 where it is traversed by the moon1 is about eight-thirds of the lunar diameter4 ,hence the moon would be eclipsed every time she is in opposition1 were it not for the inclination of her orbit to the plane of the ecliptic1 in consequence of which the moon1 when in opposition1 is either above or below the cone of the earth's shadow1 except when in or near her nodes4 ,her position with regard to them occasions all the varieties in the lunar eclipses4 ,every point of the moon's surface successively loses the light of different parts of the sun's disc before being eclipsed4 ,her brightness therefore gradually diminishes before she plunges into the earth's shadow4 ,the breadth of the space occupied by the penumbra 7,n4 #aab7 is equal to the apparent diameter of the sun1 as seen from the centre of the moon4 ,the mean duration of a revolution of the sun1 with regard to the node of the lunar orbit1 is to the duration of a synodic revolution 7,n4 #aac7 of the moon as #bbc to #ai4 ,so that1 after a period of #bbc lunar months1 the sun and moon would return to the same relative position with regard to the node of the moon's orbit1 and therefore the eclipses would recur in the same order were not the periods altered by irregularities in the motions of the sun and moon4 ,in lunar eclipses1 our atmosphere bends the sun's rays which pass through it all round into the cone of the earth's shadow4 ,and as the horizontal refraction 7,n4 #aad7 or bending of the rays surpasses half the sum of the semidiameters of the sun and moon1 divided by their mutual distance1 the centre of the lunar disc1 supposed to be in the axis of the shadow1 would receive the rays from the same point of the sun1 round all sides of the earth2 so that it would be more illuminated than in full moon1 if the greater portion of the light were not stopped or absorbed by the atmosphere4 ,instances are recorded where this feeble light has been entirely absorbed1 so that the moon has altogether disappeared in her eclipses4 ,the sun is eclipsed when the moon intercepts his rays 7,n4 #aae74 ,the moon1 though incomparably smaller than the sun1 is so much nearer the earth1 that her apparent diameter differs but little from his1 but both are liable to such variations that they alternately surpass one another4 ,were the eye of a spectator in the same straight line with the centres of the sun and moon1 he would see the sun eclipsed4 ,if the apparent diameter of the moon surpassed that of the sun1 the eclipse would be total4 ,if it were less1 the observer would see a ring of light round the disc of the moon1 and the eclipse would be annular1 as it was on the #agth of ,may1 #ahcf1 and on the #aeth of ,march1 #aheh4 ,if the centre of the moon should not be in the straight line joining the centres of the sun and the eye of the observer1 the moon might only eclipse a part of the sun4 ,the variation1 therefore1 in the distances of the sun and moon from the centre of the earth1 and of the moon from her node at the instant of conjunction1 occasions great varieties in the solar eclipses4 ,besides1 the height of the moon above the horizon changes her apparent diameter1 and may augment or diminish the apparent distances of the centres of the sun and moon1 so that an eclipse of the sun may occur to the inhabitants of one country1 and not to those of another4 ,in this respect the solar eclipses differ from the lunar1 which are the same for every part of the earth where the moon is above the horizon4 ,in solar eclipses1 the light reflected by the atmosphere diminishes the obscurity they produce4 ,even in total eclipses the higher part of the atmosphere is enlightened by a part of the sun's disc1 and reflects its rays to the earth4 ,the whole disc of the new moon is frequently visible from atmospheric reflection4 ,during the eclipse of the #aith of ,march1 #ahdi1 the spots on the lunar disc were distinctly visible1 and during that of #ahef the moon was like a beautiful rose-coloured ball floating in the ether3 the colour is owing to the refraction of the sun's light passing through the earth's atmosphere4 ,in total solar eclipses the slender luminous arc that is visible for a few seconds before the sun vanishes and also before he reappears1 resembles a string of pearls surrounding the dark edge of the moon2 it is occasioned by the sun's rays passing between the tops of the lunar mountains3 it occurs likewise in annular eclipses4 ,a phenomenon altogether unprecedented was seen during the total eclipse of the sun which happened on the #hth of ,july1 #ahdb4 ,the moon was like a black patch on the sky surrounded by a faint whitish light or corona about the eighth of the moon's diameter in breadth1 which is supposed to be the solar atmosphere rendered visible by the intervention of the moon4 ,in this whitish corona there appeared three rose-coloured flames like the teeth of a saw4 ,similar flames were also seen in the white corona of the total eclipse which took place in #ahea1 and a long rose-coloured chain of what appeared to be jagged mountains or sierras united at the base by a red band seemed to be raised into the corona by mirage2 but there is no doubt that the corona and red phenomena belong to the sun4 ,this red chain was so bright that ,mr4 ,airy saw it illuminate the northern horizon through an azimuth of #ij with red light4 ,m4 ,faye attributes the rose-coloured protuberances to the constitution of the sun1 which1 like ,sir ,william ,herschel1 he conceives to be an incandescent globe1 consisting of two concentric parts of very unequal density1 the internal part being a dark spherical mass1 the external a very extensive atmosphere1 at a certain height in which there is a stratum of luminous clouds which constitutes the photosphere of the sun2 above this rises his real atmosphere1 so rare as to be only visible as a white aureola or corona during total and annular eclipses4 ,m4 ,faye conceives that from the central mass gaseous eruptions issue1 which form the spots by dissipating and partly extinguishing the luminous clouds1 and then rising into the rare atmosphere above that they appear as rose-coloured protuberances during annular eclipses4 ,he estimates that the volume of these vapours sometimes surpasses that of the earth a thousand or even two thousand times4 ,sir ,william ,herschel attributed the spots to occasional openings in the luminous coating1 which seems to be always in motion2 but whatever the cause of the spots may be1 it is certainly periodical4 ,the white corona and beads were seen during the eclipse of the #aeth ,march1 #aheh1 but there were no rose-coloured appearances1 in ,england at least2 but the sky was clouded1 so that the eclipse was only visible at intervals4 ,planets sometimes eclipse one another4 ,on the #agth of ,may1 #agcg1 ,mercury was eclipsed by ,venus near their inferior conjunction2 ,mars passed over ,jupiter on the #ith of ,january1 #aeia2 and on the #cjth of ,october1 #ahbe1 the moon eclipsed ,saturn4 ,these phenomena1 however1 happen very seldom1 because all the planets1 or even a part of them1 are very rarely seen in conjunction at once2 that is1 in the same part of the heavens at the same time4 ,more than #bejj years before our era the five great planets were in conjunction4 ,on the #aeth of ,september1 #aahf1 a similar assemblage took place between the constellations of ,virgo and ,libra2 and in #ahja the ,moon1 ,jupiter1 ,saturn1 and ,venus were united in the heart of the ,lion4 ,these conjunctions are so rare1 that ,lalande has computed that more than seventeen millions of millions of years separate the epochs of the contemporaneous conjunctions of the six great planets4 ,the motions of the moon have now become of more importance to the navigator and geographer than those of any other heavenly body1 from the precision with which terrestrial longitude is determined by occultations of stars1 and by lunar distances4 ,in consequence of the retrograde motion of the nodes of the lunar orbit1 at the rate of #c #ajfd daily1 these points make a tour of the heavens in a little more than eighteen years and a half4 ,this causes the moon to move round the earth in a kind of spiral1 so that her disc at different times passes over every point in a zone of the heavens extending rather more than #e #i on each side of the ecliptic4 ,it is therefore evident that at one time or other she must eclipse every star and planet she meets with in this space4 ,therefore the occultation of a star by the moon is a phenomenon of frequent occurrence4 ,the moon seems to pass over the star1 which almost instantaneously vanishes at one side of her disc1 and after a short time as suddenly reappears on the other4 ,a lunar distance is the observed distance of the moon from the sun1 or from a particular star or planet1 at any instant4 ,the lunar theory is brought to such perfection1 that the times of these phenomena1 observed under any meridian1 when compared with those computed for that of ,greenwich1 and given in the ,nautical ,almanac1 furnish the longitude of the observer within a few miles 7,n4 #ie47 ,from the lunar theory1 the mean distance of the sun from the earth1 and thence the whole dimensions of the solar system1 are known2 for the forces which retain the earth and moon in their orbits are respectively proportional to the radii vectores of the earth and moon1 each being divided by the square of its periodic time4 ,and1 as the lunar theory gives the ratio of the forces1 the ratio of the distances of the sun and moon from the earth is obtained4 ,hence it appears that the sun's mean distance from the earth is #ciig or nearly #djj times greater than that of the moon4 ,the method of finding the absolute distances of the celestial bodies1 in miles1 is in fact the same with that employed in measuring the distances of terrestrial objects4 ,from the extremities of a known base 7,n4 #aaf71 the angles which the visual rays from the object form with it are measured2 their sum subtracted from two right angles gives the angle opposite the base2 therefore1 by trigonometry1 all the angles and sides of the triangle may be computed--consequently the distance of the object is found4 ,the angle under which the base of the triangle is seen from the object is the parallax of that object4 ,it evidently increases and decreases with the distance4 ,therefore the base must be very great indeed to be visible from the celestial bodies4 ,the globe itself1 whose dimensions are obtained by actual admeasurement1 furnishes a standard of measures with which we compare the distances1 masses1 densities1 and volumes of the sun and planets4 ,,section ,,vi4 ,form of the ,earth and ,planets--,figure of a ,homogeneous ,spheroid in ,rotation--,figure of a ,spheroid of variable ,density--,figure of the ,earth1 supposing it to be an ,ellipsoid of ,revolution--,mensuration of a ,degree of the ,meridian--,compression and ,size of the ,earth from ,degrees of ,meridian--,figure of ,earth from the ,pendulum4 ,,the theoretical investigation of the figure of the earth and planets is so complicated1 that neither the geometry of ,newton1 nor the refined analysis of ,la ,place1 has attained more than an approximation4 ,the solution of that difficult problem has been accomplished by our distinguished countryman ,mr4 ,ivory4 ,the investigation has been conducted by successive steps1 beginning with a simple case1 and then proceeding to the more difficult4 ,but1 in all1 the forces which occasion the revolutions of the earth and planets are omitted1 because1 by acting equally upon all the particles1 they do not disturb their mutual relations4 ,a fluid mass of uniform density1 whose particles mutually gravitate to each other1 will assume the form of a sphere when at rest4 ,but1 if the sphere begins to revolve1 every particle will describe a circle 7,n4 #aag71 having its centre in the axis of revolution4 ,the planes of all these circles will be parallel to one another and perpendicular to the axis1 and the particles will have a tendency to fly from that axis in consequence of the centrifugal force arising from the velocity of rotation4 ,the force of gravity is everywhere perpendicular to the surface 7,n4 #aah71 and tends to the interior of the fluid mass2 whereas the centrifugal force acts perpendicularly to the axis of rotation1 and is directed to the exterior4 ,and1 as its intensity diminishes with the distance from the axis of rotation1 it decreases from the equator to the poles1 where it ceases4 ,now it is clear that these two forces are in direct opposition to each other in the equator alone1 and that gravity is there diminished by the whole effect of the centrifugal force1 whereas1 in every other part of the fluid1 the centrifugal force is resolved into two parts1 one of which1 being perpendicular to the surface1 diminishes the force of gravity2 but the other1 being at a tangent to the surface1 urges the particles towards the equator1 where they accumulate till their numbers compensate the diminution of gravity1 which makes the mass bulge at the equator1 and become flattened at the poles4 ,it appears1 then1 that the influence of the centrifugal force is most powerful at the equator1 not only because it is actually greater there than elsewhere1 but because its whole effect is employed in diminishing gravity1 whereas1 in every other point of the fluid mass1 it is only a part that is so employed4 ,for both these reasons1 it gradually decreases towards the poles1 where it ceases4 ,on the contrary1 gravity is least at the equator1 because the particles are farther from the centre of the mass1 and increases towards the poles1 where it is greatest4 ,it is evident1 therefore1 that1 as the centrifugal force is much less than the force of gravity--gravitation1 which is the difference between the two1 is least at the equator1 and continually increases towards the poles1 where it is a maximum4 ,on these principles ,sir ,isaac ,newton proved that a homogeneous fluid 7,n4 #aai7 mass in rotation assumes the form of an ellipsoid of revolution 7,n4 #abj71 whose compression is #a_/bcj4 ,such1 however1 cannot be the form of the earth1 because the strata increase in density towards the centre4 ,the lunar inequalities also prove the earth to be so constructed2 it was requisite1 therefore1 to consider the fluid mass to be of variable density4 ,including this condition1 it has been found that the mass1 when in rotation1 would still assume the form of an ellipsoid of revolution 7,n4 #abj72 that the particles of equal density would arrange themselves in concentric elliptical strata 7,n4 #aba71 the most dense being in the centre2 but that the compression or flattening would be less than in the case of the homogeneous fluid4 ,the compression is still less when the mass is considered to be1 as it actually is1 a solid nucleus1 decreasing regularly in density from the centre to the surface1 and partially covered by the ocean1 because the solid parts1 by their cohesion1 nearly destroy that part of the centrifugal force which gives the particles a tendency to accumulate at the equator1 though not altogether2 otherwise the sea1 by the superior mobility of its particles1 would flow towards the equator and leave the poles dry4 ,besides1 it is well known that the continents at the equator are more elevated than they are in higher latitudes4 ,it is also necessary for the equilibrium of the ocean that its density should be less than the mean density of the earth1 otherwise the continents would be perpetually liable to inundations from storms and other causes4 ,on the whole1 it appears from theory1 that a horizontal line passing round the earth through both poles must be nearly an ellipse1 having its major axis in the plane of the equator1 and its minor axis coincident with the axis of the earth's rotation 7,n4 #abb74 ,it is easy to show1 in a spheroid whose strata are elliptical1 that the increase in the length of the radii 7,n4 #abc71 the decrease of gravitation1 and the increase in the length of the arcs of the meridian1 corresponding to angles of one degree1 from the poles to the equator1 are all proportional to the square of the cosine of the latitude 7,n4 #abd74 ,these quantities are so connected with the ellipticity of the spheroid1 that the total increase in the length of the radii is equal to the compression or flattening1 and the total diminution in the length of the arcs is equal to the compression1 multiplied by three times the length of an arc of one degree at the equator4 ,hence1 by measuring the meridian curvature of the earth1 the compression1 and consequently its figure1 become known4 ,this1 indeed1 is assuming the earth to be an ellipsoid of revolution2 but the actual measurement of the globe will show how far it corresponds with that solid in figure and constitution4 ,the courses of the great rivers1 which are in general navigable to a considerable extent1 prove that the curvature of the land differs but little from that of the ocean2 and1 as the heights of the mountains and continents are inconsiderable when compared with the magnitude of the earth1 its figure is understood to be determined by a surface at every point perpendicular to the direction of gravitation1 or of the plumb-line1 and is the same which the sea would have if it were continued all round the earth beneath the continents4 ,such is the figure that has been measured in the following manner3-- ,a terrestrial meridian is a line passing through both poles1 all the points of which have their noon contemporaneously4 ,were the lengths and curvatures of different meridians known1 the figure of the earth might be determined4 ,but the length of one degree is sufficient to give the figure of the earth1 if it be measured on different meridians1 and in a variety of latitudes4 ,for1 if the earth were a sphere1 all degrees would be of the same length2 but1 if not1 the lengths of the degrees would be greater1 exactly in proportion as the curvature is less4 ,a comparison of the length of a degree in different parts of the earth's surface will therefore determine its size and form4 ,an arc of the meridian may be measured by determining the latitude of its extreme points by astronomical observations 7,n4 #abe71 and then measuring the distance between them in feet or fathoms4 ,the distance thus determined on the surface of the earth1 divided by the degrees and parts of a degree contained in the difference of the latitudes1 will give the exact length of one degree1 the difference of the latitudes being the angle contained between the verticals at the extremities of the arc4 ,this would be easily accomplished were the distance unobstructed and on a level with the sea4 ,but1 on account of the innumerable obstacles on the surface of the earth1 it is necessary to connect the extreme points of the arc by a series of triangles 7,n4 #abf71 the sides and angles of which are either measured or computed1 so that the length of the arc is ascertained with much laborious calculation4 ,in consequence of the irregularities of the surface each triangle is in a different plane4 ,they must therefore be reduced by computation to what they would have been had they been measured on the surface of the sea4 ,and1 as the earth may in this case be esteemed spherical1 they require a correction to reduce them to spherical triangles4 ,the officers who conducted the trigonometrical survey1 in measuring #ejj feet of a base in ,ireland twice over1 found that the difference in the two measurements did not amount to the #hjjth part of an inch2 and in the ,general ,survey of ,great ,britain1 five bases were measured from #e to #g miles long1 and some of them #djj miles apart1 yet1 when connected by series of triangles1 the measured and computed lengths did not differ by more than #c inches1 an unparalleled degree of accuracy2 but such is the accuracy with which these operations are conducted4 ,arcs of the meridian have been measured in a variety of latitudes in both hemispheres1 as well as arcs perpendicular to the meridian4 ,from these measurements it appears that the length of the degrees increases from the equator to the poles1 nearly in proportion to the square of the sine of the latitude 7,n4 #abg74 ,consequently1 the convexity of the earth diminishes from the equator to the poles4 ,were the earth an ellipsoid of revolution1 the meridians would be ellipses whose lesser axes would coincide with the axis of rotation1 and all the degrees measured between the pole and the equator would give the same compression when combined two and two4 ,that1 however1 is far from being the case4 ,scarcely any of the measurements give exactly the same results1 chiefly on account of local attractions1 which cause the plumb-line to deviate from the vertical4 ,the vicinity of mountains produces that effect4 ,one of the most remarkable anomalies of this kind has been observed in certain localities of northern ,italy1 where the action of some dense subterraneous matter causes the plumb-line to deviate seven or eight times more than it did from the attraction of ,chimborazo1 in the observations of ,bouguer1 while measuring a degree of the meridian at the equator4 ,in consequence of this local attraction1 the degrees of the meridian in that part of ,italy seem to increase towards the equator through a small space1 instead of decreasing1 as if the earth was drawn out at the poles1 instead of being flattened4 ,many other discrepancies occur1 but from the mean of the five principal measurements of arcs in ,peru1 ,india1 ,france1 ,england1 and ,lapland1 ,mr4 ,ivory has deduced that the figure which most nearly follows this law is an ellipsoid of revolution whose equatorial radius is #cifbhbd miles1 and the polar radius #cidiehe miles4 ,the difference1 or #acbci miles1 divided by the equatorial radius1 is #a_/bii nearly#c 7,n4 #abh74 ,this fraction is called the compression of the earth1 and does not differ much from that given by the lunar inequalities4 ,since the preceding quantities were determined1 arcs of the meridian have been measured in various parts of the globe1 of which the most extensive are the ,russian arc of #be #bj between the ,glacial ,sea and the ,danube1 conducted under the superintendence of ,m4 ,struve1 and the ,indian arc extended to #ba #ba1 by ,colonel ,everest4 ,the compression deduced by ,bessel from the sum of ten arcs is #bih-c_/d1 the equatorial radius #cifbhjb1 and the polar #cidieed miles1 whilst ,mr4 ,airy arrives at an almost identical result 7#cifbhbd1 #cidiehe1 and #bih-hc_/ajj7 from a consideration of all the arcs1 measured up to #ahca1 including the great ,indian and ,russian ones4 ,if we assume the earth to be a sphere1 the length of a degree of the meridian is #fi-ad_/ajj ,english miles4 ,therefore #cfj degrees1 or the whole equatorial circumference of the globe1 is #bd1hii ,english miles4 ,eratosthenes1 who died #aid years before the ,christian era1 was the first to give an approximate value of the earth's circumference1 by the measurement of an arc between ,alexandria and ,syene4 ,there is another method of finding the figure of the earth1 totally different from the preceding1 solely depending upon the increase of gravitation from the equator to the poles4 ,the force of gravitation at any place is measured by the descent of a heavy body during the first second of its fall4 ,and the intensity of the centrifugal force is measured by the deflection of any point from the tangent in a second4 ,for1 since the centrifugal force balances the attraction of the earth1 it is an exact measure of the gravitating force4 ,were the attraction to cease1 a body on the surface of the earth would fly off in the tangent by the centrifugal force1 instead of bending round in the circle of rotation4 ,therefore1 the deflection of the circle from the tangent in a second measures the intensity of the earth's attraction1 and is equal to the versed sine of the arc described during that time1 a quantity easily determined from the known velocity of the earth's rotation4 ,whence it has been found that at the equator the centrifugal force is equal to the #bhith part of gravity4 ,now1 it is proved by analysis that1 whatever the constitution of the earth and planets may be1 if the intensity of gravitation at the equator be taken equal to unity1 the sum of the compression of the ellipsoid1 and the whole increase of gravitation from the equator to the pole1 is equal to five halves of the ratio of the centrifugal force to gravitation at the equator4 ,this quantity with regard to the earth is #e_/b of #a_/bhi or #a_/aaeb4 ,consequently1 the compression of the earth is equal to #a_/aaeb diminished by the whole increase of gravitation4 ,so that its form will be known1 if the whole increase of gravitation from the equator to the pole can be determined by experiment4 ,this has been accomplished by a method founded upon the following considerations3--,if the earth were a homogeneous sphere without rotation1 its attraction on bodies at its surface would be everywhere the same4 ,if it be elliptical and of variable density1 the force of gravity1 theoretically1 ought to increase from the equator to the pole1 as unity plus a constant quantity multiplied into the square of the sine of the latitude 7,n4 #abg74 ,but for a spheroid in rotation the centrifugal force varies1 by the laws of mechanics1 as the square of the sine of the latitude1 from the equator1 where it is greatest1 to the pole1 where it vanishes4 ,and1 as it tends to make bodies fly off the surface1 it diminishes the force of gravity by a small quantity4 ,hence1 by gravitation1 which is the difference of these two forces1 the fall of bodies ought to be accelerated from the equator to the poles proportionably to the square of the sine of the latitude2 and the weight of the same body ought to increase in that ratio4 ,this is directly proved by the oscillations of the pendulum 7,n4 #abi71 which1 in fact1 is a falling body2 for1 if the fall of bodies be accelerated1 the oscillations will be more rapid3 in order1 therefore1 that they may always be performed in the same time1 the length of the pendulum must be altered4 ,by numerous and careful experiments it is proved that a pendulum1 which oscillates #hf1djj times in a mean day at the equator1 will do the same at every point of the earth's surface1 if its length be increased progressively to the pole1 as the square of the sine of the latitude4 ,from the mean of these it appears that the whole decrease of gravitation from the poles to the equator is #jjjeaddi1 which1 subtracted from #a_/aaeb1 shows that the compression of the terrestrial spheroid is about #a_/bhebf4 ,this value has been deduced by the late ,mr4 ,baily1 president of the ,astronomical ,society1 who devoted much attention to this subject2 at the same time1 it may be observed that no two sets of pendulum experiments give the same result1 probably from local attractions4 ,the compression obtained by this method does not differ much from that given by the lunar inequalities1 nor from the arcs in the direction of the meridian1 and those perpendicular to it4 ,the near coincidence of these three values1 deduced by methods so entirely independent of each other1 shows that the mutual tendencies of the centres of the celestial bodies to one another1 and the attraction of the earth for bodies at its surface1 result from the reciprocal attraction of all their particles4 ,another proof may be added4 ,the nutation of the earth's axis and the precession of the equinoxes 7,n4 #adf7 are occasioned by the action of the sun and moon on the protuberant matter at the earth's equator4 ,and1 although these inequalities do not give the absolute value of the terrestrial compression1 they show that the fraction expressing it is comprised between the limits #a_/bgi and #a_/egc4 ,it might be expected that the same compression should result from each1 if the different methods of observation could be made without error4 ,this1 however1 is not the case2 for after allowance has been made for every cause of error1 such discrepancies are found1 both in the degrees of the meridian and in the length of the pendulum1 as show that the figure of the earth is very complicated4 ,but they are so small1 when compared with the general results1 that they may be disregarded4 ,the compression deduced from the mean of the whole appears not to differ much from #a_/cjj2 that given by the lunar theory has the advantage of being independent of the irregularities of the earth's surface and of local attractions4 ,the regularity with which the observed variation in the length of the pendulum follows the law of the square of the sine of the latitude proves the strata to be elliptical1 and symmetrically disposed round the centre of gravity of the earth1 which affords a strong presumption in favour of its original fluidity4 ,it is remarkable how little influence the sea has on the variation of the lengths of the arcs of the meridian1 or on gravitation2 neither does it much affect the lunar inequalities1 from its density being only about a fifth of the mean density of the earth4 ,for1 if the earth were to become fluid1 after being stripped of the ocean1 it would assume the form of an ellipsoid of revolution whose compression is #a_/cjdh1 which differs very little from that determined by observation1 and proves1 not only that the density of the ocean is inconsiderable1 but that its mean depth is very small4 ,there are profound cavities in the bottom of the sea1 but its mean depth probably does not much exceed the mean height of the continents and islands above its level4 ,on this account1 immense tracts of land may be deserted or overwhelmed by the ocean1 as appears really to have been the case1 without any great change in the form of the terrestrial spheroid4 ,the variation in the length of the pendulum was first remarked by ,richter in #afgb1 while observing transits of the fixed stars across the meridian at ,cayenne1 about five degrees north of the equator4 ,he found that his clock lost at the rate of #bm #bhs daily1 which induced him to determine the length of a pendulum beating seconds in that latitude2 and1 repeating the experiments on his return to ,europe1 he found the seconds' pendulum at ,paris to be more than the twelfth of an inch longer than that at ,cayenne4 ,the form and size of the earth being determined1 a standard of measure is furnished with which the dimensions of the solar system may be compared4 ,,section ,,vii4 ,parallax--,lunar ,parallax found from ,direct ,observation--,solar ,parallax deduced from the ,transit of ,venus--,distance of the ,sun from the ,earth--,annual ,parallax--,distance of the ,fixed ,stars4 ,,the parallax of a celestial body is the angle under which the radius of the earth would be seen if viewed from the centre of that body2 it affords the means of ascertaining the distances of the sun1 moon1 and planets 7,n4 #acj74 ,when the moon is in the horizon at the instant of rising or setting1 suppose lines to be drawn from her centre to the spectator and to the centre of the earth3 these would form a right-angled triangle with the terrestrial radius1 which is of a known length2 and1 as the parallax or angle at the moon can be measured1 all the angles and one side are given2 whence the distance of the moon from the centre of the earth may be computed4 ,the parallax of an object may be found1 if two observers under the same meridian1 but at a very great distance from one another1 observe its zenith distances on the same day at the time of its passage over the meridian4 ,by such contemporaneous observations at the ,cape of ,good ,hope and at ,berlin1 the mean horizontal parallax of the moon was found to be #cdei1 whence the mean distance of the moon is about sixty times the greatest terrestrial radius1 or #bcg1fjh miles nearly4#d ,since the parallax is equal to the radius of the earth divided by the distance of the moon1 it varies with the distance of the moon from the earth under the same parallel of latitude1 and proves the ellipticity of the lunar orbit4 ,when the moon is at her mean distance1 it varies with the terrestrial radii1 thus showing that the earth is not a sphere 7,n4 #aca74 ,although the method described is sufficiently accurate for finding the parallax of an object as near as the moon1 it will not answer for the sun1 which is so remote that the smallest error in observation would lead to a false result4 ,but that difficulty is obviated by the transits of ,venus4 ,when that planet is in her nodes 7,n4 #acb71 or within #a-a_/d of them1 that is1 in1 or nearly in1 the plane of the ecliptic1 she is occasionally seen to pass over the sun like a black spot4 ,if we could imagine that the sun and ,venus had no parallax1 the line described by the planet on his disc1 and the duration of the transit1 would be the same to all the inhabitants of the earth4 ,but1 as the semi-diameter of the earth has a sensible magnitude when viewed from the centre of the sun1 the line described by the planet in its passage over his disc appears to be nearer to his centre1 or farther from it1 according to the position of the observer2 so that the duration of the transit varies with the different points of the earth's surface at which it is observed 7,n4 #acc74 ,this difference of time1 being entirely the effect of parallax1 furnishes the means of computing it from the known motions of the earth and ,venus1 by the same method as for the eclipses of the sun4 ,in fact1 the ratio of the distances of ,venus and the sun from the earth at the time of the transit is known from the theory of their elliptical motion4 ,consequently the ratio of the parallaxes of these two bodies1 being inversely as their distances1 is given2 and as the transit gives the difference of the parallaxes1 that of the sun is obtained4 ,in #agfi the parallax of the sun was determined by observations of a transit of ,venus made at ,wardhus in ,lapland1 and at ,tahiti in the ,south ,sea4 ,the latter observation was the object of ,cook's first voyage4 ,the transit lasted about six hours at ,tahiti1 and the difference in duration at these two stations was eight minutes2 whence the sun's horizontal parallax was found to be #hgb4 ,but by other considerations it has been reduced by ,professor ,encke to #heggf2 from which the mean distance of the sun appears to be about ninety-five millions of miles4 ,this is confirmed by an inequality in the motion of the moon1 which depends upon the parallax of the sun1 and which1 when compared with observation1 gives #hf for the sun's parallax4 ,the transits of ,venus in #ahgd and #ahhb will be unfavourable for ascertaining the accuracy of the solar parallax1 and no other transit of that planet will take place till the twenty-first century2 but in the mean time recourse may be had to the oppositions of ,mars4 ,the parallax of ,venus is determined by her transits2 that of ,mars by direct observation1 and it is found to be nearly double that of the sun1 when the planet is in opposition4 ,the distance of these two planets from the earth is therefore known in terrestrial radii1 consequently their mean distances from the sun may be computed2 and as the ratios of the distances of the planets from the sun are known by ,kepler's law1 of the squares of the periodic times of any two planets being as the cubes of their mean distances from the sun1 their absolute distances in miles are easily found 7,n4 #acd74 ,this law is very remarkable1 in thus uniting all the bodies of the system1 and extending to the satellites as well as the planets4 ,far as the earth seems to be from the sun1 ,uranus is no less than nineteen1 and ,neptune thirty times farther4 ,situate on the verge of the system1 the sun must appear from ,uranus not much larger than ,venus does to us1 and from ,neptune as a star of the fifth magnitude4 ,the earth cannot even be visible as a telescopic object to a body so remote as either ,uranus or ,neptune4 ,yet man1 the inhabitant of the earth1 soars beyond the vast dimensions of the system to which his planet belongs1 and assumes the diameter of its orbit as the base of a triangle whose apex extends to the stars4 ,sublime as the idea is1 this assumption proves ineffectual1 except in a very few cases2 for the apparent places of the fixed stars are not sensibly changed by the earth's annual revolution4 ,with the aid derived from the refinements of modern astronomy1 and of the most perfect instruments1 a sensible parallax has been detected only in a very few of these remote suns4 ,centauri has a parallax of one second of space1 therefore it is the nearest known star1 and yet it is more than two hundred thousand times farther from us than the sun is4 ,at such a distance not only the terrestrial orbit shrinks to a point1 but the whole solar system1 seen in the focus of the most powerful telescope1 might be eclipsed by the thickness of a spider's thread4 ,light1 flying at the rate of #aij1jjj miles in a second1 would take more than three years to travel over that space4 ,one of the nearest stars may therefore have been kindled or extinguished more than three years before we could have been aware of so mighty an event4 ,but this distance must be small when compared with that of the most remote of the bodies which are visible in the heavens4 ,the fixed stars are undoubtedly luminous like the sun3 it is therefore probable that they are not nearer to one another than the sun is to the nearest of them4 ,in the milky way and the other starry nebul1 some of the stars that seem to us to be close to others may be far behind them in the boundless depth of space2 nay1 may be rationally supposed to be situate many thousand times farther off4 ,light would therefore require thousands of years to come to the earth from those myriads of suns of which our own is but 8the remote companion40 ,,section ,,viii4 ,masses of ,planets that have no ,satellites determined from their ,perturbations--,masses of the others obtained from the ,motions of their ,satellites--,masses of the ,sun1 the ,earth1 of ,jupiter and of the ,jovial ,system--,mass of the ,moon--,real ,diameters of ,planets1 how obtained--,size of ,sun1 ,densities of the ,heavenly ,bodies--,formation of ,astronomical ,tables--,requisite ,data and ,means of obtaining them4 ,,the masses of such planets as have no satellites are known by comparing the inequalities they produce in the motions of the earth and of each other1 determined theoretically1 with the same inequalities given by observation2 for the disturbing cause must necessarily be proportional to the effect it produces4 ,the masses of the satellites themselves may also be compared with that of the sun by their perturbations4 ,thus1 it is found1 from the comparison of a vast number of observations with ,la ,place's theory of ,jupiter's satellites1 that the mass of the sun is no less than #fe1jjj1jjj times greater than the least of these moons4 ,but1 as the quantities of matter in any two primary planets are directly as the cubes of the mean distances at which their satellites revolve1 and inversely as the squares of their periodic times 7,n4 #ace71 the mass of the sun and of any planets which have satellites may be compared with the mass of the earth4 ,in this manner it is computed that the mass of the sun is #ced1icf times that of the earth2 whence the great perturbations of the moon1 and the rapid motion of the perigee and nodes of her orbit 7,n4 #acf74 ,even ,jupiter1 the largest of the planets1 has been found by ,professor ,airy to be #ajdghga times less than the sun2 and1 indeed1 the mass of the whole ,jovial system is not more than the #ajeddth part of that of the sun4 ,so that the mass of the satellites bears a very small proportion to that of their primary4 ,the mass of the moon is determined from several sources--from her action on the terrestrial equator1 which occasions the nutation in the axis of rotation2 from her horizontal parallax2 from an inequality she produces in the sun's longitude2 and from her action on the tides4 ,the three first quantities1 computed from theory and compared with their observed values1 give her mass respectively equal to the #a_/ga1 #a_/gdb1 and #a_/fib1 part of that of the earth1 which do not differ much from each other4 ,dr4 ,brinkley has found it to be #a_/hj from the constant of lunar nutation3 but1 from the moon's action in raising the tides1 her mass appears to be about the #a_/ge part of that of the earth--a value that cannot differ much from the truth4 ,the apparent diameters of the sun1 moon1 and planets are determined by measurement2 therefore their real diameters may be compared with that of the earth2 for the real diameter of a planet is to the real diameter of the earth1 or #gibf miles1 as the apparent diameter of the planet to the apparent diameter of the earth as seen from the planet1 that is1 to twice the parallax of the planet4 ,according to ,bessel1 the mean apparent diameter of the sun is #aibcfd1 and with the solar parallax #heggf1 it will be found that the diameter of the sun is about #hhf1hgg miles4 ,therefore1 if the centre of the sun were to coincide with the centre of the earth1 his volume would not only include the orbit of the moon1 but would extend nearly as far again2 for the moon's mean distance from the earth is about sixty times the earth's equatorial radius1 or #bch1gic miles3 so that twice the distance of the moon is #dgg1ehf miles1 which differs but little from the solar radius2 his equatorial radius is probably not much less than the major axis of the lunar orbit4 ,the diameter of the moon is only #bafj miles2 and ,jupiter's diameter of #hh1bjj miles is very much less than that of the sun2 the diameter of ,pallas does not much exceed #gi miles1 so that an inhabitant of that planet1 in one of our steam carriages1 might go round his world in a few hours4 ,the diameters of ,lutetia and ,atalanta are only #h and #d miles respectively2 but the whole of the #ee telescopic planets are so small1 that their united mass is probably not more than the fifth or sixth part of that of the moon4 ,the densities of bodies are proportional to their masses1 divided by their volumes4 ,hence1 if the sun and planets be assumed to be spheres1 their volumes will be as the cubes of their diameters4 ,now1 the apparent diameters of the sun and earth1 at their mean distance1 are #aibcf and #agaeeb1 and the mass of the earth is the #ced1icfth part of that of the sun taken as the unit4 ,it follows1 therefore1 that the earth is four times as dense as the sun4 ,but the sun is so large that his attractive force would cause bodies to fall through about #ccdfe feet in a second4 ,consequently1 if he were habitable by human beings1 they would be unable to move1 since their weight would be thirty times as great as it is here4 ,a man of moderate size would weigh about two tons at the surface of the sun2 whereas at the surface of some of the new planets he would be so light that it would be impossible to stand steady1 since he would only weigh a few pounds4 ,the mean density of the earth has been determined by the following method4 ,since a comparison of the action of two planets upon a third gives the ratio of the masses of these two planets1 it is clear that1 if we can compare the effect of the whole earth with the effect of any part of it1 a comparison may be instituted between the mass of the whole earth and the mass of that part of it4 ,now a leaden ball was weighed against the earth by comparing the effects of each upon a pendulum2 the nearness of the smaller mass making it produce a sensible effect as compared with that of the larger3 for by the laws of attraction the whole earth must be considered as collected in its centre4 ,by this method it has been found that the mean density of the earth is #effj times greater than that of water at the temperature of #fb of ,fahrenheit's thermometer4 ,the late ,mr4 ,baily1 whose accuracy as an experimental philosopher is acknowledged1 was unremittingly occupied nearly four years in accomplishing this very important object4 ,in order to ascertain the mean density of the earth still more perfectly1 ,mr4 ,airy made a series of experiments to compare the simultaneous oscillations of two pendulums1 one at the bottom of the ,harton coal-pit1 #abfj feet deep1 in ,northumberland1 and the other on the surface of the earth immediately above it4 ,the oscillations of the pendulums were compared with an astronomical clock at each station1 and the time was instantaneously transmitted from one to the other by a telegraphic wire4 ,the oscillations were observed for more than #ajj hours continuously1 when it was found that the lower pendulum made #b-a_/b oscillations more in #bd hours than the upper one4 ,the experiment was repeated for the same length of time with the same result2 but on this occasion the upper pendulum was taken to the bottom of the mine and the lower brought to the surface4 ,from the difference between the oscillations at the two stations it appears that gravitation at the bottom of the mine exceeds that at the surface by the #a_/aiaij part1 and that the mean density of the earth is #fefe1 which is greater than that obtained by ,mr4 ,baily by #hi4 ,while employed on the trigonometrical survey of ,scotland1 ,colonel ,james determined the mean density of the earth to be #ecaf1 from a deviation of the plumb-line amounting to #b1 caused by the attraction of ,arthur's ,seat and the heights east of ,edinburgh3 it agrees more nearly with the density found by ,mr4 ,baily than with that deduced from ,mr4 ,airy's experiments4 ,all the planets and satellites appear to be of less density than the earth4 ,the motions of ,jupiter's satellites show that his density increases towards his centre4 ,were his mass homogeneous1 his equatorial and polar axes would be in the ratio of #da to #cf1 whereas they are observed to be only as #da to #ch4 ,the singular irregularities in the form of ,saturn1 and the great compression of ,mars1 prove the internal structure of these two planets to be very far from uniform4 ,before entering on the theory of rotation1 it may not be foreign to the subject to give some idea of the methods of computing the places of the planets1 and of forming astronomical tables4 ,astronomy is now divided into the three distinct departments of theory1 observation1 and computation4 ,since the problem of the three bodies can only be solved by approximation1 the analytical astronomer determines the position of a planet in space by a series of corrections4 ,its place in its circular orbit is first found1 then the addition or subtraction of the equation of the centre 7,n4 #dh7 to or from its mean place gives its position in the ellipse4 ,this again is corrected by the application of the principal periodic inequalities4 ,but1 as these are determined for some particular position of the three bodies1 they require to be corrected to suit other relative positions4 ,this process is continued till the corrections become less than the errors of observation1 when it is obviously unnecessary to carry the approximation further4 ,the true latitude and distance of the planet from the sun are obtained by methods similar to those employed for the longitude4 ,as the earth revolves equably about its axis in #bd hours1 at the rate of #ae in an hour1 time becomes a measure of angular motion1 and the principal element in astronomy1 where the object is to determine the exact state of the heavens and the successive changes it undergoes in all ages1 past1 present1 and to come4 ,now1 the longitude1 latitude1 and distance of a planet from the sun are given in terms of the time1 by general analytical formul4 ,these formul will consequently give the exact place of the body in the heavens1 for any time assumed at pleasure1 provided they can be reduced to numbers4 ,but before the calculator begins his task the observer must furnish the necessary data1 which are1 obviously1 the forms of the orbits1 and their positions with regard to the plane of the ecliptic 7,n4 #eg74 ,it is therefore necessary to determine by observation1 for each planet1 the length of the major axis of its orbit1 the excentricity1 the inclination of the orbit to the plane of the ecliptic1 the longitudes of its perihelion and ascending node at a given time1 the periodic time of the planet1 and its longitude at any instant arbitrarily assumed1 as an origin from whence all its subsequent and antecedent longitudes are estimated4 ,each of these quantities is determined from that position of the planet on which it has most influence4 ,for example1 the sum of the greatest and least distances of the planet from the sun is equal to the major axis of the orbit1 and their difference is equal to twice the excentricity4 ,the longitude of the planet1 when at its least distance from the sun1 is the same with the longitude of the perihelion2 the greatest latitude of the planet is equal to the inclination of the orbit3 the longitude of the planet1 when in the plane of the ecliptic in passing towards the north1 is the longitude of the ascending node1 and the periodic time is the interval between two consecutive passages of the planet through the same node1 a small correction being made for the precession of the node during the revolution of the planet 7,n4 #acg74 ,notwithstanding the excellence of instruments and the accuracy of modern observers1 unavoidable errors of observation can only be compensated by finding the value of each element from the mean of a thousand1 or even many thousands of observations4 ,for as it is probable that the errors are not all in one direction1 but that some are in excess and others in defect1 they will compensate each other when combined4 ,however1 the values of the elements determined separately can only be regarded as approximate1 because they are so connected that the estimation of any one independently will induce errors in the others4 ,the excentricity depends upon the longitude of the perihelion1 the mean motion depends upon the major axis1 the longitude of the node upon the inclination of the orbit1 and vice vers4 ,consequently1 the place of a planet computed with the approximate data will differ from its observed place4 ,then the difficulty is to ascertain what elements are most in fault1 since the difference in question is the error of all2 that is obviated by finding the errors of some thousands of observations1 and combining them1 so as to correct the elements simultaneously1 and to make the sum of the squares of the errors a minimum with regard to each element 7,n4 #ach74 ,the method of accomplishing this depends upon the ,theory of ,probabilities2 a subject fertile in most important results in the various departments of science and of civil life1 and quite indispensable in the determination of astronomical data4 ,a series of observations continued for some years will give approximate values of the secular and periodic inequalities1 which must be corrected from time to time1 till theory and observation agree4 ,and these again will give values of the masses of the bodies forming the solar system1 which are important data in computing their motions4 ,the periodic inequalities derived from a great number of observations are employed for the determination of the values of the masses till such time as the secular inequalities shall be perfectly known1 which will then give them with all the necessary precision4 ,when all these quantities are determined in numbers1 the longitude1 latitude1 and distance of the planet from the sun are computed for stated intervals1 and formed into tables1 arranged according to the time estimated from a given epoch1 so that the place of the body may be determined from them by inspection alone1 at any instant for perhaps a thousand years before and after that epoch4 ,by this tedious process1 tables have been computed for all the great planets1 and several of the small1 besides the moon and the satellites of ,jupiter4 ,in the present state of astronomy the masses and elements of the orbits are pretty well known1 so that the tables only require to be corrected from time to time as observations become more accurate4 ,those containing the motions of ,jupiter1 ,saturn1 and ,uranus have already been twice constructed within the last thirty years1 and the tables of ,jupiter and ,saturn agree almost perfectly with modern observation4 ,the following prediction will be found in the sixth edition of this book1 published in the year #ahdb3 8,those of ,uranus1 however1 are already defective1 probably because the discovery of that planet in #agha is too recent to admit of much precision in the determination of its motions1 or that possibly it may be subject to disturbances from some unseen planet revolving about the sun beyond the present boundaries of our system4 ,if1 after a lapse of years1 the tables formed from a combination of numerous observations should be still inadequate to represent the motions of ,uranus1 the discrepancies may reveal the existence1 nay1 even the mass and orbit1 of a body placed for ever beyond the sphere of vision40#e ,that prediction has been fulfilled since the seventh edition of this book was published4 ,not only the existence of ,neptune1 revolving at the distance of three thousand millions of miles from the sun1 has been discovered from his disturbing action on ,uranus1 but his mass1 the form and position of his orbit in space1 and his periodic time had been determined before the planet had been seen1 and the planet itself was discovered in the very point of the heavens which had been assigned to it4 ,it had been noticed for years that the perturbation of ,uranus had increased in an unaccountable manner 7,n4 #aci74 ,after the disturbing action of all the known planets had been determined1 it was found that1 between the years #ahcc and #ahcg1 the observed and computed distance of ,uranus from the sun differed by #bdj1jjj miles1 which is about the mean distance of the moon from the earth1 while1 in #ahda1 the error in the geocentric longitude of the planet amounted to #if4 ,these discrepancies were therefore attributed to the attraction of some unseen and unknown planet1 consequently they gave rise to a case altogether unprecedented in the history of astronomy4 ,heretofore it was required to determine the disturbing action of one known planet upon another4 ,whereas the inverse problem had now to be solved1 in which it was required to find the place of an unknown body in the heavens1 at a given time1 together with its mass1 and the form and position of its orbit1 from the disturbance it produced on the motions of another4 ,the difficulty was extreme1 because all the elements of the orbit of ,uranus were erroneous from the action of ,neptune1 and those of ,neptune's orbit were unknown4 ,in this dilemma it was necessary to form some hypothesis with regard to the unknown planet2 it was therefore assumed1 according to ,bode's empirical law on the mean distances of the planets1 that it was revolving at twice the distance of ,uranus from the sun4 ,in fact1 the periodic time of ,uranus is about #hd years1 and1 as the discrepancies in his motions increased slowly and regularly1 it was evident that it would require a planet with a much longer periodic time to produce them--moreover1 it was clear that the new planet must be exterior to ,uranus1 otherwise it would have disturbed the motions of ,saturn4 ,another circumstance tended to lessen the difficulty2 the latitude of ,uranus was not much affected1 therefore it was concluded that the inclination of the orbit of the unknown body must be very small1 and1 as that of the orbit of ,uranus is only #df #bhd1 both planets were assumed to be moving in the plane of the ecliptic1 and thus the elements of the orbit of the unknown planet were reduced from six to four4 ,having thus assumed that the unknown body was revolving in a circle in the plane of the ecliptic1 the analytical expression of its action on the motion of ,uranus1 when in numerous points of its orbit1 was compared with the observed longitude of ,uranus1 through a regular series of years1 by means of which the faulty elements of the orbit of ,uranus were eliminated1 or got rid of1 and there only remained a relation between the mass of the new planet and three of the elements of its orbit2 and it then was necessary to assume such a value for two of them as would suit the rest4 ,that was accomplished so dexterously1 that the perturbations of ,uranus were perfectly conformable to the motions of ,neptune1 moving in the orbit thus found1 and the place of the new planet exactly agreed with observation4 ,subsequently its orbit and motions have been determined more accurately4 ,the honour of this admirable effort of genius is shared by ,mr4 ,adams and ,m4 ,le ,verrier1 who1 independently of each other1 arrived at these wonderful results4 ,mr4 ,adams had determined the mass and apparent diameter of ,neptune1 with all the circumstances of its motion1 eight months before ,m4 ,le ,verrier had terminated his results1 and had also pointed out the exact spot where the planet would be found2 but the ,english observers neglected to look for it till ,m4 ,le ,verrier made known his researches1 and communicated its position to ,dr4 ,galle1 at ,berlin1 who found it the very first night he looked for it1 and then it was evident that it would have been seen in the place ,mr4 ,adams had assigned to it eight months before had it been looked for4 ,so closely did the results of these two great mathematicians agree4 ,neptune has a diameter of #ci1gic miles1 consequently he is nearly #bjj times larger than the earth1 and may be seen with a telescope of moderate power4 ,his motion is retrograde at present1 and six times slower than that of the earth4 ,at so great a distance from the sun it can only have the #a_/acjjth part of the light and heat the earth receives2 but having a satellite1 the deficiency of light may in some measure be supplied4 ,the prediction may now be transferred from ,uranus to ,neptune1 whose perturbations may reveal the existence of a planet still further removed1 which may for ever remain beyond the reach of telescopic vision--yet its mass1 the form and position of its orbit1 and all the circumstances of its motion may become known1 and the limits of the solar system may still be extended hundreds of millions of miles4 ,the mean distance of ,neptune from the sun has subsequently proved to be only #bhic millions of miles1 and the period of his revolution #aff years1 so that ,baron ,bode's law1 of the interval between the orbits of any two planets being twice as great as the inferior interval and half of the superior1 fails in the case of ,neptune1 though it was useful on the first approximation to his motions2 and since ,bode's time it has led to the discovery of fifty-five telescopic planets revolving between the orbits of ,mars and ,jupiter1 some by chance1 others by a systematic search on the faith that these minute planets are fragments of a larger body that has exploded1 because their distances from the sun are nearly the same2 the lines of the nodes of some of their orbits terminate in the same points of the heavens1 and the inclinations of their orbits are such as might have taken place from their mutual disturbances at the time of the explosion1 and while yet they were near enough for their forms to affect their motions4 ,the orbits of the more recently discovered asteroids show that this hypothesis is untenable4 ,the tables of ,mars1 ,venus1 and even those of the sun1 have been greatly improved1 and still engage the attention of our ,astronomer ,royal1 ,mr4 ,airy1 and other eminent astronomers4 ,we are chiefly indebted to the ,german astronomers for tables of the four older telescopic planets1 ,vesta1 ,juno1 ,ceres1 and ,pallas2 the others have only been discovered since the year #ahde4 ,the determination of the path of a planet when disturbed by all the others1 a problem which has employed the talents of the greatest astronomers1 from ,newton to the present day1 is only successfully accomplished with regard to the older planets1 which revolve in nearly circular orbits1 but little inclined to the plane of the ecliptic4 ,when the excentricity and inclination of the orbits are great1 their analysis fails1 because the series expressing the co-ordinates of the bodies become extremely complicated1 and do not converge when applied to comets and the telescopic planets4 ,this difficulty has been overcome by ,sir ,john ,lubbock1 and other mathematicians1 who have the honour of having completed the theory of planetary motion1 which becomes every day of more importance1 from the new planets that have been discovered1 and also with regard to comets1 many of which return to the sun at regular intervals1 and from whose perturbations the masses of the planets will be more accurately determined1 and the retarding influence of the ethereal medium better known4 ,,section ,,ix4 ,rotation of the ,sun and ,planets--,saturn's ,rings--,periods of the ,rotation of the ,moon and other ,satellites equal to the ,periods of their ,revolutions--,form of ,lunar ,spheroid--,libration1 ,aspect1 and ,constitution of the ,moon--,rotation of ,jupiter's ,satellites4 ,,the oblate form of several of the planets indicates rotatory motion4 ,this has been confirmed in most cases by tracing spots on their surface1 by which their poles and times of rotation have been determined4 ,the rotation of ,mercury is unknown1 on account of his proximity to the sun2 that of the new planets has not yet been ascertained4 ,the sun revolves in twenty-five days and ten hours about an axis which is directed towards a point half-way between the pole-star and of ,lyra1 the plane of rotation being inclined by #g #cj1 or a little more than seven degrees1 to the plane of the ecliptic3 it may therefore be concluded that the sun's mass is a spheroid1 flattened at the poles4 ,from the rotation of the sun1 there was every reason to believe that he has a progressive motion in space1 a circumstance which is confirmed by observation4 ,but1 in consequence of the reaction of the planets1 he describes a small irregular orbit about the centre of gravity of the system1 never deviating from his position by more than twice his own diameter1 or a little more than seven times the distance of the moon from the earth4 ,the sun and all his attendants rotate from west to east1 on axes that remain nearly parallel to themselves 7,n4 #adj7 in every point of their orbit1 and with angular velocities that are sensibly uniform 7,n4 #ada74 ,although the uniformity in the direction of their rotation is a circumstance hitherto unaccounted for in the economy of nature1 yet1 from the design and adaptation of every other part to the perfection of the whole1 a coincidence so remarkable cannot be accidental4 ,and1 as the revolutions of the planets and satellites are also from west to east1 it is evident that both must have arisen from the primitive cause which determined the planetary motions4#f ,indeed1 ,la ,place has computed the probability to be as four millions to one that all the motions of the planets1 both of rotation and revolution1 were at once imparted by an original common cause1 but of which we know neither the nature nor the epoch4 ,the larger planets rotate in shorter periods than the smaller planets and the earth4 ,their compression is consequently greater1 and the action of the sun and of their satellites occasions a nutation in their axes and a precession of their equinoxes 7,n4 #adg7 similar to that which obtains in the terrestrial spheroid1 from the attraction of the sun and moon on the prominent matter at the equator4 ,jupiter revolves in less than ten hours round an axis at right angles to certain dark belts or bands1 which always cross his equator4 7,see ,plate #a47 ,this rapid rotation occasions a very great compression in his form4 ,his equatorial axis exceeds his polar axis by #fjjj miles1 whereas the difference in the axes of the earth is only about twenty-six and a half4 ,it is an evident consequence of ,kepler's law of the squares of the periodic times of the planets being as the cubes of the major axes of their orbits1 that the heavenly bodies move slower the farther they are from the sun4 ,in comparing the periods of the revolutions of ,jupiter and ,saturn with the times of their rotation1 it appears that a year of ,jupiter contains nearly ten thousand of his days1 and that of ,saturn about thirty thousand ,saturnian days4 ,the appearance of ,saturn is unparalleled in the system of the world4 ,he is a spheroid nearly #ajjj times larger than the earth1 surrounded by a ring even brighter than himself1 which always remains suspended in the plane of his equator3 and1 viewed with a very good telescope1 it is found to consist of two concentric rings1 divided by a dark band4 ,the exterior ring1 as seen through ,mr4 ,lassell's great equatorial at ,malta1 has a dark-striped band through the centre1 and is altogether less bright than the interior ring1 one half of which is extremely brilliant2 while the interior half is shaded in rings like the seats in an amphitheatre4 ,mr4 ,lassell made the remarkable discovery of a dark transparent ring1 whose edge coincides with the inner edge of the interior ring1 and which occupies about half the space between it and ,saturn4 ,he compares it to a band of dark-coloured crape drawn across a portion of the disc of the planet1 and the part projected upon the blue sky is also transparent4 ,at the time these observations were made at ,malta1 ,captain ,jacob discovered the transparent ring at ,madras4 ,it is conjectured to be fluid2 even the luminous rings cannot be very dense1 since the density of ,saturn himself is known to be less than the eighth part of that of the earth4 ,a transit of the ring across a star might reveal something concerning this wonderful object4 ,the ball of ,saturn is striped by belts of different colours4 ,at the time of these observations the part above the ring was bright white2 at his equator there was a ruddy belt divided in two1 above which were belts of a bluish green alternately dark and light1 while at the pole there was a circular space of a pale colour4 7,see ,plate #b47 ,the mean distance of the interior part of the double ring from the surface of the planet is about #bb1bdj miles1 it is no less than #cc1cfj miles broad1 but1 by the estimation of ,sir ,john ,herschel1 its thickness does not much exceed #ajj miles1 so that it appears like a plane4 ,by the laws of mechanics1 it is impossible that this body can retain its position by the adhesion of its particles alone4 ,it must necessarily revolve with a velocity that will generate a centrifugal force sufficient to balance the attraction of ,saturn4 ,observation confirms the truth of these principles1 showing that the rings rotate from west to east about the planet in ten hours and a half1 which is nearly the time a satellite would take to revolve about ,saturn at the same distance4 ,their plane is inclined to the ecliptic1 at an angle of #bh #aj #dde2 in consequence of this obliquity of position1 they always appear elliptical to us1 but with an excentricity so variable as even to be occasionally like a straight line drawn across the planet4 ,in the beginning of ,october1 #ahcb1 the plane of the rings passed through the centre of the earth2 in that position they are only visible with very superior instruments1 and appear like a fine line across the disc of ,saturn4 ,about the middle of ,december1 in the same year1 the rings became invisible1 with ordinary instruments1 on account of their plane passing through the sun4 ,in the end of ,april1 #ahcc1 the rings vanished a second time1 and reappeared in ,june of that year4 ,similar phenomena will occur as often as ,saturn has the same longitude with either node of his rings4 ,each side of these rings has alternately fifteen years of sunshine and fifteen years of darkness4 ,it is a singular result of theory1 that the rings could not maintain their stability of rotation if they were everywhere of uniform thickness2 for the smallest disturbance would destroy the equilibrium1 which would become more and more deranged1 till1 at last1 they would be precipitated on the surface of the planet4 ,the rings of ,saturn must therefore be irregular solids1 of unequal breadth in different parts of the circumference1 so that their centres of gravity do not coincide with the centres of their figures4 ,professor ,struve has also discovered that the centre of the rings is not concentric with the centre of ,saturn4 ,the interval between the outer edge of the globe of the planet and the outer edge of the rings on one side is #aabgb1 and1 on the other side1 the interval is #aacij1 consequently there is an excentricity of the globe in the rings of #jbae4 ,if the rings obeyed different forces1 they would not remain in the same plane1 but the powerful attraction of ,saturn always maintains them and his satellites in the plane of his equator4 ,the rings1 by their mutual action1 and that of the sun and satellites1 must oscillate about the centre of ,saturn1 and produce phenomena of light and shadow whose periods extend to many years4 ,according to ,m4 ,bessel the mass of ,saturn's ring is equal to the #a_/aah part of that of the planet4 ,the periods of rotation of the moon and the other satellites are equal to the times of their revolutions1 consequently these bodies always turn the same face to their primaries4 ,however1 as the mean motion of the moon is subject to a secular inequality1 which will ultimately amount to many circumferences 7,n4 #ajh71 if the rotation of the moon were perfectly uniform and not affected by the same inequalities1 it would cease exactly to counterbalance the motion of revolution2 and the moon1 in the course of ages1 would successively and gradually discover every point of her surface to the earth4 ,but theory proves that this never can happen2 for the rotation of the moon1 though it does not partake of the periodic inequalities of her revolution1 is affected by the same secular variations1 so that her motions of rotation and revolution round the earth will always balance each other1 and remain equal4 ,this circumstance arises from the form of the lunar spheroid1 which has three principal axes of different lengths at right angles to each other4 ,the moon is flattened at her poles from her centrifugal force1 therefore her polar axis is the least4 ,the other two are in the plane of her equator1 but that directed towards the earth is the greatest 7,n4 #adb74 ,the attraction of the earth1 as if it had drawn out that part of the moon's equator1 constantly brings the greatest axis1 and consequently the same hemisphere1 towards us1 which makes her rotation participate in the secular variations of her mean motion of revolution4 ,even if the angular velocities of rotation and revolution had not been nicely balanced in the beginning of the moon's motion1 the attraction of the earth would have recalled the greatest axis to the direction of the line joining the centres of the moon and earth2 so that it would have vibrated on each side of that line in the same manner as a pendulum oscillates on each side of the vertical from the influence of gravitation4 ,no such libration is perceptible2 and1 as the smallest disturbance would make it evident1 it is clear that1 if the moon has ever been touched by a comet1 the mass of the latter must have been extremely small4 ,if it had been only the hundred thousandth part of that of the earth1 it would have rendered the libration sensible4 ,according to analysis1 a similar libration exists in the motions of ,jupiter's satellites1 which still remains insensible to observation1 and yet the comet of #aggj passed twice through the midst of them4 ,the moon1 it is true1 is liable to librations depending upon the position of the spectator4 ,at her rising1 part of the western edge of her disc is visible1 which is invisible at her setting1 and the contrary takes place with regard to her eastern edge4 ,there are also librations arising from the relative positions of the earth and moon in their respective orbits2 but1 as they are only optical appearances1 one hemisphere will be eternally concealed from the earth4 ,for the same reason the earth1 which must be so splendid an object to one lunar hemisphere1 will be for ever veiled from the other4 ,on account of these circumstances1 the remoter hemisphere of the moon has its day a fortnight long1 and a night of the same duration1 not even enlightened by a moon1 while the favoured side is illuminated by the reflection of the earth during its long night4 ,a planet exhibiting a surface thirteen times larger than that of the moon1 with all the varieties of clouds1 land1 and water1 coming successively into view1 must be a splendid object to a lunar traveller in a journey to his antipodes4 ,the great height of the lunar mountains probably has a considerable influence on the phenomena of her motion1 the more so as her compression is small1 and her mass considerable4 ,in the curve passing through the poles1 and that diameter of the moon which always points to the earth1 nature has furnished a permanent meridian1 to which the different spots on her surface have been referred1 and their positions are determined with as much accuracy as those of many of the most remarkable places on the surface of our globe4 ,according to the observations of ,professor ,secchi at ,rome1 the mountains of the moon are mostly volcanic and of three kinds4 ,the first and oldest have their borders obliterated1 so that they look like deep wells2 the second1 which are of an intermediate class1 have elevated1 and1 for the most part1 regular unbroken edges1 with the ground around them raised to a prodigious extent in proportion to the size of the volcano1 with generally an insulated rock in the centre of the crater4 ,the third1 and most recent class1 are very small1 and seem to be the last effort of the expiring volcanic force1 which is probably now extinct4 ,the distance and minuteness of ,jupiter's satellites render it extremely difficult to ascertain their rotation4 ,it was1 however1 accomplished by ,sir ,william ,herschel from their relative brightness4 ,he observed that they alternately exceed each other in brilliancy1 and1 by comparing the maxima and minima of their illumination with their positions relatively to the sun and to their primary1 he found that1 like the moon1 the time of their rotation is equal to the period of their revolution about ,jupiter4 ,miraldi was led to the same conclusion with regard to the fourth satellite1 from the motion of a spot on its surface4 ,,section ,x4 ,rotation of the ,earth invariable--,decrease in the ,earth's mean ,temperature--,earth originally in a state of ,fusion--,length of ,day constant--,decrease of ,temperature ascribed by ,sir ,john ,herschel to the variation in the ,excentricity of the ,terrestrial ,orbit--,difference in the ,temperature of the two ,hemispheres erroneously ascribed to the ,excess in the ,length of ,spring and ,summer in the ,southern ,hemisphere2 attributed by ,sir ,charles ,lyell to the ,operation of existing ,causes--,three principal ,axes of ,rotation--,position of the ,axis of ,rotation on the ,surface of the ,earth invariable--,ocean not sufficient to restore the ,equilibrium of the ,earth if deranged--,its ,density and mean ,depth--,internal ,structure of the ,earth4 ,the rotation of the earth1 which determines the length of the day1 may be regarded as one of the most important elements in the system of the world4 ,it serves as a measure of time1 and forms the standard of comparison for the revolutions of the celestial bodies1 which1 by their proportional increase or decrease1 would soon disclose any changes it might sustain4 ,theory and observation concur in proving that1 among the innumerable vicissitudes which prevail throughout creation1 the period of the earth's diurnal rotation is immutable4 ,the water of rivers1 falling from a higher to a lower level1 carries with it the velocity due to its revolution with the earth at a greater distance from the centre2 it will therefore accelerate1 although to an almost infinitesimal extent1 the earth's daily rotation4 ,the sum of all these increments of velocity1 arising from the descent of all the rivers on the earth's surface1 would in time become perceptible1 did not nature1 by the process of evaporation1 raise the waters back to their sources1 and thus1 by again removing matter to a greater distance from the centre1 destroy the velocity generated by its previous approach2 so that the descent of rivers does not affect the earth's rotation4 ,enormous masses projected by volcanoes from the equator to the poles1 and the contrary1 would indeed affect it1 but there is no evidence of such convulsions4 ,the disturbing action of the moon and planets1 which has so powerful an effect on the revolution of the earth1 in no way influences its rotation4 ,the constant friction of the trade winds on the mountains and continents between the tropics does not impede its velocity1 which theory even proves to be the same as if the sea1 together with the earth1 formed one solid mass4 ,but1 although these circumstances be insufficient1 a variation in the mean temperature would certainly occasion a corresponding change in the velocity of rotation4 ,in the science of dynamics it is a principle in a system of bodies or of particles revolving about a fixed centre1 that the momentum or sum of the products of the mass of each into its angular velocity and distance from the centre is a constant quantity1 if the system be not deranged by a foreign cause4 ,now1 since the number of particles in the system is the same whatever its temperature may be1 when their distances from the centre are diminished1 their angular velocity must be increased1 in order that the preceding quantity may still remain constant4 ,it follows1 then1 that1 as the primitive momentum of rotation with which the earth was projected into space must necessarily remain the same1 the smallest decrease in heat1 by contracting the terrestrial spheroid1 would accelerate its rotation1 and consequently diminish the length of the day4 ,notwithstanding the constant accession of heat from the sun's rays1 geologists have been induced to believe1 from the fossil remains1 that the mean temperature of the globe is decreasing4 ,the high temperature of mines1 hot springs1 and above all the internal fires which have produced1 and do still occasion1 such devastation on our planet1 indicate an augmentation of heat towards its centre4 ,the increase of density corresponding to the depth and the form of the spheroid1 being what theory assigns to a fluid mass in rotation1 concurs to induce the idea that the temperature of the earth was originally so high as to reduce all the substances of which it is composed to a state of fusion or of vapour1 and that in the course of ages it has cooled down to its present state2 that it is still becoming colder2 and that it will continue to do so till the whole mass arrives at the temperature of the medium in which it is placed1 or rather at a state of equilibrium between this temperature1 the cooling power of its own radiation1 and the heating effect of the sun's rays4 ,previous to the formation of ice at the poles1 the ancient lands of northern latitudes might1 no doubt1 have been capable of producing those tropical plants preserved in the coal-measures1 if indeed such plants could flourish without the intense light of a tropical sun4 ,but1 even if the decreasing temperature of the earth be sufficient to produce the observed effects1 it must be extremely slow in its operation2 for1 in consequence of the rotation of the earth being a measure of the periods of the celestial motions1 it has been proved that1 if the length of the day had decreased by the three-thousandth part of a second since the observations of ,hipparchus two thousand years ago1 it would have diminished the secular equation of the moon by #ddd4 ,it is1 therefore1 beyond a doubt that the mean temperature of the earth cannot have sensibly varied during that time4 ,if1 then1 the appearances exhibited by the strata are really owing to a decrease of internal temperature1 it either shows the immense periods requisite to produce geological changes1 to which two thousand years are as nothing1 or that the mean temperature of the earth had arrived at a state of equilibrium before these observations4 ,however strong the indications of the primitive fluidity of the earth1 as there is no direct proof of it1 the hypothesis can only be regarded as very probable4 ,but one of the most profound philosophers and elegant writers of modern times has found in the secular variation of the excentricity of the terrestrial orbit an evident cause of decreasing temperature4 ,that accomplished author1 in pointing out the mutual dependencies of phenomena1 says1 8,it is evident that the mean temperature of the whole surface of the globe1 in so far as it is maintained by the action of the sun at a higher degree than it would have were the sun extinguished1 must depend on the mean quantity of the sun's rays which it receives1 or--which comes to the same thing--on the total quantity received in a given invariable time2 and1 the length of the year being unchangeable in all the fluctuations of the planetary system1 it follows that the total amount of solar radiation will determine1 cteris paribus1 the general climate of the earth4 ,now1 it is not difficult to show that this amount is inversely proportional to the minor axis of the ellipse described by the earth about the sun 7,n4 #adc71 regarded as slowly variable2 and that1 therefore1 the major axis remaining1 as we know it to be1 constant1 and the orbit being actually in a state of approach to a circle1 and consequently the minor axis being on the increase1 the mean annual amount of solar radiation received by the whole earth must be actually on the decrease4 ,we have1 therefore1 an evident real cause to account for the phenomenon40 ,the limits of the variation in the excentricity of the earth's orbit are unknown4 ,but1 if its ellipticity has ever been as great as that of the orbit of ,mercury or ,pallas1 the mean temperature of the earth must have been sensibly higher than it is at present4 ,whether it was great enough to render our northern climates fit for the production of tropical plants1 and for the residence of the elephant and other animals now inhabitants of the torrid zone1 it is impossible to say4 ,of the decrease in temperature of the northern hemisphere there is abundant evidence in the fossil plants discovered in very high latitudes1 which could only have existed in a tropical climate1 and which must have grown near the spot where they are found1 from the delicacy of their structure and the perfect state of their preservation4 ,this change of temperature has been erroneously ascribed to an excess in the duration of spring and summer in the northern hemisphere1 in consequence of the excentricity of the solar ellipse4 ,the length of the seasons varies with the position of the perihelion 7,n4 #fd7 of the earth's orbit for two reasons4 ,on account of the excentricity1 small as it is1 any line passing through the centre of the sun divides the terrestrial ellipse into two unequal parts1 and by the laws of elliptical motion the earth moves through these two portions with unequal velocities4 ,the perihelion always lies in the smaller portion1 and there the earth's motion is the most rapid4 ,in the present position of the perihelion1 spring and summer north of the equator exceed by about eight days the duration of the same seasons south of it4 ,and #aj1dib years ago the southern hemisphere enjoyed the advantage we now possess from the secular variation of the perihelion4 ,yet ,sir ,john ,herschel has shown that by this alternation neither hemisphere acquires any excess of light or heat above the other2 for1 although the earth is nearer to the sun while moving through that part of its orbit in which the perihelion lies than in the other part1 and consequently receives a greater quantity of light and heat1 yet as it moves faster it is exposed to the heat for a shorter time4 ,in the other part of the orbit1 on the contrary1 the earth1 being farther from the sun1 receives fewer of his rays2 but because its motion is slower1 it is exposed to them for a longer time2 and1 as in both cases the quantity of heat and the angular velocity vary exactly in the same proportion1 a perfect compensation takes place 7,n4 #add74 ,so that the excentricity of the earth's orbit has little or no effect on the temperature corresponding to the difference of the seasons4 ,sir ,charles ,lyell1 in his excellent works on ,geology1 refers the increased cold of the northern hemisphere to the operation of existing causes with more probability than most theories that have been advanced in solution of this difficult subject4 ,the loftiest mountains would be represented by a grain of sand on a globe six feet in diameter1 and the depth of the ocean by a scratch on its surface4 ,consequently the gradual elevation of a continent or chain of mountains above the surface of the ocean1 or their depression below it1 is no very great event compared with the magnitude of the earth1 and the energy of its subterranean fires1 if the same periods of time be admitted in the progress of geological as in astronomical phenomena1 which the successive and various races of extinct beings show to have been immense4 ,climate is always more intense in the interior of continents than in islands or sea-coasts4 ,an increase of land within the tropics would therefore augment the general heat1 and an increase in the temperate and frigid zones would render the cold more severe4 ,now it appears that most of the ,european1 ,north ,asiatic1 and ,north ,american continents and islands were raised from the deep after the coal-measures were formed in which the fossil tropical plants are found2 and a variety of geological facts indicate the existence of an ancient and extensive archipelago throughout the greater part of the northern hemisphere4 ,sir ,charles ,lyell is therefore of opinion that the climate of these islands must have been sufficiently mild1 in consequence of the surrounding ocean1 to clothe them with tropical plants1 and render them a fit abode for the huge animals whose fossil remains are so often found2 that the arborescent ferns and the palms of these regions1 carried by streams to the bottom of the ocean1 were imbedded in the strata which were by degrees heaved up by the subterranean fires during a long succession of ages1 till the greater part of the northern hemisphere became dry land as it now is1 and that the consequence has been a continual decrease of temperature4 ,it is evident1 from the marine shells found on the tops of the highest mountains and in almost every part of the globe1 that immense continents have been elevated above the ocean which must have engulfed others4 ,such a catastrophe would be occasioned by a variation in the position of the axis of rotation on the surface of the earth2 for the seas tending to a new equator would leave some portions of the globe and overwhelm others4 ,now1 it is found by the laws of mechanics that in every body1 be its form or density what it may1 there are at least three axes at right angles to each other1 round any one of which1 if the solid begins to rotate1 it will continue to revolve for ever1 provided it be not disturbed by a foreign cause1 but that the rotation about any other axis will only be for an instant1 and consequently the poles or extremities of the instantaneous axis of rotation would perpetually change their position on the surface of the body4 ,in an ellipsoid of revolution the polar diameter and every diameter in the plane of the equator are the only permanent axes of rotation 7,n4 #ade74 ,hence1 if the ellipsoid were to begin to revolve about any diameter between the pole and the equator1 the motion would be so unstable that the axis of rotation and the position of the poles would change every instant4 ,therefore1 as the earth does not differ much from this figure1 if it did not turn round one of its principal axes1 the position of the poles would change daily2 the equator1 which is #ij distant1 would undergo corresponding variations2 and the geographical latitudes of all places1 being estimated from the equator1 assumed to be fixed1 would be perpetually changing4 ,a displacement in the position of the poles of only two hundred miles would be sufficient to produce these effects1 and would immediately be detected4 ,but1 as the latitudes are found to be invariable1 it may be concluded that the terrestrial spheroid must have revolved about the same axis for ages4 ,the earth and planets differ so little from ellipsoids of revolution1 that in all probability any libration from one axis to another1 produced by the primitive impulse which put them in motion1 must have ceased soon after their creation from the friction of the fluids at their surface4 ,theory also proves that neither nutation1 precession1 nor any of the disturbing forces that affect the system1 have the smallest influence on the axis of rotation1 which maintains a permanent position on the surface1 if the earth be not disturbed in its rotation by a foreign cause1 as the collision of a comet1 which might have happened in the immensity of time4 ,but1 had that been the case1 its effects would still have been perceptible in the variations of the geographical latitudes4 ,if we suppose that such an event had taken place1 and that the disturbance had been very great1 equilibrium could then only have been restored with regard to a new axis of rotation by the rushing of the seas to the new equator1 which they must have continued to do till the surface was everywhere perpendicular to the direction of gravity4 ,but it is probable that such an accumulation of the waters would not be sufficient to restore equilibrium if the derangement had been great1 for the mean density of the sea is only about a fifth part of the mean density of the earth1 and the mean depth of the ,pacific ,ocean is supposed not to be more than four or five miles1 whereas the equatorial diameter of the earth exceeds the polar diameter by about #bf-a_/b miles4 ,consequently the influence of the sea on the direction of gravity is very small4 ,and1 as it thus appears that a great change in the position of the axis is incompatible with the law of equilibrium1 the geological phenomena in question must be ascribed to an internal cause4 ,indeed it is now demonstrated that the strata containing marine diluvia1 which are in lofty situations1 must have been formed at the bottom of the ocean1 and afterwards upheaved by the action of subterraneous fires4 ,besides1 it is clear1 from the mensuration of the arcs of the meridian and the length of the seconds' pendulum1 as well as from the lunar theory1 that the internal strata and also the external outline of the globe are elliptical1 their centres being coincident and their axes identical with that of the surface--a state of things which1 according to the distinguished author lately quoted1 is incompatible with a subsequent accommodation of the surface to a new and different state of rotation from that which determined the original distribution of the component matter4 ,thus1 amidst the mighty revolutions which have swept innumerable races of organized beings from the earth1 which have elevated plains and buried mountains in the ocean1 the rotation of the earth and the position of the axes on its surface have undergone but slight variations4 ,the strata of the terrestrial spheroid are not only concentric and elliptical1 but the lunar inequalities show that they increase in density from the surface of the earth to its centre4 ,this would certainly have happened if the earth had originally been fluid1 for the denser parts must have subsided towards the centre as it approached a state of equilibrium4 ,but the enormous pressure of the superincumbent mass is a sufficient cause for the phenomenon4 ,professor ,leslie observes that air compressed into the fiftieth part of its volume has its elasticity fifty times augmented4 ,if it continues to contract at that rate1 it would1 from its own incumbent weight1 acquire the density of water at the depth of thirty-four miles4 ,but water itself would have its density doubled at the depth of ninety-three miles1 and would even attain the density of quicksilver at a depth of #cfb miles4 ,descending therefore towards the centre through nearly #djjj miles1 the condensation of ordinary substances would surpass the utmost powers of conception4 ,dr4 ,young says that steel would be compressed into one-fourth and stone into one-eighth of its bulk at the earth's centre4 ,however1 we are yet ignorant of the laws of compression of solid bodies beyond a certain limit2 from the experiments of ,mr4 ,perkins they appear to be capable of a greater degree of compression than has generally been imagined4 ,but a density so extreme is not borne out by astronomical observation4 ,it might seem to follow therefore that our planet must have a widely cavernous structure1 and that we tread on a crust or shell whose thickness bears a very small proportion to the diameter of its sphere4 ,possibly1 too1 this great condensation at the central regions may be counterbalanced by the increased elasticity due to a very elevated temperature4 ,,section ,,xi4 ,precession and ,nutation--,their ,effects on the ,apparent ,places of the ,fixed ,stars4 ,,it has been shown that the axis of rotation is invariable on the surface of the earth2 and observation as well as theory prove that1 were it not for the action of the sun and moon on the matter at the equator1 it would remain exactly parallel to itself in every point of its orbit4 ,the attraction of an external body not only draws a spheroid towards it1 but1 as the force varies inversely as the square of the distance1 it gives it a motion about its centre of gravity1 unless when the attracting body is situated in the prolongation of one of the axes of the spheroid4 ,the plane of the equator is inclined to the plane of the ecliptic at an angle of #bc #bg #bhbi2 and the inclination of the lunar orbit to the same is #e #h #dgi4 ,consequently1 from the oblate figure of the earth1 the sun and moon1 acting obliquely and unequally on the different parts of the terrestrial spheroid1 urge the plane of the equator from its direction1 and force it to move from east to west1 so that the equinoctial points have a slow retrograde motion on the plane of the ecliptic of #ejda annually4 ,the direct tendency of this action is to make the planes of the equator and ecliptic coincide1 but it is balanced by the tendency of the earth to return to stable rotation about the polar diameter1 which is one of its principal axes of rotation4 ,therefore the inclination of the two planes remains constant1 as a top spinning preserves the same inclination to the plane of the horizon4 ,were the earth spherical1 this effect would not be produced1 and the equinoxes would always correspond with the same points of the ecliptic1 at least as far as this kind of motion is concerned4 ,but another and totally different cause which operates on this motion has already been mentioned4 ,the action of the planets on one another and on the sun occasions a very slow variation in the position of the plane of the ecliptic1 which affects its inclination to the plane of the equator1 and gives the equinoctial points a slow but direct motion on the ecliptic of #jca annually1 which is entirely independent of the figure of the earth1 and would be the same if it were a sphere4 ,thus the sun and moon by moving the plane of the equator cause the equinoctial points to retrograde on the ecliptic3 and the planets by moving the plane of the ecliptic give them a direct motion1 though much less than the former4 ,consequently the difference of the two is the mean precession1 which is proved both by theory and observation to be about #eja annually 7,n4 #adf74 ,as the longitudes of all the fixed stars are increased by this quantity1 the effects of precession are soon detected4 ,it was accordingly discovered by ,hipparchus in the year #abh before ,christ1 from a comparison of his own observations with those of ,timocharis #aee years before4 ,in the time of ,hipparchus the entrance of the sun into the constellation ,aries was the beginning of spring2 but since that time the equinoctial points have receded #cj1 so that the constellations called the signs of the zodiac are now at a considerable distance from those divisions of the ecliptic which bear their names4 ,moving at the rate of #eja annually1 the equinoctial points will accomplish a revolution in #be1hfh years4 ,but1 as the precession varies in different centuries1 the extent of this period will be slightly modified4 ,since the motion of the sun is direct1 and that of the equinoctial points retrograde1 he takes a shorter time to return to the equator than to arrive at the same stars2 so that the tropical year of #cfe;d #e;h #dhm #dis#g must be increased by the time he takes to move through an arc of #eja1 in order to have the length of the sidereal year4 ,the time required is #bjm #ais#f1 so that the sidereal year contains #cfe;d #f;h #im #is#f mean solar days4 ,the mean annual precession is subject to a secular variation2 for1 although the change in the plane of the ecliptic in which the orbit of the sun lies be independent of the form of the earth1 yet1 by bringing the sun1 moon1 and earth into different relative positions from age to age1 it alters the direct action of the two first on the prominent matter at the equator3 on this account the motion of the equinox is greater by #jdee now than it was in the time of ,hipparchus4 ,consequently the actual length of the tropical year is about #ds#ba shorter than it was at that time4 ,the utmost change that it can experience from this cause amounts to #dc seconds4 ,such is the secular motion of the equinoxes4 ,but it is sometimes increased and sometimes diminished by periodic variations1 whose periods depend upon the relative positions of the sun and moon with regard to the earth1 and which are occasioned by the direct action of these bodies on the equator4 ,dr4 ,bradley discovered that by this action the moon causes the pole of the equator to describe a small ellipse in the heavens1 the axes of which are #ahe and #acfgd1 the longer being directed towards the pole of the ecliptic4 ,the period of this inequality is about #ai years1 the time employed by the nodes of the lunar orbit to accomplish a revolution4 ,the sun causes a small variation in the description of this ellipse2 it runs through its period in half a year4 ,since the whole earth obeys these motions1 they affect the position of its axis of rotation with regard to the starry heavens1 though not with regard to the surface of the earth2 for in consequence of precession alone the pole of the equator moves in a circle round the pole of the ecliptic in #be1hfh years1 and by nutation alone it describes a small ellipse in the heavens every #ai years1 on each side of which it deviates every half-year from the action of the sun4 ,the real curve traced in the starry heavens by the imaginary prolongation of the earth's axis is compounded of these three motions 7,n4 #adg74 ,this nutation in the earth's axis affects both the precession and obliquity with small periodic variations4 ,but in consequence of the secular variation in the position of the terrestrial orbit1 which is chiefly owing to the disturbing energy of ,jupiter on the earth1 the obliquity of the ecliptic is annually diminished1 according to ,m4 ,bessel1 by #jdeg4 ,this variation in the course of ages may amount to #aj or #aa degrees2 but the obliquity of the ecliptic to the equator can never vary more than #b #db or #c1 since the equator will follow in some measure the motion of the ecliptic4 ,it is evident that the places of all the celestial bodies are affected by precession and nutation4 ,their longitudes estimated from the equinox are augmented by precession2 but1 as it affects all the bodies equally1 it makes no change in their relative positions4 ,both the celestial latitudes and longitudes are altered to a small degree by nutation2 hence all observations must be corrected for these inequalities4 ,in consequence of this real motion in the earth's axis1 the pole-star1 forming part of the constellation of the ,little ,bear1 which was formerly #ab from the celestial pole1 is now within #a #bd of it1 and will continue to approach it till it is within #a_/b1 after which it will retreat from the pole for ages2 and #ab1icd years hence the star ,lyr will come within #e of the celestial pole1 and become the polar star of the northern hemisphere4 ,,section ,,xii4 ,mean and ,apparent ,sidereal ,time--,mean and ,apparent ,solar ,time--,equation of ,time--,english and ,french ,subdivisions of ,time--,leap ,year--,christian ,era--,equinoctial ,time--,remarkable ,eras depending upon the ,position of the ,solar ,perigee--,inequality of the ,lengths of the ,seasons in the two ,hemispheres--,application of ,astronomy to ,chronology--,english and ,french ,standards of ,weights and ,measures4 ,,astronomy has been of immediate and essential use in affording invariable standards for measuring duration1 distance1 magnitude1 and velocity4 ,the mean sidereal day measured by the time elapsed between two consecutive transits of any star at the same meridian 7,n4 #adh71 and the mean sidereal year which is the time included between two consecutive returns of the sun to the same star1 are immutable units with which all great periods of time are compared2 the oscillations of the isochronous pendulum measure its smaller portions4 ,by these invariable standards alone we can judge of the slow changes that other elements of the system may have undergone4 ,apparent sidereal time1 which is measured by the transit of the equinoctial point at the meridian of any place1 is a variable quantity1 from the effects of precession and nutation4 ,clocks showing apparent sidereal time are employed for observation1 and are so regulated that they indicate #j;h #jm #js at the instant the equinoctial point passes the meridian of the observatory4 ,and as time is a measure of angular motion1 the clock gives the distances of the heavenly bodies from the equinox by observing the instant at which each passes the meridian1 and converting the interval into arcs at the rate of #ae to an hour4 ,the returns of the sun to the meridian and to the same equinox or solstice have been universally adopted as the measure of our civil days and years4 ,the solar or astronomical day is the time that elapses between two consecutive noons or midnights4 ,it is consequently longer than the sidereal day1 on account of the proper motion of the sun during a revolution of the celestial sphere4 ,but1 as the sun moves with greater rapidity at the winter than at the summer solstice1 the astronomical day is more nearly equal to the sidereal day in summer than in winter4 ,the obliquity of the ecliptic also affects its duration2 for near the equinoxes the arc of the equator is less than the corresponding arc of the ecliptic1 and in the solstices it is greater 7,n4 #adi74 ,the astronomical day is therefore diminished in the first case1 and increased in the second4 ,if the sun moved uniformly in the equator at the rate of #ei #hcc every day1 the solar days would be all equal4 ,the time therefore which is reckoned by the arrival of an imaginary sun at the meridian1 or of one which is supposed to move uniformly in the equator1 is denominated mean solar time1 and is given by clocks and watches in common life4 ,when it is reckoned by the arrival of the real sun at the meridian1 it is true or apparent time1 and is given by dials4 ,the difference between the time shown by a clock and a dial is the equation of time given in the ,nautical ,almanac1 sometimes amounting to as much as sixteen minutes4 ,the apparent and mean time coincide four times in the year2 when the sun's daily motion in right ascension is equal to #ei #hcc in a mean solar day1 which happens about the #afth of ,april1 the #afth of ,june1 the #ast of ,september1 and the #beth of ,december4 ,the astronomical day begins at noon1 but in common reckoning the day begins at midnight4 ,in ,england it is divided into twenty-four hours1 which are counted by twelve and twelve2 but in ,france astronomers1 adopting the decimal division1 divide the day into ten hours1 the hour into one hundred minutes1 and the minute into a hundred seconds1 because of the facility in computation1 and in conformity with their decimal system of weights and measures4 ,this subdivision is not now used in common life1 nor has it been adopted in any other country2 and although some scientific writers in ,france still employ that division of time1 the custom is beginning to wear out4 ,at one period during the ,french ,revolution1 the clock in the gardens of the ,tuileries was regulated to show decimal time4 ,the mean length of the day1 though accurately determined1 is not sufficient for the purposes either of astronomy or civil life4 ,the tropical or civil year of #cfe;d #e;h #dhm #dis#g1 which is the time elapsed between the consecutive returns of the sun to the mean equinoxes or solstices1 including all the changes of the seasons1 is a natural cycle peculiarly suited for a measure of duration4 ,it is estimated from the winter solstice1 the middle of the long annual night under the north pole4 ,but although the length of the civil year is pointed out by nature as a measure of long periods1 the incommensurability that exists between the length of the day and the revolution of the sun renders it difficult to adjust the estimation of both in whole numbers4 ,if the revolution of the sun were accomplished in #cfe days1 all the years would be of precisely the same number of days1 and would begin and end with the sun at the same point of the ecliptic4 ,but as the sun's revolution includes the fraction of a day1 a civil year and a revolution of the sun have not the same duration4 ,since the fraction is nearly the fourth of a day1 in four years it is nearly equal to a revolution of the sun1 so that the addition of a supernumerary day every fourth year nearly compensates the difference4 ,but in process of time further correction will be necessary1 because the fraction is less than the fourth of a day4 ,in fact1 if a bissextile be suppressed at the end of three out of four centuries1 the year so determined will only exceed the true year by an extremely small fraction of a day2 and if in addition to this a bissextile be suppressed every #djjj years1 the length of the year will be nearly equal to that given by observation4 ,were the fraction neglected1 the beginning of the year would precede that of the tropical year1 so that it would retrograde through the different seasons in a period of about #aejg years4 ,the ,egyptian year began with the heliacal rising of ,sirius 7,n4 #aej71 and contained only #cfe days1 by which they lost one year in every #adfa years1 their ,sothaic period1 or that cycle in which the heliacal rising of ,sirius passes through the whole year and takes place again on the same day4 ,the division of the year into months is very old and almost universal4 ,but the period of seven days1 by far the most permanent division of time1 and the most ancient monument of astronomical knowledge1 was used by the ,brahmins in ,india with the same denominations employed by us1 and was alike found in the calendars of the ,jews1 ,egyptians1 ,arabs1 and ,assyrians4 ,it has survived the fall of empires1 and has existed among all successive generations1 a proof of their common origin4 ,the day of the new moon immediately following the winter solstice in the #gjgth year of ,rome was made the #ast of ,january of the first year of ,julius ,csar4 ,the #beth of ,december of his forty-fifth year is considered as the date of ,christ's nativity2 and the forty-sixth year of the ,julian ,calendar is assumed to be the first of our era4 ,the preceding year is called the first year before ,christ by chronologists1 but by astronomers it is called the year #j4 ,the astronomical year begins on the #cast of ,december at noon2 and the date of an observation expresses the days and hours which have actually elapsed since that time4 ,since solar and sidereal time are estimated from the passage of the sun and the equinoctial point across the meridian of each place1 the hours are different at different places3 while it is one o'clock at one place1 it is two at another1 three at another1 c42 for it is obvious that it is noon at one part of the globe at the same moment that it is midnight at another diametrically opposite to it3 consequently an event which happens at one and the same instant of absolute time is recorded at different places as having happened at different times4 ,therefore1 when observations made at different places are to be compared1 they must be reduced by computation to what they would have been had they been made under the same meridian4 ,to obviate this it was proposed by ,sir ,john ,herschel to employ mean equinoctial time1 which is the same for all the world1 and independent alike of local circumstances and inequalities in the sun's motion4 ,it is the time elapsed from the instant the mean sun enters the mean vernal equinox1 and is reckoned in mean solar days and parts of a day4 ,some remarkable astronomical eras are determined by the position of the major axis of the solar ellipse1 which depends upon the direct motion of the perigee 7,n4 #ajb7 and the precession of the equinoxes conjointly1 the annual motion of the one being #aah1 and that of the other #eja4 ,hence the axis1 moving at the rate of #fai annually1 accomplishes a tropical revolution in #bjihd years4 ,it coincided with the line of the equinoxes #djjj or #djhi years before the ,christian era1 much about the time chronologists assign for the creation of man4 ,in #fdhc the major axis will again coincide with the line of the equinoxes2 but then the solar perigee will coincide with the equinox of autumn1 whereas at the creation of man it coincided with the vernal equinox4 ,in the year #abdf the major axis was perpendicular to the line of the equinoxes2 then the solar perigee coincided with the solstice of summer1 and the apogee with the solstice of winter4 ,according to ,la ,place1 who computed these periods from different data1 the last coincidence happened in the year #abej of our era1 which induced him to propose that year as a universal epoch1 the vernal equinox of the year #abej to be the first day of the first year4 ,these eras can only be regarded as approximate1 since ancient observations are too inaccurate1 and modern observations too recent1 to afford data for their precise determination4 ,the variation in the position of the solar ellipse occasions corresponding changes in the length of the seasons4 ,in its present position spring is shorter than summer1 and autumn longer than winter2 and while the solar perigee continues as it now is1 between the solstice of winter and the equinox of spring1 the period including spring and summer will be longer than that including autumn and winter4 ,in this century the difference is between seven and eight days4 ,the intervals will be equal towards the year #fdhc1 when the perigee will coincide with the equinox of spring2 but1 when it passes that point1 the spring and summer taken together will be shorter than the period including the autumn and winter 7,n4 #aea74 ,these changes will be accomplished in a tropical revolution of the major axis of the earth's orbit1 which includes an interval of #bj1ihd years4 ,were the orbit circular1 the seasons would be equal2 their difference arises from the excentricity of the orbit1 small as it is2 but the changes are so trifling as to be imperceptible in the short span of human life4 ,no circumstance in the whole science of astronomy excites a deeper interest than its application to chronology4 8,whole nations10 says ,la ,place1 8have been swept from the earth1 with their languages1 arts1 and sciences1 leaving but confused masses of ruins to mark the place where mighty cities stood2 their history1 with the exception of a few doubtful traditions1 has perished2 but the perfection of their astronomical observations marks their high antiquity1 fixes the periods of their existence1 and proves that1 even at that early time1 they must have made considerable progress in science40 ,the ancient state of the heavens may now be computed with great accuracy2 and1 by comparing the results of calculation with ancient observations1 the exact period at which they were made may be verified if true1 or1 if false1 their error may be detected4 ,if the date be accurate and the observation good1 it will verify the accuracy of modern tables1 and will show to how many centuries they may be extended without the fear of error4 ,a few examples will show the importance of the subject4 ,at the solstices the sun is at his greatest distance from the equator2 consequently his declination at these times is equal to the obliquity of the ecliptic 7,n4 #aeb71 which was formerly determined from the meridian length of the shadow of the stile of a dial on the day of a solstice4 ,the lengths of the meridian shadow at the summer and winter solstices are recorded to have been observed at the city of ,layang1 in ,china1 #aajj years before the ,christian era4 ,from these the distances of the sun from the zenith 7,n4 #aec7 of the city of ,layang are known4 ,half the sum of these zenith distances determines the latitude1 and half their difference gives the obliquity of the ecliptic at the period of the observation2 and1 as the law of the variation of the obliquity is known1 both the time and place of the observations have been verified by computations from modern tables4 ,thus the ,chinese had made some advances in the science of astronomy at that early period4 ,their whole chronology is founded on the observations of eclipses1 which prove the existence of that empire for more than #dgjj years4 ,the epoch of the lunar tables of the ,indians1 supposed by ,bailly to be #cjjj years before the ,christian era1 was proved by ,la ,place1 from the acceleration of the moon1 not to be more ancient than the time of ,ptolemy1 who lived in the second century after it4 ,the great inequality of ,jupiter and ,saturn1 whose cycle embraces #iah years1 is peculiarly fitted for marking the civilization of a people4 ,the ,indians had determined the mean motions of these two planets in that part of their periods when the apparent mean motion of ,saturn was at the slowest1 and that of ,jupiter the most rapid4 ,the periods in which that happened were #cajb years before the ,christian era1 and the year #adia after it4 ,the returns of comets to their perihelia may possibly mark the present state of astronomy to future ages4 ,the places of the fixed stars are affected by the precession of the equinoxes2 and1 as the law of that variation is known1 their positions at any time may be computed4 ,now ,eudoxus1 a contemporary of ,plato1 mentions a star situate in the pole of the equator1 and it appears from computation that ,draconis was not very far from that place about #cjjj years ago2 but1 as it is only about #baej years since ,eudoxus lived1 he must have described an anterior state of the heavens1 supposed to be the same that was mentioned by ,chiron about the time of the siege of ,troy4 ,thus every circumstance concurs in showing that astronomy was cultivated in the highest ages of antiquity4 ,it is possible that a knowledge of astronomy may lead to the interpretation of hieroglyphical characters4 ,astronomical signs are often found on the ancient ,egyptian monuments1 probably employed by the priests to record dates4 ,the author had occasion to witness an instance of this most interesting application of astronomy1 in ascertaining the date of a papyrus1 sent from ,egypt by ,mr4 ,salt1 in the hieroglyphical researches of the late ,dr4 ,thomas ,young1 whose profound and varied acquirements do honour to his country1 and to the age in which he lived4 ,the manuscript was found in a mummy case2 it proved to be a horoscope of the age of ,ptolemy1 and its date was determined from the configuration of the heavens at the time of its construction4 ,the form of the earth furnishes a standard of weights and measures for the ordinary purposes of life1 as well as for the determination of the masses and distances of the heavenly bodies4 ,the length of the pendulum vibrating seconds of mean solar time1 in the latitude of ,london1 forms the standard of the ,british measure of extension4 ,its approximate length oscillating in vacuo at the temperature of #fb of ,fahrenheit1 and reduced to the level of the sea 7,n4 #aed71 was determined by ,captain ,kater to be #ciacic inches4 ,the weight of a cubic inch of water at the temperature of #fb of ,fahrenheit1 barometer #cj inches1 was also determined in parts of the imperial troy pound1 whence a standard both of weight and capacity was deduced4 ,the ,french have adopted the mtre1 equal to #cbhjhiib ,english feet1 for their unit of linear measure1 which is the ten-millionth part of the arc of the meridian which extends from the equator to the pole1 as deduced from the measures of the separate arc extending from ,formentera1 the most southern of the ,balearic ,islands1 to ,dunkirk4 ,should the national standards of the two countries ever be lost1 both may be recovered1 since they are derived from natural and invariable ones4 ,the length of the measure deduced from that of the pendulum would be found again with more facility than the mtre4 ,but1 as no measure is mathematically exact1 an error in the original standard may at length become sensible in measuring a great extent1 whereas the error that must necessarily arise in measuring the quadrant of the meridian 7,n4 #aee7 is rendered totally insensible by subdivision in taking its ten-millionth part4 ,the ,french have adopted the decimal division1 not only in time1 but also in their degrees1 weights1 and measures1 on account of the very great facility it affords in computation4 ,it has not been adopted by any other country1 though nothing is more desirable than that all nations should concur in using the same standards1 not only on account of convenience1 but as affording a more definite idea of quantity4 ,it is singular that the decimal division of the day1 of space1 weights1 and measures1 was employed in ,china #djjj years ago2 and that at the time ,ibn ,junis made his observations at ,cairo1 about the year #ajjj of the ,christian era1 the ,arabs were in the habit of employing the vibrations of the pendulum in their astronomical observations as a measure of time4 ,,section ,,xiii4 ,tides--,forces that produce them--,origin and ,course of ,tidal ,wave--,its ,speed--,three kinds of ,oscillations in the ,ocean--,the ,semidiurnal ,tides--,equinoctial ,tides--,effects of the ,declination of the ,sun and ,moon--,theory insufficient without ,observation--,direction of the ,tidal ,wave--,height of ,tides--,mass of ,moon obtained from her ,action on the ,tides--,interference of ,undulations--,impossibility of a ,universal ,inundation--,currents4 ,,one of the most immediate and remarkable effects of a gravitating force external to the earth is the alternate rise and fall of the surface of the sea twice in the course of a lunar day1 or #bd;h #ejm #bhs of mean solar time4 ,as it depends upon the action of the sun and moon1 it is classed among astronomical problems1 of which it is by far the most difficult and its explanation the least satisfactory4 ,the form of the surface of the ocean in equilibrio1 when revolving with the earth round its axis1 is an ellipsoid flattened at the poles2 but the action of the sun and moon1 especially of the moon1 disturbs the equilibrium of the ocean4 ,if the moon attracted the centre of gravity of the earth and all its particles with equal and parallel forces1 the whole system of the earth and the waters that cover it would yield to these forces with a common motion1 and the equilibrium of the seas would remain undisturbed4 ,the difference of the forces and the inequality of their directions alone disturb the equilibrium4 ,the particles of water under the moon are more attracted than the centre of gravity of the earth1 in the inverse ratio of the square of the distance4 ,hence they have a tendency to leave the earth1 but are retained by their gravitation1 which is diminished by this tendency4 ,on the contrary1 the moon attracts the centre of the earth more powerfully than she attracts the particles of water in the hemisphere opposite to her2 so that the earth has a tendency to leave the waters1 but is retained by gravitation1 which is again diminished by this tendency4 ,thus the waters immediately under the moon are drawn from the earth1 at the same time that the earth is drawn from those which are diametrically opposite to her1 in both instances producing an elevation of the ocean of nearly the same height above the surface of equilibrium2 for the diminution of the gravitation of the particles in each position is almost the same1 on account of the distance of the moon being great in comparison of the radius of the earth4 ,were the earth entirely covered by the sea1 the waters thus attracted by the moon would assume the form of an oblong spheroid whose greater axis would point towards the moon2 since the columns of water under the moon1 and in the direction diametrically opposite to her1 are rendered lighter in consequence of the diminution of their gravitation2 and1 in order to preserve the equilibrium1 the axes #ij distant would be shortened4 ,the elevation1 on account of the smaller space to which it is confined1 is twice as great as the depression1 because the contents of the spheroid always remain the same4 ,if the waters were capable of assuming the form of equilibrium instantaneously1 that is1 the form of the spheroid1 its summit would always point to the moon notwithstanding the earth's rotation4 ,but1 on account of their resistance1 the rapid motion produced in them by rotation prevents them from assuming at every instant the form which the equilibrium of the forces acting upon them requires4 ,hence1 on account of the inertia of the waters1 if the tides be considered relatively to the whole earth and open seas1 there is a meridian about #cj eastward of the moon1 where it is always high water both in the hemisphere where the moon is and in that which is opposite4 ,on the west side of this circle the tide is flowing1 on the east it is ebbing1 and on every part of the meridian at #ij distant it is low water4 ,this great wave1 which follows all the motions of the moon as far as the rotation of the earth will permit1 is modified by the action of the sun1 the effects of whose attraction are in every respect like those produced by the moon1 though greatly less in degree4 ,consequently a similar wave1 but much smaller1 raised by the sun1 tends to follow his motions1 which at times combines with the lunar wave1 and at others opposes it1 according to the relative positions of the two luminaries2 but as the lunar wave is only modified a little by the solar1 the tides must necessarily happen twice in a day1 since the rotation of the earth brings the same point twice under the meridian of the moon in that time1 once under the superior and once under the inferior meridian4 ,the periodic motions of the waters of the ocean1 on the hypothesis of an ellipsoid of revolution1 entirely covered by the sea1 are1 however1 very far from according with observation4 ,this arises from the great irregularities in the surface of the earth1 which is but partially covered by the sea1 from the variety in the depths of the ocean1 the manner in which it is spread out on the earth1 the position and inclination of the shores1 the currents1 and the resistance which the waters meet with3 causes impossible to estimate generally1 but which modify the oscillations of the great mass of the ocean4 ,however1 amidst all these irregularities1 the ebb and flow of the sea maintain a ratio to the forces producing them sufficient to indicate their nature1 and to verify the law of the attraction of the sun and moon on the sea4 ,la ,place observes1 that the investigation of such relations between cause and effect is no less useful in natural philosophy than the direct solution of problems1 either to prove the existence of the causes or to trace the laws of their effects4 ,like the theory of probabilities1 it is a happy supplement to the ignorance and weakness of the human mind4 ,since the disturbing action of the sun and moon can only become sensible in a very great extent of deep water1 the ,antarctic ,ocean is the origin and birthplace of our tides4 ,a succession of tidal waves from that source follow one another in a north-westerly direction down the ,pacific and ,atlantic ,oceans1 modified as they proceed by the depth of the water and the form of the coasts4 ,for when the sun and moon are in the same meridian1 and pass over the mass of waters lying east from ,van ,diemen's ,land1 ,new ,zealand1 and the ,south ,pole1 the resulting force of their combined attraction1 penetrating to the abyss of the deep and boundless circuit of the ,southern ,ocean1 raises a vast wave or ridge of water1 which tends to follow the luminaries to the north and west1 and continues to flow in that direction long after the bodies cease to act upon it2 but it is so retarded by the rotation of the earth and by the inertia of the water1 that it does not arrive at the different parts of the coasts till after the moon's southing 7,n4 #aef74 ,when this tidal wave leaves the ,antarctic ,ocean and enters the ,pacific1 it rushes along the western coast of ,america to its farthest end1 but it is so much obstructed by the number of islands in the middle of that ocean that it is hardly perceptible among them2 while on the east it enters the ,indian ,ocean1 strikes with violence on the coasts of ,hindostan and the shores at the mouths of the ,ganges1 and causes the terrific bore in the ,hoogly4 ,the part of this tidal wave that enters the ,atlantic passes impetuously along the coasts of ,africa and ,america1 arriving later and later at each place4 ,it is modified1 however1 by a tide raised in the ,atlantic1 which is deep and free from islands2 and this combined tidal wave1 still coming northward1 pours its surge into the ,gulf of ,fundy to the height of fifty feet2 then being deflected by the coast of ,america at right angles1 it rushes eastward1 bringing high water to the western coasts of ,ireland and ,england4 ,it then goes round ,scotland1 brings high water to ,aberdeen and the opposite coasts of ,norway and ,denmark1 and1 continuing its course to the south1 arrives at the mouth of the ,thames and fills the channels of that river on the morning of the third day after leaving the ,antarctic ,ocean4 ,thus the tides in our ports are owing to an impulse from the waters of the ,antarctic seas raised by the action of the sun and moon4 ,no doubt a similar action raised that tide in the ,north ,polar ,ocean which ,dr4 ,kane saw rolling on the northern coast of ,greenland in #hb ,n4 latitude1 but which1 in the present state of the globe1 is imprisoned by bars of ice and ice-bound lands4 ,the tidal wave extends to the bottom of the ocean1 and moves uniformly and with great speed in very deep water1 variably and slow in shallow water2 the time of propagation depends upon the depth of the sea1 as well as on the nature and form of the coasts4 ,it varies inversely as the square of the depth--a law which theoretically affords the means of ascertaining the proportionate depth of the sea in different parts4 ,it is one of the great constants of nature1 and is to fluids what the pendulum is to solids--a connecting link between time and force4 ,for example3 the tidal wave moves across the ,southern ,ocean with the velocity of #ajjj miles an hour1 and in the ,atlantic it is scarcely less on account of the deep trough which runs through the centre of that ocean2 but the sea is so shallow on the ,british coast that it takes more time to come from ,aberdeen to ,london than to travel over an arc of #abj1 between #fj ,s4 lat4 and #fj ,n4 lat4 ,in deep water the tidal wave is merely a rise and fall of the surface2 the water does not advance1 though the wave does4 ,indeed1 if so heavy a body as water were to move at the rate of #ajjj miles an hour1 it would cause universal destruction1 since in the most violent hurricanes the velocity of the wind is little more than #ajj miles an hour4 ,besides1 it is evident that no ship could either sail or steam against it4 ,when the water is shallow1 however1 there is a motion of translation in the water along with the tide4 ,in the deep ocean the undulating motion consists of two distinct things--an advancing form and a molecular movement4 ,the motion of each particle of water is in an ellipse lying wholly in the vertical plane2 so that1 after the momentary displacement during the passage of the wave1 they return to their places again4 ,the resistance of the sea-bed is insensible in deep water2 but when the tidal wave1 which extends to the very bottom of the ocean1 comes into shallow water with diminished velocity1 the particles of water moving in vertical ellipses strike the bottom1 and by reaction the wave rises higher2 and that being continually repeated1 as the form moves on the wave rises higher and higher1 bends more and more forward1 till at last it loses its equilibrium1 and then both form and water roll to the shore1 and the elliptical trajectories of the particles1 which in deep water were vertical1 incline more and more1 till at length they become horizontal4 ,the distance from the shore at which the water begins to be translated depends upon the depth1 the nature of the coast1 and the form of the shore4 ,mr4 ,scott ,russell has demonstrated that in shallow water the velocity of the wave is equal to that which a heavy body falling freely by its gravity would acquire in descending through half the depth of the fluid4 ,it is proved by daily experience1 as well as by strict mathematical reasoning1 that1 if a number of waves or oscillations be excited in a fluid by different forces1 each pursues its course and has its effect independently of the rest4 ,now1 in the tides there are three kinds of oscillations1 depending on different causes1 and producing their effects independently of each other1 which may therefore be estimated separately4 ,the oscillations of the first kind1 which are very small1 are independent of the rotation of the earth1 and1 as they depend upon the motion of the disturbing body in its orbit1 they are of long periods4 ,the second kind of oscillations depend upon the rotation of the earth1 therefore their period is nearly a day4 ,the oscillations of the third kind vary with an angle equal to twice the angular rotation of the earth1 and consequently happen twice in twenty-four hours 7,n4 #aeg74 ,the first afford no particular interest1 and are extremely small2 but the difference of two consecutive tides depends upon the second4 ,at the time of the solstices this difference1 which ought to be very great according to ,newton's theory1 is hardly sensible on our shores4 ,la ,place has shown that the discrepancy arises from the depth of the sea1 and that if the depth were uniform there would be no difference in the consecutive tides but that which is occasioned by local circumstances4 ,it follows1 therefore1 that1 as this difference is extremely small1 the sea1 considered in a large extent1 must be nearly of uniform depth1 that is to say1 there is a certain mean depth from which the deviation is not great4 ,the mean depth of the ,pacific ,ocean is supposed to be about four or five miles1 that of the ,atlantic only three or four1 which1 however1 is mere conjecture4 ,possibly the great extent and uniformly small depth of the ,atlantic over the telegraphic platform may prevent the difference of the oscillations in question from being perceptible on our shores4 ,from the formul which determine the difference of these consecutive tides it is proved that the precession of the equinoxes and the nutation of the earth's axis are the same as if the sea formed one solid mass with the earth4 ,the oscillations of the third kind are the semi-diurnal tides so remarkable on our coasts4 ,in these there are two phenomena particularly to be distinguished1 one occurring twice in a month1 the other twice in a year4 ,the first phenomenon is1 that the tides are much increased in the syzygies 7,n4 #aeh71 or at the time of new and full moon3 in both cases the sun and moon are in the same meridian2 for when the moon is new they are in conjunction1 and when she is full they are in opposition4 ,in each of these positions their action is combined to produce the highest or spring tides under that meridian1 and the lowest in those points that are #ij distant4 ,it is observed that the higher the sea rises in full tide1 the lower it is in the ebb4 ,the neap tides take place when the moon is in quadrature4 ,they neither rise so high nor sink so low as the spring tides4 ,it is evident that the spring tides must happen twice in a month1 since in that time the moon is once new and once full4 ,theory proves that each partial tide increases as the cube of the parallax or apparent diameter of the body producing it1 for the greater the apparent diameter the nearer the body and the more intense its action upon the sea2 hence the spring tides are much increased when the moon is in perigee1 for then she is nearest to the earth4 ,the second phenomenon in the tides is the augmentation occurring at the time of the equinoxes1 when the sun's declination is zero 7,n4 #aei71 which happens twice in every year4 ,the spring tides which take place at that time are often much increased by the equinoctial gales1 and1 on the hypothesis of the whole earth covered by the ocean1 would be the greatest possible if the line of the moon's nodes coincided with that of her perigee1 for then the whole action of the luminaries would be in the plane of the equator4 ,but since the ,antarctic ,ocean is the source of the tides1 it is evident that the spring tide must be greatest when the moon is in perigee1 and when both luminaries have their highest southern declination1 for then they act most directly upon the great circuit of the south polar seas4 ,the sun and moon are continually making the circuit of the heavens at different distances from the plane of the equator1 on account of the obliquity of the ecliptic and the inclination of the lunar orbit4 ,the moon takes about #bi-a_/b days to vary through all her declinations1 which sometimes extend #bh-c_/d on each side of the equator1 while the sun requires nearly #cfe-a_/d days to accomplish his motions through #bc-a_/b on each side of the same plane1 so that their combined action causes great variations in the tides4 ,both the height and time of high water are perpetually changing1 and1 although the problem does not admit of a general solution1 it is necessary to analyse the phenomena which ought to arise from the attraction of the sun and moon1 but the result must be corrected in each particular case for local circumstances1 so that the theory of the tides in each port becomes really a matter of experiment1 and can only be determined by means of a vast number of observations1 including many revolutions of the moon's nodes4 ,the mean height of the tides will be increased by a very small quantity for ages to come1 in consequence of the decrease in the mean distance of the moon from the earth2 the contrary effect will take place after that period has elapsed1 and the moon's mean distance begins to increase again1 which it will continue to do for many ages4 ,thus the mean distance of the moon and the consequent minute increase in the height of the tides will oscillate between fixed limits for ever4 ,the height to which the tides rise is much greater in narrow channels than in the open sea1 on account of the obstructions they meet with4 ,the sea is so pent up in the ,british ,channel that the tides sometimes rise as much as fifty feet at ,st4 ,malo1 on the coast of ,france2 whereas on the shores of some of the ,south ,sea islands1 near the centre of the ,pacific1 they do not exceed one or two feet4 ,the winds have great influence on the height of the tides1 according as they conspire with or oppose them4 ,but the actual effect of the wind in exciting the waves of the ocean extends very little below the surface4 ,even in the most violent storms the water is probably calm at the depth of ninety or a hundred fathoms4 ,the tidal wave of the ocean does not reach the ,mediterranean nor the ,baltic1 partly from their position and partly from the narrowness of the ,straits of ,gibraltar and of the ,categat1 but it is very perceptible in the ,red ,sea and in ,hudson's ,bay4 ,the ebb and flow of the sea are perceptible in rivers to a very great distance from their estuaries4 ,in the ,narrows of ,pauxis1 in the river of the ,amazons1 more than five hundred miles from the sea1 the tides are evident4 ,it requires so many days for the tide to ascend this mighty stream1 that the returning tides meet a succession of those which are coming up2 so that every possible variety occurs at some part or other of its shores1 both as to magnitude and time4 ,it requires a very wide expanse of water to accumulate the impulse of the sun and moon1 so as to render their influence sensible2 on that account the tides in the ,mediterranean and ,black ,sea are scarcely perceptible4 ,these perpetual commotions in the waters are occasioned by forces that bear a very small proportion to terrestrial gravitation3 the sun's action in raising the ocean is only the #a_/chddhjjj of gravitation at the earth's surface1 and the action of the moon is little more than twice as much2 these forces being in the ratio of #a to #b4ceccc1 when the sun and moon are at their mean distances from the earth4 ,from this ratio the mass of the moon is found to be only the #a_/ge part of that of the earth4 ,had the action of the sun on the ocean been exactly equal to that of the moon1 there would have been no neap tides1 and the spring tides would have been of twice the height which the action of either the sun or moon would have produced separately--a phenomenon depending upon the interference of the waves or undulations4 ,a stone plunged into a pool of still water occasions a series of waves to advance along the surface1 though the water itself is not carried forward1 but only rises into heights and sinks into hollows1 each portion of the surface being elevated and depressed in its turn4 ,another stone of the same size1 thrown into the water near the first1 will occasion a similar set of undulations4 ,then1 if an equal and similar wave from each stone arrive at the same spot at the same time1 so that the elevation of the one exactly coincides with the elevation of the other1 their united effect will produce a wave twice the size of either4 ,but1 if one wave precede the other by exactly half an undulation1 the elevation of the one will coincide with the hollow of the other1 and the hollow of the one with the elevation of the other2 and the waves will so entirely obliterate one another1 that the surface of the water will remain smooth and level4 ,hence1 if the length of each wave be represented by #a1 they will destroy one another at intervals of #a_/b1 #c_/b1 #e_/b1 c41 and will combine their effects at the intervals #a1 #b1 #c1 c4 ,it will be found according to this principle1 when still water is disturbed by the fall of two equal stones1 that there are certain lines on its surface of a hyperbolic form1 where the water is smooth in consequence of the waves obliterating each other1 and that the elevation of the water in the adjacent parts corresponds to both the waves united 7,n4 #afj74 ,now1 in the spring and neap tides arising from the combination of the simple solilunar waves1 the spring tide is the joint result of the combination when they coincide in time and place2 and the neap tide happens when they succeed each other by half an interval1 so as to leave only the effect of their difference sensible4 ,it is1 therefore1 evident that1 if the solar and lunar tides were of the same height1 there would be no difference1 consequently no neap tides1 and the spring tides would be twice as high as either separately4 ,in the port of ,batsha1 in ,tonquin1 where the tides arrive by two channels of lengths corresponding to half an interval1 there is neither high nor low water on account of the interference of the waves4 ,the initial state of the ocean has no influence on the tides2 for1 whatever its primitive conditions may have been1 they must soon have vanished by the friction and mobility of the fluid4 ,one of the most remarkable circumstances in the theory of the tides is the assurance that1 in consequence of the density of the sea being only one-fifth of the mean density of the earth1 and the earth itself increasing in density towards the centre1 the stability of the equilibrium of the ocean never can be subverted by any physical cause4 ,a general inundation arising from the mere instability of the ocean is therefore impossible4 ,a variety of circumstances1 however1 tend to produce partial variations in the equilibrium of the seas1 which is restored by means of currents4 ,winds and the periodical melting of the ice at the poles occasion temporary watercourses2 but by far the most important causes are the centrifugal force induced by the velocity of the earth's rotation1 and variations in the density of the sea4 ,the centrifugal force may be resolved into two forces--one perpendicular1 and another tangent to the earth's surface 7,n4 #afa74 ,the tangential force1 though small1 is sufficient to make the fluid particles within the polar circles tend towards the equator1 and the tendency is much increased by the immense evaporation in the equatorial regions from the heat of the sun1 which disturbs the equilibrium of the ocean4 ,to this may also be added the superior density of the waters near the poles1 from their low temperature4 ,in consequence of the combination of all these circumstances1 two great currents perpetually set from each pole towards the equator4 ,but1 as they come from latitudes where the rotatory motion of the surface of the earth is very much less than it is between the tropics1 on account of their inertia1 they do not immediately acquire the velocity with which the solid part of the earth's surface is revolving at the equatorial regions2 from whence it follows that1 within twenty-five or thirty degrees on each side of the line1 the ocean has a general motion from east to west1 which is much increased by the action of the trade winds4 ,both in the ,pacific and ,atlantic currents of enormous magnitude are deflected by the continents and islands to the north and south from this mighty mass of rushing waters1 which convey the warmth of the equator to temper the severity of the polar regions1 while to maintain the equilibrium of the seas counter currents of cold water are poured from the polar oceans to mingle with the warm waters at the line1 so that a perpetual circulation is maintained4 ,icebergs are sometimes drifted as far as the ,azores from the ,polar seas1 and from the south pole they have come even to the ,cape of ,good ,hope4 ,but the ice which encircles the south pole extends to lower latitudes by #aj than that which surrounds the north4 ,in consequence of the polar current ,sir ,edward ,parry was obliged to give up his attempt to reach the north pole in the year #ahbg1 because the fields of ice were drifting to the south faster than his party could travel over them to the north4 ,kotzebue and ,sir ,james ,ross found a stratum of constant temperature in the ocean at a depth depending upon the latitude3 at the equator it is at the depth of #gbjj feet1 from whence it gradually rises till it comes to the surface in both hemispheres about the latitude of #ef #bf1 where the water has the same temperature at all depths2 it then descends to #dejj feet below the surface about the #gjth parallel both in the ,arctic and ,antarctic ,seas4 ,the temperature of that aqueous zone is about #cie of ,fahrenheit4#g ,it divides the surface of the ocean into five great zones of temperature1 namely1 a medial region1 in which the highest mean temperature is #hb ,fahr41 two temperate zones each of #cie ,fahr41 and two polar basins at the freezing point of salt water4 ,,section ,,xiv4 ,molecular ,forces--,permanency of the ultimate ,particles of ,matter--,interstices--,mossotti's ,theory--,rankin's ,theory of ,molecular ,vortices--,gases reduced to ,liquids by ,pressure--,gravitation of ,particles--,cohesion--,crystallization- -,cleavage--,isomorphism--,minuteness of the ,particles--,height of ,atmosphere--,chemical ,affinity--,definite ,proportions and ,relative ,weights of ,atoms--,faraday's ,discovery with regard to ,affinity--,capillary ,attraction4 ,,the oscillations of the atmosphere1 and its action upon the rays of light coming from the heavenly bodies1 connect the science of astronomy with the equilibrium and movements of fluids and the laws of molecular attraction4 ,hitherto that force has been under consideration which acts upon masses of matter at sensible distances2 but now the effects of such forces are to be considered as act at inappreciable distances upon the ultimate molecules of material bodies4 ,all substances consist of an assemblage of material particles1 or molecules1 which are far too small to be visible by any means human ingenuity has yet been able to devise1 and which are much beyond the limits of our perceptions4 ,they neither can be created nor destroyed2 bodies may be burned1 but their particles are not consumed--they are merely liberated from one combination to enter into another1 nor are their peculiar properties ever changed2 whatever combinations they may enter into1 they are ever and invariably the same4 ,since every known substance may be reduced in bulk by pressure1 it follows that the particles of matter are not in actual contact1 but are separated by interstices2 and it is evident that the smaller the interstitial spaces the greater the density4 ,these spaces appear to be filled with air in some cases1 as may be inferred from certain semi-opaque minerals and other substances becoming transparent when plunged into water4 ,sometimes they may possibly contain some unknown and highly elastic fluid1 such as ,sir ,david ,brewster has discovered in the minute cavities of various minerals1 which occasionally causes them to explode under the hands of the lapidary2 but as it is inconceivable that the particles of matter should act upon one another without some means of communication1 it is presumed that the interstices of material substances contain a portion of the ethereal medium with which the regions of space are filled4 ,the various hypotheses that have been formed as to the nature and action of the forces which unite the particles of matter1 have been successively given up as science advanced1 and now nothing decisive has been attained1 although ,professor ,mossotti1 of ,pisa1 by a very able analysis1 has endeavoured to prove the identity of the cohesive force with gravitation4 ,as the particles of material bodies are not in actual contact1 he supposes that each is surrounded by an atmosphere of the ethereal medium1 which he conceives to be electricity2 moreover he assumes that the atoms of the medium repel one another1 that the particles of matter also repel one another1 but with less intensity1 and that there is a mutual attraction between the particles of matter and the atoms of the medium1 forces which are assumed to vary inversely as the square of the distance4 ,hence1 when the material molecules of a body are inappreciably near to one another1 they mutually repel each other with a force which diminishes rapidly as the infinitely small distance between the material molecules augments1 and at last vanishes4 ,when the molecules are still farther apart1 the force becomes attractive4 ,at that particular point where the change takes place the forces of repulsion and attraction balance each other1 so that the molecules of a body are neither disposed to approach nor recede1 but remain in equilibrio4 ,if we try to press them nearer1 the repulsive force resists the attempt2 and if we endeavour to break the body so as to tear the particles asunder1 the attractive force predominates and keeps them together4 ,this is what constitutes the cohesive force1 or force of aggregation1 by which the molecules of all substances are united4 ,the limits of the distance at which the negative action becomes positive vary according to the temperature and nature of the molecules1 and determine whether the body which they form be solid1 liquid1 or ariform4 ,beyond this neutral point the attractive force increases as the distance between the molecules augments till it attains a maximum2 when the particles are more apart1 it diminishes2 and1 as soon as they are separated by finite or sensible distances1 it varies directly as their mass and inversely as the square of the distance1 which is precisely the law of universal gravitation4 ,thus1 on the hypothesis that the mutual repulsion between the electric atoms is a little more powerful than the mutual repulsion between the particles of matter1 the ether and the matter attract each other with unequal intensities1 which leaves an excess of attractive force constituting gravitation4 ,as the gravitating force is in operation wherever there is matter1 the ethereal electric medium must encompass all the bodies in the universe2 and1 as it is utterly incomprehensible that the celestial bodies should exert a reciprocal attraction through a void1 the ,professor concludes that the ethereal electrical medium fills all space4 ,it is true that this connexion between the molecular forces and gravitation depends upon hypothesis2 but in the greater number of physical investigations some hypothesis is requisite in the first instance to aid the imperfection of our senses2 and when the phenomena of nature accord with the assumption1 we are justified in believing it to be a general law4 ,mr4 ,rankin's theory of molecular vortices1 or the molecular structure of matter1 is independent of electricity4 ,according to his hypothesis1 each atom of matter consists of an inappreciably small nucleus1 encompassed by an elastic ethereal atmosphere which is retained in its position by attractive forces directed towards the molecule1 whilst the molecules attract each other in the direction of straight lines joining their centres4 ,the nuclei may either be solid1 or a high condensation of the atmospheres which surround each with decreasing density4 ,when the attraction between the molecules is such that the elasticity of the atmospheres is insensible1 the body is a perfect solid1 the rigidity of which bears a certain definite proportion to the elasticity of the volume4 ,when the atmospheres are less condensed and the attraction of the molecules merely produces a cohesive force sufficient to balance the atomic elasticity of the atmosphere1 the body is a perfect liquid2 and when the attraction of the molecules is very small compared with the elasticity of their ethereal atmospheres1 the body is a perfect gas4 ,these atmospheres are supposed to be portions of the ethereal medium which penetrates into the interstices of every substance1 and their elasticity to be due to the heat generated by the centrifugal force or oscillations among their atoms1 for motion is the cause of heat1 the force producing the motions varying simply as the density of the ether4 ,in ariform fluids1 although the particles are more remote from each other than in liquids and solids1 yet the pressure may be so great as to reduce an ariform fluid to a liquid1 and a liquid to a solid4 ,dr4 ,faraday has reduced some of the gases to a liquid state by very great compression2 but although atmospheric air is capable of a diminution of volume to which we do not know a limit1 it has hitherto always retained its gaseous qualities1 and resumes its primitive volume the instant the pressure is removed4 ,substances are said to be more or less elastic1 according to the facility with which they regain their bulk or volume when the pressure is removed2 thus liquids resist compression on account of their elasticity1 and in solids the resistance is much greater but variable1 and the effort required to break a substance is a measure of the cohesive force exerted by its particles4 ,in stone1 iron1 steel1 and all brittle and hard substances1 the cohesion of the particles is powerful but of small extent2 in elastic bodies1 on the contrary1 its action is weak1 but more extensive4 ,an infinite variety of conditions may be observed in the fusion of metals and other substances passing from hardness to toughness1 viscidity1 and through all the other stages to perfect fluidity and even to vapour4 ,since all bodies expand by heat1 the cohesive force is weakened by increase of temperature4 ,the cohesion of matter or the strength of substances forms an important branch of study in engineering4 ,every particle of matter1 whether it forms a constituent part of a solid1 liquid1 or ariform fluid1 is subject to the law of gravitation4 ,the weight of the atmosphere1 of gases and vapour1 shows that they consist of gravitating particles4 ,in liquids the cohesive force is not sufficiently powerful to resist the action of gravitation4 ,therefore1 although their component particles still maintain their connexion1 the liquid is scattered by their weight1 unless when it is confined in a vessel or has already descended to the lowest point possible1 and assumed a level surface from the mobility of its particles and the influence of the gravitating forces1 as in the ocean1 or a lake4 ,solids would also fall to pieces by the weight of their particles1 if the force of cohesion were not powerful enough to resist the efforts of gravitation4 ,the phenomena arising from the force of cohesion are innumerable4 ,the spherical form of rain-drops2 the difficulty of detaching a plate of glass from the surface of water2 the force with which two plane surfaces adhere when pressed together2 the drops that cling to the window-glass in a shower of rain--are all effects of cohesion entirely independent of atmospheric pressure1 and are included in the same analytical formula 7,n4 #afb7 which expresses all the circumstances accurately1 although the laws according to which the forces of cohesion and repulsion vary are unknown4 ,it is more than probable that the spherical form of the sun and planets is due to the force of cohesion1 as they have every appearance of having been at one period in a state of fusion4 ,a very remarkable instance has occasionally been observed in plate-glass manufactories4 ,after the large plates of glass of which mirrors are to be made have received their last polish1 they are carefully wiped and laid on their edges with their surfaces resting on one another4 ,in the course of time the cohesion has sometimes been so powerful1 that they could not be separated without breaking4 ,instances have occurred where two or three have been so perfectly united1 that they have been cut and their edges polished as if they had been fused together2 and so great was the force required to make the surfaces slide that one tore off a portion of the surface of the other4 ,in liquids and gases the forms of the particles have no influence1 they are so far apart2 but the structure of solids varies according to the sides which the particles present to one another during their aggregation4 ,nothing is known of their form further than the dissimilarity of their different sides in certain cases1 which appears from their reciprocal attractions during crystallisation being more or less powerful according to the sides they present to one another4 ,crystallisation is an effect of molecular attraction regulated by certain laws1 according to which atoms of the same kind of matter unite in regular forms--a fact easily proved by dissolving a piece of alum in pure water4 ,the mutual attraction of the particles is destroyed by the water2 but1 if it be evaporated1 they unite1 and form in uniting eight-sided figures called octahedrons 7,n4 #afc74 ,these however are not all the same4 ,some have their angles cut off1 others their edges1 and some both1 while the remainder take the regular form4 ,it is quite clear that the same circumstances which cause the aggregation of a few particles would1 if continued1 cause the addition of more2 and the process would go on as long as any particles remain free round the primitive nucleus1 which would increase in size1 but would remain unchanged in form1 the figure of the particles being such as to maintain the regularity and smoothness of the surfaces of the solid and their mutual inclinations4 ,a broken crystal will by degrees resume its regular figure when put back again into the solution of alum1 which shows that the internal and external particles are similar1 and have a similar attraction for the particles held in solution4 ,the original conditions of aggregation which make the molecules of the same substance unite in different forms must be very numerous1 since of carbonate of lime alone there are many hundred varieties2 and certain it is1 from the motion of polarised light through rock crystal1 that a very different arrangement of particles is requisite to produce an extremely small change in external form4 ,a variety of substances in crystallising combine chemically with a certain portion of water which in a dry state forms an essential part of their crystals1 and1 according to the experiments of ,,mm4 ,haidinger and ,mitscherlich1 seems in some cases to give the peculiar determination to their constituent molecules4 ,these gentlemen have observed that the same substance crystallising at different temperatures unites with different quantities of water and assumes a corresponding variety of forms4 ,seleniate of zinc1 for example1 unites with three different portions of water1 and assumes three different forms1 according as its temperature in the act of crystallising is hot1 lukewarm1 or cold4 ,sulphate of soda also1 which crystallises at #ij of ,fahrenheit without water of crystallisation1 combines with water at the ordinary temperature1 and takes a different form4 ,heat appears to have a great influence on the phenomena of crystallisation1 not only when the particles of matter are free1 but even when firmly united1 for it dissolves their union1 and gives them another determination4 ,professor ,mitscherlich found that prismatic crystals of sulphate of nickel 7,n4 #afd71 exposed to a summer's sun in a close vessel1 had their internal structure so completely altered without any exterior change1 that when broken open they were composed internally of octahedrons with square bases4 ,the original aggregation of the internal particles had been dissolved1 and a disposition given to arrange themselves in a crystalline form4 ,crystals of sulphate of magnesia and of sulphate of zinc1 gradually heated in alcohol till it boils1 lose their transparency by degrees1 and when opened are found to consist of innumerable minute crystals totally different in form from the whole crystals2 and prismatic crystals of zinc 7,n4 #afe7 are changed in a few seconds into octahedrons by the heat of the sun3 other instances might be given of the influence of even moderate degrees of temperature on molecular attraction in the interior of substances4 ,it must be observed that these experiments give entirely new views with regard to the constitution of solid bodies4 ,we are led from the mobility of fluids to expect great changes in the relative positions of their molecules1 which must be in perpetual motion even in the stillest water or calmest air2 but we were not prepared to find motion to such an extent in the interior of solids4 ,that their particles are brought nearer by cold and pressure1 or removed farther from one another by heat1 might be expected2 but it could not have been anticipated that their relative positions could be so entirely changed as to alter their mode of aggregation4 ,it follows1 from the low temperature at which these changes are effected1 that there is probably no portion of inorganic matter that is not in a state of relative motion4 ,professor ,mitscherlich's discoveries with regard to the forms of crystallised substances1 as connected with their chemical character1 have thrown additional light on the constitution of material bodies4 ,there is a certain set of crystalline forms which are not susceptible of variation1 as the die or cube 7,n4 #aff71 which may be small or large1 but is invariably a solid bounded by six square surfaces or planes4 ,such also is the tetrahedron 7,n4 #afg7 or four-sided solid contained by four equal-sided triangles4 ,several other solids belong to this class1 which is called the ,tessular system of crystallisation4 ,there are other crystals which1 though bounded by the same number of sides1 and having the same form1 are yet susceptible of variation2 for instance1 the eight-sided figure with a square base1 called an octahedron 7,n4 #afh71 which is sometimes flat and low1 and sometimes acute and high4 ,it was formerly believed that identity of form in all crystals not belonging to the ,tessular system indicated identity of chemical composition4 ,professor ,mitscherlich1 however1 has shown that substances differing to a certain degree in chemical composition have the property of assuming the same crystalline form4 ,for example1 the neutral phosphate of soda and the arseniate of soda crystallise in the very same form1 contain the same quantities of acid1 alkali1 and water of crystallisation2 yet they differ so far1 that one contains arsenic and the other an equivalent quantity of phosphorus4 ,substances having such properties are said to be isomorphous1 that is1 equal in form4 ,of these there are many groups1 each group having the same form1 and similarity though not identity of chemical composition4 ,for instance1 one of the isomorphous groups is that consisting of certain chemical substances called the protoxides of iron1 copper1 zinc1 nickel1 and manganese1 all of which are identical in form and contain the same quantity of oxygen1 but differ in the respective metals they contain1 which are1 however1 nearly in the same proportion in each4 ,all these circumstances tend to prove that substances having the same crystalline form must consist of ultimate atoms having the same figure and arranged in the very same order2 so that the form of crystals is dependent on their atomic constitution4 ,all crystallised bodies have joints called cleavages1 at which they split more easily than in other directions2 on this property the whole art of cutting diamonds depends4 ,each substance splits in a manner and in forms peculiar to itself4 ,for example1 all the hundreds of forms of carbonate of lime split into six-sided figures1 called rhombohedrons 7,n4 #afi71 whose alternate angles measure #ajeee and #geje1 however far the division may be carried2 therefore the ultimate particle of carbonate of lime is presumed to have that form4 ,however this may be1 it is certain that all the various crystals of that mineral may be formed by building up six-sided solids of the form described1 in the same manner as children build houses with miniature bricks4 ,it may be imagined that a wide difference may exist between the particles of an unformed mass and a crystal of the same substance--between the common shapeless limestone and the pure and limpid crystal of ,iceland spar2 yet chemical analysis detects none2 their ultimate atoms are identical1 and crystallisation shows that the difference arises only from the mode of aggregation4 ,besides1 all substances either crystallise naturally1 or may be made to do so by art4 ,liquids crystallise in freezing1 vapours by sublimation 7,n4 #agj72 and hard bodies1 when fused1 crystallise in cooling4 ,hence it may be inferred that all substances are composed of atoms1 on whose magnitude1 density1 and form1 their nature and qualities depend2 and1 as these qualities are unchangeable1 the ultimate particles of matter must be incapable of wear--the same now as when created4 ,the size of the ultimate particles of matter must be small in the extreme4 ,organised beings1 possessing life and all its functions1 have been discovered so small1 that a million of them would occupy less space than a grain of sand4 ,the malleability of gold1 the perfume of musk1 the odour of flowers1 and many other instances might be given of the excessive minuteness of the atoms of matter4 ,supposing the density of the air at the surface of the earth to be represented by unity1 ,sir ,john ,herschel has shown that1 under any hypothesis as to its atoms1 it would require a fraction having at least #acgj figures in its denominator to express its tenuity in the interplanetary space2 yet the definite proportions of chemical compounds afford a proof that divisibility of matter has a limit4 ,the cohesive force1 which has been the subject of the preceding considerations1 only unites particles of the same kind of matter2 whereas affinity1 which is the cause of chemical compounds1 is the mutual attraction between particles of different kinds of matter1 generally producing a compound which has no sensible property in common with its component parts except that of their combined gravity1 as1 for example1 water1 which is a compound of oxygen and hydrogen gases4 ,it is merely a result of the electrical state of the particles1 chemical affinity and electricity being only forms of the same power4 ,in most cases it produces electricity1 as in the oxidation of metals and combustion1 and in every case without exception heat is evolved by bodies while combining chemically2 and as heat is an expansive force1 chemical action is changed into mechanical expansion1 but it is not known in this case why heat is produced1 nor the manner in which the particles act4 ,it is a permanent and universal law in vast numbers of unorganised bodies that their composition is definite and invariable1 the same compound always consisting of the same elements united together in the same proportions4 ,two substances may indeed be mixed2 but they will not combine to form a third substance different from both1 unless their component particles unite in definite proportions2 that is to say1 one part by weight of one of the substances will unite with one part by weight of the other1 or with two parts1 or three1 or four1 c41 so as to form a new substance2 but in any other proportions they will only be mechanically mixed4 ,for example1 one part by weight of hydrogen gas will combine with eight parts by weight of oxygen gas1 and form water2 or it will unite with sixteen parts by weight of oxygen1 and form a substance called deutoxide of hydrogen2 but1 added to any other weight of oxygen1 it will produce one or both of these compounds mingled with the portion of oxygen or hydrogen in excess4 ,the law of definite proportion established by ,dr4 ,dalton1 on the principle that every compound body consists of a combination of the atoms of its constituent parts1 is of universal application1 and is in fact one of the most important discoveries in physical science1 furnishing information previously unhoped for with regard to the most secret and minute operations of nature1 in disclosing the relative weights of the ultimate atoms of matter4 ,thus an atom of oxygen uniting with an atom of hydrogen forms the compound water2 but1 as every drop of water however small consists of eight parts by weight of oxygen and one part by weight of hydrogen1 it follows that an atom of oxygen is eight times heavier than an atom of hydrogen4 ,in the same manner sulphuretted hydrogen gas consists of sixteen parts by weight of sulphur and one of hydrogen2 therefore an atom of sulphur is sixteen times heavier than an atom of hydrogen4 ,also carbonic oxide is constituted of six parts by weight of carbon and eight of oxygen2 and1 as an atom of oxygen has eight times the weight of an atom of hydrogen1 it follows that an atom of carbon is six times heavier than one of hydrogen4 ,since the same definite proportion holds in the composition of a vast number of substances that have been examined1 it has been concluded that there are great differences in the weights of the ultimate particles of matter4 ,although ,dalton's law is fully established1 yet instances have occurred from which it appears that the atomic theory deduced from it is not always maintained4 ,m4 ,gay ,lussac discovered that gases unite together by their bulk or volumes1 in such simple and definite proportions as one to one1 one to two1 one to three1 c4 ,for example1 one volume or measure of oxygen unites with two volumes or measures of hydrogen in the formation of water4 ,dr4 ,faraday has proved1 by experiments on bodies both in solution and fusion1 that chemical affinity is merely a result of the electrical state of the particles of matter4 ,now it must be observed that the composition of bodies1 as well as their decomposition1 may be accomplished by means of electricity2 and ,dr4 ,faraday has found that this chemical composition and decomposition1 by a given current of electricity1 is always accomplished according to the laws of definite proportions2 and that the quantity of electricity requisite for the decomposition of a substance is exactly the quantity necessary for its composition4 ,thus the quantity of electricity which can decompose a grain weight of water is exactly equal to the quantity of electricity which unites the elements of that grain of water together1 and is equivalent to the quantity of atmospheric electricity which is active in a very powerful flash of lightning4 ,this law is universal1 and of that high and general order which characterises all great discoveries4 ,chemical force is extremely powerful4 ,a pound of the best coal gives when burnt sufficient heat to raise the temperature of #hjhf pounds of water one ,centigrade degree1 whence ,professor ,helmholtz of ,bonn has computed that the magnitude of the chemical force of attraction between the particles of a pound of coal and the quantity of oxygen that corresponds to it1 is capable of lifting a weight of #ajj pounds to the height of #bj miles4 ,dr4 ,faraday has given a singular instance of cohesive force inducing chemical combination1 by the following experiment1 which seems to be nearly allied to the discovery made by ,m4 ,dbereiner1 in #ahbc1 of the spontaneous combustion of spongy platinum 7,n4 #aga7 exposed to a stream of hydrogen gas mixed with common air4 ,a plate of platinum with extremely clean surfaces1 when plunged into oxygen and hydrogen gas mixed in the proportions which are found in the constitution of water1 causes the gases to combine and water to be formed1 the platinum to become red-hot1 and at last an explosion to take place2 the only conditions necessary for this curious experiment being excessive purity in the gases and in the surface of the plate4 ,a sufficiently pure metallic surface can only be obtained by immersing the platinum in very strong hot sulphuric acid and then washing it in distilled water1 or by making it the positive pole of a galvanic pile in dilute sulphuric acid4 ,it appears that the force of cohesion1 as well as the force of affinity1 exerted by particles of matter1 extends to all the particles within a very minute distance4 ,hence the platinum1 while drawing the particles of the two gases towards its surface by its great cohesive attraction1 brings them so near to one another that they come within the sphere of their mutual affinity1 and a chemical combination takes place4 ,dr4 ,faraday attributes the effect in part also to a diminution in the elasticity of the gaseous particles on their sides adjacent to the platinum1 and to their perfect mixture or association1 as well as to the positive action of the metal in condensing them against its surface by its attractive force4 ,the particles when chemically united run off the surface of the metal in the form of water by their gravitation1 or pass away as aqueous vapour and make way for others4 ,the oscillations of the atmosphere1 and the changes in its temperature1 are measured by variations in the heights of the barometer and thermometer4 ,but the actual length of the liquid columns depends not only upon the force of gravitation1 but upon the cohesive force or reciprocal attraction between the molecules of the liquid and those of the tube containing it4 ,this peculiar action of the cohesive force is called capillary attraction or capillarity4 ,if a glass tube of extremely fine bore1 such as a small thermometer tube1 be plunged into a cup of water or spirit of wine1 the liquid will immediately rise in the tube above the level of that in the cup2 and the surface of the little column thus suspended will be a hollow hemisphere1 whose diameter is the interior diameter of the tube4 ,if the same tube be plunged into a cupful of mercury1 the liquid will also rise in the tube1 but it will never attain the level of that in the cup1 and its surface will be a hemisphere whose diameter is also the diameter of the tube 7,n4 #agb74 ,the elevation or depression of the same liquid in different tubes of the same matter is in the inverse ratio of their internal diameters 7,n4 #agc71 and altogether independent of their thickness2 whence it follows that the molecular action is insensible at sensible distances1 and that it is only the thinnest possible film of the interior surface of the tubes that exerts a sensible action on the liquid4 ,so much indeed is this the case1 that1 when tubes of the same bore are completely wetted with water throughout their whole extent1 mercury will rise to the same height in all of them1 whatever be their thickness or density1 because the minute coating of moisture is sufficient to remove the internal column of mercury beyond the sphere of attraction of the tube1 and to supply the place of a tube by its own capillary attraction4 ,the forces which produce the capillary phenomena are the reciprocal attraction of the tube and the liquid1 and of the liquid particles on one another2 and1 in order that the capillary column may be in equilibrio1 the weight of that part of it which rises above or sinks below the level of the liquid in the cup must balance these forces4 ,the estimation of the action of the liquid is a difficult part of this problem4 ,la ,place1 ,dr4 ,young1 and other mathematicians1 have considered the liquid within the tube to be of uniform density2 but ,m4 ,poisson1 in one of those masterly productions in which he elucidates the most abstruse subjects1 has proved that the phenomena of capillary attraction depend upon a rapid decrease in the density of the liquid column throughout an extremely small space at its surface4 ,every indefinitely thin layer of a liquid is compressed by the liquid above it1 and supported by that below4 ,its degree of condensation depends upon the magnitude of the compressive force2 and1 as this force decreases rapidly towards the surface1 where it vanishes the density of the liquid decreases also4 ,m4 ,poisson has shown that1 when this force is omitted1 the capillary surface becomes plane1 and that the liquid in the tube will neither rise above nor sink below the level of that in the cup4 ,in estimating the forces1 it is also necessary to include the variation in the density of the capillary surface round the edges from the attraction of the tube4 ,the direction of the resulting force determines the curvature of the surface of the capillary column4 ,in order that a liquid may be in equilibrio1 the force resulting from all the forces acting upon it must be perpendicular to the surface4 ,now it appears that1 as glass is more dense than water or alcohol1 the resulting force will be inclined towards the interior side of the tube2 therefore the surface of the liquid must be more elevated at the sides of the tube than in the centre in order to be perpendicular to it1 so that it will be concave as in the thermometer4 ,but1 as glass is less dense than mercury1 the resulting force will be inclined from the interior side of the tube 7,n4 #agd71 so that the surface of the capillary column must be more depressed at the sides of the tube than in the centre1 in order to be perpendicular to the resulting force1 and is consequently convex1 as may be perceived in the mercury of the barometer when rising4 ,the absorption of moisture by sponges1 sugar1 salt1 c41 are familiar examples of capillary attraction4 ,indeed the pores of sugar are so minute1 that there seems to be no limit to the ascent of the liquid4 ,wine is drawn up in a curve on the interior surface of a glass2 tea rises above its level on the side of a cup2 but1 if the glass or cup be too full1 the edges attract the liquid downwards1 and give it a rounded form4 ,a column of liquid will rise above or sink below its level between two plane parallel surfaces when near to one another1 according to the relative densities of the plates and the liquid 7,n4 #age72 and the phenomena will be exactly the same as in a cylindrical tube whose diameter is double the distance of the plates from each other4 ,if the two surfaces be very near to one another1 and touch each other at one of their upright edges1 the liquid will rise highest at the edges that are in contact1 and will gradually diminish in height as the surfaces become more separated4 ,the whole outline of the liquid column will have the form of a hyperbola4 ,indeed1 so universal is the action of capillarity1 that solids and liquids cannot touch one another without producing a change in the form of the surface of the liquid4 ,the attractions and repulsions arising from capillarity present many curious phenomena4 ,if two plates of glass or metal1 both of which are either dry or wet1 be partly immersed in a liquid parallel to one another1 the liquid will be raised or depressed close to their surfaces1 but will maintain its level through the rest of the space that separates them4 ,at such a distance they neither attract nor repel one another2 but the instant they are brought so near as to make the level part of the liquid disappear1 and the two curved parts of it meet1 the two plates will rush towards each other and remain pressed together 7,n4 #agf74 ,if one of the surfaces be wet and the other dry1 they will repel one another when so near as to have a curved surface of liquid between them2 but1 if forced to approach a little nearer1 the repulsion will be overcome1 and they will attract each other as if they were both wet or both dry4 ,two balls of pith or wood floating in water1 or two balls of tin floating in mercury1 attract one another as soon as they are so near that the surface of the liquid is curved between them4 ,two ships in the ocean may be brought into collision by this principle4 ,but two balls1 one of which is wet and the other dry1 repel one another as soon as the liquid which separates them is curved at its surface4 ,a bit of tea-leaf is attracted by the edge of the cup if wet1 and repelled when dry1 provided it be not too far from the edge and the cup moderately full2 if too full1 the contrary takes place4 ,it is probable that the rise of the sap in vegetables is in some degree owing to capillarity4 ,,section ,,xv4 ,analysis of the ,atmosphere--,its ,pressure--,law of ,decrease in ,density--,law of ,decrease in ,temperature--,measurement of ,heights by the ,barometer--,extent of the ,atmosphere--,barometrical ,variations--,oscillations--,trade-,wind s--,cloud-,ring--,monsoons--,rotation of ,winds--,laws of ,hurricanes4 ,,the atmosphere is not homogeneous4 ,it appears from analysis that1 of #ajj parts1 #iie consist of nitrogen and oxygen gases mixed in the proportions of #gi to #ba of volume1 the remainder consists of #jje parts of carbonic acid and on an average #jde of aqueous vapour4 ,these proportions are found to be the same at all heights hitherto attained by man4 ,the air is an elastic fluid1 resisting pressure in every direction1 and is subject to the law of gravitation4 ,as the space in the top of the tube of a barometer is a vacuum1 the column of mercury suspended by the pressure of the atmosphere on the surface of that in the cistern is a measure of its weight4 ,consequently every variation in the density occasions a corresponding rise or fall in the barometrical column4 ,at the level of the sea in latitude #db1 and at the temperature of melting ice1 the mean height of the barometer is #biibb or #cj inches nearly4 ,the pressure of the atmosphere is about fifteen pounds on every square inch2 so that the surface of the whole globe sustains a weight of #aa1fga1jjj1jjj hundreds of millions of pounds4 ,shell-fish1 which have the power of producing a vacuum1 adhere to the rocks by a pressure of fifteen pounds upon every square inch of contact4 ,the atmosphere when in equilibrio is an ellipsoid flattened at the poles from its rotation with the earth4 ,in that state its strata are of uniform density at equal heights above the level of the sea2 but since the air is both heavy and elastic1 its density necessarily diminishes in ascending above the surface of the earth2 for each stratum of air is compressed only by the weight above it4 ,therefore the upper strata are less dense because they are less compressed than those below them4 ,whence it is easy to show1 supposing the temperature to be constant1 that if the heights above the earth be taken in increasing arithmetical progression1 that is1 if they increase by equal quantities1 as by a foot or a mile1 the densities of the strata of air1 or the heights of the barometer which are proportionate to them1 will decrease in geometrical progression4 ,for example1 at the level of the sea if the mean height of the barometer be #biibb inches1 at the height of #ah1jjj feet it will be #adifa inches1 or one half as great2 at the height of #cf1jjj feet it will be one-fourth as great2 at #ed1jjj feet it will be one-eighth1 and so on4 ,sir ,john ,herschel has shown that the actual decrease is much more rapid1 and that1 in any hypothesis that has been formed with regard to the divisibility of the arial atoms1 a vacuum exists at the height of #hj or #ij miles above the earth's surface1 inconceivably more perfect than any that can be produced in the best air-pumps4 ,indeed the decrease in density is so rapid that three-fourths of all the air contained in the atmosphere is within four miles of the earth2 and1 as its superficial extent is #bjj millions of square miles1 its relative thickness is less than that of a sheet of paper when compared with its breadth4 ,the air even on mountain tops is sufficiently rare to diminish the intensity of sound1 to affect respiration1 and to occasion a loss of muscular strength4 ,the blood burst from the lips and ears of ,m4 de ,humboldt as he ascended the ,andes2 and he experienced the same difficulty in kindling and maintaining a fire at great heights which ,marco ,polo1 the ,venetian1 felt on the mountains of ,central ,asia4 ,m4 ,gay-,lussac ascended in a balloon to the height of #dcf miles1 and he suffered greatly from the rarity of the air4 ,it is true that at the height of thirty-seven miles the atmosphere is still dense enough to reflect the rays of the sun when #ah below the horizon2 but the tails of comets show that extremely attenuated matter is capable of reflecting light4 ,and although1 at the height of fifty miles1 the bursting of the meteor of #aghc was heard on earth like the report of a cannon1 it only proves the immensity of the explosion of a mass half a mile in diameter1 which could produce a sound capable of penetrating air three thousand times more rare than that we breathe4 ,but even these heights are extremely small when compared with the radius of the earth4 ,the density of the air is modified by various circumstances1 chiefly by changes of temperature1 because heat dilates the air and cold contracts it1 varying #a_/dhj of the whole bulk when at #cb for every degree of ,fahrenheit's thermometer4 ,experience shows that the heat of the air decreases as the height above the surface of the earth increases4 ,it appears that the mean temperature of space is #bbf below the zero point of ,fahrenheit by the theories of ,fourier and ,pouillet1 but ,sir ,john ,herschel has computed it to be -#bci ,fahr4 from observations made during the ascent in balloons4 ,such would probably be the temperature of the surface of the earth also1 were it not for the non-conducting power of the air1 whence it is enabled to retain the heat of the sun's rays1 which the earth imbibes and radiates in all directions4 ,the decrease in heat is very irregular2 each authority gives a different estimate1 because it varies with latitude and local circumstances1 but from the mean of five different statements it seems to be about one degree for every #ccd feet2 the mean of observations made in balloons is #djj feet1 which is probably nearer the truth4 ,this is the cause of the severe cold and perpetual snow on the summits of the alpine chains4 ,in the year #aheb four ascents in a balloon took place from the meteorological observatory at ,kew1 in which the greatest height attained was #bb1cgj feet4 ,the observations then made by ,mr4 ,welsh furnished ,sir ,john ,herschel with data for computing that the temperature of space is minus #bci1 that is #bci below the zero point of ,fahrenheit1 that the limiting temperature of the atmosphere is probably #gg-a_/b degrees below that point at the equator1 and #aai-a_/b below it at the poles1 with a range of temperature from the surface of #afa-a_/b in the former case1 and #aai-a_/b in the latter4 ,during these ascents it was found that the temperature of the air decreases uniformly up to a certain point1 where it is arrested and remains constant1 or increases through a depth of #bjjj or #cjjj feet1 after which it decreases again according to the same law as before4 ,throughout this zone of constant temperature it either rains1 or there is a great fall in the dew point2 in short1 it is the region of clouds1 and the increase of temperature is owing to the latent or absorbed heat set free by the condensation of the aqueous vapour4 ,in the latitude of ,kew the cloud region begins at altitudes varying between #bjjj and #fejj feet1 according to the state of the weather4 ,were it not for the effects of temperature on the density of the air1 the heights of mountains might be determined by the barometer alone2 but as the thermometer must also be consulted1 the determination becomes more complicated4 ,mr4 ,ivory's method of computing heights from barometrical measurements has the advantage of combining accuracy with the greatest simplicity4 ,indeed the accuracy with which the heights of mountains can be obtained by this method is very remarkable4 ,admiral ,smyth1 ,r4,n41 and ,sir ,john ,herschel measured the height of ,etna by the barometer1 without any communication and in different years2 ,admiral ,smyth made it #aj1hgd feet1 and ,sir ,john ,herschel #aj1hgc1 the difference being only one foot4 ,in consequence of the diminished pressure of the atmosphere water boils at a lower temperature on mountain tops than in the valleys1 which induced ,fahrenheit to propose this mode of observation as a method of ascertaining their heights4 ,it is very simple1 as ,professor ,forbes ascertained that the temperature of the boiling point varies in arithmetical proportion with the height1 or #edie feet for every degree of ,fahrenheit1 so that the calculation of height becomes one of arithmetic only1 without the use of any table4 ,the mean pressure of the atmosphere is not the same all over the globe4 ,it is less by #jbd of an inch at the equator than at the tropics or in the higher latitudes1 in consequence of the ascent of heated air and vapour from the surface of the ocean4 ,it is less also on the shores of the ,baltic ,sea than it is in ,france1 and it was observed by ,sir ,james ,c4 ,ross that throughout the whole of the ,antarctic ,ocean1 from #fh to #gd ,s4 latitude1 and from #h to #g ,w4 longitude1 there is a depression of the barometer amounting to an inch and upwards1 which is equivalent to an elevation above the sea level of #hjj feet4 ,a similar depression was observed by ,m4 ,erman in the sea of ,ochotzk1 and in the adjacent continent of eastern ,siberia4 ,sir ,john ,herschel assigns as the cause of these singular anomalies the great system of circulation of the trade and antetrade winds1 in both hemispheres1 reacting upon the general mass of the continents as obstacles in their path1 which is modified by the configuration of the land4 ,there are various periodic oscillations in the atmosphere1 which1 rising and falling like waves in the sea1 occasion corresponding changes in the height of the barometer1 but they differ as much from the trade-winds1 monsoons1 and other currents1 as the tides of the sea do from the ,gulf-stream and other oceanic rivers4 ,the sun and moon disturb the equilibrium of the atmosphere by their attraction1 and produce annual undulations which have their maximum altitudes at the equinoxes1 and their minima at the solstices4 ,there are also lunar tides1 which ebb and flow twice in the course of a lunation4 ,the diurnal tides1 which accomplish their rise and fall in six hours1 are greatly modified by the heat of the sun4 ,between the tropics the barometer attains its maximum height about nine in the morning1 then sinks till three or four in the afternoon2 it again rises and attains a second maximum about nine in the evening1 and then it begins to fall1 and reaches a second minimum at three in the morning1 again to pursue the same course4 ,according to ,m4 ,bouvard1 the amount of the oscillations at the equator is proportional to the temperature1 and in other parallels it varies as the temperature and the square of the cosine of the latitude conjointly2 consequently it decreases from the equator to the poles1 but it is somewhat greater in the day than in the night4 ,besides these small undulations1 there are vast waves perpetually moving over the continents and oceans in separate and independent systems1 being confined to local1 yet very extensive districts1 probably occasioned by long-continued rains or dry weather over large tracts of country4 ,by numerous barometrical observations made simultaneously in both hemispheres1 the courses of several have been traced1 some of which occupy twenty-four1 and others thirty-six1 hours to accomplish their rise and fall4 ,one especially of these vast barometric waves1 many hundreds of miles in breadth1 has been traced over the greater part of ,europe2 and not its breadth only1 but also the direction of its front and its velocity1 have been clearly ascertained4 ,although1 like all other waves1 these are but moving forms1 yet winds arise dependent on them like tide streams in the ocean4 ,mr4 ,birt has determined the periods of other waves of still greater extent and duration1 two of which required seventeen days to rise and fall2 and another which takes fourteen days to complete its undulation1 called by ,mr4 ,birt the ,november wave1 passes annually over the ,british ,islands1 probably over the whole of ,europe and the seas on its northern coasts4 ,its crest1 which appears to be #fjjj miles in extent1 moves from ,n4,w4 to ,s4,e4 at the rate of about #ai miles an hour2 while the extent of its barometrical elevation from its trough to its crest is seldom less than an inch1 sometimes double that quantity4 ,the great crest is preceded and followed at about five days' interval by two lower ones1 and the beginning and end are marked by deep depressions4 ,the researches of ,m4 ,leverrier leave no doubt that the great ,crimean storm of the #adth ,november1 #ahed1 was part of this phenomenon1#h for even a very small difference of atmospheric pressure is sufficient to raise a considerable wind4 ,since each oscillation has its perfect effect independently of the others1 each one is marked by a change in the barometer1 and this is beautifully illustrated by curves constructed from a series of observations4 ,the general form of the curve shows the course of the principal wave1 while small undulations in its outline mark the maxima and minima of the minor oscillations4 ,the trade-winds1 which are the principal currents in the atmosphere1 are only a particular case of those very general laws which regulate the motion of the winds depending on the rarefaction of the air combined with the rotation of the earth on its axis4 ,they are permanent currents of wind between the tropics1 blowing to the ,n4,e4 on the ,n4 side of the equator1 and to the ,s4,e4 on the ,s4 side4 ,if currents of air come from the poles1 it is clear that equilibrium must be restored by counter-currents from the equator2 moreover1 winds coming from the poles1 where there is no rotation1 to the equator1 which revolves from ,w4 to ,e4 at the rate of #ajjj miles an hour1 must of necessity move in a direction resulting from their own progressive motion and that of rotation2 hence1 in blowing towards the equator the bias is to the ,e41 and in blowing from it the bias is to the ,w4 ,thus as ,n4 and ,s4 winds from the poles blow along the surface from the tropics to the equator1 in consequence of this composition of motions that from the ,n4 becomes the ,n4,e4 trade-wind1 and that from the ,s4 the ,s4,e4 trade-wind4 ,now these winds being in contrary directions cross at the equator1 balance each other through about #f degrees of latitude1 and produce a belt of calms of that breadth encircling the globe1 known as the calms of the equator1 or the ,variables of seamen4 ,the heat of the sun rarefies the air so much1 that the trade-winds1 after crossing at the equator1 ascend into the higher regions of the atmosphere1 where that from the ,n4 goes to the tropic of ,capricorn1 and that from the ,s4 to the tropic of ,cancer4 ,but while travelling in these lofty regions they become cold and heavy1 and1 sinking to the surface at the tropics1 each proceeds to the opposite pole from which it set out4 ,now1 however1 they have a greater rotatory motion than the places they successively arrive at1 so the bias is to the ,w41 and they become the ,n4,w4 and ,s4,w4 extra-tropical winds4 ,if on arriving at the poles the air were to accumulate there1 the circulation of the winds would cease2 but currents rise into the upper regions1 and flow back again to the tropics1 where they sink down to fill the vacuum caused by the great precipitation of vapour in these regions1 and then flow to the equator as trade-winds 7,n4 #agg74 ,so the currents of air cross again at the tropics and produce two belts of calms which surround the globe1 named by ,lieutenant ,maury the ,calms of ,cancer and the ,calms of ,capricorn1 but generally known to sailors as the ,doldrums4 ,thus the winds go from pole to pole and back again1 alternately as under and upper currents4 ,in their circuits the winds cross each other five times1 producing regions of calms at the poles1 the tropics1 and equator4 ,the trade-winds generally extend for about #bh on each side of the equator1 but1 on account of the greater quantity of land in the northern hemisphere1 the ,n4,e4 trade-wind is narrower than the ,s4,e4 ,the sun is perpetually raising enormous quantities of vapour from the ocean which the trade-winds carry to the equator3 it is condensed when it rises with the air into the higher strata1 and forms a ring of clouds along the southern side of the belt of equatorial calms that surrounds the earth1 which1 during the day1 is perpetually pouring down torrents of rain1 while the sun continually beating upon its upper surface dissolves the clouds into invisible vapour which is carried by the winds and condensed into rain on the extra-tropical regions4 ,the whole system of trade-winds1 equatorial and tropical calms1 with the cloud ring1 follow the sun in declination2 consequently in its journeys back and forwards it annually travels over #ajjj miles of latitude1 and regulates the dry and rainy season in the tropical parts of the earth4 ,the monsoons1 which are periodic winds in the ,indian ,ocean1 in part depend upon this movement4 ,for when the sun is in the northern hemisphere the trade-winds come northward with him2 and when his intense heat expands the air over the ,great ,gobi and other arid ,asiatic deserts1 it ascends2 the ,n4,e4 trade-wind is drawn in to fill the vacuum and ascends with it2 then the ,s4,e4 trade-wind1 being no longer met and balanced by the ,n4,e4 trade1 passes into the northern hemisphere1 and as it proceeds northward from the equator it is deflected to the west by the rotation of the earth1 combined with the indraught over the heated deserts1 and becomes the ,s4,w4 monsoon1 which blows while the sun is north of the equator1 but as soon as he goes south1 and no longer rarefies the air over the ,indian deserts1 the ,s4,e4 trade-wind resumes its usual course1 and is then known as the ,s4,e4 monsoon4 ,the influence of the heated deserts is perceptible to the distance of #ajjj miles from the shore2 the monsoons prevail with great steadiness over the ,arabian ,gulf1 the ,indian ,ocean1 and part of the ,china ,sea4 ,at the change1 torrents of rain and violent thunderstorms accompany the conflict between the contending winds4 ,the ,sahara desert in ,north ,africa1 and those of ,utah1 ,texas1 and ,new ,mexico1 occasion the monsoons which prevail in the ,north ,atlantic and on both sides of ,central ,america1 and the monsoons which blow to the north of ,australia show the sterility of the interior1 even if other proofs were wanting4 ,from the powerful effect of the land in drawing off the winds from their course1 it may be seen why the ,n4,e4 trade-winds are narrower than the ,s4,e4 trades4 ,in the extra-tropical winds in the ,north ,atlantic1 which blow from the #djth parallel to the pole1 the north-westerly are to the easterly as #b to #a3 hence there would be an accumulation of air at the pole at the expense of the equator1 did not a current rise at the pole and return to the equator through the high regions of the atmosphere1 which confirms the theory of the rotation of the wind4 ,there are many proofs of the existence of the counter-currents above the trade-winds4 ,on the ,peak of ,teneriffe the prevailing winds are from the west4 ,light clouds have frequently been seen moving rapidly from west to east at a very great height above the trade-winds1 which were sweeping along the surface of the ocean in a contrary direction4 ,rains1 clouds1 and nearly all the other atmospheric phenomena1 occur below the height of #ah1jjj feet1 and generally much nearer to the surface of the earth4 ,they are owing to currents of air running upon each other in horizontal strata1 differing in their electric state1 in temperature and moisture1 as well as in velocity and direction4 ,when north and south winds blow alternately1 the wind at any place will veer in one uniform direction through every point of the compass1 provided the one begins before the other has ceased4 ,in the northern hemisphere a north wind sets out with a smaller degree of rotatory motion than the places have at which it successively arrives1 consequently it passes through all the points of the compass from ,n4 to ,n4,e4 and ,e4 ,a current from the south1 on the contrary1 sets out with a greater rotatory velocity than the places have at which it successively arrives1 so by the rotation of the earth it is deflected from ,s4 to ,s4,w4 and ,w4 ,now1 if the vane at any place should have veered from the ,n4 through ,n4,e4 to ,e41 and a south wind should spring up1 it would combine its motion with the former and cause the vane to turn successively from the ,e4 to ,s4,e4 and ,s4 ,but by the earth's rotation this south wind will veer to the ,s4,w4 and ,w41 and1 if a north wind should now arise1 it would combine its motion with that of the west1 and cause it to veer to the ,n4,w4 and ,n4 ,thus two alternations of north and south wind will cause the vane at any place to go completely round the compass1 from ,n4 to ,e41 ,s41 ,w41 and ,n4 again4 ,at the ,royal ,observatory at ,greenwich the wind accomplishes five circuits in that direction in the course of a year4 ,when circumstances combine to produce alternate north and south winds in the southern hemisphere1 the gyration is in the contrary direction4 ,although the general tendency of the wind may be rotatory1 and is so in many instances1 at least for part of the year1 yet it is so often counteracted by local circumstances1 that the winds are in general very irregular1 every disturbance in atmospheric equilibrium from heat or any other cause producing a corresponding wind4 ,the most prevalent winds in ,europe are the ,n4,e4 and ,s4,w42 the former arises from the north polar current1 and the latter from causes already mentioned4 ,the law of the wind's rotation was first described by ,dr4 ,dalton1 but has been developed by ,professor ,dove1 of ,berlin4 ,hurricanes are those storms of wind in which the portion of the atmosphere that forms them revolves in a horizontal circuit round a vertical or somewhat inclined axis of rotation1 while the axis itself1 and consequently the whole storm1 is carried forward along the surface of the globe1 so that the direction in which the storm is advancing is quite different from the direction in which the rotatory current may be blowing at any point4 ,in the ,west ,indies1 where hurricanes are frequent and destructive1 they generally originate in the tropical regions near the inner boundary of the trade-winds1 and are caused by the vertical ascent of a column of rarefied air1 whose place is supplied by a rush of wind from the surrounding regions1 set into gyration by the rotation of the earth4 ,by far the greater number of ,atlantic hurricanes have begun eastward of the lesser ,antilles or ,caribbean ,islands4 ,in every case the axis of the storm moves in an elliptical or parabolic curve1 having its vertex in or near the tropic of ,cancer1 which marks the external limit of the trade-winds north of the equator4 ,as the motion before it reaches the tropic is in a straight line from ,s4,e4 to ,n4,w41 and after it has passed it from ,s4,w4 to ,n4,e41 the bend of the curve is turned towards ,florida and the ,carolinas4 ,in the southern hemisphere the body of the storm moves in exactly the opposite direction4 ,the hurricanes which originate south of the equator1 and whose initial path is from ,n4,e4 to ,s4,w41 bend round at the tropic of ,capricorn1 and then move from ,n4,w4 to ,s4,e4 ,the extent and velocity of these storms are great2 for instance1 the hurricane that took place on the #abth of ,august1 #ahcj1 was traced from eastward of the ,caribbee ,islands1 along the ,gulf ,stream1 to the bank of ,newfoundland1 a distance of more than #cjjj miles1 which it passed over in six days4 ,although the hurricane of the #ast of ,september1 #ahba1 was not so extensive1 its velocity was greater1 as it moved at the rate of #cj miles an hour3 small storms are generally more rapid than those of greater dimensions4 ,the action of these storms seems to be at first confined to the stratum of air nearest the earth1 and then they seldom appear to be more than a mile high1 though sometimes they are raised higher2 or even divided by a mountain into two separate storms1 each of which continues its new path and gyrations with increased violence4 ,this occurred in the gale of the #beth of ,december1 #ahba1 in the ,mediterranean1 when the ,spanish mountains and the ,maritime ,alps became new centres of motion4 ,by the friction of the earth the axis of the storm bends a little forward4 ,this causes a continual intermixture of the lower and warmer strata of air with those that are higher and colder1 producing torrents of rain and violent electric explosions4 ,the breadth of the whirlwind is greatly augmented when the path of the storm changes on crossing the tropic4 ,the vortex of a storm has covered an extent of the surface of the globe #ejj miles in diameter4 ,the revolving motion accounts for the sudden and violent changes observed during hurricanes4 ,in consequence of the rotation of the air1 the wind blows in opposite directions on each side of the axis of the storm1 and the violence of the blast increases from the circumference towards the centre of gyration1 but in the centre itself the air is in repose3 hence1 when the body of the storm passes over a place1 the wind begins to blow moderately1 and increases to a hurricane as the centre of the whirlwind approaches2 then1 in a moment1 a dead and awful calm succeeds1 suddenly followed by a renewal of the storm in all its violence1 but now blowing in a direction diametrically opposite to its former course4 ,this happened at the ,island of ,st4 ,thomas on the #bnd of ,august1 #ahcg1 where the hurricane increased in violence till half-past seven in the morning1 when perfect stillness took place for forty minutes1 after which the storm recommenced in a contrary direction4 ,the sudden fall of the mercury in the barometer in the regions habitually visited by hurricanes is a certain indication of a coming tempest4 ,in consequence of the centrifugal force of these rotatory storms the air becomes rarefied1 and1 as the atmosphere is disturbed to some distance beyond the actual circle of gyration or limits of the storm1 the barometer often sinks some hours before its arrival1 from the original cause of the rotatory disturbance4 ,it continues sinking under the first half of the hurricane1 is at a maximum sometimes of two inches in the centre of gyration1 and again rises during the passage of the latter half1 though it does not attain its greatest height till the storm is over4 ,the diminution of atmospheric pressure is greater and extends over a wider area in the temperate zones than in the torrid1 on account of the sudden expansion of the circle of rotation when the gale crosses the tropic4 ,as the fall of the barometer gives warning of the approach of a hurricane1 so the laws of the storm's motion afford the seaman knowledge to guide him in avoiding it4 ,in the northern temperate zone1 if the gale begins from the ,s4,e4 and veers by ,s4 to ,w41 the ship should steer to the ,s4,e42 but1 if the gale begins from the ,n4,e41 and changes through ,n4 to ,n4,w41 the vessel should go to the ,n4,w4 ,in the northern part of the torrid zone1 if the storm begin from the ,n4,e41 and veer through ,e4 to ,s4,e41 the ship should steer to the ,n4,e42 but1 if it begin from the ,n4,w41 and veer by ,w4 to ,s4,w41 the ship should steer to the ,s4,w41 because she is in the south-western side of the storm4 ,since the laws of storms are reversed in the southern hemisphere1 the rules for steering vessels are necessarily reversed also4 ,a heavy swell is peculiarly characteristic of these storms4 ,in the open sea the swell often extends many leagues beyond the range of the gale which produced it4 ,waterspouts are occasioned by small whirlwinds1 which always have their origin at a great distance from that part of the sea from which the spout begins to rise1 where it is generally calm4 ,the whirl is produced by two currents of air1 which1 running in opposite directions1 compress one another by their impetus1 so that they rise in spiral eddies to the clouds4 ,they move slowly along the surface of the sea1 sometimes in vertical1 and sometimes in twisted spirals1 putting the sea into violent agitation as they pass1 and carrying the water aloft by the force of gyration4 ,occasionally the eddies begin in the clouds and dip down to the sea4 ,,section ,,xvi4 ,sound--,propagation of ,sound illustrated by a ,field of ,standing ,corn--,nature of ,waves--,propagation of ,sound through the ,atmosphere--,intensity--,noises--,a ,musical ,sound--,quality--,pitch--,extent of ,human ,hearing--,velocity of ,sound in ,air1 ,water1 and ,solids--,causes of the ,obstruction of ,sound--,law of its ,intensity--,reflection of ,sound--,echoes--,thunder--,refraction of ,sound--,interference of ,sounds4 ,,one of the most important uses of the atmosphere is the conveyance of sound4 ,without the air1 deathlike silence would prevail through nature1 for in common with all substances it has a tendency to impart vibrations to bodies in contact with it4 ,therefore undulations received by the air1 whether it be from a sudden impulse1 such as an explosion or the vibrations of a musical chord1 are propagated in every direction1 and produce the sensation of sound upon the auditory nerves4 ,a bell rung under the exhausted receiver of an air-pump is inaudible1 which shows that the atmosphere is really the medium of sound4 ,in the small undulations of deep water in a calm1 the vibrations of the liquid particles are made in the vertical plane1 that is1 up and down1 or at right angles to the direction of the transmission of the waves4 ,but the vibrations of the particles of air which produce sound differ from these1 being performed in the same direction in which the waves of sound travel4 ,the propagation of sound has been illustrated by a field of corn agitated by the wind4 ,however irregular the motion of the corn may seem on a superficial view1 it will be found1 if the velocity of the wind be constant1 that the waves are all precisely similar and equal1 and that all are separated by equal intervals and move in equal times4 ,a sudden blast depresses each ear equally and successively in the direction of the wind1 but1 in consequence of the elasticity of the stalks and the force of the impulse1 each ear not only rises again as soon as the pressure is removed1 but bends back nearly as much in the contrary direction1 and then continues to oscillate backwards and forwards in equal times1 like a pendulum1 to a less and less extent1 till the resistance of the air puts a stop to the motion4 ,these vibrations are the same for every individual ear of corn4 ,yet1 as their oscillations do not all commence at the same time1 but successively1 the ears will have a variety of positions at any one instant4 ,some of the advancing ears will meet others in their returning vibrations1 and1 as the times of oscillation are equal for all1 they will be crowded together at regular intervals4 ,between these there will occur equal spaces where the ears will be few1 in consequence of being bent in opposite directions2 and at other equal intervals they will be in their natural upright positions4 ,so that over the whole field there will be a regular series of condensations and rarefactions among the ears of corn1 separated by equal intervals1 where they will be in their natural state of density4 ,in consequence of these changes the field will be marked by an alternation of bright and dark bands4 ,thus the successive waves which fly over the corn with the speed of the wind are totally distinct from1 and entirely independent of the extent of the oscillations of each individual ear1 though both take place in the same direction4 ,the length of a wave is equal to the space between two ears precisely in the same state of motion1 or which are moving similarly1 and the time of the vibration of each ear is equal to that which elapses between the arrival of two successive waves at the same point4 ,the only difference between the undulations of a corn-field and those of the air which produce sound is1 that each ear of corn is set in motion by an external cause1 and is uninfluenced by the motion of the rest2 whereas in air1 which is a compressible and elastic fluid1 when one particle begins to oscillate1 it communicates its vibrations to the surrounding particles1 which transmit them to those adjacent1 and so on continually4 ,hence from the successive vibrations of the particles of air the same regular condensations and rarefactions take place as in the field of corn1 producing waves throughout the whole mass of air1 though each molecule like each individual ear of corn never moves far from its state of rest4 ,the small waves of a liquid1 and the undulations of the air1 like waves in the corn1 are evidently not real masses moving in the direction in which they are advancing1 but merely outlines1 motions1 or forms passing along1 and comprehending all the particles of an undulating fluid which are at once in a vibratory state4 ,it is thus that an impulse given to any one point of the atmosphere is successively propagated in all directions1 in a wave diverging as from the centre of a sphere to greater and greater distances1 but with decreasing intensity1 in consequence of the increasing number of particles of inert matter which the force has to move2 like the waves formed in still water by a falling stone1 which are propagated circularly all around the centre of disturbance 7,n4 #afj74 ,the intensity of sound depends upon the violence and extent of the initial vibrations of air2 but1 whatever they may be1 each undulation when once formed can only be transmitted straight forwards1 and never returns back again unless when reflected by an opposing obstacle4 ,the vibrations of the arial molecules are always extremely small1 whereas the waves of sound vary from a few inches to several feet4 ,the various musical instruments1 the human voice and that of animals1 the singing of birds1 the hum of insects1 the roar of the cataract1 the whistling of the wind1 and the other nameless peculiarities of sound1 show at once an infinite variety in the modes of arial vibration1 and the astonishing acuteness and delicacy of the ear1 thus capable of appreciating the minutest differences in the laws of molecular oscillation4 ,all mere noises are occasioned by irregular impulses communicated to the ear2 and1 if they be short1 sudden1 and repeated beyond a certain degree of quickness1 the ear loses the intervals of silence1 and the sound appears continuous4 ,still such sounds will be mere noise3 in order to produce a musical sound1 the impulses1 and consequently the undulations of the air1 must be all exactly similar in duration and intensity1 and must recur after exactly equal intervals of time4 ,if a blow be given to the nearest of a series of broad1 flat1 and equidistant palisades1 set edgewise in a line direct from the ear1 each palisade will repeat or echo the sound2 and these echoes1 returning to the ear at successive equal intervals of time1 will produce a musical note4 ,the quality of a musical note depends upon the abruptness1 and its intensity upon the violence and extent of the original impulse4 ,in the theory of harmony the only property of sound taken into consideration is the pitch1 which varies with the rapidity of the vibrations4 ,the grave or low tones are produced by very slow vibrations1 which increase in frequency as the note becomes more acute4 ,the lowest man's voice makes #cif vibrations in a second1 whilst the highest woman's voice makes #baab4 ,very deep tones are not heard by all alike1 and ,dr4 ,wollaston1 who made a variety of experiments on the sense of hearing1 found that many people1 though not at all deaf1 are quite insensible to the cry of the bat or the cricket1 while to others it is painfully shrill4 ,from his experiments he concluded that human hearing is limited to about nine octaves1 extending from the lowest note of the organ to the highest known cry of insects2 and he observes with his usual originality that1 8as there is nothing in the nature of the atmosphere to prevent the existence of vibrations incomparably more frequent than any of which we are conscious1 we may imagine that animals like the ,grylli1 whose powers appear to commence nearly where ours terminate1 may have the faculty of hearing still sharper sounds which we do not know to exist1 and that there may be other insects hearing nothing in common with us1 but endowed with a power of exciting1 and a sense which perceives vibrations1 of the same nature indeed as those which constitute our ordinary sounds1 but so remote that the animals which perceive them may be said to possess another sense1 agreeing with our own solely in the medium by which it is excited40 ,m4 ,savart1 so well known for the number and beauty of his researches in acoustics1 has proved that a high note of a given intensity1 being heard by some ears and not by others1 must not be attributed to its pitch1 but to its feebleness4 ,his experiments1 and those more recently made by ,professor ,wheatstone1 show that1 if the pulses could be rendered sufficiently powerful1 it would be difficult to fix a limit to human hearing at either end of the scale4 ,m4 ,savart had a wheel made about nine inches in diameter with #cfj teeth set at equal distances round its rim1 so that while in motion each tooth successively hit on a piece of card4 ,the tone increased in pitch with the rapidity of the rotation1 and was very pure when the number of strokes did not exceed three or four thousand in a second1 but beyond that it became feeble and indistinct4 ,with a wheel of a larger size a much higher tone could be obtained1 because1 the teeth being wider apart1 the blows were more intense and more separated from one another4 ,with #gbj teeth on a wheel thirty-two inches in diameter1 the sound produced by #ab1jjj strokes in a second was audible1 which corresponds to #bd1jjj vibrations of a musical chord4 ,so that the human ear can appreciate a sound which only lasts the #bd1jjjth part of a second4 ,this note was distinctly heard by ,m4 ,savart and by several people who were present1 which convinced him that with another apparatus still more acute sounds might be rendered audible4 ,for the deep tones ,m4 ,savart employed a bar of iron1 two feet eight inches long1 about two inches broad1 and half an inch in thickness1 which revolved about its centre as if its arms were the spokes of a wheel4 ,when such a machine rotates1 it impresses a motion on the air similar to its own1 and1 when a thin board or card is brought close to its extremities1 the current of air is momentarily interrupted at the instant each arm of the bar passes before the card2 it is compressed above the card and dilated below2 but the instant the spoke has passed a rush of air to restore equilibrium makes a kind of explosion1 and1 when these succeed each other rapidly1 a musical note is produced of a pitch proportional to the velocity of the revolution4 ,when ,m4 ,savart turned this bar slowly1 a succession of single beats was heard2 as the velocity became greater1 the sound was only a rattle2 but1 as soon as it was sufficient to give eight beats in a second1 a very deep musical note was distinctly audible corresponding to sixteen single vibrations in a second1 which is the lowest that has hitherto been produced4 ,when the velocity of the bar was much increased1 the intensity of the sound was hardly bearable4 ,the spokes of a revolving wheel produce the sensation of sound1 on the very same principle that a burning stick whirled round gives the impression of a luminous circle4 ,the vibrations excited in the organ of hearing by one beat have not ceased before another impulse is given4 ,indeed it is indispensable that the impressions made upon the auditory nerves should encroach upon each other in order to produce a full and continued note4 ,on the whole1 ,m4 ,savart has come to the conclusion1 that the most acute sounds would be heard with as much ease as those of a lower pitch1 if the duration of the sensation produced by each pulse could be diminished proportionally to the augmentation of the number of pulses in a given time3 and on the contrary1 if the duration of the sensation produced by each pulse could be increased in proportion to their number in a given time1 that the deepest tones would be as audible as any of the others4 ,the velocity of sound is uniform and independent of the nature1 extent1 and intensity of the primitive disturbance4 ,consequently sounds of every quality and pitch travel with equal speed4 ,the smallest difference in their velocity is incompatible either with harmony or melody1 for notes of different pitches and intensities1 sounded together at a little distance1 would arrive at the ear in different times4 ,a rapid succession of notes would in this case produce confusion and discord4 ,but1 as the rapidity with which sound is transmitted depends upon the elasticity of the medium through which it has to pass1 whatever tends to increase the elasticity of the air must also accelerate the motion of sound4 ,on that account its velocity is greater in warm than in cold weather1 supposing the pressure of the atmosphere constant4 ,in dry air1 at the freezing temperature1 sound travels at the rate of #ajij feet in a second1 and for any higher temperature one foot must be added for every degree of the thermometer above #cb3 hence at #fb of ,fahrenheit its speed in a second is #aabj feet1 or #gfe miles an hour1 which is about three-fourths of the diurnal velocity of the earth's equator4 ,since all the phenomena of the transmission of sound are simple consequences of the physical properties of the air1 they have been predicted and computed rigorously by the laws of mechanics4 ,it was found1 however1 that the velocity of sound1 determined by observation1 exceeded what it ought to have been theoretically by #agc feet1 or about one-sixth of the whole amount4 ,la ,place suggested that this discrepancy might arise from the increased elasticity of the air in consequence of a development of latent or absorbed heat 7,n4 #agh7 during the undulations of sound1 and calculation confirmed the accuracy of his views4 ,the arial molecules being suddenly compressed give out their absorbed heat2 and1 as air is too bad a conductor to carry it rapidly off1 it occasions a momentary and local rise of temperature1 which1 increasing the elasticity of the air without at the same time increasing its inertia1 causes the movement to be propagated more rapidly4 ,analysis gives the true velocity of sound in terms of the elevation of temperature that a mass of air is capable of communicating to itself1 by the disengagement of its own absorbed heat when suddenly compressed in a given ratio4 ,this change of temperature however could not be obtained directly by any experiments which had been made at that epoch2 but by inverting the problem1 and assuming the velocity of sound as given by experiment1 it was computed that the temperature of a mass of air is raised nine-tenths of a degree when the compression is equal to #a_/aaf of its volume4 ,probably all liquids are elastic1 though considerable force is required to compress them4 ,water suffers a condensation of nearly #jjjjjdif for every atmosphere of pressure1 and is consequently capable of conveying sound even more rapidly than air1 the velocity in the former being #dgjh feet in a second4 ,a person under water hears sounds made in air feebly1 but those produced in water very distinctly4 ,according to the experiments of ,m4 ,colladon1 the sound of a bell was conveyed under water through the ,lake of ,geneva to the distance of about nine miles4 ,he also perceived that the progress of sound through water is greatly impeded by the interposition of any object1 such as a projecting wall2 consequently sound under water resembles light in having a distinct shadow4 ,it has much less in air1 being transmitted all round buildings or other obstacles1 so as to be heard in every direction1 though often with a considerable diminution of intensity1 as when a carriage turns the corner of a street4 ,the velocity of sound in passing through solids is in proportion to their hardness1 and is much greater than in air or water4 ,a sound which takes some time in travelling through the air passes almost instantaneously along a wire six hundred feet long2 consequently it is heard twice--first as communicated by the wire1 and afterwards through the medium of the air4 ,the facility with which the vibrations of sound are transmitted along the grain of a log of wood is well known4 ,indeed they pass through iron1 glass1 and some kinds of wood1 at the rate of #ah1ecj feet in a second4 ,the velocity of sound is obstructed by a variety of circumstances1 such as falling snow1 fog1 rain1 or any other cause which disturbs the homogeneity of the medium through which it has to pass4 ,m4 de ,humboldt says that it is on account of the greater homogeneity of the atmosphere during the night that sounds are then better heard than during the day1 when its density is perpetually changing from partial variations of temperature4 ,his attention was called to this subject on the plain surrounding the ,mission of the ,apures by the rushing noise of the great cataracts of the ,orinoco1 which seemed to be three times as loud by night as by day4 ,this he illustrated by experiment4 ,a tall glass half full of champagne cannot be made to ring as long as the effervescence lasts4 ,in order to produce a musical note1 the glass together with the liquid it contains must vibrate in unison as a system1 which it cannot do in consequence of the fixed air rising through the wine and disturbing its homogeneity1 because1 the vibrations of the gas being much slower than those of the liquid1 the velocity of the sound is perpetually interrupted4 ,for the same reason the transmission of sound as well as light is impeded in passing through an atmosphere of variable density4 ,sir ,john ,herschel1 in his admirable ,treatise on ,sound1 thus explains the phenomenon3--8,it is obvious10 he says1 8that sound as well as light must be obstructed1 stifled1 and dissipated from its original direction by the mixture of air of different temperatures1 and consequently elasticities2 and thus the same cause which produces that extreme transparency of the air at night1 which astronomers alone fully appreciate1 renders it also more favourable to sound4 ,there is no doubt1 however1 that the universal and dead silence generally prevalent at night renders our auditory nerves sensible to impressions which would otherwise escape notice4 ,the analogy between sound and light is perfect in this as in so many other respects4 ,in the general light of day the stars disappear4 ,in the continual hum of voices1 which is always going on by day1 and which reach us from all quarters1 and never leave the ear time to attain complete tranquillity1 those feeble sounds which catch our attention at night make no impression4 ,the ear1 like the eye1 requires long and perfect repose to attain its utmost sensibility40 ,many instances may be brought in proof of the strength and clearness with which sound passes over the surface of water or ice4 ,lieutenant ,forster was able to carry on a conversation across ,port ,bowen ,harbour1 when frozen1 a distance of a mile and a half4 ,the intensity of sound depends upon the extent of the excursions of the fluid molecules1 on the energy of the transient condensations and dilatations1 and on the greater or less number of particles which experience these effects4 ,we estimate that intensity by the impetus of these fluid molecules on our organs1 which is consequently as the square of the velocity1 and not by their inertia1 which is as the simple velocity4 ,were the latter the case1 there would be no sound1 because the inertia of the receding waves of air would destroy the equal and opposite inertia of those advancing2 whence it may be concluded that the intensity of sound diminishes inversely as the square of the distance from its origin4 ,in a tube1 however1 the force of sound does not decay as in open air1 unless perhaps by friction against the sides4 ,m4 ,biot found1 from a number of highly-interesting experiments made on the pipes of the aqueducts in ,paris1 that a continued conversation could be carried on in the lowest possible whisper through a cylindrical tube about #cabj feet long1 the time of transmission through that space being #bgi seconds4 ,in most cases sound diverges in all directions so as to occupy at any one time a spherical surface2 but ,dr4 ,young has shown that there are exceptions1 as1 for example1 when a flat surface vibrates only in one direction4 ,the sound is then most intense when the ear is at right angles to the surface1 whereas it is scarcely audible in a direction precisely perpendicular to its edge4 ,in this case it is impossible that the whole of the surrounding air can be affected in the same manner1 since the particles behind the sounding surface must be moving towards it whenever the particles before it are retreating4 ,hence in one half of the surrounding sphere of air its motions are retrograde1 while in the other half they are direct2 consequently1 at the edges where these two portions meet1 the motions of the air will neither be retrograde nor direct1 and therefore it must be at rest4 ,it appears1 from theory as well as daily experience1 that sound is capable of reflection from surfaces 7,n4 #agi7 according to the same laws as light4 ,indeed any one who has observed the reflection of the waves from a wall on the side of a river1 after the passage of a steam-boat1 will have a perfect idea of the reflection of sound and of light4 ,as every substance in nature is more or less elastic1 it may be agitated according to its own law by the impulse of a mass of undulating air2 and reciprocally the surface by its reaction will communicate its undulations back again into the air4 ,such reflections produce echoes2 and as a series of them may take place between two or more obstacles1 each will cause an echo of the original sound1 growing fainter and fainter till it dies away2 because sound1 like light1 is weakened by reflection4 ,should the reflecting surface be concave towards a person1 the sound will converge towards him with increased intensity1 which will be greater still if the surface be spherical and concentric with him4 ,undulations of sound diverging from one focus of an elliptical shell 7,n4 #ahj7 converge in the other after reflection4 ,consequently a sound from the one will be heard in the other as if it were close to the ear4 ,the rolling noise of thunder has been attributed to reverberation between different clouds1 which may possibly be the case to a certain extent4 ,sir ,john ,herschel is of opinion that an intensely prolonged peal is probably owing to a combination of sounds1 because1 the velocity of electricity being incomparably greater than that of sound1 the thunder may be regarded as originating in every point of a flash of lightning at the same instant4 ,the sound from the nearest point will arrive first2 and if the flash run in a direct line from a person1 the noise will come later and later from the remote points of its path in a continued roar4 ,should the direction of the flash be inclined1 the succession of sounds will be more rapid and intense3 and if the lightning describe a circular curve round a person1 the sound will arrive from every point at the same instant with a stunning crash4 ,in like manner the subterranean noises heard during earthquakes like distant thunder may arise from the consecutive arrival at the ear of undulations propagated at the same instant from nearer and more remote points2 or if they originate in the same point1 the sound may come by different routes through strata of different densities4 ,sounds under water are heard very distinctly in the air immediately above2 but the intensity decays with great rapidity as the observer goes farther off1 and is altogether inaudible at the distance of two or three hundred yards4 ,so that waves of sound1 like those of light1 in passing from a dense to a rare medium1 are not only refracted1 but suffer total reflection at very oblique incidences 7,n4 #ahi74 ,the laws of interference extend also to sound4 ,it is clear that two equal and similar musical strings will be in unison if they communicate the same number of vibrations to the air in the same time4 ,but if two such strings be so nearly in unison that one performs a hundred vibrations in a second1 and the other a hundred and one in the same period--during the first few vibrations the two resulting sounds will combine to form one of double the intensity of either1 because the arial waves will sensibly coincide in time and place2 but one will gradually gain on the other till at the fiftieth vibration it will be half an oscillation in advance4 ,then the waves of air which produce the sound being sensibly equal1 but the receding part of the one coinciding with the advancing part of the other1 they will destroy one another1 and occasion an instant of silence4 ,the sound will be renewed immediately after1 and will gradually increase till the hundredth vibration1 when the two waves will combine to produce a sound double the intensity of either4 ,these intervals of silence and greatest intensity1 called beats1 will recur every second2 but if the notes differ much from one another1 the alternations will resemble a rattle2 and if the strings be in perfect unison1 there will be no beats1 since there will be no interference4 ,thus by interference is meant the co-existence of two undulations in which the lengths of the waves are the same4 ,and as the magnitude of an undulation may be diminished by the addition of another transmitted in the same direction1 it follows that one undulation may be absolutely destroyed by another when waves of the same length are transmitted in the same direction1 provided that the maxima of the undulations are equal1 and that one follows the other by half the length of a wave4 ,a tuning-fork affords a good example of interference4 ,when that instrument vibrates1 its two branches alternately recede from and approach one another2 each communicates its vibrations to the air1 and a musical note is the consequence4 ,if the fork be held upright about a foot from the ear1 and turned round its axis while vibrating1 at every quarter revolution the sound will scarcely be heard1 while at the intermediate points it will be strong and clear4 ,this phenomenon arises from the interference of the undulations of air coming from the two branches of the fork4 ,when the two branches coincide1 or when they are at equal distances from the ear1 the waves of air combine to reinforce each other2 but at the quadrants1 where the two branches are at unequal distances from the ear1 the lengths of the waves differ by half an undulation1 and consequently destroy one another4 ,,section ,,xvii4 ,vibration of ,musical ,strings--,harmonic ,sounds--,nodes--,vibration of ,air in ,wind-,instruments--,vibration of ,solids--,vibrating ,plates--,bells--,harmony--,sounding ,boards--,forced ,vibrations--,resonance--,speaking ,machines4 ,,when the particles of elastic bodies are suddenly disturbed by an impulse1 they return to their natural position by a series of isochronous vibrations1 whose rapidity1 force1 and permanency depend upon the elasticity1 the form1 and the mode of aggregation which unites the particles of the body4 ,these oscillations are communicated to the air1 and on account of its elasticity they excite alternate condensations and dilatations in the strata of the fluid nearest to the vibrating body2 from thence they are propagated to a distance4 ,a string or wire stretched between two pins1 when drawn aside and suddenly let go1 will vibrate till its own rigidity and the resistance of the air reduce it to rest4 ,these oscillations may be rotatory1 in every plane1 or confined to one plane according as the motion is communicated4 ,in the piano-forte1 where the strings are struck by a hammer at one extremity1 the vibrations probably consist of a bulge running to and fro from end to end4 ,different modes of vibration may be obtained from the same sonorous body4 ,suppose a vibrating string to give the lowest ,c of the pianoforte which is the fundamental note of the string2 if it be lightly touched exactly in the middle1 so as to retain that point at rest1 each half will then vibrate twice as fast as the whole1 but in opposite directions2 the ventral or bulging segments will be alternately above and below the natural position of the string1 and the resulting note will be the octave above ,c4 ,when a point at a third of the length of the string is kept at rest1 the vibrations will be three times as fast as those of the whole string1 and will give the twelfth above ,c4 ,when the point of rest is one-fourth of the whole1 the oscillations will be four times as fast as those of the fundamental note1 and will give the double octave2 and so on4 ,these acute sounds are called the harmonics of the fundamental note4 ,it is clear1 from what has been stated1 that the string thus vibrating could not give these harmonics spontaneously unless it divided itself at its aliquot parts into two1 three1 four1 or more segments in opposite states of vibration separated by points actually at rest4 ,in proof of this1 pieces of paper placed on the string at the half1 third1 fourth1 or other aliquot points1 according to the corresponding harmonic sound1 will remain on it during its vibration1 but will instantly fly off from any of the intermediate points4 ,the points of rest1 called the nodal points of the string1 are a mere consequence of the law of interferences2 for1 if a rope fastened at one end be moved to and fro at the other extremity so as to transmit a succession of equal waves along it1 they will be successively reflected when they arrive at the other end of the rope by the fixed point1 and in returning they will occasionally interfere with the advancing waves2 and1 as these opposite undulations will at certain points destroy one another1 the point of the rope in which this happens will remain at rest4 ,thus a series of nodes and ventral segments will be produced whose number will depend upon the tension and the frequency of the alternate motions communicated to the moveable end4 ,so1 when a string fixed at both ends is put in motion by a sudden blow at any point of it1 the primitive impulse divides itself into two pulses running opposite ways1 which are each totally reflected at the extremities1 and1 running back again along the whole length1 are again reflected at the other ends4 ,and thus they will continue to run backwards and forwards1 crossing one another at each traverse1 and occasionally interfering1 so as to produce nodes2 so that the motion of a string fastened at both ends consists of a wave or pulse continually doubled back on itself by reflection at the fixed extremities4 ,harmonics generally co-exist with the fundamental sound in the same vibrating body4 ,if one of the lowest strings of the pianoforte be struck1 an attentive ear will not only hear the fundamental note1 but will detect all the others sounding along with it1 though with less and less intensity as their pitch becomes higher4 ,according to the law of co-existing undulations1 the whole string and each of its aliquot parts are in different and independent states of vibration at the same time2 and as all the resulting notes are heard simultaneously1 not only the air1 but the ear also1 vibrates in unison with each at the same instant 7,n4 #aha74 ,harmony consists in an agreeable combination of sounds4 ,when two chords perform their vibrations in the same time1 they are in unison2 but1 when their vibrations are so related as to have a common period1 after a few oscillations they produce concord4 ,thus1 when the vibrations of two strings bear a very simple relation to each other1 as where one of them makes two1 three1 four1 c41 vibrations in the time the other makes one2 or1 if it accomplishes three1 four1 c41 vibrations while the other makes two1 the result is a concord which is the more perfect the shorter the common period4 ,in discords1 on the contrary1 the beats are distinctly audible1 which produces a disagreeable and harsh effect1 because the vibrations do not bear a simple relation to one another1 as where one of two strings makes eight vibrations while the other accomplishes fifteen4 ,the pleasure afforded by harmony is attributed by ,dr4 ,young to the love of order1 and to a predilection for a regular repetition of sensations natural to the human mind1 which is gratified by the perfect regularity and rapid recurrence of the vibrations4 ,the love of poetry and dancing he conceives to arise in some degree from the rhythm of the one and the regularity of the motions in the other4 ,a blast of air passing over the open end of a tube1 as over the reeds in ,pan's pipes2 over a hole in one side1 as in the flute2 or through the aperture called a reed1 with a flexible tongue1 as in the clarinet1 puts the internal column of air into longitudinal vibrations by the alternate condensations and rarefactions of its particles4 ,at the same time the column spontaneously divides itself into nodes1 between which the air also vibrates longitudinally1 but with a rapidity inversely proportional to the length of the divisions1 giving the fundamental note or one of its harmonics4 ,the nodes are produced on the principle of interferences by the reflection of the longitudinal undulations of the air at the ends of the pipe1 as in the musical string1 only that in one case the undulations are longitudinal1 and in the other transverse4 ,a pipe1 either open or shut at both ends1 when sounded1 vibrates entire1 or divides itself spontaneously into two1 three1 four1 c41 segments separated by nodes4 ,the whole column gives the fundamental note by waves or vibrations of the same length with the pipe4 ,the first harmonic is produced by waves half as long as the tube1 the second harmonic by waves a third as long1 and so on4 ,the harmonic segments in an open and shut pipe are the same in number1 but differently placed4 ,in a shut pipe the two ends are nodes1 but in an open pipe there is half a segment at each extremity1 because the air at these points is neither rarefied nor condensed1 being in contact with that which is external4 ,if one of the ends of the open pipe be closed1 its fundamental note will be an octave lower3 the air will now divide itself into three1 five1 seven1 c41 segments2 and the wave producing its fundamental note will be twice as long as the pipe1 so that it will be doubled back 7,n4 #ahb74 ,all these notes may be produced separately by varying the intensity of the blast4 ,blowing steadily and gently1 the fundamental note will sound2 when the force of the blast is increased the note will all at once start up an octave2 when the intensity of the wind is augmented the twelfth will be heard2 and1 by continuing to increase the force of the blast1 the other harmonics may be obtained1 but no force of wind will produce a note intermediate between these4 ,the harmonics of a flute may be obtained in this manner1 from the lowest ,c or ,d upwards1 without altering the fingering1 merely by increasing the intensity of the blast and altering the form of the lips4 ,pipes of the same dimensions1 whether of lead1 glass1 or wood1 give the same tone as to pitch under the same circumstances1 which shows that the air alone produces the sound4 ,metal springs fastened at one end1 when forcibly bent1 endeavour to return to rest by a series of vibrations1 which give very pleasing tones1 as in musical boxes4 ,various musical instruments have been constructed1 consisting of metallic springs thrown into vibration by a current of air4 ,among the most perfect of these are ,mr4 ,wheatstone's ,symphonion1 ,concertina1 and olian ,organ1 instruments of different effects and capabilities1 but all possessing considerable execution and expression4 ,the ,syren is an ingenious instrument1 devised by ,m4 ,cagniard de la ,tour1 for ascertaining the number of pulsations in a second1 corresponding to each pitch3 the notes are produced by jets of air passing through small apertures1 arranged at regular distances in a circle on the side of a box1 before which a disc revolves pierced with the same number of holes4 ,during a revolution of the disc the currents are alternately intercepted and allowed to pass as many times as there are apertures in it1 and a sound is produced whose pitch depends on the velocity of rotation4 ,a glass or metallic rod1 when struck at one end1 or rubbed in the direction of its length with a wet finger1 vibrates longitudinally1 like a column of air1 by the alternate condensation and expansion of its constituent particles1 producing a clear and beautiful musical note of a high pitch1 on account of the rapidity with which these substances transmit sound4 ,rods1 surfaces1 and1 in general1 all undulating bodies1 resolve themselves into nodes4 ,but in surfaces the parts which remain at rest during their vibrations are lines which are curved or plane according to the substance1 its form1 and the mode of vibration4 ,if a little fine dry sand be strewed over the surface of a plate of glass or metal1 and if undulations be excited by drawing the bow of a violin across its edge1 it will emit a musical sound1 and the sand will immediately arrange itself in the nodal lines1 where alone it will accumulate and remain at rest1 because the segments of the surface on each side will be in different states of vibration1 the one being elevated while the other is depressed2 and1 as these two motions meet in the nodal lines1 they neutralise one another4 ,these lines vary in form and position with the part where the bow is drawn across1 and the point by which the plate is held4 ,the motion of the sand shows in what direction the vibrations take place4 ,if they be perpendicular to the surface1 the sand will be violently tossed up and down till it finds the points of rest4 ,if they be tangential1 the sand will only creep along the surface to the nodal lines4 ,sometimes the undulations are oblique1 or compounded of both the preceding4 ,if a bow be drawn across one of the angles of a square plate of glass or metal held firmly by the centre1 the sand will arrange itself in two straight lines parallel to the sides of the plate1 and crossing in the centre so as to divide it into four equal squares1 whose motions will be contrary to each other4 ,two of the diagonal squares will make their excursions on one side of the plate1 while the other two make their vibrations on the other side of it4 ,this mode of vibration produces the lowest tone of the plate 7,n4 #ahc74 ,if the plate be still held by the centre1 and the bow applied to the middle of one of the sides1 the vibrations will be more rapid1 and the tone will be a fifth higher than in the preceding case3 now the sand will arrange itself from corner to corner1 and will divide the plate into four equal triangles1 each pair of which will make their excursions on opposite sides of the plate4 ,the nodal lines and pitch vary not only with the point where the bow is applied1 but with the point by which the plate is held1 which being at rest necessarily determines the direction of one of the quiescent lines4 ,the forms assumed by the sand in square plates are very numerous1 corresponding to all the various modes of vibration4 ,the lines in circular plates are even more remarkable for their symmetry1 and upon them the forms assumed by the sand may be classed in three systems4 ,the first is the diametrical system1 in which the figures consist of diameters dividing the circumference of the plate into equal parts1 each of which is in a different state of vibration from those adjacent4 ,two diameters1 for example1 crossing at right angles1 divide the circumference into four equal parts2 three diameters divide it into six equal parts2 four divide it into eight1 and so on4 ,in a metallic plate1 these divisions may amount to thirty-six or forty4 ,the next is the concentric system1 where the sand arranges itself in circles1 having the same centre with the plate2 and the third is the compound system1 where the figures assumed by the sand are compounded of the other two1 producing very complicated and beautiful forms4 ,galileo seems to have been the first to notice the points of rest and motion in the sounding-board of a musical instrument2 but to ,chladni is due the whole discovery of the symmetrical forms of the nodal lines in vibrating plates 7,n4 #ahd74 ,professor ,wheatstone has shown1 in a paper read before the ,royal ,society in #ahcc1 that all ,chladni's figures1 and indeed all the nodal figures of vibrating surfaces1 result from very simple modes of vibration oscillating isochronously1 and superposed upon each other2 the resulting figure varying with the component modes of vibration1 the number of the superpositions1 and the angles at which they are superposed4 ,for example1 if a square plate be vibrating so as to make the sand arrange itself in straight lines parallel to one side of the plate1 and if1 in addition to this1 such vibrations be excited as would have caused the sand to form in lines perpendicular to the first had the plate been at rest1 the combined vibrations will make the sand form in lines from corner to corner 7,n4 #ahe74 ,m4 ,savart's experiments on the vibrations of flat glass rulers are highly interesting4 ,let a lamina of glass #bg;in#ef long1 #jei of an inch broad1 and #jjf of an inch in thickness1 be held by the edges in the middle1 with its flat surface horizontal4 ,if this surface be strewed with sand1 and set in longitudinal vibration by rubbing its under surface with a wet cloth1 the sand on the upper surface will arrange itself in lines parallel to the ends of the lamina1 always in one or other of two systems 7,n4 #ahf74 ,although the same one of the two systems will always be produced by the same plate of glass1 yet among different plates of the preceding dimensions1 even though cut from the same sheet side by side1 one will invariably exhibit one system1 and the other the other1 without any visible reason for the difference4 ,now1 if the positions of these quiescent lines be marked on the upper surface1 and if the plate be turned so that the lower surface becomes the upper one1 the sand being strewed1 and vibrations excited as before1 the nodal lines will still be parallel to the ends of the lamina1 but their positions will be intermediate between those of the upper surface 7,n4 #ahg74 ,thus it appears that all the motions of one half of the thickness of the lamina1 or ruler1 are exactly contrary to those of the corresponding points of the other half4 ,if the thickness of the lamina be increased1 the other dimensions remaining the same1 the sound will not vary1 but the number of nodal lines will be less4 ,when the breadth of the lamina exceeds the #jf of an inch1 the nodal lines become curved1 and are different on the two surfaces4 ,a great variety of forms are produced by increasing the breadth and changing the form of the surface2 but in all it appears that the motions in one half of the thickness are opposed to those in the other half4 ,m4 ,savart also found1 by placing small paper rings round a cylindrical tube or rod1 so as to rest upon it at one point only1 that1 when the tube or rod is continually turned on its axis in the same direction1 the rings slide along during the vibrations1 till they come to a quiescent point1 where they rest4 ,by tracing these nodal lines he discovered that they twist in a spiral or corkscrew round rods and cylinders1 making one or more turns according to the length2 but at certain points1 varying in number according to the mode of vibration of the rod1 the screw stops1 and recommences on the other side1 though it is turned in a contrary direction2 that is1 on one side it is a right-handed screw1 on the other a left 7,n4 #ahh74 ,the nodal lines in the interior surface of the tube are perfectly similar to those in the exterior1 but they occupy intermediate positions4 ,if a small ivory ball be put within the tube1 it will follow these nodal lines when the tube is made to revolve on its axis4 ,all solids which ring when struck1 such as bells1 drinking glasses1 gongs1 c41 have their shape momentarily and forcibly changed by the blow1 and from their elasticity1 or tendency to resume their natural form1 a series of undulations take place1 owing to the alternate condensations and rarefactions of the particles of solid matter4 ,these have also their harmonic tones1 and consequently nodes4 ,indeed1 generally1 when a rigid system of any form whatever vibrates either transversely or longitudinally1 it divides itself into a certain number of parts which perform their vibrations without disturbing one another4 ,these parts are at every instant in alternate states of undulation2 and1 as the points or lines where they join partake of both1 they remain at rest1 because the opposing motions destroy one another4 ,the air1 notwithstanding its rarity1 is capable of transmitting its undulations when in contact with a body susceptible of admitting and exciting them4 ,it is thus that sympathetic undulations are excited by a body vibrating near insulated tended strings1 capable of following its undulations1 either by vibrating entire1 or by separating themselves into their harmonic divisions4 ,if two chords equally stretched1 of which one is twice or three times longer than the other1 be placed side by side1 and if the shorter be sounded1 its vibrations will be communicated by the air to the other1 which will be thrown into such a state of vibration that it will be spontaneously divided into segments equal in length to the shorter string4 ,when a tuning-fork receives a blow and is made to rest upon a piano-forte during its vibration1 every string which1 either by its natural length or by its spontaneous subdivisions1 is capable of executing corresponding vibrations1 responds in a sympathetic note4 ,the same effect will be produced by the stroke of a bell near a piano or harp4 ,some one or other of the notes of an organ are generally in unison with one of the panes or with the whole sash of a window1 which consequently resounds when those notes are sounded4 ,a peal of thunder has frequently the same effect4 ,the sound of very large organ-pipes is generally inaudible till the air be set in motion by the undulations of some of the superior accords1 and then the sound becomes extremely energetic4 ,recurring vibrations occasionally influence each other's periods4 ,for example1 two adjacent organ-pipes nearly in unison may force themselves into concord2 and two clocks1 whose rates differed considerably when separate1 have been known to beat together when fixed to the same wall1 and one clock has forced the pendulum of another into motion1 when merely standing on the same stone pavement4 ,these forced oscillations1 which correspond in their periods with those of the exciting cause1 are to be traced in every department of physical science4 ,several instances of them have already occurred in this work4 ,such are the tides1 which follow the sun and moon in all their motions and periods4 ,the nutation of the earth's axis also1 which corresponds with the period1 and represents the motion of the nodes of the moon1 is again reflected back to the moon1 and may be traced in the nutation of the lunar orbit4 ,and1 lastly1 the acceleration of the moon's mean motion represents the action of the planets on the earth reflected by the sun to the moon4 ,in consequence of the facility with which the air communicates undulations1 all the phenomena of vibrating plates may be exhibited by sand strewed on paper or parchment1 stretched over a harmonica glass or large bell-shaped tumbler4 ,in order to give due tension to the paper or vellum1 it must be wetted1 stretched over the glass1 gummed round the edges1 allowed to dry1 and varnished over1 to prevent changes in its tension from the humidity of the atmosphere4 ,if a circular disc of glass be held concentrically over this apparatus1 with its plane parallel to the surface of the paper1 and set in vibration by drawing a bow across its edge1 so as to make sand on its surface take any of ,chladni's figures1 the sand on the paper will assume the very same form1 in consequence of the vibrations of the disc being communicated to the paper by the air4 ,when the disc is removed slowly in a horizontal direction1 the forms on the paper will correspond with those on the disc1 till the distance is too great for the air to convey the vibrations4 ,if the disc while vibrating be gradually more and more inclined to the horizon1 the figures on the paper will vary by degrees2 and1 when the vibrating disc is perpendicular to the horizon1 the sand on the paper will form into straight lines parallel to the surface of the disc1 by creeping along it instead of dancing up and down4 ,if the disc be made to turn round its vertical diameter while vibrating1 the nodal lines on the paper will revolve1 and exactly follow the motion of the disc4 ,it appears1 from this experiment1 that the motions of the arial molecules in every part of a spherical wave1 propagated from a vibrating body as a centre1 are parallel to each other1 and not divergent like the radii of a circle4 ,when a slow air is played on a flute near this apparatus1 each note calls up a particular form in the sand1 which the next note effaces1 to establish its own4 ,the motion of the sand will even detect sounds that are inaudible4 ,by the vibrations of sand on a drum-head the besieged have discovered the direction in which a counter-mine was working4 ,m4 ,savart1 who made these beautiful experiments1 employed this apparatus to discover nodal lines in masses of air4 ,he found that the air of a room1 when thrown into undulations by the continued sound of an organ-pipe1 or by any other means1 divides itself into masses separated by nodal curves of double curvature1 such as spirals1 on each side of which the air is in opposite states of vibration4 ,he even traced these quiescent lines going out at an open window1 and for a considerable distance in the open air4 ,the sand is violently agitated where the undulations of the air are greatest1 and remains at rest in the nodal lines4 ,m4 ,savart observed1 that when he moved his head away from a quiescent line towards the right the sound appeared to come from the right1 and when he moved it towards the left the sound seemed to come from the left1 because the molecules of air are in different states of motion on each side of the quiescent line4 ,a musical string gives a very feeble sound when vibrating alone1 on account of the small quantity of air set in motion2 but when attached to a sounding-board1 as in the harp and piano-forte1 it communicates its undulations to that surface1 and from thence to every part of the instrument2 so that the whole system vibrates isochronously1 and by exposing an extensive undulating surface1 which transmits its undulations to a great mass of air1 the sound is much reinforced4 ,the intensity is greatest when the vibrations of the string or sounding body are perpendicular to the sounding-board1 and least when they are in the same plane with it4 ,the sounding-board of the piano-forte is better disposed than that of any other stringed instrument1 because the hammers strike the strings so as to make them vibrate at right angles to it4 ,in the guitar1 on the contrary1 they are struck obliquely1 which renders the tone feeble1 unless when the sides1 which also act as a sounding-board1 are deep4 ,it is evident that the sounding-board and the whole instrument are agitated at once by all the superposed vibrations excited by the simultaneous or consecutive notes that are sounded1 each having its perfect effect independently of the rest4 ,a sounding-board not only reciprocates the different degrees of pitch1 but all the nameless qualities of tone4 ,this has been beautifully illustrated by ,professor ,wheatstone in a series of experiments on the transmission through solid conductors of musical performances1 from the harp1 piano1 violin1 clarinet1 c4 ,he found that all the varieties of pitch1 quality1 and intensity are perfectly transmitted with their relative gradations1 and may be communicated1 through conducting wires or rods of very considerable length1 to a properly disposed sounding-board in a distant apartment4 ,the sounds of an entire orchestra may be transmitted and reciprocated by connecting one end of a metallic rod with a sounding-board near the orchestra1 so placed as to resound to all the instruments1 and the other end with the sounding-board of a harp1 piano1 or guitar1 in a remote apartment4 ,professor ,wheatstone observes1 8,the effect of this experiment is very pleasing2 the sounds1 indeed1 have so little intensity as scarcely to be heard at a distance from the reciprocating instrument2 but1 on placing the ear close to it1 a diminutive band is heard in which all the instruments preserve their distinctive qualities1 and the pianos and fortes1 the crescendos and diminuendos1 their relative contrasts4 ,compared with an ordinary band heard at a distance through the air1 the effect is as a landscape seen in miniature beauty through a concave lens1 compared with the same scene viewed by ordinary vision through a murky atmosphere40 ,every one is aware of the reinforcement of sound by the resonance of cavities4 ,when singing or speaking near the aperture of a wide-mouthed vessel1 the intensity of some one note in unison with the air in the cavity is often augmented to a great degree4 ,any vessel will resound if a body vibrating the natural note of the cavity be placed opposite to its orifice1 and be large enough to cover it1 or at least to set a large portion of the adjacent air in motion4 ,for the sound will be alternately reflected by the bottom of the cavity and the undulating body at its mouth4 ,the first impulse of the undulating substance will be reflected by the bottom of the cavity1 and then by the undulating body1 in time to combine with the second new impulse4 ,this reinforced sound will also be twice reflected in time to conspire with the third new impulse2 and1 as the same process will be repeated on every new impulse1 each will combine with all its echoes to reinforce the sound prodigiously4 ,professor ,wheatstone1 to whose ingenuity we are indebted for so much new and valuable information on the theory of sound1 has given some very striking instances of resonance4 ,if one of the branches of a vibrating tuning-fork be brought near the embouchure of a flute1 the lateral apertures of which are stopped so as to render it capable of producing the same sound as the fork1 the feeble and scarcely audible sound of the fork will be augmented by the rich resonance of the column of air within the flute1 and the tone will be full and clear4 ,the sound will be found greatly to decrease by closing or opening another aperture2 for the alteration in the length of the column of air renders it no longer fit perfectly to reciprocate the sound of the fork4 ,this experiment may be made on a concert flute with a ,c tuning-fork4 ,but ,professor ,wheatstone observes1 that in this case it is generally necessary to finger the flute for ,b1 because1 when blown into with the mouth1 the under-lip partly covers the embouchure1 which renders the sound about a semitone flatter than it would be were the embouchure entirely uncovered4 ,he has also shown1 by the following experiment1 that any one among several simultaneous sounds may be rendered separately audible4 ,if two bottles be selected1 and tuned by filling them with such a quantity of water as will render them unisonant with two tuning-forks which differ in pitch1 on bringing both of the vibrating tuning-forks to the mouth of each bottle alternately1 in each case that sound only will be heard which is reciprocated by the unisonant bottle4 ,several attempts have been made to imitate the articulation of the letters of the alphabet4 ,about the year #aggi1 ,,mm4 ,kratzenstein of ,st4 ,petersburg1 and ,kempelen of ,vienna1 constructed instruments which articulated many letters1 words1 and even sentences4 ,mr4 ,willis of ,cambridge has adapted cylindrical tubes to a reed1 whose length can be varied at pleasure by sliding joints4 ,upon drawing out a tube while a column of air from the bellows of an organ is passing through it1 the vowels are pronounced in the order1 i1 e1 a1 o1 u4 ,on extending the tube1 they are repeated after a certain interval1 in the inverted order1 u1 o1 a1 e1 i4 ,after another interval they are again obtained in the direct order1 and so on4 ,when the pitch of the reed is very high1 it is impossible to sound some of the vowels1 which is in perfect correspondence with the human voice1 female singers being unable to pronounce u and o in their high notes4 ,from the singular discoveries of ,m4 ,savart on the nature of the human voice1 and the investigations of ,mr4 ,willis on the mechanism of the larynx1 it may be presumed that ultimately the utterance or pronunciation of modern languages will be conveyed1 not only to the eye1 but also to the ear of posterity4 ,had the ancients possessed the means of transmitting such definite sounds1 the civilised world would still have responded in sympathetic notes at the distance of many ages4 ,,section ,,xviii4 ,refraction--,astronomical ,refraction and its ,laws--,formation of ,tables of ,refraction--,terrestrial ,refraction--,its ,quantity--,instances of extraordinary ,refraction--,reflection--,instances of extraordinary ,reflection--,loss of ,light by the ,absorbing ,power of the ,atmosphere--,apparent ,magnitude of ,sun and ,moon in the ,horizon4 ,,not only everything we hear but all we see is through the medium of the atmosphere4 ,without some knowledge of its action upon light1 it would be impossible to ascertain the position of the heavenly bodies1 or even to determine the exact place of very distant objects upon the surface of the earth2 for1 in consequence of the refractive power of the air1 no distant object is seen in its true position4 ,all the celestial bodies appear to be more elevated than they really are2 because the rays of light1 instead of moving through the atmosphere in straight lines1 are continually inflected towards the earth4 ,light passing obliquely out of a rare into a denser medium1 as from vacuum into air1 or from air into water1 is bent or refracted from its course towards a perpendicular to that point of the denser surface where the light enters it 7,n4 #ahi74 ,in the same medium1 the sine of the angle contained between the incident ray and the perpendicular is in a constant ratio to the sine of the angle contained by the refracted ray and the same perpendicular2 but this ratio varies with the refracting medium4 ,the denser the medium1 the more the ray is bent4 ,the barometer shows that the density of the atmosphere decreases as the height above the earth increases4 ,direct experiments prove that the refractive power of the air increases with its density4 ,it follows therefore that1 if the temperature be uniform1 the refractive power of the air is greatest at the earth's surface1 and diminishes upwards4 ,a ray of light from a celestial object falling obliquely on this variable atmosphere1 instead of being refracted at once from its course1 is gradually more and more bent during its passage through it so as to move in a vertical curved line1 in the same manner as if the atmosphere consisted of an infinite number of strata of different densities4 ,the object is seen in the direction of a tangent to that part of the curve which meets the eye2 consequently the apparent altitude 7,n4 #aij7 of the heavenly bodies is always greater than their true altitude4 ,owing to this circumstance1 the stars are seen above the horizon after they are set1 and the day is lengthened from a part of the sun being visible1 though he really is behind the rotundity of the earth4 ,it would be easy to determine the direction of a ray of light through the atmosphere if the law of the density were known2 but1 as this law is perpetually varying with the temperature1 the case is very complicated4 ,when rays pass perpendicularly from one medium into another1 they are not bent2 and experience shows1 that in the same surface1 though the sines of the angles of incidence and refraction retain the same ratio1 the refraction increases with the obliquity of incidence 7,n4 #ahi74 ,hence it appears that the refraction is greatest at the horizon1 and at the zenith there is none4 ,but it is proved that1 at all heights above ten degrees1 refraction varies nearly as the tangent of the angular distance of the object from the zenith1 and wholly depends upon the heights of the barometer and thermometer4 ,for the quantity of refraction at the same distance from the zenith varies nearly as the height of the barometer1 the temperature being constant2 and the effect of the variation of temperature is to diminish the quantity of refraction by about its #dhjth part for every degree in the rise of ,fahrenheit's thermometer4 ,not much reliance can be placed on celestial observations1 within less than ten or twelve degrees of the horizon1 on account of irregular variations in the density of the air near the surface of the earth1 which are sometimes the cause of very singular phenomena4 ,the humidity of the air produces no sensible effect on its refractive power2 and it has been proved that the amount of refraction is the same whatever be the velocity of the incident light1 that is whether the light comes from a star in that part of the heavens towards which the earth is going1 or from one in that part of the sky whence it is receding4 ,bodies1 whether luminous or not1 are only visible by the rays which proceed from them4 ,as the rays must pass through strata of different densities in coming to us1 it follows that1 with the exception of stars in the zenith1 no object either in or beyond our atmosphere is seen in its true place4 ,but the deviation is so small in ordinary cases that it causes no inconvenience1 though in astronomical and trigonometrical observations due allowance must be made for the effects of refraction4 ,dr4 ,bradley's tables of refraction were formed by observing the zenith distances of the sun at his greatest declinations1 and the zenith distances of the pole-star above and below the pole4 ,the sum of these four quantities is equal to #ahj1 diminished by the sum of the four refractions1 whence the sum of the four refractions was obtained2 and1 from the law of the variation of refraction determined by theory1 he assigned the quantity due to each altitude 7,n4 #aia74 ,the mean horizontal refraction is about #ce #f1 and at the height of forty-five degrees it is #ehcf4 ,the effect of refraction upon the same star above and below the pole was noticed by ,alhazen1 a ,saracen astronomer of ,spain1 in the ninth century2 but its existence was known to ,ptolemy in the second1 though he was ignorant of its quantity4 ,the refraction of a terrestrial object is estimated differently from that of a celestial body4 ,it is measured by the angle contained between the tangent to the curvilineal path of the ray where it meets the eye1 and the straight line joining the eye and the object 7,n4 #aib74 ,near the earth's surface the path of the ray may be supposed to be circular2 and the angle at the centre of the earth corresponding to this path is called the horizontal angle4 ,the quantity of terrestrial refraction is obtained by measuring contemporaneously the elevation of the top of a mountain above a point in the plain at its base1 and the depression of that point below the top of the mountain4 ,the distance between these two stations is the chord of the horizontal angle2 and it is easy to prove that double the refraction is equal to the horizontal angle1 increased by the difference between the apparent elevation and the apparent depression4 ,whence it appears that1 in the mean state of the atmosphere1 the refraction is about the fourteenth part of the horizontal angle4 ,some very singular appearances occur from the accidental expansion or condensation of the strata of the atmosphere contiguous to the surface of the earth1 by which distant objects1 instead of being elevated1 are depressed4 ,sometimes1 being at once both elevated and depressed1 they appear double1 one of the images being direct1 and the other inverted4 ,in consequence of the upper edges of the sun and moon being less refracted than the lower1 they often appear to be oval when near the horizon4 ,the looming also or elevation of coasts1 mountains1 and ships1 when viewed across the sea1 arises from unusual refraction4 ,a friend of the author's1 while standing on the plains of ,hindostan1 saw the whole upper chain of the ,himalaya ,mountains start into view1 from a sudden change in the density of the air1 occasioned by a heavy shower after a very long course of dry and hot weather4 ,single and double images of objects at sea1 arising from sudden changes of temperature which are not so soon communicated to the water on account of its density as to the air1 occur more rarely and are of shorter duration than similar appearances on land4 ,in #ahah ,captain ,scoresby1 whose observations on the phenomena of the polar seas are so valuable1 recognised his father's ship by its inverted image in the air1 although the vessel itself was below the horizon4 ,he afterwards found that she was seventeen miles beyond the horizon1 and thirty miles distant4 ,two images are sometimes seen suspended in the air over a ship1 one direct and the other inverted1 with their topmasts or their hulls meeting1 according as the inverted image is above or below the direct image 7,n4 #aic74 ,dr4 ,wollaston has proved that these appearances are owing to the refraction of the rays through media of different densities1 by the very simple experiment of looking along a red-hot poker at a distant object4 ,two images are seen1 one direct and another inverted1 in consequence of the change induced by the heat in the density of the adjacent air4 ,he produced the same effect by a saline or saccharine solution with water and spirit of wine floating upon it 7,n4 #aid74 ,many of the phenomena that have been ascribed to extraordinary refraction seem to be occasioned by a partial or total reflection of the rays of light at the surfaces of strata of different densities 7,n4 #ahi74 ,it is well known that1 when light falls obliquely upon the external surface of a transparent medium1 as on a plate of glass or a stratum of air1 one portion is reflected and the other transmitted4 ,but1 when light falls very obliquely upon the internal surface1 the whole is reflected1 and not a ray is transmitted4 ,in all cases the angles made by the incident and reflected rays with a perpendicular to the surface being equal1 as the brightness of the reflected image depends on the quantity of light1 those arising from total reflection must be by far the most vivid4 ,the delusive appearance of water1 so well known to ,african travellers and to the ,arab of the desert as the ,lake of the ,gazelles1 is ascribed to the reflection which takes place between strata of air of different densities1 owing to radiation of heat from the arid sandy plains4 ,the mirage described by ,captain ,mundy in his ,journal of a ,tour in ,india probably arises from this cause4 8,a deep precipitous valley below us1 at the bottom of which ,i had seen one or two miserable villages in the morning1 bore in the evening a complete resemblance to a beautiful lake2 the vapour which played the part of water ascending nearly half way up the sides of the vale1 and on its bright surface trees and rocks being distinctly reflected4 ,i had not been long contemplating this phenomenon1 before a sudden storm came on and dropped a curtain of clouds over the scene40 ,an occurrence which happened on the #ahth of ,november1 #ahjd1 was probably produced by reflection4 ,dr4 ,buchan1 while watching the rising sun from the cliff about a mile to the east of ,brighton1 at the instant the solar disc emerged from the surface of the ocean1 saw the cliff on which he was standing1 a windmill1 his own figure and that of a friend1 depicted immediately opposite to him on the sea4 ,this appearance lasted about ten minutes1 till the sun had risen nearly his own diameter above the surface of the waves4 ,the whole then seemed to be elevated into the air1 and successively vanished4 ,the rays of the sun fell upon the cliff at an incidence of #gc from the perpendicular1 and the sea was covered with a dense fog many yards in height1 which gradually receded before the rising sun4 ,when extraordinary refraction takes place laterally1 the strata of variable density are perpendicular to the horizon1 and1 if combined with vertical refraction1 the objects are magnified as when seen through a telescope4 ,from this cause1 on the #bfth of ,july1 #agih1 the cliffs of ,france1 fifty miles off1 were seen as distinctly from ,hastings as if they had been close at hand2 and even ,dieppe was said to have been visible in the afternoon4 ,the stratum of air in the horizon is so much thicker and more dense than the stratum in the vertical1 that the sun's light is diminished #acjj times in passing through it1 which enables us to look at him when setting without being dazzled4 ,the loss of light1 and consequently of heat1 by the absorbing power of the atmosphere1 increases with the obliquity of incidence4 ,of ten thousand rays falling on its surface1 #habc arrive at a given point of the earth if they fall perpendicularly2 #gjbd arrive if the angle of direction be fifty degrees2 #bhca1 if it be seven degrees2 and only five rays will arrive through a horizontal stratum4 ,since so great a quantity of light is lost in passing through the atmosphere1 many celestial objects are altogether invisible from the plain1 which may be seen from elevated situations4 ,diminished splendour1 and the false estimate we make of distance from the number of intervening objects1 lead us to suppose the sun and moon to be much larger when in the horizon than at any other altitude1 though their apparent diameters are then somewhat less4 ,instead of the sudden transitions of light and darkness1 the reflective power of the air adorns nature with the rosy and golden hues of the ,aurora and twilight4 ,even when the sun is eighteen degrees below the horizon1 a sufficient portion of light remains to show that at the height of thirty miles it is still dense enough to reflect light4 ,the atmosphere scatters the sun's rays1 and gives all the beautiful tints and cheerfulness of day4 ,it transmits the blue light in greatest abundance2 the higher we ascend1 the sky assumes a deeper hue2 but1 in the expanse of space1 the sun and stars must appear like brilliant specks in profound blackness4 ,,section ,,xix4 ,constitution of ,light according to ,sir ,isaac ,newton--,absorption of ,light--,colours of ,bodies--,constitution of ,light according to ,sir ,david ,brewster--,new ,colours--,fraunhofer's ,dark ,lines--,dispersion of ,light--,the ,achromatic ,telescope--,homogeneous ,light--,accidental and ,complementary ,colours--,m4 ,plateau's ,experiments and ,theory of ,accidental ,colours4 ,,it is impossible thus to trace the path of a sunbeam through our atmosphere without feeling a desire to know its nature1 by what power it traverses the immensity of space1 and the various modifications it undergoes at the surfaces and in the interior of terrestrial substances4 ,sir ,isaac ,newton proved the compound nature of white light1 as emitted from the sun1 by passing a sunbeam through a glass prism 7,n4 #aie71 which1 separating the rays by refraction1 formed a spectrum or oblong image of the sun1 consisting of seven colours1 red1 orange1 yellow1 green1 blue1 indigo1 and violet--of which the red is the least refrangible1 and the violet the most4 ,but1 when he reunited these seven rays by means of a lens1 the compound beam became pure white as before4 ,he insulated each coloured ray1 and1 finding that it was no longer capable of decomposition by refraction1 concluded that white light consists of seven kinds of homogeneous light1 and that to the same colour the same refrangibility ever belongs1 and to the same refrangibility the same colour4 ,since the discovery of absorbent media1 however1 it appears that this is not the constitution of the solar spectrum4 ,we know of no substance that is either perfectly opaque or perfectly transparent4 ,even gold may be beaten so thin as to be pervious to light4 ,on the contrary1 the clearest crystal1 the purest air or water1 stops or absorbs its rays when transmitted1 and gradually extinguishes them as they penetrate to greater depths4 ,on this account objects cannot be seen at the bottom of very deep water1 and many more stars are visible to the naked eye from the tops of mountains than from the valleys4 ,the quantity of light that is incident on any transparent substance is always greater than the sum of the reflected and refracted rays4 ,a small quantity is irregularly reflected in all directions by the imperfections of the polish by which we are enabled to see the surface2 but a much greater portion is absorbed by the body4 ,bodies that reflect all the rays appear white1 those that absorb them all seem black2 but most substances1 after decomposing the white light which falls upon them1 reflect some colours and absorb the rest4 ,a violet reflects the violet rays alone and absorbs the others4 ,scarlet cloth absorbs almost all the colours except red4 ,yellow cloth reflects the yellow rays most abundantly1 and blue cloth those that are blue4 ,consequently colour is not a property of matter1 but arises from the action of matter upon light4 ,in fact1 the law of action and reaction obtains in light as in every other department of nature1 so that light cannot be reflected1 refracted1 much less absorbed1 by any medium without being reacted upon by it4 ,thus a white riband reflects all the rays1 but1 when dyed red1 the particles of the silk acquire the property of reflecting the red rays most abundantly and of absorbing the others4 ,upon this property of unequal absorption the colours of transparent media depend2 for they also receive their colour from their power of stopping or absorbing some of the colours of white light1 and transmitting others4 ,as1 for example1 black and red inks1 though equally homogeneous1 absorb different kinds of rays2 and1 when exposed to the sun1 they become heated in different degrees2 while pure water seems to transmit all rays equally1 and is not sensibly heated by the passing light of the sun4 ,the rich dark light transmitted by a smalt-blue finger-glass is not a homogeneous colour like the blue or indigo of the spectrum1 but is a mixture of all the colours of white light which the glass has not absorbed4 ,the colours absorbed are such as mixed with the blue tint would form white light4 ,when the spectrum of seven colours is viewed through a thin plate of this glass1 they are all visible2 and1 when the plate is very thick1 every colour is absorbed between the extreme red and the extreme violet1 the interval being perfectly black2 but1 if the spectrum be viewed through a certain thickness of the glass intermediate between the two1 it will be found that the middle of the red space1 the whole of the orange1 a great part of the green1 a considerable part of the blue1 a little of the indigo1 and a very little of the violet1 vanish1 being absorbed by the blue glass2 and that the yellow rays occupy a larger space1 covering part of that formerly occupied by the orange on one side and by the green on the other3 so that the blue glass absorbs the red light1 which when mixed with the yellow constitutes orange2 and also absorbs the blue light1 which when mixed with the yellow forms the part of the green space next to the yellow4 ,hence1 by absorption1 green light is decomposed into yellow and blue1 and orange light into yellow and red3 consequently the orange and green rays1 though incapable of decomposition by refraction1 can be resolved by absorption1 and actually consist of two different colours possessing the same degree of refrangibility4 ,difference of colour1 therefore1 is not a test of difference of refrangibility1 and the conclusion deduced by ,newton is no longer admissible as a general truth4 ,by this analysis of the spectrum1 not only with blue glass but with a variety of coloured media1 ,sir ,david ,brewster1 so justly celebrated for his optical discoveries1 is of opinion that the solar spectrum consists of three primary colours1 red1 yellow1 and blue1 each of which exists throughout its whole extent1 but with different degrees of intensity in different parts2 and that the superposition of these three produces all the seven hues according as each primary colour is in excess or defect4 ,that since a certain portion of red1 yellow1 and blue rays constitute white light1 the colour of any point of the spectrum may be considered as consisting of the predominating colour at that point mixed with white light4 ,consequently1 8by absorbing the excess of any colour at any point of the spectrum above what is necessary to form white light1 such white light will appear at that point as never mortal eye looked upon before this experiment1 since it possesses the remarkable property of remaining the same after any number of refractions1 and of being capable of decomposition by absorption alone40 ,this analysis of light has been called in question by ,professor ,challis1 of ,cambridge1 who does not admit of any resolution by absorbing media different from that by the prism1 though he admits that a mixture of blue and yellow solar light produces green4 ,professor ,stokes1 on the contrary1 does not allow that a mixture of blue and yellow solar light produces green1 although that mixture produces green when the light is from other sources1 for he found the gradation from sunlight to pass from yellow through diluted yellow1 white1 diluted blue to blue2 so he does not admit of ,sir ,david ,brewster's analysis of the spectrum2 however1 there appears to be still a doubt as to the real character of the phenomena presented by certain absorbing substances4 ,in addition to the seven colours of the ,newtonian spectrum1 ,sir ,john ,herschel has discovered a set of very dark red rays beyond the red extremity of the spectrum which can only be seen when the eye is defended from the glare of the other colours by a dark blue cobalt glass4 ,he has also found that beyond the extreme violet there are visible rays of a lavender gray colour1 which may be seen by throwing the spectrum on a sheet of paper moistened by the carbonate of soda4 ,the illuminating power of the different rays of the spectrum varies with the colour4 ,the most intense light is in the mean yellow ray1 or1 according to ,m4 ,fraunhofer1 at the boundary of the orange and yellow4 ,when the prism is very perfect and the sunbeam small1 so that the spectrum may be received on a sheet of white paper in its utmost state of purity1 it presents the appearance of a riband shaded with all the prismatic colours1 having its breadth irregularly striped or subdivided by an indefinite number of dark1 and sometimes black lines4 ,the greater number of these rayless lines are so extremely narrow that it is impossible to see them in ordinary circumstances4 ,the best method is to receive the spectrum on the object-glass of a telescope1 so as to magnify them sufficiently to render them visible4 ,this experiment may also be made1 but in an imperfect manner1 by viewing a narrow slit between two nearly closed window-shutters through a very excellent glass prism held close to the eye1 with its refracting angle parallel to the line of light4 ,the rayless lines in the red portion of the spectrum become most visible as the sun approaches the horizon1 while those in the blue extremity are most obvious in the middle of the day4 ,when the spectrum is formed by the sun's rays1 either direct or indirect--as from the sky1 clouds1 rainbow1 moon1 or planets--the black bands are always found to be in the same parts of the spectrum1 and under all circumstances to maintain the same relative positions4 ,similar dark lines are also seen in the light of the stars1 in the electric light1 and in the flame of combustible substances1 though differently arranged1 each star and each flame having a system of dark lines peculiar to itself4 ,dr4 ,wollaston and ,m4 ,fraunhofer1 of ,munich1 discovered these lines deficient of rays independently of each other4 ,m4 ,fraunhofer found that their number extends to nearly six hundred1 but they are much more numerous4 ,there are bright lines in the solar spectrum which also maintain a fixed position4 ,among the dark lines1 ,m4 ,fraunhofer selected seven of the most remarkable1 and determined their distances so accurately1 that they now form standard and invariable points of reference for measuring the refractive powers of different media on the rays of light1 which renders this department of optics as exact as any of the physical sciences4 ,these lines are designated by the letters of the alphabet1 beginning with ,b1 which is in the red near the end of the spectrum2 ,c is farther advanced in the red2 ,d is in the orange2 ,e in the green2 ,f in the blue2 ,g in the indigo2 and ,h in the violet4 ,by means of these fixed points1 ,m4 ,fraunhofer has ascertained from prismatic observation the refrangibility of seven of the principal rays in each of ten different substances solid and liquid4 ,the refraction increased in all from the red to the violet end of the spectrum4 ,the rays that are wanting in the solar spectrum1 which occasion the dark lines1 were supposed to be absorbed by the atmosphere of the sun4 ,but the annular eclipse which happened on the #aeth of ,may1 #ahcf1 afforded ,professor ,forbes the means of proving that the dark lines in question cannot be attributed to the absorption of the solar atmosphere2 they were neither broader nor more numerous in the spectrum formed during that phenomenon than at any other time1 though the rays came only from the circumference of the sun's disc1 and consequently had to traverse a greater depth of his atmosphere4 ,sir ,david ,brewster found that in certain states of the atmosphere the obscure lines become much broader1 and some of them deeply black2 and he observed also1 that1 at the time the sun was setting in a veil of red light1 part of the luminous spectrum was absorbed1 whence he concluded that the earth's atmosphere had absorbed the rays of light which occupied the dark bands4 ,by direct experiments also the atmosphere was observed to act powerfully upon the rayless lines4 ,when a lens is used along with a prism1 longitudinal dark lines of different breadths are seen to cross the bands1 already described1 at right angles2 these ,m4 ,ragona-,scina and ,m4 ,babinet believe to be lines of interference which exist in light that has passed through a convex lens4 ,the lines are different both in kind and number in the spectra of gases and flames4 ,in a highly-magnified spectrum from light passed through nitrous acid gas1 ,sir ,david ,brewster counted #bjjj dark bands4 ,in the spectrum of a lamp1 and generally of all white flames1 none of the defective lines are found2 so all such flames contain rays which do not exist in the light of the sun or stars4 ,brilliant red lines appear in the spectrum produced by the combustion of nitre upon charcoal2 and in all artificial flames dark and bright bands exist1 sometimes corresponding in position with those in the solar spectrum1 and sometimes not4 ,a sunbeam received on a screen1 after passing through a small round hole in a window-shutter1 appears like a round white spot2 but when a prism is interposed1 the beam no longer occupies the same space4 ,it is separated into the prismatic colours1 and spread over a line of considerable length1 while its breadth remains the same with that of the white spot4 ,the act of spreading or separation is called the dispersion of the coloured rays4 ,dispersion always takes place in the plane of refraction1 and is greater as the angle of incidence is greater4 ,it varies inversely as the length of a wave of light1 and directly as its velocity3 hence towards the blue end of the spectrum1 where the undulations of the rays are least1 the dispersion is greatest4 ,substances have very different dispersive powers2 that is to say1 the spectra formed by two equal prisms of different substances1 under precisely the same circumstances1 are of different lengths4 ,thus1 if a prism of flint-glass and one of crown-glass of equal refracting angles be presented to two rays of white light at equal angles1 it will be found that the space over which the coloured rays are dispersed by the flint-glass is much greater than the space occupied by that produced by the crown-glass3 and as the quantity of dispersion depends upon the refracting angle of the prism1 the angles of the two prisms may be made such that1 when the prisms are placed close together with their edges turned opposite ways1 they will exactly oppose each other's action1 and will refract the coloured rays equally1 but in contrary directions1 so that an exact compensation will be effected1 and the light will be refracted without colour 7,n4 #aie74 ,the achromatic telescope is constructed on this principle4 ,it consists of a tube with an object-glass or lens at one end to bring the rays to a focus1 and form an image of the distant object1 and a magnifying-glass at the other end to view the image thus formed4 ,now it is found that the object-glass1 instead of making the rays converge to one point1 disperses them1 and gives a confused and coloured image3 but by constructing it of two lenses in contact1 one of flint and the other of crown-glass of certain forms and proportions1 the dispersion is counteracted1 and a perfectly well-defined and colourless image of the object is formed 7,n4 #aif74 ,it was thought to be impossible to produce refraction without colour1 till ,mr4 ,hall1 a gentleman of ,worcestershire1 constructed a telescope on this principle in the year #agcc2 and twenty-five years afterwards the achromatic telescope was brought to perfection by ,mr4 ,dollond1 a celebrated optician in ,london4 ,by means of ,mr4 ,fraunhofer's determination of the refraction of the principal rays in substances1 their dispersive powers may be found 7,n4 #aig74 ,a perfectly homogeneous colour is very rarely to be found2 but the tints of all substances are most brilliant when viewed in light of their own colour4 ,the red of a wafer is much more vivid in red than in white light2 whereas1 if placed in homogeneous yellow light1 it can no longer appear red1 because there is not a ray of red in the yellow light4 ,were it not that the wafer1 like all other bodies1 whether coloured or not1 reflects white light at its outer surface1 it would appear absolutely black when placed in yellow light4 ,after looking steadily for a short time at a coloured object1 such as a red wafer1 on turning the eyes to a white substance1 a green image of the wafer appears1 which is called the accidental colour of red4 ,all tints have their accidental colours3 thus the accidental colour of orange is blue2 that of yellow is indigo2 of green1 reddish white2 of blue1 orange-red2 of violet1 yellow2 and of white1 black2 and vice vers4 ,when the direct and accidental colours are of the same intensity1 the accidental is then called the complementary colour1 because any two colours are said to be complementary to one another which produce white when combined4 ,from experiments by ,m4 ,plateau of ,brussels1 it appears that two complementary colours from direct impression1 which would produce white when combined1 produce black1 or extinguish one another1 by their union1 when accidental2 and also that the combination of all the tints of the solar spectrum produces white light if they be from a direct impression on the eye1 whereas blackness results from a union of the same tints if they be accidental2 and in every case where the real colours produce white by their combination1 the accidental colours of the same tints produce black4 ,when the image of an object is impressed on the retina only for a few moments1 the picture left is exactly of the same colour with the object1 but in an extremely short time the picture is succeeded by the accidental image4 ,m4 ,plateau attributes this phenomenon to a reaction of the retina after being excited by direct vision1 so that the accidental impression is of an opposite nature to the corresponding direct impression4 ,he conceives that when the eye is excited by being fixed for a time on a coloured object1 and then withdrawn from the excitement1 it endeavours to return to its state of repose2 but in so doing1 that it passes this point1 and spontaneously assumes an opposite condition1 like a spring which1 bent in one direction1 in returning to its state of rest bends as much the contrary way4 ,the accidental image thus results from a particular modification of the organ of sight1 in virtue of which it spontaneously gives us a new sensation after it has been excited by direct vision4 ,if the prevailing impression be a very strong white light1 its accidental image is not black1 but a variety of colours in succession4 ,according to ,m4 ,plateau1 the retina offers a resistance to the action of light1 which increases with the duration of this action2 whence1 after looking intently at an object for a long time1 it appears to decrease in brilliancy4 ,the imagination has a powerful influence on our optical impressions1 and has been known to revive the images of highly luminous objects months1 and even years1 afterwards4 ,,section ,,xx4 ,interference of ,light--,undulatory ,theory of ,light--,propagation of ,light--,newton's ,rings--,measurement of the ,length of the ,waves of ,light1 and of the ,frequency of the ,vibrations of ,ether for each ,colour--,newton's ,scale of ,colours--,diffraction of ,light--,sir ,john ,herschel's ,theory of the ,absorption of ,light--,refraction and ,reflection of ,light4 ,,newton and most of his immediate successors imagined light to be a material substance1 emitted by all self-luminous bodies in extremely minute particles1 moving in straight lines with prodigious velocity1 which1 by impinging upon the optic nerves1 produce the sensation of light4 ,many of the observed phenomena have been explained by this theory2 it is1 however1 totally inadequate to account for the following circumstances4 ,when two equal rays of red light1 proceeding from two luminous points1 fall upon a sheet of white paper in a dark room1 they produce a red spot on it which will be twice as bright as either ray would produce singly1 provided the difference in the lengths of the two beams1 from the luminous points to the red spot on the paper1 be exactly the #jjjjjbehth part of an inch4 ,the same effect will take place if the difference in the lengths be twice1 three times1 four times1 c41 that quantity4 ,but if the difference in the lengths of the two rays be equal to one-half of the #jjjjjbehth part of an inch1 or to its #a-a_/b1 #b-a_/b1 #c-a_/b1 c41 part1 the one light will entirely extinguish the other1 and will produce absolute darkness on the paper where the united beams fall4 ,if the difference in the lengths of their paths be equal to the #a-a_/d1 #b-a_/d1 #c-a_/d1 c41 of the #jjjjjbehth part of an inch1 the red spot arising from the combined beams will be of the same intensity which one alone would produce4 ,if violet light be employed1 the difference in the lengths of the two beams must be equal to the #jjjjjaegth part of an inch1 in order to produce the same phenomena2 and for the other colours1 the difference must be intermediate between the #jjjjjbehth and the #jjjjjaegth part of an inch4 ,similar phenomena may be seen by viewing the flame of a candle through two very fine slits in a card extremely near to one another 7,n4 #aih72 or by admitting the sun's light into a dark room through a pin-hole about the fortieth of an inch in diameter1 receiving the image on a sheet of white paper1 and holding a slender wire in the light4 ,its shadow will be found to consist of a bright white bar or stripe in the middle1 with a series of alternate black and brightly-coloured stripes on each side4 ,the rays which bend round the wire in two streams are of equal lengths in the middle stripe2 it is consequently doubly bright from their combined effect2 but the rays which fall on the paper on each side of the bright stripe1 being of such unequal lengths as to destroy one another1 form black lines4 ,on each side of these black lines the rays are again of such lengths as to combine to form bright stripes1 and so on alternately till the light is too faint to be visible4 ,when any homogeneous light is used1 such as red1 the alternations are only black and red2 but on account of the heterogeneous nature of white light1 the black lines alternate with vivid stripes or fringes of prismatic colours1 arising from the superposition of systems of alternate black lines and lines of each homogeneous colour4 ,that the alternation of black lines and coloured fringes actually does arise from the mixture of the two streams of light which flow round the wire1 is proved by their vanishing the instant one of the streams is interrupted4 ,it may therefore be concluded1 as often as these stripes of light and darkness occur1 that they are owing to the rays combining at certain intervals to produce a joint effect1 and at others to extinguish one another4 ,now it is contrary to all our ideas of matter to suppose that two particles of it should annihilate one another under any circumstances whatever2 while1 on the contrary1 two opposing motions may2 and it is impossible not to be struck with the perfect similarity between the interferences of small undulations of air or of water and the preceding phenomena4 ,the analogy is indeed so perfect1 that philosophers of the highest authority concur in the belief that the celestial regions are filled with an extremely rare and highly elastic medium or ether1 whose particles are capable of receiving the vibrations communicated to them by self-luminous bodies1 and of transmitting them to the optic nerves1 so as to produce the sensation of light4 ,the acceleration in the mean motion of ,encke's comet1 as well as of the comet discovered by ,m4 ,biela1 renders the existence of such a medium certain4 ,it is clear that1 in this hypothesis1 the alternate stripes of light and darkness are entirely the effect of the interference of the undulations2 for1 by actual measurement1 the length of a wave of the mean red rays of the solar spectrum is equal to the #jjjjjbehth part of an inch2 consequently1 when the elevations of the waves combine1 they produce double the intensity of light that each would do singly2 and when half a wave combines with a whole--that is1 when the hollow of one wave is filled up by the elevation of another--darkness is the result4 ,at intermediate points between these extremes1 the intensity of the light corresponds to intermediate differences in the lengths of the rays4 ,the theory of interferences is a particular case of the general mechanical law of the superposition of small motions2 whence it appears that the disturbance of a particle of an elastic medium1 produced by two co-existent undulations1 is the sum of the disturbances which each undulation would produce separately2 consequently1 the particle will move in the diagonal of a parallelogram1 whose sides are the two undulations4 ,if1 therefore1 the two undulations agree in direction1 or nearly so1 the resulting motion will be very nearly equal to their sum1 and in the same direction2 if they nearly oppose one another1 the resulting motion will be nearly equal to their difference2 and1 if the undulations be equal and opposite1 the resultant will be zero1 and the particle will remain at rest4 ,the preceding experiments1 and the inferences deduced from them1 which have led to the establishment of the doctrine of the undulations of light1 are the most splendid memorials of our illustrious countryman ,dr4 ,thomas ,young1 though ,huygens was the first to originate the idea4 ,it is supposed that the particles of luminous bodies are in a state of perpetual agitation1 and that they possess the property of exciting regular vibrations in the molecules of the ethereal medium1 corresponding to the vibrations of their own molecules2 and that1 on account of its elastic nature1 one particle of the ether when set in motion communicates its vibrations to those adjacent1 which in succession transmit them to those farther off2 so that the primitive impulse is transferred from particle to particle1 and the undulating motion darts through ether like a wave in water2 so that light is motion1 and therefore subject to the laws of dynamics and mathematical analysis4 ,although the progressive motion of light is known by experience to be uniform and in a straight line1 the vibrations of the particles are always at right angles to the direction of the ray4 ,the propagation of light is like the spreading of waves in water2 but1 if one ray alone be considered1 its motion may be conceived by supposing a rope of indefinite length stretched horizontally1 one end of which is held in the hand4 ,if it be agitated to and fro at regular intervals1 with a motion perpendicular to its length1 a series of similar and equal tremors or waves will be propagated along it2 and if the regular impulses be given in a variety of planes1 as up and down1 from right to left1 and also in oblique directions1 the successive undulations will take place in every possible plane4 ,an analogous motion in the ether1 when communicated to the optic nerves1 would produce the sensation of common light4 ,it is evident that the waves which flow from end to end of the cord in a serpentine form are altogether different from the perpendicular vibratory motion of each particle of the rope1 which never deviates far from a state of rest4 ,so1 in ether1 each particle vibrates perpendicularly to the direction of the ray2 but these vibrations are totally different from and independent of the undulations which are transmitted through it1 in the same manner as the vibrations of each particular ear of corn are independent of the waves that rush from end to end of a harvest-field when agitated by the wind4 ,the intensity of light depends upon the amplitude or extent of the vibrations of the particles of ether1 while its colour depends upon their frequency4 ,the time of the vibration of a particle of ether is1 by theory1 as the length of a wave directly1 and inversely as its velocity4 ,now1 as the velocity of light is known to be #aij1jjj miles in a second1 if the lengths of the waves of the different coloured rays could be measured1 the number of vibrations in a second corresponding to each could be computed4 ,that has been accomplished as follows3--,all transparent substances of a certain thickness1 with parallel surfaces1 reflect and transmit white light2 but1 if they be extremely thin1 both the reflected and transmitted light is coloured4 ,the vivid hues on soap-bubbles1 the iridescent colours produced by heat on polished steel and copper1 the fringes of colour between the lamin of ,iceland spar and sulphate of lime1 all consist of a succession of hues disposed in the same order1 totally independent of the colour of the substance1 and determined solely by its greater or less thickness--a circumstance which affords the means of ascertaining the length of the waves of each coloured ray1 and the frequency of the vibrations of the particles producing them4 ,if a plate of glass be laid upon a lens of almost imperceptible curvature1 before an open window1 when they are pressed together a black spot will be seen in the point of contact1 surrounded by seven rings of vivid colours1 all differing from one another 7,n4 #aii74 ,in the first ring1 estimated from the black spot1 the colours succeed each other in the following order3--black1 very faint blue1 brilliant white1 yellow1 orange1 and red4 ,they are quite different in the other rings1 and in the seventh the only colours are pale bluish green and very pale pink4 ,that these rings are formed between the two surfaces in apparent contact may be proved by laying a prism on the lens instead of the plate of glass1 and viewing the rings through the inclined side of it that is next to the eye1 which arrangement prevents the light reflected from the upper surface mixing with that from the surfaces in contact1 so that the intervals between the rings appear perfectly black--one of the strongest circumstances in favour of the undulatory theory2 for1 although the phenomena of the rings can be explained by either hypothesis1 there is this material difference1 that1 according to the undulatory theory1 the intervals between the rings ought to be absolutely black1 which is confirmed by experiment2 whereas1 by the doctrine of emanation1 they ought to be half illuminated1 which is not found to be the case4 ,m4 ,fresnel1 whose opinion is of the first authority1 thought this test conclusive4 ,it may therefore be concluded that the rings arise entirely from the interference of the rays3 the light reflected from each of the surfaces in apparent contact reaches the eye by paths of different lengths1 and produces coloured and dark rings alternately1 according as the reflected waves coincide or destroy one another4 ,the breadths of the rings are unequal2 they decrease in width1 and the colours become more crowded1 as they recede from the centre4 ,coloured rings are also produced by transmitting light through the same apparatus2 but the colours are less vivid1 and are complementary to those reflected1 consequently the central spot is white4 ,the size of the rings increases with the obliquity of the incident light1 the same colour requiring a greater thickness or space between the glasses to produce it than when the light falls perpendicularly upon them4 ,now1 if the apparatus be placed in homogeneous instead of white light1 the rings will all be of the same colour with that of the light employed1 that is to say1 if the light be red1 the rings will be red1 divided by black intervals4 ,the size of the rings varies with the colour of the light4 ,they are largest in red1 and decrease in magnitude with the succeeding prismatic colours1 being smallest in violet light4 ,since one of the glasses is plane and the other spherical1 it is evident that from the point of contact the space between them gradually increases in thickness all round1 so that a certain thickness of air corresponds to each colour1 which in the undulatory system measures the length of the wave producing it 7,n4 #bjj74 ,by actual measurement ,sir ,isaac ,newton found that the squares of the diameters of the brightest part of each ring are as the odd numbers1 #a1 #c1 #e1 #g1 c42 and that the squares of the diameters of the darkest parts are as the even numbers1 #j1 #b1 #d1 #f1 c4 ,consequently1 the intervals between the glasses at these points are in the same proportion4 ,if1 then1 the thickness of the air corresponding to any one colour could be found1 its thickness for all the others would be known4 ,now1 as ,sir ,isaac ,newton knew the radius of curvature of the lens1 and the actual breadth of the rings in parts of an inch1 it was easy to compute that the thickness of air at the darkest part of the first ring is the #a_/hijjj part of an inch1 whence all the others have been deduced4 ,as these intervals determine the length of the waves on the undulatory hypothesis1 it appears that the length of a wave of the extreme red of the solar spectrum is equal to the #jjjjjbffth part of an inch2 that the length of a wave of the extreme violet is equal to the #jjjjjafgth part of an inch2 and1 as the time of a vibration of a particle of ether producing any particular colour is directly as the length of a wave of that colour1 and inversely as the velocity of light1 it follows that the molecules of ether producing the extreme red of the solar spectrum perform #deh millions of millions of vibrations in a second2 and that those producing the extreme violet accomplish #gbg millions of millions of vibrations in the same time4 ,the lengths of the waves of the intermediate colours1 and the number of their vibrations1 being intermediate between these two1 white light1 which consists of all the colours1 is consequently a mixture of waves of all lengths between the limits of the extreme red and violet4 ,the determination of these minute portions of time and space1 both of which have a real existence1 being the actual results of measurement1 do as much honour to the genius of ,newton as that of the law of gravitation4 ,the number of advancing waves of light in an inch is known to be from #cg1fjj to #ei1hhj1 and the number of lateral vibrations is from #deh to #gbg billions in a second1 but the extent of these lateral vibrations of the particles of the ethereal medium is not known1 though both their extent and velocity are probably very small compared with the length of the advancing waves and the velocity of propagation4 ,colour is identified with the number of vibrations2 but whether reflection1 refraction1 absorption1 c41 have any relations to the lateral vibrations1 or whether they are dependent in part upon some physical action of the ethereal medium unknown and unsuspected1 are points as yet undetermined4 ,to ascertain these circumstances1 ,dr4 ,faraday instituted a series of the most refined experiments upon the relation of the minute particles of metals to the vibrations of light4 ,gold acts powerfully on light1 and possesses a real transparency1 transmitting green rays when very thin2 and being capable of extreme division by solvents without losing its metallic character1 its particles transmit rays of various colours according to their size2 those that transmit the rose-colour in ,bohemian glass are of inconceivable minuteness4 ,the progressive waves of the ether are so long compared with the dimensions of the molecules to which gold can be reduced1 that it seemed probable to ,dr4 ,faraday when the latter were placed in a sunbeam that some effective relation might be discovered between them and the smaller vibrations of the ethereal medium2 in which case1 if reflection1 refraction1 c41 depended upon such relations1 there was reason to expect that these functions would change sensibly by the substitution of different sized particles of the gold for one another4 ,at one time ,dr4 ,faraday hoped he had changed one colour into another by means of gold1 which would have been equivalent to a change in the number of vibrations2 but although he has not yet confirmed that result1 his researches are of the greatest interest4#i ,the phenomenon of the coloured rings takes place in vacuo as well as in air1 which proves that it is the distance between the lenses alone1 and not the air1 which produces the colours4 ,however1 if water or oil be put between them1 the rings contract1 but no other change ensues2 and ,newton found that the thickness of different media at which a given tint is seen is in the inverse ratio of their refractive indices1 so that the thickness of lamin which could not otherwise be measured may be known by their colour2 and1 as the position of the colours in the rings is invariable1 they form a fixed standard of comparison1 well known as ,newton's scale of colours2 each tint being estimated according to the ring to which it belongs from the central spot inclusively4 ,not only the periodical colours which have been described1 but the colours seen in thick plates of transparent substances1 the variable hues of feathers1 of insects' wings1 mother-of-pearl1 and of striated substances1 all depend upon the same principle4 ,to these may be added the coloured fringes surrounding the shadows of all bodies held in an extremely small beam of light1 and the coloured rings surrounding the small beam itself when received on a screen4 ,when a very slender sunbeam1 passing through a small pin-hole into a dark room1 is received on a white screen1 or plate of ground-glass1 at the distance of a little more than six feet1 the spot of light on the screen is larger than the pin-hole3 and1 instead of being bounded by shadow1 it is surrounded by a series of coloured rings separated by obscure intervals4 ,the rings are more distinct in proportion to the smallness of the beam 7,n4 #bja74 ,when the light is white there are seven rings1 which dilate or contract with the distance of the screen from the hole4 ,as the distance of the screen diminishes1 the white central spot contracts to a point and vanishes2 and1 on approaching still nearer1 the rings gradually close in upon it1 so that the centre assumes successively the most intense and vivid hues4 ,when the light is homogeneous--red1 for example--the rings are alternately red and black1 and more numerous2 and their breadth varies with the colour1 being broadest in red light and narrowest in violet4 ,the tints of the coloured fringes from white light1 and their obliteration after the seventh ring1 arise from the superposition of the different sets of fringes of all the coloured rays4 ,the shadows of objects are also bordered by coloured fringes when held in this slender beam of light4 ,if the edge of a knife or hair1 for example1 be held in it1 the rays1 instead of proceeding in straight lines past its edge1 are bent when quite close to it1 and proceed from thence to the screen in curved lines called hyperbolas2 so that the shadow of the object is enlarged1 and1 instead of being at once bounded by light1 is surrounded or edged with coloured fringes alternating with black bands1 which are more distinct the smaller the pin-hole 7,n4 #bjb74 ,the fringes are altogether independent of the form or density of the object1 being the same when it is round or pointed1 when of glass or platinum4 ,when the rays which form the fringes arrive at the screen1 they are of different lengths1 in consequence of the curved path they follow after passing the edge of the object4 ,the waves are therefore in different phases or states of vibration1 and either conspire to form coloured fringes or destroy one another in the obscure intervals4 ,the coloured fringes bordering the shadows of objects were first described by ,grimaldi in #affe2 but1 besides these1 he noticed that there are others within the shadows of slender bodies exposed to a small sunbeam1 a phenomenon which has already been mentioned to have afforded ,dr4 ,young the means of proving1 beyond all controversy1 that coloured rings are produced by the interference of light4 ,it may be concluded that material substances derive their colours from two different causes3 some from the law of interference1 such as iridescent metals1 peacocks' feathers1 c42 others from the unequal absorption of the rays of white light1 such as vermilion1 ultramarine1 blue1 or green cloth1 flowers1 and the greater number of coloured bodies4 ,the latter phenomena have been considered extremely difficult to reconcile with the undulatory theory of light1 and much discussion has arisen as to what becomes of the absorbed rays4 ,but that embarrassing question has been ably answered by ,sir ,john ,herschel in a most profound paper on the ,absorption of ,light by coloured ,media1 and cannot be better given than in his own words4 ,it must1 however1 be premised1 that1 as all transparent bodies are traversed by light1 they are presumed to be permeable to the ether4 ,he says1--8,now1 as regards only the general fact of the obstruction and ultimate extinction of light in its passage through gross media1 if we compare the corpuscular and undulatory theories1 we shall find that the former appeals to our ignorance1 the latter to our knowledge1 for its explanation of the absorptive phenomena4 ,in attempting to explain the extinction of light on the corpuscular doctrine1 we have to account for the light so extinguished as a material body1 which we must not suppose annihilated4 ,it may1 however1 be transformed2 and among the imponderable agents1 heat1 electricity1 c41 it may be that we are to search for the light which has become thus comparatively stagnant4 ,the heating power of the solar rays gives a prim facie plausibility to the idea of the transformation of light into heat by absorption4 ,but1 when we come to examine the matter more nearly1 we find it encumbered on all sides with difficulties4 ,how is it1 for instance1 that the most luminous rays are not the most calorific1 but that1 on the contrary1 the calorific energy accompanies1 in its greatest intensity1 rays which possess comparatively feeble illuminating powers8 ,these and other questions of a similar nature may perhaps admit of answer in a more advanced state of our knowledge2 but at present there is none obvious4 ,it is not without reason1 therefore1 that the question1 #8,what becomes of light8' which appears to have been agitated among the photologists of the last century1 has been regarded as one of considerable importance as well as obscurity by the corpuscular philosophers4 ,on the other hand1 the answer to this question1 afforded by the undulatory theory of light1 is simple and distinct4 ,the question1 #8,what becomes of light8' merges in the more general one1 #8,what becomes of motion8' ,and the answer1 on dynamical principles1 is1 that it continues for ever4 ,no motion is1 strictly speaking1 annihilated2 but it may be divided1 and the divided parts made to oppose and in effect destroy one another4 ,a body struck1 however perfectly elastic1 vibrates for a time1 and then appears to sink into its original repose4 ,but this apparent rest 7even abstracting from the inquiry that part of the motion which may be conveyed away by the ambient air7 is nothing else than a state of subdivided and mutually destroying motion1 in which every molecule continues to be agitated by an indefinite multitude of internally reflected waves1 propagated through it in every possible direction1 from every point in its surface on which they successively impinge4 ,the superposition of such waves will1 it is easily seen1 at length operate their mutual destruction1 which will be the more complete the more irregular the figure of the body1 and the greater the number of internal reflections40 ,thus ,sir ,john ,herschel1 by referring the absorption of light to the subdivision and mutual destruction of the vibrations of ether in the interior of bodies1 brings another class of phenomena under the laws of the undulatory theory4 ,according to ,mr4 ,rankin's hypothesis of ,molecular ,vortices#aj the absorption of light and radiant heat consists in the transference of motion from the molecules to their atmospheres1 and conversely the emission of light and radiant heat is the transmission of motion from the atmospheres to the molecules4 ,the great velocity of light and heat is a natural consequence of this hypothesis1 according to which the vibratory masses must be extremely small compared with the forces exerted by them4 ,the ethereal medium pervading space is supposed to penetrate all material substances1 occupying the interstices between their molecules2 but in the interior of refracting media it exists in a state of less elasticity compared with its density in vacuo2 and1 the more refractive the medium1 the less the elasticity of the ether within it4 ,hence the waves of light are transmitted with less velocity in such media as glass and water than in the external ether4 ,as soon as a ray of light reaches the surface of a diaphanous reflecting substance1 for example a plate of glass1 it communicates its undulations to the ether next in contact with the surface1 which thus becomes a new centre of motion1 and two hemispherical waves are propagated from each point of this surface2 one of which proceeds forward into the interior of the glass1 with a less velocity than the incident waves2 and the other is transmitted back into the air1 with a velocity equal to that with which it came 7,n4 #bjc74 ,thus1 when refracted1 the light moves with a different velocity without and within the glass2 when reflected1 the ray comes and goes with the same velocity4 ,the particles of ether without the glass1 which communicate their motions to the particles of the dense and less elastic ether within it1 are analogous to small elastic balls striking large ones2 for some of the motion will be communicated to the large balls1 and the small ones will be reflected4 ,the first would cause the refracted wave1 and the last the reflected4 ,conversely1 when the light passes from glass to air1 the action is similar to large balls striking small ones4 ,the small balls receive a motion which would cause the refracted ray1 and the part of the motion retained by the large ones would occasion the reflected wave2 so that1 when light passes through a plate of glass or of any other medium differing in density from the air1 there is a reflection at both surfaces2 but this difference exists between the two reflections1 that one is caused by a vibration in the same direction with that of the incident ray1 and the other by a vibration in the opposite direction4 ,a single wave of air or ether would not produce the sensation of sound or light4 ,in order to excite vision1 the vibrations of the molecules of ether must be regular1 periodical1 and very often repeated3 and1 as the ear continues to be agitated for a short time after the impulse by which alone a sound becomes continuous1 so also the fibres of the retina1 according to ,m4 d',arcet1 continue to vibrate for about the eighth part of a second after the exciting cause has ceased4 ,the interval of time during which the impression lasts is longer for the blue than for red or white light3 it must not be less than #jcd4 ,every one must have observed1 when a strong impression is made by a bright light1 that an object remains visible for a short time after shutting the eyes1 which is supposed to be in consequence of the continued vibrations of the fibres of the retina4 ,occasionally the retina becomes insensible to feebly illuminated objects when continuously presented4 ,if the eye be turned aside for a moment1 the object becomes again visible4 ,it is probably on this account that the owl makes so peculiar a motion with its head when looking at objects in the twilight4 ,it is quite possible that many vibrations may be excited in the ethereal medium incapable of producing undulations in the fibres of the human retina1 which yet have a powerful effect on those of other animals or of insects4 ,such may receive luminous impressions of which we are totally unconscious1 and at the same time they may be insensible to the light and colours which affect our eyes1 their perceptions beginning where ours end4 ,,section ,,xxi4 ,polarization of ,light--,defined--,polarization by ,refraction--,properties of the ,tourmaline--,double ,refraction--,all doubly ,refracted ,light is ,polarized--,properties of ,iceland ,spar--,tourmaline absorbs one of the two ,refracted ,rays--,undulations of ,natural ,light--,undulations of ,polarized ,light--,the ,optic ,axes of ,crystals--,m4 ,fresnel's ,discoveries on the ,rays passing along the ,optic ,axis--,polarization by ,reflection4 ,,in giving a sketch of the constitution of light1 it is impossible to omit the extraordinary property of its polarization1 8the phenomena of which10 ,sir ,john ,herschel says1 8are so singular and various1 that to one who has only studied the common branches of physical optics it is like entering into a new world1 so splendid as to render it one of the most delightful branches of experimental inquiry1 and so fertile in the views it lays open of the constitution of natural bodies1 and the minuter mechanism of the universe1 as to place it in the very first rank of the physico-mathematical sciences1 which it maintains by the rigorous application of geometrical reasoning its nature admits and requires40 ,light is said to be polarized1 which1 by being once reflected or refracted1 is rendered incapable of being again reflected or refracted at certain angles4 ,in general1 when a ray of light is reflected from a pane of plate-glass1 or any other substance1 it may be reflected a second time from another surface1 and it will also pass freely through transparent bodies4 ,but1 if a ray of light be reflected from a pane of plate-glass at an angle of #eg1 it is rendered totally incapable of reflection at the surface of another pane of glass in certain definite positions1 but it will be completely reflected by the second pane in other positions4 ,it likewise loses the property of penetrating transparent bodies in particular positions1 whilst it is freely transmitted by them in others4 ,light1 so modified as to be incapable of reflection and transmission in certain directions1 is said to be polarized4 ,light may be polarized by reflection from any polished surface1 and the same property is also imparted by refraction4 ,it is proposed to explain these methods of polarizing light1 to give a short account of its most remarkable properties1 and to endeavour to describe a few of the splendid phenomena it exhibits4 ,if a brown tourmaline1 which is a mineral generally crystallized in the form of a long prism1 be cut longitudinally1 that is1 parallel to the axis of the prism1 into plates about the thirtieth of an inch in thickness1 and the surfaces polished1 luminous objects may be seen through them1 as through plates of coloured glass4 ,the axis of each plate is in its longitudinal section parallel to the axis of the prism whence it was cut 7,n4 #bjd74 ,if one of these plates be held perpendicularly between the eye and a candle1 and turned slowly round in its own plane1 no change will take place in the image of the candle4 ,but if the plate be held in a fixed position1 with its axis or longitudinal section vertical1 when a second plate of tourmaline is interposed between it and the eye1 parallel to the first1 and turned slowly round in its own plane1 a remarkable change will be found to have taken place in the nature of the light4 ,for the image of the candle will vanish and appear alternately at every quarter revolution of the plate1 varying through all degrees of brightness down to total or almost total evanescence1 and then increasing again by the same degrees as it had before decreased4 ,these changes depend upon the relative positions of the plates4 ,when the longitudinal sections of the two plates are parallel1 the brightness of the image is at its maximum2 and1 when the axes of the sections cross at right angles1 the image of the candle vanishes4 ,thus the light1 in passing through the first plate of tourmaline1 has acquired a property totally different from the direct light of the candle4 ,the direct light would have penetrated the second plate equally well in all directions1 whereas the refracted ray will only pass through it in particular positions1 and is altogether incapable of penetrating it in others4 ,the refracted ray is polarized in its passage through the first tourmaline1 and experience shows that it never loses that property1 unless when acted upon by a new substance4 ,thus1 one of the properties of polarized light is the incapability of passing through a plate of tourmaline perpendicular to it1 in certain positions1 and its ready transmission in other positions at right angles to the former4 ,many other substances have the property of polarizing light4 ,if a ray of light falls upon a transparent medium1 which has the same temperature1 density1 and structure throughout every part1 as fluids1 gases1 glass1 c41 and a few regularly crystallized minerals1 it is refracted into a single pencil of light by the laws of ordinary refraction1 according to which the ray1 passing through the refracting surface from the object to the eye1 never quits a plane perpendicular to that surface4 ,almost all other bodies1 such as the greater number of crystallized minerals1 animal and vegetable substances1 gums1 resins1 jellies1 and all solid bodies having unequal tensions1 whether from unequal temperature or pressure1 possess the property of doubling the image or appearance of an object seen through them in certain directions2 because a ray of natural light falling upon them is refracted into two pencils which move with different velocities1 and are more or less separated1 according to the nature of the body and the direction of the incident ray4 ,whenever a ray of natural light is thus divided into two pencils in its passage through a substance1 both of the transmitted rays are polarized4 ,iceland spar1 a carbonate of lime1 which by its natural cleavage may be split into the form of a rhombohedron1 possesses the property of double refraction in an eminent degree1 as may be seen by pasting a piece of paper1 with a large pin-hole in it1 on the side of the spar farthest from the eye4 ,the hole will appear double when held to the light 7,n4 #bje74 ,one of these pencils is refracted according to the same law as in glass or water1 never quitting the plane perpendicular to the refracting surface1 and is therefore called the ordinary ray4 ,but the other does quit the plane1 being refracted according to a different and much more complicated law1 and on that account is called the extraordinary ray4 ,for the same reason one image is called the ordinary1 and the other the extraordinary image4 ,when the spar is turned round in the same plane1 the extraordinary image of the hole revolves about the ordinary image1 which remains fixed1 both being equally bright4 ,but if the spar be kept in one position1 and viewed through a plate of tourmaline1 it will be found that1 as the tourmaline revolves1 the images vary in their relative brightness--one increases in intensity till it arrives at a maximum1 at the same time that the other diminishes till it vanishes1 and so on alternately at each quarter revolution1 proving both rays to be polarized4 ,for in one position the tourmaline transmits the ordinary ray1 and reflects the extraordinary2 and1 after revolving #ij1 the extraordinary ray is transmitted1 and the ordinary ray is reflected4 ,thus another property of polarized light is1 that it cannot be divided into two equal pencils by double refraction1 in positions of the doubly refracting bodies in which a ray of common light would be so divided4 ,were tourmaline like other doubly refracting bodies1 each of the transmitted rays would be double2 but that mineral1 when of a certain thickness1 after separating the light into two polarized pencils1 absorbs that which undergoes ordinary refraction1 and consequently shows only one image of an object4 ,on this account tourmaline is peculiarly fitted for analyzing polarized light1 which shows nothing remarkable till viewed through it or something equivalent4 ,the pencils of light1 on leaving a double refracting substance1 are parallel2 and it is clear1 from the preceding experiments1 that they are polarized in planes at right angles to each other 7,n4 #bjf74 ,but that will be better understood by considering the change produced in common light by the action of the polarizing body4 ,it has been shown that the undulations of ether1 which produce the sensation of common light1 are performed in every possible plane1 at right angles to the direction in which the ray is moving4 ,but the case is very different after the ray has passed through a doubly refracting substance1 like ,iceland spar4 ,the light then proceeds in two parallel pencils1 whose undulations are still indeed transverse to the direction of the rays1 but they are accomplished in planes at right angles to one another1 analogous to two parallel stretched cords1 one of which performs its undulations only in a horizontal plane1 and the other in a vertical or upright plane 7,n4 #bjf74 ,thus the polarizing action of ,iceland spar and of all doubly refracting substances is to separate a ray of common light1 whose waves or undulations are in every plane1 into two parallel rays1 whose waves or undulations lie in planes at right angles to each other4 ,by a simple mechanical law each vibratory motion of the first is resolved into two vibratory motions at right angles to one another4 ,the ray of common light may be assimilated to a round rod1 whereas the two polarized rays are like two parallel long flat rulers1 one of which is laid horizontally on its broad surface1 and the other horizontally on its edge4 ,the alternate transmission and obstruction of one of these flattened beams by the tourmaline is similar to the facility with which a card may be passed between the bars of a grating or wires of a cage1 if presented edgeways1 and the impossibility of its passing in a transverse direction4 ,although it generally happens that a ray of light1 in passing through ,iceland spar1 is separated into two polarized rays1 yet there is one direction along which it is refracted in one ray only1 and that according to the ordinary law4 ,this direction is called the optic axis 7,n4 #bjg74 ,many crystals and other substances have two optic axes1 inclined to each other1 along which a ray of light is transmitted in one pencil by the law of ordinary refraction4 ,the extraordinary ray is sometimes refracted towards the optic axis1 as in quartz1 zircon1 ice1 c41 which are therefore said to be positive crystals2 but when it is bent from the optic axis1 as in ,iceland spar1 tourmaline1 emerald1 beryl1 c41 the crystals are negative1 which is the most numerous class4 ,the ordinary ray moves with uniform velocity within a doubly refracting substance1 but the velocity of the extraordinary ray varies with the position of the ray relatively to the optic axis1 being a maximum when its motion within the crystal is at right angles to the optic axis1 and a minimum when parallel to it4 ,between these extremes its velocity varies according to a determinate law4 ,it had been inferred1 from the action of ,iceland spar on light1 that in all doubly refracting substances one only of two rays is turned aside from the plane of ordinary refraction1 while the other follows the ordinary law2 and the great difficulty of observing the phenomena tended to confirm that opinion4 ,m4 ,fresnel1 however1 proved by a most profound mathematical inquiry1 priori1 that the extraordinary ray must be wanting in glass and other uncrystallized substances1 and that it must necessarily exist in carbonate of lime1 quartz1 and other bodies having one optic axis1 but that in a numerous class of substances1 which possess two optic axes1 both rays must undergo extraordinary refraction1 and consequently that both must deviate from their original plane2 and these results have been perfectly confirmed by subsequent experiments4 ,this theory of refraction1 which for generalization is perhaps only inferior to the law of gravitation1 has enrolled the name of ,fresnel among those which pass not away1 and makes his early loss a subject of deep regret to all who take an interest in the higher paths of scientific research4 ,when a beam of common light is partly reflected at1 and partly transmitted through a transparent surface1 the reflected and refracted pencils contain equal quantities of polarized light1 and their planes of polarization are at right angles to one another3 hence1 a pile of panes of glass will give a polarized beam by refraction4 ,for1 if a ray of common light pass through them1 part of it will be polarized by the first plate1 the second plate will polarize a part of what passes through it1 and the rest will do the same in succession1 till the whole beam is polarized1 except what is lost by reflection at the different surfaces1 or by absorption4 ,this beam is polarized in a plane at right angles to the plane of reflection1 that is1 at right angles to the plane passing through the incident and reflected ray 7,n4 #bjh74 ,by far the most convenient way of polarizing light is by reflection4 ,a plane of plate-glass laid upon a piece of black cloth1 on a table at an open window1 will appear of a uniform brightness from the reflection of the sky or clouds4 ,but if it be viewed through a plate of tourmaline1 having its axis vertical1 instead of being illuminated as before1 it will be obscured by a large cloudy spot1 having its centre quite dark1 which will readily be found by elevating or depressing the eye1 and will only be visible when the angle of incidence is #eg1 that is1 when the line from the eye to the centre of the black spot makes an angle of #cc with the surface of the reflector 7,n4 #bji74 ,when the tourmaline is turned round in its own plane1 the dark cloud will diminish1 and entirely vanish when the axis of the tourmaline is horizontal1 and then every part of the surface of the glass will be equally illuminated4 ,as the tourmaline revolves1 the cloudy spot will appear and vanish alternately at every quarter revolution4 ,thus1 when a ray of light is incident on a pane of plate-glass at an angle of #eg1 the reflected ray is rendered incapable of penetrating a plate of tourmaline whose axis is in the plane of incidence4 ,consequently it has acquired the same character as if it had been polarized by transmission through a plate of tourmaline1 with its axis at right angles to the plane of reflection4 ,it is found by experience that this polarized ray is incapable of a second reflection at certain angles and in certain positions of the incident plane4 ,for if another pane of plate-glass1 having one surface blackened1 be so placed as to make an angle of #cc with the reflected ray1 the image of the first pane will be reflected in its surface1 and will be alternately illuminated and obscured at every quarter revolution of the blackened pane1 according as the plane of reflection is parallel or perpendicular to the plane of polarization4 ,since this happens by whatever means the light has been polarized1 it evinces another general property of polarized light1 which is1 that it is incapable of reflection in a plane at right angles to the plane of polarization4 ,all reflecting surfaces are capable of polarizing light1 but the angle of incidence at which it is completely polarized is different in each substance 7,n4 #baj74 ,it appears that the angle for plate-glass is #eg2 in crown-glass it is #ef #ee1 and no ray will be completely polarized by water unless the angle of incidence be #ec #aa4 ,the angles at which different substances polarize light are determined by a very simple and elegant law1 discovered by ,sir ,david ,brewster1 8,that the tangent of the polarizing angle for any medium is equal to the sine of the angle of incidence divided by the sine of the angle of refraction of that medium40 ,whence also the refractive power even of an opaque body is known when its polarizing angle has been determined4 ,if a ray1 polarized by refraction or by reflection from any substance not metallic1 be viewed through a piece of ,iceland spar1 each image will alternately vanish and reappear at every quarter revolution of the spar1 whether it revolves from right to left or from left to right2 which shows that the properties of the polarized ray are symmetrical on each side of the plane of polarization4 ,although there be only one angle in each substance at which light is completely polarized by one reflection1 yet it may be polarized at any angle of incidence by a sufficient number of reflections4 ,for1 if a ray falls upon the upper surface of a pile of plates of glass at an angle greater or less than a polarizing angle1 a part only of the reflected ray will be polarized1 but a part of what is transmitted will be polarized by reflection at the surface of the second plate1 part at the third1 and so on till the whole is polarized4 ,this is the best apparatus2 but one plate of glass having its inferior surface blackened1 or even a polished table1 will answer the purpose4 ,,section ,,xxii4 ,phenomena exhibited by the ,passage of ,polarized ,light through ,mica and ,sulphate of ,lime--,the ,coloured ,images produced by ,polarized ,light passing through ,crystals having one and two ,optic ,axes--,circular ,polarization--,elliptical ,polarization--,discoveries of ,,mm4 ,biot1 ,fresnel1 and ,professor ,airy--,coloured ,images produced by the ,interference of ,polarized ,rays--,fluorescence4 ,,such is the nature of polarized light and of the laws it follows4 ,but it is hardly possible to convey an idea of the splendour of the phenomena it exhibits under circumstances which an attempt will now be made to describe4 ,if light polarized by reflection from a pane of glass be viewed through a plate of tourmaline1 with its longitudinal section vertical1 an obscure cloud1 with its centre totally dark1 will be seen on the glass4 ,now1 let a plate of mica1 uniformly about the thirtieth of an inch in thickness1 be interposed between the tourmaline and the glass2 the dark spot will instantly vanish1 and1 instead of it1 a succession of the most gorgeous colours will appear1 varying with every inclination of the mica1 from the richest reds1 to the most vivid greens1 blues1 and purples 7,n4 #baa74 ,that they may be seen in perfection1 the mica must revolve at right angles to its own plane4 ,when the mica is turned round in a plane perpendicular to the polarized ray1 it will be found that there are two lines in it where the colours entirely vanish4 ,these are the optic axes of the mica1 which is a doubly refracting substance1 with two optic axes1 along which light is refracted in one pencil4 ,no colours are visible in the mica1 whatever its position may be with regard to the polarized light1 without the aid of the tourmaline1 which separates the transmitted ray into two pencils of coloured light complementary to one another1 that is1 which taken together would make white light4 ,one of these it absorbs1 and transmits the other2 it is therefore called the analyzing plate4 ,the truth of this will appear more readily if a film of sulphate of lime1 between the twentieth and sixtieth of an inch thick1 be used instead of the mica4 ,when the film is of uniform thickness1 only one colour will be seen when it is placed between the analyzing plate and the reflecting glass2 as1 for example1 red4 ,but1 when the tourmaline revolves1 the red will vanish by degrees till the film is colourless2 then it will assume a green hue1 which will increase and arrive at its maximum when the tourmaline has turned through ninety degrees2 after that1 the green will vanish and the red will reappear1 alternating at each quadrant4 ,thus the tourmaline separates the light which has passed through the film into a red and a green pencil2 in one position it absorbs the green and lets the red pass1 and in another it absorbs the red and transmits the green4 ,this is proved by analyzing the ray with ,iceland spar instead of tourmaline2 for1 since the spar does not absorb the light1 two images of the sulphate of lime will be seen1 one red and the other green2 and these exchange colours every quarter revolution of the spar1 the red becoming green1 and the green red2 and1 where the images overlap1 the colour is white1 proving the red and green to be complementary to each other4 ,the tint depends on the thickness of the film4 ,films of sulphate of lime1 the #jjjabd and #jjahah of an inch respectively1 give white light in whatever position they may be held1 provided they be perpendicular to the polarized ray2 but films of intermediate thickness will give all colours4 ,consequently1 a wedge of sulphate of lime1 varying in thickness between the #jjjabd and the #jjahah of an inch1 will appear to be striped with all colours when polarized light is transmitted through it4 ,a change in the inclination of the film1 whether of mica or sulphate of lime1 is evidently equivalent to a variation in thickness4 ,when a plate of mica1 held as close to the eye as possible1 at such an inclination as to transmit the polarized ray along one of its optic axes1 is viewed through the tourmaline with its axis vertical1 a most splendid appearance is presented4 ,the cloudy spot in the direction of the optic axis is seen surrounded by a set of vividly coloured rings of an oval form1 divided into two unequal parts by a black curved band passing through the cloudy spot about which the rings are formed4 ,the other optic axis of the mica exhibits a similar image 7,n4 #bab74 ,when the two optic axes of a crystal make a small angle with one another1 as in nitre1 the two sets of rings touch externally2 and1 if the plate of nitre be turned round in its own plane1 the black transverse bands undergo a variety of changes1 till1 at last1 the whole richly coloured image assumes the form of the figure #h1 traversed by a black cross 7,n4 #bac74 ,substances with one optic axis have but one set of coloured circular rings1 with a broad black cross passing through its centre1 dividing the rings into four equal parts4 ,when the analyzing plate revolves1 this figure recurs at every quarter revolution2 but in the intermediate positions it assumes the complementary colours1 the black cross becoming white4 ,it is in vain to attempt to describe the beautiful phenomena exhibited by innumerable bodies which undergo periodic changes in form and colour when the analyzing plate revolves1 but not one of them shows a trace of colour without the aid of tourmaline1 or something equivalent1 to analyze the light1 and as it were to call these beautiful phantoms into existence4 ,tourmaline has the disadvantage of being itself a coloured substance2 but that inconvenience may be obviated by employing a reflecting surface as an analyzing plate4 ,when polarized light is reflected by a plate of glass at the polarizing angle1 it will be separated into two coloured pencils2 and1 when the analyzing plate is turned round in its own plane1 it will alternately reflect each ray at every quarter revolution1 so that all the phenomena that have been described will be seen by reflection on its surface4 ,coloured rings are produced by analyzing polarized light transmitted through glass melted and suddenly or unequally cooled2 also through thin plates of glass bent with the hand1 jelly indurated or compressed1 c4 c4 ,in short1 all the phenomena of coloured rings may be produced1 either permanently or transiently1 in a variety of substances1 by heat and cold1 rapid cooling1 compression1 dilatation1 and induration2 and so little apparatus is necessary for performing the experiments1 that1 as ,sir ,john ,herschel says1 a piece of window glass or a polished table to polarize the light1 a sheet of clear ice to produce the rings1 and a broken fragment of plate-glass placed near the eye to analyze the light1 are alone requisite to produce one of the most splendid of optical exhibitions4 ,pressure produces remarkable changes in the optical properties of crystals4 ,compression1 perpendicular to the axis1 transforms a crystal with one optic axis into one with two4 ,a slice of quartz and one of beryl1 both cut perpendicularly to their axis1 were compressed thus by ,,mm4 ,moignot and ,soleil4 ,they found that the single system in the quartz1 which is a positive crystal1 was doubled in the direction of the compression1 while in the beryl1 which is a negative crystal1 the duplication was perpendicular to the compression4 ,in the quartz the axis of the double system coincided with the line of pressure1 but in the tourmaline1 which is a negative crystal1 the line which joins the centres of the rings was perpendicular to the pressure4 ,if a positive crystal be compressed in the direction of its axis the tint of the rings descends1 and that of a negative crystal rises4 ,but if the crystals be dilated in the direction of their optic axis1 the tints in positive crystals rise1 and negative descend4 ,it has been observed1 that when a ray of light1 polarized by reflection from any surface not metallic1 is analyzed by a doubly refracting substance1 it exhibits properties which are symmetrical both to the right and left of the plane of reflection1 and the ray is then said to be polarized according to that plane4 ,this symmetry is not destroyed when the ray1 before being analyzed1 traverses the optic axis of a crystal having but one optic axis1 as evidently appears from the circular forms of the coloured rings already described4 ,regularly crystallized quartz1 however1 forms an exception4 ,in it1 even though the rays should pass through the optic axis itself1 where there is no double refraction1 the primitive symmetry of the ray is destroyed1 and the plane of primitive polarization deviates either to the right or left of the observer1 by an angle proportional to the thickness of the plate of quartz4 ,this angular motion1 or true rotation of the plane of polarization1 which is called circular polarization1 is clearly proved by the phenomena4 ,the coloured rings produced by all crystals having but one optic axis are circular1 and traversed by a black cross concentric with the rings2 so that the light entirely vanishes throughout the space enclosed by the interior ring1 because there is neither double refraction nor polarization along the optic axis4 ,but in the system of rings produced by a plate of quartz1 whose surfaces are perpendicular to the axis of the crystal1 the part within the interior ring1 instead of being void of light1 is occupied by a uniform tint of red1 green1 or blue1 according to the thickness of the plate 7,n4 #bad74 ,suppose the plate of quartz to be #a_/be of an inch thick1 which will give the red tint to the space within the interior ring2 when the analyzing plate is turned1 in its own plane through an angle of #ag-a_/b1 the red hue vanishes4 ,if a plate of rock crystal #b_/be of an inch thick be used1 the analyzing plate must revolve through #ce before the red tint vanishes1 and so on1 every additional #beth of an inch in thickness requiring an additional rotation of #ag-a_/b2 whence it is manifest that the plane of polarization revolves in the direction of a spiral within the rock crystal4 ,it is remarkable that1 in some crystals of quartz1 the plane of polarization revolves from right to left1 and in others from left to right1 although the crystals themselves differ apparently only by a very slight1 almost imperceptible1 variety in form4 ,in these phenomena the rotation to the right is accomplished according to the same laws1 and with the same energy1 as that to the left4 ,but if two plates of quartz be interposed1 which possess different affections1 the second plate undoes1 either wholly or partly1 the rotatory motion which the first had produced1 according as the plates are of equal or unequal thickness4 ,when the plates are of unequal thickness1 the deviation is in the direction of the strongest1 and exactly the same with that which a third plate would produce equal in thickness to the difference of the two4 ,m4 ,biot has discovered the same properties in a variety of liquids4 ,oil of turpentine1 and an essential oil of laurel1 cause the plane of polarization to turn to the left1 whereas the syrup of sugar-cane1 and a solution of natural camphor1 by alcohol1 turn it to the right4 ,a compensation is effected by the superposition or mixture of two liquids which possess these opposite properties1 provided no chemical action takes place4 ,a remarkable difference was also observed by ,m4 ,biot between the action of the particles of the same substances when in a liquid or solid state4 ,the syrup of grapes1 for example1 turns the plane of polarization to the left as long as it remains liquid2 but1 as soon as it acquires the solid form of sugar1 it causes the plane of polarization to revolve towards the right1 a property which it retains even when again dissolved4 ,instances occur also in which these circumstances are reversed4 ,a ray of light passing through a liquid possessing the power of circular polarization is not affected by mixing other fluids with the liquid--such as water1 ether1 alcohol1 c4--which do not possess circular polarization themselves1 the angle of deviation remaining exactly the same as before the mixture4 ,whence ,m4 ,biot infers that the action exercised by the liquids in question does not depend upon their mass1 but that it is a molecular action exercised by the ultimate particles of matter1 which depends solely upon the individual constitution1 and is entirely independent of the positions and mutual distances of the particles with regard to each other4 ,these important discoveries show that circular polarization surpasses the power of chemical analysis in giving certain and direct evidence of the similarity or difference existing in the molecular constitution of bodies1 as well as of the permanency of that constitution1 or of the fluctuations to which it may be liable4 ,for example1 no chemical difference has been discovered between syrup from the sugar-cane and syrup from grapes4 ,yet the first causes the plane of polarization to revolve to the right1 and the other to the left2 therefore some essential difference must exist in the nature of their ultimate molecules4 ,the same difference is to be traced between the juices of such plants as give sugar similar to that from the cane1 and those which give sugar like that obtained from grapes4 ,if chlorate of soda be dissolved in water1 the liquid has no circular polarization2 but if the solution be allowed to crystallize1 some of the crystals turn the light to the right and others to the left4 ,now1 if all those of one kind be gathered together and dissolved a second time1 the liquid will have no circular polarization2 but if crystals be allowed to form1 some will turn the light to the right and others to the left1 although only one kind was dissolved4#aa ,it is a fact established by ,m4 ,biot1 that in circular polarization the laws of rotation followed by the different simple rays of light are dissimilar in different substances4 ,whence he infers that the deviation of the simple rays from one another ought not to result from a special property of the luminous principle only1 but that the proper action of the molecules must also concur in modifying the deviations of the simple rays differently in different substances4 ,one of the many brilliant discoveries of ,m4 ,fresnel is the production of circular and elliptical polarization by the internal reflection of light from plate-glass4 ,he has shown that1 if light polarized by any of the usual methods be twice reflected within a glass rhomb 7,n4 #afi7 of a given form1 the vibrations of the ether that are perpendicular to the plane of incidence will be retarded a quarter of a vibration1 which causes the vibrating particles to describe circles1 and the succession of such vibrating particles throughout the extent of a wave to form altogether a circular helix1 or curve like a corkscrew4 ,however1 that only happens when the plane of polarization is inclined at an angle of #de to the plane of incidence4 ,when these two planes form an angle either greater or less1 the succession of vibrating particles forms an elliptical helix1 which curve may be represented by twisting a thread in a spiral about an oval rod4 ,these curves will turn to the right or left1 according to the position of the incident plane4 ,the motion of the ethereal medium in elliptical and circular polarization may be represented by the analogy of a stretched cord2 for1 if the extremity of such a cord be agitated at equal and regular intervals by a vibratory motion entirely confined to one plane1 the cord will be thrown into an undulating curve lying wholly in that plane4 ,if to this motion there be superadded another similar and equal1 but perpendicular to the first1 the cord will assume the form of an elliptical helix2 its extremity will describe an ellipse1 and every molecule throughout its length will successively do the same4 ,but1 if the second system of vibrations commence exactly a quarter of an undulation later than the first1 the cord will take the form of a circular helix or corkscrew1 the extremity will move uniformly in a circle1 and every molecule throughout the cord will do the same in succession4 ,it appears1 therefore1 that both circular and elliptical polarization may be produced by the composition of the motions of two rays in which the particles of ether vibrate in planes at right angles to one another4 ,professor ,airy1 in a very profound and able paper published in the ,cambridge ,transactions1 has proved that all the different kinds of polarized light are obtained from rock crystal4 ,when polarized light is transmitted through the axis of a crystal of quartz1 in the emergent ray the particles of ether move in a circular helix2 and when it is transmitted obliquely so as to form an angle with the axis of the prism1 the particles of ether move in an elliptical helix1 the ellipticity increasing with the obliquity of the incident ray2 so that1 when the incident ray falls perpendicularly to the axis1 the particles of ether move in a straight line4 ,thus quartz exhibits every variety of elliptical polarization1 even including the extreme cases where the excentricity is zero1 or equal to the greater axis of the ellipse 7,n4 #bae74 ,in many crystals the two rays are so little separated1 that it is only from the nature of the transmitted light that they are known to have the property of double refraction4 ,m4 ,fresnel discovered1 by experiments on the properties of light passing through the axis of quartz1 that it consists of two superposed rays1 moving with different velocities2 and ,professor ,airy has shown that in these two rays the molecules of ether vibrate in similar ellipses at right angles to each other1 but in different directions2 that their ellipticity varies with the angle which the incident ray makes with the axis2 and that1 by the composition of their motions1 they produce all the phenomena of polarized light observed in quartz4 ,it appears1 from what has been said1 that the molecules of ether always perform their vibrations at right angles to the direction of the ray1 but very differently in the various kinds of light4 ,in natural light the vibrations are rectilinear1 and in every plane4 ,in ordinary polarized light they are rectilinear1 but confined to one plane2 in circular polarization the vibrations are circular2 and in elliptical polarization the molecules vibrate in ellipses4 ,these vibrations are communicated from molecule to molecule1 in straight lines when they are rectilinear1 in a circular helix when they are circular1 and in an oval or elliptical helix when elliptical4 ,some fluids possess the property of circular polarization naturally1 as oil of turpentine1 the essential oils of laurel and lemon1 sugar of grapes1 and various liquids4 ,elliptical polarization is produced by reflection from metallic surfaces4 ,mr4 ,baden ,powell discovered it also in the light reflected from ,china ink1 chromate of lead1 plumbago1 c4 ,mr4 ,airy observed that the light reflected from the diamond is elliptically polarized2 and ,mr4 ,jamin has shown that this kind of polarization is generally produced by reflection from almost all transparent bodies1 whatever their refractive power may be1 especially from glass at angles very little different from the law of the tangents4 ,water polarizes light circularly when between the points of maximum density and solidification2 hence it becomes crystalline4 ,the coloured images from polarized light arise from the interference of the rays 7,n4 #baf74 ,,mm4 ,fresnel and ,arago found that two rays of polarized light interfere and produce coloured fringes if they be polarized in the same plane1 but that they do not interfere when polarized in different planes4 ,in all intermediate positions1 fringes of intermediate brightness are produced4 ,the analogy of a stretched cord will show how this happens4 ,suppose the cord to be moved backwards and forwards horizontally at equal intervals2 it will be thrown into an undulating curve lying all in one plane4 ,if to this motion there be superadded another similar and equal1 commencing exactly half an undulation later than the first1 it is evident that the direct motion every molecule will assume1 in consequence of the first system of waves1 will at every instant be exactly neutralized by the retrograde motion it would take in virtue of the second2 and the cord itself will be quiescent in consequence of the interference4 ,but1 if the second system of waves be in a plane perpendicular to the first1 the effect would only be to twist the rope1 so that no interference would take place4 ,rays polarized at right angles to each other may subsequently be brought into the same plane without acquiring the property of producing coloured fringes2 but1 if they belong to a pencil the whole of which was originally polarized in the same plane1 they will interfere4 ,the manner in which the coloured images are formed may be conceived by considering that1 when polarized light passes through the optic axis of a doubly refracting substance1--as mica1 for example1--it is divided into two pencils by the analyzing tourmaline2 and1 as one ray is absorbed1 there can be no interference4 ,but1 when polarized light passes through the mica in any other direction1 it is separated into two white rays1 and these are again divided into four pencils by the tourmaline1 which absorbs two of them2 and the other two1 being transmitted in the same plane with different velocities1 interfere and produce the coloured phenomena4 ,if the analysis be made with ,iceland spar1 the single ray passing through the optic axis of the mica will be refracted into two rays1 polarized in different planes1 and no interference will happen4 ,but1 when two rays are transmitted by the mica1 they will be separated into four by the spar1 two of which will interfere to form one image1 and the other two1 by their interference1 will produce the complementary colours of the other image when the spar has revolved through #ij2 because1 in such positions of the spar as produce the coloured images1 only two rays are visible at a time1 the other two being reflected4 ,when the analysis is accomplished by reflection1 if two rays are transmitted by the mica1 they are polarized in planes at right angles to each other4 ,and1 if the plane of reflection of either of these rays be at right angles to the plane of polarization1 only one of them will be reflected1 and therefore no interference can take place2 but in all other positions of the analyzing plate both rays will be reflected in the same plane1 and consequently will produce coloured rings by their interference4 ,it is evident that a great deal of the light we see must be polarized1 since most bodies which have the power of reflecting or refracting light also have the power of polarizing it4 ,the blue light of the sky is completely polarized at an angle of #gd from the sun in a plane passing through his centre4 ,a constellation of talent almost unrivalled at any period in the history of science has contributed to the theory of polarization1 though the original discovery of that property of light was accidental1 and arose from an occurrence which1 like thousands of others1 would have passed unnoticed had it not happened to one of those rare minds capable of drawing the most important inferences from circumstances apparently trifling4 ,in #ahjh1 while ,m4 ,malus was accidentally viewing with a doubly-refracting prism a brilliant sunset reflected from the windows of the ,luxembourg ,palace in ,paris1 on turning the prism slowly round1 he was surprised to see a very great difference in the intensity of the two images1 the most refracted alternately changing from brightness to obscurity at each quadrant of revolution4 ,a phenomenon so unlooked for induced him to investigate its cause1 whence sprung one of the most elegant and refined branches of physical optics4 ,fluorescence1 or the internal dispersion of light1 though far from possessing the beauty or extensive consequences of polarized light1 is scarcely less wonderful4 ,a variety of substances1 such as canary-glass1 a solution of sulphate of quinine1 fluor-spar1 and a great number of organic substances1 have the property of diminishing the refrangibility of light by internal dispersion1 consequently of increasing the length of the waves1 and lowering the colour in the prismatic scale2 it is therefore called degraded light1 or fluorescence1 because first discovered in fluor-spar4 ,if a piece of glass coloured by cobalt be fixed in a hole in a window-shutter of a dark room1 a slab of white porcelain placed near it will appear blue2 but if the slab be viewed through a yellow glass coloured by silver1 it will appear to be almost quite black1 because the yellow glass absorbs all the rays transmitted by the blue glass4 ,if1 however1 a piece of canary-glass be laid on the slab while it is dark1 every part of the canary-glass will shine as if it were self-luminous1 and with so bright a light that anything written on the slab that was invisible before may now be distinctly read4 ,such is the singular phenomenon of internal dispersion1 degraded light1 or fluorescence4 ,the brightness is by no means due to phosphorescence1 because the canary-glass only shines when under the influence of the active or blue rays1 whereas phosphorescent bodies shine by their own light--the latter has independent1 the former dependent1 emission2 it is possible1 however1 that a connexion may hereafter be traced between them4 ,it appears from the analytical investigation of this phenomenon that the vibrations of the fluorescent substance are analogous to those of a sonorous body1 as a bell or musical cord1 which give the fundamental note and its harmonics4 ,now since there is a reciprocal action between the molecules of matter and light1 when the light of the sun is absorbed by a substance capable of fluorescence1 it puts the whole of its molecules into vibrations the same as its own1 analogous to the fundamental note1 while at the same time a certain number of molecules take more rapid vibrations exactly like the harmonics4 ,the latter form new centres of light throughout the substance1 which impart their vibrations to the ethereal medium around1 and constitute fluorescence or degraded light4 ,for example1 in the experiment that has been described1 the blue light imparted its own vibrations to all the molecules of the canary-glass1 and also more rapid vibrations to a certain number of them4 ,all of the blue rays were excluded by the yellow glass held before the eye2 but it was pervious to the rays emanating in more rapid vibrations from the smaller number of molecules1 which thus became really new centres of light1 different from the sun's light1 though owing to it2 the one celestial1 the other terrestrial2 and the latter vibrations being more rapid than those of the blue light1 their refrangibility was less1 and therefore their colour lower in the prismatic scale4 ,mr4 ,power computed from his formul1 that fluorescent light is produced by undulations which are a major or minor third below the pitch of the general vibration of the medium--that is to say1 below the vibrations which the whole molecules of the body most readily assume4 ,professor ,stokes1 of ,cambridge1 who made the preceding experiment1 found that the chemical rays from a point in the solar spectrum produced1 in a solution of the sulphate of quinine1 light of a sky-blue colour1 which emanates in all directions from the liquid1 and that this blue fluorescent light contains1 when analysed1 all the rays of the spectrum2 hence he inferred that the dispersive power or fluorescence had lowered the refrangibility of the chemical rays1 so as to make them visible3 and ,sir ,david ,brewster observes that the new spectrum1 of all colours into which they were transformed1 must possess the extraordinary property of being a luminous spectrum1 either without chemical rays or full of them4 ,the dispersion in the quinine solution is greatest near the surface1 but the blue emanation proceeds from every part of the liquid2 and ,sir ,john ,herschel1 who discovered the fluorescent property in this liquid1 and gave it the name of epipolic light1 found that the remainder of the beam1 when it issued from the solution1 though not apparently different from the incident white light1 is yet so much changed in passing through the liquid1 that it is no longer capable of producing fluorescence1 though still capable of common dispersion4 ,the blue light from the solution of quinine1 when examined1 consisted of rays extending over a great part of the spectrum4 ,by passing a sunbeam through a bluish kind of fluor-spar1 ,sir ,david ,brewster perceived that the blue colour is not superficial1 as it appears to be1 but that some veins in the interior of the crystal disperse blue light1 others pink1 and even white light2 in short1 he met with fluorescence in such a variety of substances1 that he concludes it may prevail more or less in the greater number of solids and liquids4 ,professor ,draper1 of ,new ,york1 proved that the result is the same whether the incident light be polarized or not1 and that the dispersed or degraded light is never polarized1 but that it emanates in all directions1 as if the substance were self-luminous2 he made experiments with light from all parts of the solar spectrum1 and with various substances1 and always found that the refrangibility of the incident ray was diminished by internal dispersion1 and that the colour was changed to suit the new refrangibility4 ,professor ,draper has also shown that the law of action and reaction prevails in all the phenomena of the sunbeam1 as in every other department of nature2 so that a beam cannot be reflected1 refracted1 much less absorbed1 without producing some change upon the recipient medium2 and ,mr4 ,power proved analytically that the solar rays can exercise no action upon any medium through which they are transmitted1 without being accompanied by a diminution of refraction4 ,he says1 8,the new light emanating from the fluorescent media is just like any other light of the same prismatic composition4 ,in its physical properties it retains no trace of its parentage2 it is of terrestrial origin1 and its colour depends simply on its new refrangibility1 having nothing to do with that of the producing rays1 nor to the circumstance of their belonging to the visible or invisible part of the spectrum40 ,these phenomena can only be explained by the undulatory theory of light4 ,,section ,,xxiii4 ,objections to the ,undulatory ,theory1 from a difference in the ,action of ,sound and ,light under the same circumstances1 removed--,the ,dispersion of ,light according to the ,undulatory ,theory--,arago's final proof that the ,undulatory ,theory is the ,law of ,nature4 ,,the numerous phenomena of periodical colours arising from the interference of light1 which do not admit of satisfactory explanation on any other principle than the undulatory theory1 are the strongest arguments in favour of that hypothesis2 and even cases which at one time seemed unfavourable to that doctrine have proved upon investigation to proceed from it alone4 ,such is the erroneous objection which has been made1 in consequence of a difference in the mode of action of light and sound1 under the same circumstances1 in one particular instance4 ,when a ray of light from a luminous point1 and a diverging sound1 are both transmitted through a very small hole into a dark room1 the light goes straight forward and illuminates a small spot on the opposite wall1 leaving the rest in darkness2 whereas the sound on entering diverges in all directions1 and is heard in every part of the room4 ,these phenomena1 however1 instead of being at variance with the undulatory theory1 are direct consequences of it1 arising from the very great difference between the magnitude of the undulations of sound and those of light4 ,the undulations of light are incomparably less than the minute aperture1 while those of sound are much greater4 ,therefore when light1 diverging from a luminous point1 enters the hole1 the rays round its edges are oblique1 and consequently of different lengths1 while those in the centre are direct1 and nearly or altogether of the same lengths4 ,so that the small undulations between the centre and the edges are in different phases1 that is1 in different states of undulation4 ,therefore the greater number of them interfere1 and by destroying one another produce darkness all around the edges of the aperture2 whereas the central rays1 having the same phases1 combine1 and produce a spot of bright light on a wall or screen directly opposite the hole4 ,the waves of air producing sound1 on the contrary1 being very large compared with the hole1 do not sensibly diverge in passing through it1 and are therefore all so nearly of the same length1 and consequently in the same phase or state of undulation1 that none of them interfere sufficiently to destroy one another4 ,hence all the particles of air in the room are set into a state of vibration1 so that the intensity of the sound is very nearly everywhere the same4 ,strong as the preceding cases may be1 the following experiment1 made by ,m4 ,arago1 seems to be decisive in favour of the undulatory doctrine4 ,suppose a plano-convex lens of very great radius to be placed upon a plate of very highly polished metal4 ,when a ray of polarized light falls upon this apparatus at a very great angle of incidence1 ,newton's rings are seen at the point of contact4 ,but as the polarizing angle of glass differs from that of metal1 when the light falls on the lens at the polarizing angle of glass1 the black spot and the system of rings vanish4 ,for although light in abundance continues to be reflected from the surface of the metal1 not a ray is reflected from the surface of the glass that is in contact with it1 consequently no interference can take place2 which proves beyond a doubt that ,newton's rings result from the interference of the light reflected from both the surfaces apparently in contact 7,n4 #aii74 ,notwithstanding the successful adaptation of the undulatory system to phenomena1 the dispersion of light for a long time offered a formidable objection to that theory1 which has been removed by ,professor ,powell of ,oxford4 ,a sunbeam falling on a prism1 instead of being refracted to a single point of white light1 is separated into its component colours1 which are dispersed or scattered unequally over a considerable space1 of which the portion occupied by the red rays is the least1 and that over which the violet rays are dispersed is the greatest4 ,thus the rays of the coloured spectrum1 whose waves are of different lengths1 have different degrees of refrangibility1 and consequently move with different velocities1 either in the medium which conveys the light from the sun1 or in the refracting medium1 or in both2 whereas rays of all colours come from the sun to the earth with the same velocity4 ,if1 indeed1 the velocities of the various rays were different in space1 the aberration of the fixed stars1 which is inversely as the velocity1 would be different for different colours1 and every star would appear as a spectrum whose length would be parallel to the direction of the earth's motion1 which is not found to agree with observation4 ,besides1 there is no such difference in the velocities of the long and short waves of air in the analogous case of sound1 since notes of the lowest and highest pitch are heard in the order in which they are struck4 ,in fact1 when the sunbeam passes from air into the prism1 its velocity is diminished2 and1 as its refraction1 and consequently its dispersion1 depend solely upon the diminished velocity of the transmission of its waves1 they ought to be the same for waves of all lengths1 unless a connexion exists between the length of a wave and the velocity with which it is propagated4 ,now1 this connexion between the length of a wave of any colour1 and its velocity or refrangibility in a given medium1 has been deduced by ,professor ,powell from ,m4 ,cauchy's investigations of the properties of light on a peculiar modification of the undulatory hypothesis4 ,hence the refrangibility of the various coloured rays1 computed from this relation for any given medium1 when compared with their refrangibility in the same medium determined by actual observation1 will show whether the dispersion of light comes under the laws of that theory4 ,but1 in order to accomplish this1 it is clear that the length of the waves should be found independently of refraction1 and a very beautiful discovery of ,m4 ,fraunhofer furnishes the means of doing so4 ,that philosopher obtained a perfectly pure and complete coloured spectrum1 with all its dark and bright lines1 by the interference of light alone1 from a sunbeam passing through a series of fine parallel wires covering the object glass of a telescope4 ,in this spectrum1 formed independently of prismatic refraction1 the positions of the coloured rays depend only on the lengths of their waves1 and ,m4 ,fraunhofer found that the intervals between them are precisely proportional to the differences of these lengths4 ,he measured the lengths of the waves of the different colours at seven fixed points1 determined by seven of the principal dark and bright lines4 ,professor ,powell1 availing himself of these measures1 has made the requisite computations1 and has found that the coincidence of theory with observation is perfect for ten substances whose refrangibility had been previously determined by the direct measurements of ,m4 ,fraunhofer1 and for ten others whose refrangibility has more recently been ascertained by ,m4 ,rudberg4 ,thus1 in the case of seven rays in each of twenty different substances1 solid and fluid1 the dispersion of light takes place according to the laws of the undulatory theory3 and there can hardly be a doubt that dispersion in all other bodies will be found to follow the same law4 ,it is1 however1 an express condition of the connexion between the velocity of light and the length of its undulations1 that the intervals between the vibrating molecules of the ethereal fluid should bear a sensible relation to the length of an undulation4 ,the coincidence of the computed with the observed refractions shows that this condition is fulfilled within the refracting media2 but the aberration of the fixed stars leads to the inference that it does not hold in the ethereal regions1 where the velocities of the rays of all colours are the same4 ,strong as all that precedes is in favour of the undulatory theory1 the relative velocity of light in air and water is the final and decisive proof4 ,by the ,newtonian theory the velocity is greater in water than in air1 by the undulatory theory it is less2 hence if a comparison could be made it would decide which is the law of nature4 ,the difficulty consisted in comparing the velocity of light passing through a small extent of water with the velocity of light in air1 which is #aj1jjj times greater than the velocity of the earth in its orbit4 ,this delicate and difficult experiment was made by means of an instrument invented by ,professor ,wheatstone for measuring the velocity of electricity4 ,it consists of a small mirror which revolves in its own plane like a coin spinning on its edge4 ,when it revolves very rapidly the reflected image of an object changes its place perceptibly in an inconceivably small fraction of a second4 ,the mirrors used in the experiment were made to revolve more than #ajjj times in a second1 by which means the places of the two images--one from light passing through air1 and the other from light passing through an equal length of water--were found to be such as to prove that the velocity of light in air and in water is as #d to #c1 while by the ,newtonian theory it is as #c to #d4 ,by this final and decisive proof the undulatory theory may from henceforth be regarded as the law of nature4 ,this experiment was accomplished by ,m4 ,fizeau and ,m4 ,lon-,faucault1 at the suggestion of ,m4 ,arago1 whose eyesight did not permit him to undertake it himself4 ,,section ,,xxiv4 ,chemical or ,photographic ,rays of ,solar ,spectrum--,scheele1 ,ritter1 and ,wollaston's ,discoveries--,wedgwood's and ,sir ,humphry ,davy's ,photographic ,pictures--,the ,calotype--,the ,daguerreotype--,the ,chromatype--,the ,cyanotype--,collodion--,sir ,john ,herschel's ,discoveries in the ,chemical ,spectrum--,m4 ,becquerel's ,discoveries of ,inactive ,lines in ditto--,thermic ,spectrum--,phosphoric ,spectrum--,electrical ,properties--,parathermic ,rays--,moser and ,hunt's ,experiments--,general ,structure and antagonist ,properties of ,solar ,spectrum--,defracted ,spectrum4 ,,the ,solar ,spectrum exercises an energetic action on matter1 producing the most wonderful and mysterious changes on the organised and unorganised creation4 ,all bodies are probably affected by light1 but it acts with greatest energy on such as are of weak chemical affinity1 imparting properties to them which they did not possess before4 ,collodion and metallic salts1 especially those of silver1 whose molecules are held together by an unstable equilibrium1 are of all bodies the most susceptible of its influence2 the effects1 however1 vary with the substances employed1 and with the different rays of the solar spectrum1 the chemical properties of which are by no means alike4 ,as early as #aggb ,m4 ,scheele showed that the pure white colour of chloride of silver was rapidly darkened by the blue rays of the solar spectrum1 while the red rays had no effect upon it3 and in #ahja ,m4 ,ritter discovered that invisible rays beyond the violet extremity have the property of blackening argentine salts1 that this property diminishes towards the less refrangible part of the spectrum1 and that the red rays have an opposite quality1 that of restoring the blackened salt of silver to its original purity2 from which he inferred that the most refrangible extremity of the spectrum has an oxygenising power1 and the other that of deoxygenating4 ,dr4 ,wollaston found that gum guaiacum acquires a green colour in the violet and blue rays1 and resumes its original tint in the red4 ,no attempt had been made to trace natural objects by means of light reflected from them1 till ,mr4 ,wedgwood1 together with ,sir ,humphry ,davy1 took up the subject3 they produced profiles and tracings of objects on surfaces prepared with nitrate and chloride of silver1 but they did not succeed in rendering their pictures permanent4 ,this difficulty was overcome in #ahad by ,m4 ,niepc1 who produced a permanent picture of surrounding objects by placing in the focus of a camera-obscura a metallic plate covered with a film of asphalt dissolved in oil of lavender4 ,mr4 ,fox ,talbot1 without any knowledge of ,m4 ,niepc's experiments1 had been engaged in the same pursuit1 and must be regarded as an independent inventor of photography1 one of the most beautiful arts of modern times3 he was the first who succeeded in using paper chemically prepared for receiving impressions from natural objects2 and he also discovered a method of fixing permanently the impressions--that is1 of rendering the paper insensible to any further action of light4 ,in the calotype1 one of ,mr4 ,talbot's applications of the art1 the photographic surface is prepared by washing smooth writing-paper1 first with a solution of nitrate of silver1 then with bromide of potassium1 and again with nitrate of silver1 drying it at a fire after each washing2 the paper is thus rendered so sensitive to light that even the passage of a thin cloud is perceptible on it1 consequently it must be prepared by candle-light4 ,portraits1 buildings1 insects1 leaves of plants--in short1 every object is accurately delineated in a few seconds2 and in the focus of a camera-obscura the most minute objects are so exactly depicted that the microscope reveals new beauties4 ,since the effect of the chemical agency of light is to destroy the affinity between the salt and the silver1 ,mr4 ,talbot found that1 in order to render these impressions permanent on paper1 it was only necessary to wash it with salt and water1 or with a solution of iodide of potassium4 ,for these liquids the liquid hyposulphites have been advantageously substituted1 which are the most efficacious in dissolving and removing the unchanged salt1 leaving the reduced silver on the paper4 ,the calotype picture is negative1 that is1 the lights and shadows are the reverse of what they are in nature1 and the right-hand side in nature is the left in the picture2 but if it be placed with its face pressed against photographic paper1 between a board and a plate of glass1 and exposed to the sun a short time1 a positive and direct picture1 as it is in nature1 is formed3 engravings may be exactly copied by this simple process1 and a direct picture may be produced at once by using photographic paper already made brown by exposure to light4 ,while ,mr4 ,fox ,talbot was engaged in these very elegant discoveries in ,england1 ,m4 ,daguerre had brought to perfection and made public that admirable process by which he has compelled ,nature permanently to engrave her own works2 and thus the talents of ,france and ,england have been combined in bringing to perfection this useful art4 ,copper1 plated with silver1 was successfully employed by ,m4 ,daguerre for copying nature by the agency of light4 ,the surface of the plate is converted into an iodide of silver1 by placing it horizontally with its face downwards in a covered box1 in the bottom of which there is a small quantity of iodine which evaporates spontaneously4 ,in three or four minutes the surface acquires a yellow tint1 and then1 screening it carefully from light1 it must be placed in the focus of a camera obscura1 where an invisible image of external objects will be impressed on it in a few minutes4 ,when taken out1 the plate must be exposed in another box to the action of mercurial vapour1 which attaches itself to those parts of the plate which had been exposed to light1 but does not adhere to such parts as had been in shadow2 and as the quantity of mercury over the other parts is in exact proportion to the degree of illumination1 the shading of the picture is perfect4 ,the image is fixed1 first by removing the iodine from the plate by plunging it into hyposulphite of soda1 and then washing it in distilled water2 by this process the yellow colour is destroyed1 and in order to render the mercury permanent1 the plate must be exposed a few minutes to nitric vapour1 then placed in nitric acid containing copper or silver in solution at a temperature of #fa-a_/d of ,fahrenheit for a short time1 and lastly polished with chalk4 ,this final part of the process is due to ,dr4 ,berre1 of ,vienna4 ,nothing can be more beautiful than the shading of these chiaroscuro pictures when objects are at rest1 but the least motion destroys the effect2 the method therefore is more applicable to buildings than landscape4 ,colour is wanting2 but the researches of ,sir ,john ,herschel give reason to believe that even this will ultimately be attained4 ,the most perfect impressions of seaweeds1 leaves of plants1 feathers1 c41 may be formed by bringing the object into close contact with a sheet of photographic paper1 between a board and plate of glass2 then exposing the whole to the sun for a short time1 and afterwards fixing it by the process described4 ,the colours of the pictures vary with the preparation of the paper1 by which almost any tint may be produced4 ,in the chromatype1 a peculiar photograph discovered by ,mr4 ,hunt1 chromate of copper is used1 on which a dark brown negative image is first formed1 but by the continued action of light it is changed to a positive yellow picture on a white ground2 the farther effect of light is checked by washing the picture in pure water4 ,in cyanotypes1 a class of photographs discovered by ,sir ,john ,herschel1 in which cyanogen in its combinations with iron forms the ground1 the pictures are ,prussian blue and white4 ,in the chrysotype of the same eminent philosopher1 the image is first received on paper prepared with the ammonia-citrate of iron1 and afterwards washed with a neutral solution of gold4 ,it is fixed by water acidulated with sulphuric acid1 and lastly by hydriodate of potash1 from which a white and purple photograph results4 ,it is vain to attempt to describe the various beautiful effects which ,sir ,john ,herschel obtained from chemical compounds1 and from the juices of plants2 the juice of the red poppy gives a positive bluish purple image1 that of the ten-week stock a fine rose colour on a pale straw-coloured ground4 ,pictures may be made by exposure to sunshine1 on all compound substances having a weak chemical affinity2 but the image is often invisible1 as in the ,daguerreotype1 till brought out by washing in some chemical preparation4 ,water is frequently sufficient2 indeed ,sir ,john ,herschel brought out dormant photographs by breathing on them1 and some substances are insensible to the action of light till moistened1 as for example1 gum guaiacum4 ,argentine papers1 however1 are little subject to the influence of moisture4 ,the power of the solar rays is augmented in certain cases by placing a plate of glass in close contact over the sensitive surface4 ,all these various experiments1 though highly interesting1 have now been superseded4 ,it was found that paper did not always answer for photography1 on account of imperfections in its structure2 silver plates were too expensive2 and glass was found to be unimpressable4 ,nevertheless1 ,m4 ,niepc de ,victor obtained beautiful results upon glass coated with albumen mixed with sensitive substances1 which suggested the medium by means of which the art has been brought to its present perfection1 and that final step is due to ,mr4 ,scott ,archer4 ,he coated a plate of glass thinly with collodion1 that is1 gun-cotton dissolved in ether and alcohol1 which dries into a delicate transparent film of extreme adhesiveness1 and of such intense sensibility that the action of light upon it is so instantaneous that it arrests a stormy sea or a fleeting cloud before they have time to change4 ,now landscapes in chiaroscuro are produced of great beauty1 which by the slower methods were mere masses of deep shade and broad light4 ,architecture is even more perfectly obtained1 but it fails to give a pleasing representation of the human countenance4 ,chemical action always accompanies the sun's light1 but the analysis of the solar spectrum has partly disclosed the wonderful nature of the emanation4 ,in the research1 properties most important and unexpected have been discovered by ,sir ,john ,herschel1 who imprints the stamp of genius on all he touches--his eloquent papers can alone convey an adequate idea of their value in opening a field of inquiry vast and untrodden4 ,the following brief and imperfect account of his experiments is all that can be attempted here3-- ,a certain degree of chemical energy is distributed through every part of the solar spectrum1 and also to a considerable extent through the dark spaces at each extremity4 ,this distribution does not depend on the refrangibility of the rays alone1 but also on the nature of the rays themselves1 and on the physical properties of the analyzing medium on which the rays are received1 whose changes indicate and measure their action4 ,the length of the photographic image of the same solar spectrum varies with the physical qualities of the surface on which it is impressed4 ,when the solar spectrum is received on paper prepared with bromide of silver1 the chemical spectrum1 as indicated merely by the length of the darkened part1 includes within its limits the whole luminous spectrum1 extending in one direction far beyond the extreme violet and lavender rays1 and in the other down to the extremest red3 with tartrate of silver the darkening occupies not only all the space under the most refrangible rays1 but reaches much beyond the extreme red4 ,on paper prepared with formobenzoate of silver the chemical spectrum is cut off at the orange rays1 with phosphate of silver in the yellow1 and with chloride of gold it terminates with the green1 with carbonate of mercury it ends in the blue1 and on paper prepared with the percyanide of gold1 ammonia1 and nitrate of silver1 the darkening lies entirely beyond the visible spectrum at its most refrangible extremity1 and is only half its length1 whereas in some cases chemical action occupies a space more than twice the length of the luminous image4 ,the point of maximum energy of chemical action varies as much for different preparations as the scale of action4 ,in the greater number of cases the point of deepest blackening lies about the lower edge of the indigo rays1 though in no two cases is it exactly the same1 and in many substances it is widely different4 ,on paper prepared with the juice of the ten-week stock 7,mathiola annua7 there are two maxima1 one in the mean yellow and a weaker in the violet2 and on a preparation of tartrate of silver ,sir ,john ,herschel found three1 one in the least refrangible blue1 one in the indigo1 and a third beyond the visible violet4 ,the decrease in photographic energy is seldom perfectly alike on both sides of the maximum4 ,thus at the most refrangible end of the solar spectrum the greatest chemical power is exerted in most instances where there is least light and heat1 and even in the space where both sensibly cease4 ,not only the intensity but the kind of action is different in the different points of the solar spectrum1 as evidently appears from the various colours that are frequently impressed on the same analyzing surface1 each ray having a tendency to impart its own colour4 ,sir ,john ,herschel obtained a coloured image of the solar spectrum on paper prepared according to ,mr4 ,talbot's principle1 from a sunbeam refracted by a glass prism and then highly condensed by a lens4 ,the photographic image was rapidly formed and very intense1 and1 when withdrawn from the spectrum and viewed in common daylight1 it was found to be coloured with sombre but unequivocal tints imitating the prismatic colours1 which varied gradually from red through green and blue to a purplish black4 ,after washing the surface in water1 the tints became more decided by being kept a few days in the dark--a phenomenon1 ,sir ,john observes1 of constant occurrence1 whatever be the preparation of the paper1 provided colours are produced at all4 ,he also obtained a coloured image on nitrate of silver1 the part under the blue rays becoming a blue brown1 while that under the violet had a pinkish shade1 and sometimes green appeared at the point corresponding to the least refrangible blue4 ,mr4 ,hunt found on a paper prepared with fluoride of silver that a yellow line was impressed on the space occupied by the yellow rays1 a green band on the space under the green rays1 an intense blue throughout the space on which the blue and indigo rays fell1 and under the violet rays a ruddy brown appeared2 these colours remained clear and distinct after being kept two months4 ,notwithstanding the great variety in the scale of action of the solar spectrum1 the darkening or deoxydizing principle that prevails in the more refrangible part rarely surpasses or even attains the mean yellow ray which is the point of maximum illumination2 it is generally cut off abruptly at that point which seems to form a limit between the opposing powers which prevail at the two ends of the spectrum4 ,the bleaching or oxydizing effect of the red rays on blackened muriate of silver discovered by ,m4 ,ritter of ,jena1 and the restoration by the same rays of discoloured gum guaiacum to its original tint by ,dr4 ,wollaston1 have already been mentioned as giving the first indications of that difference in the mode of action of the chemical rays at the two ends of the visible spectrum1 now placed beyond a doubt4 ,the action exerted by the less refrangible rays beyond and at the red extremity of the solar spectrum1 in most instances1 so far from blackening metallic salts1 protects them from the action of the diffused daylight3 but1 if the prepared surface has already been blackened by exposure to the sun1 they possess the remarkable property of bleaching it in some cases1 and under other circumstances of changing the black surface into a fiery red4 ,sir ,john ,herschel1 to whom we owe most of our knowledge of the properties of the chemical spectrum1 prepared a sheet of paper by washing it with muriate of ammonia1 and then with two coats of nitrate of silver2 on this surface he obtained an impression of the solar spectrum exhibiting a range of colours very nearly corresponding with its natural hues4 ,but a very remarkable phenomenon occurred at the end of least refrangibility2 the red rays exerted a protecting influence which preserved the paper from the change which it would otherwise have undergone from the deoxydizing influence of the dispersed light which always surrounds the solar spectrum1 and this maintained its whiteness4 ,sir ,john met with another instance on paper prepared with bromide of silver1 on which the whole of the space occupied by the visible spectrum was darkened down to the very extremity of the red rays1 but an oxydizing action commenced beyond the extreme red1 which maintained the whiteness of the paper to a considerable distance beyond the last traceable limit of the visible rays1 thus evincing decidedly the existence of some chemical power over a considerable space beyond the least refrangible end of the spectrum4 ,mr4 ,hunt also found that on the ,daguerreotype plate a powerful protecting influence is exercised by the extreme red rays4 ,in these cases the red and those dark rays beyond them exert an action of an opposite nature to that of the violet and lavender rays4 ,the least refrangible part of the solar spectrum possesses also1 under certain circumstances1 a bleaching property1 by which the metallic salts are restored to their original whiteness after being blackened by exposure to common daylight1 or to the most refrangible rays of the solar spectrum4 ,paper prepared with iodide of silver1 when washed over with ferrocyanite of potash1 blackens rapidly when exposed to the solar spectrum4 ,it begins in the violet rays and extends over all the space occupied by the dark chemical rays1 and over the whole visible spectrum down to the extreme red rays4 ,this image is coloured1 the red rays giving a reddish tint and the blue a blueish4 ,in a short time a bleaching process begins under the red rays1 and extends upwards to the green1 but the space occupied by the extreme red is maintained perfectly dark4 ,mr4 ,hunt found that a similar bleaching power is exerted by the red rays on paper prepared with protocyanide of potassium and gold with a wash of nitrate of silver4 ,the application of a moderately strong hydriodate of potash to darkened photographic paper renders it peculiarly susceptible of being whitened by further exposure to light4 ,if paper prepared with bromide of silver be washed with ferrocyanate of potash while under the influence of the solar spectrum1 it is immediately darkened throughout the part exposed to the visible rays down to the end of the red1 some slight interference being perceptible about the region of the orange and yellow4 ,after this a bleaching action begins over the part occupied by the red rays1 which extends to the green4 ,by longer exposure an oval spot begins again to darken about the centre of the bleached space2 but1 if the paper receive another wash of the hydriodate of potash1 the bleaching action extends up from the green1 over the region occupied by the most refrangible rays and considerably beyond them1 thus inducing a negative action in the most refrangible part of the spectrum4 ,in certain circumstances the red rays1 instead of restoring darkened photographic paper to its original whiteness1 produce a deep red colour4 ,when ,sir ,john ,herschel received the spectrum on paper somewhat discoloured by exposure to direct sunshine1 instead of whiteness1 a red border was formed extending from the space occupied by the orange1 and nearly covering that on which the red fell4 ,when1 instead of exposing the paper in the first instance to direct sunshine1 it was blackened by the violet rays of a prismatic spectrum1 or by a sunbeam that had undergone the absorptive action of a solution of ammonia-sulphate of copper1 the red rays of the condensed spectrum produced on it1 not whiteness1 but a full and fiery red1 which occupied the whole space on which any of the visible red rays had fallen2 and this red remained unchanged1 however long the paper remained exposed to the least refrangible rays4 ,sunlight transmitted through red glass produces the same effect as the red rays of the spectrum in the foregoing experiment4 ,sir ,john ,herschel placed an engraving over a paper blackened by exposure to sunshine1 covering the whole with a dark red-brown glass previously ascertained to absorb every ray beyond the orange3 in this way a photographic copy was obtained in which the shades were black1 as in the original engraving2 but the lights1 instead of being white1 were of the red colour of venous blood1 and no other colour could be obtained by exposure to light1 however long4 ,sir ,john ascertained that every part of the spectrum impressed by the more refrangible rays is equally reddened1 or nearly so1 by the subsequent action of the less refrangible2 thus the red rays have the very remarkable property of assimilating to their own colour the blackness already impressed on photographic paper4 ,that there is a deoxydating property in the more refrangible rays1 and an oxydating action in the less refrangible part of the spectrum1 is manifest from the blackening of one and the bleaching effect of the other2 but the peculiar action of the red rays in the experiments mentioned shows that some other principle exists different from contrariety of action4 ,these opposite qualities are balanced or neutralized in the region of the mean yellow ray4 ,but1 although this is the general character of the photographic spectrum1 under certain circumstances even the red rays have a deoxydating power1 while the blue and violet exert a contrary influence2 but these are rare exceptions4 ,the photographic action of the two portions of the solar spectrum being so different1 ,sir ,john ,herschel tried the effect of their united action by superposing the less refrangible part of the spectrum over the more refrangible portion by means of two prisms2 and he thus discovered that two rays of different refrangibility1 and therefore of different lengths of undulation1 acting simultaneously1 produce an effect which neither1 acting separately1 can do4 ,some circumstances that occurred during the analysis of the chemical spectrum seem to indicate an absorptive action in the sun's atmosphere4 ,the spectral image impressed on paper prepared with nitrate of silver and ,rochelle salt commenced at1 or very little below1 the mean yellow ray1 of a delicate lead colour2 and when the action was arrested1 such was the character of the whole photographic spectrum4 ,but1 when the light of the solar spectrum was allowed to continue its action1 there was observed to come on suddenly a new and much more intense impression of darkness1 confined in length to the blue and violet rays2 and1 what is most remarkable1 confined also in breadth to the middle of the sun's image1 so far at least as to leave a border of the lead-coloured spectrum traceable1 not only round the clear and well-defined convexity of the dark interior spectrum at the less refrangible end1 but also laterally along both its edges2 and this border was the more easily traced1 and less liable to be mistaken1 from its striking contrast of colour with the interior spectrum1 the former being lead gray1 the latter an extremely rich deep velvety brown4 ,the less refrangible end of this interior brown spectrum presented a sharply terminated and regularly elliptical contour1 the more refrangible a less decided one4 8,it may seem too hazardous10 ,sir ,john continues1 8to look for the cause of this very singular phenomenon in a real difference between the chemical agencies of those rays which issue from the central portion of the sun's disc1 and those which1 emanating from its borders1 have undergone the absorptive action of a much greater depth of its atmosphere2 and yet ,i confess myself somewhat at a loss what other cause to assign for it4 ,it must suffice1 however1 to have thrown out the hint1 remarking only1 that ,i have other1 and ,i am disposed to think decisive1 evidence of the existence of an absorptive solar atmosphere extending beyond the luminous one40 ,m4 ,arago observed that the rays from the centre of the sun have a greater photographic power than those from the edges1 and the photographic images of the sun1 taken on glass by ,m4 ,niepc1 were blood-red1 much deeper in the centre1 and on one occasion the image was surrounded by an auriol4 ,several circumstances concur in showing that there are influences also concerned in the transmission of the photographic action which have not yet been explained1 as1 for example1 the influence which the time of the day exercises on the rapidity with which photographic impressions are made1 the sun being much less effective two hours after passing the meridian than two hours before4 ,there is also reason to suspect that the effect in some way depends on the latitude1 since a much longer time is required to obtain an image under the bright skies of the tropics than in ,england2 and it is even probable that there is a difference in the sun's light in high and low latitudes1 because an image of the solar spectrum1 obtained on a ,daguerreotype plate in ,virginia1 by ,dr4 ,draper1 differed from a spectral image obtained by ,mr4 ,hunt on a similar plate in ,england4 ,the inactive spaces discovered in the photographic spectrum by ,m4 ,e4 ,becquerel1 similar to those in the luminous spectrum1 and coinciding with them1 is also a phenomenon of which no explanation has yet been given2 possibly the chemical rays may be absorbed by the atmosphere with those of light4 ,although chemical action extends over the whole luminous spectrum1 and much beyond it1 in gradations of more or less intensity1 it is found by careful investigation to be by no means continuous2 numerous inactive lines cross it1 coinciding with those in the luminous image as far as it extends2 besides1 a very great number exist in the portions that are obscure1 and which overlap the visible part4 ,there are three extraspectral lines beyond the red1 and some strongly marked groups on the obscure part beyond the violet2 but the whole number of those inactive lines1 especially in the dark spaces1 is so great that it is impossible to count them4 ,notwithstanding this coincidence in the inactive lines of the two spectra1 photographic energy is independent of both light and heat1 since it exerts the most powerful influence in those rays where they are least1 and also in spaces where neither sensibly exist2 but the transmission of the sun's light through coloured media makes that independence quite evident4 ,heat and light pass abundantly through yellow glass1 or a solution of chromate of potash2 but the greater part of the chemical rays are excluded1 and chlorine gas diluted with common air1 though highly pervious to the luminous and calorific principles1 has the same effect4 ,sir ,john ,herschel found that a slight degree of yellow ,london fog had a similar effect with that of pale yellow media3 he also remarked that a weak solution of azolitmine in potash1 which admits a great quantity of green light1 excludes chemical action2 and some years ago the author1 while making experiments on the transmission of chemical rays1 observed that green glass1 coloured by oxide of copper about the #bjth of an inch thick1 excludes the photographic rays2 and1 as ,m4 ,melloni has shown that substance to be impervious to the most refrangible calorific rays1 it has the property of excluding the whole of the most refrangible part of the solar spectrum1 visible and invisible4 ,green mica1 if not too thin1 has also the same effect1 whereas amethyst1 deep blue1 and violet-coloured glasses1 though they transmit a very little light1 allow the chemical rays to pass freely4 ,thus light and photographic energy may be regarded as distinct parts of the solar beam1 and both being propagated by vibrations of the etherial medium they are merely motion4 ,excellent images have been obtained of the moon in its different phases by ,professor ,secchi1 at ,rome2 candlelight is nearly deficient of the chemical rays4 ,how far they may influence crystallization and other molecular arrangements is unknown1 but their power is universal wherever the solar beam falls1 although their effect only becomes evident in cases of unstable molecular equilibrium4 ,it is not by vision alone that a knowledge of the sun's rays is acquired3 touch proves that they have the power of raising the temperature of substances exposed to their action4 ,sir ,william ,herschel discovered that rays which produce the sensation of heat exist in the solar spectrum independent of those of light2 when he used a prism of flint glass1 he found that the warm rays are most abundant in the dark space a little beyond the red extremity of the spectrum1 that from thence they decrease towards the violet1 beyond which they are insensible4 ,it may be concluded therefore1 that the calorific rays vary in refrangibility1 and that those beyond the extreme red are less refrangible than any rays of light4 ,since ,sir ,william ,herschel's time it has been discovered that the calorific spectrum exceeds the luminous one in length in the ratio of #db to #be1 but the most singular phenomenon is its want of continuity4 ,sir ,john ,herschel blackened the under side of a sheet of very thin white paper by the smoke of a lamp1 and1 having exposed the white side to the solar spectrum1 he drew a brush dipped in spirit of wine over it1 by which the paper assumed a black hue when sufficiently saturated4 ,the heat in the spectrum evaporated the spirit first on those parts of the paper where it fell with greatest intensity1 thereby restoring their white colour1 and he thus discovered that the heat increases uniformly and gradually throughout the luminous spectrum1 and that it comes to a maximum and forms a spot at a considerable distance beyond the extreme red4 ,it then decreases1 but again increasing it forms a second maximum spot1 after which it ceases altogether through a short space1 but is again renewed and forms two more insulated spots1 and even a fifth may be traced at a little distance from the latter4 ,these circumstances are probably owing to the absorbing action of the atmospheres of the sun and earth4 8,the effect of the former10 says ,sir ,john1 8is beyond our control1 unless we could carry our experiments to such a point of delicacy as to operate separately on rays emanating from the centre and borders of the sun's disc2 that of the earth's1 though it cannot be eliminated any more than in the case of the sun's1 may yet be varied to a considerable extent by experiments made at great elevations1 under a vertical sun1 and compared with others where the sun is more oblique1 the situation lower1 and the atmospheric pressure of a temporarily high amount4 ,should it be found that this cause is in reality concerned in the production of the spots1 we should see reason to believe that a large portion of solar heat never reaches the earth's surface1 and that what is incident on the summits of lofty mountains differs not only in quantity but also in quality from what the plains receive40 ,a remarkable phosphorescent property was discovered by ,m4 ,e4 ,becquerel in the solar spectrum4 ,two luminous bands separated by a dark one are excited by the solar spectrum on paper covered with a solution of gum arabic1 and strewed with powdered sulphuret of calcium or ,canton's phosphorus4 ,one of the luminous bands occupies the space under the least refrangible violet rays1 and the other that beyond the lavender rays1 so that the dark band lies under the extreme violet and lavender rays4 ,when the action of the light is continued1 the whole surface beyond the least refrangible violet shines1 the luminous bands already mentioned brightest2 but all the space from the least refrangible violet to the extreme red remains dark4 ,if the surface1 prepared either with the sulphuret of calcium or ,bologna stone1 be exposed to the sun's light for a little time1 it becomes luminous all over2 but when1 in this state1 a solar spectrum is thrown upon it1 the whole remains luminous except the part from the least refrangible violet to the extreme red1 on which space the light is extinguished2 and when the temperature of the surface is raised by a lamp1 the bright parts become more luminous and the dark parts remain dark4 ,glass stained by the protoxide of copper1 which transmits only the red and orange rays1 has the same effect with the less refrangible part of the spectrum2 hence there can be no doubt that the most refrangible and obscure rays of the spectrum excite phosphorescence1 while all the less refrangible rays of light and heat extinguish it4 ,paper prepared with the sulphuret of barium1 when under the solar spectrum1 shows only one space of maximum luminous intensity1 and the destroying rays are the same as in the sulphuret of calcium4 ,thus the obscure rays beyond the extreme violet produce light1 while the luminous rays extinguish it4 ,the phosphoric spectrum has inactive lines which coincide with those in the luminous and chemical spectra1 at least as far as it extends2 but in order to be seen the spectrum must be received for a few seconds upon the prepared surface through an aperture in a dark room1 then the aperture must be closed1 and the temperature of the surface raised two or three hundred degrees2 the phosphorescent parts then shine brilliantly and the dark lines appear black4 ,since the parts of similar refrangibility in different spectra are traversed by the same dark lines1 rays of the same refrangibility are probably absorbed at the same time by the different media through which they pass4 ,it appears from the experiments of ,,mm4 ,becquerel and ,biot1 that electrical disturbances produce these phosphorescent effects4 ,there is thus a mysterious connexion between the most refrangible rays and electricity which the experiments of ,m4 ,e4 ,becquerel confirm1 showing that electricity is developed during chemical action by the violet rays1 that it is feebly developed by the blue and indigo1 but that none is excited by the less refrangible part of the spectrum4 ,a series of experiments by ,sir ,john ,herschel have disclosed a new set of obscure rays in the solar spectrum1 which seem to bear the same relation to those of heat that the photographic or chemical rays bear to the luminous4 ,they are situate in that part of the spectrum which is occupied by the less refrangible visible colours1 and have been named by their discoverer ,parathermic rays4 ,it must be held in remembrance that the region of greatest heat in the solar spectrum lies in the dark space beyond the visible red4 ,now1 ,sir ,john ,herschel found that in experiments with a solution of gum guaiacum in soda1 which gives the paper a green colour1 the green1 yellow1 orange1 and red rays of the spectrum invariably discharged the colour1 while no effect was produced by the extra-spectral rays of heat1 which ought to have had the greatest effect had heat been the cause of the phenomenon4 ,when an aqueous solution of chlorine was poured over a slip of paper prepared with gum guaiacum dissolved in soda1 a colour varying from a deep somewhat greenish hue to a fine celestial blue was given to it2 and1 when the solar spectrum was thrown on the paper while moist1 the colour was discharged from all the space under the less refrangible luminous rays1 at the same time that the more distant thermic rays beyond the spectrum evaporated the moisture from the space on which they fell2 so that the heat spots became apparent4 ,but the spots disappeared as the paper dried1 leaving the surface unchanged2 while the photographic impression within the visible spectrum increased in intensity2 the non-luminous thermic rays1 though evidently active as to heat1 were yet incapable of effecting that peculiar chemical change which other rays of much less heating power were all the time producing4 ,sir ,john having ascertained that an artificial heat from #ahj to #bhj of ,fahrenheit changed the green tint of gum guaiacum to its original yellow hue when moist1 but that it had no effect when dry1 he therefore tried whether heat from a hot iron applied to the back of the paper used in the last-mentioned experiment while under the influence of the solar spectrum might not assist the action of the calorific rays2 but1 instead of doing so1 it greatly accelerated the discoloration over the spaces occupied by the less refrangible rays1 but had no effect on the extra-spectral region of maximum heat4 ,obscure terrestrial heat1 therefore1 is capable of assisting and being assisted in effecting this peculiar change by those rays of the spectrum1 whether luminous or thermic1 which occupy its red1 yellow1 and green regions2 while1 on the other hand1 it receives no such assistance from the purely thermic rays beyond the spectrum acting under similar circumstances and in an equal state of condensation4 ,the conclusions drawn from these experiments are confirmed by that which follows3 a photographic picture formed on paper prepared with a mixture of the solutions of ammonia-citrate of iron and ferro-sesquicyanite of potash in equal parts1 then thrown into water and afterwards dried1 will be blue and negative1 that is to say1 the lights and shadows will be the reverse of what they are in nature4 ,if in this state the paper be washed with a solution of proto-nitrate of mercury1 the picture will be discharged2 but if it be well washed and dried1 and a hot smoothing-iron passed over it1 the picture instantly reappears1 not blue1 but brown2 if kept some weeks in this state in perfect darkness between the leaves of a portfolio1 it fades1 and almost entirely vanishes1 but a fresh application of heat restores it to its full original intensity4 ,this curious change is not the effect of light1 at least not of light alone4 ,a certain temperature must be attained1 and that suffices in total darkness2 yet1 on exposing to a very concentrated spectrum a slip of the paper used in the last experiment1 after the uniform blue colour has been discharged and a white ground left1 this whiteness is changed to brown over the whole region of the red and orange rays1 but not beyond the luminous spectrum4 ,sir ,john thence concludes3--#ast4 ,that it is the heat of these rays1 not their light1 which operates the change2 #bndly4 ,that this heat possesses a peculiar chemical quality which is not possessed by the purely calorific rays outside of the visible spectrum1 though far more intense2 and1 #crdly4 ,that the heat radiated from obscurely hot iron abounds especially in rays analogous to those of the region of the spectrum above indicated4 ,another instance of these singular transformations may be noticed4 ,the pictures formed on cyanotype paper rendered more sensitive by the addition of corrosive sublimate are blue on a white ground and positive1 that is1 the lights and shadows are the same as in nature1 but1 by the application of heat1 the colour is changed from blue to brown1 from positive to negative2 even by keeping in darkness the blue colour is restored1 as well as the positive character4 ,sir ,john attributes this1 as in the former instance1 to certain rays1 which1 regarded as rays of heat or light1 or of some influence sui generis accompanying the red and orange rays of the spectrum1 are also copiously emitted by bodies heated short of redness4 ,he thinks it probable that these invisible parathermic rays are the rays which radiate from molecule to molecule in the interior of bodies1 that they determine the discharge of vegetable colours at the boiling temperature1 and also the innumerable atomic transformations of organic bodies which take place at the temperature below redness1 that they are distinct from those of pure heat1 and that they are sufficiently identified by these characters to become legitimate objects of scientific discussion4 ,the calorific and parathermic rays appear to be intimately connected with the discoveries of ,messrs4 ,draper and ,moser4 ,daguerre has shown that the action of light on the iodide of silver renders it capable of condensing the vapour of mercury which adheres to the parts affected by it4 ,professor ,moser of ,knigsberg has proved that the same effect is produced by the simple contact of bodies1 and even by their very near juxtaposition1 and that in total darkness as well as in light4 ,this discovery he announced in the following words3--8,if a surface has been touched in any particular parts by any body1 it acquires the property of precipitating all vapours1 and these adhere to it or combine chemically with it on these spots differently from what they do on the untouched parts40 ,if we write on a plate of glass or any smooth surface whatever with blotting-paper1 a brush1 or anything else1 and then clean it1 the characters always reappear if the plate or surface be breathed upon1 and the same effect may be produced even on the surface of mercury2 nor is absolute contact necessary4 ,if a screen cut in a pattern be held over a polished metallic surface at a small distance1 and the whole breathed on1 after the vapour has evaporated so that no trace is left on the surface1 the pattern comes out when it is breathed on again4 ,professor ,moser proved that bodies exert a very decided influence upon each other1 by placing coins1 cut stones1 pieces of horn1 and other substances1 for a short time on a warm metallic plate3 when the substance was removed1 no impression appeared on the plate till it was breathed upon or exposed to the vapour of mercury1 and then these vapours adhered only to the parts where the substance had been placed1 making distinct images1 which in some cases were permanent after the vapour was removed4 ,similar impressions were obtained on glass and other substances even when the bodies were not in contact1 and the results were the same whether the experiments were performed in light or in darkness4 ,mr4 ,grove found1 when plates of zinc and copper were closely approximated1 but not in contact1 and suddenly separated1 that one was positively and the other negatively electric2 whence he inferred that the intervening medium was either polarised1 or that a radiation analogous1 if not identical1 with that which produces ,moser's images takes place from plate to plate4 ,mr4 ,hunt has shown that many of these phenomena depend on difference of temperature1 and that1 in order to obtain good impressions1 dissimilar metals must be used4 ,for example1 gold1 silver1 bronze1 and copper coins were placed on a plate of copper too hot to be touched1 and allowed to remain till the plate cooled3 all the coins had made an impression1 the distinctness and intensity of which were in the order of the metals named4 ,when the plate was exposed to the vapour of mercury the result was the same1 but1 when the vapour was wiped off1 the gold and silver coins only had left permanent images on the copper4 ,these impressions are often minutely perfect1 whether the coins are in actual contact with the plate or one-eighth of an inch above it4 ,the mass of the metal has a material influence on the result2 a large copper coin makes a better impression on a copper plate than a small silver coin4 ,when coins of different metals are placed on the same plate they interfere with each other4 ,when1 instead of being heated1 the copper plate was cooled by a freezing mixture1 and bad conductors of heat laid upon it1 as wood1 paper1 glass1 c41 the result was similar4 ,mr4 ,hunt1 observing that a black substance leaves a stronger impression on a metallic surface than a white1 applied the property to the art of copying prints1 woodcuts1 writing1 and printing1 on copper amalgamated on one surface and highly polished1 merely by placing the object to be copied smoothly on the metal1 and pressing it into close contact by a plate of glass3 after some hours the plate is subjected to the vapour of mercury1 and afterwards to that of iodine1 when a black and accurate impression of the object comes out on a grey ground4 ,effects similar to those attributed to heat may also be produced by electricity4 ,mr4 ,karsten1 by placing a glass plate upon one of metal1 and on the glass plate a medal subjected to discharges of electricity1 found a perfect image of the medal impressed on the glass1 which could be brought into evidence by either mercury or iodine2 and1 when several plates of glass were interposed between the medal and the metallic plate1 each plate of glass received an image on its upper surface after the passage of electrical discharges4 ,these discharges have the remarkable power of restoring impressions that have been long obliterated from plates by polishing--a proof that the disturbances upon which these phenomena depend are not confined to the surface of the metals1 but that a very decided molecular change has taken place to a considerable depth4 ,mr4 ,hunt's experiments prove that the electro-negative metals make the most decided images upon electro-negative plates1 and vice vers4 ,m4 ,matteucci has shown that a discharge of electricity does not visibly affect a polished silver plate1 but that it produces an alteration which renders it capable of condensing vapour4 ,the impression of an engraving was made by laying it face downwards on a silver plate iodized1 and placing an amalgamated copper plate upon it2 it was left in darkness fifteen hours1 during which time an impression of the engraving had been made on the amalgamated plate through the paper4 ,an iodized silver plate was placed in darkness with a coil of string laid on it1 and with a polished silver plate suspended one-eighth of an inch above it3 after four hours they were exposed to the vapours of mercury1 which became uniformly deposited on the iodized plate1 but on the silver one there was a sharp image of the string1 so that this image was formed in the dark1 and even without contact4 ,coins or other objects leave their impressions in the same manner with perfect sharpness and accuracy1 when brought out by vapour without contact1 in darkness1 and on simple metals4 ,red and orange coloured media1 smoked glass1 and all bodies that transmit or absorb the hot rays freely1 leave strong impressions on a plate of copper1 whether they be in contact or one-eighth of an inch above it4 ,heat must be concerned in this1 for a solar spectrum concentrated by a lens was thrown on a polished plate of copper1 and kept on the same spot by a heliostat for two or three hours3 when exposed to mercurial vapour1 a film of the vapour covered the plate where the diffused light which always accompanies the solar spectrum had fallen4 ,on the obscure space occupied by the maximum heating power of ,sir ,william ,herschel1 and also on the great heat spot in the thermic spectrum of ,sir ,john ,herschel1 the condensation of the mercury was so thick that it stood out a distinct white spot on the plate1 while over the whole space that had been under the visible spectrum the quantity of vapour was much less than that which covered the other parts1 affording distinct evidence of a negative effect in the luminous spectrum and of the power of the hot rays1 which is not always confined to the surface of the metal1 since in many instances the impressions penetrated to a considerable depth below it1 and consequently were permanent4 ,several of these singular effects appear to be owing to the mutual action of molecules in contact while in a different state1 whether of electricity or temperature3 others clearly point at some unknown influence exerted between surfaces at a distance1 and affecting their molecular structure3 possibly it may be the parathermic rays1 which have a peculiar chemical action even in total darkness4 ,in the last experiment the effect is certainly produced by the positive portion of one of those remarkable antagonist principles which characterise the solar spectrum4 ,thus it appears that the prism resolves the pure white sunbeam into three superposed spectra1 each varying in refrangibility and intensity throughout its whole length2 the visible part is overlapped at one end by the chemical or photographic rays1 and at the other by the thermic1 but the two latter so much exceed the visible part1 that the linear dimensions of the three--the luminous1 thermic1 and photographic--are in proportion to the numbers #be1 #dbaj1 and #eeaj1 so that the whole solar spectrum is twice as long as its visible part4 ,the two extremities exert a decided antagonist energy4 ,the least refrangible luminous rays obliterate the action of the photographic rays1 while the latter produce phosphorescent light1 which is extinguished by the least refrangible luminous rays4 ,according to ,mr4 ,hunt's experiment1 the hot rays condense mercurial vapour on a polished metallic plate1 while the luminous rays prevent its formation4 ,electricity is excited by the chemical rays1 while the parathermic are found in the less refrangible rays alone4 ,each of the spectra is crossed by coloured and rayless lines peculiar to itself1 and these are traversed at right angles by innumerable dark lines of various breadths1 the whole forming an inexpressibly wonderful and glorious creation4 ,the arrangement varies a little according to the material of the prism and the manner of producing the spectrum1 as in that obtained by ,professor ,draper from diffracted light4 ,it was formed by a beam diffracted by passing through a netting of fine wire1 or by reflection from a polished surface of steel1 having fine parallel lines drawn on it4 ,this diffracted spectrum is divided into two equal parts in the centre of the yellow2 and as in the prismatic spectrum1 one half is antagonist to the other half1 the red or negative end undoing what the positive or violet end has done4 ,the centre of the yellow is the hottest part1 and the heat decreases to both extremities4 ,a line of cold is supposed to exist on this spectrum answering to ,fraunhofer's dark line ,h4 ,the undulations of the ethereal medium which constitute a sunbeam must be infinitely varied1 each influence having a vibration peculiar to itself4 ,those of light are certainly transverse to the direction of the ray2 while ,professor ,draper believes that those of heat are normal1 that is1 in the direction of the ray1 like those of sound4 ,a doubt exists whether the vibrations of polarised light are perpendicular to the plane of polarisation or in that plane4 ,professor ,stokes of ,cambridge has come to the conclusion1 both from the diffracted spectrum and theory1 that they are perpendicular to the plane of polarisation1 but ,m4 ,holtzmann is of opinion that they are in that plane1 so the subject is still open to discussion4 ,,section ,,xxv4 ,size and ,constitution of the ,sun--,the ,solar ,spots--,intensity of the ,sun's ,light and ,heat--,the ,sun's ,atmosphere--,his influence on the ,planets--,atmospheres of the ,planets--,the ,moon has none--,lunar heat--,the ,differential ,telescope--,temperature of ,space--,internal ,heat of the ,earth--,zone of constant ,temperature--,increase of ,heat ,with the ,depth--,central ,heat--,volcanic ,action--,quantity of ,heat received from the ,sun--,isogeothermal ,lines--,line of perpetual ,congelation--,climate--,isothermal ,lines--,same quantity of ,heat annually received and radiated by the ,earth4 ,,the sun is a globe #hhj1jjj miles in diameter3 what his body may be it is impossible to conjecture1 but it seems to be a dark mass surrounded by an extensive atmosphere at a certain height in which there is a stratum of luminous clouds which constitutes the photosphere of the sun4 ,above it rises the true solar atmosphere1 visible as an aureola or corona during annular and total eclipses1 and probably the cause of the peculiar phenomena in the photographic image of the sun already mentioned4 ,through occasional openings in the photosphere or mottled ocean of flame1 the dark nucleus appears like black spots1 often of enormous size4 ,these spots are almost always comprised within a zone of the sun's surface1 whose breadth measured on a solar meridian does not extend beyond #cj-a_/b on each side of his equator1 though they have been seen at a distance of #ci-a_/b4 ,the dark central part of the spots is surrounded by a succession of obscure cloudy envelopes increasing in brightness up to a penumbra1 sometimes there are three or more shades1 but it requires a good telescope to distinguish the intermediate ones4 ,the spots gradually increase in size and number from year to year to a maximum1 and then as gradually decrease to a minimum1 accomplishing regular vicissitudes in periods of about eleven years1 and are singularly connected with the cycles of terrestrial magnetism4 ,from their extensive and rapid changes1 there is every reason to believe that the exterior and incandescent part of the sun is gaseous4 ,doubts have arisen as to the uniformity of the quantity of heat emitted by the sun4 ,sir ,william ,herschel was the first to suspect that it was affected by the quantity and magnitude of the spots on his surface2 ,professor ,secchi has observed that the spots are less hot than the luminous part2 and now ,professor ,wolf has perceived that the amount of heat emitted by the sun varies periodically with the spots every #aaaa years1 or nearly nine times in a century1 beginning at the commencement of the present one4 ,he has discovered a sub-period in that of the spots1 which no doubt has an effect on the quantity of solar heat4 ,so the unaccountable vicissitudes in the temperature of different years may ultimately be found to depend upon the constitution of the sun himself4 ,the intensity of the sun's light diminishes from the centre to the circumference of the solar disc4 ,his direct light has been estimated to be equal to that of #eefc wax candles of moderate size placed at the distance of one foot from an object2 that of the moon is probably only equal to the light of one candle at the distance of #ab feet3 consequently the light of the sun is more than three hundred thousand times greater than that of the moon4 ,according to ,professor ,secchi's experiments at ,rome1 the heat of the solar image is almost twice as great at the centre as at the edge4 ,the maximum heat1 however1 is not in the centre1 but in the solar equator1 and the spots are less hot than the rest of the surface4 ,the oceans of light and heat probably arising from electric or chemical processes of immense energy that continually take place at the sun's surface 7,n4 #bag7 are transmitted in undulations by the ethereal medium in all directions2 but notwithstanding the sun's magnitude and the inconceivable intensity of light and heat that must exist at his surface1 as the intensity of both diminishes as the square of the distance increases1 his kindly influence can hardly be felt at the boundaries of our system4 ,in ,uranus the sun must be seen like a small brilliant star not above the hundred and fiftieth part as bright as he appears to us1 but that is #bjjj times brighter than our moon1 so that he is really a sun to ,uranus1 and may impart some degree of warmth4 ,but if we consider that water would not remain fluid in any part of ,mars1 even at his equator1 and that1 in the temperate zones of the same planet1 even alcohol and quicksilver would freeze1 we may form some idea of the cold that must reign in ,uranus and ,neptune4 ,the climate of ,venus more nearly resembles that of the earth1 though1 excepting at her poles1 much too hot for animal and vegetable life such as they exist here1 for she receives seven times as much light and heat as the earth does2 but in ,mercury the mean heat from the intensity of the sun's rays must be above that of boiling quicksilver1 and water would boil even at his poles4 ,thus the planets1 though kindred with the earth in motion and form1 are1 according to our experience1 totally unfit for the habitation of such a being as man1 unless indeed their temperature should be modified by circumstances of which we are not aware1 and which may increase or diminish the sensible heat so as to render them habitable4 ,in our utter ignorance it may be observed1 that the earth1 if visible at all from ,neptune1 can only be a minute telescopic object2 that from the nearest fixed star the sun must dwindle to a mere point of light2 that the whole solar system would there be hid by a spider's thread2 and that the starry firmament itself is only the first series of starry systems1 the numbers of which are bounded alone by the imperfection of our space-penetrating instruments4 ,in this overwhelming majesty of creation1 it seems rash to affirm that the earth alone is inhabited by intelligent beings1 and thus to limit the ,omnipotent1 who has made nothing in vain4 ,several of the planets have extensive and dense atmospheres3 according to ,schroter the atmosphere of ,ceres is more than #ffh miles high1 and that of ,pallas has an elevation of #dfe miles1 but not a trace of an atmosphere can be perceived round ,vesta4 ,the attraction of the earth has probably deprived the moon of hers1 for the refractive power of the air at the surface of the earth is at least a thousand times as great as at the surface of the moon3 the lunar atmosphere must therefore be of a greater degree of rarity than can be produced by our best air-pumps4 ,this is confirmed by ,arago's observations during a solar eclipse1 when no trace of a lunar atmosphere could be seen4 ,since then1 however1 some indications of air have been perceived in the lunar valleys4 ,in taking photographic images of the moon and ,jupiter at ,rome1 ,professor ,secchi found that the light of the full moon is to that of the quarter moon as #c to #a4 ,jupiter gives a photographic image as bright and vigorous as the brightest part of the moon2 but although the light of ,jupiter is less than that of the moon1 he is nearly five times farther from the sun2 and as light diminishes as the square of the distance increases1 the light of ,jupiter is proportionally greater than that of the moon1 consequently ,jupiter's atmosphere reflects more light than the dark volcanic soil of the moon2 thus ,professor ,secchi observes photography may in time reveal the quality of the materials of which the celestial bodies are formed4 ,the effect of the earth's atmosphere on lunar heat is remarkable4 ,professor ,forbes proved that the direct light of the full moon is incapable of raising a thermometer the one three thousandth part of a ,centigrade degree1 at least in ,england2 but at an elevation of #hhgj feet on the ,peak of ,teneriffe1 ,mr4 ,piazzi ,smyth found a very sensible heat from the moon1 although she was then #ai south of the equator2 so it is no doubt absorbed by our atmosphere at lower levels4 ,some exceedingly interesting experiments might be made by means of a telescope having a prism attached to its objective extremity1 and furnished with a micrometer1 because by it the difference of the illumination of objects might be determined with extreme accuracy--as for example1 the comparative intensity between the bright and dark parts of the moon1 the comparative intensity of the solar light reflected by the moon1 and the lumire cendr1 or the light of the earth reflected on the moon1 whence a comparison might be made between the light of the sun and that of the earth4 ,hence also it might be known whether the terrestrial hemispheres successively visible from the moon are more or less luminous1 according as they contain more land or water1 and at the same time it might be possible to appreciate the more or less cloudy or clear state of our atmosphere1 so that in time we might ultimately find in the lumire cendr of the moon data upon the mean diaphaneity of different terrestrial hemispheres which are of different temperatures4 ,it is found by experience that heat is developed in opaque and translucent substances by their absorption of solar light1 but that the sun's rays do not sensibly alter the temperature of perfectly transparent bodies through which they pass4 ,as the temperature of the pellucid planetary space can be but little affected by the passage of the sun's light and heat1 neither can it be sensibly raised by the heat now radiated from the earth4 ,doubtless the radiation of all the bodies in the universe maintains the ethereal medium at a higher temperature than it would otherwise have1 and must eventually increase it1 but by a quantity so evanescent that it is hardly possible to conceive a time when a change will become perceptible4 ,the temperature of space being so low as -#bci ,fahrenheit1 it becomes a matter of no small interest to ascertain whether the earth may not be ultimately reduced by radiation to the temperature of the surrounding medium2 what the sources of heat are2 and whether they be sufficient to compensate the loss1 and to maintain the earth in a state fit for the support of animal and vegetable life in time to come4 ,all observations that have been made under the surface of the ground concur in proving that there is a stratum at the depth of from #dj to #ajj feet throughout the whole earth where the temperature is invariable at all times and seasons1 and which differs but little from the mean annual temperature of the country above4 ,according to ,m4 ,boussingault1 that stratum at the equator is at the depth of little more than a foot in places sheltered from the direct rays of the sun2 but in our climates it is at a much greater depth4 ,in the course of more than half a century the temperature of the earth at the depth of #ij feet1 in the cellars of the ,observatory at ,paris1 has never been above or below #ec of ,fahrenheit's thermometer1 which is only #b above the mean annual temperature at ,paris4 ,this zone1 unaffected by the sun's rays from above1 or by the internal heat from below1 serves as an origin whence the effects of the external heat are estimated on one side1 and the internal temperature of the globe on the other4 ,as early as the year #agdj ,m4 ,gensanne discovered in the lead-mines of ,giromagny1 in the ,vosges mountains1 three leagues from ,bfort1 that the heat of the ground increases with the depth below the zone of constant temperature4 ,a vast number of observations have been made since that time1 in the mines of ,europe and ,america1 by ,,mm4 ,saussure1 ,daubuisson1 ,humboldt1 ,cordier1 ,fox1 ,reich1 and others1 which agree1 without an exception1 in proving that the temperature of the earth becomes higher in descending towards its centre4 ,the greatest depth that has been attained is in the silver-mine of ,guanaxato1 in ,mexico1 where ,m4 de ,humboldt found a temperature of #ih at the depth of #bhe fathoms1 the mean annual temperature of the country being only #fa4 ,next to that is the ,dalcoath copper-mine1 in ,cornwall1 where ,mr4 ,fox's thermometer stood at #fh in a hole in the rock at the depth of #bcj fathoms1 and at #hb in water at the depth of #bdj fathoms1 the mean annual temperature at the surface being about #ej4 ,but it is needless to multiply examples1 all of which concur in showing that there is a very great difference between the temperature in the interior of the earth and at its surface4 ,mr4 ,fox's observations on the temperature of springs which rise at profound depths in mines afford the strongest testimony4 ,he found considerable streams flowing into some of the ,cornish mines at the temperature of #hj or #ij1 which is about #cj or #dj above that of the surface1 and also ascertained that nearly #b1jjj1jjj gallons of water are daily pumped from the bottom of the ,poldice mine1 which is #agf fathoms deep at #ij or #ajj4 ,as this is higher than the warmth of the human body1 ,mr4 ,fox justly observes that it amounts to a proof that the increased temperature cannot proceed from the persons of the workmen employed in the mines4 ,neither can the warmth of mines be attributed to the condensation of the currents of air which ventilate them4 ,mr4 ,fox1 whose opinion is of high authority in these matters1 states that1 even in the deepest mines1 the condensation of the air would not raise the temperature more than #e or #f2 and that1 if the heat could be attributed to this cause1 the seasons would sensibly affect the temperature of mines1 which it appears they do not where the depth is great4 ,besides1 the ,cornish mines are generally ventilated by numerous shafts opening into the galleries from the surface or from a higher level4 ,the air circulates freely in these1 descending in some shafts and ascending in others4 ,in all cases ,mr4 ,fox found that the upward currents are of a higher temperature than the descending currents2 so much so1 that in winter the moisture is often frozen in the latter to a considerable depth2 the circulation of air1 therefore1 tends to cool the mine instead of increasing the heat4 ,mr4 ,fox has also removed the objections arising from the comparatively low temperature of the water in the shafts of abandoned mines1 by showing that observations in them1 from a variety of circumstances which he enumerates1 are too discordant to furnish any conclusion as to the actual heat of the earth4 ,the high temperature of mines might be attributed to the effects of the fires1 candles1 and gunpowder used by the miners1 did not a similar increase obtain in deep wells1 and in borings to great depths in search of water1 where no such causes of disturbance occur4 ,in a well dug with a view to discover salt in the canton of ,berne1 and long deserted1 ,m4 de ,saussure had the most complete evidence of increasing heat4 ,the same has been confirmed by the temperature of many wells1 both in ,france and ,england1 especially by the ,artesian wells1 so named from a peculiar method of raising water first resorted to in ,artois1 and since become very general4 ,an ,artesian well consists of a shaft a few inches in diameter1 bored into the earth till a spring is found4 ,to prevent the water being carried off by the adjacent strata1 a tube is let down which exactly fills the bore from top to bottom1 in which the water rises pure to the surface4 ,it is clear the water could not rise unless it had previously descended from high ground through the interior of the earth to the bottom of the well4 ,it partakes of the temperature of the strata through which it passes1 and in every instance has been warmer in proportion to the depth of the well2 but it is evident that the law of increase cannot be obtained in this manner4 ,perhaps the most satisfactory experiments on record are those made by ,,mm4 ,auguste de la ,rive and ,f4 ,marcet during the year #ahcc1 in a boring for water about a league from ,geneva1 at a place #cah feet above the level of the lake4 ,the depth of the bore was #gbg feet1 and the diameter only between four and five inches4 ,no spring was ever found2 but the shaft filled with mud1 from the moisture of the ground mixing with the earth displaced in boring1 which was peculiarly favourable for the experiments1 as the temperature at each depth may be considered to be that of the particular stratum4 ,in this case1 where none of the ordinary causes of disturbance could exist1 and where every precaution was employed by scientific and experienced observers1 the temperature was found to increase regularly and uniformly with the depth at the rate of about #a of ,fahrenheit for every #eb feet4 ,professor ,reich of ,freyberg has found that the mean of a great number of observations both in mines and wells is #a of ,fahrenheit for every #ee feet of depth2 and from ,m4 ,arago's observations in the ,grenelle ,artesian well at ,paris1 the increase is #a of ,fahrenheit for every #de feet4 ,though there can be no doubt as to the increase of temperature in penetrating the crust of the earth1 there is still much uncertainty as to the law of increase1 which varies with the nature of the soil and other local circumstances2 but1 on an average1 it has been estimated at the rate of #a for every #ej or #fj feet1 which corresponds with the observations of ,,mm4 ,marcet and ,de la ,rive4 ,in consequence of the rapid increase of internal heat1 thermal springs1 or such as are independent of volcanic action1 rising from a great depth1 must necessarily be very rare and of a high temperature2 and it is actually found that none are so low as #fh of ,fahrenheit2 that of ,chaudes ,aigues1 in ,auvergne1 is about #acf4 ,in many places warm water from ,artesian wells will probably come into use for domestic purposes1 and it is even now employed in manufactories near ,stutgardt1 in ,alsace1 c4 ,it is hardly to be expected that at present any information with regard to the actual internal temperature of the earth should be obtained from that of the ocean1 on account of the mobility of fluids1 by which the colder masses sink downwards1 while those that are warmer rise to the surface4 ,nevertheless1 it may be stated that the temperature of the sea decreases with the depth between the tropics2 while1 on the contrary1 all our northern navigators found that the temperature increases with the depth in the polar seas4 ,the change takes place about the #gjth parallel of latitude4 ,some ages hence1 however1 it may be known whether the earth has arrived at a permanent state as to heat1 by comparing secular observations of the temperature of the ocean if made at a great distance from the land4 ,should the earth's temperature increase at the rate of #a for every #ej feet1 it is clear that at the depth of #bjj miles the hardest substances must be in a state of fusion1 and our globe must in that case either be encompassed by a stratum of melted lava at that depth1 or it must be a ball of liquid fire #gfjj miles in diameter1 enclosed in a thin coating of solid matter2 for #bjj miles are nothing when compared with the size of the earth4 ,no doubt the form of the earth1 as determined by the pendulum and arcs of the meridian1 as well as by the motions of the moon1 indicates original fluidity and subsequent consolidation and reduction of temperature by radiation2 but whether the law of increasing temperature is uniform at still greater depths than those already attained by man1 it is impossible to say4 ,at all events1 internal fluidity is not inconsistent with the present state of the earth's surface1 since earthy matter is as bad a conductor of heat as lava1 which often retains its heat at a very little depth for years after its surface is cool4 ,whatever the radiation of the earth might have been in former times1 certain it is that it goes on very slowly in our days2 for ,m4 ,fourier has computed that the central heat is decreasing from radiation by only about the #a_/cjjjjth part of a degree in a century4 ,if so1 there can be no doubt that it will ultimately be dissipated2 but as far as regards animal and vegetable life1 it is of very little consequence whether the centre of our planet be liquid fire or ice1 since its condition in either case could have no sensible effect on the climate at its surface4 ,the internal fire does not even impart heat enough to melt the snow at the poles1 though nearer to the centre than any other part of the globe4 ,the immense extent of active volcanic fire is one of the causes of heat which must not be overlooked4 ,the range of the ,andes from ,chile to the north of ,mexico1 probably from ,cape ,horn to ,behring ,straits1 is one vast district of igneous action1 including the ,caribbean and the ,west ,indian ,islands on one hand2 and stretching quite across the ,pacific ,ocean1 through the ,polynesian ,archipelago1 the ,new ,hebrides1 the ,georgian and ,friendly ,islands1 on the other4 ,another chain begins with the ,aleutian ,islands1 extends to ,kamtschatka1 and from thence passes through the ,kurile1 ,japanese1 and ,philippine ,islands1 to the ,moluccas1 whence it spreads with terrific violence through the ,indian ,archipelago1 even to the ,bay of ,bengal4 ,volcanic action may again be followed from the entrance of the ,persian ,gulf to ,madagascar1 ,bourbon1 the ,canaries1 and ,azores4 ,thence a continuous igneous region extends through about #ajjj geographical miles to the ,caspian ,sea1 including the ,mediterranean1 and extending north and south between the #ceth and #djth parallels of latitude2 and in central ,asia a volcanic region occupies #bejj square geographical miles4 ,the volcanic fires are developed in ,iceland in tremendous force2 and the antarctic land discovered by ,sir ,james ,ross is an igneous formation of the boldest structure1 where a volcano in high activity rises #ab1jjj feet above the perpetual ice of these polar deserts1 and within #ai-a_/b of the south pole4 ,throughout this vast portion of the world the subterraneous fire is often intensely active1 producing such violent earthquakes and eruptions that their effects1 accumulated during millions of years1 may account for many of the great geological changes of igneous origin that have already taken place in the earth1 and may occasion others not less remarkable1 should time--that essential element in the vicissitudes of the globe--be granted1 and their energy last4 ,sir ,charles ,lyell1 who has shown the power of existing causes with great ingenuity1 estimates that on an average twenty eruptions take place annually in different parts of the world2 and many must occur or have happened1 even on the most extensive and awful scale1 among people equally incapable of estimating their effects and of recording them4 ,we should never have known the extent of the fearful eruption which took place in the island of ,sumbawa1 in #ahae1 but for the accident of ,sir ,stamford ,raffles having been governor of ,java at the time4 ,it began on the #eth of ,april1 and did not entirely cease till ,july4 ,the ground was shaken through an area of #ajjj miles in circumference2 the tremors were felt in ,java1 the ,moluccas1 a great part of ,celebes1 ,sumatra1 and ,borneo4 ,the detonations were heard in ,sumatra1 at the distance of #igj geographical miles in a straight line2 and at ,ternate1 #gbj miles in the opposite direction4 ,the most dreadful whirlwinds carried men and cattle into the air2 and with the exception of #bf persons1 the whole population of the island perished to the amount of #ab1jjj4 ,ashes were carried #cjj miles to ,java in such quantities that the darkness during the day was more profound than ever had been witnessed in the most obscure night4 ,the face of the country was changed by the streams of lava1 and by the upheaving and sinking of the soil4 ,the town of ,tomboro was submerged1 and water stood to the depth of #ah feet in places which had been dry land4 ,ships grounded where they had previously anchored1 and others could hardly penetrate the mass of cinders which floated on the surface of the sea for several miles to the depth of two feet4 ,a catastrophe similar to this1 though of less magnitude1 took place in the island of ,bali in #ahjh1 which was not heard of in ,europe till years afterwards4 ,the eruption of ,coseguina in the ,bay of ,fonseca1 which began on the #aith of ,january1 #ahce1 and lasted many days1 was even more dreadful and extensive in its effects than that of ,sumbawa4 ,the ashes during this eruption were carried by the upper current of the atmosphere as far north as ,chiassa1 which is upwards of #djj leagues to the windward of that volcano4 ,many volcanoes supposed to be extinct have all at once burst out with inconceivable violence4 ,witness ,vesuvius1 on historical record2 and the volcano in the island of ,st4 ,vincent in our own days1 whose crater was lined with large trees1 and which had not been active in the memory of man4 ,vast tracts are of volcanic origin where volcanoes have ceased to exist for ages4 ,whence it may be inferred that in some places the subterraneous fires are in the highest state of activity1 in some they are inert1 and in others they appear to be extinct4 ,yet there are few countries that are not subject to earthquakes of greater or less intensity2 the tremors are propagated like a sonorous undulation to such distances that it is impossible to say in what point they originate4 ,in some recent instances their power must have been tremendous4 ,in ,south ,america1 so lately as #ahbb1 an area of #ajj1jjj square miles1 which is equal in extent to the half of ,france1 was raised several feet above its present level--a most able account of which is given in the #8,transactions of the ,geological ,society1' by an esteemed friend of the author's1 the late ,mrs4 ,graham1 who was present during the whole time of that formidable earthquake1 which recurred at short intervals for more than two months1 and who possessed talents to appreciate1 and had opportunities of observing1 its effects under the most favourable circumstances at ,valparaiso1 and for miles along the coast where it was most intense4 ,a considerable elevation of the land has again taken place along the coast of ,chile1 in consequence of the violent earthquake which happened on the #bjth of ,february1 #ahce4 ,in #ahai a ridge of land stretching for #ej miles across the delta of the ,indus1 #af feet broad1 was raised #aj feet above the plain4 ,the reader is referred to ,sir ,charles ,lyell's excellent #8,principles of ,geology1' already mentioned1 for most interesting details of the phenomena and extensive effects of volcanoes and earthquakes1 too numerous to find a place here4 ,it may however be mentioned that innumerable earthquakes are from time to time shaking the solid crust of the globe1 and carrying destruction to distant regions1 progressively though slowly accomplishing the great work of change4 ,a most disastrous instance took place on the #aeth of ,december1 #aheg1 in the ,neapolitan provinces of ,la ,basilicata and ,principato ,citeriore1 where the destruction was extensive and terrible2 the number of victims1 according to the official accounts1 being returned at upwards of ten thousand4 ,these terrible engines of ruin1 fitful and uncertain as they may seem1 must1 like all durable phenomena1 have a law which may in time be discovered by long-continued and accurate observations4 ,the shell of volcanic fire that girds the globe at a small depth below our feet has been attributed to different causes4 ,by some it is supposed to originate in an ocean of incandescent matter1 still existing in the central abyss of the earth4 ,some conceive it to be superficial1 and due to chemical action1 in strata at no very great depth when compared with the size of the globe4 ,the more so as matter on a most extensive scale is passing from old into new combinations1 which1 if rapidly effected1 are capable of producing the most intense heat4 ,according to others1 electricity1 which is so universally diffused in all its forms throughout the earth1 if not the immediate cause of the volcanic phenomena1 at least determines the chemical affinities that produce them4 ,it is clear that a subject so involved in mystery must give rise to much speculation1 in which every hypothesis is attended with difficulties that observation alone can remove4 ,but the views of ,mr4 ,babbage and ,sir ,john ,herschel on the general cause of volcanic action1 and the changes in the equilibrium of the internal heat of the globe1 accord more with the laws of mechanics and radiant heat than any that have been proposed4 ,the theory of these distinguished philosophers1 formed independently of each other1 is equally consistent with observed phenomena1 whether the earth be a solid crust encompassing a nucleus of liquid lava1 or that there is merely a vast reservoir or stratum of melted matter at a moderate depth below the superficial crust4 ,the author is indebted to the kindness of ,sir ,charles ,lyell for the perusal of a most interesting letter from ,sir ,john ,herschel1 in which he states his views on the subject4 ,supposing that the globe is merely a solid crust1 resting upon fluid or semi-fluid matter1 whether extending to the centre or not1 the transfer of pressure from one part of its surface to another by the degradation of existing continents1 and the formation of new ones1 would be sufficient to subvert the equilibrium of heat in the interior1 and occasion volcanic eruptions4 ,for1 since the internal heat of the earth is transmitted outwards by radiation1 an accession of new matter on any part of the surface1 like an addition of clothing1 by keeping it in1 would raise the temperature of the strata below1 and in the course of ages would even reduce those at a great depth to a state of fusion4 ,some of the substances might be converted into gases2 and should the accumulation of new matter take place at the bottom of the sea1 as is generally the case1 this lava would be mixed with water in a state of ignition in consequence of the enormous pressure of the ocean1 and of the newly superimposed matter which would prevent it from expanding into steam4 ,now ,sir ,charles ,lyell has shown1 with his usual talent1 that the quantity of matter carried down by rivers from the surface of the continents is comparatively trifling1 and that the great transfer to the bottom of the ocean is produced at the coast-line by the action of the sea2 hence1 says ,sir ,john ,herschel1 8the greatest accumulation of local pressure is in the central area of the deep sea1 while the greatest local relief takes place along the abraded coast-lines4 ,here then should occur the chief volcanic vents40 ,as the crust of the earth is much weaker on the coasts than elsewhere1 it is more easily ruptured1 and1 as ,mr4 ,babbage observes1 immense rents might be produced there by its contraction in cooling down after being deprived of a portion of its original thickness4 ,the pressure on the bottom of the ocean would force a column of lava mixed with ignited water and gas to rise through an opening thus formed1 and1 says ,sir ,john ,herschel1 8when the column attains such a height that the ignited water can become steam1 the joint specific gravity of the column is suddenly diminished1 and up comes a jet of mixed steam and lava1 till so much has escaped that the matter deposited at the bottom of the ocean takes a fresh bearing1 when the evacuation ceases and the crack becomes sealed up40 ,this theory perfectly accords with the phenomena of nature1 since there are very few active volcanoes at a distance from the sea1 and the exceptions that do occur are generally near lakes1 or they are connected with volcanoes on the maritime coasts4 ,many break out even in the bottom of the ocean1 probably owing to some of the supports of the superficial crust giving way1 so that the steam and lava are forced up through the fissures4 ,finally1 ,mr4 ,babbage observes that1 8in consequence of changes continually going on1 by the destruction of forests1 the filling up of seas1 the wearing down of elevated lands1 the heat radiated from the earth's surface varies considerably at different periods4 ,in consequence of this variation1 and also in consequence of the covering up of the bottom of the sea by the detritus of the land1 the surfaces of equal temperature within the earth are continually changing their form1 and exposing thick beds near the exterior to alterations of temperature4 ,the expansion and contraction of these strata may form rents and veins1 produce earthquakes1 determine volcanic eruptions1 elevate continents1 and1 possibly1 raise mountain chains40 ,the numerous vents for the internal heat formed by volcanoes1 hot springs1 and the emission of steam1 so frequent in volcanic regions1 no doubt maintain the tranquillity of the interior fluid mass1 which seems to be perfectly inert unless when put in motion by unequal pressure4 ,but1 to whatever cause the increasing heat of the earth and the subterranean fires may ultimately be referred1 it is certain that1 except in some local instances1 they have no sensible effect on the temperature of its surface4 ,it may therefore be concluded that the heat of the earth1 above the zone of uniform temperature1 is entirely owing to the sun4 ,the power of the solar rays depends much upon the manner in which they fall1 as we readily perceive from the different climates on our globe4 ,although the sun is about three millions of miles nearer to the earth in winter than in summer1 his rays strike the atmosphere in the northern hemisphere so obliquely that it absorbs a much greater quantity of heat than when they are more direct 7,n4 #bag74 ,indeed it is so great that1 when the sun has an altitude of #cj1 one half of his heat is absorbed by the atmosphere1 and it increases very rapidly as he sinks towards the horizon4 ,however1 that heat is not lost3 it is most beneficial to the earth1 being really the heat which supplies the greatest part of that which is radiated into space during the absence of the sun4 ,professor ,dove has shown1 by taking at all seasons the mean of the temperatures of points on the earth's surface diametrically opposite to each other1 that the average temperature of the whole earth's surface in ,june1 when we are farthest from the sun1 considerably exceeds that in ,december1 when we are nearest to him1 owing to the excess of water in the southern hemisphere1 and that of land in the northern1 which gives a general insular climate to the former1 and a continental climate to the latter4 ,the observations of the north polar navigators1 and those of ,sir ,john ,herschel at the ,cape of ,good ,hope1 show that the direct heating influence of the solar rays is greatest at the equator1 and that it diminishes gradually as the latitude increases4 ,at the equator the maximum is #dh-c_/d1 while in ,europe it has never exceeded #bi-a_/b4 ,m4 ,pouillet has estimated with singular ingenuity1 from a series of observations made by himself1 that the whole quantity of heat which the earth receives annually from the sun is such as would be sufficient to melt a stratum of ice covering the whole globe #df feet deep4 ,part of this heat is radiated back into space2 but by far the greater part descends into the earth during the summer1 towards the zone of uniform temperature1 whence it returns to the surface in the course of the winter1 and tempers the cold of the ground and the atmosphere in its passage to the ethereal regions1 where it is lost1 or rather where it combines with the radiation from the other bodies of the universe in maintaining the temperature of space4 ,the sun's power being greatest between the tropics1 the heat sinks deeper there than elsewhere1 and the depth gradually diminishes towards the poles2 but the heat is also transmitted laterally from the warmer to the colder strata north and south of the equator1 and aids in tempering the severity of the polar regions4 ,the mean heat of the earth1 above the stratum of constant temperature1 is determined from that of springs2 and1 if the spring be on elevated ground1 the temperature is reduced by computation to what it would be at the level of the sea1 assuming that the heat of the soil varies according to the same law as the heat of the atmosphere1 which is about #a of ,fahrenheit's thermometer for every #cccg feet4 ,from a comparison of the temperature of numerous springs with that of the air1 ,sir ,david ,brewster concludes that there is a particular line passing nearly through ,berlin1 at which the temperature of springs and that of the atmosphere coincide2 that in approaching the arctic circle the temperature of springs is always higher than that of the air1 while1 proceeding towards the equator1 it is lower4 ,since the warmth of the superficial strata of the earth decreases from the equator to the poles1 there are many places in both hemispheres where the ground has the same mean temperature4 ,if lines were drawn through all those points in the upper strata of the globe which have the same mean annual temperature1 they would be nearly parallel to the equator between the tropics1 and would become more and more irregular and sinuous towards the poles4 ,these are called isogeothermal lines4 ,a variety of local circumstances disturb their parallelism1 even between the tropics4 ,the temperature of the ground at the equator is lower on the coasts and islands than in the interior of continents2 the warmest part is in the interior of ,africa2 but it is obviously affected by the nature of the soil1 especially if it be volcanic4 ,much has been done to ascertain the manner in which heat is distributed over the surface of our planet1 and the variations of climate1 which1 in a general view1 mean every change of the atmosphere1 such as of temperature1 humidity1 variations of barometric pressure1 purity of air1 the serenity of the heavens1 the effects of winds1 and electric tension4 ,temperature depends upon the property which all bodies possess1 more or less1 of perpetually absorbing and emitting or radiating heat4 ,when the interchange is equal1 the temperature of a body remains the same2 but1 when the radiation exceeds the absorption1 it becomes colder1 and vice vers4 ,in order to determine the distribution of heat over the surface of the earth1 it is necessary to find a standard by which the temperature in different latitudes may be compared4 ,for that purpose it is requisite to ascertain1 by experiment1 the mean temperature of the day1 of the month1 and of the year1 at as many places as possible throughout the earth4 ,the annual average temperature may be found by adding the mean temperatures of all the months in the year1 and dividing the sum by twelve4 ,the average of ten or fifteen years will give it approximately2 for1 although the temperature in any place maybe subject to very great variations1 yet it never deviates more than a few degrees from its mean state1 which consequently offers a good standard of comparison4 ,as a standard1 however1 much greater accuracy is required4 ,if climate depended solely upon the heat of the sun1 all places having the same latitude would have the same mean annual temperature4 ,the motion of the sun in the ecliptic1 indeed1 occasions perpetual variations in the length of the day1 and in the direction of the rays with regard to the earth2 yet1 as the cause is periodic1 the mean annual temperature from the sun's motion alone must be constant in each parallel of latitude2 for it is evident that the accumulation of heat in the long days of summer1 which is but little diminished by radiation during the short nights1 is balanced by the small quantity of heat received during the short days in winter1 and its radiation in the long1 frosty1 and clear nights4 ,in fact1 if the globe were everywhere on a level with the surface of the sea1 and of uniform substance1 so as to absorb and radiate heat equally1 the mean heat of the sun would be regularly distributed over its surface in zones of equal annual temperature parallel to the equator1 from which it would decrease to each pole as the square of the cosine of the latitude2 and its quantity would only depend upon the altitude of the sun and atmospheric currents4 ,the distribution of heat1 however1 in the same parallel1 is very irregular in all latitudes except between the tropics1 where the isothermal lines1 or the lines passing through places of equal mean annual temperature1 are more nearly parallel to the equator4 ,the causes of disturbance are very numerous2 but such as have the greatest influence1 according to ,m4 de ,humboldt1 to whom we are indebted for the greater part of what is known on the subject1 are the elevation of the continents1 the distribution of land and water over the surface of the globe exposing different absorbing and radiating powers2 the variations in the surface of the land1 as forests1 sandy deserts1 verdant plains1 rocks1 c42 mountain-chains covered with masses of snow1 which diminish the temperature2 the reverberation of the sun's rays in the valleys1 which increases it2 and the interchange of currents1 both of air and water1 which mitigates the rigour of climates2 the warm currents from the equator softening the severity of the polar frosts1 and the cold currents from the poles tempering the intense heat of the equatorial regions4 ,to these may be added cultivation1 though its influence extends over but a small portion of the globe1 only a fourth part of the land being inhabited4 ,temperature decreases with the height above the level of the sea1 as well as with the latitude4 ,the air in the higher regions of the atmosphere is much cooler than that below1 because the warm air expands as it rises1 by which its capacity for heat is increased1 a great proportion becomes latent or absorbed1 and less of it sensible4 ,a portion of air at the surface of the earth whose temperature is #gj of ,fahrenheit1 if carried to the height of two miles and a half1 would expand so much that its temperature would be reduced #ej2 and in the ethereal regions the temperature is #bci below the zero point of ,fahrenheit4 ,the height at which snow lies perpetually decreases from the equator to the poles1 and is higher in summer than in winter2 but it varies from many circumstances4 ,snow rarely falls when the cold is intense and the atmosphere dry4 ,extensive forests produce moisture by their evaporation2 and high table-lands1 on the contrary1 dry and warm the air1 because the air at great elevations is too rare to absorb much of the sun's heat4 ,in the ,cordilleras of the ,andes1 plains of only twenty-five square leagues from their extent raise the temperature as much as #c or #d above what is found at the same altitude on the rapid declivity of a mountain1 consequently the line of perpetual snow varies according as one or other of these causes prevails4 ,aspect in general has also a great influence2 yet the line of perpetual snow is much higher on the northern than on the southern side of the ,himalaya1 partly because the air is nearly deprived of its moisture by precipitation before it arrives at the northern side of the mountains4 ,on the whole1 it appears that the mean height between the tropics at which the snow lies perpetually is about #ae1bjg feet above the level of the sea2 whereas snow does not cover the ground continually at the level of the ocean till near the north pole4 ,in the southern hemisphere1 however1 the cold is greater than in the northern4 ,in ,sandwich ,land1 between the #edth and #ehth degrees of latitude1 perpetual snow and ice extend to the sea-level2 and in the island of ,s4 ,georgia1 in the #ecrd degree of south latitude1 which corresponds with the latitude of the central counties of ,england1 perpetual snow descends even to the level of the ocean4 ,it has been shown that this excess of cold in the southern hemisphere cannot be attributed to the winter being longer than ours by #g-c_/d days4 ,it is probably owing to the open sea surrounding the south pole1 which permits the icebergs to descend to a lower latitude by #aj than they do in the northern hemisphere1 on account of the numerous obstructions opposed to them by the islands and continents about the north pole4 ,icebergs from the ,arctic seas seldom float farther to the south than the ,azores2 whereas those that come from the south pole descend to as low a latitude as that of the ,cape of ,good ,hope4 ,the influence of mountain-chains does not wholly depend upon the line of perpetual congelation4 ,they attract and condense the vapours floating in the air1 and send them down in torrents of rain4 ,they radiate heat into the atmosphere at a lower elevation1 and increase the temperature of the valleys by the reflection of the sun's rays1 and by the shelter they afford against prevailing winds4 ,but1 on the contrary1 one of the most general and powerful causes of cold arising from the vicinity of mountains is the freezing currents of wind which rush from their lofty peaks along the rapid declivities1 chilling the surrounding valleys3 such is the cutting north wind called the bise in ,switzerland4 ,next to elevation1 the difference in the radiating and absorbing powers of the sea and land has the greatest influence in disturbing the regular distribution of heat4 ,the extent of the dry land is not above the fourth part of that of the ocean2 so that the general temperature of the atmosphere1 regarded as the result of the partial temperatures of the whole surface of the globe1 is most powerfully modified by the sea4 ,besides1 the ocean acts more uniformly on the atmosphere than the diversified surface of the solid mass does1 both by the equality of its curvature and its homogeneity4 ,in opaque substances the accumulation of heat is confined to the stratum nearest the surface4 ,the seas become less heated at their surface than the land1 because the solar rays1 before being extinguished1 penetrate the transparent liquid to a greater depth and in greater numbers than in the opaque masses4 ,on the other hand1 water has a considerable radiating power1 which1 together with evaporation1 would reduce the surface of the ocean to a very low temperature1 if the cold particles did not sink to the bottom on account of their superior density4 ,the seas preserve a considerable portion of the heat they receive in summer1 and from their saltness do not freeze so soon as fresh water4 ,so that1 in consequence of all these circumstances1 the ocean is not subject to such variations of heat as the land1 and1 by imparting its temperature to the winds and by its currents1 it diminishes the rigour of climate on the coasts and in the islands1 which are never subject to such extremes of heat and cold as are experienced in the interior of continents1 though they are liable to fogs and rain from the evaporation of the adjacent seas4 ,on each side of the equator to the #dhth degree of latitude1 the surface of the ocean is in general warmer than the air above it4 ,the mean of the difference of the temperature at noon and midnight is about #acg1 the greatest deviation never exceeding from #jcf to #baf1 which is much cooler than the air over the land4 ,on land the temperature depends upon the nature of the soil and its products1 its habitual moisture or dryness4 ,from the eastern extremity of the ,sahara desert quite across ,africa1 the soil is almost entirely barren sand2 and the ,sahara desert itself extends over an area of #aid1jjj square leagues1 equal to twice the area of the ,mediterranean ,sea1 and raises the temperature of the air by radiation from #ij to #ajj1 which must have a most extensive influence4 ,on the contrary1 vegetation cools the air by evaporation and the apparent radiation of cold from the leaves of plants1 because they absorb more caloric than they give out4 ,the graminiferous plains of ,south ,america cover an extent ten times greater than ,france1 occupying no less than about #ej1jjj square leagues1 which is more than the whole chain of the ,andes1 and all the scattered mountain-groups of ,brazil4 ,these1 together with the plains of ,north ,america and the steppes of ,europe and ,asia1 must have an extensive cooling effect on the atmosphere if it be considered that in calm and serene nights they cause the thermometer to descend #ab or #ad1 and that in the meadows and heaths in ,england the absorption of heat by the grass is sufficient to cause the temperature to sink to the point of congelation during the night for ten months in the year4 ,forests cool the air also by shading the ground from the rays of the sun1 and by evaporation from the boughs4 ,hales found that the leaves of a single plant of helianthus three feet high exposed nearly forty feet of surface2 and1 if it be considered that the woody regions of the river ,amazons1 and the higher part of the ,orinoco1 occupy an area of #bfj1jjj square leagues1 some idea may be formed of the torrents of vapour which rise from the leaves of the forests all over the globe4 ,however1 the frigorific effects of their evaporation are counteracted in some measure by the perfect calm which reigns in the tropical wildernesses4 ,the innumerable rivers1 lakes1 pools1 and marshes interspersed through the continents absorb caloric1 and cool the air by evaporation2 but1 on account of the chilled and dense particles sinking to the bottom1 deep water diminishes the cold of winter1 so long as ice is not formed4 ,in consequence of the difference in the radiating and absorbing powers of the sea and land1 their configuration greatly modifies the distribution of heat over the surface of the globe4 ,under the equator only one-sixth part of the circumference is land2 and the superficial extent of land in the northern and southern hemispheres is in the proportion of three to one4 ,the effect of this unequal division is greater in the temperate than in the torrid zones1 for the area of land in the northern temperate zone is to that in the southern as thirteen to one1 whereas the proportion of land between the equator and each tropic is as five to four4 ,it is a curious fact1 noticed by ,mr4 ,gardner1 that only one twenty-seventh part of the land of the globe has land diametrically opposite to it4 ,this disproportionate arrangement of the solid part of the globe has a powerful influence on the temperature of the southern hemisphere4 ,but1 besides these greater modifications1 the peninsulas1 promontories1 and capes1 running out into the ocean1 together with bays and internal seas1 all affect temperature4 ,to these may be added the position of continental masses with regard to the cardinal points4 ,all these diversities of land and water influence temperature by the agency of the winds4 ,on this account the temperature is lower on the eastern coasts both of the ,new and ,old ,world than on the western2 for1 considering ,europe as an island1 the general temperature is mild in proportion as the aspect is open to the ,atlantic ,ocean1 the superficial temperature of which1 as far north as the #deth and #ejth degrees of latitude1 does not fall below #dh or #ea of ,fahrenheit1 even in the middle of winter4 ,on the contrary1 the cold of ,russia arises from its exposure to the northern and eastern winds4 ,but the ,european part of that empire has a less rigorous climate than the ,asiatic1 because it does not extend to so high a latitude4 ,the interposition of the atmosphere modifies all the effects of the sun's heat4 ,the earth communicates its temperature so slowly1 that ,m4 ,arago has occasionally found as much as from #ad to #ah of difference between the heat of the soil and that of the air two or three inches above it4 ,the circumstances which have been enumerated1 and many more1 concur in disturbing the regular distribution of heat over the globe1 and occasion numberless local irregularities4 ,nevertheless the mean annual temperature becomes gradually lower from the equator to the poles4 ,but the diminution of mean heat is most rapid between the #djth and #deth degrees of latitude both in ,europe and ,america1 which accords perfectly with theory2 whence it appears that the variation in the square of the cosine of the latitude 7,n4 #abg71 which expresses the law of the change of temperature1 is a maximum towards the #deth degree of latitude4 ,the mean annual temperature under the equator in ,america is about #ha-a_/b of ,fahrenheit3 in ,africa it is said to be nearly #hc4 ,the difference probably arises from the winds of ,siberia and ,canada1 whose chilly influence is sensibly felt in ,asia and ,america1 even within #ah of the equator4 ,the isothermal lines are nearly parallel to the equator1 till about the #bbnd degree of latitude on each side of it1 where they begin to lose their parallelism1 and continue to do so more and more as the latitude augments4 ,with regard to the northern hemisphere1 the isothermal line of #ei of ,fahrenheit passes between ,rome and ,florence in latitude #dc2 and near ,raleigh in ,north ,carolina1 latitude #cf3 that of #ej of equal annual temperature runs through the ,netherlands1 latitude #ea2 and near ,boston in the ,united ,states1 latitude #db-a_/b3 that of #da passes near ,stockholm1 latitude #ei-a_/b2 and ,st4 ,george's ,bay1 ,newfoundland1 latitude #dh3 and lastly1 the line of #cb1 the freezing point of water1 passes between ,ulea in ,lapland1 latitude #ff1 and ,table ,bay1 on the coast of ,labrador1 latitude #ed4 ,thus it appears that the isothermal lines1 which are nearly parallel to the equator for about #bb1 afterwards deviate more and more4 ,from observations made during the numerous voyages in the ,arctic ,seas1 it is found that the isothermal lines of ,europe and ,america entirely separate in the high latitudes1 and surround two poles of maximum cold3 one1 in #gi ,n4 lat4 and #abj ,e4 long41 has a mean temperature of #b ,fahrenheit2 and the other1 whose temperature was determined by ,sir ,david ,brewster to be #c-a_/b ,fahrenheit1 from the observations of ,sir ,edward ,parry is near ,melville ,island4 ,the pole of the earth's rotation1 whose mean temperature is probably not below #ae ,fahrenheit1 is nearly midway between the two2 and the line which joins these points of maximum cold is almost coincident with that diameter of the polar basin which bisects it1 and passes through its two great outlets into the ,pacific and ,atlantic ,oceans1 a most remarkable feature1 and strongly indicative of the absence of land1 and of the prevalence of a materially milder climate in the polar ,ocean1 probably not under #ae ,fahrenheit4#ab ,it is believed that two corresponding poles of maximum cold exist in the southern hemisphere1 though observations are wanting to trace the course of the southern isothermal lines with the same accuracy as the northern4 ,the isothermal lines1 or such as pass through places where the mean annual temperature of the air is the same1 do not always coincide with the isogeothermal lines1 which are those passing through places where the mean temperature of the ground is the same4 ,sir ,david ,brewster1 in discussing this subject1 finds that the isogeothermal lines are always parallel to the isothermal lines2 consequently the same general formula will serve to determine both1 since the difference is a constant quantity obtained by observation1 and depending upon the distance of the place from the neutral isothermal line4 ,these results are confirmed by the observations of ,m4 ,kupffer of ,kasan during his excursions to the north1 which show that the ,european and the ,american portions of the isogeothermal line of #cb of ,fahrenheit actually separate1 and go round the two poles of maximum cold4 ,this traveller remarked1 also1 that the temperature both of the air and of the soil decreases most rapidly towards the #deth degree of latitude4 ,it is evident that places may have the same mean annual temperature1 and yet differ materially in climate4 ,in one1 the winters may be mild and the summers cool2 whereas another may experience the extremes of heat and cold4 ,lines passing through places having the same mean summer or winter temperature are neither parallel to the isothermal1 the geothermal lines1 nor to one another1 and they differ still more from the parallels of latitude4 ,in ,europe1 the latitude of two places which have the same annual heat never differs more than #h or #i2 whereas the difference in the latitude of those having the same mean winter temperature is sometimes as much as #ah or #ai4 ,at ,kasan1 in the interior of ,russia1 in latitude #eedh1 nearly the same with that of ,edinburgh1 the mean annual temperature is about #cgf2 at ,edinburgh it is #dghd4 ,at ,kasan the mean summer temperature is #fdhd1 and that of winter #bab2 whereas at ,edinburgh the mean summer temperature is #ehbh1 and that of winter #chff4 ,whence it appears that the difference of winter temperature is much greater than that of summer4 ,at ,quebec the summers are as warm as those in ,paris1 and grapes sometimes ripen in the open air3 whereas the winters are as severe as in ,petersburgh2 the snow lies five feet deep for several months1 wheel carriages cannot be used1 the ice is too hard for skating1 travelling is performed in sledges1 and frequently on the ice of the river ,st4 ,lawrence4 ,the cold at ,melville ,island on the #aeth of ,january1 #ahbj1 according to ,sir ,edward ,parry1 was #ee below the zero of ,fahrenheit's thermometer2 and when ,dr4 ,kane was on the northern coast of ,greenland it was #gj below that point2 yet the summer heat during the day in these high latitudes is insupportable4 ,observations tend to prove that all the climates of the earth are stable1 and that their vicissitudes are only periods or oscillations of more or less extent1 which vanish in the mean annual temperature of a sufficient number of years4 ,this constancy of the mean annual temperature of the different places on the surface of the globe shows that the same quantity of heat which is annually received by the earth is annually radiated into space2 and that would be the case even if the quantity of heat emitted by the sun should vary with his spots1 for1 if more were received1 more would be radiated4 ,nevertheless1 a variety of causes may disturb the climate of a place2 cultivation may make it warmer2 but it is at the expense of some other place1 which becomes colder in the same proportion4 ,there may be a succession of cold summers and mild winters1 but in some other country the contrary takes place to effect the compensation2 wind1 rain1 snow1 fog1 and the other meteoric phenomena1 are the ministers employed to accomplish the changes4 ,the distribution of heat may vary with a variety of circumstances2 but the absolute quantity lost and gained by the whole earth in the course of a year1 if not invariably the same1 is at least periodical4 ,,section ,,xxvi4 ,influence of ,temperature on ,vegetation--,vegetation varies with the ,latitude and ,height above the ,sea--,geographical ,distribution of ,land ,plants--,distribution of ,marine ,plants--,corallines1 ,shell-fish1 ,reptiles1 ,insects1 ,birds1 and ,quadrupeds--,varieties of ,mankind1 yet identity of ,species4 ,,the gradual decrease of temperature in the air and in the earth1 from the equator to the poles1 is clearly indicated by its influence on vegetation4 ,in the valleys of the torrid zone1 where the mean annual temperature is very high1 and where there is abundance of light and moisture1 nature adorns the soil with all the luxuriance of perpetual summer4 ,the palm1 the bombax ceiba1 and a variety of magnificent trees1 tower to the height of #aej or #bjj feet above the banana1 the bamboo1 the arborescent fern1 and numberless other tropical productions1 so interlaced by creeping and parasitical plants1 as often to present an impenetrable barrier4 ,but the richness of vegetation gradually diminishes with the temperature2 the splendour of the tropical forest is succeeded by the regions of the vine and olive2 these again yield to the verdant meadows of more temperate climes2 then follow the birch and the pine1 which probably owe their existence in very high latitudes more to the warmth of the soil than to that of the air4 ,but even these enduring plants become dwarfish shrubs1 till a verdant carpet of mosses and lichens1 enamelled with flowers1 exhibits the last sign of vegetable life during the short but fervid summers at the polar regions4 ,such is the effect of cold and diminished light on the vegetable kingdom1 that the number of species growing under the equator and in the northern latitudes of #de and #fh are in the proportion of the numbers #ab1 #d1 and #a4 ,notwithstanding the remarkable difference between a tropical and polar flora1 light and moisture seem to be almost the only requisites for vegetation1 since neither heat1 cold1 nor even comparative darkness1 absolutely destroy the fertility of nature4 ,in salt plains and sandy deserts alone hopeless barrenness prevails4 ,plants grow on the borders of hot springs3 they form the oases wherever moisture exists among the burning sands of ,africa2 they are found in caverns almost void of light1 though generally blanched and feeble4 ,the ocean teems with vegetation4 ,the snow itself not only produces a red lichen1 discovered by ,saussure in the frozen declivities of the ,alps1 found in abundance by the author crossing the ,col de ,bonhomme from ,savoy to ,piedmont1 and by the polar navigators in the ,arctic regions1 but it affords shelter to the productions of these inhospitable climes against the piercing winds that sweep over fields of everlasting ice4 ,those undaunted mariners narrate that under this cold defence plants spring up1 dissolve the snow a few inches round1 and the part above1 being again quickly frozen into a transparent sheet of ice1 admits the sun's rays1 which warm and cherish the plants in this natural hothouse1 till the returning summer renders such protection unnecessary4 ,the chemical action of light is1 however1 absolutely requisite for the growth of plants which derive their principal nourishment from the atmosphere4 ,they consume the carbonic acid gas1 nitrogen1 aqueous vapour1 and ammonia it contains2 but it is the chemical agency of light that enables them to absorb1 decompose1 and consolidate these substances into wood1 leaves1 flowers1 and fruit4 ,the atmosphere would soon be deprived of these elements of vegetable life were they not perpetually supplied by the animal creation2 while1 in return1 plants decompose the moisture they imbibe1 and1 having assimilated the carbonic acid gas1 they exhale oxygen for the maintenance of the animated creation1 and thus preserve a just equilibrium4 ,hence it is the combined and powerful influences of the whole solar beams that give such brilliancy to the tropical forests1 while1 with their decreasing energy in the higher latitudes1 vegetation becomes less vigorous4 ,on that account it is vain to expect that the fruit and flowers raised in our hothouses can ever have the flavour1 perfume1 or colouring equal to that which they acquire from the vivid light of their native skies4 ,by far the greater number of the known species of plants are indigenous in equinoctial ,america2 ,europe contains about half the number2 ,asia1 with its islands1 somewhat less than ,europe2 ,australia1 with the islands in the ,pacific1 still less2 and in ,africa there are fewer known vegetable productions than in any part of the globe of equal extent1 for that rich and luxuriant region discovered by ,dr4 ,livingstone has yet to be explored botanically4 ,very few social plants1 such as grasses and heaths that cover large tracts of land1 are to be found between the tropics1 except on the sea-coasts and elevated plains4 ,some exceptions to this1 however1 are to be met with in the jungles of the ,deccan1 c4 ,in the equatorial regions1 where the heat is always great1 the distribution of plants depends upon the mean annual temperature2 whereas in temperate zones the distribution is regulated in some degree by the summer heat4 ,some plants require a gentle heat of long continuance1 others flourish most where the extremes of heat and cold are greater4 ,the range of wheat is very great2 it may be cultivated as far north as the #fjth degree of latitude2 but in the torrid zone it will seldom form an ear below an elevation of #dejj feet above the level of the sea from exuberance of vegetation2 nor will it ripen generally above the height of #ab1jjj feet2 in ,tibet it ripens at a still greater elevation4 ,colonel ,sykes states that in the ,deccan wheat thrives as low as #ahjj feet above the sea4 ,the best wines are produced between the #cjth and #deth degrees of north latitude4 ,with regard to the vegetable kingdom1 elevation is equivalent to latitude as far as temperature is concerned4 ,in ascending the mountains of the torrid zone1 the richness of the tropical vegetation diminishes with the height2 a succession of plants similar to1 though not identical with1 those found in latitudes of corresponding mean temperature takes place2 the lofty forests by degrees lose their splendour2 stunted shrubs succeed2 till at last the progress of the lichen is checked by perpetual snow4 ,on the volcano of ,teneriffe there are five successive zones1 each producing distinct families of plants4 ,the first is the region of vines1 the next that of laurels2 these are followed by the region of pines1 of ,ericas or heaths1 of grass2 the whole covering the declivity of the peak through an extent of #aa1bjj feet of perpendicular height4 ,near the equator oaks flourish at the height of #ibjj feet above the sea2 and1 on the lofty range of the ,himalaya1 the primula1 the convallaria1 and the veronica flower1 but not the primrose1 the lily of the valley1 or the veronica1 which adorn our meadows2 for1 although the herbarium collected by ,moorcroft1 on his route from ,neetee to ,daba and ,gartope in ,chinese ,tartary1 at elevations as high or even higher than ,mont ,blanc1 abound in ,alpine and ,european genera1 the species are universally different1 with the single exception of the ,rhodiola rosea1 which is identical with the species that blooms in ,scotland4 ,it is not in this instance alone that similarity of climate obtains without identity of productions2 throughout the whole globe a certain analogy both of structure and appearance is frequently discovered between plants under corresponding circumstances which are yet specifically different4 ,it is even said that a difference of #be of latitude occasions a total change1 not only of vegetable productions1 but of organised beings4 ,certain it is that each separate region both of land and water1 from the frozen shores of the polar circles to the burning regions of the torrid zone1 possesses a flora peculiarly its own4 ,the whole globe has been divided by physical geographers into various botanical districts1 differing almost entirely in their specific vegetable productions1 the limits of which are most decided when they are separated by a wide expanse of ocean1 mountain chains1 sandy deserts1 salt plains1 or internal seas4 ,a considerable number of plants are common to the northern regions of ,asia1 ,europe1 and ,america1 where the continents almost unite2 but1 in approaching the south1 the floras of these three great divisions of the globe differ more and more even in the same parallels of latitude1 which shows that temperature alone is not the cause of the almost complete diversity of species that everywhere prevails4 ,the floras of ,china1 ,siberia1 ,tartary1 of the ,european district including central ,europe and the coast of the ,mediterranean1 and the ,oriental region comprising the countries round the ,black and ,caspian ,seas1 all differ in specific character4 ,only twenty-four species were found by ,,mm4 ,humboldt and ,bonpland in ,equinoctial ,america identical with those of the ,old ,world2 and ,dr4 ,robert ,brown not only found that a peculiar vegetation exists in ,australia between the #ccrd and #ceth parallels of south latitude1 but that at the eastern and western extremities of these parallels not one species is common to both1 and that certain genera also are almost entirely confined to these spots4 ,the number of species common to ,australia and ,europe are only #aff out of #dajj1 and probably some of these have been conveyed thither by the colonists2 but the greater part of that continent is still unexplored4 ,however1 this proportion exceeds what has hitherto been observed in southern ,africa1 and1 from what has been already stated1 the proportion of ,european species in ,equinoctial ,america is still less4 ,islands partake of the vegetation of the nearest continents2 but1 when very remote from land1 their floras are altogether peculiar4 ,the ,aleutian ,islands1 extending between ,asia and ,america1 partake of the vegetation of the northern parts of both continents1 and may have served as a chain of communication4 ,in ,madeira and ,teneriffe1 the plants of ,portugal1 ,spain1 the ,azores1 and of the northern coast of ,africa1 are found2 and the ,canaries contain a great number of plants belonging to the ,african coast4 ,but each of these islands possesses a flora that exists nowhere else2 and ,st4 ,helena1 standing alone in the midst of the ,atlantic ,ocean1 produces only two or three species of plants recognised as belonging to any other part of the world4 ,it appears from the investigations of ,m4 de ,humboldt that between the tropics the plants1 such as grasses and palms1 which have only one seed-lobe1 are to the tribe which have two seed-lobes1 like most of the ,european species1 in the proportion of one to four2 in the temperate zones they are as one to six2 and in the ,arctic regions1 where mosses and lichens1 which form the lowest order of the vegetable creation1 abound1 the proportion is as one to two4 ,annuals with one and two seed-lobes1 in the temperate zones1 amount to one-sixth of the whole1 omitting the cryptogamia 7,n4 #bah72 in the torrid zone they scarcely form one-twentieth1 and in ,lapland one-thirtieth part4 ,in approaching the equator the ligneous exceed the number of herbaceous plants2 in ,america there are #abj different species of forest trees1 whereas in the same latitudes in ,europe only #cd are to be found4 ,similar laws regulate the distribution of marine plants4 ,groups of alg1 or marine plants1 affect particular temperatures or zones of latitude and different depths1 though some few genera prevail throughout the ocean4 ,the polar ,atlantic basin to the #djth degree of north latitude presents a well-defined vegetation4 ,the ,west ,india seas1 including the ,gulf of ,mexico1 the eastern coast of ,south ,america1 the ,indian ,ocean and its gulfs1 the shores of ,new ,holland1 and the neighbouring islands1 have each their distinct species4 ,the ,mediterranean possesses a vegetation peculiar to itself1 extending to the ,black ,sea2 and the species of marine plants on the coast of ,syria and in the port of ,alexandria differ almost entirely from those of ,suez and the ,red ,sea4 ,it is observed that shallow seas have a different set of plants from such as are deeper and colder2 and1 unlike terrestrial vegetation1 the alg are more numerous in the mean latitudes than either towards the equator or the poles4 ,they vary also with the depth3 completely different kinds affect different depths1 their seeds being of such specific gravity as to remain and germinate where the parent plant grew4 ,the quantity of alg in that accumulation known as the sargassa or grassy sea is so great1 that the early navigators1 ,columbus and ,lerius1 compared it to extensively inundated meadows3 it impeded their ships1 and alarmed their sailors4 ,it is in the ,north ,atlantic1 a little to the west of the meridian of ,fayal1 one of the ,azores1 between the #beth and #cfth parallels of latitude4 ,a smaller bank lies between the #bbnd and #bfth degrees of north latitude1 about #hj leagues west of the meridian of the ,bahama ,islands4 ,these masses chiefly consist of one or two species of sargassa1 the most extensive genus of the order ,fucoide4 ,some of the seaweeds grow to enormous lengths1 and all are highly coloured1 though many of them must grow in deep water4 ,light1 however1 may not be the only principle on which the colour of vegetables depends1 since ,baron ,humboldt met with green plants growing in complete darkness in one of the mines at ,freyberg4 ,in the dark and tranquil caves of the ocean1 on the shores alternately covered and deserted by the restless waves1 on the lofty mountain and extended plain1 in the chilly regions of the north1 and in the genial warmth of the south1 specific diversity is a general law of the vegetable kingdom1 which cannot be accounted for by diversity of climate2 and yet the similarity1 though not identity1 of species is such1 under the same isothermal lines1 that if the number of species belonging to one of the great families of plants be known in any part of the globe1 the whole number of the flowering or more perfect plants1 and also the number of species composing the other vegetable families1 may be estimated with considerable accuracy4 ,various opinions have been formed on the original or primitive distribution of plants over the face of the globe2 but1 since botanical geography has become a science1 the phenomena observed have led to the conclusion that vegetable creation must have taken place in a number of distinctly different centres1 as the islands and continents rose above the ocean1 each of which was the original seat of a certain number of peculiar species which at first grew there and nowhere else4 ,heaths are exclusively confined to the ,old ,world2 and no indigenous rose-tree has ever been seen in the ,new1 the whole southern hemisphere being destitute of that beautiful and fragrant plant4 ,but this is still more confirmed by multitudes of particular plants1 having an entirely local and insulated existence1 growing spontaneously in some particular spot1 and in no other place3 for example1 the cedar of ,lebanon1 which grows indigenously on that mountain1 and in no other part of the world4 ,on the other hand1 as there can be no doubt that many races of plants have been extinguished1 ,sir ,john ,herschel thinks it possible that these solitary instances may be the last surviving remnants of the same group universally disseminated1 but in course of extinction1 or that perhaps two processes may be going on at the same time3--8,some groups may be spreading from their foci1 others retreating to their last holds40 ,the same laws obtain in the distribution of the animal creation4 ,even the microscopic existences1 which seem to be the most widely spread1 have their specific localities2 and the zoophyte 7,n4 #bai71 occupying the next lowest place in animated nature1 is widely scattered through the seas of the torrid zone1 each species being confined to the district and depth best suited to its wants4 ,mollusks1 or the animals of shells1 decrease in size and beauty with their distance from the equator2 and not only each sea and every basin of the ocean1 but each depth1 is inhabited by its peculiar tribe of fish4 ,indeed1 ,,mm4 ,peron and ,le ,sueur assert that1 among the many thousands of marine animals which they had examined1 there is not a single animal of the southern regions which is not distinguishable by essential characters from the analogous species in the northern seas4 ,reptiles are not exempt from the general law4 ,the saurian 7,n4 #bbj7 tribes of the four quarters of the globe differ in species2 and1 although warm countries abound in venomous snakes1 they are specifically different in different localities1 and decrease both in numbers and in the virulence of their poison with decrease of temperature4 ,the dispersion of insects necessarily follows that of the vegetables which supply their food2 and in general it is observed that each kind of plant is peopled by its peculiar inhabitants4 ,each species of bird has its peculiar haunt1 notwithstanding the locomotive powers of the winged tribes4 ,the emu is confined to ,australia1 the condor to the ,andes and their declivities1 and the bearded vulture or lemmergeyer to the ,alps4 ,some birds1 like the common sparrow1 have a wide range2 but those met with in every country are few in number4 ,quadrupeds are distributed in the same manner wherever man has not interfered4 ,such as are indigenous in one country are not the same with their congeners in another2 and1 with the exception of some kind of bats1 no mammiferous animal is indigenous in the ,polynesian ,archipelago1 nor in any of the islands on the borders of the central part of the ,pacific4 ,in reviewing the infinite variety of organised beings that people the surface of the globe1 nothing is more remarkable than the distinctions which characterise the different tribes of mankind1 from the ebony skin of the torrid zone to the fair and ruddy complexion of the ,scandinavian--a difference which existed in the earliest recorded times1 since the ,african is represented in the sacred writings to have been as black as he is at the present day1 and the most ancient ,egyptian paintings confirm that truth2 yet it appears1 from a comparison of the principal circumstances relating to the animal economy or physical character of the various tribes of mankind1 that the different races are identical in species4 ,many attempts have been made to trace the various tribes back to a common origin1 by collating the numerous languages which are or have been spoken4 ,some classes of these have few or no words in common1 yet exhibit a remarkable analogy in the laws of their grammatical construction4 ,the languages spoken by the native ,american nations afford examples of these2 indeed1 the refinement in the grammatical construction of the tongues of the ,american savages leads to the belief that they must originally have been spoken by a much more civilised class of mankind4 ,some tongues have little or no resemblance in structure1 though they correspond extensively in their vocabularies1 as the ,syrian dialects4 ,in all these cases it may be inferred that the nations speaking the languages in question descended from the same stock2 but the probability of a common origin is much greater in the ,indo-,european nations1 whose languages1 such as the ,sanscrit1 ,greek1 ,latin1 ,german1 c41 have an affinity both in structure and correspondence of vocables4 ,in many tongues not the smallest resemblance can be traced2 length of time1 however1 may have obliterated original identity2 but so many ages have passed before the subject became a study1 and so many languages have worn out of use1 that it may be doubted whether any satisfactory result will ever be arrived at with regard to the original speech of mankind4 ,,section ,,xxvii4 ,terrestrial ,heat--,radiation--,transmission--,mello ni's experiments--,heat in ,solar ,spectrum--,polarization of ,heat--,nature of ,heat--,absorptions--,dew--,rain--,combu stion--,expansion--,compensation ,pendulum--,transmission through ,crystals--,propagation--,dynamic ,theory of ,heat--,mechanical equivalent of ,heat--,latent ,heat is the ,force of ,expansion--,steam--,work performed by ,heat--,conservation of ,force--,mechanical ,power in the ,tides--,dynamical ,power of ,light--,analogy between ,light1 ,heat1 and ,sound4 ,,that heat producing rays exist independently of those of light is a matter of constant experience in the abundant emission of them from boiling water4 ,they dart in divergent straight lines from flame and from each point in the surfaces of hot bodies1 in the same manner as diverging rays of light proceed from every point of those that are luminous4 ,according to the experiments of ,sir ,john ,leslie1 radiation proceeds not only from the surface of substances1 but also from the particles at a minute depth below it4 ,he found that the emission is most abundant in a direction perpendicular to the radiating surface1 and that it is more rapid from a rough than from a polished surface3 radiation1 however1 can only take place in air and in vacuo2 it is altogether imperceptible when the hot body is enclosed in a solid or liquid4 ,heated substances1 when exposed to the open air1 continue to radiate heat till they become nearly of the temperature of the surrounding medium4 ,the radiation is very rapid at first1 but diminishes according to a known law with the temperature of the heated body4 ,it appears1 also1 that the radiating power of a surface is inversely as its reflecting power2 and bodies that are most impermeable to heat radiate least4 ,substances1 however1 have an elective power1 only reflecting heat of a certain refrangibility4 ,mr4 ,grove gives paper1 snow1 and lime as instances1 which1 although all white1 radiate heat of different refrangibilities1 while metals1 whatever their colour may be1 radiate all kinds alike4 ,rays of heat1 whether they proceed from the sun1 from flame1 or other terrestrial sources1 luminous or non-luminous1 are instantaneously transmitted through solid and liquid substances1 there being no appreciable difference in the time they take to pass through layers of any nature or thickness whatever4 ,they pass also with the same facility whether the media be agitated or at rest2 and in these respects the analogy between light and heat is perfect4 ,radiant heat passes through the gases with the same facility as light2 but a remarkable difference obtains in the transmission of light and heat through most solid and liquid substances1 the same body being often perfectly permeable to the luminous1 and altogether impermeable to the calorific rays4 ,for example1 thin and perfectly transparent plates of alum and citric acid sensibly transmit all the rays of light from an argand lamp1 but stop eight or nine tenths of the concomitant heat2 whilst a large piece of brown rock-crystal gives a free passage to the radiant heat1 but intercepts almost all the light4 ,alum united to green glass is also capable of transmitting the brightest light1 but it gives not the slightest indication of heat2 while rock-salt covered thickly over with soot1 so as to be perfectly opaque to light1 transmits a considerable quantity of heat4 ,m4 ,melloni has established the general law in uncrystallized substances such as glass and liquids1 that the property of instantaneously transmitting heat is in proportion to their refractive powers4 ,the law1 however1 is entirely at fault in bodies of a crystalline texture4 ,carbonate of lead1 for instance1 which is colourless1 and possesses a very high refractive power with regard to light1 transmits less radiant heat than ,iceland spar or rock-crystal1 which are very inferior to it in the order of refrangibility2 whilst rock-salt1 which has the same transparency and refractive power with alum and citric acid1 transmits six or eight times as much heat4 ,this remarkable difference in the transmissive power of substances having the same appearance is attributed by ,m4 ,melloni to their crystalline form1 and not to the chemical composition of their molecules1 as the following experiments prove4 ,a block of common salt cut into plates entirely excludes calorific radiation2 yet1 when dissolved in water1 it increases the transmissive power of that liquid3 moreover1 the transmissive power of water is increased in nearly the same degree1 whether salt or alum be dissolved in it2 yet these two substances transmit very different quantities of heat in their solid state4 ,notwithstanding the influence of crystallization on the transmissive power of bodies1 no relation has been traced between that power and the crystalline form4 ,the transmission of radiant heat is analogous to that of light through coloured media4 ,when common white light passes through a red liquid1 almost all the more refrangible rays1 and a few of the red1 are intercepted by the first layer of the fluid2 fewer are intercepted by the second1 still less by the third1 and so on3 till at last the losses become very small and invariable1 and those rays alone are transmitted which give the red colour to the liquid4 ,in a similar manner1 when plates of the same thickness of any substance1 such as glass1 are exposed to an argand lamp1 a considerable portion of the radiant heat is arrested by the first plate1 a less portion by the second1 still less by the third1 and so on1 the quantity of lost heat decreasing till at last the loss becomes a constant quantity4 ,the transmission of radiant heat through a solid mass follows the same law4 ,the losses are very considerable on first entering it1 but they rapidly diminish in proportion as the heat penetrates deeper1 and become constant at a certain depth4 ,indeed1 the only difference between the transmission of radiant heat through a solid mass1 or through the same mass when cut into plates of equal thickness1 arises from the small quantity of heat that is reflected at the surface of the plates4 ,it is evident1 therefore1 that the heat gradually lost is not intercepted at the surface1 but absorbed in the interior of the substance1 and that heat which has passed through one stratum of air experiences a less absorption in each of the succeeding strata1 and may therefore be propagated to a greater distance before it is extinguished4 ,the experiments of ,m4 de ,laroche show that glass1 however thin1 totally intercepts the obscure rays of heat when they flow from a body whose temperature is lower than that of boiling water2 that1 as the temperature increases1 the calorific rays are transmitted more and more abundantly2 and1 when the body becomes highly luminous1 that they penetrate the glass with perfect ease4 ,the extreme brilliancy of the sun is probably the reason why his heat1 when brought to a focus by a lens1 is more intense than any that has been produced artificially4 ,it is owing to the same cause that glass screens1 which entirely exclude the heat of a common fire1 are permeable by the solar heat4 ,the results obtained by ,m4 de ,laroche have been confirmed by the experiments of ,m4 ,melloni on heat radiated from sources of different temperatures1 whence it appears that the calorific rays pass less abundantly not only through glass1 but through rock-crystal1 ,iceland spar1 and other diaphanous bodies1 both solid and liquid1 according as the temperature of their origin is diminished1 and that they are altogether intercepted when the temperature is about that of boiling water4 ,in fact1 he has proved that the heat emanating from the sun or from a bright flame consists of rays which differ from each other as much as the coloured rays do which constitute white light4 ,this explains the reason of the loss of heat as it penetrates deeper and deeper into a solid mass1 or in passing through a series of plates2 for1 of the different kinds of rays which dart from a vivid flame1 all are successively extinguished by the absorbing nature of the substance through which they pass1 till those homogeneous rays alone remain which have the greatest facility in passing through that particular substance2 exactly as in a red liquid the violet1 blue1 green1 orange1 and yellow rays are extinguished1 and the red are transmitted4 ,m4 ,melloni employed four sources of heat1 two of which were luminous and two obscure2 namely1 an oil-lamp without a glass1 incandescent platina1 copper heated to #fif1 and a copper vessel filled with water at the temperature of #agh-a_/b of ,fahrenheit4 ,rock-salt transmitted heat in the proportion of #ib rays out of #ajj from each of these sources2 but all other substances pervious to radiant heat1 whether solid or liquid1 transmitted more heat from sources of high temperature than from such as are low4 ,for instance1 limpid and colourless fluate of lime transmitted in the proportion of #gh rays out of #ajj from the lamp1 #fi from the platina1 #db from the copper1 and #cc from the hot water2 while transparent rock-crystal transmitted #ch rays in #ajj from the lamp1 #bh from the platina1 #f from the copper1 and #i from the hot water4 ,pure ice transmitted only in the proportion of #f rays in the #ajj from the lamp1 and entirely excluded those from the other three sources4 ,out of #ci different substances1 #cd were pervious to the calorific rays from hot water1 #ad excluded those from the hot copper1 and #d did not transmit those from the platinum4 ,thus it appears that heat proceeding from these four sources is of different kinds3 this difference in the nature of the calorific rays is also proved by another experiment1 which will be more easily understood from the analogy of light4 ,red light1 emanating from red glass1 will pass in abundance through another piece of red glass1 but it will be absorbed by green glass2 green rays will more readily pass through a green medium than through one of any other colour4 ,this holds with regard to all colours2 so in heat4 ,rays of heat of the same intensity1 which have passed through different substances1 are transmitted in different quantities by the same piece of alum1 and are sometimes stopped altogether2 showing that rays which emanate from different substances possess different qualities4 ,it appears that a bright flame furnishes rays of heat of all kinds1 in the same manner as it gives light of all colours2 and1 as coloured media transmit some coloured rays and absorb the rest1 so bodies transmit some rays of heat and exclude the others4 ,rock-salt alone resembles colourless transparent media in transmitting all kinds of heat1 even that of the hand1 just as they transmit white light1 consisting of rays of all colours4 ,radiant heat is unequally refracted by a prism of rock-salt like light1 and the rays of heat thus dispersed are found to possess properties analogous to the rays of the coloured spectrum4 ,the property of transmitting the calorific rays diminishes to a certain degree with the thickness of the body they have to traverse1 but not so much as might be expected4 ,a piece of very transparent alum transmitted three or four times less radiant heat from the flame of a lamp than a piece of nearly opaque quartz about a hundred times as thick4 ,however1 the influence of thickness upon the phenomena of transmission increases with the decrease of temperature in the origin of the rays1 and becomes very great when that temperature is low4 ,this is a circumstance intimately connected with the law established by ,m4 de ,laroche2 for ,m4 ,melloni observed that the difference between the quantities of heat transmitted by the same plate of glass1 exposed successively to several sources of heat1 diminished with the thinness of the plate1 and vanished altogether at a certain limit2 and that a film of mica transmitted the same quantity of heat1 whether it was exposed to incandescent platinum or to a mass of iron heated to #cfj4 ,coloured glasses transmit rays of light of certain degrees of refrangibility1 and absorb those of other degrees4 ,for example1 red glass absorbs the more refrangible rays1 and transmits the red1 which are the least refrangible4 ,on the contrary1 violet glass absorbs the least refrangible1 and transmits the violet1 which are the most refrangible4 ,now ,m4 ,melloni has found1 that1 although the colouring matter of glass diminishes its power of transmitting heat1 yet red1 orange1 yellow1 blue1 violet1 and white glass transmit calorific rays of all degrees of refrangibility2 whereas green glass possesses the peculiar property of transmitting the least refrangible calorific rays1 and stopping those that are most refrangible4 ,it has therefore the same elective action for heat that coloured glass has for light1 and its action on heat is analogous to that of red glass on light4 ,alum and sulphate of lime are exactly opposed to green glass in their action on heat1 by transmitting the most refrangible rays with the greatest facility4 ,the heat which has already passed through green or opaque black glass will not pass through alum1 whilst that which has been transmitted through glasses of other colours traverses it readily4 ,by reversing the experiment1 and exposing different substances to heat that had already passed through alum1 ,m4 ,melloni found that the heat emerging from alum is almost totally intercepted by opaque substances1 and is abundantly transmitted by all such as are transparent and colourless1 and that it suffers no appreciable loss when the thickness of the plate is varied within certain limits4 ,the properties of the heat therefore which issues from alum nearly approach to those of light and solar heat4 ,radiant heat in traversing various media is not only rendered more or less capable of being transmitted a second time1 but1 according to the experiments of ,professor ,powell1 it becomes more or less susceptible of being absorbed in different quantities by black or white surfaces4 ,m4 ,melloni has proved that solar heat contains rays which are affected by different substances in the same way as if the heat proceeded from a terrestrial source2 whence he concludes that the difference observed between the transmission of terrestrial and solar heat arises from the circumstance of solar heat containing all kinds of heat1 whilst in other sources some of the kinds are wanting4 ,radiant heat1 from sources of any temperature whatever1 is subject to the same laws of reflection and refraction as rays of light4 ,the index of refraction from a prism of rock-salt1 determined experimentally1 is nearly the same for light and heat4 ,liquids1 the various kinds of glass1 and probably all substances1 whether solid or liquid1 that do not crystallize regularly1 are more pervious to the calorific rays according as they possess a greater refractive power4 ,for example1 the chloride of sulphur1 which has a high refractive power1 transmits more of the calorific rays than the oils1 which have a less refractive power3 oils transmit more radiant heat than the acids2 the acids more than aqueous solutions2 and the latter more than pure water1 which of all the series has the least refractive power1 and is the least pervious to heat4 ,m4 ,melloni observed also that each ray of the solar spectrum follows the same law of action with that of terrestrial rays having their origin in sources of different temperatures2 so that the very refrangible rays may be compared to the heat emanating from a focus of high temperature1 and the least refrangible to the heat which comes from a source of low temperature4 ,thus1 if the calorific rays emerging from a prism be made to pass through a layer of water contained between two plates of glass1 it will be found that these rays suffer a loss in passing through the liquid as much greater as their refrangibility is less4 ,the rays of heat that are mixed with the blue or violet light pass in great abundance1 while those in the obscure part which follows the red light are almost totally intercepted4 ,the first1 therefore1 act like the heat of a lamp1 and the last like that of boiling water4 ,these circumstances explain the phenomena observed by several philosophers with regard to the point of greatest heat in the solar spectrum1 which varies with the substance of the prism4 ,sir ,william ,herschel1 who employed a prism of flint glass1 found that point to be a little beyond the red extremity of the spectrum2 but1 according to ,m4 ,seebeck1 it is found to be upon the yellow1 upon the orange1 on the red1 or at the dark limit of the red1 according as the prism consists of water1 sulphuric acid1 crown or flint glass4 ,if it be recollected that1 in the spectrum from crown glass1 the maximum heat is in the red part1 and that the solar rays1 in traversing a mass of water1 suffer losses inversely as their refrangibility1 it will be easy to understand the reason of the phenomenon in question4 ,the solar heat which comes to the anterior face of the prism of water consists of rays of all degrees of refrangibility4 ,now1 the rays possessing the same index of refraction with the red light suffer a greater loss in passing through the prism than the rays possessing the refrangibility of the orange light1 and the latter lose less in their passage than the heat of the yellow4 ,thus the losses1 being inversely proportional to the degree of refrangibility of each ray1 cause the point of maximum heat to tend from the red towards the violet1 and therefore it rests upon the yellow part4 ,the prism of sulphuric acid1 acting similarly1 but with less energy than that of water1 throws the point of greatest heat on the orange2 for the same reason1 the crown and flint glass prisms transfer that point respectively to the red and to its limit4 ,m4 ,melloni1 observing that the maximum point of heat is transferred farther and farther towards the red end of the spectrum1 according as the substance of the prism is more and more permeable to heat1 inferred that a prism of rock-salt1 which possesses a greater power of transmitting the calorific rays than any known body1 ought to throw the point of greatest heat to a considerable distance beyond the visible part of the spectrum1--an anticipation which experiment fully confirmed1 by placing it as much beyond the dark limits of the red rays as the red part is distant from the blueish green band of the spectrum4 ,in all these experiments ,m4 ,melloni employed a thermomultiplier1--an instrument that measures the intensity of the transmitted heat with an accuracy far beyond what any thermometer ever attained4 ,it is a very elegant application of ,m4 ,seebeck's discovery of thermo-electricity2 but the description of this instrument is reserved for a future occasion1 because the principle on which it is constructed has not yet been explained4 ,in the beginning of the present century1 not long after ,m4 ,malus had discovered the polarization of light1 he and ,m4 ,berard proved that the heat which accompanies the sun's light is capable of being polarized2 but their attempts totally failed with heat derived from terrestrial1 and especially from non-luminous sources4 ,m4 ,berard1 indeed1 imagined that he had succeeded2 but1 when his experiments were repeated by ,mr4 ,lloyd and ,professor ,powell1 no satisfactory result could be obtained4 ,m4 ,melloni resumed the subject1 and endeavoured to effect the polarization of heat by tourmaline1 as in the case of light4 ,it was already shown that two slices of tourmaline1 cut parallel to the axis of the crystal1 transmit a great portion of the incident light when looked through with their axes parallel1 and almost entirely exclude it when they are perpendicular to one another4 ,should radiant heat be capable of polarization1 the quantity transmitted by the slices of tourmaline in their former position ought greatly to exceed that which passes through them in the latter1 yet ,m4 ,melloni found that the quantity of heat was the same in both cases3 whence he inferred that heat from a terrestrial source is incapable of being polarized4 ,professor ,forbes of ,edinburgh1 who prosecuted this subject with great acuteness and success1 came to the same conclusion in the first instance2 but it occurred to him1 that1 as the pieces of tourmaline became heated by being very near the lamp1 the secondary radiation from them rendered the very small difference in the heat that was transmitted in the two positions of the pieces of tourmaline imperceptible4 ,nevertheless he succeeded in proving1 by numerous observations1 that heat from various sources is polarized by the tourmaline2 but that the effect with non-luminous heat is very minute and difficult to perceive1 on account of the secondary radiation4 ,though light is almost entirely excluded in one position of the pieces of tourmaline1 and transmitted in the other1 a vast quantity of radiant heat passes through them in all positions4 ,eighty-four per cent4 of the heat from an argand lamp passed through them in the case where light was altogether stopped4 ,it is only the difference in the quantity of transmitted heat that gives evidence of its polarization4 ,the second slice of tourmaline1 when perpendicular to the first1 stops all the light1 but transmits a great proportion of heat2 alum1 on the contrary1 stops almost all the heat1 and transmits the light2 whence it may be concluded that heat1 though intimately partaking the nature of light1 and accompanying it under certain circumstances1 as in reflection and refraction1 is capable of almost complete separation from it under others4 ,the separation has since been perfectly effected by ,m4 ,melloni1 by passing a beam of light through a combination of water and green glass1 coloured by the oxide of copper4 ,even when the transmitted light was concentrated by lenses1 so as to render it almost as brilliant as the direct light of the sun1 it showed no sensible heat4 ,professor ,forbes next employed two bundles of lamin of mica1 placed at the polarizing angle1 and so cut that the plane of incidence of the heat corresponded with one of the optic axes of this mineral4 ,the heat transmitted through this apparatus was polarized from a source whose temperature was even as low as #bjj2 heat was also polarized by reflection2 but the experiments1 though perfectly successful1 are more difficult to conduct4 ,it appears1 from the various experiments of ,m4 ,melloni and ,professor ,forbes1 that all the calorific rays emanating from the sun and terrestrial sources are equally capable of being polarized by reflection and by refraction1 whether double or single1 and that they are also capable of circular polarization by all the methods employed in the circular polarization of light4 ,plates of quartz cut at right angles to the axis of the prism possess the property of turning the calorific rays in one direction1 while other plates of the same substance from a differently modified prism cause the rays to rotate in the contrary direction2 and two plates combined1 when of different affection1 and of equal thickness1 counteract each other's effects as in the case of light4 ,tourmaline separates the heat into two parts1 one of which it absorbs1 while it transmits the other2 in short1 the transmission of radiant heat is precisely similar to that of light4 ,since heat is polarized in the same manner as light1 it may be expected that polarized heat transmitted through doubly refracting substances should be separated into two pencils1 polarized in planes at right angles to each other2 and that when received on an analyzing plate they should interfere and produce invisible phenomena1 perfectly analogous to those described in ,section ,,xxii4 with regard to light 7,n4 #bba74 ,it was shown1 in the same section1 that if light polarized by reflection from a pane of glass be viewed through a plate of tourmaline1 with its longitudinal section vertical1 an obscure cloud1 with its centre wholly dark1 is seen on the glass4 ,when1 however1 a plate of mica uniformly about the thirteenth of an inch in thickness is interposed between the tourmaline and the glass1 the dark spot vanishes1 and a succession of very splendid colours are seen2 and1 as the mica is turned round in a plane perpendicular to the polarized ray1 the light is stopped when the plane containing the optic axis of the mica is parallel or perpendicular to the plane of polarization4 ,now1 instead of light1 if heat from a non-luminous source be polarized in the manner described1 it ought to be transmitted and stopped by the interposed mica under the same circumstances under which polarized light would be transmitted or stopped4 ,professor ,forbes found that this is really the case1 whether he employed heat from luminous or non-luminous sources3 and he had evidence1 also1 of circular and elliptical polarization of heat4 ,it therefore follows1 that if heat were visible1 under similar circumstances we should see figures perfectly similar to those given in ,note #bac1 and those following2 and1 as these figures are formed by the interference of undulations of light1 it may be inferred that heat1 like light1 is propagated by undulations of the ethereal medium1 which interfere under certain conditions1 and produce figures analogous to those of light4 ,it appears also1 from ,mr4 ,forbes's experiments1 that the undulations of heat are longer than the undulations of light2 and it has already been mentioned that ,professor ,draper considers them to be normal1 like those of sound4 ,that light and heat are both vibrations of the ethereal medium is not the less true on account of the rays of heat being unseen1 for the condition of visibility or invisibility may only depend upon the construction of our eyes1 and not upon the nature of the motion which produces these sensations in us4 ,the sense of seeing may be confined within certain limits4 ,the chemical rays beyond the violet end of the spectrum may be too rapid1 or not sufficiently excursive1 in their vibrations1 to be visible to the human eye2 and the calorific rays beyond the other end of the spectrum may not be sufficiently rapid1 or too extensive1 in their undulations1 to affect our optic nerves1 though both may be visible to certain animals or insects4 ,we are altogether ignorant of the perceptions which direct the carrier-pigeon to his home1 or of those in the antenn of insects which warn them of the approach of danger2 nor can we understand the telescopic vision which directs the vulture to his prey before he himself is visible even as a speck in the heavens4 ,so1 likewise1 beings may exist on earth1 in the air1 or in the waters1 which hear sounds our ears are incapable of hearing1 and which see rays of light and heat of which we are unconscious4 ,our perceptions and faculties are limited to a very small portion of that immense chain of existence which extends from the ,creator to evanescence4 ,the identity of action under similar circumstances is one of the strongest arguments in favour of the common nature of the chemical1 visible1 and calorific rays4 ,they are all capable of reflection from polished surfaces1 of refraction through diaphanous substances1 of polarization by reflection and by doubly refracting crystals2 their velocity is prodigious2 they may be concentrated and dispersed by convex and concave mirrors2 they pass with equal facility through rock-salt and are capable of radiation2 and they are subject to the same law of interference with those of light3 hence there can be no doubt that the whole assemblage of rays visible and invisible which constitute a solar beam are propagated by the undulations of the ethereal medium1 and consequently as motions they come under the same laws of analysis4 ,when radiant heat falls upon a surface1 part of it is reflected and part of it is absorbed2 consequently1 the best reflectors possess the least absorbing powers4 ,the temperature of very transparent fluids is not raised by the passage of the sun's rays1 because they do not absorb any of them2 and1 as his heat is very intense1 transparent solids arrest a very small portion of it4 ,the absorption of the sun's rays is the cause both of the colour and temperature of solid bodies4 ,a black substance absorbs all the rays of light1 and reflects none2 and since it absorbs1 at the same time1 all the calorific rays1 it becomes sooner warm1 and rises to a higher temperature1 than bodies of any other colour4 ,blue bodies come next to black in their power of absorption4 ,and1 since substances of a blue tint absorb all the other colours of the spectrum1 they absorb by far the greatest part of the calorific rays1 and reflect the blue where they are least abundant4 ,next in order come the green1 yellow1 red1 and1 last of all1 white bodies1 which reflect nearly all the rays both of light and heat4 ,however1 there are certain limpid and colourless media1 which in some cases intercept calorific radiations and become heated1 while in other cases they transmit them and undergo no change of temperature4 ,all substances may be considered to radiate heat1 whatever their temperature may be1 though with different intensities1 according to their nature1 the state of their surfaces1 and the temperature of the medium into which they are brought4 ,but every surface absorbs as well as radiates heat2 and the power of absorption is always equal to that of radiation2 for1 under the same circumstances1 matter which becomes soon warm also cools rapidly4 ,there is a constant tendency to an equal diffusion of heat1 since every body in nature is giving and receiving it at the same instant2 each will be of uniform temperature when the quantities of heat given and received during the same time are equal--that is1 when a perfect compensation takes place between each and all the rest4 ,our sensations only measure comparative degrees of heat3 when a body1 such as ice1 appears to be cold1 it imparts fewer calorific rays than it receives2 and when a substance seems to be warm--for example1 a fire--it gives more heat than it takes4 ,the phenomena of dew and hoar-frost are owing to this inequality of exchange2 the heat radiated during the night by substances on the surface of the earth1 into a clear expanse of sky1 is lost to us1 and no return is made from the blue vault1 so that their temperature sinks below that of the air1 whence they abstract a part of that heat which holds the atmospheric humidity in solution1 and a deposition of dew takes place4 ,if the radiation be great1 the dew is frozen and becomes hoar-frost1 which is the ice of dew4 ,cloudy weather is unfavourable to the formation of dew1 by preventing the free radiation of heat2 and actual contact is requisite for its deposition1 since it is never suspended in the air like fog4 ,plants derive a great part of their nourishment from this source2 and1 as each possesses a power of radiation peculiar to itself1 they are capable of procuring a sufficient supply for their wants4 ,the action of the chemical rays imparts to all substances more or less the power of condensing vapour on those parts on which they fall1 and must therefore have a considerable influence on the deposition of dew4 ,there may be a low degree of humidity in the air which may yet contain a great quantity of aqueous vapour1 for vapour while it exists as gas is dry4 ,the temperature at which the atmosphere can contain no more vapour without precipitation is called the dew point1 and is measured by the hygrometer4 ,in foretelling the changes of weather it is scarcely inferior to the barometer4 ,steam is formed throughout the whole mass of a boiling liquid1 whereas evaporation takes place only at the free surface of liquids1 and that under the ordinary temperature and pressure of the atmosphere4 ,there is a constant evaporation from the land and water all over the earth4 ,the rapidity of the formation does not depend altogether on the dryness of the air2 according to ,dr4 ,dalton's experiments1 it depends also on the difference between the tension of the vapour which is forming1 and that which is already in the atmosphere4 ,in calm weather vapour accumulates in the stratum of air immediately above the evaporating surface1 and retards the formation of more2 whereas a strong wind accelerates the process by carrying off the vapour as soon as it rises1 and making way for a succeeding portion of dry air4 ,rain is formed by the mixing of two masses of air of different temperatures2 the colder part1 by abstracting from the other the heat which holds it in solution1 occasions the particles to approach each other and form drops of water1 which1 becoming too heavy to be sustained by the atmosphere1 sink to the earth by gravitation in the form of rain4 ,the contact of two strata of air of different temperatures1 moving rapidly in opposite directions1 occasions an abundant precipitation of rain4 ,when the masses of air differ very much in temperature1 and meet suddenly1 hail is formed4 ,this happens frequently in hot plains near a ridge of mountains1 as in the south of ,france1 from the sudden descent of an intensely cold current of wind into a mass of air nearly saturated with vapour4 ,such also is the cause of the severe hail-storms which occasionally take place on extensive plains within the tropics4 ,an accumulation of heat invariably produces light3 with the exception of the gases1 all bodies which can endure the requisite degree of heat without decomposition begin to emit light at the same temperature2 but1 when the quantity of heat is so great as to render the affinity of their component particles less than their affinity for the oxygen of the atmosphere1 a chemical combination takes place with the oxygen1 light and heat are evolved1 and fire is produced4 ,combustion--so essential for our comfort1 and even existence--takes place very easily from the small affinity between the component parts of atmospheric air1 the oxygen being nearly in a free state2 but1 as the cohesive force of the particles of different substances is very variable1 different degrees of heat are requisite to produce their combustion4 ,the tendency of heat to a state of equal diffusion or equilibrium1 either by radiation or contact1 makes it necessary that the chemical combination which occasions combustion should take place instantaneously2 for1 if the heat were developed progressively1 it would be dissipated by degrees1 and would never accumulate sufficiently to produce a temperature high enough for the evolution of flame4 ,it is a general law that all bodies expand by heat and contract by cold4 ,the expansive force of heat has a constant tendency to overcome the attraction of cohesion1 and to separate the constituent particles of solids and fluids2 by this separation the attraction of aggregation is more and more weakened1 till at last it is entirely overcome1 or even changed into repulsion4 ,by the continual addition of heat1 solids may be made to pass into liquids1 and from liquids to the ariform state1 the dilatation increasing with the temperature2 and every substance expands according to a law of its own4 ,gases expand more than liquids1 and liquids more than solids4 ,the expansion of air is more than eight times that of water1 and the increase in the bulk of water is at least forty-five times greater than that of iron4 ,metals dilate uniformly from the freezing to the boiling points of the thermometer2 the uniform expansion of the gases extends between still wider limits2 but1 as liquidity is a state of transition from the solid to the ariform condition1 the equable dilatation of liquids has not so extensive a range4 ,this change of bulk1 corresponding to the variation of heat1 is one of the most important of its effects1 since it furnishes the means of measuring relative temperature by the thermometer and pyrometer4 ,the rate of expansion of solids varies at their transition to liquidity1 and that of liquidity is no longer equable near their change to an ariform state4 ,there are exceptions1 however1 to the general laws of expansion2 some liquids have a maximum density corresponding to a certain temperature1 and dilate whether that temperature be increased or diminished4 ,for example--water expands whether it be heated above or cooled below #dj4 ,the solidification of some liquids1 and especially their crystallization1 is always accompanied by an increase of bulk4 ,water dilates rapidly when converted into ice1 and with a force sufficient to split the hardest substances4 ,the formation of ice is therefore a powerful agent in the disintegration and decomposition of rocks1 operating as one of the most efficient causes of local changes in the structure of the crust of the earth2 of which we have experience in the tremendous boulemens of mountains in ,switzerland4 ,but ,professor ,w4 ,thomson has proved experimentally that it requires a lower temperature to freeze water under pressure than when free4 ,the dilatation of substances by heat1 and their contraction by cold1 occasion such irregularities in the rate of clocks and watches as would render them unfit for astronomical or nautical purposes1 were it not for a very beautiful application of the laws of unequal expansion4 ,the oscillations of a pendulum are the same as if its whole mass were united in one dense particle1 in a certain point of its length1 called the centre of oscillation4 ,if the distance of this point from the point by which the pendulum is suspended were invariable1 the rate of the clock would be invariable also4 ,the difficulty is to neutralize the effects of temperature1 which is perpetually increasing or diminishing its length4 ,among many contrivances1 ,graham's compensation pendulum is the most simple4 ,he employed a glass tube containing mercury4 ,when the tube expands from the effects of heat1 the mercury expands much more2 so that its surface rises a little more than the end of the pendulum is depressed1 and the centre of oscillation remains stationary4 ,harrison invented a pendulum which consists of seven bars of steel and of brass1 joined in the shape of a gridiron1 in such a manner that1 if by change of temperature the bars of brass raise the weight at the end of the pendulum1 the bars of steel depress it as much4 ,in general1 only five bars are used2 three being of steel1 and two a mixture of silver and zinc4 ,the effects of temperature are neutralized in chronometers upon the same principle2 and to such perfection are they brought1 that the loss or gain of one second in twenty-four hours for two days running would render one unfit for use4 ,accuracy in surveying depends upon the compensation rods employed in measuring bases4 ,thus1 the laws of the unequal expansion of matter judiciously applied have an immediate influence upon our estimation of time2 of the motions of bodies in the heavens1 and of their fall upon the earth2 on our determination of the figure of the globe1 and on our system of weights and measures2 on our commerce abroad1 and the mensuration of our lands at home4 ,the expansion of the crystalline substances takes place under very different circumstances from the dilatation of such as are not crystallized4 ,the latter become both longer and thicker by an accession of heat1 whereas ,m4 ,mitscherlich has found that the former expand differently in different directions2 and1 in a particular instance1 extension in one direction is accompanied by contraction in another3 for example1 ,iceland spar is dilated in the direction of its axis of double refraction 7,n4 #bje71 but at right angles to that axis it is contracted1 which brings the crystal nearer to the form of the cube and diminishes its double refractive power4 ,when heat is applied to crystals of sulphate of lime1 the two optical axes 7,n4 #bjg7 gradually approach1 and at last coincide2 when the heat is increased1 the axes open again1 but in a direction at right angles to their former position4 ,by experiment ,m4 ,senarmont has concluded1 that in media constituted like crystals of the rhomboidal 7,n4 #afi7 system the conducting power varies in such a manner1 that1 supposing a centre of heat to exist within them1 and the medium to be indefinitely extended in all directions1 the isothermal surfaces are concentric ellipsoids of revolution round the axes of symmetry1 or at least surfaces differing but little from them4 ,the internal structure of crystallized matter must be very peculiar thus to modify the expansive power of heat4 ,heat applied to the surface of a fluid is propagated downwards very slowly1 the warmer1 and consequently lighter strata1 always remaining at the top4 ,this is the reason why the water at the bottom of lakes fed from ,alpine chains is so cold2 for the heat of the sun is transfused but a little way below the surface4 ,when the heat is applied below a liquid1 the particles continually rise as they become specifically lighter1 and diffuse the heat through the mass1 their place being perpetually supplied by those that are more dense4 ,the power of conducting heat varies materially in different liquids4 ,mercury conducts twice as fast as an equal bulk of water1 and therefore it appears to be very cold4 ,a hot body diffuses its heat in the air by a double process3 the air in contact with it becoming lighter ascends and scatters its heat by transmission1 while at the same time another portion is discharged in straight lines by the radiating power of the surface4 ,hence a substance cools more rapidly in air than in vacuo1 because in the latter case the process is carried on by radiation alone4 ,it is probable that the earth having been originally of very high temperature has become cooler by radiation alone1 the ethereal medium being too rare to carry off much heat by contact4 ,heat is propagated with more or less rapidity through all bodies2 air is the worst conductor1 and consequently mitigates the severity of cold climates by preserving the heat imparted to the earth by the sun4 ,on the contrary1 dense bodies1 especially metals1 possess the power of conduction in the greatest degree1 but the transmission requires time4 ,if a bar of iron twenty inches long be heated at one extremity1 the heat takes four minutes in passing to the other4 ,the particle of the metal that is first heated communicates the heat to the second1 and the second to the third3 so that the temperature of the intermediate molecule at any instant is increased by the excess of the temperature of the first above its own1 and diminished by the excess of its own temperature above that of the third4 ,that however will not be the temperature indicated by the thermometer1 because as soon as the particle is more heated than the surrounding atmosphere it loses its heat by radiation1 in proportion to the excess of its actual temperature above that of the air4 ,the velocity of the discharge is directly proportional to the temperature1 and inversely as the length of the bar4 ,as there are perpetual variations in the temperature of all terrestrial substances1 and of the atmosphere1 from the rotation of the earth1 and its revolution round the sun1 from combustion1 friction1 fermentation1 electricity1 and an infinity of other causes1 the tendency to restore the equability of temperature by the transmission of heat must maintain all the particles of matter in a state of perpetual oscillation1 which will be more or less rapid according to the conducting powers of the substances4 ,from the motion of the heavenly bodies about their axes1 and also round the sun1 exposing them to perpetual changes of temperature1 it may be inferred that similar causes will produce like effects in them too4 ,the revolutions of the double stars show that they are not at rest2 and although we are totally ignorant of the changes that may be going on in the nebul and millions of other remote bodies1 it is hardly possible that they should be in absolute repose2 so that1 as far as our knowledge extends1 motion is a law of the universe and the immediate cause of heat1 as in the sunbeam so also in all terrestrial phenomena4 ,this is by no means hypothetical1 but founded upon fact and experiment4 ,heat is produced by motion and is equivalent to it1 for we measure heat by motion in the thermometer4 ,the heat evolved by percussion is proportional to the force of the blow2 by repeated blows iron becomes red hot2 and the quantity of heat produced by friction1 whether the matter be solid or fluid1 is always in proportion to the force employed3 in cold weather we rub our hands to make them warm1 and the harder we rub the warmer they become4 ,the warmth of the sea after a storm is in proportion to the force of the wind2 and in ,sir ,humphry ,davy's experiment of melting ice by friction in the receiver of an air-pump kept at the freezing point1 the heat which melted the ice was exactly proportional to the force of friction4 ,this experiment proves the immateriality of heat1 since the capacity of ice for heat is less than that of water4 ,thus mechanical action and heat are equivalent to one another4 ,mr4 ,joule of ,manchester#ac has proved that the quantity of heat requisite to raise the temperature of a pound of water one degree of ,fahrenheit's thermometer1 is equivalent to the mechanical force developed by the fall of a body weighing #ggbfi pounds through the perpendicular height of one foot4 ,this quantity is the mechanical equivalent of heat4 ,thus heat is motion1 and it is measured by force4 ,in fact1 for every unit of force expended in friction or percussion1 a definite quantity of heat is generated2 and conversely1 when work is performed by the consumption of heat1 for each unit of force gained1 a unit of heat disappears4 ,for since heat is a dynamical force of mechanical effect1 there must be an equivalent between mechanical work and heat as between cause and effect4 7,n4 #bbb47 ,besides the temperature indicated by the thermometer1 bodies absorb heat1 and their capacity for heat is so various that very different quantities of heat are required to raise different substances to the same sensible temperature4 ,it is evident1 therefore1 that much of the heat is absorbed and becomes insensible to the thermometer4 ,that portion of heat requisite to raise a body to a given temperature is its specific heat1 but the latent or absorbed heat is an expansive force or energy1 which1 acting upon the ether surrounding the ultimate particles of bodies1 changes them from solid to liquid1 and from liquid to vapour or gas4 ,according to the law of absorption1 the transfer of heat from a warm body to one that is cold is a mere transfer of force1 in which the force of compression is exactly proportional to the force of expansion4 ,ice remains at the temperature of #cb ,fahrenheit till it has absorbed #adj of heat1 and then it melts1 but without raising the temperature of the water above #cb4 ,on the contrary1 when a liquid is converted into a solid1 a quantity of heat leaves it without any diminution of temperature4 ,thus water at #cb must part with #adj of heat before it freezes4 ,the slowness with which water freezes or ice thaws1 is a consequence of the time required for the ethereal atmospheres round the particles of the water to contract or expand with a force equivalent to #adj of heat4 ,a considerable degree of cold is felt during a thaw1 because the ice in its transition from a solid to a liquid state absorbs sensible heat from the atmosphere and surrounding objects4 ,the heat absorbed and evolved by the rarefaction and condensation of air is exactly proportional to the force evolved and absorbed in these operations4 ,in fact1 the changes of temperature produced by these rarefactions and condensations of air show that the heat of elastic fluids is the mechanical force possessed by them2 and since the temperature of a gas determines its elastic force1 it follows that the elastic force or pressure must be the effect of the motion of the constituent particles in any gas4 ,sir ,humphry ,davy1 who first demonstrated the immateriality of heat1 assumed the hypothesis that the motion we call heat is a rotation or vibration among the particles of the fluid1 which1 according to ,mr4 ,joule1 agrees perfectly with the observed phenomena1 but he prefers the more simple view of ,mr4 ,herapath1 that the elastic force or pressure is due to the impact of the particles against any surface presented to them4 ,absorbed or latent heat may be regarded as a quiescent energy ready to be restored to the form of sensible heat when called forth3 its vibrations as heat are extinguished for the time by being transferred to the internal expansive force1 and are restored by compression4 ,the absorbed heat of air and all elastic fluids may be forced out by sudden compression like squeezing water out of a sponge4 ,the quantity of heat brought into action in this way is well illustrated by the experiment of igniting tinder by the sudden compression of air by a piston thrust into a cylinder closed at one end4 ,the development of heat on a stupendous scale is exhibited in lightning3 it is proportional to the square of the quantity of electricity discharged1 and is due to its excessive velocity and the violent compression of the air in its transit through the atmosphere4 ,prodigious quantities of heat are constantly absorbed or disengaged by the changes to which substances are liable in passing from the solid to the liquid and from the liquid to the gaseous form and the contrary1 causing endless vicissitudes of temperature over the globe1 and endless expansions and contractions1 which are correlative terms for heat and cold1 while radiation of heat is merely a transfer of motion from the particles on the surface of bodies to the adjacent particles of the atmosphere4 ,by the continual application of heat1 that is of the expansive force1 liquids are converted into steam or vapour1 which is invisible and highly elastic4 ,under the mean pressure of the atmosphere1 that is when the barometer stands at #cj inches1 water in a boiler absorbs heat continually till it attains the temperature of the boiling point1 which is #bab ,fahrenheit4 ,after that it ceases to show any increase of sensible heat2 but when it has absorbed an additional #ajjj of heat or expansive energy1 that energy converts it into steam1 and a condensing force equivalent to #ajjj of heat reduces it again to water4 ,water boils at different temperatures under different degrees of pressure4 ,it boils at a lower temperature on the top of a mountain than on the plain below1 because the weight of the atmosphere is less at the higher station4 ,there is no limit to the temperature to which water might be raised3 it might even be made red hot1 could a vessel be found strong enough to resist the pressure1 for the intensity of the expansive force prevented from having effect by the extreme pressure of the boiler would be converted into sensible heat which might eventually render the water red hot4 ,thus1 since the force of steam is in proportion to the temperature at which the water boils1 or to the pressure1 it is under control1 and1 perhaps with the exception of electricity1 it is the greatest power that has been made subservient to the wants of man4 ,it is found that the absolute quantity of heat consumed in the process of converting water into steam is the same at whatever temperature water may boil1 but that the absolute heat of the steam is greater exactly in proportion as its sensible heat is less4 ,thus1 steam raised at #bab ,fahrenheit under the mean pressure of the atmosphere1 and steam raised at #ahj under half the pressure1 contain the same quantity of heat1 with this difference1 that the one has more absorbed heat and less sensible heat than the other4 ,it is evident that1 as the same quantity of heat is requisite for converting a given weight of water into steam1 at whatever temperature or under whatever pressure the water may be boiled1 therefore1 in the steam engine1 equal weights of steam at a high pressure and a low pressure are produced by the same quantity of fuel2 and whatever the pressure of the steam may be1 the consumption of fuel is proportional to the quantity of water converted into vapour4 ,steam of whatever tension expands on being set free1 but the expansion of high pressure steam at the expense of its sensible heat is so great1 that the hand may be plunged into it without injury the instant it issues from the orifice of a boiler4 ,the steam becomes hotter by friction in issuing through the orifice which maintains it in its dry form1 for there is no doubt that high-pressure steam is dry4 ,the elasticity or tension of steam1 like that of common air1 varies inversely as its volume--that is1 when the space it occupies is doubled1 its elastic force is reduced to one half4 ,the expansion of steam is indefinite2 the smallest quantity of water expanded into vapour will occupy many millions of cubic feet2 a wonderful illustration of the minuteness of the ultimate particles of matter4 ,the force of steam1 tremendous as the lightning itself when uncontrolled1 is merely the result of chemical affinity3 it is the chemical attraction between the particles of carbon1 of coal or wood1 and the oxygen of the atmosphere4 ,mr4 ,joule has ascertained that a pound of the best coal when burnt gives sufficient heat to raise the temperature of #hjhf pounds of water one degree of the ,centigrade thermometer1 whence it has been computed by ,m4 ,helmholtz that the chemical force arising from the combustion of that pound of coal is capable of lifting a body of one hundred pounds weight to the height of twenty miles4 ,that is the work performed by the heat arising from the combustion of a pound of coal4 ,in all cases where work is produced by heat1 a quantity of heat proportional to the work done is expended2 and conversely1 by the expenditure of a like quantity of work1 the same amount of heat may be produced4 ,the equivalence of heat and work is a law of nature4 ,the mechanical force exerted by the steam engine for example is exactly proportional to the consumption of heat1 nor more nor less2 if we could produce a greater quantity than its equivalent we should have perpetual motion1 which is impossible4 ,mechanical engines generate no force4 ,we cannot create force2 we can only avail ourselves of the inexhaustible stores of nature1 the lightning1 fire1 water1 wind1 chemical action1 c4 ,the quantity of mechanical power in nature is ever the same2 it is never increased1 it is never diminished1 throughout the whole circuit of natural powers4 ,the conservation of force is as permanent and unchangeable as matter4 ,it may be dormant for a time1 but it ever exists4 ,we are unconscious of the enormous dynamic power that is either active or latent throughout the globe1 because we do not attend to it4 ,by the ebb and flow of the tide alone a power is exerted by which #be1jjj cubic miles of water is moved over a quarter of the globe every twelve hours2 and ,professor ,w4 ,thomson has computed1 by means of ,pouillet's data of solar radiation and ,mr4 ,joule's mechanical equivalent of heat1 that the mechanical value of the whole energy active and potential of the disturbances kept up in the ethereal medium by the vibrations of the solar light within a cubic mile of our atmosphere is equal to #ab1jej times the unit of mechanical force1 that is to say1 #ab1jej times the force that would raise a pound of matter to the height of one foot1 whence some idea may be formed of the vast amount of force exerted by the sun's light within the limits of the whole terrestrial atmosphere4 7,n4 #bbc47 ,the dynamic energy of the undulations of the solar light gives the leaves of plants the power of decomposing carbonic acid1 and of separating the particles of carbon and hydrogen from the oxygen for which they have so strong an affinity4 ,in this operation the undulations of the sunbeam are extinguished as light and heat1 and ,professor ,w4 ,thomson has proved that the quantity of these undulations thus extinguished is precisely equal to the potential or quiescent energy thus created1 and that precisely that very quantity of light and heat is restored when the plants are burned1 whatever state they may be in2 and that thus1 as ,mr4 ,george ,stephenson#ad has truly and beautifully observed1 our coal fires and gas lamps restore to our use the light and heat of the sun of the early geological epochs which have rested as dormant powers under the seas and mountains for unnumbered ages4 ,the sun is therefore the source of the mechanical energy of all the heat and motion of inanimate things1 of all the motions of the heat and light of fires and artificial flames1 and of the heat of all living creatures4 ,for animal heat1 and weights raised or resistance overcome1 are mechanical effects of the chemical combination of food with oxygen2 and food is either directly or indirectly vegetable1 consequently dependent upon the sun4 ,professor ,helmholtz of ,bonn has put in a strong point of view the enormous store of force possessed by our system by comparing it with its equivalent of heat4 ,the force with which the earth moves in its orbit is such1 that if brought to rest by a sudden shock1 a quantity of heat would be generated by the blow equal to that produced by the combustion of fourteen such earths of solid coal2 and supposing the capacity of the earth for heat as low as that of water1 the globe would be heated to #aa1bjj ,cent4 ,it would be quite fused and for the most part reduced to vapour4 ,if it should fall to the sun1 which it would certainly do1 the quantity of heat developed by the shock would be four hundred times as great4 ,the application of heat to the various branches of the mechanical and chemical arts has within the present century effected a greater change in the condition of man than had been accomplished in any equal period of his existence4 ,armed by the expansion and condensation of fluids with a power equal to that of the lightning itself1 conquering time and space1 he flies over plains1 and travels on paths cut by human industry even through mountains with a velocity and smoothness more like planetary than terrestrial motion2 he crosses the deep in opposition to wind and tide2 by releasing the strain on the cable1 he rides at anchor fearless of the storm2 he makes the lightning his messenger2 and like a magician he raises from the gloomy abyss of the mine the sunbeam of former ages to dispel the midnight darkness4 ,the principal phenomena of heat may be illustrated by a comparison with those of sound4 ,their excitation is not only similar but identical1 as in friction and percussion2 they are both communicated by contact and radiation2 and ,dr4 ,young observes that the effect of radiant heat in raising the temperature of a body upon which it falls1 resembles the sympathetic agitation of a string when the sound of another string which is in unison with it is transmitted through the air4 ,light1 heat1 sound1 and the waves of fluids are all subject to the same laws2 their undulatory theories are perfectly similar3 hence the interference of two hot rays must produce cold1 that is1 they must extinguish one another3 darkness results from the interference of two undulations of light1 silence ensues from the interference of two undulations of sound1 and still water or no tide is the consequence of the interference of two tides4 ,the propagation of sound1 however1 requires a much denser medium than that of light and heat2 its intensity diminishes as the rarity of the air increases3 so that1 at a very small height above the surface of the earth1 the noise of the tempest ceases1 and the thunder is heard no more in those boundless regions where the heavenly bodies accomplish their periods in eternal and sublime silence4 ,a consciousness of the fallacy of our senses is one of the most important consequences of the study of nature4 ,this study teaches us that no object is seen by us in its true place1 owing to aberration2 that the colours of substances are solely the effects of the action of matter upon light2 and that light itself as well as heat and sound are not real beings1 but mere motions communicated to our perceptions by the nerves4 ,the human frame may therefore be regarded as an elastic system1 the different parts of which are capable of receiving the tremors of elastic media1 and of vibrating in unison with any number of superimposed undulations1 all of which have their perfect and independent effect4 ,here our knowledge ends3 the mysterious influence of matter on mind will in all probability be for ever hid from man4 ,,section ,,xxviii4 ,common or ,static ,electricity1 or ,electricity of ,tension--,a ,dual ,power--,methods of exciting it--,attraction and ,repulsion--,conduction--,electrics and ,non-electrics--,induction--,dielectrics --,tension--,law of the ,electric ,force--,distribution--,laws of ,distribution--,heat of ,electricity--,electrical ,light and its ,spectrum--,velocity--,atmospheric ,electricity--,its cause--,electric ,clouds--,violent effects of ,lightning--,back ,stroke--,electric ,glow--,phosphorescence4 ,,electricity is a dual power which gives no visible sign of its existence when in equilibrio1 but when elicited forces are developed capable of producing the most sudden1 violent1 and destructive effects in some cases1 while in others their action1 though generally less energetic1 is of indefinite and uninterrupted continuance4 ,these modifications of the electric forces1 incidentally depending upon the manner in which they are excited1 present phenomena of great diversity1 but yet so connected as to justify the conclusion that they originate in a common principle4 ,the hypothesis of electricity being a fluid is untenable in the present advanced state of the science2 we only know that it is a force whose action is twofold2 that bodies in one electric state attract1 and in another repel each other2 in the former the electricity is said to be positive1 in the latter negative2 and thus regarding it as a force1 its modes of action come under the laws of mechanics and mathematical analysis4 ,electricity may be called into activity by the friction of heterogeneous substances1 as in the common electrifying machine1 by mechanical power1 heat1 chemical action1 and the influence of magnetism4 ,we are totally ignorant why it is roused from its neutral state by these means1 or of the manner of its existence in bodies2 but when excited it seems to produce a molecular polarity or chemical change in the ultimate particles of matter4 ,the science is divided into various branches1 of which static or common electricity comes first under consideration1 including that of the atmosphere4 ,substances in a neutral state neither attract nor repel4 ,there is a numerous class called electrics in which the electric equilibrium is destroyed by friction2 then the positive and negative electricities are called into action or separated2 the positive is impelled in one direction1 and the negative in another4 ,electricities of the same kind repel1 whereas those of different kinds attract each other4 ,the attractive power is exactly equal to the repulsive power at equal distances1 and when not opposed they coalesce with great rapidity and violence1 producing the electric flash1 explosion1 and shock2 then the equilibrium is restored4 ,one kind of electricity cannot be evolved without the evolution of an equal quantity of the opposite kind4 ,thus when a glass rod is rubbed with a piece of silk1 as much positive electricity is elicited in the glass as there is negative in the silk4 ,the kind of electricity depends more upon the mechanical condition than on the nature of the surface2 for when two plates of glass1 one polished and the other rough1 are rubbed against each other1 the polished surface acquires positive and the rough negative electricity4 ,the manner in which friction is performed also alters the kind of electricity4 ,equal lengths of black and white ribbon applied longitudinally to one another1 and drawn between the finger and thumb so as to rub their surfaces together1 become electric4 ,when separated the white ribbon is found to have acquired positive electricity1 and the black negative2 but if the whole length of the black ribbon be drawn across the breadth of the white1 the black will be positively and the white negatively electric when separated4 ,the friction of the rubber on the glass plate of the electrifying machine produces abundance of static electricity4 ,the friction of the steam on the valve of an insulated locomotive steam-engine produces seven times the quantity of electricity that an electrifying machine would do with a plate three feet in diameter1 worked at the rate of #gj revolutions in a minute4 ,pressure is a source of electricity which ,m4 ,becquerel has found to be common to all bodies2 but it is necessary to separate them to prevent the reunion of the electricities4 ,when two substances of any kind whatever are insulated and pressed together they assume different electric states1 but they only show contrary electricities when one of them is a good conductor4 ,when both are good conductors they must be separated with extreme rapidity to prevent a return to equilibrium4 ,when the separation is very sudden the tension of the two electricities may be great enough to produce light4 ,m4 ,becquerel attributes the light produced by the collision of icebergs to this cause4 ,iceland spar is made electric by the smallest pressure between the finger and the thumb1 and retains it for a long time4 ,all these circumstances are modified by the temperature of the substances1 the state of their surfaces and that of the atmosphere4 ,several crystalline bodies become electric when heated1 especially tourmaline1 one end of which acquires positive1 and the other negative electricity1 while the intermediate part is neutral4 ,if the tourmaline be broken through the middle1 each fragment is found to possess positive electricity at one end and negative at the other4 ,electricity is evolved by substances passing from a liquid to a solid state1 and by chemical action during the production and condensation of vapour1 which is a great source of atmospheric electricity4 ,in short1 it may be generally stated1 that when any cause whatever tends to destroy molecular attraction there is a development of electricity2 if1 however1 the substances be not immediately separated1 there will be an instantaneous restoration of equilibrium4 ,electricity may be transferred from one body to another in the same manner as heat is communicated1 and like it too the body loses by the transmission4 ,although no substance is altogether impervious to electricity1 nor is there any that does not offer some resistance to its passage1 yet it moves with more facility through a certain class of substances called conductors1 such as metals1 water1 the human body1 c41 than through atmospheric air1 glass1 silk1 c41 which are therefore called non-conductors4 ,the conducting power is affected both by temperature and moisture4 ,the terrestrial globe is a conductor on account of its moisture1 though dry earth is not4 ,though metals are the best conductors of electricity1 it affects their molecular structure1 for the heat which accompanies its passage acts as a transverse expansive force1 which increases their breadth by diminishing their length1 as may be seen by passing electricity through a platinum wire sufficiently thick to resist fusion4 ,through air the force is disruptive on account of its non-conducting quality1 and it seems to act chemically on the oxygen1 producing the substance known as ozone during its passage through the atmosphere4 ,if a conductor be good and of sufficient size the electricity passes imperceptibly but it is shivered to pieces in an instant if it be a bad conductor or too small to carry off the charge4 ,in that case the physical change is generally a separation of the particles1 or expansion from the heat1 as in trees1 where it turns the moisture into steam1 but all these effects are in proportion to the obstacles opposed to the freedom of its course4 ,bodies surrounded by non-conductors are said to be insulated1 because when charged the electricity cannot escape4 ,when that is not the case1 the electricity is conveyed to the earth3 consequently it is impossible to accumulate electricity in a conducting substance that is not insulated4 ,there are a great many substances called non-electrics in which electricity is not sensibly developed by friction unless they be insulated1 because it is carried off by their conducting power as soon as elicited4 ,metals1 for example1 which are said to be non-electrics can be excited1 but being conductors they cannot retain this state if in communication with the earth4 ,it is probable that no bodies exist which are either perfect non-electrics or perfect non-conductors4 ,but it is evident that electrics must be non-conductors to a certain degree1 otherwise they could not retain their electric state4 ,a body charged with electricity1 although perfectly insulated1 so that all escape of electricity is prevented1 tends to produce an electric state of the opposite kind in all bodies in its vicinity4 ,positive electricity tends to produce negative electricity in a body near to it1 and vice vers1 the effect being greater as the distance diminishes4 ,this power which electricity possesses of causing an opposite electrical state in its vicinity is called induction4 ,a ,leyden jar1 for example1 or glass jar coated half way up both outside and in with tin foil1 when charged with positive electricity1 immediately induces negative electricity on the tin foil outside4 ,notwithstanding their strong mutual attraction they are prevented from coalescing by the glass1 which is a non-conductor2 but if the tin inside and out be connected by a conducting wire they instantly unite4 ,when a body in either electric state is presented to a neutral one1 its tendency in consequence of the law of induction is to disturb the condition of the neutral body by inducing electricity contrary to its own in the adjacent side1 and therefore an electrical state similar to its own in the remote part4 ,hence the neutrality of the second body is destroyed by the action of the first1 and the adjacent parts of the two1 having now opposite electricities1 will attract each other4 ,the attraction between electrified and unelectrified substances is a consequence of the altered state of their molecules4 ,induction depends upon the facility with which the equilibrium of the neutral body can be overcome1 a facility which is proportional to its conducting power4 ,consequently the attraction exerted by an electrified substance upon another substance previously neutral will be much more energetic if the latter be a conductor than if it be a non-conductor4 ,it is clear that one body cannot act upon another at a distance without some means of communication4 ,dr4 ,faraday has proved that the intervening non-conducting substance or dielectric has a great influence upon induction4 ,thus the inductive force is greater when sulphur is interposed between the two bodies than when shellac is the dielectric1 and greater when shellac is the dielectric than glass1 c4 ,professor ,matteucci has proved by the following experiment that the intervening substance is itself polarized by induction4 ,a number of plates of mica in contact were placed between two plates of metal1 one of which was electrified1 so that the whole was charged like a ,leyden jar4 ,on separating the plates with insulating handles1 each plate of mica was electrified2 one side of it was positive and the other negative1 showing decidedly a polarization by induction throughout the whole intervening non-conducting substance2 and thus1 although the interposed substance or dielectric is incapable of conducting the electrical force from one body to the other1 it becomes by induction capable of transmitting it4 ,in the atmosphere induction is transmitted by that of the intervening strata of air4 ,it is true that induction takes place through the most perfect vacuum we can make1 but there always remains some highly elastic air2 and even if air could be altogether excluded1 the ethereal medium cannot1 and it must be capable of induction1 since1 however attenuated1 it must consist of material atoms1 otherwise it would be a nonentity4 ,the law of electrical attraction and repulsion has been determined by suspending a needle of gum-lac horizontally by a silk fibre1 the needle carrying at one end a piece of electrified gold leaf4 ,a globe in the same or opposite electrical state when presented to the gold leaf will repel or attract it1 and will therefore cause the needle to vibrate more or less rapidly according to the distance of the globe4 ,a comparison of the number of oscillations performed in a given time at different distances will determine the law of the variation of the electrical intensity1 in the same manner that the force of gravitation is measured by the oscillations of the pendulum4 ,coulomb invented an instrument which balances the forces in question by the force of the torsion of a thread1 which consequently measures the intensity2 and ,sir ,william ,snow ,harris has constructed an instrument with which he has measured the intensity of the electrical force in terms of the weight requisite to balance it4 ,by these methods it has been found that the intensity of electrical attraction and repulsion varies inversely as the square of the distance4 ,however1 the law of repulsive force is liable to great disturbances from inductive action1 which ,sir ,william ,snow ,harris has found to exist not only between a charged and neutral body1 but also between bodies similarly charged2 and that1 in the latter case1 the inductive process may be indefinitely modified by the various circumstances of the quantity and intensity of the electricity and the distance between the charged bodies4 ,the quantity of electricity bodies are capable of receiving does not follow the proportion of their bulk1 but depends principally upon the form and extent of their surface4 ,it appears from the experiments of ,sir ,w4 ,s4 ,harris that a given quantity of electricity1 divided between two perfectly equal and similar bodies1 exerts upon external bodies only one fourth of the attractive force apparent when disposed upon one of them2 and if it be distributed among three equal and similar bodies1 the force is one ninth of that apparent when it is disposed on one of them4 ,hence1 if the quantity of electricity be the same1 the force varies inversely as the square of the surface on which it is disposed2 and if the surface be the same1 the force varies directly as the square of the quantity of electricity4 ,these laws however do not hold when the form of the surface is changed4 ,a given quantity of electricity disposed on a given surface has the greatest intensity when the surface has a circular form1 and the least intensity when the surface is expanded into an indefinite straight line4 ,the decrease of intensity seems to arise from some peculiar arrangement of the electricity depending on the extension of the surface4 ,it is quite independent of the extent of the edge1 the area being the same2 for ,sir ,w4 ,s4 ,harris found that the electrical intensity of a charged sphere is the same with that of a plane circular area of the same superficial extent1 and that of a charged cylinder the same as if it were cut open and expanded into a plane surface4 ,the same able electrician has shown that the attractive force between an electrified and a neutral uninsulated body is the same whatever be the forms of their unopposed parts4 ,thus two hemispheres attract each other with precisely the same force as if they were spheres2 and as the force is as the number of attracting points in operation directly1 and as the squares of the respective distances inversely1 it follows that the attraction between a mere ring and a circular area is no greater than that between two similar rings1 and the force between a sphere and an opposed spherical segment of the same curvature is no greater than that of two similar segments1 each equal to the given segment4 ,electricity may be accumulated to a great extent in insulated bodies1 and so long as it is quiescent it occasions no sensible change in their properties4 ,when restrained by the non-conducting power of the atmosphere1 its tension or the pressure it exerts is proportional to the coercive force of the air4 ,if the pressure be less than the coercive force1 the electricity is retained2 but the instant it exceeds that force in any one point it escapes1 and that more readily when the air is attenuated or saturated with moisture1 for the resistance of the air is proportional to the square of its density1 but the inductive action of electricity on distant bodies is independent of atmospheric pressure4 ,the power of retaining electricity depends also on the shape of the charged body4 ,it is most easily retained by a sphere1 next to that by a spheroid1 but it readily escapes from a point1 and a pointed object receives it with most facility4 ,the heat produced by the electric shock is proportional to the square of the quantity of electricity discharged1 and is so intense that it fuses metals and volatilizes substances1 but its intensity is not felt to its full extent on account of the shortness of its duration4 ,it is only accompanied by light when the electricity is obstructed in its passage through substance4 ,electrical light when analysed by a prism differs very much from solar light4 ,fraunhofer found that1 instead of the fixed dark lines1 the spectrum of an electric spark is crossed by numerous bright lines2 and ,professor ,wheatstone has observed that the number and position of the lines differ with the metal from which the spark is taken1 and believes the spark itself results from the ignition and volatilization of the matter of the conductor4 ,according to the experiments of ,sir ,humphry ,davy1 the density of the air has an influence on the colour4 ,he passed the electric spark through a vacuum over mercury1 which from green became successively sea-green1 blue1 and purple1 on admitting different quantities of air4 ,when the vacuum was made over a fusible alloy of tin and bismuth1 the spark was yellowish and extremely pale4 ,sir ,humphry thence concluded that electrical light principally depends upon some properties belonging to the ponderable matter through which it passes1 and that space is capable of exhibiting luminous appearances1 though it does not contain an appreciable quantity of matter4 ,he thought that the superficial particles of bodies which form vapour1 when detached by the repulsive power of heat1 might be equally separated by the electric forces1 and produce luminous appearances in vacuo by the destruction of their opposite electric states4 ,the velocity of electricity is so great that the most rapid motion which can be produced by art appears to be actual rest when compared with it4 ,a wheel revolving with celerity sufficient to render its spokes invisible1 when illuminated by a flash of lightning1 is seen for an instant with all its spokes distinct1 as if it were in a state of absolute repose2 because1 however rapid the rotation may be1 the light has come and already ceased before the wheel has had time to turn through a sensible space4 ,this beautiful experiment is due to ,professor ,wheatstone1 as well as the following variation of it1 which is not less striking3 ,if a circular piece of pasteboard be divided into three sectors1 one of which is painted blue1 another yellow1 and a third red1 it will appear to be white when revolving quickly1 because of the rapidity with which the impressions of the colours succeed each other on the retina4 ,but1 the instant it is illuminated by an electric spark1 it seems to stand still1 and each colour is as distinct as if it were at rest4 ,this transcendent speed of electricity has been ingeniously measured1 as follows1 by ,professor ,wheatstone1 who has ascertained that it much surpasses the velocity of light4 ,in the horizontal diameter of a small disc1 fixed on the wall of a darkened room1 are disposed six small brass balls1 well insulated from each other4 ,an insulated copper wire1 half a mile long1 is disjointed in its middle1 and also near its two extremities2 the six ends thus obtained are connected with the six-balls on the disc4 ,when an electric discharge is sent through the wire by connecting its two extremities1 one with the positive1 and the other with the negative coating of a ,leyden jar1 three sparks are seen on the disc1 apparently at the same instant4 ,at the distance of about ten feet a small revolving mirror is placed so as to reflect these three sparks during its revolution4 ,from the extreme velocity of the electricity1 it is clear that1 if the three sparks be simultaneous1 they will be reflected1 and will vanish before the mirror has sensibly changed its position1 however rapid its rotation may be1 and they will be seen in a straight line4 ,but if the three sparks be not simultaneously transmitted to the disc--if one1 for example1 be later than the other two--the mirror will have time to revolve through an indefinitely small arc in the interval between the reflection of the two sparks and that of the single one4 ,however1 the only indication of this small motion of the mirror will be1 that the single spark will not be reflected in the same straight line with the other two1 but a little above or below it1 for the reflection of all three will still be apparently simultaneous1 the time intervening being much too short to be appreciated4 ,since the number of revolutions which the revolving mirror makes in a second is known1 and the angular deviation of the reflection of the single spark from the reflection of the other two can be measured1 the time elapsed between their consecutive reflections can be ascertained4 ,and1 as the length of that part of the wire through which the electricity has passed is given1 its velocity may be found4 ,the number of pulses in a second1 requisite to produce a musical note of any pitch1 are known2 hence the number of revolutions accomplished by the mirror in a given time may be determined from the musical note produced by a tooth or peg1 in its axis of rotation1 striking against a card1 or from the notes of a siren attached to the axis4 ,it was thus that ,professor ,wheatstone found the mirror which he employed in his experiments made #hjj revolutions in a second2 and1 as the angular velocity of the reflected image in a revolving mirror is double that of the mirror itself1 an angular deviation of one degree in the appearance of the two sparks would indicate an interval of the #egf1jjjth of a second2 the deviation of half a degree would1 therefore1 indicate more than the millionth of a second4 ,the use of sound as a measure of velocity is a happy illustration of the connexion of the physical sciences4 ,the earth possesses a powerful electrical tension1 and the atmosphere when clear is almost always positively electric4 ,its electricity is stronger in winter than in summer1 during the day than in the night4 ,the intensity increases for two or three hours from the time of sunrise1 comes to a maximum between seven and eight1 then decreases towards the middle of the day1 arrives at its minimum between one and two1 and again augments as the sun declines till about the time of sunset1 after which it diminishes and continues feeble during the night4 ,the mere condensation of vapour is a source of atmospheric electricity2 but although it is also produced by the vapour that rises from the surface of the earth1 it is not under all circumstances4 ,m4 ,pouillet found that electricity is only developed when accompanied by chemical action3 for example1 when the water whence the vapour proceeds contains lime1 chalk1 or any solid alkali1 negative electricity is produced2 and when it holds in solution either gas1 acid1 or some of the salts1 the vapour is positively electric4 ,besides1 the contact of earth with salt and fresh water generates positive electricity1 and the contact of fresh and salt currents of water negative1 so that the ocean must afford a great supply to the atmosphere2 hence thunderstorms are most frequent near the coasts3 but as electricity of one kind or another is developed whenever the molecules of matter are deranged from their natural state of equilibrium1 there must be many partial variations in the electric state of the air4 ,when the invisible vapour rises charged with electricity into the cold regions of the atmosphere1 it is condensed into cloud1 in which the tension is increased because the electricity is confined to a smaller space2 and if the condensation be sufficient to produce drops of rain1 they carry the electricity to the ground1 so that in general a shower is a conductor between the clouds and the earth4 ,when two clouds charged with opposite kinds1 but of equal tension1 approach within a certain distance1 the intensity increases on the sides of the clouds that are nearest to one another2 and when the tension is great enough to overcome the coercive pressure of the atmosphere1 a discharge takes place which causes a flash of lightning1 the stroke being given either by the cloud or the rain4 ,the actual quantity of electricity in any part of a cloud is extremely small4 ,the intensity of the flash arises from the great extent of surface over which it is spread1 so that clouds may be compared to enormous ,leyden jars thinly coated with electricity1 which only acquires its intensity by its instantaneous condensation4 ,the rapid and irregular motions of thunder clouds are probably more owing to strong electrical attractions and repulsions among themselves than to currents of air1 though both are no doubt concerned in these hostile movements4 ,the atmosphere becomes intensely electric on the approach of rain1 hail1 snow1 sleet1 and wind2 but it varies afterwards1 and the transitions are very rapid on the approach of a thunderstorm4 ,since air is a non-conductor1 it does not convey the electricity from the clouds to the earth1 but it acquires from them an opposite kind1 and when the tension is very great the force of the electricity becomes irresistible1 and an interchange takes place between the clouds and the earth2 but so rapid is the motion of lightning1 that it is difficult to ascertain whether it goes from the clouds to the earth or shoots upwards from the earth to the clouds1 though there can be no doubt that it does both4 ,in a storm that occurred at ,manchester in ,june #ahce1 the lightning was observed to issue from various points of a road1 attended by explosions as if pistols had been fired out of the ground1 and a man seems to have been killed by one of these explosions taking place under his foot4 ,m4 ,gay ,lussac ascertained that a flash of lightning sometimes darts more than three miles in a straight line4 ,a person may be killed by lightning1 although the explosion takes place at a distance of twenty miles1 by what is called the back stroke4 ,suppose that the two extremities of a highly charged cloud hang down towards the earth1 they will repel the electricity from the earth's surface if it be of the same kind with their own1 and will attract the other kind2 and if a discharge should suddenly take place at one end of the cloud1 the equilibrium will be instantly restored by a flash at that point of the earth which is under the other4 ,though the back stroke is often sufficiently powerful to destroy life1 it is never so terrible in its effects as the direct stroke1 which is often of inconceivable intensity4 ,instances have occurred when large masses of iron and stone1 and even many feet of a stone wall1 have been carried to a considerable distance by a stroke of lightning4 ,rocks and the tops of mountains often bear the marks of fusion from its intense heat2 and occasionally vitreous tubes descending many feet into banks of sand mark its path4 ,dr4 ,fiedler exhibited several of these fulgorites in ,london of considerable length1 which had been dug out of the sandy plains of ,silesia and ,eastern ,prussia4 ,one found at ,paderborn was forty feet long4 ,their ramifications generally terminate in pools or springs of water below the sand1 which are supposed to determine the course of the lightning4 ,no doubt the soil and substrata must influence its direction1 since it is found by experience that places which have been once struck by lightning are often struck again4 ,an insulated conductor on the approach of a storm gives out such quantities of sparks that it is dangerous to approach it1 as was fatally experienced by ,professor ,richman at ,petersburg1 who was struck dead by a globe of fire from the extremity of a conductor1 while making experiments on atmospheric electricity4 ,copper conductors afford the best protection1 especially if they expose a broad surface1 since electricity is conveyed along the surface of bodies4 ,there is no instance of an electric cloud of high tension being dispelled by a conductor1 yet those invented by ,sir ,william ,snow ,harris1 and universally employed in the navy1 afford a complete protection in the most imminent danger4 ,the ,shannon1 a #ej-;gun frigate1 commanded by the brave and lamented ,sir ,william ,peel1 was enveloped in a thunder-storm when about #ij miles to the north-west of ,java4 ,it began at fifty minutes past four in the afternoon2 the ship was driven before the storm1 in a high sea1 amid streams of vivid lightning1 deafening thunder1 hail1 and rain4 ,at five o'clock an immense ball of fire covered the maintopgallant mast1 ran up the royal pole1 and exploded in the air with a terrific concussion1 covering all the surrounding space with sparks of electric light1 which were driven rapidly to leeward by the wind4 ,fifteen minutes later an immense mass of lightning struck the mainmast1 attended by a violent gust of wind2 and another heavy discharge fell on it a quarter of an hour afterwards4 ,from that time till six o'clock the ship was continually encompassed by sharp forked lightning1 accompanied by incessant peals of thunder4 ,though actually enveloped in electricity1 and struck three times1 neither the hull nor the rigging sustained the slightest injury4 ,when the air is rarefied by heat1 its coercive power is diminished1 so that the electricity escapes from the clouds in those lambent diffuse flashes without thunder so frequent in warm summer evenings2 and when the atmosphere is highly charged with electricity1 it not unfrequently happens that electric light1 in the form of a star1 is seen on the topmasts and yard-arms of ships4 ,in #ahca the ,french officers at ,algiers were surprised to see brushes of light on the heads of their comrades1 and at the points of their fingers when they held up their hands4 ,this phenomenon was well known to the ancients1 who reckoned it a lucky omen4 ,many substances1 in decaying1 emit light1 which is attributed to electricity1 such as fish and rotten wood4 ,oyster-shells1 and a variety of minerals1 become phosphorescent at certain temperatures when exposed to electric shocks or friction3 indeed1 most of the causes which disturb molecular equilibrium give rise to phosphoric phenomena4 ,the minerals possessing this property are generally coloured or imperfectly transparent2 and1 though the colour of this light varies in different substances1 it has no fixed relation to the colour of the mineral4 ,an intense heat entirely destroys this property1 and the phosphorescent light developed by heat has no connexion with light produced by friction2 for ,sir ,david ,brewster observed that bodies deprived of the faculty of emitting the one are still capable of giving out the other4 ,among the bodies which generally become phosphorescent when exposed to heat1 there are some specimens which do not possess this property2 wherefore phosphorescence cannot be regarded as an essential character of the minerals possessing it4 ,sulphuret of calcium1 known as ,canton's phosphorus1 and the sulphuret of barium1 or ,bologna stone1 possess the phosphorescent property in an eminent degree4 ,multitudes of fish are endowed with the power of emitting light at pleasure1 no doubt to enable them to pursue their prey at depths where the sunbeams cannot penetrate4 ,flashes of light are frequently seen to dart along a shoal of herrings or pilchards2 and the ,medusa tribes are noted for their phosphorescent brilliancy1 many of which are extremely small1 and so numerous as to make the wake of a vessel look like a stream of silver4 ,nevertheless1 the luminous appearance which is frequently observed in the sea during the summer months cannot always be attributed to marine animalcul1 as the following narrative will show3-- ,captain ,bonnycastle1 coming up the ,gulf of ,st4 ,lawrence on the #gth of ,september1 #ahbf1 was roused by the mate of the vessel in great alarm from an unusual appearance4 ,it was a starlight night1 when suddenly the sky became overcast in the direction of the high land1 and an instantaneous and intensely vivid light1 resembling the aurora1 shot out of the hitherto gloomy and dark sea on the lee bow1 which was so brilliant that it lighted everything distinctly even to the mast-head4 ,the light spread over the whole sea between the two shores1 and the waves1 which before had been tranquil1 now began to be agitated4 ,captain ,bonnycastle describes the scene as that of a blazing sheet of awful and most brilliant light4 ,a long and vivid line of light1 superior in brightness to the parts of the sea not immediately near the vessel1 showed the base of the high1 frowning1 and dark land abreast2 the sky became lowering and more intensely obscure4 ,long tortuous lines of light showed immense numbers of very large fish darting about as if in consternation4 ,the sprit-sail yard and mizen-boom were lighted by the glare1 as if gaslights had been burning directly below them2 and until just before daybreak1 at four o'clock1 the most minute objects were distinctly visible4 ,day broke very slowly1 and the sun rose of a fiery and threatening aspect4 ,rain followed4 ,captain ,bonnycastle caused a bucket of this fiery water to be drawn up2 it was one mass of light when stirred by the hand1 and not in sparks as usual1 but in actual coruscations4 ,a portion of the water preserved its luminosity for seven nights4 ,on the third night1 the scintillations of the sea reappeared2 the sun went down very singularly1 exhibiting in its descent a double sun2 and1 when only a few degrees high1 its spherical figure changed into that of a long cylinder1 which reached the horizon4 ,in the night the sea became nearly as luminous as before1 but on the fifth night the appearance entirely ceased4 ,captain ,bonnycastle did not think it proceeded from animalcul1 but imagined it might be some compound of phosphorus1 suddenly evolved and disposed over the surface of the sea4 ,it had probably been that peculiar form of electricity known as the glow discharge1 of which the author once saw a very remarkable instance4 ,m4 ,e4 ,becquerel assures us that almost all substances are phosphorescent after being exposed to the sun if instantly withdrawn into darkness1 and that it depends upon the arrangement of the particles and not upon chemical action4 ,the salts of uranium give the same kind of phosphorescent light as that produced by the violet rays of the solar spectrum4 ,a solution of the bisulphate of quinine emits a yellow phosphorescent light1 whereas the fluorescent light of that liquid is blue4 ,the colours of these two kinds of light are generally complementary to one another4 ,phosphorescence is probably more or less concerned in some1 at least1 of a series of very curious experiments made by ,m4 ,niepc de ,saint-,victor1 on what he calls the saturation of substances with light4 ,it has long been known that1 if a person in an intensely dark room should expose his arm to the sun through a hole in a window-shutter1 it will shine on being drawn into the darkness4 ,now1 ,m4 de ,saint-,victor found that if an engraving be exposed for a certain time to the sun1 and instantly brought into darkness1 it will make a photographic impression on a collodion or argentine surface1 and that anything written or drawn with tartaric acid1 or a solution of the salts of uranium1 in large characters1 is reproduced even at a small distance from a sensitive surface4 ,it may be presumed that the light communicates its vibrations to the surfaces exposed to it with sufficient force to enable them to disturb the unstable equilibrium of such sensitive substances as collodion or the argentine salts4 ,m4 de ,saint-,victor has shown that tartaric acid1 which is readily impressed by sunlight1 is neither fluorescent nor phosphorescent1 whence he concludes that his experiments are independent of both of these modes of action4 ,uranium appears to have very peculiar properties3 its salts are strongly luminous when exposed to the sun2 they are very fluorescent2 and the crystallized azitote of uranium becomes phosphorescent by percussion4 ,,section ,,xxix4 ,voltaic ,electricity--,the ,voltaic ,battery--,intensity--,quantity--,static ,electricity1 and ,electricity in ,motion--,luminous ,effects--,mr4 ,grove on the ,electric ,arc and ,light--,decomposition of ,water--,formation of ,crystals by ,voltaic ,electricity--,photo-galvanic ,engraving--,conduction--,heat of ,voltaic ,electricity--,electric ,fish4 ,,voltaic or ,dynamic electricity is elicited by the force of chemical action4 ,it is connected with some of the most brilliant periods of ,british science1 from the splendid discoveries to which it led ,sir ,humphry ,davy and ,dr4 ,faraday4 ,in #agij1 while ,galvani1 ,professor of ,anatomy in ,bologna1 was making experiments on electricity1 he was surprised to see convulsive motions in the limbs of a dead frog accidentally lying near the machine during an electrical discharge4 ,though a similar action had been noticed long before his time1 he was so much struck with this singular phenomenon1 that he examined all the circumstances carefully1 and at length found that convulsions take place when the nerve and muscle of a frog are connected by a metallic conductor4 ,this excited the attention of all ,europe2 and it was not long before ,volta1 ,professor at ,pavia1 showed that the mere contact of different bodies is sufficient to disturb electrical equilibrium1 and that a current of electricity flows in one direction through a circuit of three conducting substances4 ,from this he was led1 by acute reasoning and experiment1 to the construction of the ,voltaic pile1 which1 in its early form1 consisted of alternate discs of zinc and copper1 separated by pieces of wet cloth1 the extremities being connected by wires4 ,this simple apparatus1 perhaps the most wonderful instrument that has been invented by the ingenuity of man1 by divesting electricity of its sudden and uncontrollable violence1 and giving in a continued stream a greater quantity at a diminished intensity1 has exhibited that force under a new and manageable form1 possessing powers the most astonishing and unexpected4 ,the expression current has no relation to a fluid1 which is now considered to be as inconsistent with the phenomena of dynamic as with static electricity4 ,it was shown by ,grotthus that the transmission of ,voltaic electricity through liquids consists of a series of chemical affinities acting in definite directions2 and ,mr4 ,grove1 from an examination of its action on the various kinds of matter1 has come to the same conclusion4 ,indeed it is now the generally received opinion that a current of electricity is merely a continuous transmission of chemical affinity from particle to particle of the substance through which it is passing1 and consequently that it is a continuous transmission of force4 ,as the ,voltaic battery has become one of the most important engines of physical research1 some account of its present condition may not be out of place4 ,the disturbance of electric equilibrium1 and a development of electricity1 invariably accompany the chemical action of a fluid on metallic substances1 and the electricity is most plentiful when that action occasions oxidation4 ,metals vary in the quantity of electricity afforded by their combination with oxygen4 ,but the greatest abundance is developed by the oxidation of zinc by weak sulphuric acid4 ,and1 in conformity with the law that one kind of electricity cannot be evolved without an equal quantity of the other being brought into activity1 it is found that the acid is positively1 and the zinc negatively electric4 ,it has not yet been ascertained why equilibrium is not restored by the contact of these two substances1 which are both conductors1 and in opposite electrical states4 ,however1 the electrical and chemical changes are so connected1 that1 unless equilibrium be restored1 the action of the acid will go on languidly1 or stop as soon as a certain quantity of electricity is accumulated in it4 ,equilibrium1 nevertheless1 will be restored1 and the action of the acid will be continuous1 if a plate of copper be placed in contact with the zinc1 both being immersed in the fluid2 for the copper1 not being acted upon by the acid1 will serve as a conductor to convey the positive electricity from the acid to the zinc1 and will at every instant restore the equilibrium1 and then the oxidation of the zinc will go on rapidly4 ,thus three substances are concerned in forming a ,voltaic circuit1 but it is indispensable that one of them should be a fluid4 ,the electricity so obtained will be very feeble in overcoming resistances offered by imperfect conductors interposed in the circuit1 or by very long wires1 but it may be augmented by increasing the number of plates4 ,in the common ,voltaic battery1 the electricity which the fluid has acquired from the first plate of zinc exposed to its action is taken up by the copper plate belonging to the second pair1 and transferred to the second zinc plate1 with which it is connected4 ,the second plate of zinc1 possessing equal powers1 and acting in conformity with the first1 having thus acquired a larger portion of electricity than its natural share1 communicates a larger quantity to the fluid in the second cell4 ,this increased quantity is again transferred to the next pair of plates2 and thus every succeeding alternation is productive of a further increase in the quantity of the electricity developed4 ,this action1 however1 would stop unless a vent were given to the accumulated electricity1 by establishing a communication between the positive and negative poles of the battery by means of wires attached to the extreme plate at each end4 ,when the wires are brought into contact1 the ,voltaic circuit is completed1 the electricities meet and neutralize each other1 producing the shock and other electrical phenomena2 and then the electric current continues to flow uninterruptedly in the circuit1 as long as the chemical action lasts4 ,the stream of positive electricity flows from the zinc to the copper4 ,the construction and power of the ,voltaic battery have been much improved of late years1 but the most valuable improvement is the constant battery of ,professor ,daniell4 ,in all batteries of the ordinary construction1 the power1 however energetic at first1 rapidly diminishes1 and ultimately becomes very feeble4 ,professor ,daniell found that this diminution of power is occasioned by the adhesion of the evolved hydrogen to the surface of the copper1 and by the precipitation of the sulphate formed by the action of the acid on the zinc4 ,he prevents the latter by interposing between the copper and the zinc1 in the cell containing the liquid1 a membrane which1 without impeding the electric current1 prevents the transfer of the salt2 and the former1 by placing between the copper and the membrane solution of sulphate of copper1 which being reduced by the hydrogen prevents the adhesion of this gas to the metallic surface4 ,each element of the battery consists of a hollow cylinder of copper1 in the axis of which is placed a cylindrical rod of zinc2 between the zinc and the copper a membranous bag is placed1 which divides the cell into two portions1 the inner of which is filled with dilute acid1 and the one nearer the copper is supplied with crystals of the sulphate of that metal4 ,the battery consists of several of these elementary cells connected together by metallic wires1 the zinc rod of one with the copper cylinder of that next to it4 ,the zinc rods are amalgamated1 so that local action1 which1 in ordinary cases1 is so destructive of the zinc1 does not take place1 and no chemical action is manifested unless the circuit be completed4 ,the rods are easily detached1 and others substituted for them when worn out4 ,this battery1 which possesses considerable power1 and is constant in its effects for a very long time1 is greatly superior to all former arrangements1 either as an instrument of research1 or for exhibiting the ordinary phenomena of ,voltaic electricity4 ,a battery charged with water alone1 instead of acid1 is constant in its action1 but the quantity of electricity it develops is comparatively very small4 ,mr4 ,cross1 of ,broomfield in ,somersetshire1 kept a battery of this kind in full force during twelve months4 ,m4 ,becquerel had invented an instrument for comparing the intensities of the different kinds of electricity by means of weights2 but1 as it is impossible to make the comparison with ,voltaic electricity produced by the ordinary batteries1 on account of the perpetual variation to which the intensity of the current is liable1 he has constructed a battery which affords a continued stream of electricity of uniform power1 but it is also of very feeble force4 ,the current is produced by the chemical combination of an acid with an alkali4 ,metallic contact is not necessary for the production of ,voltaic electricity1 which is entirely due to chemical action4 ,the intensity of the ,voltaic electricity is in proportion to the intensity of the affinities concerned in its production1 and the quantity produced is in proportion to the quantity of matter which has been chemically active during its evolution4 ,dr4 ,faraday considers this definite production to be one of the strongest proofs that electricity is of chemical origin4 ,galvanic or ,voltaic electricity is manifested by two continuous forces or currents passing in opposite directions through the circuit3 the zinc is the positive end or pole of the battery1 and the copper the negative4 ,voltaic electricity is distinguished by two marked characters4 ,its intensity increases with the number of plates1 its quantity with the extent of their surfaces4 ,the most intense concentration of force is displayed by a numerous series of large plates3 light and heat are copiously evolved1 and chemical decomposition is accomplished with extraordinary energy2 whereas the electricity from one pair of plates1 whatever their size may be1 is so feeble that it gives no sign either of attraction or repulsion4 ,common or static electricity is of greater intensity and has a greater power of overcoming resistance than ,voltaic electricity1 but it acts upon a smaller quantity of matter4 ,however1 by diminishing the size of the plates1 and increasing their number1 the intensity of a battery may be increased till it becomes equal to that of the electrical machine4 ,the action of ,voltaic electricity differs in some respects materially from that of the ordinary kind4 ,when a quantity of common electricity is accumulated1 the restoration of equilibrium is attended by an instantaneous violent explosion1 accompanied by the development of light1 heat1 and sound4 ,the concentrated power of the electricity forces its way through every obstacle1 disrupting and destroying the cohesion of the particles of the bodies through which it passes1 and occasionally increasing its destructive effects by the conversion of fluids into steam from the intensity of the momentary heat1 as when trees are torn to pieces by a stroke of lightning4 ,even the vivid light which marks the path of the electricity is probably owing in part to the sudden compression of the air and the rapidity of its passage4 ,but the instant equilibrium is restored by this energetic action the whole is at an end4 ,on the contrary1 when an accumulation takes place in a ,voltaic battery1 equilibrium is restored the moment the circuit is completed4 ,but so far is the electric stream from being exhausted1 that it continues to flow silently and invisibly in an uninterrupted current supplied by a perpetual reproduction4 ,and1 although its action on bodies is neither so sudden nor so intense as that of common electricity1 yet it acquires such power from constant accumulation and continued action1 that it ultimately surpasses the energy of the other4 ,the two kinds of electricity differ in no circumstance more than in the development of heat4 ,instead of a momentary evolution1 the circulation of the ,voltaic electricity is accompanied by a continued development of heat1 lasting as long as the circuit is complete1 without producing either light or sound4 ,its intensity from a very powerful battery is greater than that of any heat that can be obtained by artificial means1 so that it fuses substances which resist the action of the most powerful furnaces4 ,the temperature of every part of a ,voltaic battery itself is raised during its activity4 ,with the greater number of metals ,mr4 ,grove found that the positive terminal or pole is hotter than the negative4 ,according to ,mr4 ,joule1 the quantity of heat generated in a unit of time is proportional to the strength of the current1 and when a galvanic current is employed in chemical analysis1 the heat in the entire circuit generated in a unit of time is equal to the work expended in producing it1 minus that employed in the analysis4 ,in fact1 a current of electricity cannot pass through a homogeneous conductor without generating heat in overcoming resistance1 an effect proved by ,mr4 ,joule to be proportional to the square of the force of the current1 and the same in whatever direction the current may be flowing4 ,any other thermal action that can take place must depend upon the heterogeneousness of the circuit1 and must be reversible with the current4 ,for example1 if a semicircle of bismuth be joined to a semicircle of antimony1 an electric current in passing through it produces cold where it passes from the bismuth to the antimony by absorption1 and heat where it passes from the antimony to the bismuth4 ,the transit of the electricity from pole to pole is accompanied by light1 and in consequence of the continuous current sparks occur every time the contact of the wires is either broken or renewed2 but considerable intensity is requisite to enable the electricity to force its way through atmospheric air or gas4 ,both its length and colour are affected by the density of the medium through which it passes4 ,if the medium be gradually rarefied the discharge increases from a spark to a luminous glow1 differing in colour in different gases1 but white in air4 ,when very much attenuated a discharge may be made to pass across #f or #g feet of space1 while in air of the ordinary density it will not pass through an inch4 ,in rarefied gas it resembles the ,aurora by its continuous flashes4 ,when the battery is powerful the luminous effects are very brilliant4 ,the most splendid artificial light known is produced by fixing pencils of charcoal at the extremities of the wires1 and bringing them into contact4 ,this light is the more remarkable as it is independent of combustion1 since the charcoal suffers no apparent change1 and1 likewise1 because it is equally vivid in such gases as do not contain oxygen4 ,it depends upon the molecular arrangement of the charcoal2 for ,mr4 ,grove observes that 8carbon in a transparent crystalline state1 as diamond1 is as perfect a non-conductor as we know1 while in an opaque amorphous state1 as graphite or charcoal1 it is one of the best conductors3 thus in one state it transmits light and stops electricity1 in the other it transmits electricity and stops light4 ,it is a circumstance worthy of remark1 that the arrangement of molecules which renders a solid body capable of transmitting light is most unfavourable to the transmission of electricity1 transparent solids being very imperfect conductors of electricity2 so all gases readily transmit light1 but are amongst the worst conductors of electricity1 if indeed they can be said to conduct it at all4 ,the fact that the molecular structure or arrangement of a body influences1 indeed ,i may say determines1 its conducting power1 is by no means explained by the theory of a fluid2 but if electricity be only a transmission of force or motion1 the influence of the molecular state is just what would be expected40 ,professor ,wheatstone1 by fixing metallic points at the extremities of the wires or poles1 has found that the appearance of the spectrum of the voltaic arc or vivid flame that is seen between the terminals of a battery1 depends1 as in static electricity1 upon the metal from whence it is taken4 ,the spectrum of that from mercury consists of seven definite rays1 separated from each other by dark intervals2 these visible rays are two orange lines close together1 a bright green line1 two blueish-green lines near each other1 a very bright purple line1 and1 lastly1 a blue line4 ,it is the same when it passes through carbonic acid gas1 oxygen gas1 air1 or vacuum4 ,the light from zinc1 cadmium1 tin1 bismuth1 and lead1 in a melted state1 gives similar results2 but the number1 position1 and colour of the lines vary so much in each case1 and the appearances are so different1 that the metals may easily be distinguished from one another by this mode of investigation4 ,the electric spark is considered by ,m4 ,angstrm to be the overlapping of two spectra1 one of which belongs to the metal1 and the other to the gas through which the spark passes1 and that the bright lines vary with the gas as well as with the metal4 ,in an oxygen spectrum the greatest number of bright lines occur in the blue and violet1 in nitrogen in the green and yellow1 and in hydrogen in the red4 ,these effects must necessarily be connected with the chemical and thermal properties of the gases4 ,mr4 ,grove considers that the colour of the voltaic arc1 or flame1 which appears between the poles of a very powerful battery1 depends upon the substance of the metal from whence it proceeds and on the medium through which it passes4 ,the spark from zinc is blue1 from silver it is green1 from iron it is red and scintillating--precisely the colours afforded by these metals in their ordinary combustion4 ,but the colour varies also with the medium through which the light passes1 for when the medium is changed a change takes place in the colour1 showing an affection of the intervening matter4 ,a portion of the metal terminals or poles is actually transmitted with every electrical or ,voltaic discharge1 whence ,mr4 ,grove concludes that the electrical discharge arises1 at least in part1 from an actual repulsion and severance of the electrified matter itself1 which flies off at the points of least resistance4 ,he observes that 8the phenomena attending the electric spark or ,voltaic arc tends to modify considerably our previous idea of the nature of the electric force as a producer of ignition and combustion4 ,the ,voltaic arc is perhaps1 strictly speaking1 neither ignition nor combustion4 ,it is not simply ignition2 because the matter of the terminals is not merely brought to a state of incandescence1 but is physically separated1 and partially transferred from one terminal to another1 much of it being dissipated in a vaporous state4 ,it is not combustion2 for the phenomena will take place independently of atmospheric air1 oxygen gas1 or any of the bodies usually called supporters of combustion2 combustion being in fact chemical union attended with heat and light4 ,in the ,voltaic arc we may have no chemical union1 for if the experiment be performed in an exhausted receiver1 or in nitrogen1 the substance forming the terminals is condensed and precipitated upon the interior of the vessel1 in1 chemically speaking1 an unaltered state4 ,thus1 to take a very striking example1 if the ,voltaic discharge be taken between zinc terminals in an exhausted receiver1 a fine black powder of zinc is deposited on the sides of the receiver2 this can be collected1 and takes fire readily in air by being touched with a match1 or ignited wire1 instantly burning into white oxide of zinc4 ,to an ordinary observer the zinc would appear to be burned twice--first in the receiver1 where the phenomenon presents all the appearance of combustion1 and1 secondly1 in the real combustion in air4 ,with iron the experiment is equally instructive4 ,iron is volatilized by the ,voltaic arc in nitrogen1 or in an exhausted receiver2 and when a scarcely perceptible film has lined the receiver1 if it be washed with an acid1 it then gives1 with ferrocyanide of potassium1 the ,prussian-blue precipitate4 ,in this case we readily distil iron1 a metal by ordinary means fusible only at a very high temperature40 ,another strong evidence that the ,voltaic discharge consists of the material itself of which the terminals are composed1 is the peculiar rotation which is observed in the light when iron is employed1 the magnetic character of this metal causing its particles to rotate by the influence of the ,voltaic current4 ,in short1 ,mr4 ,grove concludes that1 although it would be hasty to assert that the electrical disruptive discharge can in no case take place without the terminals being affected1 yet he had met with no instance of such a result1 provided the discharge had been sufficiently prolonged1 and the terminals in such a state as could be expected to render manifest slight changes6#ae ,some years ago ,mr4 ,grove discovered that the electrical discharge possesses certain phases or fits of an alternate character1 forming rings of alternate oxidation and deoxidation on metallic surfaces4 ,a highly polished silver plate in an air-pump was connected with the pole of a powerful inductive battery1 while a fine metallic wire1 or even a common sewing needle1 was fixed at the other pole1 and so arranged as to be perpendicular to the silver plate1 and very near1 but not touching it4 ,by means of this apparatus the electrical discharge could be sent through any kind of rarefied media4 ,in some of the experiments a series of concentric coloured rings of oxide alternating with rings of polished or unoxidated silver were formed on the plate under the point of the needle or wire4 ,when the plate was previously coated with a film of oxide1 the oxide was removed in concentric spaces by the discharge1 and increased on the alternate ones1 showing an alternate positive and negative electricity1 or electricity of an opposite character in the same discharge4 ,when the silver plate was polished the centre of the rings formed on it was yellow-green surrounded by blue-green2 then a ring of polished silver1 followed by a crimson ring with a slight orange tint on the inner side and deep purple on the outer2 lastly the indication of a polished one4 ,when the air-pump was filled with attenuated olefiant gas the rings were precisely the same with those seen in thin plates2 hence the effect is the same as that produced by the interference of light4 ,in these experiments the luminous appearance extended from three quarters of an inch to an inch round the point of the needle or wire4 ,when the silver plate was connected with the negative pole of the battery a polished point appeared upon it opposite the needle1 surrounded by a dusky ill-defined areola of a brown colour tinged with purple when viewed in one direction1 and greenish-white when seen in another4 ,in the present year ,mr4 ,gassiot1 ,vice-,president of the ,royal ,society1 has shown that the stratified character of the electric discharge is remarkably developed in the ,torricellian vacuum4 ,among the various experiments made by that gentleman two may be selected as strongly illustrative of this new and singular property of electrical light4 ,in a closed glass tube about an inch internal diameter and #ch inches long1 in which a vacuum had been made1 two platinum wires were hermetically sealed1 #cb inches apart1 and connected with the poles of an inductive battery4 ,the luminous appearance at the two poles was very different when electricity passed through the wires4 ,a glow surrounded the negative pole1 and in close approximation to the glow a well-defined dark space appeared1 while from the positive pole or wire the light proceeded in a stream2 but unless the charge be great or the tube short1 the stream will not extend to the black band1 which is totally different from the intervening space4 ,when discharges of electricity were sent through this vacuum tube a series of bands or stratifications were formed which were concave towards the positive pole2 and as in the changes in making and breaking the circuit the electricity emanates from the different terminals or wires1 their concavities were in opposite directions4 ,when instead of platinum wires narrow tinfoil coatings were placed round the exterior of the glass tube and connected with the wires of the battery1 brilliant stratifications filled the interior of the tube between the foil coatings1 but no dark band appeared4 ,at present ,mr4 ,gassiot is inclined to believe that the dark band is due to interference2 but that the stratifications arise from pulsations or impulses of a force acting in a highly attenuated but resisting medium1 for even with the best air-pumps it is impossible to make a perfect void2 he is still occupied with experiments on this new subject1 and no doubt will obtain very remarkable results1 of which none can be more extraordinary than his discovery of the powerful influence of the magnet on this electric light4 ,the stratifications are formed in rapid succession in the tube with platinum wires and are turned different ways1 but they can be separated at any part of the tube by the pole of a magnet round which the whole stratifications have a tendency to revolve4 ,in the second experiment1 where the tinfoil was used1 the discharge was divided in two by the pole of a magnet1 and the two parts had a tendency to rotate round the magnet in opposite directions4 ,voltaic electricity is a powerful agent in chemical analysis4 ,when transmitted through conducting fluids1 it separates them into their constituent parts1 which it conveys in an invisible state through a considerable space or quantity of liquid to the poles1 where they come into evidence4 ,numerous instances might be given1 but the decomposition of water is perhaps the most simple and elegant4 ,suppose a glass tube filled with water1 and corked at both ends2 if one of the wires of an active ,voltaic battery be made to pass through one cork1 and the other through the other cork1 into the water1 so that the extremities of the two wires shall be opposite and about a quarter of an inch asunder1 chemical action will immediately take place1 and gas will continue to rise from the extremities of both wires till the water has vanished4 ,if an electric spark be then sent through the tube1 the water will reappear4 ,by arranging the experiment so as to have the gas given out by each wire separately1 it is found that water consists of two volumes of hydrogen and one of oxygen4 ,the hydrogen is given out at the positive wire of the battery1 and the oxygen at the negative4 ,the oxides are also decomposed2 the oxygen appears at the positive pole1 and the metal at the negative4 ,the decomposition of the alkalies and earths by ,sir ,humphry ,davy formed a remarkable era in the history of science4 ,soda1 potash1 lime1 magnesia1 and other substances heretofore considered to be simple bodies incapable of decomposition1 were resolved by electric agency into their constituent parts1 and proved to be metallic oxides1 by that illustrious philosopher4 ,all chemical changes produced by electricity are accomplished on the same principle2 and it appears that1 in general1 combustible substances1 metals1 and alkalies go to the negative wire1 while acids and oxygen are evolved at the positive4 ,the transfer of these substances to the poles is not the least wonderful effect of the ,voltaic battery4 ,though the poles be at a considerable distance from one another1 nay1 even in separate vessels1 if a communication be only established by a quantity of wet thread1 as the decomposition proceeds the component parts pass through the thread in an invisible state1 and arrange themselves at their respective poles4 ,according to ,dr4 ,faraday1 electro-chemical decomposition is simply a case of the preponderance of one set of chemical affinities more powerful in their nature over another set which are less powerful4 ,and in electro-chemical action of any kind produced by a continuous current1 the amount of action in a given time is nearly1 if not rigorously1 proportional to the strength of the current4 ,the great efficacy of ,voltaic electricity in chemical decomposition arises not from its tension1 but from the quantity set in motion and the continuance of its action4 ,its agency appears to be most exerted on fluids and substances which by conveying the electricity partially and imperfectly impede its progress4 ,but it is now proved to be as efficacious in the composition as in the decomposition or analysis of bodies4 ,it had been observed that1 when metallic solutions are subjected to galvanic action1 a deposition of metal1 sometimes in the form of minute crystals1 takes place on the negative wire4 ,by extending this principle1 and employing a very feeble ,voltaic action1 ,m4 ,becquerel has succeeded in forming crystals of a great proportion of the mineral substances1 precisely similar to those produced by nature4 ,the electric state of metallic veins makes it possible that many natural crystals may have taken their form from the action of electricity bringing their ultimate particles1 when in solution1 within the narrow sphere of molecular attraction4 ,both light and motion favour crystallization4 ,crystals which form in different liquids are generally more abundant on the side of the jar exposed to the light2 and it is well known that still water1 cooled below #cb1 starts into crystals of ice the instant it is agitated4 ,a feeble action is alone necessary1 provided it be continued for a sufficient time4 ,crystals formed rapidly are generally imperfect and soft1 and ,m4 ,becquerel found that even years of constant ,voltaic action were necessary for the crystallization of some of the hard substances4 ,if this law be general1 how many ages may be required for the formation of a diamond6 ,the deposition of metal from a metallic solution by galvanic electricity has been most successfully applied to the arts of plating and gilding1 as well as to the more delicate process of copying medals and copper plates4 ,indeed1 not medals only1 but any object of art or nature1 may be coated with precipitated metal1 provided it be first covered with the thinnest film of plumbago1 which renders a non-conductor sufficiently conducting to receive the metal4 ,photo-galvanic engraving depends upon this4 ,gelatine mixed with bichromate of potash1 nitrate of silver1 and iodide of potassium1 is spread over a plate of glass1 and when dry a positive print is laid upon it with its face downwards1 which1 when exposed to the sun1 leaves its impression4 ,when soaked in water the gelatine swells around all those parts where the light had fallen1 thus forming an intaglio1 a cast of which is taken in gutta-percha1 which is then coated with copper by the electro process1 whence a copper plate in relief is obtained4 ,static electricity1 on account of its high tension1 passes through water and other liquids as soon as it is formed1 whatever the length of its course may be4 ,voltaic electricity1 on the contrary1 is weakened by the distance it has to traverse4 ,pure water is a very bad conductor2 but ice absolutely stops a current of ,voltaic electricity altogether1 whatever be the power of the battery1 although static or common electricity has sufficient power to overcome its resistance4 ,dr4 ,faraday has discovered that this property is not peculiar to ice2 that1 with a few exceptions1 bodies which do not conduct electricity when solid acquire that property1 and are immediately decomposed1 when they become fluid1 and1 in general1 that decomposition takes place as soon as the solution acquires the capacity of conduction1 which has led him to suspect that the power of conduction may be only a consequence of decomposition4 ,heat increases the conducting power of some substances for ,voltaic electricity1 and of the gases for both kinds4 ,dr4 ,faraday has given a new proof of the connexion between heat and electricity1 by showing that1 in general1 when a solid1 which is not a metal1 becomes fluid1 it almost entirely loses its power of conducting heat1 while it acquires a capacity for conducting electricity in a high degree4 ,m4 ,becquerel regards the production of heat and that of electricity to be concomitant2 their dependence being such1 that when one is increased the other diminishes1 and vice vers1 so that one may altogether disappear with the increase of the other4 ,for instance1 when electricity circulates in a metallic wire1 the greater the heat produced1 the less the quantity of electricity which passes1 and the contrary1 so that the affair proceeds as if electricity were converted into heat1 and heat into electricity4 ,again1 in a closed galvanic circuit the sum of the heat produced in the chemical action of the acidulated water upon the zinc and in the conducting wire is constant1 so that the quantity of heat disengaged in the reaction is greater in proportion as less electricity passes through the wire4 ,these1 and other circumstances1 prove such an intimate connexion between the production of heat and electricity1 that in the change of condition of substances the electrical effects might disappear or be annulled by the calorific effects4 ,the galvanic current affects all the senses3 nothing can be more disagreeable than the shock1 which may even be fatal if the battery be very powerful4 ,a bright flash of light is perceived with the eyes shut1 when one of the wires touches the face1 and the other the hand4 ,by touching the ear with one wire1 and holding the other1 strange noises are heard2 and an acid taste is perceived when the positive wire is applied to the tip of the tongue1 and the negative wire touches some other part of it4 ,by reversing the poles the taste becomes alkaline4 ,it renders the pale light of the glow-worm more intense4 ,dead animals are roused by it1 as if they started again into life1 and it may ultimately prove to be the cause of muscular action in the living4 ,several fish possess the faculty of producing electrical effects4 ,the most remarkable are the gymnotus electricus1 found in ,south ,america2 and the torpedo1 a genus of ray1 frequent in the ,mediterranean4 ,the electrical action of the torpedo depends upon an apparatus apparently analogous to the ,voltaic pile1 which the animal has the power of charging at will1 consisting of membranous columns filled throughout with lamin1 separated from one another by a fluid4 ,the absolute quantity of electricity brought into circulation by the torpedo is so great1 that it effects the decomposition of water1 has power sufficient to make magnets1 gives very severe shocks and the electric spark4 ,it is identical in kind with that of the galvanic battery1 the electricity of the under surface of the fish being the same with the negative pole1 and that in the upper surface the same with the positive pole4 ,its manner of action is1 however1 somewhat different2 for1 although the evolution of the electricity is continued for a sensible time1 it is interrupted1 being communicated by a succession of discharges4 ,,section ,,xxx4 ,discovery of ,electro-magnetism--,deflection of the ,magnetic ,needle by a ,current of ,electricity--,direction of the ,force--,rotatory ,motion by ,electricity--,rotation of a ,wire and a ,magnet--,rotation of a ,magnet about its ,axis--,of ,mercury and ,water--,electro-,magnetic ,cylinder or ,helix--,suspension of a ,needle in a ,helix--,electro-,magnetic ,induction--,temporary ,magnets--,the ,galvanometer4 ,,the disturbing effects of the aurora and lightning on the mariner's compass had been long known4 ,in the year #ahai ,m4 ,oersted1 ,professor of ,natural ,philosophy at ,copenhagen1 discovered that a current of ,voltaic electricity exerts a powerful influence on a magnetized needle4 ,this observation has given rise to the theory of electro-magnetism--one of the most interesting sciences of modern times1 whether it be considered as leading us a step farther in generalization1 by identifying two agencies hitherto referred to different causes1 or as developing a new force1 unparalleled in the system of the world1 which1 overcoming the retardation from friction1 and the obstacle of a resisting medium1 maintains a perpetual motion as long as the action of a ,voltaic battery is continued4 ,when the two poles of a ,voltaic battery are connected by a metallic wire1 so as to complete a circuit1 the electricity flows without ceasing4 ,if a straight portion of that wire be placed parallel to1 and horizontally above1 a magnetized needle at rest in the magnetic meridian1 but freely poised like the mariner's compass1 the action of the electric current flowing through the wire will instantly cause the needle to change its position4 ,its extremity will deviate from the north towards the east or west1 according to the direction in which the current is flowing2 and1 on reversing the direction of the current1 the motion of the needle will be reversed also4 ,the numerous experiments that have been made on magnetism and electricity1 as well as those on the various relative motions of a magnetic needle under the influence of galvanic electricity1 arising from all possible positions of the conducting wire1 and every direction of the ,voltaic current1 together with all the other phenomena of electro-magnetism1 are explained by ,dr4 ,roget in some excellent articles on these subjects in the ,library of ,useful ,knowledge4 ,all experiments tend to prove that the force emanating from the electric current1 which produces such effects on the magnetic needle1 acts at right angles to the current4 ,the action of an electrical current upon either pole of a magnet has no tendency to cause the pole to approach or recede1 but to rotate about it4 ,if the stream of electricity be supposed to pass through the centre of a circle whose plane is perpendicular to the current1 the direction of the force exerted by the electricity will always be in the tangent to the circle1 or at right angles to its radius 7,n4 #bbc74 ,consequently1 the tangential force of the electricity has a tendency to make the pole of a magnet move in a circle round the wire of the battery4 ,rotatory motion was suggested by ,dr4 ,wollaston4 ,dr4 ,faraday was the first who actually succeeded in making the pole of a magnet rotate about a vertical conducting wire4 ,in order to limit the action of the electricity to one pole1 about two-thirds of a small magnet were immersed in mercury1 the lower end being fastened by a thread to the bottom of the vessel containing the mercury4 ,when the magnet was thus floating almost vertically with its north pole above the surface1 a current of positive electricity was made to descend perpendicularly through a wire touching the mercury1 and immediately the magnet began to rotate from left to right about the wire4 ,the force being uniform1 the rotation was accelerated till the tangential force was balanced by the resistance of the mercury1 when it became constant4 ,under the same circumstances the south pole of the magnet rotates from right to left4 ,it is evident1 from this experiment1 that the wire may also be made to perform a rotation round the magnet1 since the action of the current of electricity on the pole of the magnet must necessarily be accompanied by a corresponding reaction of the pole of the magnet on the electricity in the wire4 ,this experiment has been accomplished by a vast number of contrivances1 and even a small battery1 consisting of two plates1 has performed the rotation4 ,dr4 ,faraday produced both motions at the same time in a vessel containing mercury2 the wire and the magnet revolved in one direction about a common centre of motion1 each following the other4 ,the next step was to make a magnet1 and also a cylinder1 revolve about their own axes1 which they do with great rapidity4 ,mercury has been made to rotate by means of ,voltaic electricity1 and ,professor ,ritchie exhibited in the ,royal ,institution the singular spectacle of the rotation of water by the same means1 while the vessel containing it remained stationary4 ,the water was in a hollow double cylinder of glass1 and1 on being made the conductor of electricity1 was observed to revolve in a regular vortex1 changing its direction as the poles of the battery were alternately reversed4 ,professor ,ritchie found that all the different conductors hitherto tried by him1 such as water1 charcoal1 c41 give the same electro-magnetic results when transmitting the same quantity of electricity1 and that they deflect the magnetic needle in an equal degree when their respective axes of conduction are at the same distance from it4 ,but one of the most extraordinary effects of this force is exhibited by coiling a copper wire1 so as to form a helix or corkscrew1 and connecting the extremities of the wire with the poles of a galvanic battery4 ,if a magnetized steel bar or needle be placed within the screw1 so as to rest upon the lower part1 the instant a current of electricity is sent through the wire of the helix1 the steel bar starts up by the influence of this invisible power1 and remains suspended in the air in opposition to the force of gravitation 7,n4 #bbd74 ,the effect of the electro-magnetic power exerted by each turn of the wire is to urge the north pole of the magnet in one direction1 and the south pole in the other4 ,the force thus exerted is multiplied in degree and increased in extent by each repetition of the turns of the wire1 and in consequence of these opposing forces the bar remains suspended4 ,this helix has all the properties of a magnet while the electrical current is flowing through it1 and may be substituted for one in almost every experiment4 ,it acts as if it had a north pole at one extremity and a south pole at the other1 and is attracted and repelled by the poles of a magnet exactly as if it were one itself4 ,all these results depend upon the course of the electricity2 that is1 on the direction of the turns of the screw1 according as it is from right to left1 or from left to right1 being contrary in the two cases4 ,the action of ,voltaic electricity on a magnet is not only precisely the same with the action of two magnets on one another1 but its influence in producing temporary magnetism in iron and steel is also the same with magnetic induction4 ,the term induction1 when applied to electric currents1 expresses the power which these currents possess of inducing a particular state upon matter in their immediate neighbourhood1 otherwise neutral or indifferent4 ,for example1 the connecting wire of a galvanic battery holds iron filings suspended like a magnet as long as the current continues to flow through it3 the iron becomes magnetic by the induction of the current4 ,the most powerful temporary magnets are obtained by bending a thick cylinder of soft iron into the form of a horseshoe1 and surrounding it with a coil of thick copper wire covered with silk to prevent communication between its coils4 ,when this wire forms part of a galvanic circuit the iron becomes so highly magnetic by the induction of the current flowing through the wire that a temporary magnet of this kind made by ,professor ,henry of the ,albany ,academy in the ,united ,states sustained a weight of nearly a ton4 ,another by ,mr4 ,gage has been applied with considerable success as a moving power3 its spark is a bright flash1 and the snap as loud as a pistol4 ,but the most powerful known is that employed by ,mr4 ,joule in his experiments1 which sustains a weight of #bjhj lbs4 ,the iron loses its magnetism the instant the electricity ceases to flow1 and acquires it again as instantaneously when the circuit is renewed4 ,the action of an electric current causes a deviation of the compass from the plane of the magnetic meridian4 ,in proportion as the needle recedes from the meridian1 the intensity of the force of terrestrial magnetism increases1 while at the same time the electro-magnetic force diminishes2 the number of degrees at which the needle stops1 showing where the equilibrium between these two forces takes place1 will indicate the intensity of the galvanic current4 ,the galvanometer1 constructed upon this principle1 is employed to measure the intensity of galvanic currents collected and conveyed to it by wires4 ,this instrument is rendered much more sensible by neutralizing the effects of the earth's magnetism on the needle1 which is accomplished by placing a second magnetised needle so as to counteract the action of the earth on the first--a precaution requisite in all delicate magnetical experiments4 ,it has been ascertained by means of this instrument that the action of an electrical current upon a magnet is inversely as the square of the distance1 and the energy with which an electro magnet acts is directly as the power of the galvanic battery and the number of coils round the core1 and inversely as the resistance of the wire4 ,,section ,,xxxi4 ,electro-,dynamics--,reciprocal ,action of ,electric ,currents--,identity of ,electro-,dynamic ,cylinders and ,magnets--,differences between the ,action of ,voltaic ,electricity and ,electricity of ,tension--,effects of a ,voltaic ,current--,ampre's ,theory--,dr4 ,faraday's ,experiment of ,electrifying and ,magnetising a ,ray of ,light4 ,,the science of electro-magnetism1 which must render the name of ,m4 ,oersted ever memorable1 relates to the reciprocal action of electrical and magnetic currents3 ,m4 ,ampre1 by discovering the mutual action of electrical currents on one another1 has added a new branch to the subject1 to which he has given the name of electro-dynamics4 ,when electric currents are passing through two conducting wires1 so suspended or supported as to be capable of moving both towards and from one another1 they show mutual attraction or repulsion1 according as the currents are flowing in the same or in contrary directions2 the phenomena varying with the relative inclinations and positions of the streams of electricity4 ,the mutual action of such currents1 whether they flow in the same or in contrary directions1 whether they be parallel1 perpendicular1 diverging1 converging1 circular1 or heliacal1 all produce different kinds of motion in a conducting wire1 both rectilineal and circular1 and also the rotation of a wire helix1 such as that described1 now called an electro-dynamic cylinder on account of some improvements in its construction 7,n4 #bbe74 ,and1 as the hypothesis of a force varying inversely as the square of the distance accords perfectly with all the observed phenomena1 these motions come under the same laws of dynamics and analysis as any other branch of physics4 ,electro-dynamic cylinders act on each other precisely as if they were magnets during the time the electricity is flowing through them4 ,all the experiments that can be performed with the cylinder might be accomplished with a magnet4 ,that end of the cylinder in which the current of positive electricity is moving in a direction similar to the motion of the hands of a watch1 acts as the south pole of a magnet1 and the other end1 in which the current is flowing in a contrary direction1 exhibits northern polarity4 ,the phenomena mark a very decided difference between the action of electricity in motion or at rest1 that is1 between ,voltaic and static electricity2 the laws they follow are in many respects of an entirely different nature1 though the electricities themselves are identical4 ,since ,voltaic electricity flows perpetually1 it cannot be accumulated1 and consequently has no tension1 or tendency to escape from the wires which conduct it4 ,nor do these wires either attract or repel light bodies in their vicinity1 whereas static or ordinary electricity can be accumulated in insulated bodies to a great degree1 and in that state of rest the tendency to escape is proportional to the quantity accumulated and the resistance it meets with4 ,in ordinary electricity1 the law of action is1 that dissimilar electricities attract and similar electricities repel one another4 ,in ,voltaic electricity1 on the contrary1 similar currents1 or such as are moving in the same direction1 attract one another1 while a mutual repulsion is exerted between dissimilar currents1 or such as flow in opposite directions4 ,common electricity escapes when the pressure of the atmosphere is removed1 but the electro-dynamical effects are the same whether the conductors be in air or in vacuo4 ,the effects produced by a current of electricity depend upon the celerity of its motion through a conducting wire4 ,yet we are ignorant whether the motion be uniform or varied1 but the method of transmission has a marked influence on the results2 for1 when it flows without intermission1 it occasions a deviation in the magnetic needle1 but it has no effect whatever when its motion is discontinuous or interrupted1 like the current produced by the common electrical machine when a communication is made between the positive and negative conductors4 ,m4 ,ampre has established a theory of electro-magnetism suggested by the analogy between electro-dynamic cylinders and magnets1 founded upon the reciprocal attraction of electro-currents1 to which he reduces all the phenomena of magnetism and electro-magnetism1 by assuming that the magnetic properties which bodies possess derive these properties from currents of electricity1 circulating about every part in one uniform direction4 ,although every particle of a magnet possesses like properties with the whole1 yet the general effect is the same as if the magnetic properties were confined to the surface4 ,consequently1 ,ampre concludes that the internal electro-currents must compensate one another1 and that the magnetism of a body must therefore arise from a superficial current of electricity constantly circulating in a direction perpendicular to the axis of the magnet2 so that the reciprocal action of magnets and all the phenomena of electro-magnetism are reduced to the action and reaction of superficial currents of electricity1 acting at right angles to their direction4 ,notwithstanding the experiments made by ,ampre to elucidate the subject1 there is still an uncertainty in the theory of the induction of magnetism by an electric current in a body near it4 ,it does not appear whether electric currents which did not previously exist are actually produced by induction1 or if its effect be only to give one uniform direction to the infinite number of electric currents previously existing in the particles of the body1 and thus rendering them capable of exhibiting magnetic phenomena1 in the same manner as polarization reduces the undulations of light to one plane1 which had previously been performed in every plane4 ,possibly both may be combined in producing the effect2 for the action of the electric current may not only give a common direction to those already existing1 but may also increase their intensity4 ,however that may be1 by assuming that the attractions and repulsions of the elementary portions of electric currents vary inversely as the square of the distance1 the actions being at right angles to the direction of the current1 it is found that the attraction and repulsion of a current of indefinite length on the elementary portion of a parallel current at any distance from it are in the simple ratio of the shortest distance between them3 consequently1 the reciprocal action of electric currents is reduced to the composition and resolution of forces1 so that the phenomena of electro-magnetism are brought under the laws of mechanics by the theory of ,ampre4 ,it appears that ,dr4 ,faraday's very remarkable experiment of electrifying and magnetising a ray of polarized light may possibly afford a demonstration of the reality of ,ampre's explanation of the ultimate nature of magnetism4 ,in this experiment a copper wire #eja feet long was arranged in four concentric spirals1 the extremities of which were connected with the poles of a powerful galvanic battery1 and a polished prism of heavy glass1 or silicated borate of lead1 was placed in the axis of the spiral as a core1 through the length or axis of which a ray of polarized light was sent4 ,this ray1 viewed through a piece of tourmaline or a ,nichol's eye-piece1 vanished and reappeared as usual at each quarter revolution of the eye-piece2 but when a current of electricity was sent through the spiral at the time the ray had vanished1 it instantly reappeared1 and remained as long as the electric current continued to flow2 but the instant the electricity ceased the light vanished1 and as often as the electric current flowed through the spiral1 or was interrupted1 so often did the polarized ray appear and vanish4 ,the character of the force thus impressed on the heavy glass is that of rotation1 for the stopping and renewing of the electric current had the same effect as the revolving motion of the eye-piece in making the light alternately appear and vanish4 ,accordingly1 ,dr4 ,faraday found that1 when the electricity flowed through the spiral in one direction1 the rotation of the plane of polarization was right-handed2 and when it flowed in the other direction1 the rotation of the plane of polarization was left-handed1 the rotation increasing with the length of the prism and the intensity of the electricity4 ,the same phenomena were produced by a very powerful magnet when a ray of polarized light was sent through the heavy glass parallel to the line of magnetic force4 ,heavy glass or silico-borate of lead has the property more than any other substance of making light rotate under electric and magnetic influence2 but many substances have the property more or less1 as flint and crown glass1 rock salt1 all the fixed and essential oils1 water1 and many other liquids1 but none of the gases possess it4 ,in those substances that have the power of circular polarization naturally1 the magnetic and electric influences increase or diminish the rotation according to its direction4 ,polarized heat is made to revolve in the same manner1 when the medium through which it passes is subject to magnetic influence4 ,mr4 ,grove observes that if light and heat be merely modes of force1 which there is every reason to believe that they are1 it may be fairly stated that in these experiments magnetism affects these forces directly2 for light and heat being1 in that view1 motions of ordinary matter1 magnetism in affecting these movements affects the forces which occasion them4 ,if1 however1 this effect of magnetism be a molecular change of the matter transmitting the light and heat1 then it follows that the light and heat are indirectly affected by the electricity or magnetism4 ,dr4 ,faraday says that the magnetic forces do not act on the ray of light directly1 without the intervention of matter1 but through the mediation of the substance in which they and the ray have a simultaneous existence2 the substances and the forces giving to and receiving from each other the power of acting on the light4 ,dr4 ,thomson has shown1 by a mathematical investigation of the subject1 that ,dr4 ,faraday's discovery seems to prove the truth of ,ampre's explanation of the ultimate nature of magnetism4 ,however1 in ,ampre's theory1 the current of electricity flowing round the iron makes it a permanent magnet1 but it does not make the heavy glass or the other bodies1 which have the same property1 either temporary magnets when the light is rotating within them1 or permanent magnets when the inductive action of the current of electricity ceases4 ,hence the molecular condition of the substances1 when the light is rotating in them1 must be specifically distinct from that of magnetised iron3 it must therefore be a new magnetic condition1 and the force which the matter in this state possesses must be a new magnetic force4 ,after describing his admirable experiment1 ,dr4 ,faraday observes that 8it has established for the first time a true1 direct relation and dependence between light and the magnetic and electric forces2 and thus a great addition is made to the facts and considerations which tend to prove that all natural forces are tied together1 and have one common origin4 ,it is no doubt difficult1 in the present state of our knowledge1 to express our expectations in exact terms2 and though ,i have said that another of the powers of nature is in these experiments directly related to the rest1 ,i ought perhaps rather to say that another form of the great power is distinctly and directly related to the other forms2 or that the great power manifested by particular phenomena in particular forms is here further identified and recognised by the direct relation of its form of light to its forms of electricity and magnetism4 ,the relation existing between polarized light and magnetism and electricity is even more interesting than if it had been shown to exist with common light only4 ,it cannot but extend to common light2 and1 as it belongs to light made in a certain respect more precise in its character and properties by polarization1 it collates and connects it with these powers in that duality of character which they possess1 and yields an opening1 which before was wanting to us1 for the appliances of these to the investigation of the nature of this and other radiant agencies40 ,thus ,dr4 ,faraday's experiment not only shows the increasing connexion between the sciences1 but the tendency of all the forces of nature to merge in one great and universal power4 ,in the action of a magnet upon the stratifications of an electrical discharge ,mr4 ,gassiot has discovered a new instance of the connexion between magnetism and light4 ,,section ,,xxxii4 ,magneto-,electricity--,volta-,electric ,induction--,magneto-,electric ,induction--,identity in the ,action of ,electricity and ,magnetism--,description of a ,magneto-,electric ,apparatus and its ,effects--,identity of ,magnetism and ,electricity--,the ,submarine ,telegraph4 ,,from the law of action and reaction being equal and contrary1 it might be expected that1 as electricity powerfully affects magnets1 so1 conversely1 magnetism ought to produce electrical phenomena4 ,by proving this very important fact from the following series of interesting and ingenious experiments1 ,dr4 ,faraday has added another branch to the science which he has named magneto-electricity4 ,a great quantity of copper wire was coiled in the form of a helix round one half of a ring of soft iron1 and connected with a galvanic battery2 while a similar helix connected with a galvanometer was wound round the other half of the ring1 but not touching the first helix4 ,as soon as contact was made with the battery1 the needle of the galvanometer was deflected4 ,but the action was transitory2 for1 when the contact was continued1 the needle returned to its usual position1 and was not affected by the continual flow of the electricity through the wire connected with the battery4 ,as soon1 however1 as the contact was broken1 the needle of the galvanometer was again deflected1 but in the contrary direction4 ,similar effects were produced by an apparatus consisting of two helices of copper wire coiled round a block of wood1 instead of iron1 from which ,dr4 ,faraday infers that the electric current passing from the battery through one wire induces a similar current through the other wire1 but only at the instant of contact1 and that a momentary current is induced in a contrary direction when the passage of the electricity is suddenly interrupted4 ,these brief currents or waves of electricity were found to be capable of magnetizing needles1 of passing through a small extent of fluid1 and1 when charcoal points were interposed in the current of the induced helix1 a minute spark was perceived as often as the contacts were made or broken1 but neither chemical action nor any other electric effects were obtained4 ,a deviation of the needle of the galvanometer took place when common magnets were employed instead of the ,voltaic current2 so that the magnetic and electric forces are identical in their effects in this experiment4 ,again1 when a helix formed of #bbj feet of copper wire1 into which a cylinder of soft iron was introduced1 was placed between the north and south poles of two bar magnets1 and connected with the galvanometer by means of wires from each extremity1 as often as the magnets were brought into contact with the iron cylinder it became magnetic by induction1 and produced a deflection in the needle of the galvanometer4 ,on continuing the contact the needle resumed its natural position1 and1 when the contact was broken1 deflection took place in the opposite direction2 when the magnetic contacts were reversed1 the deflection was reversed also4 ,with strong magnets1 so powerful was the action1 that the needle of the galvanometer whirled round several times successively2 and similar effects were produced by the mere approximation or removal of the helix to the poles of the magnets4 ,thus it was proved that magnets produce the very same effects on the galvanometer that electricity does4 ,though at that time no chemical decomposition was effected by these momentary currents which emanate from the magnets1 they agitated the limbs of a frog2 and ,dr4 ,faraday justly observes1 that 8an agent which is conducted along metallic wires in the manner described1 which1 whilst so passing1 possesses the peculiar magnetic actions and force of a current of electricity1 which can agitate and convulse the limbs of a frog1 and which finally can produce a spark by its discharge through charcoal1 can only be electricity40 ,soon after he completely established the identity of the two powers by producing the spark1 heating metallic wires1 and accomplishing chemical decomposition4 ,hence it appears that electrical currents are evolved by magnets1 which produce the same phenomena with the electrical currents from the ,voltaic battery3 they1 however1 differ materially in this respect--that time is required for the exercise of the magnetico-electric induction1 whereas ,volta-electric induction is instantaneous4 ,thus the effect of induction or the influence of the spiral wire in increasing the electric and magnetic power is very great indeed1 and to that we are indebted for the electric telegraph1 for ,voltaic electricity alone is too feeble to overcome the resistance of a long wire4 ,electric currents1 whatever their tension may be1 produce the phenomena of induction2 these again induce other currents in bodies capable of induction1 and so on indefinitely2 the first and second flow in the same direction1 the others alternately opposite and direct4 ,they all give the shock and can decompose water1 but with ,volta-electric currents the elevation of temperature as well as their physiological and magnetic effects are produced by instantaneous actions1 which only depend upon the quantity and tension of the current1 and by no means on its duration1 for induced currents only exist for a moment when the circuit of the battery is broken4 ,the most energetic physiological effects are produced by a small quantity of electricity moving with great velocity4 ,the apparatus first employed by ,dr4 ,faraday is in effect a battery1 where the agent is the magnetic instead of the ,voltaic force1 or1 in other words1 electricity1 and is thus constructed3-- ,a very powerful horseshoe magnet1 formed of twelve steel plates in close approximation1 is placed in a horizontal position4 ,an armature1 consisting of a bar of the purest soft iron1 has each of its ends bent at right angles1 so that the faces of those ends may be brought directly opposite and close to the poles of the magnet when required4 ,ten copper wires--covered with silk1 in order to insulate them--are wound round one half of the bar of soft iron1 as a compound helix3 ten other wires1 also insulated1 are wound round the other half of the bar4 ,the extremities of the first set of wires are in metallic connexion with a circular disc1 which dips into a cup of mercury1 while the ends of the other ten wires in the opposite direction are soldered to a projecting screw-piece1 which carries a slip of copper with two opposite points4 ,the steel magnet is stationary2 but when the armature1 together with its appendages1 is made to rotate vertically1 the edge of the disc always remains immersed in the mercury1 while the points of the copper slip alternately dip in it and rise above it4 ,by the ordinary laws of induction1 the armature becomes a temporary magnet while its bent ends are opposite the poles of the steel magnet1 and ceases to be magnetic when they are at right angles to them4 ,it imparts its temporary magnetism to the helices which concentrate it2 and1 while one set conveys a current to the disc1 the other set conducts the opposite current to the copper slip4 ,as the edge of the revolving disc is always immersed in the mercury1 one set of wires is constantly maintained in contact with it1 and the circuit is only completed when a point of the copper slip dips in the mercury also2 but the circuit is broken the moment that point rises above it4 ,thus1 by the rotation of the armature1 the circuit is alternately broken and renewed2 and as it is only at these moments that electric action is manifested1 a brilliant spark takes place every time the copper point leaves the surface of the mercury4 ,platinum wire is ignited1 shocks smart enough to be disagreeable are given1 and water is decomposed with astonishing rapidity1 by the same means2 which proves1 beyond a doubt1 the identity of the magnetic and electric agencies1 and places ,dr4 ,faraday1 whose experiments established the principle1 in the first rank of experimental philosophers4 ,a magneto-electric machine has been recently constructed by ,mr4 ,henley1 of enormous power4 ,it consists of two permanent magnets1 from which the induction is obtained2 each of these is formed of thirty horseshoe steel magnets1 two feet and a half long1 and from four to five inches broad1 and each is surrounded by a coil of wire six miles long1 coated with silk to insulate the coils4 ,a shock from these wires would be instantaneous death4 ,this apparatus will ultimately be employed to send a stream of electricity through long submarine and subterraneous wires2 but a ,volta-electric machine has hitherto been used1 in which the electricity is generated by a galvanic battery instead of magnets4 ,induction1 or the effect of the spiral wires in augmenting the power of ,voltaic electricity1 is admirably illustrated in the ,atlantic telegraph4 ,wires that are to convey electricity under ground1 or through water1 must be defended from injury and insulated to prevent the lateral escape of the electricity4 ,for that purpose the cable that is laid at the bottom of the ,atlantic1 from near ,valentia in ,ireland to ,trinity ,bay in ,newfoundland1 is formed of seven fine copper wires which convey the electricity1 bound together by a coating of gutta percha1 over which there are layers of cloth dipped in pitch1 and then the whole is covered by steel wires twisted together in strands and twined round in long close spirals1 forming a cord or cable not more than an inch and a quarter in diameter1 and #bajj miles long4 ,the use of the gutta percha is to insulate the wires2 the other coatings are merely for protection4 ,the ,voltaic battery which generates the electricity consists of #dj cells1 the plates of which are alternately of zinc and platinized silver1 each about nine inches square1 the exciting fluid being dilute sulphuric acid4 ,although the force developed by this battery is so great that a piece of iron three inches long and three eighths of an inch in diameter placed in contact with the poles may be consumed in a few minutes1 it is absolutely incapable of sending a current of electricity through wires #bejj miles long1 on account of their resistance1 without the aid of ,dr4 ,faraday's inductive action4 ,it is only the primary agent for inducing a current of sufficient strength4 ,to accomplish that1 many thousand yards of fine copper wire coated with silk are wound round a hollow soft iron cylinder2 the whole is then coated by gutta percha1 and the end of the wire is joined to the wires in the cable so as to form a continuous line from ,valentia to ,newfoundland4 ,a second copper wire1 shorter but thicker than the preceding1 and also insulated by a coating of silk1 is wound round the cylinder above the gutta percha3 when the ends of this thick wire are brought into contact with the poles of the battery1 currents of electricity flow through it1 between pole and pole1 and in their passage temporarily convert the hollow iron cylinder into a powerful electro-magnet1 which by its reaction induces a current of electricity in the fine wire of sufficient power to cross the ,atlantic4 ,the efficiency of the electric telegraph depends upon the power we possess of breaking and renewing the current at pleasure1 since by that means distinct and successive signals are made from station to station4 ,in the ,atlantic cable positive and negative electricity are transmitted alternately2 the electricity is sent to ,america from alternate poles1 and the current returns again through the water1 which completes the circuit4 ,the passage of electricity through a cable or telegraphic wire in air is sensibly instantaneous2 that through a cable1 whether extended in water or under ground1 requires time on account of lateral induction through the gutta percha2 for the electricity1 in passing through the wires1 induces the opposite electricity on the surface of the water or moist earth in contact with the cable1 and in that respect it is precisely like a ,leyden jar1 the gutta percha representing the glass4 ,as the power of induction is proportional to the tension of the electricity1 and as the tension is continually diminished by the resistance of the wires1 the induction is continually diminished and requires a longer time4 ,electricity took two seconds to pass through a cable #gfh miles long1 laid under ground from ,london to ,manchester1 and back again twice2 while in air it was all but instantaneous1 because the inductive capacity of air is very much less than that of water or moist earth4 ,in the experiment with the cable under ground it took two-thirds of a second to overcome the resistance of the wires1 and then the velocity of the electricity was #ajjj miles in a second1 and it was the same whatever the intensity of the electricity4 ,it has already been mentioned that the efficiency of the electric telegraph depends upon the breaking and renewing the current of electricity by means of which a succession of waves of electricity are sent through the conducting wires4 ,now it has been ascertained that three electric waves may travel simultaneously through the wires of the ,atlantic telegraph with sufficient intervals between them to record the indications they are intended to convey2 that is1 three signals can be intelligibly and practically transmitted in two seconds4 ,the original design1 structure1 and difficulty of depositing the cable are only equalled by the talent and perseverance with which it has been done4 ,the #eth of ,august1 #aheh1 will be memorable for the accomplishment of the boldest enterprise that ever was undertaken by man1 and which is only the beginning of a vast submarine communication that will ultimately encircle the globe4 ,it has been granted to ,british genius thus to annihilate time and space1 in order to connect all mankind into one great family for their moral and religious advancement2 and1 whatever may be the fate of the ,british ,islands in the course of ages1 to their energetic race the glory will remain of having been the chief instruments in the hands of ,providence for the civilization of the world--a civilization which will extend with the development of their numerous colonies into great independent ,christian states1 like those of the ,union in ,north ,america4 ,the thunderbolt snatched from heaven by ,franklin now passes through the depths of the ,atlantic as a messenger of peace between the kindred nations4#af ,in telegraphs on land the intensity of the battery or magnets is increased by induction on the same principle4 ,it is by intensity that the electric current is enabled to pass through the wires1 and that is augmented by increasing the number of coils round the cylinder3 however1 it is only advantageous when the distance between the stations is great1 for then the resistance in the additional coils bears a small proportion to the resistance offered by very long wires1 but a very great proportion to that opposed in very short ones4 ,the nice adjustment for each case has been determined by the experiments of eminent electricians1 and all the arrangements have been brought to great perfection in this wonderful triumph of science1 which is due to ,volta1 who called into existence the fiery stream1 and to ,faraday1 who has given it the energy of the lightning4 ,when the length of the wire in the helices of an electro-magnet is very great1 it offers increasing resistance to the passage of the electricity1 so that the cessation of magnetism is not instantaneous when the contact with the ,voltaic battery is broken4 ,to remedy that defect an instrument has been invented which instantaneously deprives the apparatus of the remaining electricity4 ,a great length of fine wire gives the severest shocks1 while a shorter and thicker wire gives the longest sparks and ignites the greatest quantity of platinum wire4 ,ruhmkorff's electro-inductive apparatus has either been improved1 or new machines constructed1 by ,messrs4 ,grove1 ,gassiot1 and ,joule1 of intense energy4 ,indeed1 so great is the energy of electro-induction1 that hopes were entertained of its superseding steam as a motive power4 ,for the current of electricity from an electro-magnet can be made to flow in opposite directions1 so as to produce alternate attractions and repulsions1 and consequently a continued motion1 which might be applied as a motive force to machinery4 ,however1 ,mr4 ,joule has proved that the power developed by one pound of coal in combustion is to that produced by one pound of zinc consumed in ,mr4 ,grove's powerful electro-magnetic apparatus as nine to one1 so that1 even if zinc were as cheap as coal1 and a ,voltaic battery as easily kept in order as an engine-furnace1 electricity will not supersede steam as a motive power4 ,a current of electricity traversing a conductor gives out a quantity of heat determined by fixed laws1 the amount of which is invariable as long as the machine to which it is applied remains at rest2 but the instant the machine is set in motion a reaction takes place in the intensity of the current1 causing a diminution in the quantity of heat1 because the heat that disappears is converted into the mechanical force exerted by the engine4 ,mr4 ,joule's experiments prove that1 whenever a current of electricity is generated by a magneto-electric machine1 the quantity of heat evolved by that current has a constant relation to the power required to work the machine2 and on the other hand1 whenever an engine is worked by a ,voltaic battery1 that the power developed is at the expense of the calorific force of the battery for a given consumption of zinc1 the mechanical effect produced having a fixed relation to the heat lost in the ,voltaic current4 ,the obvious conclusion ,mr4 ,joule draws from these experiments is1 that heat and mechanical power are convertible into one another1 and it becomes evident1 therefore1 that heat is either the vis viva or living force of ponderable particles1 or a state of attraction and repulsion capable of generating vis viva 7,n4 #bbb74 ,,section ,,xxxiii4 ,electricity produced by ,rotation--,direction of the ,currents--,electricity from the ,rotation of a ,magnet--,m4 ,arago's ,experiment explained--,rotation of a ,plate of ,iron between the ,poles of a ,magnet--,relation of ,substances to ,magnets of three ,kinds--,thermo-,electricity4 ,m4 ,,arago discovered a source of magnetism in rotatory motion4 ,if a circular plate of copper be made to revolve immediately above or below a magnetic needle or magnet1 suspended in such a manner that it may rotate in a plane parallel to that of the copper plate1 the magnet tends to follow the circumvolution of the plate2 or1 if the magnet revolves1 the plate tends to follow its motion2 so powerful is the effect1 that magnets and plates of many pounds weight have been carried round4 ,this is quite independent of the motion of the air1 since it is the same when a pane of glass is interposed between the magnet and the copper4 ,when the magnet and the plate are at rest1 not the smallest effect1 attractive1 repulsive1 or of any kind1 can be perceived between them4 ,in describing this phenomenon1 ,m4 ,arago states that it takes place not only with metals1 but with all substances1 solids1 liquids1 and even gases1 although the intensity depends upon the kind of substance in motion4 ,experiments made by ,dr4 ,faraday explain this singular action4 ,he found that1 if a piece of metal or a metallic wire forming a circuit of any form be moved from right to left across the lines of force proceeding from the pole of a bar magnet1 these lines of force induce a current of electricity flowing in one direction2 and when the motion of the metal or wire is reversed1 the direction of the current is reversed also3 the rotation of the magnet about its axis has no effect on these results1 and no current is induced when the metal or wire is at rest4 ,a plate of copper1 twelve inches in diameter and one fifth of an inch thick1 was placed between the poles of a powerful horseshoe magnet1 consequently crossing the magnetic lines of force at right angles1 and connected at certain points with a galvanometer by copper wires4 ,when the plate was at rest no effect was produced2 but as soon as the plate was made to revolve rapidly the galvanometer needle was deflected sometimes as much as #ij1 and by a uniform rotation the deflection was constantly maintained at #de4 ,when the motion of the copper plate was reversed1 the needle was deflected in the contrary direction1 and thus a permanent current of electricity was evolved by an ordinary magnet4 ,the intensity of the electricity collected by the wires1 and conveyed by them to the galvanometer1 varied with the position of the plate relatively to the poles of the magnet4 ,the motion of the electricity in the copper plate may be conceived by considering that1 merely by moving a single wire1 like the spoke of a wheel1 before a magnetic pole1 a current of electricity tends to flow through it from one end to the other4 ,hence1 if a wheel be constructed of a great many such spokes1 and revolved near the pole of a magnet in the manner of the copper disc1 each radius or spoke will tend to have a current produced in it as it passes the pole4 ,now1 as the circular plate is nothing more than an infinite number of radii or spokes in contact1 the currents will flow in the direction of the radii if a channel be open for their return2 and1 in a continuous plate1 that channel is afforded by the lateral portions on each side of the particular radius close to the magnetic pole4 ,this hypothesis is confirmed by observation2 for the currents of positive electricity set from the centre to the circumference1 and the negative from the circumference to the centre1 and vice vers1 according to the position of the magnetic poles and the direction of rotation2 so that a collecting wire at the centre of the copper plate conveys positive electricity to the galvanometer in one case1 and negative in another2 that collected by a conducting wire in contact with the circumference of the plate is always the opposite of the electricity conveyed from the centre4 ,it is evident that1 when the plate and magnet are both at rest1 no effect takes place1 since the electric currents which cause the deflection of the galvanometer are only induced by motion across the magnetic lines of force4 ,when the plate is placed edgewise so as to be parallel to these lines of force1 no revolution of it with the most powerful magnet produces the slightest signs of a current at the galvanometer4 ,the same phenomena may be produced by electro-magnets4 ,the effects are similar when the magnet rotates and the plate remains at rest4 ,when the magnet revolves uniformly about its own axis1 electricity of the same kind is collected at its poles1 and the opposite electricity at its equator4 ,the phenomena which take place in ,m4 ,arago's experiments may be explained on this principle4 ,when both the copper plate and the magnet are revolving1 the action of the induced electric current tends continually to diminish their relative motion1 and to bring the moving bodies into a state of relative rest2 so that1 if one be made to revolve by an extraneous force1 the other will tend to revolve about it in the same direction1 and with the same velocity4 ,when a plate of iron1 or of any substance capable of being made either a temporary or permanent magnet1 revolves between the poles of a magnet1 it is found that dissimilar poles on opposite sides of the plate neutralize each other's effects1 so that no electricity is evolved2 while similar poles on each side of the revolving plate increase the quantity of electricity1 and a single pole end-on is sufficient4 ,but when copper1 and substances not sensible to ordinary magnetic impressions1 revolve1 similar poles on opposite sides of the plate neutralize each other2 dissimilar poles on each side exalt the action2 and a single pole at the edge of the revolving plate1 or end-on1 does nothing4 ,this forms a test for distinguishing the ordinary magnetic force from that produced by rotation4 ,if unlike poles1 that is1 a north and south pole1 produce more effect than one pole1 the force will be due to electric currents2 if similar poles produce more effect than one1 then the power is not electric4 ,these investigations show that there are really very few bodies magnetic in the manner of iron4 ,dr4 ,faraday therefore arranges substances in three classes1 with regard to their relation to magnets3--those affected by the magnet when at rest1 like iron1 steel1 and nickel1 which possess ordinary magnetic properties2 those affected when in motion1 in which electric currents are evolved by the inductive force of the magnet1 such as copper2 and1 lastly1 those which are perfectly indifferent to the magnet1 whether at rest or in motion4 ,it has already been observed that three bodies are requisite to form a galvanic circuit1 one of which must be fluid4 ,but1 in #ahbb1 ,professor ,seebeck1 of ,berlin1 discovered that electric currents may be produced by the partial application of heat to a circuit formed of two solid conductors4 ,for example1 when a semicircle of bismuth1 joined to a semicircle of antimony1 so as to form a ring1 is heated at one of the junctions by a lamp1 a current of electricity flows through the circuit from the antimony to the bismuth2 and such thermo-electric currents produce all the electro-magnetic effects4 ,a compass needle1 placed either within or without the circuit1 and at a small distance from it1 is deflected from its natural position1 in a direction corresponding to the way in which the electricity is flowing4 ,if such a ring be suspended so as to move easily in any direction1 it will obey the action of a magnet brought near it1 and may even be made to revolve4 ,according to the researches of ,m4 ,seebeck1 the same substance1 unequally heated1 exhibits electrical currents2 and ,m4 ,nobili observed1 that in all metals1 except zinc1 iron1 and antimony1 the electricity flows from the hot part towards that which is cold4 ,that philosopher attributes terrestrial magnetism to a difference in the action of heat on the various substances of which the crust of the earth is composed2 and1 in confirmation of his views1 he has produced electrical currents by the contact of two pieces of moist clay1 of which one was hotter than the other4 ,m4 ,becquerel constructed a thermo-electric battery of one kind of metal1 by which he has determined the relation between the heat employed and the intensity of the resulting electricity4 ,he found that1 in most metals1 the intensity of the current increases with the heat to a certain limit1 but that this law extends much farther in metals that are difficult to fuse1 and which do not rust4 ,the experiments of ,professor ,cumming show that the mutual action of a magnet and a thermo-electric current is subject to the same laws as those of magnets and galvanic currents2 consequently all the phenomena of repulsion1 attraction1 and rotation may be exhibited by a thermo-electric current4 ,m4 ,botto1 of ,turin1 has decomposed water and some solutions by thermo-electricity2 and the ,cav4 ,antinori of ,florence succeeded in obtaining a brilliant spark with the aid of an electro-dynamic coil4 ,the principle of thermo-electricity has been employed by ,,mm4 ,nobili and ,melloni for measuring extremely minute quantities of heat in their experiments on the instantaneous transmission of radiant heat4 ,the thermo-multiplier1 which they constructed for that purpose1 consists of a series of alternate bars1 or rather fine wires of bismuth and antimony1 placed side by side1 and the extremities alternately soldered together4 ,when heat is applied to one end of this apparatus1 the other remaining at its natural temperature1 currents of electricity flow through each pair of bars1 which are conveyed by wires to a delicate galvanometer1 the needle of which points out the intensity of the electricity conveyed1 and consequently that of the heat employed4 ,this instrument is so delicate that the comparative warmth of different insects has been ascertained by means of it4 ,the conservation of force is strictly maintained throughout the whole science and different forms of electricity4 ,in static electricity the positive and negative forces exactly balance one another2 they are always simultaneous1 and related often by curved lines of force2 there is no defect or surplus1 and the existence of one kind without the other is utterly impossible--it is absolutely a dual force4 ,the very same may be said of electric currents1 whether produced by the ,voltaic battery or in any other way--the current in one part of the circuit is absolutely the same in amount and dual character as the other2 and in the insulated ,voltaic battery1 where the sustaining power is internal1 not the slightest development of the forces of either of these can occur till the circuit is completed or induction allowed at the extremities2 for if when there is no circuit the induction be prevented1 not merely no current1 but no quantity of electricity at the poles ready to produce a current1 can be evolved in the slightest degree4#ag ,,section ,,xxxiv4 ,magnetism a ,dual ,power--,antithetic ,character of ,paramagnetism and ,diamagnetism--,the ,earth ,paramagnetic--,properties of ,paramagnetic ,bodies--,polarity--,induction--,lines of ,magnetic ,force--,currents of ,electricity induced by them--,proved to be ,closed ,curves--,analogy and ,identity of ,electricity and ,magnetism--,terrestrial ,magnetism--,mean ,values of the ,three ,magnetic ,elements--,their ,variations in ,double ,progression proved to consist of ,two ,superposed ,variations--,discovery of the ,periodicity of the ,magnetic ,storms--,the ,decennial ,period of the ,magnetic ,elements the same with that of the ,solar ,spots--,magnetism of the ,atmosphere--,diamagnetism--,action of ,electro-,magnetism on ,paramagnetic1 ,diamagnetic ,bodies1 and on ,copper1 very different--,proof of ,diamagnetic ,polarity and ,induction--,magnecrystallic ,action--,effects of ,compression1 ,heat1 and ,cleavage on ,magnetic ,bodies--,mutual ,dependence of ,light1 ,heat1 ,electricity1 c4 c4--,the ,conservation of ,force and the ,permanency of ,matter ,primary ,laws of ,nature--,definition of ,gravity not according to that ,law--,gravity only the ,residual ,force of a ,universal ,power--,magnetism of the ,ethereal ,medium4 ,,magnetism may be regarded as a new science in consequence of the profound researches and admirable discoveries of ,dr4 ,faraday4 ,since the magnetism of matter is only known by the action of a magnet or of electricity upon it1 by using an extremely energetic magnet or electro-magnet he has proved that all known substances1 whether solid1 liquid1 or ariform1 are more or less magnetic1 but that the magnetism is very different in different substances4 ,for example1 if a bar of iron be freely suspended between the poles of a very powerful magnet or electro-magnet1 it will be attracted by both poles1 and will set or rest in the direction of a straight line joining them2 but if a similar bar of bismuth be freely suspended in the same manner1 it will rest in a direction at right angles to that which the iron bar assumed4 ,thus the direction in which the iron sets is axial or in the line of force1 while that which the bismuth assumes is equatorial or perpendicular to the line of force4 ,substances that are magnetic after the manner of iron are said to be paramagnetic1 those that are magnetic after the manner of bismuth are diamagnetic4 ,as far as we know1 all matter comes under one or other of these laws4 ,many bodies are paramagnetic besides iron1 as the loadstone1 which consists of the peroxide and protoxide of iron mixed with small portions of silica and alumina2 also some of the gems and metals1 as cobalt1 nickel1 c4 ,a substance is often paramagnetic if it contains only the #acj1jjjth part of its weight of iron2 but by far the greater number are diamagnetic1 as all animal and vegetable matter1 acids1 oils1 sugar1 starch1 bread1 c41 and all the gases except oxygen1 which is highly paramagnetic2 and its force increases with its density3 but notwithstanding the predominance of diamagnetic matter at the surface1 the terrestrial globe is paramagnetic--in fact it is a powerful magnet4 ,besides the substances which are paramagnetic naturally1 that property may be imparted by a variety of methods1 as by friction with magnets or even juxtaposition with them2 and a bar of hard steel held at the angle of the dip will become a magnet on receiving a few strokes with a hammer on its upper end4 ,polarity is one of the most distinguishing characters of magnetism3 it is the property which a magnet possesses when freely suspended of resting spontaneously in the magnetic meridian1 or nearly north and south1 and always returning to that position when disturbed in consequence of the mean magnetic attraction of the earth2 yet the magnet has no tendency to move to the north or south even when floating on water1 because the same pole that attracts one end repels the other4 ,both poles of a magnet attract iron1 which in return attracts either pole of the magnet with an equal and contrary force4 ,the action of a magnet on unmagnetised iron is confined to attraction1 whereas the reciprocal agency of magnets is characterised by a repulsive as well as by an attractive force2 for a north pole repels a north pole1 and a south pole repels a south pole2 but a north and south pole mutually attract one another--which proves that paramagnetism is a dual power in which the conservation of force is perfectly maintained1 for the force of attraction is exactly equal to the force of repulsion4 ,one kind of polarity cannot exist without the other3 they are absolutely simultaneous1 dependent1 and of equal intensity4 ,induction is the power which a magnet possesses of exciting temporary or permanent paramagnetism in such bodies in its vicinity as are capable of receiving it4 ,by this property the mere approach of a magnet renders iron and steel paramagnetic1 the more powerfully the less the distance1 but the induced force is always exactly equal to the force which produces it4 ,when the north end of a magnet is brought near to1 and in the line with1 an unmagnetised iron bar1 the bar acquires all the properties of a perfect magnet2 the end next the north pole of the magnet becomes a south pole1 while the remote end becomes a north pole4 ,exactly the reverse takes place when the south end is presented to the bar1 so that each pole of a magnet induces the opposite polarity in the adjacent end of the bar1 and the same polarity in the remote extremity2 consequently the nearest extremity of the bar is attracted1 and the farther repelled2 but as the action is greater on the adjacent than on the distant part1 the resulting force is that of attraction4 ,by induction the iron bar not only acquires polarity1 but the power of inducing paramagnetism in a third body2 and although all these properties vanish from the iron as soon as the magnet is removed1 a lasting increase of intensity is generally imparted to the magnet itself by the reaction of the temporary paramagnetism of the iron4 ,iron acquires the inductive force more rapidly than steel1 yet it loses it as quickly on the removal of the magnet1 whereas the steel is impressed with a lasting polarity4 ,a certain time is requisite for induction1 and it may be accelerated by anything that excites a vibratory motion in the particles of the steel2 such as the smart stroke of a hammer1 or heat succeeded by sudden cold4 ,a steel bar may be converted into a magnet by the transmission of an electric discharge through it2 and as its efficacy is the same in whatever direction the electricity passes1 the effect arises from its mechanical operation exciting a vibration among the particles of the steel4 ,it has been observed that the particles of iron easily resume their neutral state after induction1 while those of steel resist the restoration of equilibrium1 or a return to the neutral state3 it is therefore evident that any cause which removes or diminishes the resistance of the particles will tend to destroy the paramagnetism of the steel2 consequently the same mechanical means which develop the power will also destroy it4 ,on that account a steel bar may lose its paramagnetism by any mechanical concussion1 such as by falling on a hard substance1 a blow with a hammer1 and heating to redness1 which makes the steel soft4 ,the circumstances which determine whether it shall gain or lose are its position with respect to the magnetic equator1 and the higher or lower intensity of its previous magnetic state4 ,a comparison of the number of vibrations accomplished by the same magnetised needle during the same time at different distances from a magnet gives the law of paramagnetic intensity1 which follows the inverse ratio of the square of the distance--a law that is not affected by the intervention of any substance whatever between the magnet and the needle1 provided the substance be not itself susceptible of magnetism4 ,induction and the reciprocal action of magnets are therefore subject to the laws of mechanics2 but the composition and resolution of the forces are complicated in consequence of four forces being constantly in activity1 two in each magnet4 ,mr4 ,were ,fox discovered that the law of the paramagnetic force changes from the inverse square of the distance to the simple inverse ratio when the distance between two magnets is as small as from the fourth to the eighth of an inch1 or even as much as half an inch when the magnets are large2 and in the case of repulsion1 that the change takes place at a still greater distance1 especially when the two magnets differ materially in intensity4 ,without assuming any hypothesis of what magnetism is1 or how that force is originated or sustained1 ,dr4 ,faraday regards a magnet as a source of power surrounded by curved lines of force which are not only representants of the magnetic power in quality and direction1 but also in quantity--an hypothesis which accords perfectly with experiment1 and with the action both of electricity and magnetism4 ,the nature and form of these lines may be seen by placing a bar magnet upon a table1 spreading a sheet of stiff paper over it so as to be perfectly level and free from creases1 and then sifting very clean iron filings through a fine sieve equably over it4 ,the filings will instantly assume the form of the curved lines represented by fig4 #a1 plate #g1 in consequence of the action of the magnet4 ,these lines are the true representatives of the magnetic forces1 and being related to a polar power1 they have opposite qualities in opposite directions4 ,when a magnet is broken across the middle1 each part is at once converted into a perfect magnet2 the part that originally had a south pole acquires a north pole at the fractured end2 the part that had originally a north pole gets a south pole2 and as far as mechanical division can be carried1 it is found that each fragment is a perfect magnet4 ,fig4 #b1 plate #g1 shows the lines of force in a fractured magnet when the ends are not yet separated2 fig4 #c shows them when they are4 ,currents of electricity are produced in conducting bodies moved across these lines of magnetic force4 ,if a copper wire at a little distance above the north pole of a bar magnet be moved from left to right1 at any angle across the lines of magnetic force1 they will induce a current of electricity in the wire flowing from right to left2 if the wire be moved with the same velocity in the contrary direction1 the induced current will be of equal intensity1 but it will flow from left to right4 ,similar results are obtained from the south pole1 and the phenomena are the same when the magnet is moved and the wire is at rest2 in both cases the intensity is greater the swifter the motion4 ,it appears that the quantity of electricity induced is directly as the amount of the magnetic curves intersected1 and when a wire is moving uniformly in a field of equal magnetic force1 the current of electricity generated is proportional to the time1 and also to the velocity of motion2 for when a metallic disc is made to revolve through the lines of force1 the current induced is strongest near the edge where the velocity is greatest2 and in different substances moving across the lines of force the intensity of the induced current is directly as the conducting power of the substance4 ,thus bodies moved near a magnet have an electrical current developed in them1 and conversely bodies affected by an electric current are definitely moved by a magnet near them4 ,by the preceding experiments it appears that magnetic polarity is manifested in two ways2 in the magnetised needle1 by attraction and repulsion1 and in a wire moving across lines of magnetic force it is shown by the opposite directions in which the induced current flows according as the body is moved from the right to the left1 or left to right4 ,hence polarity consists in the opposite and antithetical actions manifested at the opposite ends or opposite sides of a limited or unlimited line of force4 ,antithesis is the true and most general character of magnetism1 whatever may be its mode of action4 ,it was by the induction of electric currents in copper wires moving across the lines of magnetic force that ,dr4 ,faraday proved that the lines of force issuing from a magnet are closed curves which return again and pass through the interior of the magnet4 ,he placed two bar magnets of the same length1 size1 and intensity with their similar poles together1 so that they might act as one magnet4 ,a copper wire was then passed between their axes1 which after extending through half their length was bent up equatorially and turned back along the outside1 so that the whole wire formed a loop1 the two ends being connected with a galvanometer4 ,when the whole wire was made to revolve1 no effect was produced1 although it crossed the lines of magnetic force2 but when it was cut in two1 so as to separate the external from the internal part1 electrical currents of equal intensity1 but in contrary directions1 were induced in each portion of the wire as they were made separately to cross the lines of force1 for the apparatus was so constructed that that could be done4 ,the exterior wire crossed the lines of force which issued from the magnets at right angles to their axes1 while the equatorial part of the interior wire traversed the returning lines of force4 ,it is evident that these forces neutralized each other when the whole wire revolved3 consequently the internal and external lines of force must have been of equal intensity and opposite in direction1 so as to balance one another4 ,by this and a very great number of other experiments ,dr4 ,faraday has proved that the magnetic lines of force are continuous closed curves alike in shape1 size1 and power4 ,they extend indefinitely beyond the magnet1 and undergo no change by distance4 ,thus the magnetic force pervades the interior of the mass2 if electricity does the same1 a compensation must either take place1 or it also must move in lines of force1 sensible only at the surface4 ,electricity has a perpetual tendency to escape1 and does escape1 when not prevented by the coercive power of the air1 and other non-conducting substances4 ,such a tendency does not exist in magnetism1 which never leaves the substance containing it under any circumstances whatever4 ,there must be some coercive force1 analogous to friction1 which arrests the magnetic forces1 so as first to oppose their separation1 and then to prevent their reunion4 ,in soft iron the coercive force is either wanting or extremely feeble1 since iron is easily rendered paramagnetic by induction1 and as easily loses that quality2 whereas in steel the coercive force is extremely energetic1 because it prevents the steel from acquiring the paramagnetic properties rapidly1 and entirely hinders it from losing them when acquired4 ,the feebleness of the coercive force in iron1 and its energy in steel1 with regard to the paramagnetic force1 is perfectly analogous to the facility of transmission afforded to electricity by non-electrics1 and the resistance it experiences in electrics4 ,at every step the analogy between electricity and magnetism becomes more striking4 ,the agency of attraction and repulsion is common to both2 the positive and negative electricities are similar to the northern and southern polarities1 and are governed by the same laws--namely1 that between like powers there is repulsion1 and between unlike powers there is attraction4 ,each of these four forces is capable of acting most energetically when alone2 but as the electric equilibrium is restored by the union of the two electric states1 and magnetic neutrality by the combination of the two polarities1 they respectively neutralise each other when joined4 ,all these forces vary inversely as the square of the distance1 and consequently come under the same mechanical laws4 ,a like analogy extends to magnetic and electric induction4 ,iron and steel are in a state of equilibrium when neutral2 but this equilibrium is immediately disturbed on the approach of the pole of a magnet1 which by induction transfers one kind of polarity to one end of an iron or steel bar1 and the opposite kind to the other--effects exactly similar to electrical induction4 ,there is even a correspondence between the fracture of a magnet and that of an electric conductor2 for if an oblong conductor be electrified by induction1 its two extremities will have opposite electricities2 and if in that state it be divided across the middle1 the two portions1 when removed to a distance from one another1 will each retain the electricity that has been induced upon it4 ,the analogy1 however1 does not extend to transference4 ,a body may transfer a redundant quantity of positive electricity to another1 or deprive another of its electricity--the one gaining at the expense of the other2 but a body cannot possess only one kind of polarity4 ,with that exception1 there is such perfect correspondence between the theories of magnetic attractions and repulsions1 and electric forces in conducting bodies1 that they not only are the same in principle1 but are determined by the same formul4 ,experiment concurs with theory in proving the identity of these two influences4 ,hence1 if the electrical phenomena be due to a modification of the ethereal medium1 the magnetic phenomena must be owing to an analogous cause4 ,curved lines of magnetic force issue from every point of the earth's surface where there is sensible dip1 and bending round enter the earth again at the magnetic equator4 ,they induce electric currents in conducting-wires1 moving across them exactly the same as in artificial magnets2 and when a hollow helix1 or coil of copper wire1 whose extremities are connected with a galvanometer1 is placed in the magnetic dip1 and suddenly moved across the lines of force1 the needle of the galvanometer will vibrate through an arc of #hj or #ij1 in consequence of the electric current induced by these lines of magnetic force in the wire1 and the action is greater when a core of soft iron is placed in the helix1 which becomes a temporary magnet by induction4 ,again1 if a copper plate be connected with a galvanometer by two copper wires1 one from the centre1 and another from the circumference1 in order to collect and convey the electricity1 it is found that1 when the plate is made to revolve in a plane passing through the line of the dip1 the galvanometer is not affected4 ,but as soon as the plate is inclined to that plane1 electricity begins to be developed by its motion across the lines of magnetic force2 it becomes more powerful as the inclination increases1 and arrives at a maximum when the plate revolves at right angles to the line of dip4 ,when the revolution is in the same direction with that of the hands of a watch1 the current of electricity flows from its centre to the circumference2 and when the rotation is in a contrary direction1 the current sets the opposite way4 ,thus a copper plate1 revolving at right angles to the line of the dip1 becomes a new electrical machine1 differing from the common plate-glass machine by the copper being the most perfect conductor1 whereas glass is the most perfect non-conductor2 besides insulation1 which is essential to the glass machine1 is fatal to the copper one4 ,the quantity of electricity evolved by the metal does not appear to be inferior to that devolved by the glass1 though very different in intensity4 ,even a ship crossing the lines of force must have electric currents running through her4 ,dr4 ,faraday observes that such is the facility with which electricity is generated by the magnetic lines of force1 that scarcely any piece of metal can be moved without a development of it2 consequently1 among the arrangements of steam-engines and metallic machinery1 curious electro-magnetic combinations probably exist which have never yet been noticed4 ,thus magnetic lines of force certainly issue from the surface of the globe4 ,no doubt the earth is a magnet on a vast scale1 but it differs from all others in having four poles of maximum magnetic force of different intensities1 the two in the northern hemisphere having a secular motion in a contrary direction from the two in the southern4 ,they are not even symmetrically placed2 hence the magnetic intensity varies so much in the different points on the earth's surface1 that the dynamic equator1 or line passing through all the points of least intensity1 is a very irregular curve surrounding the globe1 but by no means coinciding with the terrestrial equator4 ,in consequence of the mean action of these four forces1 the north end of a magnetised needle1 arranged so as to revolve in a vertical plane1 dips or inclines beneath the horizon in the northern hemisphere1 and the south end in the southern4 ,the two hemispheres are separated by a line encircling the earth1 called the magnetic equator1 or line of no dip1 in which the dipping or inclination needle is horizontal4 ,on each side of this line the inclination increases till at last the needle becomes perpendicular to the horizon in two points1 or rather small spaces1 in each hemisphere1 known as the magnetic poles1 which are quite different from the poles of the earth's rotation4 ,the mean action of the four poles of magnetic intensity causes the mariner's compass1 or a magnetic needle suspended so as to revolve in a horizontal plane1 to remain at rest when pointing to the two magnetic poles4 ,it is then in the magnetic meridian of the place of observation1 which is thus determined by the mean action of all the four magnetic forces4 ,these mean values of the three magnetic elements1 namely1 the declination1 inclination or dip1 and magnetic intensity1 are well known to be subject to secular1 annual1 and diurnal variations4 ,the secular only become sensible after some years1 but the annual and diurnal variations have a double progression--that is to say1 two maximum and two minimum values in their respective periods of a year and twenty-four hours2 for example1 the declination needle makes two deviations to the west and two to the east in the course of twenty-four hours1 and that with great regularity4 ,now ,general ,sabine discovered that the double progression arises from two combined or superposed variations having different hours of maxima and minima1 and that they are due to two distinctly different causes--the one being the difference in the sun's position relatively to the place of observation at the different seasons of the year1 and hours of the day and night2 the other being a mean annual and diurnal variation proved by ,general ,sabine to exist in those great magnetic storms or casual disturbances which affect the magnetic elements simultaneously over enormously extensive tracts of the globe4 ,moreover the ,general discovered that1 besides these annual and diurnal variations1 the magnetic storms have a variation which accomplishes its vicissitudes in ten or more nearly eleven years1 the increase from year to year being gradual1 till its maximum becomes twice as great as its minimum value4 ,in consequence of this inequality in the storms or casual disturbances1 each of the magnetic elements has a variation of similar period and similar maxima and minima4 ,now the number and magnitude of the spots on the sun had been observed by ,m4 ,schwabe1 of ,dessau1 to increase to a maximum1 and decrease again to a minimum1 regularly in the very same period of between ten and eleven years2 and ,general ,sabine found that this variation in the solar spots1 and that in the magnetic elements1 not only have the same periods of maxima and minima1 but that they correspond in all their minutest vicissitudes4 ,thus a very remarkable and unexpected connexion exists between terrestrial and solar magnetism4 ,the dual and antagonist principle is perfectly maintained in the earth's magnetism1 all the phenomena and their variations being in opposite directions in the two hemispheres4 7,n4 #bbf47 ,no doubt the magnetic lines of force in the earth are closed curves1 as in artificial magnets2 but in their circuitous courses they may extend to any distance in space1 or rather in the ethereal medium1 even to thousands or tens of thousands of miles2 for the ethereal medium is permeable to lines of magnetic force1 or rather transmits them1 otherwise the solar spots could not affect the variations of terrestrial magnetism2 besides1 they pass through the ,torricellian vacuum1 which is nearly a void with respect to air1 but not to the ethereal medium4 ,the atmosphere which surrounds the earth to the height of about fifty miles with sensible density1 consists of three and a half parts by weight of nitrogen gas and one part of oxygen1 uniformly mixed4 ,the nitrogen is neutral whether dense or rare1 hot or cold1 while the oxygen is highly paramagnetic2 but it loses a great part of its force when rarefied by heat2 consequently the magnetic force of the atmosphere must increase from the equator to the poles of maximum cold2 it must vary summer and winter1 night and day4 ,its effect upon terrestrial magnetism is unknown2 but it can hardly be without some influence4 ,m4 ,e4 ,becquerel observes--8,if we reflect that the earth is encompassed by a mass of air equivalent in weight to a layer of mercury of #cj inches1 we may inquire whether such a mass of magnetic gas1 continually agitated1 and submitted to the regular and irregular variations of pressure and temperature1 does not intervene in some of the phenomena dependent upon terrestrial magnetism4 ,if we calculate1 in fact1 what is the magnetic force of this fluid mass1 we find that it is equivalent to an immense plate of iron1 of a thickness little more than #a_/bej of an inch1 which covers the whole surface of the globe40 ,both the conducting power of the air and its density are increased by cold2 and as the sum of the magnetic forces which issue from the earth on one side of the line of no dip is equal to their sum on the other side1 the intensity and concentration in our winter are coincident with a diffusion and feebleness in the opposite hemisphere1 so that the line of no dip will move annually from north to south and back again4 ,the same holds with regard to day and night4 ,thus the law of the conservation of force is rigorously maintained2 and it is equally so in the effect of the atmosphere on the magnetic lines of force1 which refracts them as they pass through it1 in one direction in summer1 and in the opposite direction in winter--in one direction in the enlightened hemisphere1 in the other in that which is dark4 ,the whole of the magnetic lines about the earth are held by their mutual tension in one connected1 sensitive system1 which feels in every part1 even to the antipodes1 a change in any particular place4 ,it may be mentioned as a well-known fact1 that apparent anomalies have been found in the diurnal variation of the declination in the high magnetic latitudes of the northern hemisphere when compared with their great regularity in other parts of the same hemisphere1 and that the magnetic storms are of much greater magnitude there than in lower latitudes4 ,moreover1 although ,captain ,maguire's observations at ,cape ,barrow1 in the ,north ,polar ,ocean1 show that the annual and diurnal variations of the casual disturbances or magnetic storms1 as well as those of the decennial period1 are maintained1 yet it appears that at certain hours of the day the disturbance in the declination may be easterly at ,point ,barrow1 and westerly at the ,magnetic ,observatory at ,toronto1 in ,upper ,canada1 and vice vers3 in fact1 the magnetic storms are simultaneous at these two stations1 but in opposite directions--a circumstance not yet accounted for1 and may possibly be due to the increased magnetism of the air in these cold regions4 ,the heat of the sun has no effect upon terrestrial magnetism unless possibly by its indirect action on the oxygen of the atmosphere2 but hitherto it has been imperceptible4 ,it is hardly possible that the aurora can be independent of the magnetic character of the air1 since it occurs in the high latitudes1 where the atmospheric magnetism is most powerful4 ,captain ,maguire remarked that it frequently appeared at ,point ,barrow when the magnetic storms were at a maximum4 ,we are totally ignorant of the cause of terrestrial magnetism1 though the powerful influence of the solar spots renders it highly probable that it will ultimately be found to originate in the sun himself4 ,mr4 ,barlow's theory of electric currents revolving round the globe is borne out by ,mr4 ,fox's observations in the ,cornish mines1 which show that electro-magnetism is extremely active in metallic veins2 that not only the nature of the metalliferous deposits must have been determined by their relative electrical conditions1 but that the direction of the metallic veins must have been influenced by the direction of the magnetic meridians1 and in fact almost all the metallic deposits in the world tend from east to west1 or from north-east to south-west4 ,however1 these currents of electricity may be regarded as magnetic lines of force1 and are more likely to be the effect than the cause of terrestrial magnetism4 ,they are found to have a powerful inductive effect on the ,atlantic telegraph1 disturbing the needles and galvanometers at each end of the line to a considerable degree1 and on the night of the #fth of ,september1 #aheh1 a magnetic storm passed over the cable1 which violently agitated the reflecting galvanometer in connection with the telegraphic wires4 ,we are equally ignorant of the cause of the secular magnetic variations1 but we have no reason to believe that the earth is alone magnetic2 on the contrary1 the planets are probably magnets1 and we know that the sun and moon are magnetic2 hence1 as the magnetic1 like the gravitating force1 is transmitted through the ethereal medium1 the induction of the sun1 moon1 and planets1 in all their secular and periodic changes1 may cause perpetual variations in terrestrial magnetism1 and it may not be beyond the delicacy of modern observation to ascertain whether a planet1 when nearest to the earth1 has any sensible magnetism4 ,diamagnetism is also a dual power1 but in complete antithesis to paramagnetism under the same circumstances4 ,dr4 ,faraday first discovered this property in heavy glass1 or silico-borate of lead1 a piece of which was repelled by the pole of a powerful electro-magnet1 and an elongated prism of the same heavy glass1 when freely suspended between the poles1 set equatorially4 ,he then found that so great a number of substances followed the same law1 that it established the very remarkable fact of a hitherto unknown force having acted upon the substances submitted to its influence1 a discovery which he subsequently confirmed by many experiments1 all of which proved the antithesis between the two modes of magnetic action4 ,he also discovered that magnetic bodies differ exceedingly in their magnetic power3 of paramagnetic bodies iron is the most powerful2 then follow nickel1 cobalt1 and a long gradation down to osmium and a vacuum4 ,the body that seems to have the lowest diamagnetic power is arsenic1 and the series ascends to heavy glass1 antimony1 phosphorus1 and bismuth2 so iron and bismuth are the most powerful in their respective classes1 and both have a small conducting power for electricity4 ,it may be presumed that many remarkable instances of diamagnetism are to be met with in nature2 among others1 ,dr4 ,faraday has suggested the idea that ,saturn's ring1 from its position1 may be diamagnetic with regard to the planet4 ,with very powerful magnets or electro-magnets1 which are absolutely necessary for all these experiments1 it is found that no simple substance is neutral1 but that such may be compounded by mixing in due proportion a diamagnetic and paramagnetic liquid1 as water and protosulphate of iron4 ,professor ,tyndall proved diamagnetic polarity by placing two bismuth bars within two vertical coils or spirals of insulated copper wire1 through which electric currents were transmitted from a galvanic battery1 and caused to act upon a steel magnet freely suspended without the spirals4 ,now1 when the excited magnetism is merely by induction1 the electric current1 being momentary1 only causes a shock or momentary deviation in the magnet1 which returns to its original position when the current ceases4 ,when1 on the contrary1 the magnetism is permanent1 the suspended magnet does not return to its original position when the current ceases4 ,in ,professor ,tyndall's experiment the deviation was permanent1 and it was equally so when a bismuth bar was freely suspended and the cores within the spirals were steel magnets4 ,had the effect been from currents induced in the mass of the bar of bismuth1 division of the bar would have stopped them1 but the result was the same with powdered bismuth as with the solid mass4 ,moreover1 since the strength of induced currents depends upon the conducting power of the substance1 and as the conducting power of copper is forty times as great as that of bismuth1 had the polarity been induced and not real1 the effect ought to have been forty times greater when copper instead of bismuth cores were put in the spirals1 whereas it was scarcely sensible4 ,besides these proofs1 ,dr4 ,tyndall made experiments with eleven different diamagnetic substances1 of which water was one1 with similar results4 ,he then determined the polarity of twelve paramagnetic bodies by the same method1 whence it appeared that the same action which produced a north pole in the paramagnetic bodies produced a south pole in those that were diamagnetic1 and vice vers1 whence he concludes that diamagnetic polarity is one of the most firmly established truths of science4 ,it follows from this that1 when a man is standing1 his head is a north pole and his feet a south1 and the top of an iron railing on which he may be leaning is a south pole and the lower end a north4 ,diamagnetic bodies thus possess a polarity1 the same in kind but opposite in direction to that possessed by paramagnetic ones4#ah ,they are both dual powers1 and the two diamagnetic forces like the two paramagnetic being coexistent1 simultaneous1 and mutually dependent1 there can be no doubt that the diamagnetic forces also are represented1 or rather consist of curved and closed lines of force passing through the interior of the substance4 ,dr4 ,tyndall has proved that the attraction of iron1 and the repulsion of bismuth1 are as the square of the electro-magnetic current producing them1 and that diamagnetic substances are capable of induction4 ,the molecular structure of substances freely suspended between the poles of a magnet has a decided effect upon the position they assume4 ,it has already been mentioned that the optic axis is a symmetrical line in a doubly refracting crystal in which there is no double refraction1 and that in some crystals there are two such symmetrical lines4 ,now1 ,professor ,plcker of ,bonn discovered1 when such crystals are submitted to powerful magnetic influence1 that the single optic axis in the one1 and the resultant or mean line between the double optic axes in the other1 set diametrically or at right angles to the line of magnetic force2 and so powerful did the ,professor find the action of magnetism on crystalline form1 that the mineral cyanite1 when suspended1 arranges itself so definitely with regard to terrestrial magnetism1 that it might be used as a compass needle4 ,dr4 ,faraday afterwards observed that amorphous substances1 cut in the form of a sphere1 have no tendency to set or be attracted or repelled in one direction in preference to any other2 but if the sphere be formed of a crystallized substance1 it is a general fact that1 whether it be paramagnetic or diamagnetic1 it is more powerfully attracted or repelled in one direction than in any other--a property named by ,dr4 ,faraday magnecrystallic action4 ,for example1 a sphere of calcareous spar1 which is a diamagnetic crystal1 is most strongly repelled in the direction of its principal optic axis1 and least strongly in the direction of its least axis4 ,in a sphere of carbonate of iron1 which has exactly the same crystalline form and is highly paramagnetic1 the line which in carbonate of lime sets equatorially1 in this case sets axially1 and more strongly in that direction than in any other4 ,the law according to which the attraction of the carbonate of iron increases from the least to its greatest or principal optic axis1 is precisely the same as that according to which the repulsion of the calcareous spar increases from the least to the principal optic axis4 ,these relations are not altered by the immersion of the spheres in liquids of either magnetism4 ,dr4 ,faraday observed that a line at right angles to the planes of principal cleavage in crystals takes the axial position1 and on that account he called it the magnecrystallic axis4 ,its position was proved by ,,mm4 ,tyndall and ,knoblauch to depend upon the general fact1 that the mass is most strongly repelled in the direction of the planes of principal cleavage1 and that the elective position of crystals depends more upon the direction of these planes with respect to the electric force1 than upon the optic axis4 ,the planes of principal cleavage set themselves equatorially in diamagnetic1 and axially in paramagnetic substances3 it was thence inferred that the phenomena offered by crystals in the magnetic field is a particular case of the general law1 that the superior action of magnets upon matter in a particular direction is due to the particles of the body being closer together in that direction than in any other3 in short1 the line of maximum density2 the force exerted being attractive or repulsive according as the particles are paramagnetic or diamagnetic4 ,it appears1 however1 that the set of crystals with regard to the line of magnetic force does not depend solely upon their density in particular directions4 ,professor ,matteucci1 of ,pisa1 has proved that the diamagnetic force is inversely as the conducting power of substances for electricity1 that the conducting power is a maximum in the planes of principal cleavage1 and that a needle of crystallized bismuth1 in which the planes of cleavage are parallel to its length1 places itself equatorially with more force when these planes are vertical1 or at right angles to the force1 than when they are horizontal or parallel to it4 ,experiments had hitherto been made only with diamagnetic or slightly paramagnetic bodies1 which induced ,m4 le ,roux to try the effect of magnetism on pulverized iron compressed by the hydraulic press1 which reduced the grains of iron to lamell equivalent to planes of cleavage4 ,cubes of this substance1 suspended by a thread over a horseshoe magnet1 oscillated for a longer time when the lamell were perpendicular than when they were horizontal2 that is1 the force was stronger when the lamell were equatorial than when they were axial1 exactly the same result as in ,professor ,matteucci's experiment with the needle of bismuth4 ,thus the vertical position of the cleavages1 which increases the diamagnetism of the bismuth1 increases also the paramagnetism of the iron4 ,m4 le ,roux observes that these results are independent of the influence of the currents of electricity induced in the oscillating body1 for the fundamental character of the phenomena of ,arago's discovery of rotation by induction is1 that the oscillations diminish rapidly in extent without any sensible diminution in their duration1 while in his experiments the time of the oscillations varied4 ,he concludes that the arrangement of the molecules must be intimately connected with paramagnetism or diamagnetism itself1 since the effect of that arrangement is equally sensible in bismuth and iron1 although the diamagnetism of the former is #be1jjj times weaker than the paramagnetism of the latter4 ,the diamagnetism of conducting substances and metals1 such as gold1 silver1 and copper1 is augmented by division4 ,compression has also a great effect on magnetic action4 ,for example1 a bar of soft iron sets with its longest dimensions from pole to pole of a magnet1 but a bar of compressed carbonate of iron-dust1 whose shortest dimensions coincide with the line of pressure1 sets equatorially4 ,a bar of bismuth whose plane of principal cleavage is parallel to its length sets equatorially1 but a bar of compressed bismuth dust1 whose shortest dimensions coincide with the line of pressure1 or a bar of bismuth whose principal planes of cleavage are transverse to its length1 sets with its length axially4 ,the antithesis is perfect whether the bars are under the influence of a magnet or electro-magnet4 ,for since the diamagnetic force is inversely as the conducting power of a body for electricity1 and that the latter is a maximum in the direction of the planes of principal cleavage1 therefore when these planes are parallel to the axis of the bismuth bar it sets equatorially2 but as the conducting power is augmented when the bismuth dust is compressed in the direction of the force1 the diamagnetic power is diminished1 and the bar sets axially4 ,again1 since the paramagnetic force augments with the conducting power1 the action of the magnet on the iron is antithetic to that on the bismuth4 ,the action of an electro-magnet on copper is strongly contrasted with that which it exerts on iron or bismuth4 ,for when a copper bar suspended by a thread revolves before its pole1 it is brought to a dead halt as soon as the electric current acts upon it1 and maintains its position with considerable tenacity1 for it does not return when pushed out of it1 but keeps its new place with stiffness2 however1 as soon as the electric current ceases1 there is a strong revulsion1 the bar revolving the contrary way4 ,even when swinging with considerable force it may be caught and retained in any position at pleasure1 but there is no revulsion when it is arrested either in the axial or equatorial position2 at any angle between these two1 but especially midway1 the electricity will make it move towards the axis1 but it is arrested before it comes to it4 ,the action depends much on the form and dimensions of the bar and the magnetic pole1 which ought to be flat4 ,the phenomena are due to the high electro-conducting power of the copper1 and are met with in some of the other pure metals1 though in a far inferior degree4 ,great magnetic power is requisite for all these experiments4 ,dr4 ,faraday employed a magnet that could sustain a weight of #dej lbs4 at each pole1 and the poles were either pointed or flat surfaces at pleasure1 as the kind of experiment required4 ,heat strongly affects the magnetic properties of bodies4 ,dr4 ,faraday found that1 when the temperature of nickel is increased1 its magnetic force diminishes2 when that of iron is increased its magnetic force remains the same1 while that of cobalt increases2 which seems to indicate that there is a temperature at which the magnetic force is a maximum1 above and below which it diminishes4 ,nickel loses its magnetism at the temperature of boiling oil1 iron at a red heat1 and cobalt near the temperature at which copper melts4 ,calcareous spar retains its magnetic character at a very high temperature2 but the same substance when it contains iron1 and also oxide of iron1 loses it entirely at a dull red heat4 ,a crystal of the ferrocarbonate of lime was absolutely reversed by change of temperature1 for at a low heat the optic axis pointed axially1 and at a high temperature equatorially4 ,with the exception of these substances1 magnecrystals1 whether paramagnetic or diamagnetic1 are generally all affected alike by heat4 ,the difference between the forces in any two different directions1 as for instance the greatest and least principal axes1 diminishes as the temperature is raised1 increases as the temperature is lowered1 and is constant for a given temperature4 ,no unmixed or pure substance has as yet passed by heat from the paramagnetic to the diamagnetic state4 ,no simple magnecrystal has shown any inversion of this kind1 nor have any of the chief axes of power changed their characters or relations to one another4 ,it appears that1 as the molecules of crystals and compressed bodies affect magnetism1 so magnetism acts upon the molecules of matter1 for torsion diminishes the magnetic force1 and the elasticity of iron and steel is altered by magnetism4 ,m4 ,matteucci has found that the mechanical compression of glass alters the rotatory power of a polarized ray of light transmitted through it1 and that a change takes place in the temper of glass under the influence of powerful magnetism4 ,even from the limited view of the powers of nature which precedes1 it is evident that the progress of science based upon experiment tends to show that the various forces of light1 heat1 motion1 chemical affinity1 electricity1 and magnetism will ultimately be traced to one common origin2 that they are so directly related1 and mutually dependent1 that they are convertible1 motion producing heat1 and heat motion2 chemical affinity producing electricity1 and electricity chemical action1 c41 each mediately or immediately producing the other4 ,these forces are transmitted through substances2 they act upon matter1 causing changes in the molecular structure of bodies either momentary or permanent1 and reciprocally the changes indicate the action of these forces4 ,matter and force are only known to us as manifestations of ,almighty power3 we are assured that we can neither create nor destroy them--that their amount is the same now as in the beginning4 ,in chemical attraction the powers with which a molecule of matter is endowed1 and which give rise to various qualities1 never change2 even when passing through a thousand combinations1 the molecule and its power are ever the same4 ,machinery does not create force2 it only enables us to turn the forces of nature to the best advantage2 it is by the force of wind or falling water that our corn is ground1 and the steam engine owes its power to the force of heat and chemical action4 ,as force cannot be created1 neither can it be annihilated4 ,it may be dispersed in various directions1 and subdivided so as to become evanescent to our perceptions2 it may be balanced so as to be in abeyance1 or become potential as in static electricity2 but the instant the impediment is removed the force is manifested by motion2 it may also be turned into heat by friction1 but it is never lost4 ,every motion we make1 every breath1 every word we utter1 is a force that produces pulsations which are communicated to continually increasing particles of air1 and conveyed through countless channels so as to become indeed imperceptible to our senses1 yet they are demonstrated to exist as witnesses of the words we have spoken or the actions we have performed1 by analysis1 that all-powerful instrument of human reason4#ai ,a body acquires heat in the exact proportion that the adjacent substances become cold1 and when heat is absorbed by a body it becomes an expansive force at the expense of those around that contract1 but it is not lost4 ,in chemical action at a distance the principle of the conservation of force is maintained1 for a chemical action may be produced miles away from an electro-magnet1 perfectly equivalent to the dominant chemical action in the battery4 ,the two electricities are developed in equal proportions1 which may be combined so as to produce many changes in their respective relations1 yet the sum of the force of one kind can never be made in the smallest degree either to exceed or to come short of the sum of the other4 ,experimental research proves that the conservation of force is an unalterable law of nature--8a principle in physics as large and sure as that of the indestructibility of matter or the invariability of gravity4 ,no hypothesis should be admitted1 nor any assertion of a fact credited1 that denies this principle4 ,no view should be inconsistent or incompatible with it4 ,many of our hypotheses in the present state of science may not comprehend it1 and may be unable to suggest its consequences1 but none should oppose or contradict it40 ,having thus expressed his conviction of the truth of this great principle1 ,dr4 ,faraday considers the case of gravity1 and concludes that 8the definition of gravity as an attractive force between the particles of matter varying inversely as the square of the distance1 while it stands as a full definition of the power1 is inconsistent with the principle of the conservation of force40 ,for while in this definition the principle is maintained of the constancy of the force at the same distance1 it implies a creation of force to an enormous amount when the distance is diminished1 and an equal amount annihilated when the distance is increased1--8an effect10 he says1 8which is equal in its infinity and its consequences with creation1 and only within the power of ,him who creates40 ,he continues1 8,it will not be imagined for a moment that ,i am opposed to what may be called the law of gravitating action1 that is1 the law by which all the known effects of gravity are governed2 what ,i am considering is the definition of the force of gravitation4 ,that the result of one exercise of a power may be inversely as the square of the distance1 ,i believe and admit2 and ,i know that it is so in the case of gravity1 and has been verified to an extent that could hardly have been within the conception of ,newton himself when he gave utterance to the law2 but that the totality of a force can be employed according to that law ,i do not believe either in relation to gravitation1 or electricity1 or magnetism1 or any other supposed form of power4 ,that there should be a power of gravitation existing by itself1 having no relation to the other natural powers1 and no respect to the law of the conservation of force1 is as little likely as that there should be a principle of levity as well as gravity4 ,gravity may be only the residual part of the other forces of nature1 as ,mossotti has tried to show2 but that it should fall out from the law of all other forces1 and should be outside the reach either of farther experiment or philosophical conclusions1 is not probable4 ,so we must strive to learn more of this outstanding power1 and endeavour to avoid any definition of it which is incompatible with the principles of force generally1 for all the phenomena of nature lead us to believe that the great and governing law is one4 ,thus gravitation can only be considered as part of a more general force whose law has yet to be discovered40 ,the definition of the gravitating force immediately suggests the question of how it is transmitted2 the full force of that question was felt by ,newton himself when1 in his third letter to ,bentley1 he wrote1 8,that gravity should be innate1 inherent1 and essential to matter1 so that one body may act upon another at a distance1 through a vacuum1 without the mediation of anything else by and through which their action and force may be conveyed from one to another1 is to me so great an absurdity that ,i believe no man who has in philosophic matters a competent faculty of thinking can ever fall into it4 ,gravity must be caused by an agent1 acting constantly according to certain laws2 but whether this agent be material or immaterial ,i have left to the consideration of my readers40 ,since ,newton's time the continual decrease in the periodic times of the comets belonging to our system1 and the undulatory theory of light and heat1 have proved the existence of an extremely rare elastic medium filling space even to the most distant regions of which we are cognizant4 ,but1 rare as it may be1 it has inertia enough to resist the motion of comets1 and therefore must be material1 whether considered to be ether or1 according to ,mr4 ,grove1 the highly attenuated atmospheres of the celestial bodies4 ,professor ,william ,thomson of ,glasgow has computed that in the space traversed by the earth in its annual revolution1 a cube whose side is #ajjj miles would contain not less than a pound weight of the ethereal medium1 and that the earth1 in moving through it1 would not displace the #bejth part of that pound of matter4 ,yet that is enormously more dense than the continuation of the earth's atmosphere would be in interplanetary space1 if rarefied according to ,bayle's law4 ,but whatever be the density or nature of the ether1 there is every reason to believe that it is the medium which transmits the gravitating force from one celestial object to another1 or possibly it may possess a higher attribute with regard to gravity than its mere transmission4 ,dr4 ,faraday1 who discovered the magnetism of the atmosphere1 is led to believe that the ethereal medium too is magnetic by the following experiment4 ,three solutions of the protosulphate of iron1 l1 m1 n1 the first of which contained #d grains of the salt dissolved in a cubic inch of water1 the second #h grains1 and the third #af grains--these were respectively enclosed in three glass globules1 all of which were attracted by the pole of a magnet4 ,a quantity of the mean solution m was then put into a vessel1 and the globule containing the strongest solution n was immersed in it1 which was attracted as before1 but the globule l1 containing the weakest solution1 was repelled when plunged into the same liquid4 ,here there was a diamagnetic phenomenon1 although the glass globules and the liquid in which they were immersed contained iron4 ,the effect was evidently differential1 for when the liquid was less attracted than the globule1 the globule approached the pole1 and when the liquid was more attracted than the globule1 the latter appeared to recede from the pole4 ,in fact1 the effect is the same as that of gravity on a body immersed in water2 if it be more forcibly attracted than the water1 it sinks2 if less forcibly attracted1 it rises1 the effect being the same as if it were repelled by the earth4 ,hence the question1 are all magnetic phenomena the result of a differential action of this kind1 and is the ethereal medium less strongly attracted than soft iron1 and more strongly attracted than bismuth1 thus permitting the approach of the iron1 but causing the bismuth to recede from the pole of a magnet8 ,if such a medium exist1 that is1 if the ethereal medium be magnetic1 then diamagnetism is the same with paramagnetism1 and the polarity of the magnetic force in iron and bismuth is one and the same4 ,the ethereal medium may be presumed to transmit the gravitating force2 it transmits the magnetism of the solar spots1 its undulations constitute light1 heat1 and all the influences bound up in the solar beam2 and the most perfect vacuum we can make is capable of transmitting mechanical energy in enormous quantities1 some of which differ but little from that of air or oxygen at an ordinary barometric pressure2 and why not thus admit1 says ,mr4 ,thomson1 the magnetic property1 of which we know so little that we have no right to pronounce a negative8 ,mr4 ,waterstone is also of opinion that it would be taking too narrow a view if we limited the function of the luminiferous ether to the conveying of physical pulses only4 ,the atmosphere also conveys physical pulses1 but that is the least important of its functions in the economy of nature4 ,there is nothing that should hinder us attributing to the media concerned in the radiation of light and heat the higher functions of electrical polarity and gravitation4 ,the special dynamic arrangements by which this is effected may ever elude our research2 but as there is no limit to the vis viva 7,n4 #bbb7 which such media may conserve in their minutest parts1 so there is no physical impossibility in that vis viva being suddenly transferred to the molecules of ordinary matter in the proportion and sequence required to carry out the order and system of nature4 ,the fundamental principle of action in such media must be in accordance with elastic impact1 for upon that the dynamic theory of heat and conservation of force rests as a foundation4 ,the statical and dynamical characteristics of gravitation and transfusion of force conform to it1 so that all the forces that hold the molecules of bodies together must also be in subjection to it4#bj ,,section ,,xxxv4 ,ethereal ,medium--,comets--,do not disturb the ,solar ,system--,their ,orbits and ,disturbances--,m4 ,faye's ,comet probably the same with ,lexel's--,periods of other three known--,acceleration in the mean ,motions of ,encke's and ,biela's ,comets--,the ,shock of a ,comet--,disturbing ,action of the ,earth and ,planets on ,encke's and ,biela's ,comets--,velocity of ,comets--,the ,comet of #abfd--,the great ,comet of #acdc--,physical ,constitution--,shine by borrowed ,light--,estimation of their ,number4 ,,in considering the constitution of the earth1 and the fluids which surround it1 various subjects have presented themselves to our notice1 of which some1 for aught we know1 are confined to the planet we inhabit2 some are common to it and to the other bodies of our system4 ,but an all-pervading ether must fill the whole visible creation1 since it conveys1 in the form of light1 tremors which may have been excited in the deepest recesses of the universe thousands of years before we were called into being4 ,the existence of such a medium1 though at first hypothetical1 is proved by the undulatory theory of light1 and rendered certain by the motion of comets1 and by its action upon the vapours of which they are chiefly composed4 ,it has often been imagined that the tails of comets have infused new substances into our atmosphere4 ,possibly the earth may attract some of that nebulous matter1 since the vapours raised by the sun's heat1 when the comets are in perihelio1 and which form their tails1 are scattered through space in their passage to their aphelion2 but it has hitherto produced no effect1 nor have the seasons ever been influenced by these bodies4 ,the light of the comet of the year #ahaa1 which was so brilliant1 did not impart any heat even when condensed on the bulb of a thermometer of a structure so delicate that it would have made the hundredth part of a degree evident4 ,in all probability1 the tails of comets may have passed over the earth without its inhabitants being conscious of their presence2 and there is reason to believe that the tail of the great comet of #ahdc did so4 ,m4 ,valz observed that the light of a brilliant comet was eclipsed as it passed over a star of the #gth magnitude1 whence ,m4 ,babinet computed that the light of the comet must have been sixty times less than that of the star1 and that matter so attenuated could not penetrate the earth's atmosphere1 but the constitution of these bodies is still a matter of conjecture4 ,the passage of comets has never sensibly disturbed the stability of the solar system2 their nucleus1 being in general only a mass of vapour1 is so rare1 and their transit so rapid1 even when they had a solid part1 that the time has not been long enough to admit of a sufficient accumulation of impetus to produce a perceptible action4 ,indeed1 ,m4 ,dusejour has shown that1 under the most favourable circumstances1 a comet cannot remain longer than two hours and a half at a less distance from the earth than #aj1ejj leagues4 ,the comet of #aggj passed within about six times the distance of the moon from the earth1 without even affecting our tides4 ,according to ,la ,place1 the action of the earth on the comet of #aggj augmented the period of its revolution by more than two days2 and1 if comets had any perceptible disturbing energy1 the reaction of the comet ought to have increased the length of our year4 ,had the mass of that comet been equal to the mass of the earth1 its disturbing action would have increased the length of the sidereal year by #b;h #ecm2 but1 as ,delambre's computations from the ,greenwich observations of the sun show that the length of the year has not been increased by the fraction of a second1 its mass could not have been equal to the #a_/ejjjth part of that of the earth4 ,this accounts for the same comet having twice swept through the system of ,jupiter's satellites without deranging the motion of these moons4 ,m4 ,dusejour has computed that a comet1 equal in mass to the earth1 passing at the distance of #ab1aej leagues from our planet1 would increase the length of the year to #cfg;d #af;h #em1 and the obliquity of the ecliptic as much as #b4 ,so the principal action of comets would be to alter the calendar1 even if they were dense enough to affect the earth4 ,comets traverse all parts of the heavens2 their paths have every possible inclination to the plane of the ecliptic1 and1 unlike the planets1 the motion of more than half of those that have appeared has been retrograde1 that is1 from east to west4 ,they are only visible when near their perihelia2 then their velocity is such1 that its square is twice as great as that of a body moving in a circle at the same distance3 they consequently remain but a very short time within the planetary orbits4 ,and1 as all the conic sections of the same focal distance sensibly coincide1 through a small arc1 on each side of the extremity of their axis1 it is difficult to ascertain in which of these curves the comets move1 from observations made1 as they necessarily must be1 near their perihelia 7,n4 #bbg74 ,probably they all move in extremely excentric ellipses2 although1 in most cases1 the parabolic curve coincides most nearly with their observed motions4 ,some few seem to describe hyperbolas2 such1 being once visible to us1 would vanish for ever1 to wander through boundless space1 to the remote systems of the universe4 ,if a planet be supposed to revolve in a circular orbit1 whose radius is equal to the perihelion distance of a comet moving in a parabola1 the areas described by these two bodies in the same time will be as unity to the square root of two1 which forms such a connexion between the motion of comets and planets1 that1 by ,kepler's law1 the ratio of the areas described during the same time by the comet and the earth may be found2 so that the place of a comet may be computed at any time in its parabolic orbit1 estimated from the instant of its passage at the perihelion4 ,it is a problem of very great difficulty to determine all the other elements of parabolic motion--namely1 the comet's perihelion distance1 or shortest distance from the sun1 estimated in parts of the mean distance of the earth from the sun2 the longitude of the perihelion2 the inclination of the orbit on the plane of the ecliptic2 and the longitude of the ascending node4 ,three observed longitudes and latitudes of a comet are sufficient for computing the approximate values of these quantities2 but an accurate estimation of them can only be obtained by successive corrections1 from a number of observations1 distant from one another4 ,when the motion of a comet is retrograde1 the place of the ascending node is exactly opposite to what it is when the motion is direct4 ,hence the place of the ascending node1 together with the direction of the comet's motion1 show whether the inclination of the orbit is on the north or south side of the plane of the ecliptic4 ,if the motion be direct1 the inclination is on the north side2 if retrograde1 it is on the south side4 ,the identity of the elements is the only proof of the return of a comet to our system4 ,should the elements of a new comet be the same1 or nearly the same1 with those of any one previously known1 the probability of the identity of the two bodies is very great1 since the similarity extends to no less than four elements1 every one of which is capable of an infinity of variations4 ,but1 even if the orbit be determined with all the accuracy the case admits of1 it may be difficult1 or even impossible1 to recognize a comet on its return1 because its orbit would be very much changed if it passed near any of the large planets of this or of any other system1 in consequence of their disturbing energy1 which would be very great on bodies of so rare a nature4 ,by far the most curious and interesting instance of the disturbing action of the great bodies of our system is found in the comet of #aggj4 ,the elements of its orbit1 determined by ,messier1 did not agree with those of any comet that had hitherto been computed1 yet ,lexel ascertained that it described an ellipse about the sun1 whose major axis was only equal to three times the length of the diameter of the terrestrial orbit1 and consequently that it must return to the sun at intervals of five years and a half4 ,this result was confirmed by numerous observations1 as the comet was visible through an arc of #agj2 yet this comet had never been observed before the year #aggj1 nor has it ever again been seen till #ahdc1 though very brilliant4 ,the disturbing action of the larger planets affords a solution of this anomaly1 as ,lexel ascertained that in #agfg the comet must have passed ,jupiter at a distance less than the fifty-eighth part of its distance from the sun1 and that in #aggi it would be #ejj times nearer ,jupiter than the sun2 consequently the action of the sun on the comet would not be the fiftieth part of what it would experience from ,jupiter1 so that ,jupiter became the primum mobile4 ,assuming the orbit to be such as ,lexel had determined in #aggj1 ,la ,place found that the action of ,jupiter1 previous to the year #aggj1 had so completely changed the form of it1 that the comet which had been invisible to us before #aggj was then brought into view1 and that the action of the same planet1 producing a contrary effect1 has subsequently to that year removed it from our sight1 since it was computed to be revolving in an orbit whose perihelion was beyond the orbit of ,ceres4 ,however1 the action of ,jupiter during the summer of #ahdj must have been so great1 from his proximity to that singular body1 that he seems to have brought it back to its former path as he had done in #agfg1 for the elements of the orbit of a comet which was discovered in ,november #ahdc1 by ,m4 ,faye1 agree so nearly with those of the orbit of ,lexel's comet that the two bodies were supposed to be identical2 by the subsequent computation of ,m4 le ,verrier1 it appears1 however1 that they are not the same1 that they were both brought to our system by ,jupiter's attraction1 and that they have been in it more than a century1 and have frequently come near the earth without having been seen4 ,from the smallness of the excentricity of ,lexel's comet1 the orbit resembles those of the planets1 but this comet is liable to greater perturbations than any other body in the system1 because it comes very near the orbit of ,mars when in perihelion1 and very near that of ,jupiter when in aphelion2 besides1 it passes within a comparatively small distance of the orbits of the minor planets2 and as it will continue to cross the orbit of ,jupiter at each revolution till the two bodies meet1 its periodic time1 now about seven years1 will again be changed1 but in the mean time it ought to have returned to its perihelion in the year #ahea4 ,this comet might have been seen from the earth in #aggf1 had its light not been eclipsed by that of the sun4 ,there is still so much doubt with regard to ,lexel's comet that during the present year1 #aheh1 ,m4 le ,verrier has constructed a table of all the orbits in which the comet may have moved after leaving ,jupiter in #aggj1 which will enable astronomers to recognise the comet even should the elements of its orbit be much altered4 ,he thinks it possible that its path may have become hyperbolic1 but that it is more likely an augmentation of its periodic time may have taken place4 ,it is quite possible that comets frequenting our system may be turned away1 or others brought to the sun1 by the attraction of planets revolving beyond the orbit of ,neptune1 or by bodies still farther removed from the solar influence4 ,other comets1 liable to less disturbance1 return to the sun at stated intervals4 ,halley computed the elements of the orbit of a comet that appeared in the year #afhb1 which agreed so nearly with those of the comets of #afjg and #aeca1 that he concluded it to be the same body returning to the sun at intervals of about seventy-five years4 ,he consequently predicted its reappearance in the year #ageh1 or in the beginning of #agei4 ,science was not sufficiently advanced in the time of ,halley to enable him to determine the perturbations this comet might experience2 but ,clairaut computed that1 in consequence of the attraction of ,jupiter and ,saturn1 its periodic time would be so much shorter than during its revolution between #afjg and #afhb1 that it would pass its perihelion on the #ahth of ,april1 #agei4 ,the comet did arrive at that point of its orbit on the #abth of ,march1 which was thirty-seven days before the time assigned4 ,clairaut subsequently reduced the error to twenty-three days2 and ,la ,place has since shown that it would only have been thirteen days if the mass of ,saturn had been as well known as it is now4 ,it appears1 from this1 that the path of the comet was not quite known at that period2 and1 although many observations were then made1 they were far from attaining the accuracy of those of the present day4 ,besides1 since the year #agei1 the orbit of the comet has been altered by the attraction of ,jupiter in one direction1 and that of ,saturn1 ,uranus1 and ,neptune in the other2 yet1 notwithstanding these sources of uncertainty1 and our ignorance of all the possible causes of derangement from unknown bodies on the confines of our system1 or in the regions beyond it1 the comet appeared exactly at the time1 and not far from the place assigned to it by astronomers2 and its actual arrival at its perihelion a little before noon on the #afth of ,november1 #ahce1 only differed from the computed time by a very few days1 which was probably owing to the attraction of ,neptune4 ,the fulfilment of this astronomical prediction is truly wonderful1 if it be considered that the comet is seen only for a few weeks during its passage through our system1 and that it wanders from the sun for seventy-five years to twice the distance of ,uranus4 ,this enormous orbit is four times longer than it is broad2 its length is about #cdbj millions of miles1 or about thirty-six times the mean distance of the earth from the sun4 ,at its perihelion the comet comes within nearly fifty-seven millions of miles of the sun1 and at its aphelion it is sixty times more distant4 ,on account of this extensive range it must experience #cfjj times more light and heat when nearest to the sun than in the most remote point of its orbit4 ,in the one position the sun will seem to be four times larger than he appears to us1 and at the other he will not be apparently larger than a star 7,n4 #bbh47 ,on the first appearance of ,halley's comet1 early in ,august #ahce1 it seemed to be merely a globular mass of dim vapour1 without a tail4 ,a concentration of light1 a little on one side of the centre1 increased as the comet approached the sun and earth1 and latterly looked so like the disc of a small planet1 that it might have been mistaken for a solid nucleus4 ,m4 ,struve1 however1 saw a central occultation of a star of the ninth magnitude by the comet1 at ,dorpat1 on the #bith of ,september4 ,the star remained constantly visible1 without any considerable diminution of light2 and1 instead of being eclipsed1 the nucleus of the comet disappeared at the moment of conjunction from the brilliancy of the star4 ,the tail increased as the comet approached its perihelion1 and shortly before it was lost in the sun's rays it was between thirty and forty degrees in length4 ,according to the observations of ,m4 ,valz1 the nebulosity increased in magnitude as it approached the sun2 but no other comet on record has exhibited such sudden and unaccountable changes of aspect4 ,it was invisible for two months when near its perihelion passage1 and when it reappeared on the #bdth of ,january1 #ahcf1 its aspect was completely changed2 it had no tail1 and to the naked eye was like a hazy star2 but with a powerful telescope it presented a small1 round1 planetary-looking nucleus #b in diameter1 surrounded by an extensive coma1 and in the centre it had a small1 bright1 solid part4 ,the nucleus1 clear and well defined1 like the disc of a planet1 was observed on one occasion to become obscure and enlarged in the course of a few hours4 ,but by far the most remarkable circumstance was the sudden appearance of certain luminous brushes or sectors1 diverging from the centre of the nucleus through the nebulosity4 ,m4 ,struve describes the nucleus of the comet1 in the beginning of ,october1 as elliptical1 and like a burning coal1 out of which there issued1 in a direction nearly opposite to the tail1 a divergent flame1 varying in intensity1 form1 and direction1 appearing occasionally even double1 and suggesting the idea of luminous gas bursting from the nucleus4 ,on one occasion ,m4 ,arago saw three of these divergent flames on the side opposite the tail1 rising through the nebulosity1 which they greatly exceeded in brilliancy3 after the comet had passed its perihelion1 it acquired another of these luminous fans1 which was observed by ,sir ,john ,herschel at the ,cape of ,good ,hope4 ,hevelius describes an appearance precisely similar1 which he had witnessed in this comet at its approach to the sun in the year #afhb1 and something of the kind seems to have been noticed in the comet of #agdd4 ,possibly the second tail of the comet of #agbd1 which was directed towards the sun1 may have been of this nature4 ,the influence of the ethereal medium on the motions of ,halley's comet will be known after another revolution1 and future astronomers will learn1 by the accuracy of its returns1 whether it has met with any unknown cause of disturbance in its distant journey4 ,undiscovered planets1 beyond the visible boundary of our system1 may change its path and the period of its revolution1 and thus may indirectly reveal to us their existence1 and even their physical nature and orbit4 ,the secrets of the yet more distant heavens may be disclosed to future generations by comets which penetrate still farther into space1 such as that of #agfc1 which1 if any faith may be placed in the computation1 goes nearly forty-three times farther from the sun than ,halley's does1 and shows that the sun's attraction is powerful enough1 at the enormous distance of #ae1ejj millions of miles1 to recall the comet to its perihelion4 ,the periods of some comets are said to be of many thousand years1 and even the average time of the revolution of comets generally is about a thousand years2 which proves that the sun's gravitating force extends very far4 ,la ,place estimates that the solar attraction is felt throughout a sphere whose radius is a hundred millions of times greater than the distance of the earth from the sun4 ,authentic records of ,halley's comet do not extend beyond the year #adef1 yet it may be traced1 with some degree of probability1 even to a period preceding the ,christian era4 ,but as the evidence only rests upon coincidences of its periodic time1 which may vary as much as eighteen months from the disturbing action of the planets1 its identity with comets of such remote times must be regarded as extremely doubtful4 ,this is the first comet whose periodicity has been established4 ,it is also the first whose elements have been determined from observations made in ,europe2 for1 although the comets which appeared in the years #bdj1 #eci1 #efe1 and #hcg1 are the most ancient of those whose orbits have been traced1 their elements were computed from ,chinese observations4 ,besides ,halley's and ,lexel's comets1 ten or twelve others are now known to form part of the solar system2 that is to say1 they return to the sun at stated periods4 ,six of them have periods of less than eight years4 ,that generally called ,encke's comet1 or the comet of the short period1 was first seen by ,,mm4 ,messier and ,mechain in #aghf1 again by ,miss ,herschel in #ahje1 and its returns1 in the years #ahje and #ahai1 were observed by other astronomers1 under the impression that all four were different bodies4 ,however1 ,professor ,encke not only proved their identity1 but determined the circumstances of the comet's motion4 ,its reappearance in the years #ahbe1 #ahbh1 and #ahcb1 accorded with the orbit assigned by ,m4 ,encke1 who thus established the length of its period to be #abjd days1 nearly4 ,this comet is very small1 of feeble light1 and invisible to the naked eye1 except under very favourable circumstances1 and in particular positions4 ,it has no tail1 it revolves in an ellipse of great excentricity inclined at an angle of #ac #bb to the plane of the ecliptic1 and is subject to considerable perturbations from the attraction of the planets1 which occasion variations in its periodic time4 ,among the many perturbations to which the planets are liable1 their mean motions1 and therefore the major axes of their orbits1 experience no change2 while1 on the contrary1 the mean motion of the moon is accelerated from age to age--a circumstance at first attributed to the resistance of an ethereal medium pervading space1 but subsequently proved to arise from the secular diminution of the excentricity of the terrestrial orbit4 ,although the resistance of such a medium has not hitherto been perceived in the motions of such dense bodies as the planets and satellites1 its effects on the revolutions of the comets leave no doubt of its existence4 ,from the numerous observations that have been made on each return of the comet of the short period1 the elements have been computed with great accuracy on the hypothesis of its moving in vacuo4 ,its perturbations occasioned by the disturbing action of the planets have been determined2 and1 after everything that could influence its motion had been duly considered1 ,m4 ,encke found that an acceleration of about two days in each revolution has taken place in its mean motion1 precisely similar to that which would be occasioned by the resistance of an ethereal medium4 ,and1 as it cannot be attributed to a cause like that which produces the acceleration of the moon1 it must be concluded that the celestial bodies do not perform their revolutions in an absolute void1 and that1 although the medium be too rare to have a sensible effect on the masses of the planets and satellites1 it nevertheless has a considerable influence on so rare a body as a comet4 ,contradictory as it may seem that the motion of a body should be accelerated by the resistance of an ethereal medium1 the truth becomes evident if it be considered that both planets and comets are retained in their orbits by two forces which exactly balance one another2 namely1 the centrifugal force producing the velocity in the tangent1 and the attraction of the gravitating force directed to the centre of the sun4 ,if one of these forces be diminished by any cause1 the other will be proportionally increased4 ,now1 the necessary effect of a resisting medium is to diminish the tangential velocity1 so that the balance is destroyed1 gravity preponderates1 the body descends towards the sun till equilibrium is again restored between the two forces2 and1 as it then describes a smaller orbit1 it moves with increased velocity4 ,thus1 the resistance of an ethereal medium actually accelerates the motion of a body2 but1 as the resisting force is confined to the plane of the orbit1 it has no influence whatever on the inclination of the orbit1 or on the place of the nodes4 ,in computing its effect1 ,m4 ,encke assumed the increase to be inversely as the square of the distance1 and that its resistance acts as a tangential force proportional to the squares of the comet's actual velocity in each point of its orbit4 ,another comet belonging to our system1 which returns to its perihelion after a period of #f-c_/d years1 has been accelerated in its motion by a whole day during one revolution1 which puts the existence of ether beyond a doubt1 and confirms the undulatory theory of light4 ,since this comet1 which revolves nearly between the orbits of the earth and ,jupiter1 is only accelerated one day at each revolution1 while ,encke's1 revolving nearly between the orbits of ,mercury and ,pallas1 is accelerated two1 the ethereal medium must increase in density towards the sun4 ,the comet in question was discovered by ,m4 ,biela at ,josephstadt on the #bgth of ,february1 #ahbf1 and ten days afterwards it was seen by ,m4 ,gambart at ,marseilles1 who computed its parabolic elements1 and found that they agreed with those of the comets which had appeared in the years #aghi and #agie1 whence he concluded them to be the same body moving in an ellipse1 and accomplishing its revolution in #bdfj days4 ,the perturbations of this comet were computed by ,m4 ,damoiseau1 who predicted that it would cross the plane of the ecliptic on the #bith of ,october1 #ahcb1 a little before midnight1 at a point nearly #ah1dhd miles within the earth's orbit2 and as ,m4 ,olbers of ,bremen1 in #ahje1 had determined the radius of the comet's head to be about #ba1acf miles1 it was evident that its nebulosity would envelop a portion of the earth's orbit1--a circumstance which caused some alarm in ,france1 from the notion that1 if any disturbing cause had delayed the arrival of the comet for one month1 the earth must have passed through its head4 ,m4 ,arago dispelled these fears by his excellent treatise on comets1 in the ,annuaire of #ahcb1 where he proves that1 as the earth would never be nearer the comet than #ah1jjj1jjj ,british leagues1 there could be no danger of collision4 ,the earth is in more danger from these two small comets than from any other4 ,encke's crosses the terrestrial orbit sixty times in a century1 and may ultimately come into collision1 but both are so extremely rare1 that little injury is to be apprehended4 ,the earth would fall to the sun in #fd-a_/b days1 if it were struck by a comet with sufficient impetus to destroy its centrifugal force4 ,what the earth's primitive velocity may have been it is impossible to say4 ,therefore a comet may have given it a shock without changing the axis of rotation1 but only destroying part of its tangential velocity1 so as to diminish the size of the orbit--a thing by no means impossible1 though highly improbable4 ,at all events1 there is no proof of this having occurred2 and it is manifest that the axis of the earth's rotation has not been changed1 because1 as the ether offers no sensible resistance to so dense a body as the earth1 the libration would to this day be evident in the variation it must have occasioned in the terrestrial latitudes4 ,supposing the nucleus of a comet to have a diameter only equal to the fourth part of that of the earth1 and that its perihelion is nearer to the sun than we are ourselves1 its orbit being otherwise unknown1 ,m4 ,arago has computed that the probability of the earth receiving a shock from it is only one in #bha millions1 and that the chance of our coming in contact with its nebulosity is about ten or twelve times greater4 ,only comets with retrograde motions can come into direct collision with the earth1 and if the momentum were great the event might be fatal2 but in general the substance of comets is so rare1 that it is likely they would not do much harm if they were to impinge2 and even then the mischief would probably be local1 and the equilibrium soon restored1 provided the nucleus were gaseous1 or very small4 ,it is1 however1 more probable that the earth would only be deflected a little from its course by the approach of a comet1 without being touched by it4 ,the comets that have come nearest to the earth were that of the year #hcg1 which remained four days within less than #a1bdj1jjj leagues from our orbit3 that of #aggj1 which approached within about six times the distance of the moon4 ,the celebrated comet of #afhj also came very near to us2 and the comet whose period is #f-c_/d years was ten times nearer the earth in #ahje than in #ahcb1 when it caused so much alarm4 ,encke's and ,biela's comets are at present far removed from the influence of ,jupiter1 but they will not always remain so1 because1 the aphelia and nodes of the orbits of these two comets being the points which approach nearest to the orbit of ,jupiter at each meeting of the planet and comets1 the major axis of ,encke's comet will be increased and that of ,biela's diminished1 till in the course of time1 when the proximity has increased sufficiently1 the orbits will be completely changed1 as that of ,lexel's was in #aggj4 ,every twenty-third year1 or after seven revolutions of ,encke's comet1 its greatest proximity to ,jupiter takes place1 and at that time his attraction increases the period of its revolution by nine days--a circumstance which took place in the end of the years #ahbj and #ahdc4 ,but from the position of the bodies there is a diminution of three days in the six following revolutions1 which reduces the increase to six days in seven revolutions4 ,thus1 before the year #ahai1 the periodic time of ,encke's comet was #abjd days1 and it was #abai days in accomplishing the revolution that ended in #ahde4 ,by this progressive increase the orbit of the comet will reach that of ,jupiter in seven or eight centuries1 and then by the very near approach of the two bodies it will be completely changed4 ,at present the ,earth and ,mercury have the most powerful influence on the motions of ,encke's and ,biela's comets2 and have had for so long a time that1 according to the computation of ,mr4 ,airy1 the present orbit of the latter was formed by the attraction of the ,earth1 and that of ,encke's by the action of ,mercury4 ,with regard to the latter comet1 that event must have taken place in ,february #aggf4 ,in #aghf ,encke's comet had both a tail and a nucleus1 now it has neither2 a singular instance of the possibility of their disappearance4 ,it was in perihelio in #ahee4 ,in #ahdf ,biela's comet was divided into two distinct bodies1 by what strange accident is altogether a mystery4 ,the nuclei of the two comets were separated by about #aej1jjj miles1 and they travelled together with their tails parallel1 and an arch of light over their heads4 ,till that time ,biela's comet never had been seen with a tail4 ,the new head was dull at first1 but increased in size and brightness till it surpassed its companion in both2 besides1 it had a bright flashing diamond-like point in its centre--gradually it resumed its dull appearance1 and its period was computed to be eight days longer than that of the original head4 ,they had separated to a greater distance from one another in #ahec1 but were still travelling together1 one having become smaller than the other4 ,a comet discovered by ,m4 ,brorsen of ,kiel1 on the #bfth of ,february1 #ahdf1 came1 on the #bjth of ,april following1 nearly as close to ,jupiter as his fourth satellite1 when ,jupiter's attraction must have been ten times greater than that of the sun2 so there is every reason to believe that the comet's orbit will be as much altered as that of ,lexel's2 and another discovered by ,padre de ,vico at ,rome1 on the #bbnd of ,august1 will1 in all probability1 be as much disturbed by the same cause4 ,one of the comets found by that astronomer has a period which varies1 according to different computations1 from #ee to #ii years2 it certainly has an elliptical orbit4 ,that discovered at ,naples by ,mr4 ,peters revolves about the sun in #af years2 but ,olbers's comet of #ahae must go nearly the same distance into space with ,halley's1 since its period is #gd years4 ,two discovered by ,m4 ,brorsen have periods1 one of #ejj and the other of #bh years2 but of the latter there is some uncertainty4 ,the comet which appeared in #aeif and #ahde has a period of #bdi years2 and should ,m4 ,argelander's computation be accurate1 the orbit which has hitherto been assigned to the great comet of #ahaa must be erroneous1 since he has ascertained its period to be #cjff years4 ,the great comet of #abfd1 which had a tail that extended over #ajj of the celestial vault1 was observed and recorded by the ,chinese1 and was ascertained to be the same that had appeared in #aeef1 and of whose motions observations were taken at ,vienna in the reign of the ,emperor ,charles ,v41 but it was then less brilliant4 ,in consequence of the discovery of the original observations of the comet of #aeef1 by ,fabricius at ,vienna1 and by ,heller at ,nuremburg1 ,mr4 ,hind was induced to compute its orbit for that year2 but after much labour1 aided by all the improved methods of calculation1 he found ,heller's observations so confused1 and even erroneous1 that he could not determine the curve described by the comet at that time with any precision1 and therefore could only predict that the epoch of its return would be some time between #ahdh and #ahfa4 ,before comets reach the sun they are rarely conspicuous2 but if after passing their perihelion they come near the earth1 then they have tails1 and become brilliant in consequence of the sun's action upon the matter of which they are formed4 ,now if the comet in question should pass its perihelion between the months of ,march and ,october1 it possibly may be as remarkable as ever2 but should it come nearest to the sun in winter1 such is the position of its orbit with regard to the earth1 that it may pass unnoticed--which is very unlikely1 as search is being made for it at almost all the observatories in ,europe and in the ,united ,states4 ,nearly the whole of its orbit lies below the plane of the ecliptic1 and far from the paths of the larger planets1 but it extends into space more than twice the distance of ,neptune1 or nearly six thousand millions of miles from the sun4 ,comets in or near their perihelion move with prodigious velocity4 ,that of #afhj appears to have gone half round the sun in ten hours and a half1 moving at the rate of #hhj1jjj miles an hour4 ,if its enormous centrifugal force had ceased when passing its perihelion1 it would have fallen to the sun in about three minutes1 as it was then less than #adg1jjj miles from his surface4 ,so near the sun1 it would be exposed to a heat #bg1ejj times greater than that received by the earth2 and as the sun's heat is supposed to be in proportion to the intensity of his light1 it is probable that a degree of heat so intense would be sufficient to convert into vapour every terrestrial substance with which we are acquainted4 ,at the perihelion distance the sun's diameter would be seen from the comet under an angle of #gc1 so that the sun1 viewed from the comet1 would nearly cover the whole extent of the heavens from the horizon to the zenith4 ,as this comet is presumed to have a period of #ege years1 the major axis of its orbit must be so great1 that at the aphelion the sun's diameter would only subtend an angle of about fourteen seconds1 which is not so great by half as the diameter of ,mars appears to us when in opposition4 ,the sun would consequently impart no heat1 so that the comet would then be exposed to the temperature of the ethereal regions1 which is #bci below the zero point of ,fahrenheit4 ,a body of such tenuity as the comet1 moving with such velocity1 must have met with great resistance from the dense atmosphere of the sun1 while passing so near his surface at its perihelion4 ,the centrifugal force must consequently have been diminished1 and the sun's attraction proportionally augmented1 so that it must have come nearer to the sun in #afhj than in its preceding revolution1 and would subsequently describe a smaller orbit4 ,as this diminution of its orbit will be repeated at each revolution1 the comet will infallibly end by falling on the surface of the sun1 unless its course be changed by the disturbing influence of some large body in the unknown expanse of creation4 ,our ignorance of the actual density of the sun's atmosphere1 of the density of the comet1 and of the period of its revolution1 renders it impossible to form any idea of the number of centuries which must elapse before this event takes place4 ,the same cause may affect the motions of the planets1 and ultimately be the means of destroying the solar system4 ,but1 as ,sir ,john ,herschel observes1 they could hardly all revolve in the same direction round the sun for so many ages without impressing a corresponding motion on the ethereal medium1 which may preserve them from the accumulated effects of its resistance4 ,should this material medium revolve about the sun like a vortex1 it will accelerate the revolutions of such comets as have direct motions1 and retard those that have retrograde motions4 ,the comet which appeared unexpectedly in the beginning of the year #ahdc was one of the most splendid that ever visited the solar system4 ,it was in the constellation of ,antinous in the end of ,january1 at a distance of #aae millions of miles from the earth1 and it passed through its perihelion on the #bgth of ,february1 when it was lost in the sun's rays2 but it began to be visible about the #crd of ,march1 at which time it was near the star ,iota ,cet1 and its tail extended towards the ,hare4 ,before the passage at the perihelion it had no tail2 but at that epoch the tail suddenly darted out1 and extended to a distance of #ahbf millions of miles in about an hour and a half--a most inexplicable speed of development1 which indicates some powerful repulsive force at the moment of the greatest proximity to the sun1 at which time the tails are formed4 ,the brightness of the comet and the length of its tail continued to increase till the latter stretched far beyond the constellation of the ,hare towards a point above ,sirius4 ,stars were distinctly seen through it1 and when near perihelion the comet was so bright that it was seen in clear sunshine1 in the ,united ,states1 like a white cloud4 ,the motion was retrograde1 and on leaving the solar system it retreated so rapidly at once from the sun and earth that it was soon lost sight of for want of light4 ,on the #ast of ,april it was between the sun and the earth1 and only #dj millions of miles from the latter2 and as its tail was at least #fj millions of miles long1 and #bj millions of miles broad1 we probably passed through it without being aware of it4 ,there is some discrepancy in the different computations of the elements of the orbit1 but in the greater number of cases the perihelion distance was found to be less than the semidiameter of the sun1 so that the comet must have grazed his surface1 if it did not actually impinge obliquely on him4 ,the perihelion distance of this comet differs little from that of the great comet of #affh1 which came so near the sun4 ,the motion of both was retrograde1 and a certain resemblance in the two orbits makes it probable that they are the same body performing a revolution in #age years4 ,though already so well acquainted with the motions of comets1 we know nothing of their physical constitution4 ,a vast number1 especially of telescopic comets1 are only like clouds or masses of vapour1 often without tails4 ,the head commonly consists of a concentrated mass of light1 like a planet1 surrounded by a very transparent atmosphere1 and the whole1 viewed with a telescope1 is so diaphanous1 that the smallest star may be seen even through the densest part of the nucleus2 in general their solid parts1 when they have any1 are so minute1 that they have no sensible diameter1 like that of the comet of #ahaa1 which appeared to ,sir ,william ,herschel like a luminous point in the middle of the nebulous matter4 ,the nuclei1 which seem to be formed of the denser strata of that nebulous matter in successive coatings1 are sometimes of great magnitude4 ,those comets which came to the sun in the years #agii and #ahjg had nuclei whose diameters measured #ahj and #bge leagues respectively1 and the second comet of #ahaa had a nucleus #acej leagues in diameter4 ,it must1 however1 be stated that1 as comets are generally at prodigious distances from the earth1 the solid parts of the nuclei appear like mere points of light1 so minute that it is impossible to measure them with any kind of accuracy1 so that the best astronomers often differ in the estimation of their size by one-half of the whole diameter4 ,the transit of a comet across the sun would afford the best information with regard to the nature of the nuclei4 ,it was computed that such an event was to take place in the year #ahbg2 unfortunately the sun was hid by clouds from the ,british astronomers1 but it was examined at ,viviers and at ,marseilles at the time the comet must have been projected on its disc1 but no spot or cloud was to be seen1 so that it must have had no solid part whatever4 ,the nuclei of many comets which seemed solid and brilliant to the naked eye have been resolved into mere vapour by telescopes of high powers2 in ,halley's comet there was no solid part at all4 ,the nebulosity immediately round the nucleus is so diaphanous1 that it gives little light2 but at a small distance the nebulous matter becomes suddenly brilliant1 so as to look like a bright ring round the body4 ,sometimes there are two or three of these luminous concentric rings separated by dark intervals1 but they are generally incomplete on the part next the tail4 ,these annular appearances are an optical effect1 arising from a succession of envelopes of the nebulous matter with intervals between them1 of which the first is sometimes in contact with the nucleus and sometimes not4 ,the thickness of these bright diaphanous coatings in the comets of #agii and #ahjg was about #gjjj and #aj1jjj leagues respectively2 and in the first comet of #ahaa the luminous ring was #hjjj leagues thick1 and the distance between its interior surface and the centre of the head was #aj1jjj leagues4 ,the latter comet was by much the most brilliant that has been seen in modern times2 it was first discovered in this country by ,mr4 ,james ,vietch of ,inchbonny1 and was observed in all its changes by ,sir ,william ,herschel and ,m4 ,olbers4 ,to the naked eye1 the head had the appearance of an ill-defined round mass of light1 which was resolved into several distinct parts when viewed with a telescope4 ,a very brilliant interior circular mass of nebulous matter was surrounded by a black space having a parabolic form1 very distinct from the dark blue of the sky4 ,this dark space was of a very appreciable breadth4 ,exterior to the black interval there was a luminous parabolic contour of considerable thickness1 which was prolonged on each side in two diverging branches1 which formed the bifid tail of the comet4 ,sir ,william ,herschel found that the brilliant interior circular mass lost the distinctness of its outline as he increased the magnifying power of the telescope1 and presented the appearance of a more and more diffuse mass of greenish or blueish green light1 whose intensity decreased gradually1 not from the centre1 but from an eccentric brilliant speck1 supposed to be the truly solid part of the comet4 ,the luminous envelope was of a decided yellow1 which contrasted strongly with the greenish tint of the interior nebulous mass4 ,stars were nearly veiled by the luminous envelope1 whilst1 on the contrary1 ,sir ,william ,herschel saw three extremely small stars shining clearly in the black space1 which was singularly transparent4 ,as the envelopes were formed in succession as the comet approached the sun1 ,sir ,william ,herschel conceived them to be vapours raised by his heat at the surface of the nucleus1 and suspended round it like a vault or dome by the elastic force of an extensive and highly transparent atmosphere4 ,in coming to the sun1 the coatings began to form when the comet was as distant as the orbit of ,jupiter1 and in its return they very soon entirely vanished2 but a new one was formed after it had retreated as far as the orbit of ,mars1 which lasted for a few days4 ,indeed1 comets in general are subject to sudden and violent convulsions in their interior1 even when far from the sun1 which produce changes that are visible at enormous distances1 and baffle all attempts at explanation--probably arising from electricity1 or even causes with which we are unacquainted4 ,the envelopes surrounding the nucleus of the comet on the side next to the sun diverge on the opposite side1 where they are prolonged into the form of a hollow cone1 which is the tail4 ,two repulsive forces seem to be concerned in producing this effect2 one from the comet and another from the sun1 the latter being the most powerful4 ,the envelopes are nearer the centre of the comet on the side next to the sun1 where these forces are opposed to one another2 but on the other side the forces conspire to form the tail1 conveying the nebulous particles to enormous distances4 ,the lateral edges of the tail reflect more light than the central part1 because the line of vision passes through a greater depth of nebulous matter1 which produces the effect of two streams somewhat like the aurora4 ,stars shine with undiminished lustre through the central part of the tail1 because their rays traverse it perpendicularly to its thickness2 but1 though distinctly seen through its edges1 their light is weakened by its oblique transmission4 ,the tail of the great comet of #ahaa was of wonderful tenuity2 stars which would have been entirely concealed by the slightest fog were seen through #fd1jjj leagues of nebulous matter without the smallest refraction4 ,possibly some part of the changes in the appearance of the tails arises from rotation4 ,several comets have been observed to rotate about an axis passing through the centre of the tail4 ,that of #ahbe performed its rotation in #bj-a_/b hours1 and the rapid changes in the luminous sectors which issued from the nucleus of ,halley's comet in all probability were owing to rotatory motion4 ,the two streams of light which form the edges of the tail in most cases unite at a greater or less distance from the nucleus1 and are generally situate in the plane of the orbit4 ,the tails follow comets in their descent towards the sun1 but precede them in their return1 with a small degree of curvature2 their apparent extent and form vary according to the positions of the orbits with regard to the ecliptic4 ,in some cases the tail has been at right angles to the line joining the sun and comet4 ,the curvature is in part owing to the resistance of the ether1 and partly to the velocity of the comet being greater than that of the particles at the extremity of its tail1 which lag behind4 ,the tails are generally of enormous lengths2 the comet of #ahaa had one no less than a hundred millions of miles in length1 and those which appeared in the years #afah1 #afhj1 and #agfi1 had tails which extended respectively over #ajd1 #ij1 and #ig degrees of space4 ,consequently1 when the heads of these comets were set1 a portion of the extremity of their tails was still in the zenith4 ,sometimes the tail is divided into several branches1 like the comet of #agdd1 which had six1 separated by dark intervals1 each of them about #d broad1 and from #cj to #dd long4 ,they were probably formed by three hollow cones of the nebulous matter proceeding from the different envelopes1 and enclosing one another1 with intervals between2 the lateral edges of these cones would give the appearance of six streams of light4 ,the tails do not attain their full magnitude till the comet has left the sun4 ,when comets first appear1 they resemble round films of vapour1 with little or no tail4 ,as they approach the sun1 they increase in brilliancy1 and their tail in length1 till they are lost in his rays2 and it is not till they emerge from the sun's more vivid light that they assume their full splendour4 ,they then gradually decrease1 their tails diminish1 and they disappear1 nearly or altogether1 before they are beyond the sphere of telescopic vision4 ,many comets have no tails1 as1 for example1 ,encke's comet4 ,those which appeared in the years #aehe1 #agfc1 and #afhb1 were also without tails1 though the latter is recorded to have been as bright as ,jupiter4 ,the matter of the tail must be extremely buoyant to precede a body moving with such velocity3 indeed1 the rapidity of its ascent cannot be accounted for4 ,it has been attributed to that power in the sun which produces those vibrations of ether which constitute light2 but as this theory will not account for the comet of #ahbd1 which is said to have had two tails1 one directed towards the sun1 and a very short one diametrically opposite to it1 our ignorance on this subject must be confessed4 ,in this case the repelling power of the comet seems to have been greater than that of the sun4 ,whatever that unknown power may be1 there are instances in which its effects are enormous2 for1 immediately after the great comet of #afhj had passed its perihelion1 its tail was #ajj1jjj1jjj miles in length1 and was projected from the comet's head in the short space of two days4 ,a body of such extreme tenuity as a comet is most likely incapable of an attraction powerful enough to recall matter sent to such an enormous distance2 it is therefore1 in all probability1 scattered in space or absorbed by the zodiacal light or nebula that surrounds the sun1 which may account for the rapid decrease observed in the tails of comets every time they return to their perihelia4 ,should the great comet of #ahdc prove to be the same with that of #affh1 its tail must have diminished considerably4 ,it is remarkable that1 although the tails of comets increase in length as they approach their perihelia1 there is reason to believe that the real diameter of the head contracts on coming near the sun1 and expands rapidly on leaving him4 ,hevelius first observed this phenomenon1 which ,encke's comet has exhibited in a very extraordinary degree4 ,on the #bhth of ,october1 #ahbh1 this comet was about three times as far from the sun as it was on the #bdth of ,december2 yet at the first date its apparent diameter was twenty-five times greater than at the second1 the decrease being progressive4 ,m4 ,valz attributes the circumstance to a real condensation of volume from the pressure of the ethereal medium1 which increases most rapidly in density towards the surface of the sun1 and forms an extensive atmosphere around him4 ,it did not occur to ,m4 ,valz1 however1 that the ethereal fluid would penetrate the nebulous matter instead of compressing it4 ,sir ,john ,herschel1 on the contrary1 conjectures that it may be owing to the alternate conversion of evaporable materials in the upper regions of the transparent atmosphere of comets into the states of visible cloud and invisible gas by the effects of heat and cold2 or that some of the external nebulous envelopes may come into view when the comet arrives at a darker part of the sky1 which were overpowered by the superior light of the sun while in his vicinity4 ,the first of these hypotheses he considers to be perfectly confirmed by his observations on ,halley's comet1 made at the ,cape of ,good ,hope1 after its return from the sun4 ,he thinks that1 in all probability1 the whole comet1 except the densest part of its head1 vanished1 and was reduced to a transparent and invisible state during its passage at its perihelion3 for when it first came into view1 after leaving the sun1 it had no tail1 and its aspect was completely changed4 ,a parabolic envelope soon began to appear1 and increased so much and so rapidly that its augmentation was visible to the eye4 ,this increase continued till it became so large and so faint1 that at last it vanished entirely1 leaving only the nucleus and a tail1 which it had again acquired1 but which also vanished2 so that at last the nucleus alone remained4 ,not only the tails1 but the nebulous part of comets1 diminishes every time they return to their perihelia2 after frequent returns they ought to lose it altogether1 and present the appearance of a fixed nucleus3 this ought to happen sooner to comets of short periods4 ,m4 de la ,place supposes that the comet of #afhb must be approaching rapidly to that state4 ,should the substances be altogether1 or even to a great degree1 evaporated1 the comet would disappear for ever4 ,possibly comets may have vanished from our view sooner than they would otherwise have done from this cause4 ,the comet discovered at ,florence by ,signore ,donati1 on the #bnd of ,june1 #aheh1 was one of the most beautiful that has been seen from our planet for many years1 whether for the brightness of the nucleus1 or the length and graceful form of the coma2 when first discovered it was near the star in the constellation of the ,lion1 being then at a distance of #bhh1jjj1jjj miles from the earth2 during the month of ,august its nucleus assumed an almost planetary aspect from the concentration of its light2 on the #bgth of ,september the head appeared almost as bright as ,mercury1 but smaller2 when near its perihelion passage1 on ,september #cjth1 its diameter1 as ascertained by ,signore ,donati1 was #c2 during the early part of ,october it continued to increase in brilliancy1 the tail becoming more elongated1 and describing a beautiful arc in the heavens1 occupying a space of nearly #dj1 or a length of #dj1jjj1jjj miles in the solar system4 ,on the evening of the #eth of ,october it was seen from most parts of ,britain1 within #bj of ,arcturus1 the brightest star in the northern heavens1 across which the densest part nearly of the tail passed1 and through which notwithstanding the star shone with undiminished brilliancy4 ,on the #cjth of ,october1 when in perihelio1 the comet was only #ee1jjj1jjj miles from the sun2 on the #ajth it approached nearest to the earth1 from which it was then distant #ea1jjj1jjj miles2 and on the #aeth of the same month near to ,venus1 being at that time less than one-tenth the distance of the earth from the ,sun2 if the comet had reached its perihelion a few days earlier1 ,venus might have passed through its nucleus1 the consequences of which to the planet it would be very difficult to imagine4 ,the motion of ,donati's comet is what astronomer's call retrograde1 or from east to west4 ,it ceased to be visible in our northern latitudes in the last week in ,october1 having passed into the southern heavens1 where it will traverse the constellations of ,sagittarius1 ,telescopium1 and ,indus1 approaching the large star of ,toucan2 after which it will disappear until it has nearly completed its revolution round the sun4 ,the observed orbit of this remarkable comet coincides more nearly with an ellipse than a parabola2 the longer diameter of the ellipse being #ahd times that of the earth's orbit1 or the immense distance of #ce1ajj1jjj1jjj miles--a space which1 however great1 is less than the thousandth of the distance of the nearest fixed star4 ,according to the calculations of ,m4 ,loewy1 and adopting an elliptic orbit1 ,donati's comet will not return to the same places in the heavens for #bdie years1 being #ejj less than the period of revolution of the great comet of #ahaa4 ,signore ,donati observed that between the #beth and #cjth ,september two concentric1 luminous1 semicircular envelopes1 with a dark space between them1 were formed in the head4 ,from the extremities of these the cone of the tail extended1 and a non-luminous or dark space stretched for #bj from the nucleus into the tail4 ,on the #ast ,october the two envelopes were combined into one4 ,this comet1 like ,halley's1 has shown some singular irregularities1 supposed to arise from the action of the sun when near its perihelion4 ,at different periods of its apparition a violent agitation was observed in its nucleus1 with luminous jets1 spiral offshoots1 c41 as in the great comets of #afhj1 #agdd1 #ahaa4 ,a ray of light was thrown out from one side of the nucleus towards the sun1 while a gas-like jet proceeded from the other side1 which appeared to form the origin of a second tail within the great tail1 and which was traced for half a degree by ,mr4 ,hind on the #aith ,september4 ,he observed decided spiral convolutions in the tail1 which show that this comet has a rotatory motion about an axis passing through the tail4 ,if comets shine by borrowed light1 they ought1 in certain positions1 to exhibit phases like the moon2 but no such appearance has been detected1 except in one instance1 when they are said to have been observed by ,hevelius and ,la ,hire1 in the year #afhb4 ,in general1 the light of comets is dull--that of the comet of #ahaa was only equal to the tenth part of the light of the full moon--yet some have been brilliant enough to be visible in full daylight1 especially the comet of #agdd1 which was seen without a telescope at one o'clock in the afternoon1 while the sun was shining4 ,hence it may be inferred that1 although some comets may be altogether diaphanous1 others seem to possess a solid mass resembling a planet4 ,but whether they shine by their own or by reflected light has never been satisfactorily made out till now4 ,even if the light of a comet were polarized1 it would not afford a decisive test1 since a body is capable of reflecting light1 though it shines by its own4 ,m4 ,arago1 however1 has1 with great ingenuity1 discovered a method of ascertaining this point1 independent both of phases and polarization4 ,since the rays of light diverge from a luminous point1 they will be scattered over a greater space as the distance increases1 so that the intensity of the light on a screen two feet from the object is four times less than at the distance of one foot2 three feet from the object it is nine times less2 and so on1 decreasing in intensity as the square of the distance increases4 ,as a self-luminous surface consists of an infinite number of luminous points1 it is clear that1 the greater the extent of surface1 the more intense will be the light2 whence it may be concluded that the illuminating power of such a surface is proportional to its extent1 and decreases inversely as the square of the distance4 ,notwithstanding this1 a self-luminous surface1 plane or curved1 viewed through a hole in a plate of metal1 is of the same brilliancy at all possible distances as long as it subtends a sensible angle1 because1 as the distance increases1 a greater portion comes into view2 and1 as the augmentation of surface is as the square of the diameter of the part seen through the whole1 it increases as the square of the distance4 ,hence1 though the number of rays from any one point of the surface which pass through the hole decreases inversely as the square of the distance1 yet1 as the extent of surface which comes into view increases also in that ratio1 the brightness of the object is the same to the eye as long as it has a sensible diameter4 ,for example--,uranus is about nineteen times farther from the sun than we are1 so that the sun1 seen from that planet1 must appear like a star with a diameter of a hundred seconds1 and must have the same brilliancy to the inhabitants that he would have to us if viewed through a small circular hole having a diameter of a hundred seconds4 ,for it is obvious that light comes from every point of the sun's surface to ,uranus1 whereas a very small portion of his disc is visible through the hole2 so that extent of surface exactly compensates distance4 ,since1 then1 the visibility of a self-luminous object does not depend upon the angle it subtends as long as it is of sensible magnitude1 if a comet shines by its own light1 it should retain its brilliancy as long as its diameter is of a sensible magnitude2 and1 even after it has lost an apparent diameter1 it ought to be visible1 like the fixed stars1 and should only vanish in consequence of extreme remoteness4 ,that1 however1 is far from being the case--comets gradually become dim as their distance increases1 and vanish merely from loss of light1 while they still retain a sensible diameter1 which is proved by observations made the evening before they disappear4 ,it may therefore be concluded that comets shine by reflecting the sun's light4 ,the most brilliant comets have hitherto ceased to be visible when about five times as far from the sun as we are4 ,most of the comets that have been visible from the earth have their perihelia within the orbit of ,mars1 because they are invisible when as distant as the orbit of ,saturn3 on that account there is not one on record whose perihelion is situate beyond the orbit of ,jupiter4 ,indeed1 the comet of #agef1 after its last appearance1 remained five whole years within the ellipse described by ,saturn without being once seen4 ,more than a hundred and forty comets have appeared within the earth's orbit during the last century that have not again been seen4 ,if a thousand years be allowed as the average period of each1 it may be computed1 by the theory of probabilities1 that the whole number which range within the earth's orbit must be #adjj2 but1 ,uranus being about nineteen times more distant1 there may be no less than #aa1bjj1jjj comets that come within the orbit of ,uranus4 ,m4 ,arago makes a different estimate2 he considers that1 as thirty comets are known to have their perihelion distance within the orbit of ,mercury1 if it be assumed that comets are uniformly distributed in space1 the number having their perihelion within the orbit of ,uranus must be to thirty as the cube of the radius of the orbit of ,uranus to the cube of the radius of the orbit of ,mercury1 which makes the number of comets amount to #c1ebi1dgj4 ,but that number may be doubled1 if it be considered that1 in consequence of daylight1 fogs1 and great southern declination1 one comet out of two must be hid from us4 ,according to ,m4 ,arago1 more than seven millions of comets come within the orbit of ,uranus4 ,the different degrees of velocity with which the planets and comets were originally propelled in space is the sole cause of the diversity in the form of their orbits1 which depends only upon the mutual relation between the projectile force and the sun's attraction4 ,when the two forces are exactly equal to one another1 circular motion is produced2 when the ratio of the projectile to the central force is exactly that of #a to the square root of #b1 the motion is parabolic2 any ratio between these two will cause a body to move in an ellipse1 and any ratio greater than that of #a to the square root of #b will produce hyperbolic motion 7,n4 #bbi74 ,the celestial bodies might move in any one of these four curves by the law of gravitation3 but1 as one particular velocity is necessary to produce either circular or parabolic motion1 such motions can hardly be supposed to exist in the solar system1 where the bodies are liable to such mutual disturbances as would infallibly change the ratio of the forces1 and cause them to move in ellipses in the first case1 and hyperbolas in the other4 ,on the contrary1 since every ratio between equality and that of #a to the square root of #b will produce elliptical motion1 it is found in the solar system in all its varieties1 from that which is nearly circular to such as borders on the parabolic from excessive ellipticity4 ,on this depends the stability of the system2 the mutual disturbances only cause the orbits to become more or less excentric without changing their nature4 ,for the same reason the bodies of the solar system might have moved in an infinite variety of hyperbolas1 since any ratio of the forces1 greater than that which causes parabolic motion1 will make a body move in one of these curves4 ,hyperbolic motion is however very rare2 only two comets appear to move in orbits of that nature1 those of #agga and #ahbd2 probably all such comets have already come to their perihelia1 and consequently will never return4 ,the ratio of the forces which fixed the nature of the celestial orbits is thus easily explained2 but the circumstances which determined these ratios1 which caused some bodies to move nearly in circles and others to wander towards the limits of the solar attraction1 and which made all the heavenly bodies to rotate and revolve in the same direction1 must have had their origin in the primeval state of things2 but as it pleases the ,supreme ,intelligence to employ gravitation alone in the maintenance of this fair system1 it may be presumed to have presided at its creation4 ,,section ,,xxxvi4 ,the ,fixed ,stars--,their ,number--,the ,milky ,way--,double ,stars--,binary ,systems--,their ,orbits and ,periodic ,times--,colours of the ,stars--,stars that have vanished--,variable ,stars--,variation in ,sun's ,light--,parallax and ,distances of the ,fixed ,stars--,masses of the ,stars--,comparative ,light of the ,stars--,proper ,motions of the ,stars--,apparent ,motions of the ,stars--,motion and ,velocity of the ,sun and ,solar ,system--,the ,nebul--,their ,number--,catalogue of them--,consist of ,two ,classes--,diffuse ,nebul--,definitely formed ,nebul--,globular ,clusters--,splendour of ,milky ,way--,distribution of the ,nebul--,the ,magellanic ,clouds--,nebul round ,args--,constitution of ,nebul1 and the ,forces that maintain them--,meteorites and ,shooting ,stars4 ,,great as the number of comets appears to be1 it is absolutely nothing in comparison of the multitude of the fixed stars4 ,about #bjjj only are visible to the naked eye2 but when the heavens are viewed through a telescope1 their number seems to be limited only by the imperfection of the instrument4 ,the number registered amounts to #bjj1jjj2 their places are determined with great precision1 and they are formed into a catalogue1 not only for the purpose of ascertaining geographical positions by the occultations of the brightest among them1 but also to serve as points of reference for marking the places of comets and other celestial phenomena4 ,sirius1 ,centauri1 and ,arcturus are the brightest stars in the heavens2 the others are classed according to their apparent lustre1 from the first to the seventeenth magnitudes4 ,capella1 ,lyr1 ,procyon1 and twenty or twenty-one more1 are of the first magnitude2 ,persei1 ,orionis1 ,cygni1 and in all fifty or sixty1 are of the second2 and of the third there are about #bjj1 such as ,bootis and ,draconis1 the numbers increasing as the magnitude diminishes4 ,those of the eighth magnitude are scarcely visible to the naked eye1 and it requires a very good telescope to see stars of the seventeenth4 ,this sequence is perfectly arbitrary2 but ,sir ,john ,herschel has ascertained by actual measurement the comparative lustre of a great many--for example1 he found that the light of a star of the sixth magnitude is #ajj times less than that of one of the first magnitude1 and that ,sirius would make between three and four hundred of such little stars4 ,were the photometric scale completed1 it would be of the greatest importance with regard to the variable stars4 ,the three or four brightest classes of stars are scattered pretty equably over the sky1 with the exception of a zone or belt following the course of the great circle passing through ,orionis and ,crucis1 where they are very numerous1 especially in the southern hemisphere4 ,the stars of all magnitudes visible to the naked eye increase in numbers towards the borders of the ,milky ,way1 which derives its lustre and name from the diffused light of myriads of stars2 so numerous are they in some parts of it that more than #ej1jjj passed through the field of ,sir ,william ,herschel's telescope in the course of an hour1 in a zone only two degrees broad2 in many places they are numerous beyond estimation1 and most of them are extremely small on account of their enormous distances4 ,the ,milky ,way1 which forms so conspicuous a part of the firmament1 is a vast and somewhat flattened stratum or congeries of stars1 encircling the heavens in a broad band1 split through one part of its circumference into two streams of stars1 bearing a strong resemblance to fig4 #e1 plate #e4 ,it is contorted and broken in some places1 and occasionally lengthened into branches stretching far into space4 ,its thickness is small compared with its length and breadth2 yet in some places it is unfathomable even with the best telescopes2 in others there is reason to believe that it is possible to see through it1 and even beyond it1 in its own plane4 ,there is a gradual but rapid increase in the crowding of the stars on each side of the flat stratum towards the centre4 ,the solar system is deeply though excentrically plunged into this mass of stars1 near that point where the circular stratum splits into two streams4 ,sir ,john ,herschel's description of the stars of the southern hemisphere shows that the ,milky ,way is a most magnificent object there4 8,the general aspect of the southern circumpolar regions 7including in that expression #fj or #gj of south polar distance7 is in a high degree rich and magnificent1 owing to the superior brilliancy and large development of the ,milky ,way1 which1 from the constellation of ,orion to that of ,antinous1 is a blaze of light1 strangely interrupted1 however1 with vacant and entirely starless patches1 especially in ,scorpio1 near ,centauri and the ,cross1 while to the north it fades away pale and dim1 and is in comparison hardly traceable4 ,i think it is impossible to view this splendid zone1 with the astonishingly rich and evenly distributed fringe of stars of the #crd and #dth magnitude1 which forms a broad skirt to its southern border like a vast curtain1 without an impression amounting almost to conviction1 that the ,milky ,way is not a mere stratum1 but annular1 or at least that our system is placed within one of the poorer or almost vacant parts of its general mass1 and that eccentrically1 so as to be much nearer to the region about the ,cross than to that diametrically opposite to it40 ,those dark vacuities called 8,coal ,sacks0 by the ancient navigators1 which are so numerous between ,centauri and ,antaris1 are among the most extraordinary phenomena in the southern hemisphere2 they are of intense blackness1 though by no means void of extremely small telescopic stars2 the darkness arises from the contrast these nearly vacant spaces form with the excessive brilliancy of the surrounding part of the ,milky ,way1 and the sudden sharp transition from light to darkness4 ,the largest and most conspicuous of them is a pear-shaped vacuity close to the ,southern ,cross4 ,that portion of the ,milky ,way that is split longitudinally through its centre lies between ,centauri and the constellation of ,cygnus3 the two bands are joined here and there by narrow bridges of condensed stars1 stretching across the darker space between them4 ,in ,scorpio and ,sagittarius ,sir ,john ,herschel describes the ,milky ,way as composed of definite clouds of light running into clusters of extremely minute stars like sand1 not strewed evenly as with a sieve1 but as if thrown down by handfuls1 and by both hands at once1 leaving dark intervals4 ,in this astonishing profusion the stars are of all sizes1 from the #adth to the #bjth magnitude1 and even down to nebulosity4 ,after an interval the same profusion is renewed1 the stars being inconceivably minute and numerous beyond description--they are in millions and millions4 ,thus there is great irregularity in their diffusion as well as magnitude--in some places intensely crowded1 in others the deep blackness of the sky1 over which they are thinly scattered1 irresistibly led to believe that in these regions the power of our telescopes fairly penetrates through the starry stratum1 and beyond it4 ,sometimes we look through a sheet of stars nearly of the same size1 of no great thickness compared with their distance from us1 and not unfrequently there is a double stratum1 one of large stars spread over another of very small ones4 ,the most southerly of the two streams of stars which form the ,milky ,way in this part of the firmament maintains an unbroken course of extreme brilliancy1 containing some of the finest clusters of stars in the heavens4 ,one round ,sagittarii is an intense aggregate of stars1 in some parts of which they are so crowded as to exceed enumeration2 at a very moderate estimate ,sir ,john ,herschel thinks this group cannot contain fewer than a hundred thousand stars4 ,other two groups between the constellations of the ,shield and ,ophiuchus stand out like promontories of intense brilliancy in the dark space that separates the starry streams of the ,milky ,way4 ,the distance of the fixed stars is too great to admit of their exhibiting a sensible disc1 but they must be spherical if gravitation pervades all space1 as there is every reason to believe it does4 ,with a powerful telescope the stars are like points of light3 their occultations by the moon are therefore instantaneous4 ,their twinkling arises from sudden changes in the refractive power of the air1 which would not be sensible if they had discs like planets4 ,thus nothing can be known of their distance from us or from one another by their apparent diameters4 ,although from the appearance of the stars no inference can be drawn as to their distance1 yet among the multitudes in the heavens a few are found near enough to exhibit distinct parallactic motions arising from the revolution of the earth in its orbit1 from whence their distance from the sun has been computed3 ,centauri1 the brightest star in the southern hemisphere1 is a very remarkable instance4 ,professor ,henderson at the ,cape of ,good ,hope determined its parallax to be #a by a series of observations on its position at opposite periods of the year1 that is1 from opposite points in the earth's orbit4 ,the result was afterwards confirmed by ,mr4 ,maclear1 who found the amount to be #jiac4 ,the difference between the two is wonderfully small1 considering the many unavoidable sources of error in the determination of such minute quantities 7,n4 #bcj74 ,since no star in the northern hemisphere has so great an amount of parallax1 an arc of #a is assumed as the parallactic unit4 ,now radius is to the sine of #a as #bjf1bfe is to #a2 hence1 ,centauri is #bjf1bfe times more distant from the sun than the sun is from the earth4 ,light flying at the rate of #aib1jjj miles in a second must take #c years and #hc days to come to us from that star4 ,one or two tenths of a second becomes a very great error when the maximum amount of parallax is only #a1 and on that account1 with the exception of ,centauri1 it has been found impracticable to determine the annual changes in the apparent motions of single stars affected by precession1 nutation1 aberration1 and the variations of temperature of the instruments used in observing4 ,however1 as two stars in juxtaposition are equally affected by all of these2 the difference in their motions is independent of them4 ,of two stars apparently in close approximation1 one may be far behind the other in space4 ,they may seem near to one another when viewed from the earth in one part of its orbit1 but may separate widely when seen from the earth in another position1 just as two terrestrial objects appear to be one when viewed in the same straight line1 but separate as the observer changes his position4 ,in this case the stars would not have real1 but only apparent motion4 ,one of them would seem to oscillate annually to and fro in a straight line on each side of the other1 a motion that could not be mistaken for that of a binary system where one star describes an ellipse about the other2 or if the edge of the orbit be turned towards the earth1 where the oscillations require years for their accomplishment4 ,the only circumstances that can affect the stars unequally1 and which must be eliminated1 are the proper motion of the stars in space1 and specific aberration1 a very minute quantity arising from peculiarities in the star's light4 ,this method of finding the distances of the fixed stars was proposed by ,galileo and attempted by ,dr4 ,long without success4 ,sir ,william ,herschel afterwards applied it to some of the binary groups2 and although he did not find the thing he sought for1 it led to the discovery of the orbital motions of the double stars4 ,m4 ,struve was the first to apply this method1 and that in a very difficult case4 ,he perceived that a very small star is close to ,lyr1 and by a series of most accurate differential measurements from #ahce to #ahch he found that ,lyr has a parallax of #jbfa1 which was afterwards corroborated by the observations of ,m4 ,peters2 hence ,lyr is #ghi1fjj times more distant from the sun than the earth is4 ,it was natural to suppose that in general the large stars are nearer to the earth than the small ones2 but there is now reason to believe that some stars1 though by no means brilliant1 are nearer to us than others which shine with greater splendour4 ,this is inferred from the comparative velocity of their proper motions2 all the stars have a general motion of translation1 which tends ultimately to mix those of the different constellations2 but none that we know of moves so rapidly as #fa ,cygni1 and on that account it was reckoned to be nearer to us than any other1 for an object seems to move more quickly the nearer it is4 ,now ,m4 ,bessel saw that two minute and probably very remote stars are very near #fa ,cygni1 their directions from that star being at right angles to one another2 so that1 during the revolution of the earth1 one of these distances was a maximum and the other a minimum alternately every three months4 ,this alternation1 although it indicated a parallax or difference of parallaxes of only #jcdh1 was maintained with such perfect regularity every three months1 that it leaves not a doubt of its accuracy1 which was afterwards confirmed by the observations of ,m4 ,peters at ,polkova4 ,it follows from that small parallax that #fa ,cygni must be #eib1gjj times farther from the earth than the sun is--a distance that light would not pass over in less than nine years and three months4 ,mr4 ,henderson found the parallax of ,sirius1 the brightest of all the stars1 to be only #jbcj2 it is consequently more distant than #fa ,cygni1 though the latter is but of the #fth magnitude4 ,m4 ,argelander has calculated that the apparent magnitude of the stars depends upon their distance4 ,supposing them all to be of the same size1 the smallest visible in ,sir ,william ,herschel's #bj feet reflecting telescope1 namely those of the #agth magnitude1 would be #bbh times farther off than those of the first magnitude2 and ,m4 ,peters of ,polkova from the annual parallax of thirty-five1 seven of which are now very accurately determined1 has ascertained the distance of the nearest of them to be such1 that light flying at the rate of #ie millions of miles in a second would take #ae years and a half to come from them to the earth1 and that a star of the #agth magnitude might be extinguished for #ceda years before we should be aware of it4 7,n4 #bca47 ,the great gulfs that separate the stars from the sun1 and probably from one another1 no doubt maintain the stability of the stellar system1 in the same manner that in the solar system the distances of the planets from the sun and the satellites from their primaries are so arranged as to preserve their mutual disturbances within due limits4 ,the stars supposed to be nearest the sun are probably in a great zone which crosses the ,milky ,way between ,args and ,crucis4 ,it comprises the bright stars of the constellations ,orion1 ,canis ,major1 the ,southern ,cross1 ,centaur1 ,lupus1 and ,scorpio4 ,the axis of the zone is inclined at an angle of #bj to the medial line1 or circle1 passing through the centre of the ,milky ,way4 ,a very great number of stars undergo periodical changes of lustre1 varying in some cases from complete extinction to their original brilliancy1 strongly suggesting the idea that they are temporarily obscured1 and sometimes completely hid1 by opaque bodies revolving round them in regular periodic times1 as the planets do about the sun4 ,the star ,mira1 or ,ceti1 which was first noticed to be periodical by ,fabricius1 in #aeif1 appears about twelve times in eleven years1 or in periods of #cca;d #h;h #dm #afs2 it remains at its greatest brightness about a fortnight1 being then on some occasions equal to a large star of the second magnitude2 then it decreases during about three months1 till it becomes completely invisible to the naked eye1 in which state it remains about five months2 after that it continues increasing during the remainder of its period4 ,such is the general course of its changes2 but it does not always return to the same degree of brightness1 nor increase and diminish by the same gradations1 neither are the successive intervals of its maxima equal4 ,from the observations and investigations of ,m4 ,argelander1 the mean period given is subject to fluctuation1 embracing #hh such periods1 and having the effect of gradually lengthening and shortening alternately those intervals to the extent of #be days one way and the other4 ,the irregularities in the degree of brightness attained at the maximum are probably also periodical4 ,for four years previous to #afgf it did not appear at all2 and on ,october #e1 #ahci1 it exceeded ,ceti1 and equalled ,aurig1 in lustre4 ,these irregularities may be occasioned by periodical perturbations among opaque bodies revolving about the star4 ,algol1 or ,persei1 is another very remarkable instance of a variable star4 ,it has the size of a star of the second magnitude for two days and thirteen and a half hours1 and then suddenly begins to diminish in splendour1 till1 in about three hours and a half1 it is reduced to the size of a star of the fourth magnitude2 it then begins again to increase1 and in three hours and a half more regains its brightness1 going through all these vicissitudes in #b;d #bj;h #dhm #eds#g4 ,sir ,john ,herschel and ,mr4 ,goodricke1 by whom the variable nature of this star was discovered in #aghb1 considered this to be a case strongly indicative of the revolution of an opaque body1 which1 coming between us and ,algol1 cuts off a large portion of the light4 ,this star has been constantly observed1 and the more recent observations1 compared with the ancient ones1 indicated a diminution in the periodic time4 ,it is even proved that this decrease is not uniformly progressive1 but is actually proceeding with accelerated rapidity1 which1 however1 will probably not continue1 but will by degrees relax1 and then be changed into increase1 according to the laws of periodicity1 which1 as well as their causes1 remain to be discovered4 ,the first minimum of this star1 in #ahdd1 happened on ,january #crd1 at #d;h #adm ,greenwich time4 ,hydr also vanishes and reappears every #did days4 ,lyr was discovered to be variable1 in #aghd1 by ,mr4 ,goodricke1 and its period was ascertained by ,argelander to be #ab;d #ba;h #ecm #ajs1 in which time a double maximum and minimum takes place1 the two maxima being nearly equal1 but the two minima unequal2 besides these semi-periods1 there is a slow aberration of period1 which appears to be periodical itself3 from its discovery to #ahdj the time was continually lengthening1 but more and more slowly1 till1 in #ahdj1 it ceased to increase1 and has since been slowly on the decrease4 ,the stars ,cephei and ,aquil1 or ,antinoi1 were discovered to be variable in #aghd2 their respective periods1 being #e;d #h;h #dgm #cis and #g;d #d;h #acm #ecs1 have since been accurately determined4 ,besides these1 the variations of between #cj and #dj have been approximately ascertained1 and a great many more among the smaller stars have been discovered to be variable by ,mr4 ,hind1 who has remarked that many of those stars which continue visible at their minimum appear hazy and indistinct1 as though some cloudy or nebulous medium intervened4 ,some of the variable stars are red1 and others present successive changes through blue1 yellow1 and red4 ,when the brightness is increasing the star has a blueish tinge1 when it is past its maximum lustre it assumes a yellow tint1 and while on its decrease it becomes ruddy with flashes of bright red light4 ,these changes are very marked in a small star near the star #gg1 at the extremity of the south wing of ,virgo4 ,sir ,john ,herschel1 after having described the glory of the starry heavens1 asks1 8,for what purpose are we to suppose such magnificent bodies scattered through the abyss of space8 ,surely not to illuminate our nights1 which an additional moon of the thousandth part the size of our own would do much better1 nor to sparkle as a pageant void of meaning and reality1 and bewilder us with vain conjectures4 ,useful1 it is true1 they are to man as points of exact and permanent reference2 but he must have studied astronomy to little purpose who can suppose man to be the only object of his ,creator's care1 or who does not see in the vast and wonderful apparatus around us provision for other races of animated beings4 ,the planets1 we have seen1 derive their light from the sun1 but that cannot be the case with the stars4 ,these doubtless then are themselves suns1 and may perhaps1 each in its sphere1 be the presiding centre round which other planets or bodies1 of which we can form no conception from any analogy offered by our own system1 may be circulating40 ,another circumstance shows how probable it is that dark bodies are revolving among the stars4 ,the proper motion of ,sirius is very irregular--sometimes it is rapid1 and at other times slow2 the cause is ascribed by ,,mm4 ,bessel and ,peters to a dark companion which revolves with ,sirius about their common centre of gravity1 and by its attraction disturbs the equable motion of the star4 ,sometimes stars have all at once appeared1 shone with a bright light1 and vanished4 ,several instances of these temporary stars are on record4 ,a remarkable one occurred in the year #abe1 which is said to have induced ,hipparchus to form the first catalogue of stars4 ,another star appeared suddenly near ,aquil in the year #chi1 which vanished after remaining for three weeks as bright as ,venus4 ,on the #ajth of ,october1 #afjd1 a brilliant star burst forth in the constellation of ,serpentarius1 which continued visible for a year2 and on the #aath of ,november1 #aegb1 a star all at once shone forth in ,cassiopeia1 which rapidly increased in brightness till it surpassed that of ,jupiter so much as to be visible at midday4 ,it began to decrease in ,december of the same year1 and1 in ,march1 #aegd1 it had entirely disappeared1 having exhibited a variety of tints4 ,it is suspected1 however1 that this star is periodically variable and identical with stars which appeared in the years #ide and #abfd4 ,a more recent case occurred in the year #afgj1 when a new star was discovered in the head of the ,swan1 which1 after becoming invisible1 reappeared1 and1 having undergone many variations in light1 vanished after two years1 and has never since been seen4 ,on the #bhth of ,april1 #ahdh1 ,mr4 ,hind discovered a star of the #eth magnitude in the constellation ,ophiuchus1 which was very conspicuous to the naked eye1 and where he was certain no star even so bright as the #ith magnitude had ever existed1 nor was there any record of such a star4 ,from the time of its discovery it continued to diminish till it became extinct4 ,its colour was ruddy1 and was thought to undergo remarkable changes1 probably an effect of its low position1 as its polar distance was #ajb #ci #ad4 ,sir ,john ,herschel discovered very singular variations in the star of the constellation ,argo4 ,it is surrounded by a wonderful nebula1 and between the years #afgg and #ahbf it varied twice from the #dth to the #bnd magnitude2 but in the beginning of #ahch it suddenly increased in lustre1 so as to be nearly as bright as ,centauri4 ,thence it diminished1 but not below the first magnitude till ,april #ahdc1 when it had again increased1 so as to surpass ,canopus1 and nearly equal ,sirius in splendour4 ,with regard to this singular phenomenon1 ,sir ,john ,herschel observes1 that 8,temporary stars heretofore recorded have all become totally extinct4 ,variable stars1 as far as they have been carefully attended to1 have exhibited periodical and regular alternations 7in some degree at least7 of splendour and comparative obscurity2 but here we have a star fitfully variable to an astonishing extent1 and whose fluctuations are spread over centuries1 apparently in no settled period1 and in no regular progression4 ,what origin can we ascribe to these sudden flashes and relapses8 ,what conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so variable a source8 ,its future career will be a subject of high physical interest4 ,to this account ,i will only add1 that in the beginning of #ahch the brightness of this star was so great as materially to interfere with the observations of that part of the nebula surrounding it40 ,sir ,john has also discovered that ,orionis is variable1 a circumstance the more remarkable as it is one of the conspicuous stars of our hemisphere1 and yet its changes had never been remarked4 ,the inferences ,sir ,john draws from the phenomena of variable stars are too interesting not to be given in his own words4 8,a periodic change existing to so great an extent in so large and brilliant a star as ,orionis cannot fail to awaken attention to the subject1 and to revive the consideration of those speculations respecting the possibility of a change in the lustre of our sun itself1 which were first put forth by my father4 ,if there be really a community of nature between the sun and the fixed stars1 every proof that we obtain of the extensive prevalence of such periodical changes in those remote bodies adds to the probability of finding something of the kind nearer home4 ,if our sun were ever intrinsically much brighter than at present1 the mean temperature of the surface of our globe would of course be proportionally greater4 ,i speak now not of periodical1 but secular changes4 ,but the argument is complicated with the consideration of the possible imperfect transparency of space1 which may be due to material non-luminous particles1 diffused irregularly in patches analogous to nebul1 but of great extent--to cosmical clouds1 in short1 of whose existence we have1 ,i think1 some indication in the singular and apparently capricious phenomena of temporary stars1 and perhaps in the recent extraordinary increase1 and hardly less sudden diminution1 of ,args40 ,mr4 ,hind has come to the same conclusion with ,goodricke and ,sir ,john ,herschel1 that the changes in the variable stars are owing to opaque bodies revolving round them2 indeed there are strong reasons to believe that there are solar systems analogous to our own in the remote regions of space4 ,our sun requires nine times the period of ,algol to perform a revolution on its axis1 while1 on the other hand1 the periodic time of an opaque revolving body1 sufficiently large to produce a similar temporary obscuration of the sun seen from a fixed star1 would be less than fourteen hours4 ,it is possible that the decrease of light in some of the variable stars may arise from large spots on their surface1 like those occasionally seen in the radiant fluid masses on the surface of the sun4 ,one of these spots which was measured by ,sir ,john ,herschel on the #bjth of ,march1 #ahcf1 with its penumbra1 occupied an area of #cghj millions of square miles2 and the black central part of a spot that appeared on the #beth of ,may following would have allowed the globe of the earth to drop through it1 leaving a thousand miles clear of contact all around this tremendous abyss4 ,all the variable stars on record of which the places are distinctly indicated have occurred without exception in1 or close upon1 the borders of the ,milky ,way1 and that only within the following semicircle1 the preceding having offered no example of the kind4 ,many stars have actually disappeared from the heavens4 #db ,virginis seems to be of the number1 having been missed by ,sir ,john ,herschel on the #ith of ,may1 #ahbh1 and not again found1 though he frequently had occasion to observe that part of the sky4 ,mr4 ,cooper1 of the ,markree ,observatory1 has given a list of fifty stars that are missing since the publication of his list of stars in #ahdg4 ,comparing the present state of the heavens with more ancient catalogues1 a much greater number have disappeared4 ,thousands of stars that seem to be only brilliant points of light1 when carefully examined are found to be in reality systems of two or more suns1 many of which are known to revolve about one another4 ,these binary and multiple systems are very remote1 requiring powerful telescopes to show the stars separately4 ,they are divided into eight classes1 according to the proximity of the two stars4 ,the first class comprises only such as are less than #a of space apart2 those of the second class are more apart than #a and less than #b1 c4 c4 ,sometimes the two stars are of equal magnitude1 but more frequently a conspicuous star is accompanied by a smaller companion4 ,in some cases the conspicuous star itself is double1 as in ,cancri1 ,scorpio1 #aa ,monocerotis1 and #ab ,lyncis1 which are triple stars4 ,each of the two stars of ,lyr is a beautiful and close double star2 so that which in a common telescope appears merely to be a double star1 is found to be quadruple with a very excellent instrument4 ,the multiple system of ,orionis is one of the most remarkable objects in our hemisphere4 ,to the naked eye and with an ordinary telescope it seems to be a single star1 but it really consists of four brilliant stars forming a trapezium1 and accompanied by two excessively minute and very close companions1 to perceive both of which is the severest test of a telescope4 ,the first catalogue of double stars in which their places and relative positions are given was accomplished by the talent and industry of ,sir ,william ,herschel1 who made so many great discoveries1 and with whom the idea of their combination in binary and multiple systems originated2 and that important fact he established by the discovery of a revolving motion in #ej or #fj1 and by the determination of the revolution of one star about the other of ,castor or ,geminorum1 the largest and finest double star in the northern hemisphere4 ,he even assigned the approximate periodic times of this and of several other binary systems4 ,more than #ajj stars are now known to be stellar systems4 ,the positions of many hundreds were measured by ,sir ,john ,herschel and ,sir ,james ,south2 and the catalogue of the double stars in the northern hemisphere1 which have been micrometrically measured1 has been increased to more than #fjjj by ,,mm4 ,bessel1 ,struve1 and ,british astronomers4 ,extensive catalogues of double stars in the southern hemisphere have been published by the astronomers in our colonial establishments4 ,to these ,sir ,john ,herschel added #ajha during his residence at the ,cape of ,good ,hope3 the angles of position and distances of the stars from one another he measured1 and found that many of them have very rapid orbital motions4 ,the elliptical elements of the orbits and periodic times of fifteen have been determined by the most eminent astronomers with wonderful accuracy1 considering the enormous distances and the extreme delicacy and difficulty of the subject4 ,m4 ,savary has the merit of having first determined the elements of the orbit of a double star from observation4 ,the difficulty of doing so is great1 because the nearest fixed star is #baa1jjj times farther from the sun than the earth is1 and the orbit itself is only visible with the best telescopes2 consequently a very small error in observation occasions an enormous error in the determination of quantities at that distance4 ,in observing the relative position of the stars of a binary system1 the distance between them1 and also the angle of position1 that is1 the angle which the meridian1 or a parallel to the equator1 makes with the line joining the two stars1 are measured4 ,the different values of the angle of position show whether the revolving star moves from east to west1 or the contrary2 whether the motion be uniform or variable1 and at what points it is greatest or least4 ,the measures of the distances show whether the two stars approach or recede from one another4 ,from these the form and nature of the orbit are determined4 ,were observations perfectly accurate1 four values of the angle of position1 and of the corresponding distances at given epochs1 would be sufficient to assign the form and position of the curve described by the revolving star2 this1 however1 scarcely ever happens4 ,the accuracy of each result depends upon taking the mean of a great number of the best observations1 and eliminating error by mutual comparison4 ,the distances between the stars are so minute that they cannot be measured with the same accuracy as the angles of position2 therefore1 in order to determine the orbit of a star independently of the distance1 it is necessary to assume1 as the most probable hypothesis1 that the stars are subject to the law of gravitation1 and consequently that one of the two stars revolves in an ellipse about the other1 supposed to be at rest1 though not necessarily in the focus4 ,a curve is thus constructed graphically by means of the angles of position and the corresponding times of observation4 ,the angular velocities of the stars are obtained by drawing tangents to this curve at stated intervals1 whence the apparent distances1 or radii vectores of the revolving star1 become known for each angle of position1 because1 by the laws of elliptical motion1 they are equal to the square roots of the apparent angular velocities4 ,now that the angles of position estimated from a given line1 and the corresponding distances of the two stars1 are known1 another curve may be drawn1 which will represent on paper the actual orbit of the star projected on the visible surface of the heavens2 so that the elliptical elements of the true orbit1 and its position in space1 may be determined by a combined system of measurements and computation4 ,but1 as this orbit has been obtained on the hypothesis that gravitation prevails in these distant regions1 which could not be known priori1 it must be compared with as many observations as can be obtained1 to ascertain how far the computed ellipse agrees with the curve actually described by the star4 ,virginis consists of two stars of nearly the same magnitude2 they were so far apart in the beginning and middle of last century1 that they were mentioned by ,bradley1 and marked in ,mayer's catalogue1 as two distinct stars4 ,since that time they have been continually approaching each other1 till in ,january1 #ahcf1 one star was seen to eclipse the other1 by ,admiral ,smyth at his ,observatory at ,bedford1 and by ,sir ,john ,herschel at the ,cape of ,good ,hope4 ,a series of observations since the beginning of the present century has enabled ,sir ,john to determine the form and position of the elliptical orbit of the revolving star with extraordinary truth by the preceding method4 ,according to his calculation1 it came to its perihelion on the #ahth of ,august of the year #ahcd4 ,its previous velocity was so great that the revolving star described an angle of #fh in one year4 ,by the laws of elliptical motion its angular velocity must diminish till it arrives at its aphelion4 ,the accuracy with which the motions of the binary systems are measured1 and the skill employed in the deduction of the elliptical elements1 are now so great1 that the periodic time of ,virginis1 determined by ,sir ,john ,herschel and ,admiral ,smyth from their respective observatories1 combined with those of ,sir ,william ,herschel1 only differ by two years1 ,sir ,john having obtained a period of #ahb years1 ,admiral ,smyth that of #ahj4 ,by the aid of more numerous observations ,mr4 ,fletcher has found that the true period is #ahdec years1 and that the revolving star passed its perihelion in #ahcg4 ,it is by such successive steps that astronomy is brought to perfection 7,n4 #bcb74 ,some of the double stars have very long periods1 such as ,coron1 where the revolving star takes #gcg years nearly to accomplish a circuit4 ,others again have very short periods1 as ,coron1 ,cancri1 and ,urs ,majoris1 whose periodic times are #dbejj1 #ehia1 and #ehbf years respectively3 therefore each of these has performed more than one entire revolution since their motions were observed4 ,herculis1 whose periodic time is only about #cj-a_/d years1 has accomplished two complete circuits1 the lesser star having been eclipsed by the greater each time4 ,the first of these two truly wonderful events1 of one sun eclipsing another sun1 was seen by ,sir ,william ,herschel in #aghb4 ,the orbits and periodic times of so many of these binary systems having been determined proves beyond a doubt that sun revolves about sun in the starry firmament by the same law of gravitation that makes the earth and planets revolve about the sun 7,n4 #bcb74 ,since the parallax of #fa ,cygni and that of ,centauri have been determined1 ,sir ,john ,herschel has made the following approximation to the dimensions of their orbits and masses4 ,the distance between the two stars of #fa ,cygni1 that is the radius vector of the revolving star1 has hardly varied from #aee ever since the earliest observations2 while in that time the star has moved through #ej2 it is evident therefore that the orbit must be nearly circular4 ,it is at right angles to the visual ray1 and the periodic time is #ead years4 ,the parallax or radius of the earth's orbit as seen from the star is #jcdh1 while the radius of the star's orbit as seen from the earth is #aee2 hence the radius of the star's orbit is to that of the earth's orbit as #aee to #jcdh1 or nearly as #de to #a4 ,so the orbit described by the two stars of #fa ,cygni about one another greatly exceeds that which ,neptune describes about the sun4 ,since the mean distance of the stars and their periodic time are given1 the sum of the masses of the two stars is computed to be #jcebi1 that of the sun being #a4 ,thus our sun is not vastly greater nor vastly less than the stars composing #fa ,cygni1 which is a small inconspicuous star to the naked eye1 not exceeding the #fth magnitude4 ,of all the double stars ,centauri is the most beautiful3 it is the brightest star in the southern hemisphere1 equal1 if not superior1 to ,arcturus in lustre4 ,the distance between the two stars has been decreasing at the rate of half a second annually since the year #ahbb1 while the angular motion has undergone very little change1 which shows that the plane of the orbit passes through the earth like the orbits of #dd ,botes1 and ,serpentarii2 that is to say1 the edge of the orbit in these three stellar systems is presented to the earth1 so that the revolving star seems to move in a straight line1 and to oscillate on each side of its primary4 ,were this libration owing to parallax1 it would be annual from the revolution of the earth about the sun2 but as years elapse before it amounts to a sensible quantity1 it can only arise from a real orbital motion seen obliquely4 ,in this case five observations are sufficient for the determination of the orbit1 provided they be exact2 but the quantities to be measured are so minute1 that it is only by a very long series of observations that accuracy can be attained4 ,in #ahcd ,captain ,jacob determined the periodic time of the revolving star of ,centauri to be #gg years1 and the distance between the two to be #age2 and since the decrease is half a second annually1 the distance or radius vector of the revolving star was #abe in the year #ahbb2 and as ,mr4 ,henderson had determined the parallax or radius of the earth's orbit as seen from the star to be #iac1 it follows that the real semi-axis of the revolving star's orbit is #ac-a_/b times greater than the semi-axis of the earth's orbit as a minimum4 ,the real dimensions of the ellipse therefore cannot be so small as the orbit of ,saturn1 and may possibly exceed that of ,uranus4 ,it is very probable that an occultation of one of the suns by the other will take place in #ahfg1 or a very close appulse of the two stars4 ,singular anomalies have appeared in the motions of #gj ,ophiuchi1 which was discovered to be a binary system by ,sir ,william ,herschel in #aggi1 and which has since nearly accomplished a revolution4 ,various orbits have been computed3 those which best represent the angles of position fail with regard to the distances of the stars from one another1 and vice vers4 ,but it is a very remarkable fact that the errors are periodical1 being for considerable periods of time alternately in excess and defect4 ,captain ,w4 ,s4 ,jacob1 who determined the periodic time of the revolving star to be #ic years1 attributes this anomaly to the disturbing action of an opaque body revolving round the lesser star4 ,assuming that to be the case1 and computing1 he found that the errors were considerably diminished both in the angle of position and distance4 ,it is a subject of the highest interest1 and well worthy of the attention of such astronomers as have the means of making the necessary observations4 ,among the triple systems1 as ,cancri1 two of the stars revolve about one another in #ehi years2 but the motion of the third and most distant is so slow1 that it has only accomplished a tenth part of its revolution about the other two since the system was discovered4 ,it appears from the calculations of ,mr4 ,dunlop that ,eridani accomplishes a revolution in little more than #cj years4 ,the motion of ,mercury is more rapid than that of any of the planets1 being at the rate of #ajg1jjj miles an hour4 ,the perihelion velocity of the comet of #afhj was #hhj1jjj miles an hour2 but1 if the two stars of ,eridani1 or of ,urs ,majoris1 be as remote from one another as the nearest fixed star is from the sun1 the velocity of the revolving star must exceed the power of imagination to conceive4 ,the elliptical motion of the double stars shows that gravitation is not confined to the planetary motions1 but that systems of suns in the far distant regions of the universe are also obedient to its laws4 ,the stellar systems present a kind of sidereal chronometer1 by which the chronology of the heavens will be marked out to future ages by epochs of their own1 liable to no fluctuations from such disturbances as take place in our system4 ,some stars are apparently double1 though altogether unconnected1 one being far behind the other in space1 as ,lyr1 which apparently consists of two stars1 one of the first1 the other of the eleventh magnitude4 ,aldebaran1 ,aquil1 and ,pollux are remarkable instances of these optically double stars4 ,it has been shown how favourable that circumstance is for ascertaining the parallax of the nearest of the two4 7,n4 #bcb47 ,the double stars are of various hues3 sometimes both stars are of the same colour1 as in ,centauri and #fa ,cygni1 where the larger stars are of a bright orange and the smaller ones a deeper tint of the same1 but they most frequently exhibit the contrasted colours4 ,the large star is generally yellow1 orange1 or red2 and the small star blue1 purple1 or green4 ,sometimes a white star is combined with a blue or a purple1 and more rarely a red and white are united4 ,in many cases these appearances are due to the influence of contrast on our judgment of colours4 ,for example1 in observing a double star1 where the large one is a full ruby red1 or almost blood colour1 and the small one a fine green1 the latter loses its colour when the former is hid by the cross wires of the telescope4 ,that is the case with ,andromed1 which is a triple star1 the small one1 which appears green1 being closely double4 ,cancri is an instance of a large yellow star and a small one which appears blue by contrast4 ,still there are a vast number where the colours are decidedly different1 and suggest the curious idea of two suns1 a red and a green1 or a yellow and a blue1 so that a planet circulating round one of them may have the variety of a red day and a green day1 a yellow day and a blue day4 ,sir ,john ,herschel observes1 in one of his papers in the ,philosophical ,transactions1 as a very remarkable fact1 that1 although red stars are common enough1 no example of a solitary blue1 green1 or purple star has yet been produced4 ,sirius is the only star on record whose colour has changed4 ,in the time of ,ptolemy it was red2 now it is one of the whitest stars in the heavens4 ,m4 ,struve has found that1 out of #eif bright double stars1 #cge pairs have the same intensity of light and colour2 #aja pairs have different intensity1 but the same colour2 and #abj pairs have the colours of the two stars decidedly different4 ,certain rays1 which exist in the sun's light1 are wanting in the spectra of every coloured star1 and probably never existed in the light of these stars1 as there is no reason to believe that they are absorbed by the stars' atmosphere1 though they may be by the earth's4 ,there are no defective rays in the white light of ,sirius1 ,procyon1 and others2 but ,sir ,david ,brewster found in the spectrum of the orange-coloured light of ,herculis a defective band in the red space1 and two or more in the blue2 consequently1 the orange colour of the star is owing to a want of blue rays2 for flames in which certain rays are wanting take the colour of the predominating rays4 ,if the black rays in the solar spectrum were owing to the absorption of the sun's atmosphere1 the light from the margin of his disc1 having to pass through a greater thickness of it1 would exhibit deeper lines than that which comes from his centre2 but1 as no difference is perceptible1 it may be inferred that the analogous bands in the light of the coloured stars are not due to the absorption of their atmospheres1 but that they arise from the different kinds of combustion by which these bodies are lighted up4 ,all the ordinary methods fail for finding the parallax when the distances of the stars are very great4 ,an angle even of one or two seconds1 viewed in the focus of our largest telescopes1 does not equal the thickness of a spider's thread1 which makes it impossible to measure such minute quantities with any degree of accuracy4 ,in some cases1 however1 the binary systems of stars furnish a method of estimating an angle of even the tenth of a second1 which is thirty times more accurate than by any other means4 ,from them the actual distances of some of the more remote stars will ultimately be known4 ,suppose that one star revolves round another in an orbit which is so obliquely seen from the earth as to look like an ellipse in a horizontal position1 then it is clear that one-half of the orbit will be nearer to us than the other half4 ,now1 in consequence of the time which light takes to travel1 we always see the satellite star in a place which it has already left4 ,hence1 when that star sets out from the point of its orbit which is nearest to us1 its light will take more and more time to come to us in proportion as the star moves round to the most distant point in its orbit4 ,on that account the star will appear to us to take more time in moving through that half of its orbit than it really does4 ,exactly the contrary takes place on the other half2 for the light will take less and less time to arrive at the earth in proportion as the star approaches nearer to us2 and therefore it will seem to move through this half of its orbit in less time than it really does4 ,this circumstance furnishes the means of finding the absolute breadth of the orbit in miles1 and from that the true distance of the star from the earth4 ,for1 since the greatest and least distances of the satellite star from the earth differ by the breadth of its orbit1 the time which the star takes to move from the nearest to the remotest point of its orbit is greater than it ought to be by the whole time its light takes to cross the orbit1 and the period of moving through the other half is exactly as much less4 ,hence the difference between the observed times of these two semi-revolutions of the star is equal to twice the time that its light employs to cross its orbit2 and1 as we know the velocity of light1 the diameter of the orbit may be found in miles1 and from that its whole dimensions2 for the position of the orbit with regard to us is known by observation1 as well as the place1 inclination1 and apparent magnitude of its major axis1 or1 which is the same thing1 the angle under which it is seen from the earth4 ,since1 then1 three things are known in this great triangle1 namely1 the base or major axis of the orbit in miles1 the angle opposite to it at the earth1 and the angle it makes with the visual ray1 the distance of the satellite star from the earth may be found by the most simple of calculations4 ,the merit of having first proposed this very ingenious method of finding the distance of the stars is due to ,m4 ,savary2 but1 unfortunately1 it is not of general application1 as it depends upon the position of the orbit1 and a long time must elapse before observation can furnish data1 since the shortest period of any revolving star that we know of is #cj years4 ,still the distances of a vast number of stars may ultimately be made out in this way2 and1 as one important discovery almost always leads to another1 their masses may thus be weighed against that of the earth or sun4 ,the only data employed for finding the mass of the earth1 as compared with that of the sun1 are1 the angular motion of our globe round the sun in a second of time1 and the distance of the earth from the sun in miles 7,n4 #bcc74 ,now1 by observations of the binary systems1 we know the angular velocity of the small star round the great one2 and1 when we know the distance between the two stars in miles1 it will be easy to compute how many miles the small star would fall through by the attraction of the great one in a second of time4 ,a comparison of this space with the space through which the earth would descend towards the sun in a second will give the ratio of the mass of the great star to that of the sun or earth4 ,according to ,m4 ,bessel1 the weight of the two stars of #fa ,cygni is equal to half the weight of the sun4 ,little as we know of the absolute magnitude of the fixed stars1 the quantity of light emitted by many of them shows that they must be much larger than the sun4 ,dr4 ,wollaston determined the approximate ratio which the light of a wax candle bears to that of the sun1 moon1 and stars1 by comparing their respective images reflected from small glass globes filled with mercury1 whence a comparison was established between the quantities of light emitted by the celestial bodies themselves4 ,by this method he found that the light of ,lyr is five and a half times greater than that of the sun4 ,sir ,john ,herschel reflected the moon's light totally by a prism1 which1 concentrated by a lens1 was compared directly with that of ,centauri4 ,after making allowance for the quantity of the moon's light lost in passing through the lens and prism1 he found that the mean quantity of light sent to the earth by a full moon exceeds that sent by ,centauri in the proportion of #bg1djh to #a4 ,now1 ,dr4 ,wollaston found the proportion of the sun's light to that of the full moon to be that of #hja1jgb to #a4 ,hence1 the light sent to us by the sun is to that sent by ,centauri as about twenty-two thousand millions to one4 ,but1 as the parallax of ,centauri is #a1 it really is two and a half times brighter than the sun4 ,the light of ,sirius is four times that of ,centauri1 but its parallax is only #jbcj3 hence it has an intrinsic splendour #fcjb times that of our luminary4 ,it is therefore estimated to be a hundred times as large2 so that1 were ,sirius in the earth's place1 its surface would extend #aej times as far as the orbit of the moon4 ,the light of ,sirius1 according to the observations of ,sir ,john ,herschel1 is #cbd times greater than that of a star of the sixth magnitude2 if we suppose the two to be really of the same size1 their distances from us must be in the ratio of #egc to #a1 because light diminishes as the square of the distance of the luminous body increases4 ,so many of the stars have proper motions altogether independent of the annual rotation of the earth in its orbit1 that it may be doubted whether there be such a thing as a fixed star4 ,groombridge is the most rapid known3 it has a proper motion of #g of arc annually2 ,centauri moves at the rate of #ceh annually1 and #fa ,cygni describes a line in space of #eab in the same time4 ,these motions are probably in curves1 but at the distance of the earth they will appear to be rectilineal for ages to come4 ,the motion of little more than five seconds of space1 which #fa ,cygni describes annually1 seems to us to be extremely small2 but at the distance of that star an angle of one second corresponds to twenty-four millions of millions of miles2 consequently the annual motion of #fa ,cygni is #abj millions of millions of miles1 and yet1 as ,m4 ,arago observes1 we call it a fixed star4 ,from the same cause it is evident that the crowding of the stars in the ,milky ,way may be apparent only1 and that the stars may be at vast distances from one another1 and no doubt are4 ,were the solar system and the whole of the stars visible to us carried forward in space by a motion common to all1 like ships drifting in a current1 it would be impossible for us1 moving with the rest1 to ascertain its direction4 ,sir ,william ,herschel perceived that a great part of the motions of the stars is only apparent1 arising from a real motion of the sun in a contrary direction4 ,among many discrepancies he found that the stars in the northern hemisphere have a general tendency to move towards a point diametrically opposite to ,herculis1 which he attributed to a motion of the solar system in a contrary direction4 ,for it was evident to him1 that the stars1 from the effects of perspective alone1 would seem to diverge in the direction to which the solar system was going1 and would converge towards the space it had left1 and that there would be a regularity in these apparent motions which would hereafter be detected4 ,since ,sir ,william ,herschel's time the proper motions of the stars have been determined with much greater accuracy1 and many have been added to the list by comparing the ancient and modern tables of their places2 his views have been established by four of the greatest astronomers of the age1 ,,mm4 ,lundahles1 ,argelander1 ,otto ,struve1 and ,peters1 who have clearly proved the motion of the sun from that of the stars in the northern hemisphere1 and ,mr4 ,galloway has come to the same conclusion from the motions of the stars in the southern hemisphere 7,n4 #bcd74 ,the result is1 that the sun1 accompanied by all his attendant planets1 is moving at the rate of #dbb1dbd miles--or over a space nearly equal to his own diameter--in the course of a day1 and that the motion is directed towards a point in a line joining the two stars and ,herculis at a quarter of the apparent distance of these two stars1 reckoning from ,herculis4 ,this investigation was founded upon no law assumed or observed1 such as the circulation of all the stars of our firmament about a common centre1 though philosophers have speculated as to the probability of such a motion in the sun and stars in the plane of the ,milky ,way4 ,should the sun and his stellar companions be moving in a nearly circular orbit1 the centre of motion would be in the plane passing through the sun perpendicular to the direction of his motion4 ,the constellations through which that great circle would pass are ,pisces1 ,australis1 ,pegasus1 ,andromeda1 ,perseus1 c4 ,m4 ,argelander is of opinion that the sun's orbit is nearly in the plane of the ,milky ,way1 and1 therefore1 that its centre must probably be in ,perseus1 while ,m4 ,mdler places it in the ,pleiades1 which seems to be inadmissible2 but the data are too uncertain at present to admit of any absolute conclusion as to the sun's orbit and the general motion of the stellar firmament3 for though the stars in every region of the sky tend towards a point in ,hercules1 it is not yet known whether their motions are uniform or variable1 whether the sun's motion be gradually changing1 and whether the stars form different independent systems1 each having its own centre of attraction1 or if all obey one powerful controlling force which pervades the whole universe4 ,accurate observations of the places of a select number of stars of all dimensions in the ,milky ,way continued for a series of years would no doubt decide this point4 ,the proper motion of a star combined with the progressive velocity of light alters the apparent periodic time of the revolving star of a binary system4 ,if the orbit of a double star be at right angles to the visual ray1 and both the sun and the star at rest1 the periodic time of the revolving star1 say of #aj1jjj days1 would always be the same4 ,but if the centre of gravity of the star were to recede in a direct line from the sun with the velocity of one tenth of the radius of the earth's orbit in a day1 then at the end of #aj1jjj days it would be more remote from us by #ajjj of such radii--a space light would take #eg days to traverse3 hence1 although the periodic time of the star would really be the same1 the completion of its period would only be known to us #eg days after it had taken place1 so that the periodic time would appear to us to be #aj1jeg days instead of #aj1jjj4 ,were the star to approach to the sun by the same quantity instead of receding1 the apparent periodic time would be diminished by #eg days4 ,as the sun is only a unit in the stellar system1 so the ,milky ,way1 and all the stars that adorn the firmament of both hemispheres1 constitute a group which is but a unit among the infinite numbers of starry clusters and nebul that are profusely scattered throughout the universe4 ,by the aid of a good telescope there may be seen on the clear vault of heaven1 between the stars of our own stellar system1 and far in the depths of space1 an immense multitude of objects like comets or clouds of white vapour of all forms and sizes4 ,some are mixed with stars1 others are entirely formed of them4 ,many appear as if they were stellar1 but required a telescope of higher power to resolve them1 and vast numbers appear to be matter rarefied in the highest possible degree1 giving no indication of a stellar nature2 and these are in every state of condensation1 from a vague film hardly to be discerned to such as have actually arrived at a solid nucleus of stars4 ,the cloudy appearance is merely the blending of the rays of innumerable stars which are themselves invisible from their extreme distance1 like parts of the ,milky ,way4 ,sir ,william ,herschel was at first of that opinion1 and the nebul that have been resolved by ,lord ,rosse's telescope have led astronomers to believe that such is the case4 ,yet the tails of comets1 the zodiacal light1 and the extensive luminous atmospheres which encompass many of the stars1 show that1 in all probability1 a self-luminous phosphorescent material substance in a highly diluted or gaseous form exists in vast abundance4 ,the number of the nebul1 like that of the stars1 is only limited by the imperfection of our instruments1 for each improvement in the telescope only enables us to penetrate a little farther into the infinity of space--to see a few more of these shadowy existences in the far distance1 and to resolve a few more of those that are comparatively near4 ,sir ,william ,herschel examined the nature and determined the position of #bejj nebul in the northern hemisphere whose places were computed from his observations1 reduced to a common epoch1 and arranged into a catalogue1 in order of right ascension1 by his sister1 ,miss ,caroline ,herschel1 who added lustre to the name she bore by her eminence in astronomical knowledge and discovery4 ,sir ,john ,herschel revised his father's observations1 and added #hjj nebul to the catalogue before he went to the ,cape of ,good ,hope1 in order to complete the survey of the heavens4 ,on his return he published a catalogue of #bjdi nebul of the southern hemisphere1 of which #ejj were previously unknown1 with their position in the heavens4 ,in a work unparalleled for elegance of style1 depth of knowledge1 and originality of views1 he has given engravings from his drawings of the most remarkable objects1 so that whatever changes may take place in their form1 place1 or condensation1 will be known by astronomers of future ages4 ,though infinite in variety1 the nebul are of two distinct classes2 one consists of patches of great dimensions1 capriciously irregular1 assuming all the fantastic forms of clouds1 now bright1 now obscure2 sometimes like vapour flying before the wind2 sometimes stretching long arms into space4 ,many present an ill-defined surface1 in which it is difficult to say where the centre of the greatest brightness is4 ,large portions are resolvable into stars2 many have a granulated appearance1 as if they were resolvable2 and others probably are not so merely from the smallness and closeness of the stars1 and possibly from their remoteness1 indicating the complex and irregular form the ,milky ,way would present if seen from a distance4 ,a wonderful nebula of this kind is visible to the naked eye in the constellation of ,orion2 it is of vast extent1 sending branches even into the southern hemisphere2 and1 although ,lord ,rosse's telescope has resolved much that had hitherto resisted others1 there are parts that still maintain their nebulous appearance from extreme remoteness1 presenting a kind of mottled aspect1 like flocks or wisps of wool1 or mackerel sky4 ,there can be no doubt of its being an unfathomable congeries of stars1 which there is reason to believe has changed its form in some parts within the last fifty years4 ,vast multitudes of nebul of this kind are so faint as to be with difficulty discerned at all till they have been for some time in the field of the telescope1 or are just about to quit it4 ,occasionally they are so vague1 that the eye is conscious of something being present1 without being able to define what it is2 but the unchangeableness of its position convinces the mind that it is a real object--8an image was before mine eyes1 but ,i could not discern the form thereof40 ,no drawing can give an idea of the boundaries of such nebul as that of ,orion2 even with ,lord ,rosse's telescope the edges either fade into a luminous mist1 which becomes more rare till it is imperceptible1 or end in a tissue of faintish flocculi1 or in filaments which become finer and more scattered till they cease to be visible1 showing that the real boundaries have not been seen4 ,the other class of nebul1 vastly inferior in size1 of definite forms and great variety of character1 are scattered through the remote heavens1 or congregated in a great nebulous district far from the ,milky ,way4 ,many cling to stars like wisps of clouds1 others are exactly like comets with com and tails2 but the most definite forms are annular and lenticular nebul1 nebulous stars1 planetary and elliptical nebul1 and starry clusters4 ,however1 there are two in the northern hemisphere differing from all of these1 which are described by ,sir ,john ,herschel as amazing objects4 ,one in ,vulpecula is like an hourglass or dumb bell of bright matter1 surrounded by a thin hazy atmosphere so as to give the whole an oval form1 or the appearance of an oblate spheroid2 with a higher optical power its form is much the same1 but the brighter part is resolved into stars1 and the hazy part1 though still nebulous1 assumes that mottled appearance which shows that the whole is a stellar system of the most peculiar structure3 it is a phenomenon that bears no resemblance to any known object4 7,fig4 #c1 plate #h1 and fig4 #c1 plate #i74 ,the other is indeed most wonderful1 and its history shows the gradual increase in the space-penetrating power of telescopes4 ,to ,messier it appeared merely to be a double nebula with stars2 with ,sir ,william ,herschel's telescope it presented the appearance of a bright round nebula encompassed at a little distance by a halo or glory1 and accompanied by a companion2 while in ,sir ,john ,herschel's #bj feet reflector it appeared to 8consist of a bright round nucleus1 surrounded at a distance by a nebulous ring split through half its circumference1 and having the split portions separated at an angle of #de degrees each to the plane of the other40 7,fig4 #a1 plate #aj47 ,this nebula appeared to ,sir ,john to 8bear a strong similitude to the ,milky ,way1 suggesting the idea of a brother system bearing a real physical resemblance and strong analogy of structure to our own40 ,this object1 which disclosed to ,lord ,rosse the astonishing phenomenon of spiral nebul seen in his telescope1 presents the appearance of the fig4 #a in plate #aj1 in which the partial division of the limb of the ring into two branches is at once recognised in the bright convolutions of the spiral4 ,the outlying nebula is connected by a narrow curved band of light with the ring2 the whole is either resolved into stars1 or evidently might be with a still higher optical power4 ,with regard to the marvellous nebula in question ,lord ,rosse observes1 that 8with each increase of optical power the structure has become more complicated1 and more unlike anything that could result from any form of a dynamical law of which we find a counterpart in our system4 ,the connection of the companion with this great nebula1 of which there is not the least doubt1 adds to the difficulty of forming any hypothesis4 ,it is impossible that such a system could exist without internal movement1 to which may be added a resisting medium2 but it cannot be regarded as a case of mere static equilibrium40 ,this is by no means the only instance of a spiral nebula2 ,lord ,rosse has discovered several others3 some are easily seen--others require the highest powers of his telescope4 ,from the numerous offsets that branch from the ,milky ,way and run far into space1 it may possibly partake also of the spiral form4 ,there are seven annular nebul in the northern hemisphere1 since ,lord ,rosse has discovered that five of the planetary nebul belong to this class4 ,one of the finest examples of an annular nebula is to be seen midway between and ,lyr 7fig4 #b1 plate #i74 ,according to ,sir ,john ,herschel1 it is elliptical in the ratio of #d to #e1 and is sharply defined--the internal opening occupying about half the diameter4 ,this opening is not entirely dark1 but filled with a faint hazy light like fine gauze stretched over a hoop4 ,its diameter1 if it is as far from us as #fa ,cygni1 must be #acjj times greater than the diameter of the earth's orbit--dimensions that are most astounding4 ,lord ,rosse's telescope resolves this object into stars of extreme minuteness1 with filaments of stars adhering to its edges and a pretty bright star in its interior4 ,these rings are like hollow shells whose borders seem brighter because the nebulous substance1 whatever it may be1 is more condensed to appearance than the central part4 ,the other annular nebula in the northern hemisphere described by ,sir ,john ,herschel is a small faint object1 and more easily resolvable into stars4 ,one of the annular nebul seen by ,lord ,rosse is surrounded by a faint external flat ring2 another has ans1 as if an annular nebulous ring encompassed it and was foreshortened4 ,two annular nebul have perforations as if the black sky was seen through openings in the interior haze1 for in no instance is the central opening quite dark4 ,some nebul are like very elliptical annular systems seen obliquely4 ,if they be elliptical flat rings1 the dark centre may be a real opening2 but should the systems be a series of very long elliptical concentric shells surrounding a hollow1 the dark axis may be merely a line of comparative darkness4 ,the connection of the elliptical nebul with double stars is mentioned as very remarkable4 ,in one elliptical nebula whose longer axis is #ej there are two individuals of a double star each of the #ajth magnitude symmetrically placed rather nearer the vertex of the ellipse than the foci2 in another the stars are unequal1 but placed exactly at the extremities of the major axis1 as in plate #h3 besides these there are several other instances4 ,double nebul are not unfrequent in both hemispheres1 exhibiting all the varieties of distance1 position1 and relative brightness1 with their counterparts the double stars4 ,the rarity of single nebul as large1 faint1 and as little condensed in the centre as these1 makes it extremely improbable that two such bodies should be accidentally so near as to touch1 and often in part to overlap each other1 as these do4 ,it is much more likely that they constitute systems2 and1 if so1 it will form an interesting object of future inquiry to discover whether they possess orbital motion4 ,nebulous stars are beautiful objects1 quite different from all the preceding4 ,they are round or oval1 increasing in density towards the centre4 ,sometimes the central matter is so vividly and sharply condensed and defined that the nebula might be taken for a bright star surrounded by a thin atmosphere4 ,one is a star of the #hth magnitude exactly in the centre of a round bright atmosphere #be in diameter2 the star is quite stellar1 and not a nucleus3 it has not the smallest appearance of being resolvable4 ,another nebulous star is ,orionis1 which has a broad atmosphere in which is a dark cavity not symmetrical with the star1 and a small double star with a similar opening on the edge of the atmosphere4 ,lord ,rosse observes that these openings appear to be of the same nature with that within the bright stars in the trapezium of ,orion1 the stars being at its edge2 and he is convinced that the stars are not only connected with the nebula1 but that they are equidistant with it2 hence1 if their parallax can be found1 the distance of this nebula would be determined4 ,the zodiacal light or lenticular shaped luminous haze surrounding the sun which may be seen extending beyond the orbits of ,mercury and ,venus soon after sunset in the months of ,april and ,may1 or before dawn in ,november and ,december1 seems to place our luminary in the class of nebulous stars4 ,the extensive and delicate atmosphere of these nebulous stars assumes all degrees of ellipticity1 from the circular to the spindle-shaped ray1 or almost the right line4 ,planetary nebul have exactly the appearance of planets with round or oval discs1 sometimes sharply terminated1 at other times hazy and ill-defined4 ,their surface1 which is blue or blueish white1 is equable or slightly mottled1 and their light occasionally rivals that of the planets in vividness4 ,they are generally attended by minute stars1 which give the idea of accompanying satellites4 ,these nebul are of enormous dimensions4 ,one near ,aquarii has a sensible diameter of about twenty seconds1 and another presents a diameter of twelve4 ,sir ,john ,herschel has computed that1 if these objects be as far from us as the stars1 their real magnitude1 on the lowest estimation1 must be such as would fill the orbit of ,uranus4 ,he concludes that1 if they be solid bodies of a solar nature1 their intrinsic splendour must be far inferior to that of the sun1 because a circular portion of the sun's disc subtending an angle of twenty seconds would give a light equal to that of a hundred full moons2 while1 on the contrary1 the objects in question are hardly1 if at all1 visible to the naked eye4 ,from the uniformity of the discs of these planetary nebul1 and their apparent want of condensation1 he presumes that they may be hollow shells emitting light from their surface only4 ,the southern hemisphere is very rich in them1 where twenty-eight or twenty-nine have been discovered1 some in the midst of a cluster of stars1 with which they form a beautiful contrast4 ,three are of a decided blue colour1 one ,prussian blue1 or verditer green1 the other two of a bright sky blue1 of great beauty and delicacy4 ,one seems to belong to the class of double nebul or double stellar nebul of the utmost remoteness4 ,since ,lord ,rosse's telescope has shown that five of the planetary nebul are annular1 some of those in the southern hemisphere may ultimately be found to belong to the same class4 ,probably nine tenths at least of the nebulous contents of the heavens consist of spherical or elliptical forms presenting every variety of elongation and central condensation4 ,of these a great number have been resolved into stars1 and a great many present that mottled appearance which renders it certain that an increase of optical power would decompose them4 ,those which resist do so on account of the smallness and closeness of the stars of which they consist4 ,elliptical nebul are very common2 by much the finest may be seen near the star in the girdle of ,andromeda4 ,it is visible to the naked eye1 and has frequently been taken for a comet4 ,with a low optical power it has the spindle-shaped form of fig4 #f1 plate #e1 the brightness being at first gradually and then rapidly condensed towards the centre1 so that it has been compared to a star shining through horn1 but had never appeared resolvable even with high optical powers till ,mr4 ,bond examined it at the observatory of ,cambridge in the ,united ,states4 ,he found that its brightness extends over #b-a_/b degrees in length1 and more than a degree in breadth1 including two small adjacent nebul1 so that it is oval4 ,it is strongly and rapidly condensed into a nucleus on its northern side2 and although it was not all resolved1 it was seen to be strewed over with star dust1 or extremely minute visible stars1 which leaves not a doubt of its being a starry system4 ,the most remarkable part of ,mr4 ,bond's discovery are two very narrow dark lines which extend along one side of the oval parallel to its major axis4 ,these black streaks1 difficult to distinguish1 indicate a stratified structure1 and are not the only instance of that arrangement in nebul4 ,fig4 #a1 in plate #i1 is from ,mr4 ,bond's drawing of this nebula4 ,multitudes of nebul appear to the unassisted eye1 or are seen with ordinary telescopes1 like round comets without tails2 but when viewed with powerful instruments they convey the idea of a globular space1 insulated in the heavens and full of stars1 constituting a family or society apart from the rest1 subject only to its own internal laws4 ,to attempt to count the stars in one of these globular clusters1 ,sir ,john ,herschel says1 would be a vain task2 they are not to be reckoned by hundreds4 ,on a rough computation1 it appears that many clusters of this description must contain ten or twenty thousand stars compacted and wedged together in a round space1 whose apparent area is not more than a tenth part of that covered by the moon2 so that its centre1 where the stars are seen projected on each other1 is one blaze of light4 ,if each of these stars be a sun1 and if they be separated by intervals equal to that which separates our sun from the nearest fixed star1 the distance which renders the whole cluster barely visible to the naked eye must be so great1 that the existence of such a splendid assemblage can only be known to us by light which must have left it at least a thousand years ago4 ,these magnificent globular or spheroidal aggregates of stars are so arranged that the interior strata are more crowded and become more nearly spherical as they approach the centre4 ,a most splendid object of this nature may be seen in the constellation ,hercules 7,n4 #bce74 ,of #aca of these magnificent objects in the southern hemisphere1 two of them are pre-eminently splendid4 ,the globular cluster of ,centauri is beyond comparison the finest of its kind3 it is perfectly spherical1 and occupies a quarter of a square degree2 the stars in it are literally innumerable1 crowding and densely aggregated towards the centre2 and1 as its light is not more to the naked eye than that of a star of the #dth or #eth magnitude1 their minuteness is extreme4 ,it has a dark hole in its centre1 with a bridge of stars across1--a circumstance peculiar to this cluster4 ,lacaille's globular cluster1 or #dg ,toucani1 is completely insulated in a very dark part of the sky not far from the lesser of the ,magellanic clouds4 ,the stars1 which are of the #adth magnitude1 immensely numerous1 compressed and white1 form three distinct stages round a centre1 where they suddenly change in hue1 and form a blaze of rose-coloured light4 ,one cluster consists of large ruddy stars and small white ones2 another of greater beauty consists of shells or coats of stars of the #aath and #aeth magnitude4 ,there are thirty globular clusters of extreme beauty collected within a circular space of not more than eighteen degrees radius1 which lies in the part of the sky occupied by the constellations ,corona ,australis1 the body and head of ,sagittarius1 the tail of ,scorpio1 part of ,telescopium and ,ara4 ,the ,milky ,way passes diametrically across the circular area in question1 which gives prodigious brilliancy to this part of the sky4 ,for besides these globular clusters1 which all lie in the starry part1 and not in the dark spaces1 there are the only two annular nebul known to exist in the southern hemisphere4 ,no part of the heavens is fuller of objects beautiful and remarkable in themselves1 and rendered still more so by their mode of association1 and by the peculiar features assumed by the ,milky ,way1 which are without a parallel for richness and magnificence in any other part of the sky4 ,some of the globular clusters are so remote that the stars are scarcely discernible--mere star dust4 ,there is a double globular cluster in the southern hemisphere of very small dimensions1 separated by a minute interval1--a combination which suggests the idea of a globular cluster revolving about a very oblate spheroidal one in the plane of the equator1 and in an orbit which is circular1 and seen obliquely like the central nebula itself1 with a diameter somewhat more than four times the latter1--a stupendous system doubtless1 but of which the reality can hardly be supposed improbable4 ,there appears to be some connexion between ellipticity of form and difficulty of resolution1 for spherical clusters are in general easily resolved into their component stars1 while there is scarcely an instance of an elliptical cluster yielding except to a very high optical power4 ,vast masses of the nebul have never been resolved4 ,lord ,rosse's great telescope has resolved parts of the nebula of ,orion1 and various others which had not yielded to instruments of less power2 it enables the astronomer to penetrate farther into space1 and shows objects with greater clearness1 than any other4 ,but1 excellent as this instrument is1 thousands of nebul are not to be resolved even by it4 ,those who imagine that any work of man can resolve all the nebulous matter in the heavens must have a very limited idea of the extent and sublimity of creation4 ,innumerable nebul in both hemispheres take the form of clusters of stars1 but are totally different from the globular clusters1 inasmuch as they are of irregular form and follow no uniform law of condensation4 ,the ,pleiades is an instance in our own stellar system2 for although only #g or #h stars are visible to the naked eye1 telescopes show that more than #bjj belong to the group4 ,in the constellation ,cancer there is a luminous spot called the ,prsepe or ,beehive1 which a very low power resolves into stars2 and the constellation ,coma ,berenices is another stellar group4 ,many are of exquisite beauty1 as that round ,crucis1 which1 though consisting of only a hundred and ten stars1 is like a piece of fancy jewellery1 from the colours of the stars1 which are greenish white1 green1 blueish green1 and red4 ,many of these clusters contain thousands of stars1 and are frequently in the poorer parts of the sky1 as if in the course of ages the stars had been attracted towards a centre4 ,the existence of every degree of ellipticity in the nebul--from long lenticular rays to the exact circular form--and of every shade of central condensation1 from the slightest increase of density to apparently a solid nucleus--may be accounted for by supposing the general constitutions of those nebul to be that of oblate spheroidal masses of every degree of flatness from the sphere to the disc1 and of every variety in their density and ellipticity towards the centre4 ,it would be erroneous1 however1 to imagine that the forms of these systems are maintained by forces identical with those already described1 which determine the form of a fluid mass in rotation2 because1 if the nebul be only clusters of separate stars1 as in the greater number of cases there is every reason to believe them to be1 no pressure can be propagated through them4 ,consequently1 since no general rotation of such a system as one mass can be supposed1 it may be conceived to be a quiescent form1 comprising within its limits an indefinite number of stars1 each of which may be moving in an orbit about the common centre of the whole1 in virtue of a law of internal gravitation resulting from the compound gravitation of all its parts4 ,sir ,john ,herschel has proved that the existence of such a system is not inconsistent with the law of gravitation under certain conditions4 ,the distribution of the nebul over the heavens is even more irregular than that of the stars4 ,in some places they are so crowded together as scarcely to allow one to pass through the field of the telescope before another appears1 while in other parts hours elapse without a single nebula occurring4 ,they are in general only to be seen with the best telescopes1 and are most abundant in a zone whose general direction is not far from the hour circles #j;h and #ab;h1 and which crosses the ,milky ,way nearly at right angles4 ,where that nebulous zone passes over the constellations ,virgo1 ,coma ,berenices1 and the ,great ,bear1 they are to be found in multitudes4 ,the nebulous system is nearly divided into two parts by the ,milky ,way4 ,one-third of the whole visible nebulous contents of the heavens forms a broad irregular mass1 interspersed with vacant intervals1 which fills about an eighth of the surface of the northern hemisphere4 ,it occupies the constellations ,leo1 ,leo ,minor1 the body1 tail1 and hind legs of ,ursa ,major1 the nose of ,camelopard1 the point of the tail of ,draco1 ,canis ,venatica1 ,coma ,berenices1 the preceding leg of ,botes1 and the head1 wings1 and shoulder of ,virgo1 which is the richest part4 ,there is a lesser nebulous region in this hemisphere1 but entirely separated from the preceding1 which occupies the chest and wing of ,pegasus1 the constellations ,pisces and ,andromeda4 ,if we could imagine the ring or zone of the ,milky ,way to encircle or coincide with the horizon1 the great nebulous mass would form a canopy over head1 descending down to a considerable distance on all sides1 chiefly towards the north pole2 and the richest part1 which is in ,virgo1 would then be directly over head in the north pole of the ,milky ,way1 that is in #ab;h #dgm right ascension1 and #fd north polar distance4 ,with the exception of the ,magellanic clouds1 there is a much greater uniformity in the distribution of the nebul in the southern hemisphere than in the northern4 ,they are separated by spaces of vacuity of greater or less dimensions4 ,one of these barren regions extends for nearly fifteen degrees all around the south pole1 and close on its border2 the lesser of the ,magellanic clouds occurs completely insulated2 while the greater ,magellanic cloud is in connexion with something approaching to a zone of connected nebulous patches which extends along the back of ,doradus1 through a portion of ,horologium and ,eridanus1 part of ,fornix1 and over the paws of ,cetus to the equator1 where it unites with the nebulous regions of ,pisces4 ,the ,magellanic clouds form two of the most striking features in the southern hemisphere2 both of these nebul are visible to the unassisted eye1 being nearly of the same intensity as the brighter portions of the ,milky ,way2 but the smaller is entirely effaced by moonlight1 and the larger nearly so4 ,they are altogether unconnected with the ,milky ,way and with one another4 ,the ,nubecula ,major is far superior to the ,nubecula ,minor in every respect1 though they are similar in internal structure4 ,the former consists of large tracts and ill-defined patches of irresolvable nebul1 and nebulosity in every stage of resolution1 up to perfectly resolved stars like the ,milky ,way2 and also of regular and irregular nebul1 properly so called2 of globular clusters in every stage of resolvability2 and of clustering groups sufficiently insulated and condensed to come under the designation of clusters of stars4 ,of these the nebula known as ,lacaille's #cj ,doradus is too remarkable to be passed over4 ,it is very large1 situate within the ,nubecula ,major1 and consists of an assemblage of nearly circular loops uniting in a centre1 in or near which there is a circular black hole4 ,in short1 for the number and variety of the objects1 there is nothing like this cloud4 ,within an area of only forty-two square degrees1 ,sir ,john ,herschel has determined the places1 and registered #bgh nebul and clusters of stars1 with fifty or sixty in outlying members immediately adjacent4 ,even the most crowded parts of the stratum of ,virgo1 in the wing of that constellation1 or in ,coma ,berenices1 offer nothing approaching to it4 ,it is evident1 from the intermixture of stars and unresolved nebulosity1 which probably might be resolved with a higher optical power1 that the nubecul are to be regarded as systems sui generis1 to which there is nothing analogous in our hemisphere4 ,next to the ,magellanic clouds the great nebula round ,args is one of the most wonderful objects of the southern sky4 ,it is situate in that part of the ,milky ,way which lies between the ,centaur and the body of ,argus1 in the midst of one of those rich and brilliant masses1 a succession of which is so curiously contrasted with the profoundly dark adjacent spaces1 and surrounded by one of the most beautiful parts of the southern heavens4 ,sir ,john ,herschel says3 8,it would be impossible1 by verbal description1 to give any just idea of the capricious forms and irregular gradations of light affected by the different branches and appendages of this nebula4 ,nor is it easy for language to convey a full impression of the beauty and sublimity of the spectacle it offers when viewed in a sweep1 ushered in as it is by so glorious and innumerable a procession of stars1 to which it forms a sort of climax1 justifying expressions which1 though ,i find them written in my journal in the excitement of the moment1 would be thought extravagant if transferred to these pages4 ,in fact1 it is impossible for any one1 with the least spark of astronomical enthusiasm about him1 to pass soberly in review with a powerful telescope1 and on a fine night1 that portion of the southern sky which is comprised between the #fth and #acth degrees of right ascension1 and from #adf to #adi of north polar distance2 such are the variety and interest of the objects he will encounter1 and such the dazzling richness of the starry ground on which they are represented to his gaze40 ,in that portion of the sky there are many fine double stars--rich starry clusters2 the elegant cluster of variously coloured stars round ,crucis2 a large planetary nebula with a satellite star2 another of a bright blue colour1 exquisitely beautiful and unique2 and1 lastly1 ,args itself1 the most extraordinary instance of a variable star in astronomical history4 ,it frequently occurred1 during ,sir ,john ,herschel's survey of the southern heavens1 that some parts of the sky were noted for deeper blackness than others1 and no stars could be seen2 it frequently happened that far from the ,milky ,way1 or any large nebula or cluster of stars1 there were some indications of very remote branches of the ,milky ,way1 or of an independent sidereal system or systems1 bearing a resemblance to such branches4 ,these were indicated by an exceedingly delicate and uniform dotting or stippling of the sky by points of light too small to admit of any one of them being steadily and fixedly viewed1 and too numerous to be counted even if possible to view them4 ,the truth of this existence was felt at the moment of observation2 but the conviction1 though often renewed1 was not permanent4 ,the places where these appearances occurred are given1 in order that those who wish to verify them may have it in their power4 ,such is a brief account of a very few of the discoveries contained in ,sir ,john ,herschel's great work on the ,nebul and other ,phenomena of the ,southern ,hemisphere1--a work which will rise in estimation with the lapse of years4 ,no doubt the form and internal structure of many of these nebul will be changed by telescopes of higher power2 but as the places of the leading phenomena have been determined1 the date of that great work may be regarded as the epoch of nebular time whence the relative changes that take place in the most distant regions of the universe will be estimated for ages to come2 and in the inimitable writings of the highly gifted father and son the reader will find these subjects treated of in a style worthy of it and of them4 ,of late years the excellence of the instruments1 and still more of the astronomers1 in the foreign observatories1 have aided the progress of sidereal astronomy immensely4 ,nor has it been cultivated with less success in our home and colonial establishments3 certainly one of the most remarkable features of the times is the number of private observatories1 built and furnished with the best instruments by private gentlemen1 whose zeal has been rewarded by eminent success in all departments of the science4 7,n4 #bcf47 ,so numerous are the objects which meet our view in the heavens1 that we cannot imagine a point of space where some light would not strike the eye2--innumerable stars1 thousands of double and multiple systems1 clusters in one blaze with their tens of thousands of stars1 and the nebul amazing us by the strangeness of their forms and the incomprehensibility of their nature1 till at last1 from the limit of our senses1 even these thin and airy phantoms vanish in the distance4 ,if such remote bodies shone by reflected light1 we should be unconscious of their existence4 ,each star must then be a sun1 and may be presumed to have its system of planets1 satellites1 and comets1 like our own2 and1 for aught we know1 myriads of bodies may be wandering in space unseen by us1 of whose nature we can form no idea1 and still less of the part they perform in the economy of the universe4 ,even in our own system1 or at its farthest limits1 minute bodies may be revolving like the telescopic planets1 which are so small that their masses have hitherto been inappreciable1 and there may be many still smaller4 ,nor is this an unwarranted presumption2 many such do come within the sphere of the earth's attraction1 are ignited by the velocity with which they pass through the atmosphere1 but leave no residuum4 ,these1 which are known as falling stars and meteors1 are periodical2 but that is by no means the case with arolites1 which are also ignited by the sudden condensation of the air on entering our atmosphere1 and are precipitated in solid masses with such violence on the earth's surface that they are often deeply buried in the ground4 ,the fall of meteoric stones is much more frequent than is generally believed4 ,hardly a year passes without some instances occurring2 and1 if it be considered that only a small part of the earth is inhabited1 it may be presumed that numbers fall in the ocean1 or on the uninhabited part of the land1 unseen by man4 ,they are sometimes of great magnitude2 the volume of several has exceeded that of the planet ,ceres1 which is about #gj miles in diameter4 ,one which passed within #be miles of us was estimated to weigh about #fjj1jjj tons1 and to move with a velocity of about #bj miles in a second2 a fragment of it alone reached the earth4 ,the obliquity of the descent of meteorites1 the peculiar substances they are composed of1 and the explosion accompanying their fall1 show that they are foreign to our system2 but whence derived is still a mystery4 ,shooting stars and meteors burst from the clear azure sky1 and1 darting along the heavens1 are extinguished without leaving any residuum except a vapour-like smoke1 and generally without noise4 ,their parallax shows them to be very high in the atmosphere1 sometimes even beyond its supposed limit1 and the direction of their motion is for the most part diametrically opposite to the motion of the earth in its orbit4 ,the astonishing multitudes of shooting stars and fire-balls that have appeared at stated periods over different parts of the globe1 warrant the conclusion that there is either a nebula or that there are myriads of bodies revolving in groups round the sun which only become visible when inflamed by entering our atmosphere4 ,one of these nebul or groups seems to meet the earth in its annual revolution on the #abth and #acth of ,november4 ,on the morning of the #abth of ,november1 #agii1 thousands of shooting stars1 mixed with large meteors1 illuminated the heavens for many hours over the whole continent of ,america1 from ,brazil to ,labrador3 it extended to ,greenland1 and even ,germany4 ,meteoric showers were seen off the coast of ,spain1 and in the ,ohio country1 on the morning of the #acth of ,november1 #ahca2 and during many hours on the morning of the #acth ,november1 #ahcb1 prodigious multitudes of shooting stars and meteors fell at ,mocha on the ,red ,sea1 in the ,atlantic1 in ,switzerland1 and at many places in ,england4 ,but by much the most splendid meteoric shower on record began at nine o'clock in the evening of the #abth of ,november1 #ahcc1 and lasted till sunrise next morning4 ,it extended from ,niagara and the northern lakes of ,america to the south of ,jamaica1 and from #fa of longitude in the ,atlantic to #ajj of longitude in central ,mexico4 ,shooting stars and meteors1 of the apparent size of ,jupiter1 ,venus1 and even the full moon1 darted in myriads towards the horizon1 as if every star in the heavens had started from their spheres4 ,they are described as having been frequent as flakes of snow in a snow-storm1 and to have been seen with equal brilliancy over the greater part of the continent of ,north ,america4 ,those who witnessed this grand spectacle were surprised to see that every one of the luminous bodies1 without exception1 moved in lines which converged in one point in the heavens3 none of them started from that point2 but their paths1 when traced backwards1 met in it like rays in a focus1 and the manner of their fall showed that they descended from it in nearly parallel straight lines towards the earth4 ,by far the most extraordinary part of the whole phenomenon is1 that this radiant point was observed to remain stationary near the star ,leonis for more than two hours and a half1 which proved the source of the meteoric shower to be altogether independent of the earth's rotation1 and its parallax showed it to be far above the atmosphere4 ,as a body could not be actually at rest in that position1 the group or nebula must either have been moving round the earth or the sun4 ,had it been moving about the earth1 the course of the meteors would have been tangential to its surface2 whereas they fell almost perpendicularly1 so that the earth in its annual revolution must have met with the group4 ,the bodies or the parts of the nebula that were nearest must have been attracted towards the earth by its gravity1 and1 as they were estimated to move at the rate of fourteen miles in a second1 they must have taken fire on entering our atmosphere1 and been consumed in their passage through it4 ,as all the circumstances of the phenomena were similar on the same day and during the same hours in #ahcb1 and as extraordinary flights of shooting stars were seen at many places both in ,europe and ,america on the #acth of ,november1 #ahcd1 #ahce1 and #ahcf1 tending also from a fixed point in the constellation ,leo1 it has been conjectured1 with much apparent probability1 that this nebula or group of bodies performs its revolution round the sun in a period of about #ahb days1 in an elliptical orbit1 whose major axis is #aai millions of miles2 and that its aphelion distance1 where it comes in contact with the earth's atmosphere1 is about #ie millions of miles1 or nearly the same with the mean distance of the earth from the sun4 ,this body must have met with disturbances after #agii1 which prevented it from encountering the earth for #cb years1 and it may again deviate from its path from the same cause4 ,it is now well ascertained that great showers of shooting stars occur also on the #abth of ,august1 whose point of divergence is ,camelopardali1 so that the earth's atmosphere comes into contact with a zone of these small bodies twice in the year4 ,by a systematic series of observations1 ,,mm4 ,benzenberg and ,brand have clearly made out that the heights at which the falling stars appear and vanish vary from #af miles to #adj1 and their velocities from #ah to #cf miles in a second1 velocities so great as certainly to indicate a planetary revolution round the sun4 ,as shooting stars are seen almost every night when the sky is clear1 ,sir ,john ,lubbock has thought it probable that some of these bodies may have come so near1 that the attraction of the earth has overcome that of the sun1 and caused them to revolve as satellites round it4 ,should that be the case1 they might shine by the reflected light of the sun1 and suddenly cease to be visible on entering the earth's shadow4 ,the splitting of the falling stars like a rocket1 and the trains of light1 may be accounted for by supposing the stars to graze the surface of the shadow before being eclipsed2 and the disappearance would be more or less rapid according to the breadth of the penumbra traversed4 ,the calculations of ,m4 ,petit1 ,director of the ,observatory of ,toulouse1 not only render probable the existence of small satellites1 but tend to establish the identity of a body revolving round the earth in three hours and twenty minutes1 at a distance of #ejjj miles above its surface4 ,it is evident that in this case the same satellite would be seen very often1 and a very few would be sufficient to account for their nightly appearance4 ,it is possible1 however1 that some shooting stars may belong to one class1 and some to the other1 since one group may be revolving about the sun1 and another round the earth4 ,in the case of a satellite shooting star1 geometry furnishes the means of ascertaining its exact distance from the spectator1 or from the centre of the earth1 if the time and place of its disappearance be known with regard to the neighbouring stars4 ,since the falling stars are consumed in the atmosphere1 their masses must be small1 but it is possible that occasionally one may be large enough to arrive at the surface of the earth as an arolite4 ,,section ,,xxxvii4 ,diffusion of ,matter through ,space--,gravitation--,its ,velocity--,simplicity of its ,laws--,gravitation independent of the ,magnitude and ,distances of the ,bodies--,not impeded by the intervention of any ,substance--,its ,intensity invariable--,general ,laws--,recapitulation and ,conclusion4 ,,the known quantity of matter bears a very small proportion to the immensity of space4 ,large as the bodies are1 the distances which separate them are immeasurably greater2 but1 as design is manifest in every part of creation1 it is probable that1 if the various systems in the universe had been nearer to one another1 their mutual disturbances would have been inconsistent with the harmony and stability of the whole4 ,it is clear that space is not pervaded by atmospheric air of such density as that we breathe1 since its resistance would long ere this have arrested the motion of the planets3 it certainly is not a void1 but replete with a medium possibly in itself electric or magnetic1 but at all events capable of transmitting light1 heat1 magnetism1 gravity1 and probably influences of which we can form no idea4 ,whatever the laws may be that obtain in the more distant regions of creation1 we are assured that one alone regulates the motions1 not only of our own system1 but also of the binary systems of the fixed stars2 and1 as general laws form the ultimate object of philosophical research1 we cannot conclude these remarks without considering the nature of gravitation--that extraordinary power whose effects we have been endeavouring to trace through some of their mazes4 ,it was at one time imagined that the acceleration in the moon's mean motion was occasioned by the successive transmission of the gravitating force4 ,it has been proved that1 in order to produce this effect1 its velocity must be about fifty millions of times greater than that of light1 which flies at the rate of #aib1jjj miles in a second4 ,its action1 even at the distance of the sun1 may therefore be regarded as instantaneous2 yet1 remote as the fixed stars are1 the solar gravitation must have some influence on the nearest of them1 as1 for example1 ,centauri1 which is only #bj1fjb times the radius of the earth's orbit from the sun1 while ,la ,place has computed that the solar gravitation extends a hundred millions of times farther than the semidiameter of the terrestrial orbit4 ,possibly the star dust in the ,milky ,way may be beyond1 or on the verge of1 that enormous limit2 yet it is very unlikely that either the sun1 or any of the stars which form the great cluster to which we belong1 should be unconnected bodies4 ,the curves in which the celestial bodies move by the force of gravitation are only lines of the second order4 ,the attraction of spheroids1 according to any other law of force than that of gravitation1 would be much more complicated2 and1 as it is easy to prove that matter might have been moved according to an infinite variety of laws1 it may be concluded that gravitation must have been selected by ,divine ,wisdom out of an infinity of others1 as being the most simple1 and that which gives the greatest stability to the celestial motions4 ,it is a singular result of the simplicity of the laws of nature1 which admit only of the observation and comparison of ratios1 that the gravitation and theory of the motions of the celestial bodies are independent of their absolute magnitudes and distances4 ,consequently1 if all the bodies of the solar system1 their mutual distances1 and their velocities1 were to diminish proportionally1 they would describe curves in all respects similar to those in which they now move2 and the system might be successively reduced to the smallest sensible dimensions1 and still exhibit the same appearances4 ,the action of the gravitating force is not impeded by the intervention even of the densest substances4 ,if the attraction of the sun for the centre of the earth1 and of the hemisphere diametrically opposite to him1 were diminished by a difficulty in penetrating the interposed matter1 the tides would be more obviously affected4 ,its attraction is the same also1 whatever the substances of the celestial bodies may be2 for1 if the action of the sun upon the earth differed by a millionth part from his action upon the moon1 the difference would occasion a periodical variation in the moon's parallax1 whose maximum would be the #a_/ae of a second1 and also a variation in her longitude amounting to several seconds--a supposition proved to be impossible by the agreement of theory with observation4 ,thus all matter is pervious to gravitation1 and is equally attracted by it4 ,gravitation is a feeble force1 vastly inferior to electric action1 chemical affinity1 and cohesion2 yet1 as far as human knowledge extends1 the intensity of gravitation has never varied within the limits of the solar system2 nor does even analogy lead us to expect that it should3 on the contrary1 there is every reason to be assured that the great laws of the universe are immutable1 like their ,author4 ,nor can we suppose the structure of the globe alone to be exempt from the universal fiat of general laws1 though ages may pass before the changes it has undergone1 or that are now in progress1 can be referred to existing causes with the same certainty with which the motions of the planets1 and all their periodic and secular variations1 are referable to the law of gravitation4 ,the traces of extreme antiquity perpetually occurring to the geologist give that information1 as to the origin of things1 in vain looked for in the other parts of the universe4 ,they date the beginning of time with regard to our system1 since there is ground to believe that the formation of the earth was contemporaneous with that of the rest of the planets2 but they show that creation is the work of ,him with whom 8a thousand years are as one day1 and one day as a thousand years40 ,in the work now brought to a conclusion1 it has been necessary to select from the whole circle of the sciences a few of the most obvious of those proximate links which connect them together1 and to pass over innumerable cases both of evident and occult alliance4 ,any one branch traced through its ramifications would alone have occupied a volume2 it is hoped1 nevertheless1 that the view here given will suffice to show the extent to which a consideration of the reciprocal influence of even a few of these subjects may ultimately lead4 ,it thus appears that the theory of dynamics1 founded upon terrestrial phenomena1 is indispensable for acquiring a knowledge of the revolutions of the celestial bodies and their reciprocal influences4 ,the motions of the satellites are affected by the forms of their primaries1 and the figures of the planets themselves depend upon their rotations4 ,the symmetry of their internal structure proves the stability of these rotatory motions1 and the immutability of the length of the day1 which furnishes an invariable standard of time2 and the actual size of the terrestrial spheroid affords the means of ascertaining the dimensions of the solar system1 and provides an invariable foundation for a system of weights and measures4 ,the mutual attraction of the celestial bodies disturbs the fluids at their surfaces1 whence the theory of the tides and of the oscillations of the atmosphere4 ,the density and elasticity of the air1 varying with every alternation of temperature1 lead to the consideration of barometrical changes1 the measurement of heights1 and capillary attraction2 and the doctrine of sound1 including the theory of music1 is to be referred to the small undulations of the arial medium4 ,a knowledge of the action of matter upon light is requisite for tracing the curved path of its rays through the atmosphere1 by which the true places of distant objects are determined1 whether in the heavens or on the earth4 ,by this we learn the nature and properties of the sunbeam1 the mode of its propagation through the ethereal medium1 or in the interior of material bodies1 and the origin of colour4 ,by the eclipses of ,jupiter's satellites the velocity of light is ascertained2 and that velocity1 in the aberration of the fixed stars1 furnishes a direct proof of the real motion of the earth 7,n4 #bcg74 ,the effects of the invisible rays of the spectrum are immediately connected with chemical action2 and heat1 forming a part of the solar ray1 so essential to animated and inanimated existence1 is too important an agent in the economy of creation not to hold a principal place in the connexion of physical sciences2 whence follows its distribution in the interior and over the surface of the globe1 its power on the geological convulsions of our planet1 its influence on the atmosphere and on climate1 and its effects on vegetable and animal life1 evinced in the localities of organized beings on the earth1 in the waters1 and in the air4 ,the correlation between molecular and chemical action1 light1 heat1 electricity1 and magnetism1 is continually becoming more perfect1 and there is every reason to believe that these different modes of force1 as well as gravity itself1 will ultimately be found to merge in one great and universal power4 ,many more instances might be given in illustration of the immediate connexion of the physical sciences1 most of which are united still more closely by the common bond of analysis1 which is daily extending its empire1 and will ultimately embrace almost every subject in nature in its formul4 ,these formul1 emblematic of ,omniscience1 condense into a few symbols the immutable laws of the universe4 ,this mighty instrument of human power itself originates in the primitive constitution of the human mind1 and rests upon a few fundamental axioms1 which have eternally existed in ,him who implanted them in the breast of man when ,he created him after ,his own image4 ,,notes4 ,,note #a1 page #b4 ,diameter4 ,a straight line passing through the centre1 and terminated both ways by the sides or surface of a figure1 such as of a circle or sphere4 ,in fig4 #a1 q ,q1 ,n ,s1 are diameters4 ,,note #b1 p4 #b4 ,mathematical and mechanical sciences4 ,mathematics teach the laws of number and quantity2 mechanics treat of the equilibrium and motion of bodies4 ,,note #c1 p4 #b4 ,analysis is a series of reasoning conducted by signs or symbols of the quantities whose relations form the subject of inquiry4 ,,note #d1 p4 #c4 ,oscillations are movements to and fro1 like the swinging of the pendulum of a clock1 or waves in water4 ,the tides are oscillations of the sea4 ,,note #e1 p4 #c4 ,gravitation4 ,gravity is the reciprocal attraction of matter on matter2 gravitation is the difference between gravity and the centrifugal force induced by the velocity of rotation or revolution4 ,sensible gravity1 or weight1 is a particular instance of gravitation4 ,it is the force which causes substances to fall to the surface of the earth1 and which retains the celestial bodies in their orbits4 ,its intensity increases as the squares of the distance decrease4 ,,note #f1 p4 #d4 ,particles of matter are the indefinitely small or ultimate atoms into which matter is believed to be divisible4 ,their form is unknown2 but1 though too small to be visible1 they must have magnitude4 ,,note #g1 p4 #d4 ,a hollow sphere4 ,a hollow ball1 like a bomb-shell4 ,a sphere is a ball or solid body1 such1 that all lines drawn from its centre to its surface are equal4 ,they are called radii1 and every line passing through the centre and terminated both ways by the surface is a diameter1 which is consequently equal to twice the radius4 ,in fig4 #c1 ,q q or ,n ,s is a diameter1 and ,c ,q1 ,c ,n are radii4 ,a great circle of the sphere has the same centre with the sphere as the circles ,q ,e q d and ,q ,n q ,s4 ,the circle ,a ,b is a lesser circle of the sphere4 ,,note #h1 p4 #d4 ,concentric hollow spheres4 ,shells1 or hollow spheres1 having the same centre1 like the coats of an onion4 ,illustration3 ,fig4 #a4 ,,note #i1 p4 #d4 ,spheroid4 ,a solid body1 which sometimes has the shape of an orange1 as in fig4 #a2 it is then called an oblate spheroid1 because it is flattened at the poles ,n and ,s4 ,such is the form of the earth and planets4 ,when1 on the contrary1 it is drawn out at the poles like an egg1 as in fig4 #b1 it is called a prolate spheroid4 ,it is evident that in both these solids the radii ,c q1 ,c a1 ,c ,n1 c41 are generally unequal2 whereas in the sphere they are all equal4 ,illustration3 ,fig4 #b4 ,,note #aj1 p4 #d4 ,centre of gravity4 ,a point in every body1 which if supported1 the body will remain at rest in whatever position it may be placed4 ,about that point all the parts exactly balance one another4 ,the celestial bodies attract each other as if each were condensed into a single particle situate in the centre of gravity1 or the particle situate in the centre of gravity of each may be regarded as possessing the resultant power of the innumerable oblique forces which constitute the whole attraction of the body4 ,,note #aa1 pp4 #d1 #f4 ,poles and equator4 ,let fig4 #a or #c represent the earth1 ,c its centre1 ,n ,c ,s the axis of rotation1 or the imaginary line about which it performs its daily revolution4 ,then ,n and ,s are the north and south poles1 and the great circle q ,e ,q1 which divides the earth into two equal parts1 is the equator4 ,the earth is flattened at the poles1 fig4 #a1 the equatorial diameter1 q ,q1 exceeding the polar diameter1 ,n ,s1 by about #bf-a_/b miles4 ,lesser circles1 ,a ,b ,g1 which are parallel to the equator1 are circles or parallels of latitude1 which is estimated in degrees1 minutes1 and seconds1 north and south of the equator1 every place in the same parallel having the same latitude4 ,greenwich is in the parallel of #ea #bh #dj4 ,thus terrestrial latitude is the angular distance between the direction of a plumb-line at any place and the plane of the equator4 ,lines such as ,n ,q ,s1 ,n ,g ,e ,s1 fig4 #c1 are called meridians2 all the places in any one of these lines have noon at the same instant4 ,the meridian of ,greenwich has been chosen by the ,british as the origin of terrestrial longitude1 which is estimated in degrees1 minutes1 and seconds1 east and west of that line4 ,if ,n ,g ,e ,s be the meridian of ,greenwich1 the position of any place1 ,b1 is determined1 when its latitude1 ,q ,c ,b1 and its longitude1 ,e ,c ,q1 are known4 ,illustration3 ,fig4 #c4 ,,note #ab1 p4 #d4 ,mean quantities are such as are intermediate between others that are greater and less4 ,the mean of any number of unequal quantities is equal to their sum divided by their number4 ,for instance1 the mean between two unequal quantities is equal to half their sum4 ,,note #ac1 p4 #d4 ,a certain mean latitude4 ,the attraction of a sphere on an external body is the same as if its mass were collected into one heavy particle in its centre of gravity1 and the intensity of its attraction diminishes as the square of its distance from the external body increases4 ,but the attraction of a spheroid1 fig4 #a1 on an external body at m in the plane of its equator1 ,e ,q1 is greater1 and its attraction on the same body when at m in the axis ,n ,s less1 than if it were a sphere4 ,therefore1 in both cases1 the force deviates from the exact law of gravity4 ,this deviation arises from the protuberant matter at the equator2 and1 as it diminishes towards the poles1 so does the attractive force of the spheroid4 ,but there is one mean latitude1 where the attraction of a spheroid is the same as if it were a sphere4 ,it is a part of the spheroid intermediate between the equator and the pole4 ,in that latitude the square of the sine is equal to #a_/c of the equatorial radius4 ,,note #ad1 p4 #d4 ,mean distance4 ,the mean distance of a planet from the centre of the sun1 or of a satellite from the centre of its planet1 is equal to half the sum of its greatest and least distances1 and1 consequently1 is equal to half the major axis of its orbit4 ,for example1 let ,p ,q ,a ,d1 fig4 #f1 be the orbit or path of the moon or of a planet2 then ,p ,a is the major axis1 ,c the centre1 and ,c ,s is equal to ,c ,f4 ,now1 since the earth or the sun is supposed to be in the point ,s according as ,p ,d ,a ,q is regarded as the orbit of the moon or that of a planet1 ,s ,a1 ,s ,p are the greatest and least distances4 ,but half the sum of ,s ,a and ,s ,p is equal to half of ,a ,p1 the major axis of the orbit4 ,when the body is at ,q or ,d1 it is at its mean distance from ,s1 for ,s ,q1 ,s ,d1 are each equal to ,c ,p1 half the major axis by the nature of the curve4 ,,note #ae1 p4 #d4 ,mean radius of the earth4 ,the distance from the centre to the surface of the earth1 regarded as a sphere4 ,it is intermediate between the distances of the centre of the earth from the pole and from the equator4 ,,note #af1 p4 #e4 ,ratio4 ,the relation which one quantity bears to another4 ,,note #ag1 p4 #e4 ,square of moon's distance4 ,in order to avoid large numbers1 the mean radius of the earth is taken for unity3 then the mean distance of the moon is expressed by #fj2 and the square of that number is #cfjj1 or #fj times #fj4 ,illustration3 ,fig4 #d ,,note #ah1 p4 #e4 ,centrifugal force4 ,the force with which a revolving body tends to fly from the centre of motion3 a sling tends to fly from the hand in consequence of the centrifugal force4 ,a tangent is a straight line touching a curved line in one point without cutting it1 as m ,t1 fig4 #d4 ,the direction of the centrifugal force is in the tangent to the curved line or path in which the body revolves1 and its intensity increases with the angular swing of the body1 and with its distance from the centre of motion4 ,as the orbit of the moon does not differ much from a circle1 let it be represented by m d g h1 fig4 #d1 the earth being in ,c4 ,the centrifugal force arising from the velocity of the moon in her orbit balances the attraction of the earth4 ,by their joint action1 the moon moves through the arc m n during the time that she would fly off in the tangent m ,t by the action of the centrifugal force alone1 or fall through m p by the earth's attraction alone4 ,t n1 the deflection from the tangent1 is parallel and equal to m p1 the versed sine of the arc m n1 supposed to be moved over by the moon in a second1 and therefore so very small that it may be regarded as a straight line4 ,t n1 or m p1 is the space the moon would fall through in the first second of her descent to the earth1 were she not retained in her orbit by her centrifugal force4 ,,note #ai1 p4 #e4 ,action and reaction4 ,when motion is communicated by collision or pressure1 the action of the body which strikes is returned with equal force by the body which receives the blow4 ,the pressure of a hand on a table is resisted with an equal and contrary force4 ,this necessarily follows from the impenetrability of matter1 a property by which no two particles of matter can occupy the same identical portion of space at the same time4 ,when motion is communicated without apparent contact1 as in gravitation1 attraction1 and repulsion1 the quantity of motion gained by the one body is exactly equal to that lost by the other1 but in a contrary direction2 a circumstance known by experience only4 ,,note #bj1 p4 #e4 ,projected4 ,a body is projected when it is thrown3 a ball fired from a gun is projected2 it is therefore called a projectile4 ,but the word has also another meaning4 ,a line1 surface1 or solid body1 is said to be projected upon a plane1 when parallel straight lines are drawn from every point of it to the plane4 ,the figure so traced upon a plane is a projection4 ,the projection of a terrestrial object is therefore its daylight shadow1 since the sun's rays are sensibly parallel4 ,,note #ba1 p4 #e4 ,space4 ,the boundless region which contains all creation4 ,illustration3 ,fig4 #e4 ,illustration3 ,fig4 #f4 ,,note #bb1 pp4 #e1 #aa4 ,conic sections4 ,lines formed by any plane cutting a cone4 ,a cone is a solid figure1 like a sugar-loaf1 fig4 #e1 of which ,a is the apex1 ,a ,d the axis1 and the plane ,b ,e ,c ,f the base4 ,the axis may or may not be perpendicular to the base1 and the base may be a circle1 or any other curved line4 ,when the axis is perpendicular to the base1 the solid is a right cone4 ,if a right cone with a circular base be cut at right angles to the base by a plane passing through the apex1 the section will be a triangle4 ,if the cone be cut through both sides by a plane parallel to the base1 the section will be a circle4 ,if the cone be cut slanting quite through both sides1 the section will be an ellipse1 fig4 #f4 ,if the cone be cut parallel to one of the sloping sides as ,a ,b1 the section will be a parabola1 fig4 #g4 ,and if the plane cut only one side of the cone1 and be not parallel to the other1 the section will be a hyperbola1 fig4 #h4 ,thus there are five conic sections4 ,illustration3 ,fig4 #g4 ,illustration3 ,fig4 #h4 ,,note #bc1 p4 #e4 ,inverse square of distance4 ,the attraction of one body for another at the distance of two miles is four times less than at the distance of one mile2 at three miles1 it is nine times less than at one2 at four miles1 it is sixteen times less1 and so on4 ,that is1 the gravitating force decreases in intensity as the squares of the distance increase4 ,,note #bd1 p4 #e4 ,ellipse4 ,one of the conic sections1 fig4 #f4 ,an ellipse may be drawn by fixing the ends of a string to two points1 ,s and ,f1 in a sheet of paper1 and then carrying the point of a pencil round in the loop of the string kept stretched1 the length of the string being greater than the distance between the two points4 ,the points ,s and ,f are called the foci1 ,c the centre1 ,s ,c or ,c ,f the excentricity1 ,a ,p the major axis1 ,q ,d the minor axis1 and ,p ,s the focal distance4 ,it is evident that1 the less the excentricity ,c ,s1 the nearer does the ellipse approach to a circle2 and from the construction it is clear that the length of the string ,s m ,f is equal to the major axis ,p ,a4 ,if ,t t be a tangent to the ellipse at m1 then the angle ,t m ,s is equal to the angle t m ,f2 and1 as this is true for every point in the ellipse1 it follows that1 in an elliptical reflecting surface1 rays of light or sound coming from one focus ,s will be reflected by the surface to the other focus ,f1 since the angle of incidence is equal to the angle of reflection by the theories of light and sound4 ,,note #be1 p4 #e4 ,periodic time4 ,the time in which a planet or comet performs a revolution round the sun1 or a satellite about its planet4 ,,note #bf1 p4 #e4 ,kepler discovered three laws in the planetary motions by which the principle of gravitation is established3--#ast law1 ,that the radii vectores of the planets and comets describe areas proportional to the time4--,let fig4 #i be the orbit of a planet2 then1 supposing the spaces or areas ,p ,s p1 p ,s a1 a ,s b1 c41 equal to one another1 the radius vector ,s ,p1 which is the line joining the centres of the sun and planet1 passes over these equal spaces in equal times2 that is1 if the line ,s ,p passes to ,s p in one day1 it will come to ,s a in two days1 to ,s b in three days1 and so on4 #bnd law1 ,that the orbits or paths of the planets and comets are conic sections1 having the sun in one of their foci4 ,the orbits of the planets and satellites are curves like fig4 #f or #i1 called ellipses1 having the sun in the focus ,s4 ,several comets are known to move in ellipses2 but the greater part seem to move in parabolas1 fig4 #g1 having the sun in ,s1 though it is probable that they really move in very long flat ellipses2 others appear to move in hyperbolas1 like fig4 #h4 ,the third law is1 that the squares of the periodic times of the planets are proportional to the cubes of their mean distances from the sun4 ,the square of a number is that number multiplied by itself1 and the cube of a number is that number twice multiplied by itself4 ,for example1 the squares of the numbers #b1 #c1 #d1 c41 are #d1 #i1 #af1 c41 but their cubes are #h1 #bg1 #fd1 c4 ,then the squares of the numbers representing the periodic times of two planets are to one another as the cubes of the numbers representing their mean distances from the sun4 ,so that1 three of these quantities being known1 the other may be found by the rule of three4 ,the mean distances are measured in miles or terrestrial radii1 and the periodic times are estimated in years1 days1 and parts of a day4 ,kepler's laws extend to the satellites4 ,illustration3 ,fig4 #i4 ,,note #bg1 p4 #e4 ,mass4 ,the quantity of matter in a given bulk4 ,it is proportional to the density and volume or bulk conjointly4 ,,note #bh1 p4 #e4 ,gravitation proportional to mass4 ,but for the resistance of the air1 all bodies would fall to the ground in equal times4 ,in fact1 a hundred equal particles of matter at equal distances from the surface of the earth would fall to the ground in parallel straight lines with equal rapidity1 and no change whatever would take place in the circumstances of their descent1 if #ii of them were united in one solid mass2 for the solid mass and the single particle would touch the ground at the same instant1 were it not for the resistance of the air4 ,,note #bi1 p4 #e4 ,primary signifies1 in astronomy1 the planet about which a satellite revolves4 ,the earth is primary to the moon4 ,,note #cj1 p4 #f4 ,rotation4 ,motion round an axis1 real or imaginary4 ,,note #ca1 p4 #g4 ,compression of a spheroid4 ,the flattening at the poles4 ,it is equal to the difference between the greatest and least diameters1 divided by the greatest1 these quantities being expressed in some standard measure1 as miles4 ,,note #cb1 p4 #g4 ,,satellites4 ,small bodies revolving about some of the planets4 ,the moon is a satellite to the earth4 ,,note #cc1 p4 #g4 ,nutation4 ,a nodding motion in the earth's axis while in rotation1 similar to that observed in the spinning of a top4 ,it is produced by the attraction of the sun and moon on the protuberant matter at the terrestrial equator4 ,,note #cd1 p4 #g4 ,axis of rotation4 ,the line1 real or imaginary1 about which a body revolves4 ,the axis of the earth's rotation is that diameter1 or imaginary line1 passing through the centre and both poles4 ,fig4 #a being the earth1 ,n ,s is the axis of rotation4 ,,note #ce1 p4 #g4 ,nutation of lunar orbit4 ,the action of the bulging matter at the earth's equator on the moon occasions a variation in the inclination of the lunar orbit to the plane of the ecliptic4 ,suppose the plane ,n p n1 fig4 #ac1 to be the orbit of the moon1 and ,n m n the plane of the ecliptic1 the earth's action on the moon causes the angle p ,n m to become less or greater than its mean state4 ,the nutation in the lunar orbit is the reaction of the nutation in the earth's axis4 ,,note #cf1 p4 #g4 ,translated4 ,carried forward in space4 ,,note #cg1 p4 #g4 ,force proportional to velocity4 ,since a force is measured by its effect1 the motions of the bodies of the solar system among themselves would be the same whether the system be at rest or not4 ,the real motion of a person walking the deck of a ship at sea is compounded of his own motion and that of the ship1 yet each takes place independently of the other4 ,we walk about as if the earth were at rest1 though it has the double motion of rotation on its axis and revolution round the sun4 ,,note #ch1 p4 #h4 ,tangent4 ,a straight line which touches a curved line in one point without cutting it4 ,in fig4 #d1 m ,t is tangent to the curve in the point m4 ,in a circle the tangent is at right angles to the radius1 ,c m4 ,,note #ci1 p4 #h4 ,motion in an elliptical orbit4 ,a planet m1 fig4 #f1 moves round the sun at ,s in an ellipse ,p ,d ,a ,q1 in consequence of two forces1 one urging it in the direction of the tangent m ,t1 and another pulling it towards the sun in the direction m ,s4 ,its velocity1 which is greatest at ,p1 decreases throughout the arc to ,p ,d ,a to ,a1 where it is least1 and increases continually as it moves along the arc ,a ,q ,p till it comes to ,p again4 ,the whole force producing the elliptical motion varies inversely as the square of the distance4 ,see note #bc4 ,,note #dj1 p4 #h4 ,radii vectores4 ,imaginary lines adjoining the centre of the sun and the centre of a planet or comet1 or the centres of a planet and its satellite4 ,in the circle1 the radii are all equal2 but in an ellipse1 fig4 #f1 the radius vector ,s ,a is greater1 and ,s ,p less than all the others4 ,the radii vectores ,s ,q1 ,s ,d1 are equal to ,c ,a or ,c ,p1 half the major axis ,p ,a1 and consequently equal to the mean distance4 ,a planet is at its mean distance from the sun when in the points ,q and ,d4 ,,note #da1 p4 #h4 ,equal areas in equal times4 ,see ,kepler's #ast law1 in note #bf1 p4 #e4 ,,note #db1 p4 #h4 ,major axis4 ,the line ,p ,a1 fig4 #f or #aj4 ,,note #dc1 p4 #h4 ,if the planet described a circle1 c4 ,the motion of a planet about the sun1 in a circle ,a ,b ,p1 fig4 #aj1 whose radius ,c ,a is equal to the planet's mean distance from him1 would be equable1 that is1 its velocity1 or speed1 would always be the same4 ,whereas1 if it moved in the ellipse ,a ,q ,p1 its speed would be continually varying1 by note #ci2 but its motion is such1 that the time elapsing between its departure from ,p and its return to that point again would be the same whether it moved in the circle or in the ellipse2 for these curves coincide in the points ,p and ,a4 ,,note #dd1 p4 #h4 ,true motion4 ,the motion of a body in its real orbit ,p ,d ,a ,q1 fig4 #aj4 ,illustration3 ,fig4 #aj4 ,,note #de1 p4 #i4 ,mean motion4 ,equable motion in a circle ,p ,e ,a ,b1 fig4 #aj1 at the mean distance ,c ,p or ,c m1 in the time that the body would accomplish a revolution in its elliptical orbit ,p ,d ,a ,q4 ,,note #df1 p4 #i4 ,the equinox4 ,fig4 #aa represents the celestial sphere1 and ,c its centre1 where the earth is supposed to be4 q ,q is the equinoctial or great circle1 traced in the starry heavens by an imaginary extension of the plane of the terrestrial equator1 and ,e e is the ecliptic1 or apparent path of the sun round the earth4 1 the intersection of these two planes1 is the line of the equinoxes2 is the vernal equinox1 and the autumnal4 ,when the sun is in these points1 the days and nights are equal4 ,they are distant from one another by a semicircle1 or two right angles4 ,the points ,e and e are the solstices1 where the sun is at his greatest distance from the equinoctial4 ,the equinoctial is everywhere ninety degrees distant from its poles ,n and ,s1 which are two points diametrically opposite to one another1 where the axis of the earth's rotation1 if prolonged1 would meet the heavens4 ,the northern celestial pole ,n is within #a #bd of the pole star4 ,as the latitude of any place on the surface of the earth is equal to the height of the pole above the horizon1 it is easily determined by observation4 ,the ecliptic ,e e is also everywhere ninety degrees distant from its poles ,p and p4 ,the angle ,p ,c ,n1 between the poles ,p and ,n of the equinoctial and ecliptic1 is equal to the angle e ,c ,q1 called the obliquity of the ecliptic4 ,illustration3 ,fig4 #aa4 ,,note #dg1 p4 #i4 ,longitude4 ,the vernal equinox1 1 fig4 #aa1 is the zero point in the heavens whence celestial longitudes1 or the angular motions of the celestial bodies1 are estimated from west to east1 the direction in which they all revolve4 ,the vernal equinox is generally called the first point of ,aries1 though these two points have not coincided since the early ages of astronomy1 about #bbcc years ago1 on account of a motion in the equinoctial points1 to be explained hereafter4 ,if ,s 1 fig4 #aj1 be the line of the equinoxes1 and the vernal equinox1 the true longitude of a planet p is the angle ,s p1 and its mean longitude is the angle ,c m1 the sun being in ,s4 ,celestial longitude is the angular distance of a heavenly body from the vernal equinox2 whereas terrestrial longitude is the angular distance of a place on the surface of the earth from a meridian arbitrarily chosen1 as that of ,greenwich4 ,,note #dh1 pp4 #i1 #eh4 ,equation of the centre4 ,the difference between ,c m and ,s p1 fig4 #aj2 that is1 the difference between the true and mean longitudes of a planet or satellite4 ,the true and mean places only coincide in the points ,p and ,a2 in every other point of the orbit1 the true place is either before or behind the mean place4 ,in moving from ,a through the arc ,a ,q ,p1 the true place p is behind the mean place m2 and through the arc ,p ,d ,a the true place is before the mean place4 ,at its maximum1 the equation of the centre measures ,c ,s1 the excentricity of the orbit1 since it is the difference between the motion of a body in an ellipse and in a circle whose diameter ,a ,p is the major axis of the ellipse4 ,,note #di1 p4 #i4 ,apsides4 ,the points ,p and ,a1 fig4 #aj1 at the extremities of the major axis of an orbit4 ,p is commonly called the perihelion1 a ,greek term signifying round the sun2 and the point ,a is called the aphelion1 a ,greek term signifying at a distance from the sun4 ,,note #ej1 p4 #i4 ,ninety degrees4 ,a circle is divided into #cfj equal parts1 or degrees2 each degree into #fj equal parts1 called minutes2 and each minute into #fj equal parts1 called seconds4 ,it is usual to write these quantities thus1 #ae #af #aj1 which means fifteen degrees1 sixteen minutes1 and ten seconds4 ,it is clear that an arc m n1 fig4 #d1 measures the angle m ,c n2 hence we may say1 an arc of so many degrees1 or an angle of so many degrees2 for1 if there be ten degrees in the angle m ,c n1 there will be ten degrees in the arc m n4 ,it is evident that there are #ij in a right angle1 m ,c d1 or quadrant1 since it is the fourth part of #cfj4 ,,note #ea1 p4 #i4 ,quadratures4 ,a celestial body is said to be in quadrature when it is #ij degrees distant from the sun4 ,for example1 in fig4 #ad1 if d be the sun1 ,s the earth1 and p the moon1 then the moon is said to be in quadrature when she is in either of the points ,q or ,d1 because the angles ,q ,s d and ,d ,s d1 which measure her apparent distance from the sun1 are right angles4 ,,note #eb1 p4 #i4 ,excentricity4 ,deviation from circular form4 ,in fig4 #f1 ,c ,s is the excentricity of the orbit ,p ,q ,a ,d4 ,the less ,c ,s1 the more nearly does the orbit or ellipse approach the circular form2 and1 when ,c ,s is zero1 the ellipse becomes a circle4 ,,note #ec1 p4 #i4 ,inclination of an orbit4 ,let ,s1 fig4 #ab1 be the centre of the sun1 ,p ,n ,a n the orbit of a planet moving from west to east in the direction ,n p4 ,let ,e ,n m e n be the shadow or projection of the orbit on the plane of the ecliptic1 then ,n ,s n is the intersection of these two planes1 for the orbit rises above the plane of the ecliptic towards ,n p1 and sinks below it at ,n ,p4 ,the angle p ,n m1 which these two planes make with one another1 is the inclination of the orbit ,p ,n p ,a to the plane of the ecliptic4 ,,note #ed1 p4 #i4 ,latitude of a planet4 ,the angle p ,s m1 fig4 #ab1 or the height of the planet p above the ecliptic ,e ,n m4 ,in this case the latitude is north4 ,thus1 celestial latitude is the angular distance of a celestial body from the plane of the ecliptic1 whereas terrestrial latitude is the angular distance of a place on the surface of the earth from the equator4 ,illustration3 ,fig4 #ab4 ,,note #ee1 p4 #i4 ,nodes4 ,the two points ,n and n1 fig4 #ab1 in which the orbit ,n ,a n ,p of a planet or comet intersects the plane of the ecliptic e ,n ,e n4 ,the part ,n ,a n of the orbit lies above the plane of the ecliptic1 and the part n ,p ,n below it4 ,the ascending node ,n is the point through which the body passes in rising above the plane of the ecliptic1 and the descending node n is the point in which the body sinks below it4 ,the nodes of a satellite's orbit are the points in which it intersects the plane of the orbit of the planet4 ,,note #ef1 p4 #aj4 ,distance from the sun4 ,s p in fig4 #ab4 ,if be the vernal equinox1 then ,s p is the longitude of the planet p1 m ,s p is its latitude1 and ,s p its distance from the sun4 ,when these three quantities are known1 the place of the planet p is determined in space4 ,,note #eg1 pp4 #aj1 #ei4 ,elements of an orbit4 ,of these there are seven4 ,let ,p ,n ,a n1 fig4 #ab1 be the elliptical orbit of a planet1 ,c its centre1 ,s the sun in one of the foci1 the point of ,aries1 and ,e ,n e n the plane of the ecliptic4 ,the elements are--the major axis ,a ,p2 the excentricity ,c ,s2 the periodic time1 that is1 the time of a complete revolution of the body in its orbit2 and the fourth is the longitude of the body at any given instant--for example1 that at which it passes through the perihelion ,p1 the point of its orbit nearest to the sun4 ,that instant is assumed as the origin of time1 whence all preceding and succeeding periods are estimated4 ,these four quantities are sufficient to determine the form of the orbit1 and the motion of the body in it4 ,three other elements are requisite for determining the position of the orbit in space4 ,these are1 the angle ,s ,p1 the longitude of the perihelion2 the angle ,a ,n e1 which is the inclination of the orbit to the plane of the ecliptic2 and1 lastly1 the angle ,s ,n1 the longitude of ,n the ascending node4 ,,note #eh1 p4 #aj4 ,whose planes1 c4 ,the planes of the orbits1 as ,p ,n ,a n1 fig4 #ab1 in which the planets move1 are inclined or make small angles e ,n ,a with the plane of the ecliptic ,e ,n e n1 and cut it in straight lines1 ,n ,s n passing through ,s1 the centre of the sun4 ,,note #ei1 p4 #aa4 ,momentum4 ,force measured by the weight of a body and its speed1 or simple velocity1 conjointly4 ,the primitive momentum of the planets is1 therefore1 the quantity of motion which was impressed upon them when they were first thrown into space4 ,,note #fj1 p4 #aa4 ,unstable equilibrium4 ,a body is said to be in equilibrium when it is so balanced as to remain at rest4 ,but there are two kinds of equilibrium1 stable and unstable4 ,if a body balanced in stable equilibrium be slightly disturbed1 it will endeavour to return to rest by a number of movements to and fro1 which will continually decrease till they cease altogether1 and then the body will be restored to its original state of repose4 ,but1 if the equilibrium be unstable1 these movements to and fro1 or oscillations1 will become greater and greater till the equilibrium is destroyed4 ,,note #fa1 p4 #ad4 ,retrograde4 ,going backwards1 as from east to west1 contrary to the motion of the planets4 ,,note #fb1 p4 #ad4 ,parallel directions4 ,such as never meet1 though prolonged ever so far4 ,illustration3 ,fig4 #ac4 ,illustration3 ,fig4 #ad4 ,,note #fc1 pp4 #ad1 #af4 ,the whole force1 c4 ,let ,s1 fig4 #ac1 be the sun1 ,n m n the plane of the ecliptic1 p the disturbed planet moving in its orbit n p ,n1 and d the disturbing planet4 ,now1 d attracts the sun and the planet p with different intensities in the directions d ,s1 d p3 the difference only of these forces disturbs the motion of p2 it is therefore called the disturbing force4 ,but this whole disturbing force may be regarded as equivalent to three forces1 acting in the directions p ,s1 p ,t1 and p m4 ,the force acting in the radius vector p ,s1 joining the centres of the sun and planet1 is called the radial force4 ,it sometimes draws the disturbed planet p from the sun1 and sometimes brings it nearer to him4 ,the force which acts in the direction of the tangent p ,t is called the tangential force4 ,it disturbs the motion of p in longitude1 that is1 it accelerates its motion in some parts of its orbit and retards it in others1 so that the radius vector ,s p does not move over equal areas in equal times4 7,see note #bf47 ,for example1 in the position of the bodies in fig4 #ad1 it is evident that1 in consequence of the attraction of d1 the planet p will have its motion accelerated from ,q to ,c1 retarded from ,c to ,d1 again accelerated from ,d to ,o1 and lastly retarded from ,o to ,q4 ,the disturbing body is here supposed to be at rest1 and the orbit circular2 but1 as both bodies are perpetually moving with different velocities in ellipses1 the perturbations or changes in the motions of p are very numerous4 ,lastly1 that part of the disturbing force which acts in the direction of a line p m1 fig4 #ac1 at right angles to the plane of the orbit ,n p n1 may be called the perpendicular force4 ,it sometimes causes the body to approach nearer1 and sometimes to recede farther from1 the plane of the ecliptic ,n m n1 than it would otherwise do4 ,the action of the disturbing forces is admirably explained in a work on gravitation1 by ,mr4 ,airy1 the ,astronomer ,royal4 ,,note #fd1 pp4 #af1 #gd4 ,perihelion4 ,fig4 #aj1 ,p1 the point of an orbit nearest the sun4 ,,note #fe1 p4 #af4 ,aphelion4 ,fig4 #aj1 ,a1 the point of an orbit farthest from the sun4 ,,note #ff1 pp4 #af1 #ag4 ,in fig4 #ae the central force is greater than the exact law of gravity2 therefore the curvature ,p p a is greater than ,p p ,a the real ellipse2 hence the planet p comes to the point a1 called the aphelion1 sooner than if it moved in the orbit ,p p ,a1 which makes the line ,p ,s ,a advance to a4 ,in fig4 #af1 on the contrary1 the curvature ,p p a is less than in the true ellipse1 so that the planet p must move through more than the arc ,p p ,a1 or #ahj1 before it comes to the aphelion a1 which causes the greater axis ,p ,s ,a to recede to a4 ,illustration3 ,fig4 #ae4 ,illustration3 ,fig4 #af4 ,,note #fg1 pp4 #af1 #ag4 ,motion of apsides4 ,let ,p ,s ,a1 fig4 #ag1 be the position of the elliptical orbit of a planet1 at any time2 then1 by the action of the disturbing forces1 it successively takes the position ,p ,s ,a1 ,p ,s ,a1 c41 till by this direct motion it has accomplished a revolution1 and then it begins again2 so that the motion is perpetual4 ,illustration3 ,fig4 #ag4 ,,note #fh1 p4 #ag4 ,sidereal revolution4 ,the consecutive return of an object to the same star4 ,,note #fi1 p4 #ag4 ,tropical revolution4 ,the consecutive return of an object to the same tropic or equinox4 ,,note #gj1 p4 #ag4 ,the orbit only bulges1 c4 ,in fig4 #ah the effect of the variation in the excentricity is shown where ,p p ,a is the elliptical orbit at any given instant2 after a time it will take the form ,p p ,a1 in consequence of the decrease in the excentricity ,c ,s2 then the forms ,p p ,a1 ,p p ,a1 c41 consecutively from the same cause2 and1 as the major axis ,p ,a always retains the same length1 the orbit approaches more and more nearly to the circular form4 ,but1 after this has gone on for some thousands of years1 the orbit contracts again1 and becomes more and more elliptical4 ,illustration3 ,fig4 #ah4 ,,note #ga1 pp4 #ah1 #ai4 ,the ecliptic is the apparent path of the sun in the heavens4 ,see note #df4 ,,note #gb1 p4 #ah4 ,this force tends to pull1 c4 ,the force in question1 acting in the direction p m1 fig4 #ac1 pulls the planet p towards the plane ,n m n1 or pushes it farther above it1 giving the planet a tendency to move in an orbit above or below its undisturbed orbit ,n p n1 which alters the angle p ,n m1 and makes the node ,n and the line of nodes ,n n change their positions4 ,illustration3 ,fig4 #ai4 ,,note #gc1 p4 #ah4 ,motion of the nodes4 ,let ,s1 fig4 #ai1 be the sun2 ,s ,n n the plane of the ecliptic2 ,p the disturbing body2 and p a planet moving in its orbit p n1 of which p n is so small a part that it is represented as a straight line4 ,the plane ,s n p of this orbit cuts the plane of the ecliptic in the straight line ,s n4 ,suppose the disturbing force begins to act on p1 so as to draw the planet into the arc p p2 then1 instead of moving in the orbit p n1 it will tend to move in the orbit p p n1 whose plane cuts the ecliptic in the straight line ,s n4 ,if the disturbing force acts again upon the body when at p1 so as to draw it into the arc p p1 the planet will now tend to move in the orbit p p n1 whose plane cuts the ecliptic in the straight line ,s n4 ,the action of the disturbing force on the planet when at p will bring the node to n1 and so on4 ,in this manner the node goes backwards through the successive points n1 n1 n1 n1 c41 and the line of nodes ,s n has a perpetual retrograde motion about ,s1 the centre of the sun4 ,the disturbing force has been represented as acting at intervals for the sake of illustration3 in nature it is continuous1 so that the motion of the node is continuous also2 though it is sometimes rapid and sometimes slow1 now retrograde and now direct2 but1 on the whole1 the motion is slowly retrograde4 ,,note #gd1 p4 #ah4 ,when the disturbing planet is anywhere in the line ,s ,n1 fig4 #ai1 or in its prolongation1 it is in the same plane with the disturbed planet2 and1 however much it may affect its motions in that plane1 it can have no tendency to draw it out of it4 ,but when the disturbing planet is in ,p1 at right angles to the line ,s ,n1 and not in the plane of the orbit1 it has a powerful effect on the motion of the nodes3 between these two positions there is great variety of action4 ,,note #ge1 p4 #ai4 ,the changes in the inclination are extremely minute when compared with the motion of the node1 as evidently appears from fig4 #ai1 where the angles n p n1 n p n1 c41 are much smaller than the corresponding angles n ,s n1 ,s n1 c4 ,,note #gf1 p4 #bj4 ,sines and cosines4 ,figure #d is a circle2 n p is the sine1 and ,c p is the cosine of an arc m n4 ,suppose the radius ,c m to begin to revolve at m1 in the direction m n a2 then at the point m the sine is zero1 and the cosine is equal to the radius ,c m4 ,as the line ,c m revolves and takes the successive positions ,c n1 ,c a1 ,c b1 c41 the sines n p1 a q1 b r1 c41 of the arcs m n1 m a1 m h1 c41 increase1 while the corresponding cosines ,c p1 ,c q1 ,c r1 c41 decrease2 and when the revolving radius takes the position ,c d1 at right angles to the diameter g m1 the sine becomes equal to the radius ,c d1 and the cosine is zero4 ,after passing the point d1 the contrary happens2 for the sines e ,k1 l ,v1 c41 diminish1 and the cosines ,c ,k1 ,c ,v1 c41 go on increasing1 till at g the sine is zero1 and the cosine is equal to the radius ,c g4 ,the same alternation takes place through the remaining parts g h1 h m1 of the circle1 so that a sine or cosine never can exceed the radius4 ,as the rotation of the earth is invariable1 each point of its surface passes through a complete circle1 or #cfj degrees1 in twenty-four hours1 at a rate of #ae degrees in an hour4 ,time1 therefore1 becomes a measure of angular motion1 and vice vers1 the arcs of a circle a measure of time1 since these two quantities vary simultaneously and equably2 and1 as the sines and cosines of the arcs are expressed in terms of the time1 they vary with it4 ,therefore1 however long the time may be1 and how often soever the radius may revolve round the circle1 the sines and cosines never can exceed the radius2 and1 as the radius is assumed to be equal to unity1 their values oscillate between unity and zero4 ,,note #gg1 p4 #bj4 ,the small excentricities and inclinations of the planetary orbits1 and the revolutions of all the bodies in the same direction1 were proved by ,euler1 ,la ,grange1 and ,la ,place1 to be conditions necessary for the stability of the solar system4 ,subsequently1 however1 the periodicity of the terms of the series expressing the perturbations was supposed to be sufficient alone1 but ,m4 ,poisson has shown that to be a mistake2 that these three conditions are requisite for the necessary convergence of the series1 and that therefore the stability of the system depends on them conjointly with the periodicity of the sines and cosines of each term4 ,the author is aware that this note can only be intelligible to the analyst1 but she is desirous of correcting an error1 and the more so as the conditions of stability afford one of the most striking instances of design in the original construction of our system1 and of the foresight and supreme wisdom of the ,divine ,architect4 ,,note #gh1 p4 #bb4 ,resisting medium4 ,a fluid which resists the motions of bodies1 such as atmospheric air1 or the highly elastic fluid called ether1 with which space is filled4 ,,note #gi1 p4 #bc4 ,obliquity of the ecliptic4 ,the angle e q1 fig4 #aa1 between the plane of the terrestrial equator q ,q1 and the plane of the ecliptic ,e e4 ,the obliquity is variable4 ,,note #hj1 p4 #bc4 ,invariable plane4 ,in the earth the equator is the invariable plane which nearly maintains a parallel position with regard to itself while revolving about the sun1 as in fig4 #bj1 where ,e ,q represents it4 ,the two hemispheres balance one another on each side of this plane1 and would still do so if all the particles of which they consist were moveable among themselves1 provided the earth were not disturbed by the action of the sun and moon1 which alters the parallelism of the equator by the small variation called nutation1 to be explained hereafter4 ,illustration3 ,fig4 #bj4 ,illustration3 ,fig4 #ba4 ,,note #ha1 p4 #bd4 ,if each particle1 c4 ,let ,p1 ,p1 ,p1 c41 fig4 #ba1 be planets moving in their orbits about the centre of gravity of the system4 ,let ,p ,s ,m1 ,p ,s ,m1 c41 be portions of these orbits moved over by the radii vectores ,s ,p1 ,s ,p1 c41 in a given time1 and let p ,s m1 p ,s m1 c41 be their shadows or projections on the invariable plane4 ,then1 if the numbers which represent the masses of the planets ,p1 ,p1 c41 be respectively multiplied by the numbers representing the areas or spaces p ,s m1 p ,s m1 c41 the sum of the whole will be greater for the invariable plane than it would be for any plane that could pass through ,s1 the centre of gravity of the system4 ,,note #hb1 p4 #bd4 ,the centre of gravity of the solar system lies within the body of the sun1 because his mass is much greater than the masses of all the planets and satellites added together4 ,,note #hc1 pp4 #be1 #cf4 ,conjunction4 ,a planet is said to be in conjunction when it has the same longitude with the sun1 and in opposition when its longitude differs from that of the sun by #ahj degrees4 ,thus two bodies are said to be in conjunction when they are seen exactly in the same part of the heavens1 and in opposition when diametrically opposite to one another4 ,mercury and ,venus1 which are nearer to the sun than the earth1 are called inferior planets2 while all the others1 being farther from the sun than the earth1 are said to be superior planets4 ,suppose the earth to be at ,e1 fig4 #bd2 then a superior planet will be in conjunction with the sun at ,c1 and in opposition to him when at ,o4 ,again1 suppose the earth to be in ,o1 then an inferior planet will be in conjunction when at ,e1 and in opposition when at ,f4 ,,note #hd1 p4 #bf4 ,the periodic inequalities are computed for a given time2 and consequently for a given form and position of the orbits of the disturbed and disturbing bodies4 ,although the elements of the orbits vary so slowly that no sensible effect is produced on inequalities of a short period1 yet1 in the course of time1 the secular variations of the elements change the forms and relative positions of the orbits so much1 that ,jupiter and ,saturn1 which would have come to the same relative positions with regard to the sun and to one another after #hej years1 do not arrive at the same relative positions till after #iah years4 ,,note #he1 p4 #bf4 ,configuration4 ,the relative position of the planets with regard to one another1 to the sun1 and to the plane of the ecliptic4 ,,note #hf1 p4 #bg4 ,in the same manner that the excentricity of an elliptical orbit may be increased or diminished by the action of the disturbing forces1 so a circular orbit may acquire less or more ellipticity from the same cause4 ,it is thus that the forms of the orbits of the first and second satellites of ,jupiter oscillate between circles and ellipses differing very little from circles4 ,illustration3 ,fig4 #bb4 ,,note #hg1 p4 #bh4 ,the plane of ,jupiter's equator is the imaginary plane passing through his centre at right angles to his axis of rotation1 and corresponds to the plane q ,e ,q e1 in fig4 #a4 ,the satellites move very nearly in the plane of ,jupiter's equator2 for1 if ,j be ,jupiter1 fig4 #bb1 ,p p his axis of rotation1 e ,q his equatorial diameter1 which is #fjjj miles longer than ,p p1 and if ,j ,o and ,j ,e be the planes of his orbit and equator seen edgewise1 then the orbits of his four satellites seen edgewise will have the positions ,j#a1 ,j#b1 ,j#c1 ,j#d4 ,these are extremely near to one another1 for the angle ,e ,j ,o is only #c #e #cj4 ,,note #hh1 p4 #bh4 ,in consequence of the satellites moving so nearly in the plane of ,jupiter's equator1 when seen from the earth1 they appear to be always very nearly in a straight line1 however much they may change their positions with regard to one another and to their primary4 ,for example1 on the evenings of the #crd1 #dth1 #eth1 and #fth of ,january1 #ahce1 the satellites had the configurations given in fig4 #bc1 where ,o is ,jupiter1 and #a1 #b1 #c1 #d1 are the first1 second1 third1 and fourth satellites4 ,the satellite is supposed to be moving in a direction from the figure towards the point4 ,on the sixth evening the second satellite was seen on the disc of the planet4 ,illustration3 ,fig4 #bc4 ,,note #hi1 p4 #bh4 ,angular motion or velocity is the swiftness with which a body revolves--a sling1 for example2 or the speed with which the surface of the earth performs its daily rotation about its axis4 ,,note #ij1 p4 #bi4 ,displacement of ,jupiter's orbit4 ,the action of the planets occasions secular variations in the position of ,jupiter's orbit ,j ,o1 fig4 #bb1 without affecting the plane of his equator ,j ,e4 ,again1 the sun and satellites themselves1 by attracting the protuberant matter at ,jupiter's equator1 change the position of the plane ,j ,e without affecting ,j ,o4 ,both of these cause perturbations in the motions of the satellites4 ,,note #ia1 p4 #bi4 ,precession1 with regard to ,jupiter1 is a retrograde motion of the point where the lines ,j ,o1 ,j ,e1 intersect fig4 #bb4 ,,note #ib1 p4 #cj4 ,synodic motion of a satellite4 ,its motion during the interval between two of its consecutive eclipses4 ,illustration3 ,fig4 #bd4 ,,note #ic1 p4 #cj4 ,opposition4 ,a body is said to be in opposition when its longitude differs from that of the sun by #ahj4 ,if ,s1 fig4 #bd1 be the sun1 and ,e the earth1 then ,jupiter is in opposition when at ,o1 and in conjunction when at ,c4 ,in these positions the three bodies are in the same straight line4 ,,note #id1 p4 #cj4 ,eclipses of the satellites4 ,let ,s1 fig4 #be1 be the sun1 ,j ,jupiter1 and a ,b b his shadow4 ,let the earth be moving in its orbit1 in the direction ,e ,a ,r ,t ,h1 and the third satellite in the direction a b m n4 ,when the earth is at ,e1 the satellite1 in moving through the arc a b1 will vanish at a1 and reappear at b1 on the same side of ,jupiter4 ,if the earth be in ,r1 ,jupiter will be in opposition2 and then the satellite1 in moving through the arc a b1 will vanish close to the disc of the planet1 and will reappear on the other side of it4 ,but1 if the satellite be moving through the arc m n1 it will appear to pass over the disc1 and eclipse the planet4 ,illustration3 ,fig4 #be4 ,,note #ie1 pp4 #cj1 #dc4 ,meridian4 ,a terrestrial meridian is a line passing round the earth and through both poles4 ,in every part of it noon happens at the same instant4 ,in figures #a and #c1 the lines ,n ,q ,s and ,n ,g ,s are meridians1 ,c being the centre of the earth1 and ,n ,s its axis of rotation4 ,the meridian passing through the ,observatory at ,greenwich is assumed by the ,british as a fixed origin from whence terrestrial longitudes are measured4 ,and as each point on the surface of the earth passes through #cfj1 or a complete circle1 in twenty-four hours1 at the rate of #ae in an hour1 time becomes a representative of angular motion4 ,hence1 if the eclipse of a satellite happens at any place at eight o'clock in the evening1 and the ,nautical ,almanac shows that the same phenomenon will take place at ,greenwich at nine1 the place of observation will be in the #ae of west longitude4 ,,note #if1 p4 #ca4 ,conjunction4 ,let ,s be the sun1 fig4 #bd1 ,e the earth1 and ,j ,o ,j ,c the orbit of ,jupiter4 ,then the eclipses which happen when ,jupiter is in ,o are seen #afm #bfs sooner than those which take place when the planet is in ,c4 ,jupiter is in conjunction when at ,c1 and in opposition when in ,o4 ,illustration3 ,fig4 #bf4 ,,note #ig1 p4 #ca4 ,in the diagonal1 c4 ,were the line ,a ,s1 fig4 #bf1 #ajj1jjj times longer than ,a ,b1 ,jupiter's true place would be in the direction ,a ,s1 the diagonal of the figure ,a ,b ,s ,s1 which is1 of course1 out of proportion4 ,,note #ih1 p4 #ca4 ,aberration of light4 ,the celestial bodies are so distant that the rays of light coming from them may be reckoned parallel4 ,therefore1 let ,s ,a1 ,s ,b1 fig4 #bf1 be two rays of light coming from the sun1 or a planet1 to the earth moving in its orbit in the direction ,a ,b4 ,if a telescope be held in the direction ,a ,s1 the ray ,s ,a1 instead of going down the tube1 will impinge on its side1 and be lost in consequence of the telescope being carried with the earth in the direction ,a ,b4 ,but1 if the tube be held in the position ,a ,e1 so that ,a ,b is to ,a ,s as the velocity of the earth to the velocity of light1 the ray will pass through ,s ,e ,a4 ,the star appears to be in the direction ,a ,s1 when it really is in the direction ,a ,s2 hence the angle ,s ,a ,s is the angle of aberration4 ,,note #ii1 p4 #cb4 ,density proportional to elasticity4 ,the more a fluid1 such as atmospheric air1 is reduced in dimensions by pressure1 the more it resists the pressure4 ,,note #ajj1 p4 #cb4 ,oscillations of pendulum retarded4 ,if a clock be carried from the pole to the equator1 its rate will be gradually diminished1 that is1 it will go slower and slower3 because the centrifugal force1 which increases from the pole to the equator1 diminishes the force of gravity4 ,,note #aja1 p4 #cd4 ,disturbing action4 ,the disturbing force acts here in the very same manner as in note #fc2 only that the disturbing body d1 fig4 #ad1 is the sun1 ,s the earth1 and p the moon4 ,,note #ajb1 pp4 #ce1 #cf1 #hf4 ,perigee4 ,a ,greek word1 signifying round the earth4 ,the perigee of the lunar orbit is the point ,p1 fig4 #f1 where the moon is nearest to the earth4 ,it corresponds to the perihelion of a planet4 ,sometimes the word is used to denote the point where the sun is nearest to the earth4 ,,note #ajc1 p4 #ce4 ,evection4 ,the evection is produced by the action of the radial force in the direction ,s p1 fig4 #ad1 which sometimes increases and sometimes diminishes the earth's attraction to the moon4 ,it produces a corresponding temporary change in the excentricity1 which varies with the position of the major axis of the lunar orbit in respect of the line ,s d1 joining the centres of the earth and sun4 ,,note #ajd1 p4 #ce4 ,variation4 ,the lunar perturbation called the variation is the alternate acceleration and retardation of the moon in longitude1 from the action of the tangential force4 ,she is accelerated in going from quadratures in ,q and ,d1 fig4 #ad1 to the points ,c and ,o1 called syzygies1 and is retarded in going from the syzygies ,c and ,o to ,q and ,d again4 ,,note #aje1 p4 #cf4 ,square of time4 ,if the times increase at the rate of #a1 #b1 #c1 #d1 c41 years or hundreds of years1 the squares of the times will be #a1 #d1 #i1 #af1 c41 years or hundreds of years4 ,,note #ajf1 p4 #cg4 ,in all investigations hitherto made with regard to the acceleration1 it was tacitly assumed that the areas described by the radius vector of the moon were not permanently altered2 that is to say1 that the tangential disturbing force produced no permanent effect4 ,but ,mr4 ,adams has discovered that1 in consequence of the constant decrease in the excentricity of the earth's orbit1 there is a gradual change in the central disturbing force which affects the arial velocity1 and consequently it alters the amount of the acceleration by a very small quantity1 as well as the variation and other periodical inequalities of the moon4 ,on the latter1 however1 it has no permanent effect1 because it affects them in opposite directions in very moderate intervals of time1 whereas a very small error in the amount of the acceleration goes on increasing as long as the excentricity of the earth's orbit diminishes1 so that it would ultimately vitiate calculations of the moon's place for distant periods of time4 ,this shows how complicated the moon's motions are1 and what rigorous accuracy is required in their determination4 ,to give an idea of the labour requisite merely to perfect or correct the lunar tables1 the moon's place was determined by observation at the ,greenwich ,observatory in #fjjj different points of her orbit1 each of which was compared with the same points calculated from ,baron ,plana's formul1 and to do that sixteen computers were constantly employed for eight years4 ,since the longitude is determined by the motions of the moon1 the lunar tables are of the greatest importance4 ,,note #ajg1 p4 #cg4 ,mean anomaly4 ,the mean anomaly of a planet is its angular distance from the perihelion1 supposing it to move in a circle4 ,the true anomaly is its angular distance from the perihelion in its elliptical orbit4 ,for example1 in fig4 #aj1 the mean anomaly is ,p ,c m1 and the true anomaly is ,p ,s p4 ,,note #ajh1 pp4 #ch1 #fh4 ,many circumferences4 ,there are #cfj degrees or #a1bif1jjj seconds in a circumference2 and1 as the acceleration of the moon only increases at the rate of eleven seconds in a century1 it must be a prodigious number of ages before it accumulates to many circumferences4 ,,note #aji1 p4 #ci4 ,phases of the moon4 ,the periodical changes in the enlightened part of her disc1 from a crescent to a circle1 depending upon her position with regard to the sun and earth4 ,,note #aaj1 p4 #ci4 ,lunar eclipse4 ,let ,s1 fig4 #bg1 be the sun1 ,e the earth1 and m the moon4 ,the space a ,a b is a section of the shadow1 which has the form of a cone or sugar-loaf1 and the spaces ,a a c1 ,a b d1 are the penumbra4 ,the axis of the cone passes through ,a1 and through ,e and ,s1 the centres of the sun and earth1 and n m n is the path of the moon through the shadow4 ,illustration3 ,fig4 #bg4 ,,note #aaa1 p4 #ci4 ,apparent diameter4 ,the diameter of a celestial body as seen from the earth4 ,,note #aab1 p4 #dj4 ,penumbra4 ,the shadow or imperfect darkness which precedes and follows an eclipse4 ,,note #aac1 p4 #dj4 ,synodic revolution of the moon4 ,the time between two consecutive new or full moons4 ,,note #aad1 p4 #dj4 ,horizontal refraction4 ,the light1 in coming from a celestial object1 is bent into a curve as soon as it enters our atmosphere2 and that bending is greatest when the object is in the horizon4 ,illustration3 ,fig4 #bh4 ,,note #aae1 p4 #dj4 ,solar eclipse4 ,let ,s1 fig4 #bh1 be the sun1 m the moon1 and ,e the earth4 ,then a ,e b is the moon's shadow1 which sometimes eclipses a small portion of the earth's surface at e1 and sometimes falls short of it4 ,to a person at e1 in the centre of the shadow1 the eclipse may be total or annular2 to a person not in the centre of the shadow a part of the sun will be eclipsed2 and to one at the edge of the shadow there will be no eclipse at all4 ,the spaces ,p b ,e1 ,p a ,e1 are the penumbra4 ,,note #aaf1 p4 #dc4 ,from the extremities1 c4 ,if the length of the line a b1 fig4 #bi1 be measured1 in feet or fathoms1 the angles ,s b a1 ,s a b1 can be measured1 and then the angle a ,s b is known1 whence the length of the line ,s ,c may be computed4 a ,s b is the parallax of the object ,s2 and it is clear that1 the greater the distance of ,s1 the less the base a b will appear1 because the angle a ,s b is less than a ,s b4 ,illustration3 ,fig4 #bi4 ,,note #aag1 p4 #dd4 ,every particle will describe a circle1 c4 ,if ,n ,s1 fig4 #c1 be the axis about which the body revolves1 then particles at ,b1 ,q1 c41 will whirl in the circles ,b ,g ,a a1 ,q ,e q d1 whose centres are in the axis ,n ,s1 and their planes parallel to one another4 ,they are1 in fact1 parallels of latitude1 ,q ,e q d being the equator4 ,,note #aah1 p4 #dd4 ,the force of gravity1 c4 ,gravity at the equator acts in the direction ,q ,c1 fig4 #cj4 ,whereas the direction of the centrifugal force is exactly contrary1 being in the direction ,c ,q2 hence the difference of the two is the force called gravitation1 which makes bodies fall to the surface of the earth4 ,at any point1 m1 not at the equator1 the direction of gravity is m b1 perpendicular to the surface1 but the centrifugal force acts perpendicularly to ,n ,s1 the axis of rotation4 ,now the effect of the centrifugal force is the same as if it were two forces1 one of which acting in the direction b m1 diminishes the force of gravity1 and another which1 acting in the direction m t1 tangent to the surface at m1 urges the particles towards ,q1 and tends to swell out the earth at the equator4 ,illustration3 ,fig4 #cj4 ,,note #aai1 p4 #de4 ,homogeneous mass4 ,a quantity of matter1 everywhere of the same density4 ,,note #abj1 p4 #de4 ,ellipsoid of revolution4 ,a solid formed by the revolution of an ellipse about its axis4 ,if the ellipse revolve about its minor axis ,q ,d1 fig4 #f1 the ellipsoid will be oblate1 or flattened at the poles like an orange4 ,if the revolution be about the greater axis ,a ,p1 the ellipsoid will be prolate1 like an egg4 ,,note #aba1 p4 #de4 ,concentric elliptical strata4 ,strata1 or layers1 having an elliptical form and the same centre4 ,,note #abb1 p4 #df4 ,on the whole1 c4 ,the line ,n ,q ,s q1 fig4 #a1 represents the ellipse in question1 its major axis being ,q q1 its minor axis ,n ,s4 ,,note #abc1 p4 #df4 ,increase in the length of the radii1 c4 ,the radii gradually increase from the polar radius ,c ,n1 fig4 #cj1 which is least1 to the equatorial radius ,c ,q1 which is greatest4 ,there is also an increase in the lengths of the arcs corresponding to the same number of degrees from the equator to the poles2 for1 the angle ,n ,c r being equal to q ,c d1 the elliptical arc ,n r is less than q d4 ,,note #abd1 p4 #df4 ,cosine of latitude4 ,the angles m ,c a1 m ,c b1 fig4 #d1 being the latitudes of the points a1 b1 c41 the cosines are ,c q1 ,c r1 c4 ,,note #abe1 p4 #dg4 ,an arc of the meridian4 ,let ,n ,q ,s q1 fig4 #cj1 be the meridian1 and m n the arc to be measured4 ,then1 if ,z m1 ,z n1 be verticals1 or lines perpendicular to the surface of the earth1 at the extremities of the arc m n they will meet in p4 ,q a n1 ,q b m1 are the latitudes of the points m and n1 and their difference is the angle m p n4 ,since the latitudes are equal to the height of the pole of the equinoctial above the horizon of the places m and n1 the angle m p n may be found by observation4 ,when the distance m n is measured in feet or fathoms1 and divided by the number of degrees and parts of a degree contained in the angle m p n1 the length of an arc of one degree is obtained4 ,,note #abf1 p4 #dg4 ,a series of triangles4 ,let ,m ,m1 fig4 #ca1 be the meridian of any place4 ,a line ,a ,b is measured with rods1 on level ground1 of any number of fathoms1 ,c being some point seen from both ends of it4 ,as two of the angles of the triangle ,a ,b ,c can be measured1 the lengths of the sides ,a ,c1 ,b ,c1 can be computed2 and if the angle m ,a ,b1 which the base ,a ,b makes with the meridian1 be measured1 the length of the sides ,b m1 ,a m1 may be obtained by computation1 so that ,a m1 a small part of the meridian1 is determined4 ,again1 if ,d be a point visible from the extremities of the known line ,b ,c1 two of the angles of the triangle ,b ,c ,d may be measured1 and the length of the sides ,c ,d1 ,b ,d1 computed4 ,then1 if the angle ,b m m be measured1 all the angles and the side ,b m of the triangle ,b m m are known1 whence the length of the line m m may be computed1 so that the portion ,a m of the meridian is determined1 and in the same manner it may be prolonged indefinitely4 ,illustration3 ,fig4 #ca4 ,,note #abg1 pp4 #dg1 #di4 ,the square of the sine of the latitude4 ,q b m1 fig4 #cj1 being the latitude of m1 e m is the sine and b e the cosine4 ,then the number expressing the length of e m1 multiplied by itself1 is the square of the sine of the latitude2 and the number expressing the length of b e1 multiplied by itself1 is the square of the cosine of the latitude4 ,,note #abh1 p4 #dh4 ,the polar diameter of the earth determined by the survey of ,great ,britain is #gijj miles2 the equatorial is #gibf1 which gives a compression of #a_/biicc4 ,,note #abi1 p4 #ej4 ,a pendulum is that part of a clock which swings to and fro4 ,illustration3 ,fig4 #cb4 ,,note #acj1 p4 #eb4 ,parallax4 ,the angle a ,s b1 fig4 #bi1 under which we view an object a b3 it therefore diminishes as the distance increases4 ,the parallax of a celestial object is the angle which the radius of the earth would be seen under1 if viewed from that object4 ,let ,e1 fig4 #cb1 be the centre of the earth1 ,e ,h its radius1 and m ,h ,o the horizon of an observer at ,h4 ,then ,h m ,e is the parallax of a body m1 the moon for example4 ,as m rises higher and higher in the heavens to the points m1 m1 c41 the parallax ,h m ,e1 ,h m ,e1 c41 decreases4 ,at ,z1 the zenith1 or point immediately above the head of the observer1 it is zero2 and at m1 where the body is in the horizon1 the angle ,h m ,e is the greatest possible1 and is called the horizontal parallax4 ,it is clear that with regard to celestial bodies the whole effect of parallax is in the vertical1 or in the direction m m ,z2 and as a person at ,h sees m in the direction ,h m ,a1 when it really is in the direction ,e m ,b1 it makes celestial objects appear to be lower than they really are4 ,the distance of the moon from the earth has been determined from her horizontal parallax4 ,the angle ,e m ,h can be measured4 ,e ,h m is a right angle1 and ,e ,h1 the radius of the earth1 is known in miles2 whence the distance of the moon ,e m is easily found4 ,annual parallax is the angle under which the diameter of the earth's orbit would be seen if viewed from a star4 ,,note #aca1 p4 #eb4 ,the radii n ,b1 n ,g1 c41 fig4 #c1 are equal in any one parallel of latitude1 ,a a ,b ,g2 therefore a change in the parallax observed in that parallel can only arise from a change in the moon's distance from the earth2 and when the moon is at her mean distance1 which is a constant quantity equal to half the major axis of her orbit1 a change in the parallax observed in different latitudes1 ,g and ,e1 must arise from the difference in the lengths of the radii n ,g and ,c ,e4 ,,note #acb1 p4 #eb4 ,when ,venus is in her nodes4 ,she must be in the line ,n ,s n where her orbit ,p ,n ,a n cuts the plane of the ecliptic ,e ,n e n1 fig4 #ab4 ,,note #acc1 p4 #ec4 ,the line described1 c4 ,let ,e1 fig4 #cc1 be the earth1 ,s the centre of the sun1 and ,v the planet ,venus4 ,the real transit of the planet1 seen from ,e the centre of the earth1 would be in the direction ,a ,b4 ,a person at ,w would see it pass over the sun in the line v a1 and a person at ,o would see it move across him in the direction v a4 ,illustration3 ,fig4 #cc4 ,,note #acd1 p4 #ed4 ,kepler's law4 ,suppose it were required to find the distance of ,jupiter from the sun4 ,the periodic times of ,jupiter and ,venus are given by observation1 and the mean distance of ,venus from the centre of the sun is known in miles or terrestrial radii2 therefore1 by the rule of three1 the square root of the periodic time of ,venus is to the square root of the periodic time of ,jupiter as the cube root of the mean distance of ,venus from the sun to the cube root of the mean distance of ,jupiter from the sun1 which is thus obtained in miles or terrestrial radii4 ,the root of a number is that number which1 once multiplied by itself1 gives its square2 twice multiplied by itself1 gives its cube1 c4 ,for example1 twice #b are #d1 and twice #d are #h2 #b is therefore the square root of #d1 and the cube root of #h4 ,in the same manner #c times #c are #i1 and #c times #i are #bg2 #c is therefore the square root of #i1 and the cube root of #bg4 ,,note #ace1 p4 #ee4 ,inversely1 c4 ,the quantities of matter in any two primary planets are greater in proportion as the cubes of the numbers representing the mean distances of their satellites are greater1 and also in proportion as the squares of their periodic times are less4 ,,note #acf1 p4 #ee4 ,as hardly anything appears more impossible than that man should have been able to weigh the sun as it were in scales and the earth in a balance1 the method of doing so may have some interest4 ,the attraction of the sun is to the attraction of the earth as the quantity of matter in the sun to the quantity of matter in the earth2 and1 as the force of this reciprocal attraction is measured by its effects1 the space the earth would fall through in a second by the sun's attraction is to the space which the sun would fall through by the earth's attraction as the mass of the sun to the mass of the earth4 ,hence1 as many times as the fall of the earth to the sun in a second exceeds the fall of the sun to the earth in the same time1 so many times does the mass of the sun exceed the mass of the earth4 ,thus the weight of the sun will be known if the length of these two spaces can be found in miles or parts of a mile4 ,nothing can be easier4 ,a heavy body falls through #afjfig feet in a second at the surface of the earth by the earth's attraction2 and1 as the force of gravity is inversely as the square of the distance1 it is clear that #afjfig feet are to the space a body would fall through at the distance of the sun by the earth's attraction1 as the square of the distance of the sun from the earth to the square of the distance of the centre of the earth from its surface2 that is1 as the square of #ie1jjj1jjj miles to the square of #djjj miles4 ,and thus1 by a simple question in the rule of three1 the space which the sun would fall through in a second by the attraction of the earth may be found in parts of a mile4 ,the space the earth would fall through in a second1 by the attraction of the sun1 must now be found in miles also4 ,suppose m n1 fig4 #d1 to be the arc which the earth describes round the sun in ,c1 in a second of time1 by the joint action of the sun and the centrifugal force4 ,by the centrifugal force alone the earth would move from m to ,t in a second1 and by the sun's attraction alone it would fall through ,t n in the same time4 ,hence the length of ,t n1 in miles1 is the space the earth would fall through in a second by the sun's attraction4 ,now1 as the earth's orbit is very nearly a circle1 if #cfj degrees be divided by the number of seconds in a sidereal year of #cfe-a_/d days1 it will give m n1 the arc which the earth moves through in a second1 and then the tables will give the length of the line ,c ,t in numbers corresponding to that angle2 but1 as the radius ,c n is assumed to be unity in the tables1 if #a be subtracted from the number representing ,c ,t1 the length of ,t n will be obtained2 and1 when multiplied by #ie1jjj1jjj1 to reduce it to miles1 the space which the earth falls through1 by the sun's attraction1 will be obtained in miles4 ,by this simple process it is found that1 if the sun were placed in one scale of a balance1 it would require #ced1icf earths to form a counterpoise4 ,,note #acg1 p4 #ei4 ,the sum of the greatest and least distances ,s ,p1 ,s ,a1 fig4 #ab1 is equal to ,p ,a1 the major axis2 and their difference is equal to twice the excentricity ,c ,s4 ,the longitude ,s ,p of the planet1 when in the point ,p1 at its least distance from the sun1 is the longitude of the perihelion4 ,the greatest height of the planet above the plane of the ecliptic ,e ,n e n1 is equal to the inclination of the orbit ,p ,n ,a n to that plane4 ,the longitude of the planet1 when in the plane of the ecliptic1 can only be the longitude of one of the points ,n or n2 and1 when one of these points is known1 the other is given1 being #ahj distant from it4 ,lastly1 the time included between two consecutive passages of the planet through the same node ,n or n1 is its periodic time1 allowance being made for the recess of the node in the interval4 ,,note #ach1 p4 #fj4 ,suppose that it were required to find the position of a point in space1 as of a planet1 and that one observation places it in n1 fig4 #cd1 another observation places it in n1 another in n1 and so on2 all the points n1 n1 n1 n1 c41 being very near to one another4 ,the true place of the planet ,p will not differ much from any of these positions4 ,it is evident1 from this view of the subject1 that ,p n1 ,p n1 ,p n1 c41 are the errors of observation4 ,the true position of the planet ,p is found by this property1 that the squares of the numbers representing the lines ,p n1 ,p n1 c41 when added together1 is the least possible4 ,each line ,p n1 ,p n1 c41 being the whole error in the place of the planet1 is made up of the errors of all the elements2 and1 when compared with the errors obtained from theory1 it affords the means of finding each4 ,the principle of least squares is of very general application2 its demonstration cannot find a place here2 but the reader is referred to ,biot's ,astronomy1 vol4 ii4 p4 #bjc4 ,illustration3 ,fig4 #cd4 ,,note #aci1 p4 #fa4 ,the true longitude of ,uranus was in advance of the tables previous to #agie1 and continued to advance till #ahbb1 after which it diminished rapidly till #ahcj-a1 when the observed and calculated longitudes agreed1 but then the planet fell behind the calculated place so rapidly that it was clear the tables could no longer represent its motion4 ,,note #adj1 p4 #fe4 ,an axis that1 c4 ,fig4 #bj represents the earth revolving in its orbit about the sun in ,s1 the axis of rotation ,p p being everywhere parallel to itself4 ,,note #ada1 p4 #fe4 ,angular velocities that are sensibly uniform4 ,the earth and planets revolve about their axis with an equable motion1 which is never either faster or slower4 ,for example1 the length of the day is never more nor less than twenty-four hours4 ,,note #adb1 p4 #fh4 ,if fig4 #a be the moon1 her polar diameter ,n ,s is the shortest2 and of those in the plane of the equator1 ,q ,e q1 that which points to the earth is greater than all the others4 ,,note #adc1 p4 #gc4 ,inversely proportional1 c4 ,that is1 the total amount of solar radiation becomes less as the minor axis ,c ,c1 fig4 #bj1 of the earth's orbit becomes greater4 ,,note #add1 p4 #ge4 ,fig4 #ce represents the position of the apparent orbit of the sun as it is at present1 the earth being in ,e4 ,the sun is nearer to the earth in moving through ,p than in moving through ,a 1 but its motion through ,p is more rapid than its motion through ,a 2 and1 as the swiftness of the motion and the quantity of heat received vary in the same proportion1 a compensation takes place4 ,illustration3 ,fig4 #ce4 ,,note #ade1 p4 #gf4 ,in an ellipsoid of revolution1 fig4 #a1 the polar diameter ,n ,s1 and every diameter in the equator q ,e ,q e1 are permanent axes of rotation1 but the rotation would be unstable about any other4 ,were the earth to begin to rotate about ,c a1 the angular distance from a to the equator at q would no longer be ninety degrees1 which would be immediately detected by the change it would occasion in the latitudes4 ,,note #adf1 pp4 #ej1 #hj4 ,let q ,q1 and ,e e1 fig4 #aa1 be the planes of the equator and ecliptic4 ,the angle e ,q1 which separates them1 called the obliquity of the ecliptic1 varies in consequence of the action of the sun and moon upon the protuberant matter at the earth's equator4 ,that action brings the point ,q towards e1 and tends to make the plane q ,q coincide with the ecliptic ,e e1 which causes the equinoctial points and to move slowly backwards on the plane e ,e1 at the rate of #ejda annually4 ,this part of the motion1 which depends upon the form of the earth1 is called luni-solar precession4 ,another part1 totally independent of the form of the earth1 arises from the mutual action of the earth1 planets1 and sun1 which1 altering the position of the plane of the ecliptic e ,e1 causes the equinoctial points and to advance at the rate of ,o#ca annually2 but1 as this motion is much less than the former1 the equinoctial points recede on the plane of the ecliptic at the rate of #eja annually4 ,this motion is called the precession of the equinoxes4 ,,note #adg1 p4 #ha4 ,let q ,q1 e ,e1 fig4 #cf1 be the planes of the equinoctial or celestial equator and ecliptic1 and p1 ,p1 their poles4 ,then suppose p1 the pole of the equator1 to revolve with a tremulous or wavy motion in the little ellipse p c d b in about #ai years1 both motions being very small1 while the point a is carried round in the circle a ,a ,b in #be1hfh years4 ,the tremulous motion may represent the half-yearly variation1 the motion in the ellipse gives an idea of the nutation discovered by ,bradley1 and the motion in the circle a ,a ,b arises from the precession of the equinoxes4 ,the greater axis p d of the small ellipse is #ahe1 its minor axis b c is #acgd4 ,these motions are so small that they have very little effect on the parallelism of the axis of the earth's rotation during its revolution round the sun1 as represented in fig4 #bj4 ,as the stars are fixed1 this real motion in the pole of the earth must cause an apparent change in their places4 ,illustration3 ,fig4 #cf4 ,illustration3 figure3 equidistant wires in an eye-piece ,,note #adh1 p4 #hc4 ,by means of a transit instrument1 which is a telescope mounted so as to revolve only in the plane of the meridian1 the instant of the transit or passage of a celestial body across the meridian can be determined4 ,the transits of the principal stars are used to ascertain the time1 or1 which is the same thing1 the amount of the error of clocks4 ,a system of equidistant wires1 as represented in the figure1 is placed in the focus of the eye-piece1 so that the middle wire is perpendicular and at right angles to the axis of the telescope4 ,it consequently represents a portion of the celestial meridian2 and when a star is seen to cross that wire it then crosses the celestial meridian of the place of observation4 ,a clock beating seconds being close at hand1 the duty of an observer is to note the exact second and part of a second at which a star crosses each wire successively in consequence of the rotation of the earth4 ,then the mean of all these observations will give the time at which the star crosses the celestial meridian of the place of observation to the tenth of a second1 provided the observations are accurate4 ,now it happens that the simultaneous impression on the eye and ear is estimated differently by different observers1 so that one person will note the transit of a star1 for example1 as happening the fraction of a second sooner or later than another person2 and as that is the case in every observation he makes1 it is called his personal equation1 that is to say1 it is a correction that must be applied to all the observations of the individual1 and a curious instance of individuality it is4 ,for instance1 ,m4 ,otto ,struve notes every observation ,o#aa too soon1 ,m4 ,peters ,o#ac too late2 ,m4 ,struve noted every observation one second later than ,m4 ,bessel1 and ,m4 ,argelander estimated the transit of a star #ab later than ,m4 ,bessel4 ,all these gentlemen were or are first-rate observers2 and when the personal equation is known it is easy to correct the observations4 ,however1 to avoid that inconvenience ,mr4 ,bond has introduced a method in the ,observatory at ,cambridge in the ,united ,states in which touch is combined with sight instead of hearing1 which is now used also at ,greenwich4 ,the observer at the moment of the observation presses his fingers on a machine which by means of a galvanic battery conveys the impression to where time is measured and marked1 so that the observation is at once recorded and the personal equation avoided4 ,,note #adi1 p4 #hd4 ,let ,n be the pole1 fig4 #aa1 e ,e the ecliptic1 and ,q q the equator4 ,then1 ,n n m ,s being a meridian1 and at right angles to the equator1 the arc m is less than the arc n4 ,,note #aej1 p4 #he4 ,heliacal rising of ,sirius4 ,when the star appears in the morning1 in the horizon1 a little before the rising of the sun4 ,,note #aea1 p4 #hg4 ,let ,p ,a 1 fig4 #ce1 be the apparent orbit or path of the sun1 the earth being in ,e4 ,its major axis1 ,a ,p1 is at present situate as in the figure1 where the solar perigee ,p is between the solstice of winter and the equinox of spring4 ,so that the time of the sun's passage through the arc ,a is greater than the time he takes to go through the arc ,p 4 ,the major axis ,a ,p coincided with 1 the line of the equinoxes1 #djjj years before the ,christian era2 at that time ,p was in the point 4 ,in #fdfh of the ,christian era the perigee ,p will coincide with 4 ,in #abcd ,a4,d4 the major axis was perpendicular to 1 and then ,p was in the winter solstice4 ,,note #aeb1 p4 #hh4 ,at the solstices1 c4 ,since the declination of a celestial object is its angular distance from the equinoctial1 the declination of the sun at the solstice is equal to the arc ,q e1 fig4 #aa1 which measures the obliquity of the ecliptic1 or angular distance of the plane e from the plane ,q 4 ,,note #aec1 p4 #hh4 ,zenith distance is the angular distance of a celestial object from the point immediately over the head of an observer4 ,,note #aed1 p4 #hi4 ,reduced to the level of the sea4 ,the force of gravitation decreases as the square of the height above the surface of the earth increases1 so that a pendulum vibrates slower on high ground2 and1 in order to have a standard independent of local circumstances1 it is necessary to reduce it to the length that would exactly make #hf1djj vibrations in a mean solar day at the level of the sea4 ,,note #aee1 p4 #ij4 ,a quadrant of the meridian is a fourth part of a meridian1 or an arc of a meridian containing #ij1 as ,n ,q1 fig4 #aa4 ,,note #aef1 p4 #ic4 ,moon's southing4 ,the time when the moon is on the meridian of any place1 which happens about forty-eight minutes later every day4 ,,note #aeg1 p4 #if4 ,the angular velocity of the earth's rotation is at the rate of #ahj in twelve hours1 which is the time included between the passages of the moon at the upper and under meridian4 ,,note #aeh1 p4 #if4 ,if ,s be the earth1 fig4 #ad1 d the sun1 and ,c ,q ,o ,d the orbit of the moon1 then ,c and ,o are the syzygies4 ,when the moon is new1 she is at ,c1 and when full she is at ,o2 and1 as both sun and moon are then on the same meridian1 it occasions the spring-tides1 it being high water at places under ,c and ,o1 while it is low water at those under ,q and ,d4 ,the neap-tides happen when the moon is in quadrature at ,q or ,d1 for then she is distant from the sun by the angle d ,s ,q1 or d ,s ,d1 each of which is #ij4 ,,note #aei1 p4 #ig4 ,declination4 ,if the earth be in ,c1 fig4 #aa1 and if q ,q be the equinoctial1 and ,n m ,s a meridian1 then m ,c n is the declination of a body at n4 ,therefore the cosine of that angle is the cosine of the declination4 ,,note #afj1 pp4 #ii1 #aca4 ,fig #cg shows the propagation of waves from two points ,c and ,c1 where stones are supposed to have fallen4 ,those points in which the waves cross each other are the places where they counteract each other's effects1 so that the water is smooth there1 while it is agitated in the intermediate spaces4 ,,note #afa1 p4 #ajj4 ,the centrifugal force may1 c4 ,the centrifugal force acts in a direction at right angles to ,n ,s1 the axis of rotation1 fig4 #cj4 ,its effects are equivalent to two forces1 one of which is in the direction b m perpendicular to the surface ,q m n of the earth1 and diminishes the force of gravity at m4 ,the other acts in the direction of the tangent m ,t1 which makes the fluid particles tend towards the equator4 ,illustration3 ,fig4 #cg4 ,,note #afb1 p4 #ajf4 ,analytical formula or expression4 ,a combination of symbols or signs expressing or representing a series of calculation1 and including every particular case that can arise from a general law4 ,,note #afc1 p4 #ajf4 ,fig4 #ch is a perfect octahedron4 ,sometimes its angles1 ,a1 ,x1 a1 a1 c41 are truncated1 or cut off4 ,sometimes a slice is cut off its edges ,a a1 ,x a1 a a1 c4 ,occasionally both these modifications take place4 ,illustration3 ,fig4 #ch4 ,,note #afd1 p4 #ajg4 ,prismatic crystals of sulphate of nickel are somewhat like fig4 #fb1 only that they are thin1 like a hair4 ,,note #afe1 p4 #ajh4 ,zinc1 a metal either found as an ore or mixed with other metals4 ,it is used in making brass4 ,,note #aff1 p4 #ajh4 ,a cube is a solid contained by six plane square surfaces1 as fig4 #ci4 ,illustration3 ,fig4 #ci4 ,,note #afg1 p4 #ajh4 ,a tetrahedron is a solid contained by four triangular surfaces1 as fig4 #dj3 of this solid there are many varieties4 ,illustration3 ,fig4 #dj4 ,,note #afh1 p4 #ajh4 ,there are many varieties of the octahedron4 ,in that mentioned in the text1 the base a a a a1 fig4 #ch1 is a square1 but the base may be a rhomb2 this solid may also be elongated in the direction of its axis ,a ,x1 or it may be depressed4 ,,note #afi1 pp4 #aji1 #aib1 #bgc4 ,a rhombohedron is a solid contained by six plane surfaces1 as in fig4 #fc1 the opposite planes being equal and similar rhombs parallel to one another2 but all the planes are not necessarily equal or similar1 nor are its angles right angles4 ,in carbonate of lime the angle ,c ,a ,b is #ajeee1 and the angle ,b or ,c is #geje4 ,,note #agj1 p4 #aji4 ,sublimation4 ,bodies raised into vapour which is again condensed into a solid state4 ,,note #aga1 p4 #aab4 ,platinum4 ,the heaviest of metals2 its colour is between that of silver and lead4 ,,note #agb1 p4 #aac4 ,the surface of a column of water1 or spirit of wine1 in a capillary tube1 is hollow2 and that of a column of quicksilver is convex1 or rounded1 as in fig4 #da4 ,,note #agc1 p4 #aac4 ,inverse ratio1 c4 ,the elevation of the liquid is greater in proportion as the internal diameter of the tube is less4 ,,note #agd1 p4 #aad4 ,in fig4 #da the line c d shows the direction of the resulting force in the two cases4 ,illustration3 ,fig4 #da4 ,,note #age1 p4 #aae4 ,when two plates of glass are brought near to one another in water1 the liquid rises between them2 and1 if the plates touch each other at one of their upright edges1 the outline of the water will become an hyperbola4 ,,note #agf1 p4 #aae4 ,let ,a ,a1 fig4 #db1 be two plates1 both of which are wet1 and ,b ,b two that are dry4 ,when partly immersed in a liquid1 its surface will be curved close to them1 but will be of its usual level for the rest of the distance4 ,at such a distance they will neither attract nor repel one another4 ,but1 as soon as they are brought near enough to have the whole of the liquid surface between them curved1 as in a a1 b b1 they will rush together4 ,if one be wet and another dry1 as ,c ,c1 they will repel one another at a certain distance2 but1 as soon as they are brought very near1 they will rush together1 as in the former cases4 ,illustration3 ,fig4 #db4 ,,note #agg1 p4 #abc4 ,in a paper on the atmospheric changes that produce rain and wind1 by ,thomas ,hopkins1 ,esq41 in the ,geographical ,journal1 it is shown that1 when vapour is condensed and falls in rain1 a partial vacuum is formed1 and that heavier air presses in as a current of wind4 ,thus the vacuum arising from the great precipitation at the tropics causes the polar winds to descend from the upper regions of the atmosphere and blow along the surface to the equator as trade winds to supply the place of the hot currents that are continually raising them into the higher regions4 ,this circumstance removes the only difficulty in ,lieutenant ,maury's theory of the winds4 ,,note #agh1 p4 #acd4 ,latent or absorbed heat4 ,there is a certain quantity of heat in all bodies1 which cannot be detected by the thermometer1 but which may become sensible by compression4 ,,note #agi1 p4 #acg4 ,reflected waves4 ,a series of waves of light1 sound1 or water1 diverge in all directions from their origin ,i1 fig4 #dc1 as from a centre4 ,when they meet with an obstacle ,s ,s1 they strike against it1 and are reflected or turned back by it in the same form as if they had proceeded from the centre ,c1 at an equal distance on the other side of the surface ,s ,s4 ,illustration3 ,fig4 #dc4 ,,note #ahj1 p4 #ach4 ,elliptical shell4 ,if fig4 #f be a section of an elliptical shell1 then all sounds coming from the focus ,s to different points on the surface1 as m1 are reflected back to ,f1 because the angle ,t m ,s is equal to t m ,f4 ,in a spherical hollow shell1 a sound diverging from the centre is reflected back to the centre again4 ,,note #aha1 p4 #adb4 ,fig4 #dd represents musical strings in vibration2 the straight lines are the strings when at rest4 ,the first figure of the four would give the fundamental note1 as1 for example1 the low ,c4 ,the second and third figures would give the first and second harmonics2 that is1 the octave and the #abth above ,c1 n n n being the points at rest2 the fourth figure shows the real motion when compounded of all three4 ,illustration3 ,fig4 #dd4 ,,note #ahb1 p4 #adc4 ,fig4 #de represents sections of an open and of a shut pipe1 and of a pipe open at one end4 ,when sounded1 the air spontaneously divides itself into segments4 ,it remains at rest in the divisions or nodes n n1 c41 but vibrates between them in the direction of the arrow-heads4 ,the undulations of the whole column of air give the fundamental note1 while the vibrations of the divisions give the harmonics4 ,illustration3 ,fig4 #de4 ,,note #ahc1 p4 #add4 ,fig4 #a1 plate #a1 shows the vibrating surface when the sand divides it into squares1 and fig4 #b represents the same when the nodal lines divide it into triangles4 ,the portions marked a a are in different states of vibration from those marked b b4 ,,note #ahd1 p4 #ade4 ,plates #a and #b contain a few of ,chladni's figures4 ,the white lines are the forms assumed by the sand1 from different modes of vibration1 corresponding to musical notes of different degrees of pitch4 ,plate #c contains six of ,chladni's circular figures4 ,,note #ahe1 p4 #ade4 ,mr4 ,wheatstone's principle is1 that when vibrations producing the forms of figs4 #a and #b1 plate #c1 are united in the same surface1 they make the sand assume the form of fig4 #c4 ,in the same manner1 the vibrations which would separately cause the sand to take the forms of figs4 #d and #e1 would make it assume the form in fig4 #f when united4 ,the figure #i results from the modes of vibration of #g and #h combined4 ,the parts marked a a are in different states of vibration from those marked b b4 ,figs4 #a1 #b1 and #c1 plate #d1 represent forms which the sand takes in consequence of simple modes of vibration2 #d and #e are those arising from two combined modes of vibration2 and the last six figures arise from four superimposed simple modes of vibration4 ,these complicated figures are determined by computation independent of experiment4 ,,note #ahf1 p4 #adf4 ,the long cross-lines of fig4 #df show the two systems of nodal lines given by ,m4 ,savart's lamin4 ,illustration3 ,fig4 #df4 ,,note #ahg1 p4 #adf4 ,the short lines on fig4 #df show the positions of the nodal lines on the other sides of the same lamin4 ,,note #ahh1 p4 #adf4 ,fig4 #dg gives the nodal lines on a cylinder1 with the paper rings that mark the quiescent points4 ,illustration3 ,fig4 #dg4 ,illustration3 ,fig4 #dh4 ,,note #ahi1 pp4 #ach1 #aec1 #aef4 ,reflection and ,refraction4 ,let ,p ,c p1 fig4 #dh1 be perpendicular to a surface of glass or water ,a ,b4 ,when a ray of light1 passing through the air1 falls on this surface in any direction ,i ,c1 part of it is reflected in the direction ,c ,s1 and the other part is bent at ,c1 and passes through the glass or water in the direction ,c ,r4 ,i ,c is called the incident ray1 and ,i ,c ,p the angle of incidence2 ,c ,s is the reflected ray1 and ,p ,c ,s the angle of reflection2 ,c ,r is the refracted ray1 and p ,c ,r the angle of refraction4 ,the plane passing through ,s ,c and ,i ,c is the plane of reflection1 and the plane passing through ,i ,c and ,c ,r is the plane of refraction4 ,in ordinary cases1 ,c ,i1 ,c ,s1 ,c ,r1 are all in the same plane4 ,we see the surface by means of the reflected light1 which would otherwise be invisible4 ,whatever the reflecting surface may be1 and however obliquely the light may fall upon it1 the angle of reflection is always equal to the angle of incidence4 ,thus ,i ,c1 ,i ,c1 being rays incident on the surface at ,c1 they will be reflected into ,c ,s1 ,c ,s1 so that the angle ,s ,c ,p will be equal to the angle ,i ,c ,p1 and ,s ,c ,p equal to ,i ,c ,p4 ,that is by no means the case with the refracted rays4 ,the incident rays ,i ,c1 ,i ,c1 are bent at ,c towards the perpendicular1 in the direction ,c ,r1 ,c ,r2 and the law of refraction is such1 that the sine of the angle of incidence has a constant ratio to the sine of the angle of refraction2 that is to say1 the number expressing the length of ,i m1 the sine of ,i ,c ,p1 divided by the number expressing the length of ,r n1 the sine of ,r ,c p1 is the same for all the rays of light that can fall upon the surface of any one substance1 and is called its index of refraction4 ,though the index of refraction be the same for any one substance1 it is not the same for all substances4 ,for water it is #accf2 for crown-glass it is #aece2 for flint-glass1 #af2 for diamond1 #bdhg2 and for chromate of lead it is #c1 which substance has a higher refractive power than any other known4 ,light falling perpendicularly on a surface passes through it without being refracted4 ,if the light be now supposed to pass from a dense into a rare medium1 as from glass or water into air1 then ,r ,c1 ,r ,c1 become the incident rays2 and in this case the refracted rays1 ,c ,i1 ,c ,i1 are bent from the perpendicular instead of towards it4 ,when the incidence is very oblique1 as r ,c1 the light never passes into the air at all1 but it is totally reflected in the direction ,c r1 so that the angle p ,c r is equal to p ,c r2 that frequently happens at the second surface of glass4 ,when a ray ,i ,c falls from air upon a piece of glass ,a ,b1 it is in general refracted at each surface4 ,at ,c it is bent towards the perpendicular1 and at ,r from it1 and the ray emerges parallel to ,i ,c2 but1 when the ray is very oblique to the second surface1 it is totally reflected4 ,an object seen by total reflection is nearly as vivid as when seen by direct vision1 because no part of the light is refracted4 ,when light falls upon a plate of crown-glass1 at an angle of #d #cb counted from the surface1 the glass reflects #d times more light than it transmits4 ,at an angle of #g #a the reflected light is double of the transmitted2 at an angle of #aa #h the light reflected is equal to that transmitted2 at #ag #ag the reflected is equal to #a_/b the transmitted light2 at #bf #ch it is equal to #a_/d1 the variation1 according to ,arago1 being as the square of the cosine4 ,,note #ahi1 p4 #aed4 ,atmospheric refraction4 ,let a b1 a b1 c41 fig4 #di1 be strata1 or extremely thin layers1 of the atmosphere1 which increase in density towards m n1 the surface of the earth4 ,a ray coming from a star meeting the surface of the atmosphere at ,s would be refracted at the surface of each layer1 and would consequently move in the curved line ,s v v v ,a2 and as an object is seen in the direction of the ray that meets the eye1 the star1 which really is in the direction ,a ,s1 would seem to a person at ,a to be in s4 ,so that refraction1 which always acts in a vertical direction1 raises objects above their true place4 ,for that reason1 a body at ,s1 below the horizon ,h ,a ,o1 would be raised1 and would be seen in s4 ,the sun is frequently visible by refraction after he is set1 or before he is risen4 ,there is no refraction in the zenith at ,z4 ,it increases all the way to the horizon1 where it is greatest1 the variation being proportional to the tangent of the angles ,z ,a ,s1 ,z ,a ,s1 the distances of the bodies ,s ,s from the zenith4 ,the more obliquely the rays fall1 the greater the refraction4 ,illustration3 ,fig4 #di4 ,illustration3 ,fig4 #ej4 ,,note #aij1 p4 #aed4 ,bradley's method of ascertaining the amount of refraction4 ,let ,z1 fig4 #ej1 be the zenith or point immediately above an observer at ,a2 let ,h ,o be his horizon1 and ,p the pole of the equinoctial ,a ,q4 ,hence ,p ,a ,q is a right angle4 ,a star as near to the pole as s would appear to revolve about it1 in consequence of the rotation of the earth4 ,at noon1 for example1 it would be at s above the pole1 and at midnight it would be in s below it4 ,the sum of the true zenith distances1 ,z ,a s1 ,z ,a s1 is equal to twice the angle ,z ,a ,p4 ,again1 ,s and ,s being the sun at his greatest distances from the equinoctial ,a ,q when in the solstices1 the sum of his true zenith distances1 ,z ,a ,s1 ,z ,a ,s1 is equal to twice the angle ,z ,a ,q4 ,consequently1 the four true zenith distances1 when added together1 are equal to twice the right angle ,q ,a ,p2 that is1 they are equal to #ahj4 ,but the observed or apparent zenith distances are less than the true on account of refraction2 therefore the sum of the four apparent zenith distances is less than #ahj by the whole amount of the four refractions4 ,,note #aia1 p4 #aee4 ,terrestrial refraction4 ,let ,c1 fig4 #ea1 be the centre of the earth1 ,a an observer at its surface1 ,a ,h his horizon1 and ,b some distant point1 as the top of a hill4 ,let the arc ,b ,a be the path of a ray coming from ,b to ,a2 ,e ,b1 ,e ,a1 tangents to its extremities2 and ,a ,g1 ,b ,f1 perpendicular to ,c ,b4 ,however high the hill ,b may be1 it is nothing when compared with ,c ,a1 the radius of the earth2 consequently1 ,a ,b differs so little from ,a ,d that the angles ,a ,e ,b and ,a ,c ,b are supplementary to one another2 that is1 the two taken together are equal to #ahj4 ,a ,c ,b is called the horizontal angle4 ,now ,b ,a ,h is the real height of ,b1 and ,e ,a ,h its apparent height2 hence refraction raises the object ,b1 by the angle ,e ,a ,b1 above its real place4 ,again1 the real depression of ,a1 when viewed from ,b1 is ,f ,b ,a1 whereas its apparent depression is ,f ,b ,e1 so ,e ,b ,a is due to refraction4 ,the angle ,f ,b ,a is equal to the sum of the angles ,b ,a ,h and ,a ,c ,b2 that is1 the true elevation is equal to the true depression and the horizontal angle4 ,but the true elevation is equal to the apparent elevation diminished by the refraction2 and the true depression is equal to the apparent depression increased by refraction4 ,hence twice the refraction is equal to the horizontal angle augmented by the difference between the apparent elevation and the apparent depression4 ,illustration3 ,fig4 #ea4 ,,note #aib1 p4 #aee4 ,fig4 #eb represents the phenomenon in question4 ,s ,p is the real ship1 with its inverted and direct images seen in the air4 ,were there no refraction1 the rays would come from the ship ,s ,p to the eye ,e in the direction of the straight lines2 but1 on account of the variable density of the inferior strata of the atmosphere1 the rays are bent in the curved lines ,p c ,e1 ,p d ,e1 ,s m ,e1 ,s n ,e4 ,since an object is seen in the direction of the tangent to that point of the ray which meets the eye1 the point ,p of the real ship is seen at p and p1 and the point ,s seems to be in s and s2 and1 as all the other points are transferred in the same manner1 direct and inverted images of the ship are formed in the air above it4 ,illustration3 ,fig4 #eb4 ,,note #aic1 p4 #aef4 ,fig4 #ec represents the section of a poker1 with the refraction produced by the hot air surrounding it4 ,illustration3 ,fig4 #ec4 ,,note #aid1 p4 #aef4 ,the solar spectrum4 ,a ray from the sun at ,s1 fig4 #ed1 admitted into a dark room1 through a small round hole ,h in a window-shutter1 proceeds in a straight line to a screen ,d1 on which it forms a bright circular spot of white light1 of nearly the same diameter with the hole ,h4 ,but when the refracting angle ,b ,a ,c of a glass prism is interposed1 so that the sunbeam falls on ,a ,c the first surface of the prism1 and emerges from the second surface ,a ,b at equal angles1 it causes the rays to deviate from the straight path ,s ,d1 and bends them to the screen ,m ,n1 where they form a coloured image ,v ,r of the sun1 of the same breadth with the diameter of the hole ,h1 but much longer4 ,the space ,v ,r consists of seven colours--violet1 indigo1 blue1 green1 yellow1 orange1 and red4 ,the violet and red1 being the most and least refrangible rays1 are at the extremities1 and the green occupy the middle part at ,g4 ,the angle ,d g ,g is called the mean deviation1 and the spreading of the coloured rays over the angle ,v g ,r the dispersion4 ,the deviation and dispersion vary with the refracting angle ,b ,a ,c of the prism1 and with the substance of which it is made4 ,illustration3 ,fig4 #ed4 ,,note #aie1 pp4 #aei1 #afd4 ,under the same circumstances1 and where the refracting angles of the two prisms are equal1 the angles ,d g ,g and ,v g ,r1 fig4 #ed1 are greater for flint-glass than for crown-glass4 ,but1 as they vary with the angle of the prism1 it is only necessary to augment the refracting angle of the crown-glass prism by a certain quantity1 to produce nearly the same deviation and dispersion with the flint-glass prism4 ,hence1 when the two prisms are placed with their refracting angles in opposite directions1 as in fig4 #ed1 they nearly neutralize each other's effects1 and refract a beam of light without resolving it into its elementary coloured rays4 ,sir ,david ,brewster has come to the conclusion that there may be refraction without colour by means of two prisms1 or two lenses1 when properly adjusted1 even though they be made of the same kind of glass4 ,,note #aif1 p4 #afe4 ,the object glass of the achromatic telescope consists of a convex lens ,a ,b1 fig4 #ee1 of crown-glass placed on the outside1 towards the object1 and of a concave-convex lens ,c ,d of flint-glass1 placed towards the eye4 ,the focal length of a lens is the distance of its centre from the point in which the rays converge1 as ,f1 fig4 #fj4 ,if1 then1 the lenses ,a ,b and ,c ,d be so constructed that their focal lengths are in the same proportion as their dispersive powers1 they will refract rays of light without colour4 ,illustration3 ,fig4 #ee4 ,,note #aig1 p4 #afe4 ,if the mean refracting angle of the prism ,d g ,g1 fig4 #ed1 were the same for all substances1 then the difference ,d g ,v - ,d g ,r would be the dispersion4 ,but the angle of the prism being the same1 all these angles are different in each substance1 so that in order to obtain the dispersion of any substance the angle ,d g ,v - ,d g ,r must be divided by the angle ,d g ,g or its excess above unity1 to which the mean refraction is always proportional4 ,according to ,mr4 ,fraunhofer the refraction of the extreme violet and red rays in crown-glass is #aedff and #aebeh2 so ,d g ,v - ,d g ,r #aedff - #aebeh #jbjh1 and half the sum of the excess of each above unity is #ecfb2 consequently 7,d g ,v - ,d g ,r7_/,d g ,g #jbjh_/ecfb #jjchgi2 for diamond 7,d g ,v - ,d g ,r7_/,d g ,g 7#bdfg - #bdaa7_/adci #jjchi2 so that the dispersive power of diamond is a little less than that of crown-glass2 hence the splendid refracted colours which distinguish diamond from every other precious stone are not owing to its high dispersive power1 but to its great mean refraction4--,,sir ,,david ,,brewster4 ,,note #aih1 p4 #afh4 ,when a sunbeam1 after having passed through a coloured glass ,v ,v1 fig4 #ef1 enters a dark room by two small slits ,o ,o in a card1 or piece of tin1 they produce alternate bright and black bands on a screen ,s ,s at a little distance4 ,when either one or other of the slits ,o or ,o is stopped1 the dark bands vanish1 and the screen is illuminated by a uniform light1 proving that the dark bands are produced by the interference of the two sets of rays4 ,again1 let ,h m1 fig4 #eg1 be a beam of white light passing through a hole at ,h1 made with a fine needle in a piece of lead or a card1 and received on a screen ,s ,s4 ,when a hair1 or a small slip of card h h1 about the #cjth of an inch in breadth1 is held in the beam1 the rays bend round on each side of it1 and1 arriving at the screen in different states of vibration1 interfere and form a series of coloured fringes on each side of a central white band m4 ,when a piece of card is interposed at ,c1 so as to intercept the light which passes on one side of the hair1 the coloured fringes vanish4 ,when homogeneous light is used1 the fringes are broadest in red1 and become narrower for each colour of the spectrum progressively to the violet1 which gives the narrowest and most crowded fringes4 ,these very elegant experiments are due to ,dr4 ,thomas ,young4 ,illustration3 ,fig4 #ef4 ,illustration3 ,fig4 #eg4 ,illustration3 ,fig4 #eh4 ,,note #aii1 pp4 #aga1 #bjj4 ,fig4 #eh shows ,newton's rings1 of which there are seven1 formed by screwing two lenses of glass together4 ,provided the incident light be white1 they always succeed each other in the following order3-- #ast ring1 or #ast order of colours3 ,black1 very faint blue1 brilliant white1 yellow1 orange1 red4 #bnd ring3 ,dark purple1 or rather violet1 blue1 a very imperfect yellow green1 vivid yellow1 crimson red4 #crd ring3 ,purple1 blue1 rich grass green1 fine yellow1 pink1 crimson4 #dth ring3 ,dull blueish green1 pale yellowish pink1 red4 #eth ring3 ,pale blueish green1 white1 pink4 #fth ring3 ,pale blue green1 pale pink4 #gth ring3 ,very pale blueish green1 very pale pink4 ,after the seventh order the colours become too faint to be distinguished4 ,the rings decrease in breadth1 and the colours become more crowded together1 as they recede from the centre4 ,when the light is homogeneous1 the rings are broadest in the red1 and decrease in breadth with every successive colour of the spectrum to the violet4 ,,note #bjj1 p4 #agb4 ,the absolute thickness of the film of air between the glasses is found as follows3--,let ,a ,f ,b ,c1 fig4 #ei1 be the section of a lens lying on a plane surface or plate of glass ,p ,p1 seen edgewise1 and let ,e ,c be the diameter of the sphere of which the lens is a segment4 ,if ,a ,b be the diameter of any one of ,newton's rings1 and ,b ,d parallel to ,c ,e1 then ,b ,d or ,c ,f is the thickness of the air producing it4 ,e ,c is a known quantity2 and when ,a ,b1 the diameter1 is measured with compasses1 ,b ,d or ,f ,c can be computed4 ,newton found that the length of ,b ,d1 corresponding to the darkest part of the first ring1 is the #ih1jjjth part of an inch when the rays fall perpendicularly on the lens1 and from this he deduced the thickness corresponding to each colour in the system of rings4 ,by passing each colour of the solar spectrum in succession over the lenses1 ,newton also determined the thickness of the film of air corresponding to each colour1 from the breadth of the rings1 which are always of the same colour with the homogeneous light4 ,illustration3 ,fig4 #ei4 ,,note #bja1 p4 #agd4 ,the focal length or distance of a lens is the distance from its centre to the point ,f1 fig4 #fj1 in which the refracted rays meet4 ,let ,l ,l be a lens of very short focal distance fixed in the window-shutter of a dark room4 ,a sunbeam ,s ,l ,l passing through the lens will be brought to a focus in ,f1 whence it will diverge in lines ,f ,c1 ,f ,d1 and will form a circular image of light on the opposite wall4 ,suppose a sheet of lead1 having a small pin-hole pierced through it1 to be placed in this beam2 when the pin-hole is viewed from behind with a lens at ,e1 it is surrounded with a series of coloured rings1 which vary in appearance with the relative positions of the pin-hole and eye with regard to the point ,f4 ,when the hole is the #cjth of an inch in diameter and at the distance of #f-a_/b feet from ,f1 when viewed at the distance of #bd inches1 there are seven rings of the following colours3-- #ast order3 ,white1 pale yellow1 yellow1 orange1 dull red4 #bnd order3 ,violet1 blue1 whitish1 greenish yellow1 fine yellow1 orange red4 #crd order3 ,purple1 indigo blue1 greenish blue1 brilliant green1 yellow green1 red4 #dth order3 ,blueish green1 blueish white1 red4 #eth order3 ,dull green1 faint blueish white1 faint red4 #fth order3 ,very faint green1 very faint red4 #gth order3 ,a trace of green and red4 ,illustration3 ,fig4 #fj4 ,illustration3 ,fig4 #fa4 ,illustration3 ,fig4 #fb4 ,,note #bjb1 p4 #age4 ,let ,l ,l1 fig4 #fa1 be the section of a lens placed in a window-shutter1 through which a very small beam of light ,s ,l ,l passes into a dark room1 and comes to a focus in ,f4 ,if the edge of a knife ,k ,n be held in the beam1 the rays bend away from it in hyperbolic curves ,k r1 ,k r1 c41 instead of coming directly to the screen in the straight line ,k ,e1 which is the boundary of the shadow4 ,as these bending rays arrive at the screen in different states of undulation1 they interfere1 and form a series of coloured fringes1 r r1 c41 along the edge of the shadow ,k ,e ,s ,n of the knife4 ,the fringes vary in breadth with the relative distances of the knife-edge and screen from ,f4 ,,note #bjc1 p4 #agg4 ,fig4 #dc represents the phenomena in question1 where ,s ,s is the surface1 and ,i the centre of incident waves4 ,the reflected waves are the dark lines returning towards ,i1 which are the same as if they had originated in ,c on the other side of the surface4 ,,note #bjd1 p4 #ahj4 ,fig4 #fb represents a prismatic crystal of tourmaline1 whose axis is ,a ,x4 ,the slices that are used for polarising light are cut parallel to ,a ,x4 ,,note #bje1 p4 #aha4 ,double refraction4 ,if a pencil of light ,r r1 fig4 #fc1 falls upon a rhombohedron of ,iceland spar ,a ,b ,x ,c1 it is separated into two equal pencils of light at r1 which are refracted in the directions r ,o1 r ,e3 when these arrive at ,o and ,e they are again refracted1 and pass into the air in the directions ,o o1 ,e o1 parallel to one another and to the incident ray ,r r4 ,the ray r ,o is refracted according to the ordinary law1 which is1 that the sines of the angles of incidence and refraction bear a constant ratio to one another 7see ,note #ahd71 and the rays ,r r1 r ,o1 ,o o1 are all in the same plane4 ,the pencil r ,e1 on the contrary1 is bent aside out of that plane1 and its refraction does not follow the constant ratio of the sines2 r ,e is therefore called the extraordinary ray1 and r ,o the ordinary ray4 ,in consequence of this bisection of the light1 a spot of ink at ,o is seen double at ,o and ,e1 when viewed from r ,i2 and when the crystal is turned round1 the image ,e revolves about ,o1 which remains stationary4 ,illustration3 ,fig4 #fc4 ,,note #bjf1 p4 #ahb4 ,both of the parallel rays ,o o and ,e o1 fig4 #fc1 are polarised on leaving the doubly refracting crystal1 and in both the particles of light make their vibrations at right angles to the lines ,o o1 ,e o4 ,in the one1 however1 these vibrations lie1 for example1 in the plane of the horizon1 while the vibrations of the other lie in the vertical plane perpendicular to the horizon4 ,,note #bjg1 p4 #ahc4 ,if light be made to fall in various directions on the natural faces of a crystal of ,iceland spar1 or on faces cut and polished artificially1 one direction ,a ,x1 fig4 #fc1 will be found1 along which the light passes without being separated into two pencils4 ,a ,x is the optic axis4 ,in some substances there are two optic axes forming an angle with each other4 ,the optic axis is not a fixed line1 it only has a fixed direction2 for if a crystal of ,iceland spar be divided into smaller crystals1 each will have its optic axis2 but if all these pieces be put together again1 their optic axes will be parallel to ,a ,x4 ,every line1 therefore1 within the crystal parallel to ,a ,x is an optic axis2 but as these lines have all the same direction1 the crystal is still said to have but one optic axis4 ,,note #bjh1 p4 #ahd4 ,if ,i ,c1 fig4 #dh1 be the incident and ,c ,s the reflected rays1 then the particles of polarised light make their vibrations at right angles to the plane of the paper4 ,,note #bji1 p4 #ahd4 ,let ,a ,a1 fig4 #dh1 be the surface of the reflector1 ,i ,c the incident and ,c ,s the reflected rays2 then1 when the angle ,s ,c ,b is #eg1 and consequently the angle ,p ,c ,s equal to #cc1 the black spot will be seen at ,c by an eye at ,s4 ,,note #baj1 p4 #ahe4 ,let ,a ,b1 fig4 #dh1 be a reflecting surface1 ,i ,c the incident and ,c ,s the reflected rays2 then1 if the surface be plate-glass1 the angle ,s ,c ,b must be #eg1 in order that ,c ,s may be polarised4 ,if the surface be crown-glass or water1 the angle ,s ,c ,b must be #ef #ee for the first1 and #ec #aa for the second1 in order to give a polarised ray4 ,,note #baa1 p4 #ahf4 ,a polarising apparatus is represented in fig4 #fd1 where ,r r is a ray of light falling on a piece of glass r at an angle of #eg3 the reflected ray r s is then polarised1 and may be viewed through a piece of tourmaline in s1 or it may be received on another plate of glass1 ,b1 whose surface is at right angles to the surface of r4 ,the ray r s is again reflected in s1 and comes to the eye in the direction s ,e4 ,the plate of mica1 ,m ,i1 or of any substance that is to be examined1 is placed between the points r and s4 ,illustration3 ,fig4 #fd4 ,,note #bab1 p4 #ahg4 ,in order to see these figures1 the polarised ray r s1 fig4 #fd1 must pass through the optic axis of the crystal1 which must be held as near as possible to s on one side1 and the eye placed as near as possible to s on the other4 ,fig4 #fe shows the image formed by a crystal of ,iceland spar which has one optic axis4 ,the colours in the rings are exactly the same with those of ,newton's rings given in ,note #aii1 and the cross is black4 ,if the spar be turned round its axis1 the rings suffer no change2 but if the tourmaline through which it is viewed1 or the plate of glass1 ,b1 be turned round1 this figure will be seen at the angles #j1 #ij1 #ahj1 and #bgj of its revolution4 ,but in the intermediate points1 that is1 at the angles #de1 #ace1 #bbe1 and #cae1 another system will appear1 such as represented in fig4 #ff1 where all the colours of the rings are complementary to those of fig4 #fe1 and the cross is white4 ,the two systems of rings1 if superposed1 would produce white light4 ,illustration3 ,fig4 #fe4 ,illustration3 ,fig4 #ff4 ,,note #bac1 p4 #ahh4 ,saltpetre1 or nitre1 crystallises in six-sided prisms having two optic axes inclined to one another at an angle of #e4 ,a slice of this substance about the #fth or #hth of an inch thick1 cut perpendicularly to the axis of the prism1 and placed very near to s1 fig4 #fd1 so that the polarised ray r s may pass through it1 exhibits the system of rings represented in fig4 #fg1 where the points ,c and ,c mark the position of the optic axes4 ,when the plate ,b1 fig4 #fd1 is turned round1 the image changes successively to those given in figs4 #fh1 #fi1 and #gj4 ,the colours of the rings are the same with those of thin plates1 but they vary with the thickness of the nitre4 ,their breadth enlarges or diminishes also with the colour1 when homogeneous light is used4 ,illustration3 ,fig4 #fg4 ,illustration3 ,fig4 #fh4 ,illustration3 ,fig4 #fi4 ,illustration3 ,fig4 #gj4 ,illustration3 ,fig4 #ga4 ,,note #bad1 p4 #ahi4 ,fig4 #ga represents the appearance produced by placing a slice of rock crystal in the polarised ray r s1 fig4 #fd4 ,the uniform colour in the interior of the image depends upon the thickness of the slice2 but whatever that colour may be1 it will alternately attain a maximum brightness and vanish with the revolution of the glass ,b4 ,it may be observed1 that the two kinds of quartz1 or rock crystal1 mentioned in the text1 are combined in the amethyst1 which consists of alternate layers of right-handed and left-handed quartz1 whose planes are parallel to the axis of the crystal4 ,,note #bae1 p4 #aic4 ,suppose the major axis ,a ,p of an ellipse1 fig4 #ah1 to be invariable1 but the excentricity ,c ,s continually to diminish1 the ellipse would bulge more and more2 and when ,c ,s vanished1 it would become a circle whose diameter is ,a ,p4 ,again1 if the excentricity were continually to increase1 the ellipse would be more and more flattened till ,c ,s was equal to ,c ,p1 when it would become a straight line ,a ,p4 ,the circle and straight line are therefore the limits of the ellipse4 ,,note #baf1 p4 #aid4 ,the coloured rings are produced by the interference of two polarised rays in different states of undulation1 on the principle explained for common light4 ,,note #bag1 p4 #bbe4 ,according to ,mr4 ,joule1 that heat is produced by motion1 and that it is equivalent to it1 ,mr4 ,thompson of ,glasgow investigates from whence the sun derives his heat1 since he shows that neither combustion nor his primitive heat could have supplied the waste during #fjjj years4 ,he concludes that the solar heat is maintained by myriads of minute bodies that are revolving at the edge of his dense nebulosity or atmosphere1 some of which are often seen by us as falling stars4 ,these1 vaporized by his heat1 and drawn by his attraction1 meet with intense resistance on entering the solar atmosphere as a shower of meteoric rain2 through it they descend in spiral lines to the sun's surface1 producing enormous heat by friction during their fall1 and serving for fuel on their arrival4 ,,note #bah1 p4 #beb4 ,the class ,cryptogamia contains the ferns1 mosses1 funguses1 and sea-weeds2 in all of which the parts of the flowers are in general too minute to be evident4 ,,note #bai1 p4 #bed4 ,zoophytes are the animals which form madrepores1 corals1 sponges1 c4 ,,note #bbj1 p4 #bed4 ,the ,saurian tribe are creatures of the crocodile and lizard kind4 ,,note #bba1 p4 #bff4 ,if heat from a non-luminous source be polarised by reflection or refraction at r1 fig4 #fd1 the polarised ray r s will be stopped or transmitted by a plate of mica ,m ,i1 under the same circumstances that it would stop or transmit light2 and if heat were visible1 images analogous to those of figs4 #fe1 #fg1 c41 would be seen at the point s4 ,,note #bbb1 pp4 #bge1 #cbi1 #ceg4 ,the foot-pound1 or unit of mechanical force established by ,mr4 ,joule1 is the force that would raise one pound weight of matter to the height of one foot2 or it is the impetus or force generated by a body of one pound weight falling by its gravitation through the height of one foot4 ,impetus1 vis viva1 or living force1 is equal to the mass of a body multiplied by the square of the velocity with which it is moving1 and is the true measure of work or labour4 ,for if a weight be raised #aj feet1 it will require four times the labour to raise an equal weight #dj feet4 ,if both these weights be allowed to descend freely by their gravitation1 at the end of their fall their velocities will be as #a to #b2 that is1 as the square roots of their heights2 but the effect produced will be as their masses multiplied by #a and #d2 but these are the squares of their velocities3 hence the impetus or vis viva is as the mass into the square of the velocity4 ,thus impetus is the true measure of the labour employed to raise the weights1 and of the effect of their descent1 and is entirely independent of time4 ,now heat is proportional to impetus1 and impetus is the true measure of labour4 ,in percussion the heat evolved is in proportion to the force of the impetus1 and is thus measured by labour4 ,travail is a word used in mechanics1 to express that work done is equal to the labouring force employed4 ,the work done may be resistance overcome or any other effect produced1 while the labouring force may be a horse1 a steam-engine1 wind1 falling water1 c4 ,,note #bbc1 p4 #cac4 ,when a stream of positive electricity descends from ,p to n1 fig4 #gb1 in a vertical wire at right angles to the plane of the horizontal circle ,a ,b1 the negative electricity ascends from n to ,p1 and the force exerted by the current makes the north pole of a magnet revolve about the wire in the direction of the arrow-heads in the circumference1 and it makes the south pole revolve in the opposite direction4 ,when the current of positive electricity flows upwards from n to ,p1 these effects are reversed4 ,illustration3 ,fig4 #gb4 ,illustration3 ,fig4 #gc4 ,,note #bbd1 p4 #cad4 ,fig4 #gc represents a helix or coil of copper wire1 terminated by two cups containing a little quicksilver4 ,when the positive wire of a ,voltaic battery is immersed in the cup p1 and the negative wire in the cup n1 the circuit is completed4 ,the quicksilver ensures the connection between the battery and the helix1 by conveying the electricity from the one to the other4 ,while the electricity flows through the helix1 the magnet ,s ,n remains suspended within it1 but falls down the moment it ceases4 ,the magnet always turns its south pole ,s towards ,p1 the positive wire of the battery1 and its north pole towards the negative wire4 ,,note #bbe1 p4 #caf4 ,a copper wire coiled in the form represented in fig4 #gc was the first and most simple form of the electro-dynamic cylinder4 ,when its extremities ,p and n are connected with the positive and negative poles of a ,voltaic battery1 it becomes a perfect magnet during the time that a current of electricity is flowing through it1 ,p and n being its north and south poles4 ,,note #bbf1 p4 #cdd4 ,it is to ,halley we are indebted for the first declination chart and the theory of #d poles of maximum magnetic intensity1 since confirmed by observation1 as well as the earliest authentic values of the magnetic elements in ,london and ,st4 ,helena1 where he went on purpose to make observations on terrestrial magnetism4 ,since that time ,m4 ,gauss has formed charts of the magnetic lines1 and published a theory which very nearly represents the magnetic state of the globe4 ,the mass of observations daily making by our cruizers and our ,government surveys in every part of the earth is enormous4 ,,note #bbg1 p4 #cfj4 ,in fig4 #gd the hyperbola ,h ,p ,y1 the parabola p ,p ,r1 and the ellipse ,a ,e ,p ,l1 have the focal distance ,s ,p1 and coincide through a small space on each side of the perihelion ,p2 and1 as a comet is only visible when near ,p1 it is difficult to ascertain which of the three curves it moves in4 ,illustration3 ,fig4 #gd4 ,,note #bbh1 p4 #cfc4 ,in fig4 #ge1 ,e ,a represents the orbit of ,halley's comet1 ,e ,t the orbit of the earth1 and ,s the sun4 ,the proportions are very nearly exact4 ,illustration3 ,fig4 #ge4 ,,note #bbi1 p4 #chb4 ,fig4 #gd represents the curves in question4 ,it is evident that1 for the same focal distance ,s ,p1 there can be but one circle and one parabola p ,p ,r1 but that there may be an infinity of ellipses between the circle and the parabola1 and an infinity of hyperbolas ,h ,p ,y exterior to the parabola p ,p ,r4 ,,note #bcj1 p4 #chg4 ,let ,a ,b1 fig4 #bf1 be the diameter of the earth's orbit1 and suppose a star to be seen in the direction ,a ,s from the earth when at ,a4 ,six months afterwards1 the earth1 having moved through half of its orbit1 would arrive at ,b1 and then the star would appear in the direction ,b ,s1 if the diameter ,a ,b1 as seen from ,s1 had any sensible magnitude4 ,but ,a ,b1 which is #aij1jjj1jjj of miles1 does not appear to be greater than the thickness of a spider's thread1 as seen from #fa ,cygni1 supposed to be the nearest of the fixed stars4 ,,note #bca1 p4 #chi4 ,stars whose parallax and proper motions are known4 ,name of ,star4 ,proper ,motion4 ,parallax4 ,observers and ,computers4 ,centauri #cgfd #jib ,maclear4 44 #a ,henderson4 #fa ,cygni #eabc #jcgd ,bessel4 ,lyr #jcfd #jbjg ,peters4 ,sirius #abcd #jbcj ,henderson4 ,arcturus #bbfi #jabg ,peters4 ,pole ,star #jjce #jajf ,peters4 ,capella 44 #jjdf ,peters4 ,la ,chevre #jdfa #jjdf ,peters4 ,great ,bear #jgdf #jacc ,peters4 ,the space run through in one second by these stars is therefore-- ,centauri #e leagues ,henderson and ,maclear4 #fa ,cygni #aj leagues ,bessel4 ,lyr #b leagues ,struve and ,peters4 ,sirius #f leagues ,henderson and ,maclear4 ,arcturus #bb leagues ,peters4 ,pole ,star league ,lindenau and ,struve4 ,la ,chevre #ab leagues ,peters4 ,great ,bear #g leagues ,peters4 ,there are three great discrepancies in the parallax of the star ,argelander or #ahcj ,groombridge4 ,m4 ,otto ,struve makes it #jjcd1 which gives it a velocity of #bea leagues per second1 while ,m4 ,faye finds the parallax to be between #jjc and #jja1 which makes its velocity from #cj to #he leagues per second4 ,these are all minimum velocities1 because we can only determine on the celestial vault a projection perhaps much foreshortened of the real motions of the stars4 ,,note #bcb1 pp4 #cih1 #dja4 ,the following are the binary systems whose orbits have been accurately determined3-- ,name of ,star4 ,period in ,perihelion ,by whom ,computed4 ,years4 ,passage4 ,herculis #cjbaf #ahcada ,madler4 ,coron #dbejj #ahjgba ,madler4 ,cancri #ehiaj #aheccg ,madler4 ,urs ,majoris #ehbfb #ahagbe ,savary4 ,leonis #hbecc #ahdigf ,villarceaux4 ,ophiuchi #gchfb #ahjfhc ,encke4 #cjfb in ,dorpat #idgfe #ahcgda ,madler4 ,catalogue ,bootis #aagadj #aggihh ,sir ,j4 ,herschel4 ,cygni #aghgjj #ahfbhg ,hind4 ,virginis #ahbabj #ahcfdc ,sir ,j4 ,herschel4 ,castor #bebffj #aheehc ,sir ,j4 ,herschel4 ,coron #gcfhhj #ahbfdh ,hind4 ,virginis #fcbbgj #afii ,hind4 ,centauri #ggjjj #aheaej ,jacob4 ,orbit of ,virginis4 ,perihelion passage #ahcfdj ,inclination #bg #cf ,position of ascending ,node #ai #g ,angle between line of ,nodes and #bie #ac ,apsides ,excentricity #jhgid ,period in years #ahdec ,orbit of ,herculis4 ,perihelion passage #ahcjef ,inclination #adj #ci ,position of ascending ,node #bag #ad ,angle between line of ,nodes and ,apsides #bffec ,eccentricity #jdcha ,period in years #cgba ,computed by ,j4 ,fletcher1 ,esq41 #ahec4 ,,note #bcc1 p4 #djc4 ,the mass is found in the manner explained in the text2 but the method of computing the distance of the star may be made more clear by what follows4 ,though the orbit of the satellite star is really and apparently elliptical1 let it be represented by ,c ,d ,o1 fig4 #ad1 for the sake of illustration1 the earth being in d4 ,it is clear that1 when the star moves through ,c ,d ,o1 its light will take longer in coming to the earth from ,o than from ,c1 by the whole time it employs in passing through ,o ,c1 the breadth of its orbit4 ,when that time is known by observation1 reduced to seconds1 and multiplied by #aij1jjj1 which is the number of miles light darts through in a second1 the product will be the breadth of the orbit in miles4 ,from this the dimensions of the ellipse will be obtained by the aid of observation2 the length and position of any diameter as ,s p may be found2 and as all the angles of the triangle d ,s p can be determined by observation1 the distance of the star from the earth may be computed4 ,,note #bcd1 p4 #dje4 ,the mean results of ,,mm4 ,argelander1 ,otto ,struve1 and ,luhndahl for stars in the northern hemisphere and the epoch #agij1 places the point to which the sun is tending in #bei #e of right ascension and #ee #bc of north polar distance4 ,mr4 ,gallaway computed from stars in the southern hemisphere1 at the same epoch1 the point to have been in #bfj #a right ascension and #ee #cg north polar distance1 results nearly identical1 though from very different data4 ,,note #bce1 p4 #dad4 ,one of the globular clusters mentioned in the text is represented in fig4 #a1 plate #h4 ,the stars are gradually condensed towards the centre1 where they run together in a blaze4 ,the more condensed part is projected on a ground of irregularly scattered stars1 which fills the whole field of the telescope4 ,there are few stars near this cluster4 ,,note #bcf1 p4 #dbj4 ,plate #h shows five nebul as seen in ,sir ,john ,herschel's #bj-;feet telescope4 #a4 ,an enormous ring seen obliquely with a dark centre and a small star at each extremity4 #b4 ,the ring in the constellation ,lyra4 #c4 ,the dumb-bell nebula in ,vulpicula4 #d4 ,the spiral nebula or brother system in the #bj-;feet telescope4 #e4 ,a spindle-shaped nebula4 ,plate #i represents some of the same objects as seen by ,lord ,rosse4 #a4 ,nebula in the girdle of ,andromeda4 #b4 ,the circular nebula of ,lyra4 #c4 ,the dumb-bell nebula in ,vulpicula4 ,the spiral nebul of #ea ,messier1 as seen by ,lord ,rosse1 #a in plate #aj1 represents fig4 #d of plate #h2 and fig4 #b in the same plate is part of the great nebula in ,orion1 for the whole has never been seen1 on account of extreme remoteness4 ,,note #bcg1 pp4 #cb1 #dbg4 ,the motion of the earth is visibly proved by ,m4 ,foucault's experiments4 ,if a pendulum be left to oscillate quite freely1 the forces producing the oscillations being in the vertical plane1 there is no cause that can produce an absolute change in its position with regard to space2 but the motion of the earth changes the position of a spectator with respect to the vertical plane1 and he refers his own motion to it1 which seems gradually to turn away from its position1 precisely as a person in a boat refers his own motion to that of the land1 and thus the motion of the earth is truly and visibly proved4 ,,index4 ,aberdeen1 high water at1 #id4 ,absorption1 influence of1 on temperature1 #bci2 difference of sea and land in power of1 #bdb2 gradually decreasing1 in transmission of radiant heat1 #bei2 of radiant heat1 varying with substances1 #bfh2 a transfer of force1 #bge1 #bgf4 ,acceleration of the moon's mean motion1 #cg1 #ch4 ,adams1 ,mr41 perturbation in ,uranus's motion computed by1 #bb2 discovery of ,neptune1 #fb4 ,arolites1 theory of1 #dbj1 #dbc4 ,africa1 tidal wave passing1 #id2 mean annual equatorial temperature in1 #bde2 indigenous productions of1 #bdi1 #bej4 ,air1 comparative velocity of light in water and1 #bjb4 ,see ,atmosphere4 ,airy1 ,professor1 periodic inequality in the solar system worked out by1 #bf2 phenomenon observed by1 during an eclipse1 #da2 mass of ,jupiter ascertained by1 #ee2 experiments ascertaining its density1 #eg2 astronomical tables improved by1 #fc2 discoveries in polarization of light1 #aib1 #aic4 ,aldebaran1 an optically double star1 #dja4 ,aleutian ,islands1 the1 vegetation of1 #beb4 ,alexandria1 arc of the meridian measured between ,syene and1 #di4 ,alg1 districts of distinct species of1 #beb2 banks of1 in the ,atlantic1 #bec4 ,algol1 fluctuations in lustre of1 #cij1 #cia4 ,alhazen1 effects of refraction observed by1 #aee4 ,alkalies1 resolved into metallic oxides1 #cjg4 ,alpha ,antaris1 8,coal ,sacks0 between ,centauri and1 #chf4 ,alpha ,aquil1 an optically double star1 #dja4 ---- ,centauri1 the parallax of1 #ed2 its rank1 #chd2 the ,milky ,way near1 #chf2 parallax1 as determined by ,henderson and ,maclear1 #chg2 distance from the sun1 #chh2 orbit and mass of1 #cii1 #djj2 colour1 #dja2 amount of light emitted by1 #djd2 rate of its proper motion1 #djd1 #dje2 globular nebulous cluster1 #dad4 ---- ,crucis1 zone of stars passing through1 #che2 zone between ,args and1 #cij2 nebulous cluster round1 #dae4 ---- ,lyr1 the polar star of the northern hemisphere1 #hb2 parallax of1 #chh2 distance from the sun1 #chi2 an optically double star1 #djj2 amount of light emitted by1 #djd4 ---- ,orionis1 a variable star1 #cic1 #cid4 ,alum1 experiments on the crystallization of1 #ajf1 #ajg2 heat transmitted through1 #bfa1 #bfb4 ,amazons1 the river of1 distance from its mouth where tides are perceptible1 #ih2 area occupied by forests on1 #bdc4 ,america1 course of the tidal wave along its coasts1 #ic1 #id2 mean annual equatorial temperature in1 #bde2 separation of isothermal lines in high latitudes1 ib42 number of known species of plants indigenous in1 #bdi2 number of species of trees1 #beb2 shooting stars over the continent of1 #dba4 ----1 ,south1 area of country raised by an earthquake in1 #bcd4 ,ampre1 ,m41 his discovery in electricity1 #caf2 theory of magnetism1 #cag1 #cah2 experiment testing his theory1 #cai1 #cbj4 ,analysis1 boundless dominion of1 #dbg1 #dbh4 ,andes1 the1 proportion of1 to the earth's mass1 #f2 increasing rarity of the air experienced in ascending1 #aah4 ,andromeda1 nebula in1 #dac2 nebulous region of1 #dag4 ,angstrm1 the electric spark defined by1 #cjc4 ,animals1 specific diversity of1 laws regulating their distribution1 #bed1 #bee4 ,annual equation1 the1 of the moon1 #ce1 #cf4 ---- variations in mean values of the magnetic elements1 #cdc4 ,annular nebul1 #dji2 in the northern hemisphere1 #daj1 #daa4 ,antarctic ,ocean1 tidal wave rising in #ic2 period of its passage to the ,thames1 #id2 depth of the stratum of constant temperature in1 #aja2 depression of the barometer observed in1 #abj4 ,antilles ,islands1 hurricanes beginning at1 #abf4 ,antinori1 ,cav41 experiments of1 in electricity1 #ccc4 ,antinous1 comet observed in the constellation of1 #cgb2 the ,milky ,way between ,orion and1 #chf4 ,antithesis1 the general character of magnetism1 #cci4 ,aphelion of a planet's path defined1 #af4 ,apogee1 solar1 its coincidence with the solstices1 #hf1 #hg4 ,april1 #ahcc1 disappearance of ,saturn's rings1 #fg2 apparent and mean time coinciding in1 #hd4 ,apsides of an axis defined1 #i2 direct1 variable motion of1 #ad2 cause of their advance1 or recession1 #af4 ,apures1 the mission of the1 ,humboldt's observations on sound at1 #ace4 ,aqueous vapour1 proportion of1 in the atmosphere1 #aag4 ,ara1 nebula in1 #dad4 ,arabian ,gulf1 the1 monsoons blowing over1 #abd4 ,arabs1 the1 their observations on planetary irregularities1 #bf2 lunar eclipses observed by1 #ch2 their division of time1 #he2 the pendulum used as a measure of time by1 #ij4 ,arago1 ,franois1 experiment by1 in proof of the undulatory theory of light1 #bjj2 decisive experiment suggested by1 #bjb2 observations in photography1 #bac2 observations on the moon's atmosphere1 #bbf2 increase of temperature below the earth's surface calculated by1 #bcj2 slow communication of temperature from the earth1 observed1 #bdd2 source of magnetism discovered1 #ccj2 theory of his magnetic experiments1 #ccb2 divergent flames of a comet described by1 #cfd2 his treatise on comets1 #cfh2 nature of comet's light determined by1 #chj1 #cha2 numbers of comets computed1 #cha1 #chb2 remark of1 on fixed stars1 #dje4 ,arc1 the ,voltaic1 #cjc-cje4 ,arcet1 ,m4 d'1 vibration of fibres of the retina according to1 #agh4 ,archer1 ,scott1 stimulus given to photography by1 #bjg4 ,arcs of the meridian1 mode of measuring1 #dg4 ,arctic ,sea1 depth of the zone of constant temperature1 #aja4 ---- regions1 vegetation found in1 #bdi4 ,arcturus1 comet bearing comparison with1 #cgi2 rank of1 #chd4 ,areas1 described by the radii vectores of planets1 a test of disturbing forces1 #aj2 unequable description of1 #ae4 ,argelander1 ,m41 period of a comet calculated by1 #cgj2 his mode of estimating distance of fixed stars1 #chi2 periods of fluctuation in stars computed by1 #cij1 #cia2 sun's motion proved1 #dje4 ,argentine preparations in photography1 chemical energy varying with1 #bjg1 #bjh2 changes effected by washing with alkalies1 #baj1 #baa4 ,argo1 variable star in1 #cic4 ,aries1 season of the sun's entrance into1 in ,hipparchus' age1 #hj4 ,arseniate of soda1 its crystals1 #aji4 ,artesian wells1 mode of sinking1 origin of the name1 #bcj4 ,asia1 indigenous productions of1 #bdi4 ,assyrians1 the1 division of time by1 #he4 ,astronomers1 fruits of their labours1 #c2 question still to be resolved by1 #bd2 terrestrial orbit differently measured by1 #cf4 ,astronomical distances1 method of measuring1 #dc2 tables1 method of forming1 #eh-fd4 ,astronomy1 its rank in the physical sciences1 an important office of1 #a2 studies necessary to the study of1 #b2 the key to divers problems in physical science1 #c2 the two greatest discoveries in1 #bc2 the three departments of1 #eh2 standards for measurement afforded by1 #hc2 application of1 to chronology1 #hg-hi2 furnishing standards of weights and measures1 #hi1 #ij2 atmospheric effects connecting the laws of molecular attraction with1 #ajb2 progress lately made by1 #dai1 #dbj4 ,atalanta1 diameter of1 #ef4 ,atlantic ,ocean1 direction of tidal waves in1 #ic2 conditions modifying tides1 #id2 depth of1 #if2 currents1 #ajj2 origin of hurricanes1 #abf2 superficial temperature of1 #bdd2 distinct vegetation of the polar basin1 #beb2 beds of alg in1 #bec2 meteors falling in1 #dba4 ---- telegraph1 #cbe1 #cbf2 terrestrial magnetism disturbing1 #cdf4 ,atmosphere of nebulous stars1 #daa1 #dab4 ---- of planets1 #bbf1 #bbg4 ---- of the sun1 its constitution1 #db2 indications of an absorptive surrounding the luminous1 #bac2 the true1 #bbd4 ---- terrestrial1 solar rays bent by1 in lunar eclipses1 #dj2 influence of1 in solar eclipses1 #da2 its analysis1 pressure on the surface of the globe1 #aag2 form of1 gradual decrease in density of its strata1 #aag1 #aah2 influence of temperature on its density1 #aai2 mean pressure of1 variable1 #abj2 the medium conveying sound1 #abi2 sympathetic vibrations transmitted by1 #adg1 #adh2 its action on light1 falsifying vision1 #aec2 phenomena produced by accidental changes in its strata1 #aee-aef2 effects of increased density in the stratum in the horizon1 #aeg1 #aeh2 lunar heat absorbed by1 #bbg2 cause of the cooler air in higher regions of1 #bdj1 #bda2 sun's heat modified by1 #bdd2 action of electricity in1 #bhd2 transmission of electricity by induction1 #bhf2 periodical variations of electricity in1 #bia2 accidental developments of electricity1 #bia1 #bib2 cause of variations in its magnetism1 #cdd1 #cde2 nebulous bodies made visible by1 #dba-dbc4 ,atmospheric air1 extreme elasticity of1 #aje4 ---- pressure1 effect of1 on electricity1 #bhh4 ,atomic constitution determining crystalline forms1 #aji4 ,atoms1 qualities of1 determining the nature of substances1 #aaj2 differences in weight of1 #aaa4 ,attraction1 modes of1 in spheres1 in the celestial bodies1 #d2 determining the forms of planets1 #f2 determining the motions of planets1 #g2 solar1 compelling the elliptical revolutions of planets1 #h2 mutual1 of planets1 complicating their motions1 #aj2 interference of1 disturbing the motions of heavenly bodies1 #aa2 disturbances from the operation of reciprocal1 #ac2 disturbances from inequality of1 #ad2 of satellites to primaries1 little disturbed1 #bf2 disturbing force of1 in spheroids1 #bg2 its effects on ,jupiter's satellites1 #bh2 sun's1 of the moon1 #cd2 principle modifying the earth's1 #cg2 local1 affecting the plumb-line1 #dh2 comparative force of the sun's1 #eg2 of an external body affecting a spheroid1 #gi2 producing tides1 #ia1 #ib2 of particles of matter1 #ajc2 capillary1 #aac2 producing annual atmospheric undulations1 #aba2 the lunar atmosphere affected by1 #bbf2 expansive force of heat overcoming1 #bga2 of electricities1 #bhc2 destruction of1 producing electricity1 #bhd2 laws of electrical1 #bhf-bhh2 modes of1 in static and in voltaic electricity1 #cag2 action of planetary1 on comet's orbits1 #cfa-cfc2 range of solar1 #cfe4 ,aurora1 the1 affecting the compass1 #cab4 ,australia1 evidence of deserts in the interior of1 #abd2 species of plants common to ,europe and1 #bea4 ,auvergne1 temperature of hot springs in1 #bca4 ,axes1 change in form of masses revolving round1 #f4 ----1 major1 length of1 in orbits1 invariable1 #bj2 of the orbits of ,jupiter's satellites1 cause of the direct motion observed in1 #bh2 position of1 in the solar system1 #fe2 a nutation in planetary1 #ff2 of the moon1 #fh1 #fi2 mechanical law affecting1 #gf4 ----1 optic1 of crystals1 #ahc4 ,axis1 greater1 of the earth's orbit1 period of its revolution1 #ch2 period of the earth's revolution1 #eh2 excess of ,jupiter's equatorial over his polar1 #ff2 of rotation1 proof of its being invariable1 #gf1 #gg4 ----1 major1 of a planet's orbit1 distance from the sun measured by1 #h2 designation of its extremities1 #i2 length of1 determining the form of the orbit1 #aj2 periods of its revolutions1 #ag2 length of1 not permanently changed1 #bj2 ,jupiter's periodically diminished1 ,saturn's increased1 #bf2 of the solar ellipse1 period of its revolution1 #hf4 ----1 magnecrystallic1 #cdi4 ,azores1 the1 icebergs reaching1 #ajj4 ,babbage1 ,charles1 his theory of volcanic action1 #bce-bcg2 quotation from1 on the nature of force1 #cec4 ,babinet1 ,m41 his theory of dark lines observed in the solar spectrum1 #afc2 comet's light computed by1 #cei4 ,babylon1 eclipse observed at1 #cf4 ,bacon1 ,francis1 anticipation of discovery by1 #cb4 ,baily1 ,mr41 compression of the terrestrial spheroid calculated by1 #ej2 density of the earth determined1 #eg2 fictitious antiquity ascribed to ,indian astronomical observations1 #hh4 ,bali1 volcanic eruption in1 #bcc4 ,balloon1 rarity of the air felt in a1 #aah2 observations made from1 #aai4 ,baltic1 the1 a tideless sea1 #ih2 decreased atmospheric pressure on the shores of1 #abj4 ,barlow1 ,mr41 observations supporting his theory of electric currents1 #cdf4 ,barometer1 the1 principles of cohesion and attraction applied to the construction of1 #aac2 density of the atmosphere measured by1 #aag2 mean heights of1 varying with atmospheric densities1 #aah2 mountain heights measured by1 #aai1 #abj2 atmospheric phenomena affecting1 #abj2 used to trace the course of atmospheric waves1 #aba2 cause of sudden fall in1 before hurricanes1 #abg2 refraction varying with1 #aed4 ,barrow1 ,cape1 observations on magnetic storms at1 #cde1 #cdf4 ,battery1 voltaic1 construction of1 #bih1 #bii2 ,professor ,daniell's improvements1 #bii1 #cjj2 action of1 charged with water1 #cjj2 constant flow of electricity obtained by means of1 #cab4 ----1 magnetic1 constructed by ,dr4 ,faraday1 #cbd1 #cbe2 ,mr4 ,henley's magneto-electric1 #cbe2 ,atlantic telegraph1 #cbf2 structure of1 for land telegraphs1 #cbh2 relation of heat to power of1 #cbi2 thermo-electric1 #ccc4 ,batsha1 port of1 tides neutralised in1 #ii4 ,bayle1 comparative density of the atmosphere in interplanetary space according to his law1 #cef4 ,bear1 ,little1 the1 the polar star in1 #hb4 ,becquerel1 ,m4 ,e41 unexplained photographic phenomenon observed by1 #bac2 phosphorescent property in the solar spectrum discovered1 #baf2 cause of phosphorescence1 #bag2 electricity excited by pressure1 #bhc2 light attributed to electricity by1 #bhd2 cause of phosphorescence investigated1 #bif2 instrument comparing intensities of electricities invented1 #cjj2 crystals formed by agency of electricity1 #cjh2 thermo-electric battery constructed by1 #ccc2 effect of atmospheric on terrestrial magnetism estimated1 #cde4 ,beehive1 the1 a nebulous star1 #dae4 ,berard1 ,m41 experiments of1 in polarizing heat1 #bfd4 ,berlin1 line of coincidence in temperature passing through1 #bch4 ,berne1 increasing temperature of a deserted mine in1 #bcj4 ,berre1 ,dr41 photographic pictures perfected by1 #bje4 ,bessel1 ,m41 his calculations from measurements of arcs of the meridian1 #dh2 calculation of the sun's mean apparent diameter1 #ef2 his computation of the mass of ,saturn's ring1 #fh2 diminished obliquity of the ecliptic observed by1 #ha2 parallax calculated1 #chi2 his theory of ,sirius's irregular motions1 #cib2 catalogue of double stars1 #cif2 mass of #fa ,cygni found by1 #djd4 ,beta ,lyr1 a variable star1 #cia2 nebula between ,lyr and1 #daj4 ,benzenberg1 ,m41 velocities of falling stars computed by1 #dbc4 ,biela1 ,m41 date of the discovery of his comet1 #cfg2 possibility of collision with the earth1 #cfh2 present and prospective planetary influence on1 #cfi2 becoming two distinct bodies1 #cfi1 #cgj4 ,binary systems of stars1 #cie-djf4 ,see ,double stars4 ,biot1 ,m41 his ascent in a balloon1 #aah2 experiments of1 on the transmission of sounds through pipes1 #acg2 liquids possessing the power of circular polarization discovered by1 #aij2 his theory of circular polarization1 #aia2 cause of phosphorescence in the solar spectrum investigated by1 #bag4 ,birds1 distribution of distinct species of1 #bee4 ,birt1 ,mr41 atmospheric waves measured by1 #aba1 #abb4 ,bise1 in ,switzerland1 cause of1 #bdb4 ,bismuth1 its magnetic and electric properties1 #cdg4 ,black ,sea1 the1 scarcely affected by tides1 #ih4 ,bode1 ,baron1 law of1 assumed in computing ,neptune's position1 #fa2 failing in the case of ,neptune1 #fc4 ,bond1 ,mr41 satellite of ,saturn discovered by1 #cb2 elliptical nebula resolved1 #dac4 ,bonnycastle1 ,captain1 phosphorescent phenomenon observed by1 #bie1 #bif4 ,bonpland1 ,m41 identical productions of the ,old and ,new ,world found by1 #bea4 ,botes1 nebulous system in1 #dag4 ,bore1 the1 of the ,hoogly1 its origin1 #id4 ,botanical districts1 distinct1 of the globe1 #bea1 #beb4 ,botto1 ,m41 thermo-electricity used in decomposition by1 #ccc4 ,bouguer1 degrees of the meridian measured by1 #dh4 ,boussingault1 ,m41 depth of the underground stratum of constant heat calculated by1 #bbh4 ,bouvard1 ,m41 atmospheric undulations estimated by1 #aba4 ,bradley1 ,dr41 motion of the pole of the equator discovered by1 #hd2 his tables of refraction1 #aee4 ,brahmins1 measurement of time by1 #he4 ,brand1 ,m41 observation of1 on meteors1 #dbc4 ,brewster1 ,sir ,david1 his analysis of the solar spectrum1 #afa2 experiments on rayless lines1 #afc2 experiments on spectra of flames1 #afd2 law discovered by1 determining angles of polarization for light1 #ahc2 experiments on fluorescence of light1 #aig2 line of coincidence in temperature of springs and of the atmosphere determined by1 #bch2 temperature of a pole of maximum cold determined1 #bde2 isogeothermal lines determined by1 #bdf2 observations on the light of fixed stars1 #djb4 ,brighton1 phenomenon caused by reflection observed from1 #aeg4 ,brinkley1 ,bishop1 mass of the moon determined by1 #ef4 ,british ,channel1 height of tides in1 #ih4 ---- ,isles1 atmospheric wave passing over1 #aba4 ,brorsen1 ,m41 periods of comets discovered by1 #cgj4 ,brown1 ,dr4 ,robert1 peculiar vegetation found by1 in ,australia1 #bea4 ,buchan1 ,dr41 phenomenon caused by reflection observed by1 #aeg4 ,csar1 ,julius1 era computed from his reign1 #he4 ,cagniard de la ,tour1 ,m41 instrument designed by1 measuring musical notes1 #adc4 ,calms produced by the trade-winds1 #abb1 #abc4 ,calorific rays4 ,see ,rays of heat4 ,calotype1 the invention of1 #bjd4 ,camelopard1 nebulous system in1 #dag4 ,canaries1 the1 vegetation of1 #beb4 ,canary-glass1 fluorescence of light in1 #aif4 ,cancer1 the calms of1 #abc2 the tropic of1 marking the limit of the trade-winds1 #abf2 nebulous cluster in1 #dae4 ,canis ,major1 position of1 #cij4 ---- ,venatica1 nebulous system in1 #dag4 ,capillarity1 theory of1 #aac2 forces producing1 #aad2 familiar examples of1 #aae2 curious phenomena1 #aae1 #aaf4 ,capricorn1 the calms of1 #abc2 the tropic of1 hurricanes changing their direction at1 #abf4 ,carbon1 its powers contrasted as a crystal and as an opaque amorphous substance1 #cjb1 #cjc4 ,carbonate of lime4 ,see ,lime4 ,carbonic oxide1 its constituent parts1 #aaa4 ---- acid1 proportion of1 in the atmosphere1 #aag4 ,cardinal points1 the1 position of continental masses with regard to1 influencing temperature1 #bdd4 ,caribbean ,islands1 hurricanes beginning at1 #abf4 ,castor1 discovered by ,sir ,william ,herschel1 #cif4 ,cassiopeia1 star appearing and vanishing in1 #cib1 #cic4 ,categat1 the1 consequence of its narrowness1 #ih4 ,cauchy1 ,m41 data furnished by1 for investigation of the theory of light1 #bja4 ,cayenne1 variation in length of the pendulum between ,paris and1 #ea4 ,celestial bodies3 law of their mutual attraction1 #d2 of the solar system3 law determining their attraction to the sun1 #e2 problem to fix the positions of1 on occurrence of disturbance in their motions through counteracting attractions1 #aa2 theory of their mutual connection and dependence1 #bd2 mode of finding the absolute distances of1 #dc2 distances of1 computed from their parallax1 #eb1 #ed2 apparent position of1 affected by refraction1 #aec1 #aed2 apparent infinity of1 #dbj4 ,centaur1 position of1 #cij2 brilliant double star in1 #cii4 ,central ,asia1 the mountains of1 their ascent by ,marco ,polo1 #aah4 ,centre of gravity4 ,see ,gravity4 ,centrifugal force1 moon's motions modified by1 #e2 influence of1 on planet-forms1 #f2 retarding oscillations of the pendulum1 #cb2 action of1 in determining the figure of the earth1 #dd1 #de2 measurement of its intensity1 #di2 resolved into two forces1 its action on the sea1 #ajj4 ,ceres1 astronomical tables of1 #fc2 height of her atmosphere1 #bbf2 comet of #aggj revolving beyond the orbit of1 #cfa4 ,cetus1 nebulous patches crossing1 #dag4 ,chaldeans1 the1 mean longitude found from observations of1 #cf2 result of comparison of their observations with modern1 #ch4 ,challis1 ,professor1 ,brewster's analysis of light questioned by1 #afa4 ,charcoal1 light produced by electricity from1 #cjb-cjc4 ,charles ,v41 the ,emperor1 observations on comets1 made in his reign1 #cgj4 ,chaudes ,aigues1 temperature of1 #bca4 ,chemical action of rays of the solar spectrum1 #bjc1 #bjg2 varying maximum of energy1 #bjh2 action varying with refrangibility1 #bji-bab2 action in luminous spectrum not continuous1 #bac2 energy an independent property of rays1 #bad2 properties of the parathermic rays1 #bai2 action of light maintaining vegetation1 #bdi2 affinities the source of the power of steam1 #bgh2 of electricity on oxygen1 #bhd2 eliciting voltaic electricity1 #big1 #cjj2 voltaic electricity1 an agent in1 analysis1 #cjg1 #cjh4 ---- combinations1 theory of1 #aaj2 invariable proportions of1 #aaa2 cohesive force inducing1 #aab2 producing combustion1 #bgj4 ---- force1 the power of1 #aab4 ---- rays1 causing the deposition of dew1 #bfi4 ,chile1 elevation of land by an earthquake in1 #bcd4 ,china1 distinct flora of1 #bea4 ---- ,sea1 the1 monsoons blowing over1 #abd4 ---- ink1 polarized light reflected from1 #aic4 ,chinese1 the1 observations of1 on the mean motions of ,jupiter and ,saturn1 #be2 proof of their early study of astronomy1 #hh2 decimal divisions used by1 #ij2 elements of comets computed from their observation1 #cfe2 comet of #abfd recorded by1 #cgj4 ---- ,tartary1 herbarium collected in1 #bej1 #bea4 ,chladni1 discovery of1 in musical science1 #ade4 ,christian era1 traces of astronomical records before1 #cfe4 ,chromatype1 the invention of1 #bjf4 ,chronology1 dependent on astronomy1 #hg-hi4 ,chrysotype1 the1 coloured photographs obtained from1 #bjf4 ,circuit1 galvanic1 modes of obtaining1 #ccb4 ,circular arcs1 principle with regard to their sines and cosines1 a pledge for the stability of the solar system1 #bj4 ---- motion1 ratio of forces procuring1 #chb4 ---- orbits of planets distinguished from elliptical1 #h2 of satellites1 #bg4 ---- polarization of light1 #ahi-aib2 of heat1 #bff4 ,circumference of the earth1 #di4 ,civil time1 measure of its periods1 #hc2 not precisely adjusted to solar revolutions1 #he4 ,clairaut1 periodic time of ,halley's comet computed by1 #cfb1 #cfc4 ,cleavages of crystals1 #aji2 position of1 affecting the intensity of magnetic action1 #cej4 ,climates1 planetary1 #bbe1 #bbf2 cause of the different terrestrial1 #bcg2 phenomena affecting1 #bci1 #bdj2 causes of variety of1 #bdc1 #bdd2 milder1 of the ,polar ,ocean1 #bde1 #bdf2 like mean annual temperatures not ensuring like1 #bdf2 compensations of irregularities1 #bdg4 ,clocks1 showing apparent sidereal time1 #hc2 regulated to show decimal time1 #hd2 irregular action of1 corrected by the laws of unequal expansion1 #bgb4 ,clouds1 circling the belt of equatorial calms1 #abc2 region of1 #abd2 electricity evolved from1 #bia-bib4 ,cloyne1 ,bishop of1 his calculation of the moon's mass1 #ef4 ,coal-measures1 tropical plants in1 #gb1 #gc2 age of their formation1 #ge4 ,coal1 chemical force evolved from1 by combustion1 #bgh2 source of its combustible qualities1 #bgi1 #bhj4 8,coal ,sacks0 in the ,milky ,way1 #chf4 ,cohesion1 influence of1 on matter1 #aje2 phenomena arising from its force1 #ajf2 attraction of1 overcome by the expansive power of heat1 #bga4 ,cohesive force1 properties of material molecules constituting1 #ajc2 effectual only to unite particles of like nature1 #aaj2 inducing chemical combination1 #aab2 capillary attraction1 an action of1 #aac4 ,coins1 impressions taken from1 by contact1 #bbj2 by electricity1 #bba4 ,cold1 contraction caused by1 #bga1 #bgb2 mitigated by slow propagation of heat in air1 #bgc2 generated by voltaic electricity1 #cjb2 increasing the conducting power of the air1 #cde4 ,colladon1 ,m41 experiments of1 testing the velocity of sound1 #ace4 ,collision between the earth and comets1 possibilities1 possible effects of1 #cfg1 #cfi4 ,collodion1 sensitiveness of1 to light1 #bjc2 properties of1 as an agent in photography1 #bjg4 ,colours1 seven primary1 #aei2 theory of the decomposition of white light into1 #afj2 degree of refrangibility not invariable1 #afa2 three primary1 ib42 new1 discovered by ,sir ,john ,herschel1 #afb2 rays refracted without1 #afd2 rarely homogeneous1 #afe2 experiments on accidental and complementary1 #afe1 #aff2 determined by undulations of ether1 experiments1 #agj-age2 of material substances1 whence derived1 #age2 produced by analyzing polarized light1 #ahf-ahh2 varying with refrangibility of rays1 #aih2 obtained in photography1 #bjf2 images of the solar spectrum imitating the prismatic1 #bjh-bji2 of seaweeds1 #bec2 not invariably dependent on light1 ib42 affected by absorption and reflection1 #bfh2 of the electric spark1 affected by the atmosphere1 #bhi2 of the voltaic spectrum1 #cjc2 of the electric spark1 #cjd2 produced by oxidation on silver1 #cje2 of the fixed stars1 #dja1 #djb2 of planetary nebul1 #dab2 of nebulous clusters1 #dae4 ,columbus1 beds of alg found by1 #bec4 ,column1 capillary1 forces producing changes in its form1 #aad1 #aae4 ,coma ,berenices1 a nebulous cluster1 #dae2 nebulous zone passing1 #daf1 #dag4 ,combustion1 cause of1 #bgj2 defined1 #cjd4 ,comets1 attraction by the sun of1 #e2 disturbances in the motion of1 a key to the nature of the ethereal medium1 #bb2 retrograde motion in1 #cc2 passing through ,jupiter's satellites1 #fi2 return of1 to their perihelia1 furnishing historical data1 #hh2 existence of the luminous ether demonstrated by1 #afh1 #afi2 terrestrial atmosphere unaffected by1 #ceh2 amount of their light computed1 #ceh1 #cei2 passages of1 through the solar system1 #cei2 velocity1 paths of1 #cei1 #cfj2 proof of the return of1 #cfj2 disturbing action of planets on their orbits1 #cfa2 of #aggj1 an example1 #cfa1 #cfb2 computed return of ,halley's1 #cfb1 #cfc2 aspects1 records of ,halley's1 #cfc-cfe2 discoveries made by the revolutions of1 #cfe2 of the solar system1 ,encke's1 #cfe1 #cff2 ,biela's1 possibility of collision with1 #cfg1 #cgj2 periods of various1 #cgj2 cause of their brilliancy1 #cga2 velocity1 sun's influence on1 #cga1 #cgb2 of #ahdc1 #cgb1 #cgc2 their constitution1 #cgc1 #cgd2 of #ahaa1 its luminous envelopes1 #cgd1 #cge2 sudden convulsions in1 #cge2 tails1 #cge-cgg2 causes assigned for contraction of diameter in1 #cgg1 #cgh2 ,donati's1 #cgh1 #cgi2 nature of their light1 #cgi-cha2 computations of their numbers1 #cha1 #chb2 orbits of1 #chc2 nebula resembling1 #dac4 ,compass1 mariner's1 phenomena disturbing1 #cab2 intensity of a galvanic current measured by1 #cae4 ,compression of the terrestrial spheroid1 calculations of1 #dh-ea2 cause of the great1 in ,jupiter1 #ff2 measures of1 from pressure of superincumbent mass1 #gh2 effect of1 on magnetic action1 #cea4 ,concord1 a1 in music1 #adb4 ,conductors of electricity1 #bhd1 #bhe2 lightning1 #bic2 molecular structure determining the power of1 #cjc4 ,conic sections1 conditions compelling bodies in space to move in1 #e2 principle determining their nature1 #aa4 ,constellations1 nearest the sun1 #cij2 where the orbit of the solar system lies1 #djf2 occupied by the nebulous system1 #dag4 ,contraction caused by cold1 #bga1 #bgb4 ,cook1 ,captain1 object of his first voyage1 #ec4 ,cooper1 ,mr41 list of missing stars drawn up by1 #cie4 ,copper1 electricity communicated to plates of1 #bbj2 lightning-conductors of1 #bic2 action of an electro-magnet on1 #cea1 #ceb4 ,cordier1 temperature of mines observed by1 #bbh4 ,cordilleras1 effect on temperature of their table-lands1 #bda4 ,corn1 a1 field used to illustrate the propagation of sound1 #abi1 #acj4 ,cornwall1 hot-springs in mines of1 #bbi4 ,corona ,australis1 nebula in1 #dad4 ,corpuscular theory of light1 #afg2 phenomena disproving1 #aga1 #age1 #agf4 ,coseguina1 volcanic irruption of1 #bcc4 ,coulomb1 instrument measuring electrical intensity1 invented by1 #bhg4 ,creation1 vastness and magnificence of1 #b4 ,crimea1 cause of the great storm in the1 #abb4 ,cross1 ,mr41 voltaic battery with constant action invented by1 #cjj4 ,cross1 the ,southern1 vacant patches of the ,milky ,way near1 #chf4 ,crystallization defined1 #ajf2 forms of1 their variety affected by temperature1 #ajg1 #ajh2 permanent and variable forms1 #ajh1 #aji2 cleavages in1 #aji2 common to all substances1 ib42 by the agency of electricity1 #cjh1 #cji4 ,crystals1 conditions determining their forms1 #ajg-aji2 optic axes of1 #ahc2 used in polarizing light1 #ahf1 #ahh2 changes in1 effected by compression1 #ahi2 transmission of rays of heat by1 #beh2 expansion of1 by heat1 #bgb1 #bgc2 formed by electricity1 #cjh2 action of magnetism in1 #cdi1 #cej2 circumstances determining the set of1 #cej1 #cea2 effect of temperature on magnetized1 #ceb4 ,cumming1 ,professor1 experiments of1 in thermo-electricity1 #ccc4 ,currents1 two great1 setting from each pole towards the equator1 #ajj2 proving the rotation of winds1 #abd1 #abe4 ----1 electric1 flow of1 regulated by ,volta1 #big-bii2 characteristics of ,voltaic1 #cja2 conductors1 non-conductors of1 #cji2 continuous flow of ,voltaic1 #cab2 action of1 on magnets1 #cac-cae2 reciprocal and mutual action of magnetic and electric1 #caf1 #cag2 ,ampre's theory of1 unsolved difficulties1 #cag1 #cah2 effect of1 on polarized rays1 #cai2 electric1 evolved by magnets1 #cbb1 #cbc2 their power of producing induction1 #cbd2 direction of1 produced by rotation1 #ccj-ccb2 evolved by application of heat1 #ccb1 #ccc2 produced by intersecting magnetic curves1 #cci2 induced by crossing terrestrial lines of magnetic force1 #cdb4 ,curves1 described by bodies projected in space1 #e4 ----1 magnetic1 #cch2 electricity produced by intersecting1 #cci2 nature of1 proved by ,dr4 ,faraday1 #cci1 #cdj2 terrestrial1 #cda1 #cdb2 extent of the range of terrestrial1 #cdd2 complete connected system of the terrestrial1 #cde2 inductive effect on the ,atlantic telegraph1 #cdf2 diamagnetic1 #cdh4 ,cyanite1 changes effected in1 by magnetism1 #cdi4 ,cyanotypes1 coloured photographs obtained by1 #bjf4 ,cygni #fa1 distance from the sun of1 #chi2 orbit and mass of1 #cih1 #cii2 colours1 #dja2 mass1 #djd2 proper motion1 #dje4 ,cygnus1 portion of the ,milky ,way lying between ,centauri and1 #chf4 ,cylinders1 rotating by electricity1 #cac2 electro-dynamic1 #caf4 ,dalcoath copper-mine1 its temperature1 #bbh4 ,daguerre1 ,m41 his inventions in photography1 #bje2 action of light on the iodide of silver explained by1 #bai4 ,daguerreotype1 the1 invention of1 #bje4 ,dalton1 ,dr41 law of definite proportion established by1 #aaa2 law of the wind's rotation observed by1 #abe4 ,damoiseau1 ,m41 perturbations of a comet computed by1 #cfg4 ,daniell1 ,professor1 ,voltaic battery improved by1 #bii4 ,daubuisson1 ,m41 observations of1 in mines1 #bbh4 ,davy1 ,sir ,humphry1 his first attempts to produce photographic pictures1 #bjc-bjd2 experiment of1 proving identity of heat and motion1 #bge2 experiments on the electric spectrum1 #bhi2 alkalies1 earths decomposed by1 #cjg4 ,days1 law determining the length of1 #ga2 period of the mean sidereal and solar1 #hc2 varying with the seasons1 #hd2 decimal division of1 #hd2 seven1 the most permanent division of time1 #he4 ,deccan1 the1 wheat ripening in1 #bej4 ,december1 #ahcb1 disappearance of ,saturn's rings in1 #fg2 coincidence of mean and apparent time in1 #hd2 date of ,christ's nativity1 #he2 the astronomical year beginning in1 #hf4 ,decimal division of time1 #hd4 ,declinations of the moon1 #ig4 ,decomposition1 effected by electricity1 #cjg-cjh2 by magnetism1 #cbc2 by thermo-electricity1 #ccc4 ,delambre1 his computations of the length of the year1 #cei4 ,delta ,cephei1 a variable star1 #cia4 ,denmark1 course of the tidal wave to1 #id4 ,density1 variable1 impeding sound1 #ace1 #acf3 of media1 modifying refraction1 #aec4 ,densities of heavenly bodies1 formula finding1 #ef2 experiments1 #eg1 #eh2 comparative of the terrestrial globe1 #gg1 #gh4 ,deserts1 causing monsoons1 #abd2 influence of1 on temperature1 #bdc4 ,dew1 cause of its deposition1 #bfi4 ,diamagnetic substances1 #cce1 #ccf4 ,diamagnetism defined1 #cce2 substances it is resident in1 #ccf2 discovery1 characteristics of1 #cdg2 neutral substances obtained by proportionate combination of1 with paramagnetism1 ib42 polarity of1 #cdh2 connected with arrangement of molecules1 #cej-cea2 affected by division and compression1 #cea2 possibly identical with paramagnetism1 #cef1 #ceg4 ,diameter of the earth1 #ba2 ,jupiter's polar1 #bg2 excess of his equatorial1 #ci2 apparent1 of the sun and moon1 nearly equal1 #dj2 of the earth1 #di2 of bodies composing the solar system1 #ef2 of ,neptune1 #fc2 comets lacking a sensible1 #cgc2 contraction of1 in comets1 #cgg2 causes assigned for1 #cgg1 #cgh4 ---- of an annular nebula1 #daj2 sensible1 of a planetary nebula1 #dab4 ,diamond1 the1 polarized light reflected from1 #aic4 ,dielectrics in electricity1 #bhf4 ,dieppe1 seen from ,hastings1 #aeg4 ,differential telescope1 the1 experiments to be made by1 #bbg4 ,discord1 a1 in music1 #adb4 ,diurnal tides of the atmosphere1 their duration1 #aba4 ---- variations in mean values of the magnetic elements1 #cdc4 ,dbereiner1 ,m41 spontaneous combustion discovered by1 #aab4 ,doldrums1 region of the1 #abc4 ,dollond1 ,mr41 achromatic telescope perfected by1 #afe4 ,donati1 ,signore1 discovery of his comet1 #cgh2 changes in1 its irregularities1 #cgi4 ,doradus1 nebulous patches on1 #dag4 ,dorpat1 occultation of a star observed from1 #cfd4 ,double nebul1 #daa4 ,double stars1 catalogues of1 #cie1 #cif2 formul obtaining the relative position and motions1 #cif1 #cig2 eclipse in ,virginis1 #cig2 orbit of1 determined1 #cih2 eclipse in ,herculis1 ib42 orbits and periodic times of1 #cih1 #cii2 anomalies in motions1 #djj2 optically double1 #djj1 #dja2 colours of1 #dja2 rays composing the light of1 #dja1 #djb2 passage of light from1 furnishing data to ascertaining their actual distance1 #djb1 #djc2 data for finding their masses1 #djc1 #djd2 calculations founded on the quantity of light emitted from1 #djd2 real and apparent motions of1 #djd-djf2 apparent periodic time1 #djf1 #djg2 connection of elliptical nebul with1 #daa4 ,dove1 ,professor1 law of the wind's rotation developed by1 #abe2 average temperature of the earth's surface estimated by1 #bcg4 ,draco1 nebulous system in1 #dag4 ,draper1 ,professor1 experiments of1 on fluorescence of light1 #aih2 experiments in photography1 #bac2 properties of parathermic rays discovered by1 #bai2 spectrum produced from diffracted light1 #bbc2 theory of heat propagated by undulations1 #bfg4 ,dunlop1 ,mr41 revolution of a double star calculated by1 #djj4 ,dusejour1 ,m41 distances of comets computed by1 #cei4 ,dynamic electricity1 #big4 ,see ,voltaic4 ---- theory of heat1 fundamental principle of1 #ceg4 ,dynamic equator of the earth1 #cdc4 ,dynamical theory of heat1 #bgd1 #bge2 illustrated by liquefaction and condensation1 #bgh2 by generation of steam1 #bgf1 #bgg2 power of nature1 #bgi-bha4 ,dynamics1 principle in1 a law1 with regard to the earth's rotation1 #gb2 electro1 discovery of action of currents in1 #caf2 the theory of1 universal application of1 #dbf1 #dbg4 ,earth1 the1 influence of its form on attraction1 #d2 square of the moon's distance from1 #e2 form of1 #f1 #g2 moon's influence on its rotations1 #g2 diameter of1 #ba2 mean distance from the sun1 ib4 note2 permanence of revolution in its times and seasons1 #bc2 perturbation in the mean motion of ,venus and1 #bf2 proof of the motion of1 in its orbit1 of its rotation1 #cb2 variations in its attraction of the moon1 #cg2 compression of its spheroid1 #ch2 internal structure of1 #ci2 its mean distance from the sun1 #dc2 theoretical investigation of its figure1 #dd-df2 dimensions of1 determined1 #dh1 #di2 figure of1 found by calculating its variations in gravitation1 #di-ea2 density compared with the sun1 #ef2 experiments finding its mean density1 #eg1 #eh2 rate of revolution round its axis1 #eh2 its diurnal rotation immutable1 #ga1 #gb2 changes in temperature and their causes1 #gc1 #gd2 nature of the revolutions producing geological changes1 #gf1 #gg2 conjectures touching its internal structure1 #gh2 effects produced by solar and lunar attraction affecting its equator1 #gi-ha2 its form furnishing standards of weight and measure1 #hi2 rotation of1 acting on tides1 #ib2 attraction of1 affecting the lunar atmosphere1 #bbf2 conjectured constitution of its interior1 #bca1 #bcb2 principles regulating the diffusion of solar heat1 #bcg-bdg2 distribution of known species of plants over1 #bdi-beb2 electric tension of1 #bia2 lines of magnetic force issuing from1 #cda2 magnetic properties of1 #cdb1 #cdc2 effect of its collision with a comet1 #cfh2 nearest approach of comets to1 #cfi2 passage of light from ,centauri to1 #chh2 theories of meteors falling on1 #dba-dbc4 ,earthquakes in ,south ,america1 #bcd4 ,earths1 decomposed by voltaic electricity1 #cjg4 ,eastern coasts1 cause of their colder climates1 #bdd4 ,ebb1 see ,tides4 boulemens of mountains in ,switzerland1 cause of1 #bga4 ,echoes1 theory of their origin1 #acg1 #ach4 ,eclipses1 lunar1 accelerated revolutions proved by observations of1 #cf2 observations of1 confirming results of analysis1 #ch2 principle regulating their return1 #ci2 refraction of rays by the terrestrial atmosphere1 #dj4 ----1 solar1 #dj2 effects of light in1 #da4 ----1 planetary1 #db2 the solar atmosphere visible in1 #bbd2 of double stars1 #cig1 #cih4 ,ecliptic1 the1 forming the equinoxes1 #i2 latitude reckoned from the plane of1 ib42 deviations of planetary orbits from1 #aj2 forces affecting their position towards1 #ae2 their compensated and uncompensated variations to the plane of1 #ah1 #ai2 secular variation in the plane of1 #bc2 orbits of satellites1 nearly perpendicular to1 #cc2 lunar motions towards1 #ce2 inclination of the sun's plane of rotation to1 #fe2 inclination of the plane of ,saturn's rings1 #fg2 inclination of the plane of the terrestrial equator1 #gi2 tendency of its plane to coincide with the equatorial1 ib42 retrograde motion of the equinoctial points on1 #hj2 obliquity of1 affecting the duration of time1 #hd4 ,edinburgh1 comparatively equal mean annual temperature of1 #bdf4 ,egypt1 hieroglyphic manuscript from1 interpreted by astronomy1 #hi4 ,egyptians1 the civil year of1 #he4 ,elastic impact1 the foundation of dynamical theories1 #ceg4 ,elasticity1 property of1 resisting compression1 #aje4 ,electric telegraphs1 experiment suggesting the principle of1 #cbc2 construction of1 #cbe-cbh4 ,electricity assumed as the medium attracting particles of matter1 #ajc1 #ajd2 identical with chemical affinity1 #aaj2 in composition and decomposition1 subject to laws of definite proportion1 #aab2 influencing winds1 #abe2 its comparative velocity1 #ach2 producing phosphorescence1 #bag2 communicated to metal plates by juxtaposition1 #bbj2 impressions traced on glass by1 #bba2 rays exciting1 #bbc2 a dual power1 #bhb2 modes of exciting by disturbing equilibrium1 #bhb-bhd2 transmission of1 #bhd1 #bhe2 transmission by induction1 #bhe1 #bhf2 laws of attraction and repulsion determining intensity of1 #bhf-bhh2 heat and light produced by1 #bhh2 velocity of1 #bhi2 experiment determining its velocity1 #bij2 development of1 in the atmosphere1 #bia1 #bib2 phosphorescence excited by1 #bid2 ,voltaic1 see ,voltaic2 conduction of static1 contrasted with ,voltaic1 #cji2 laws of action in1 distinguishing it from ,voltaic1 #cag2 relation between #cbb1 #cbc2 telegraphs working by1 #cbc-cbh2 produced by rotation1 #ccj1 #cca2 thermo1 #ccb1 #ccc2 exact balance of its dual force1 #ccd2 points of analogy between magnetism and1 #cdj1 #cda2 causing convulsions in comets1 #cge4 ,electro-dynamics1 see ,dynamics4 ---- magnetism1 see ,magnetism4 ,elements1 the three terrestrial magnetic1 #cdc2 variations in1 ib42 storms affecting1 #cdd4 ,elevation1 effect of1 on temperature1 #bdj-bdb2 on vegetation1 #bej4 ,ellipses1 described by planets1 #e2 paths of planets describing1 #aj2 preventing compensation of disturbance1 #ae2 cause and measures of variation in1 #ag2 described by comets1 #cfc1 #cff4 ,ellipsoid1 an1 of revolution1 mass assuming the form of1 #de2 its equatorial and its polar radius1 #dh2 permanent axes of rotation1 #gf4 ,elliptic motion1 ratio of forces procuring1 #chb4 ,elliptical polarization of light1 #aib1 #aic2 of heat1 #bfg4 ---- nebul1 #dji2 their connection with double stars1 #daa2 frequency1 #dac2 difficult of resolution1 #dae4 ,encke1 ,professor1 sun's parallax found by1 #ec2 his comet1 #afi2 aspects1 period of his comet1 #cfe1 #cff2 cause of acceleration in its revolution1 #cff1 #cfg2 crossing the terrestrial orbit1 #cfh2 prospective and present planetary influence on1 #cfi2 disappearance of its tail and nucleus1 #cfi2 referred to1 #cgg2 contraction of diameter1 ib4 ,england1 arcs of the meridian measured in1 #dh2 course of the tidal wave towards its west coast1 #id2 peculiarities of photography in1 #bac2 meteors falling in1 #dba4 ,engravings copied by photography1 #bjd2 impressions taken by contact with iodized silver1 #bba2 impressions taken from1 by galvanism1 #cji4 ,epipolic light1 #aig4 ,epsilon ,orionis1 zone of stars passing through1 #che4 ,equation of the centre1 defined1 #i2 lunar1 #ce4 ,equator1 the1 forces compelling the wider circle of1 #f2 inclination of the terrestrial to the plane of the ecliptic1 #bc2 of the solar system1 #bd2 measure of the centrifugal force at1 #di2 calculation from lunar action on the terrestrial1 #ee2 effects produced by external attraction influencing the direction of its plane1 #gi1 #hj2 inequality in its polar motion1 #ha2 cause of the calms at1 #abb2 depth of the underground stratum of constant temperature at1 #bbh2 maximum of solar heating influence1 #bch2 superficial extent of land1 #bdd2 mean annual temperature1 #bde4 ,equator of the sun1 maximum of solar heat attained in1 #bbe4 ----1 dynamic1 surrounding the terrestrial globe1 #cdc4 ----1 magnetic1 of the earth1 #cdc4 ,equinoctial circle1 the1 defined1 #i4 ---- points1 effects of solar and lunar attraction on1 #gi2 period of their revolution1 #hj2 measuring time1 #hc4 ,equinoxes1 the1 defined1 #i2 venial1 a point whence planetary motions are estimated1 ib42 of the planets1 cause of a precession in1 #ff2 causes preventing their invariable correspondence with points of the ecliptic1 #gi2 precession affecting the seasons1 #hj2 secular motion of1 periodic variations1 #hj1 #ha2 eras depending on the precession of1 #hf1 #hg2 tides augmented in1 #ig4 ,eras1 astronomical1 determined by the position of the major axis of the solar ellipse1 #hf1 #hg4 ,eratosthenes1 the earth's circumference measured by1 #di4 ,eridanus1 nebulous patches crossing1 #dag4 ,erman1 ,m41 depression of the barometer observed by1 #abj4 ,eruptions1 volcanic1 recorded1 #bcd4 ,eta ,aquil1 a variable star1 #cia4 ---- ,args1 zone stretching from1 #cij2 nebula round1 #dah1 #dai4 ---- ,coron1 periodic time of1 #cih4 ,etna1 measurements of1 #abj4 ,ethereal medium1 undulations of1 propagating heat1 #bfg2 permeable to lines of magnetic force1 #cdd2 its density1 #cef2 transmitting gravity1 ib42 magnetic1 #cef1 #ceg2 offices discharged by1 #ceg2 pervading the visible creation1 #ceh2 influence of1 on comet motion1 #cfe2 astral revolutions accelerated by1 #cff2 probable increase in density of1 #cfg4 ,europe1 atmospheric wave passing over1 #aba2 causes of variation of climate in1 #bdd2 separation of isothermal lines in high latitudes of1 #bde2 differences of latitude enjoying the same mean temperature1 #bdf2 indigenous productions of1 #bdi2 number of indigenous productions common to ,australia and1 #bea2 number of species of forest trees1 #beb4 ,eudoxus1 ,plato's contemporary1 astronomical observation of1 #hh4 ,evaporation1 conditions affecting1 #bfi1 #bgj4 ,everest1 ,colonel1 arc of the meridian measured by1 #dh4 ,excentricity of planetary orbits measured1 #ag4 ,expansion1 universal law of1 #bga2 accuracy in measurement ensured by laws of unequal1 #bgb2 of crystals1 #bgb1 #bgc2 theory of1 #bge1 #bgg2 of steam1 #bgh2 by electricity1 #bhe4 ,extra-tropical winds1 #abd4 ,fabricius1 the comet of #aeef observed by1 #cgj2 variable star1 #cij4 ,fahrenheit1 mode of ascertaining heights proposed by1 #abj4 ,falling stars1 #dbj2 theories of1 #dbb1 #dbc4 ,faraday1 ,dr41 gases reduced to liquids by1 #aje2 experiments testing chemical affinity1 #aaa2 instance of cohesive force inducing chemical combination1 #aab2 experiments on vibrations producing colour1 #agc2 influence of dialectrics1 #bhf2 chemical origin of electricity defended by1 #cjj2 electro-chemical decomposition defined by1 #cjh2 remarks of1 on conduction of voltaic electricity1 #cji2 experiments on magnetic rotation1 #cac2 experiment magnetizing polarized light1 #cah1 #cai2 importance of his experiment1 #cbj2 experiment establishing the identity of magnetism and electricity1 #cbb1 #cbc2 his magnetic battery1 #cbd1 #cbe2 aid given by1 in construction of telegraphs1 #cbf1 #cbh2 electricity produced by rotatory motion explained1 #ccj2 his classification of substances according to magnetic qualities1 #ccb2 quotation from1 on conservation of force in electricity1 #ccd2 magnetism raised to a new science by1 #cce2 the magnet as represented by1 #cch2 experiment determining the forms of magnetic lines of force1 #cci1 #cdj2 accidental electro-magnetic combinations pointed out by1 #cdb2 his discovery of diamagnetism1 #cdg2 experiments on magnetic action in crystals1 #cdi2 observations on influence of heat in magnetism1 #ceb2 definition of gravity questioned by1 #ced1 #cee2 magnetism of the ethereal medium tested1 #cef4 ,fauna1 distinct1 of separate regions1 #bed1 #bee4 ,faye1 ,m41 his conception of the sun's constitution1 #da2 his theory of phenomena observed in eclipses1 #db2 comet of #ahdc discovered by1 #cfa4 ,fiedler1 ,dr41 fulgorites exhibited by1 #bic4 ,fire1 chemical combination producing1 #bgj4 ---- balls1 theory of1 #dba4 ,fires1 central1 subterranean1 #bca-bcg4 ,fish1 phosphorescent1 #bid1 #bie2 electric1 #caj4 ,fixed stars4 ,see ,stars4 ,fizeau1 ,m41 decisive experiment in proof of the undulatory theory of light accomplished by1 #bjb4 ,flame1 chemical combination evolving1 #bgj1 #bga4 ,flames1 lambent1 caused by electricity1 #bid4 ---- divergent from the nucleus of a comet1 #cfd4 ,fletcher1 ,mr41 periodic time of ,virginis determined by1 #cih4 ,flora of the ,himalaya1 #bej2 distinct1 in separate regions1 #bea2 condition establishing distinct1 in islands1 #beb4 ,florence1 comet discovered from1 #cgh4 ,fluor-spar1 its property of diminishing refrangibility of light1 #aif4 ,fluorescence of light1 definition of1 #aie2 vibrations of the substance producing1 #aif2 experiments1 #aig1 #aih4 ,focus of a meteoric shower1 #dbb4 ,fog1 yellow1 excluding the chemical action of rays1 #bad4 ,forbes1 ,professor1 temperature of the boiling point ascertained by1 #abj2 observations of1 on rayless lines1 #afc2 lunar heat tested by1 #bbg2 experiments of1 in polarization of heat1 #bfd1 #bfg4 ,force1 relation of1 to heat1 #bge2 transforming solids to liquids and to vapour1 #bge1 #bgg2 a power of nature1 #bgi2 light and heat modes of1 #bai1 #bbj2 heat a living1 #cbi2 lines of magnetic1 #cch1 #cdj2 conservation of1 maintained in periodic variation of atmospheric magnetism1 #cde2 increatable1 indestructible1 #cec2 examples of conservation of1 #ced2 fundamental principle of conservation1 #ceg2 influence and action of the gravitating1 #dbd1 #dbf4 ,forces1 the unknown cause of motion1 #e et passim2 counteraction of solar and tangential1 in planetary motion1 #h2 adjustment of1 ensuring the permanence of the solar system1 #aa1 #ab2 three partial1 causing perturbation in planetary motion1 #ad1 #ae2 excess of equatorial diameter the origin of1 #bg1 #bh2 three1 disturbing lunar motions1 #cd1 #ce2 determining planet forms1 #dd1 #de2 producing tides1 #ia1 #ib2 combining to form the centrifugal1 #ajj2 acting on molecules of matter1 #ajb1 #aje2 producing capillary phenomena1 #aad2 latent1 in nature1 #bgi1 #bhj2 one universal power1 the root of all1 #cba2 exact balance of1 in electricity1 #ccd2 kindred and convertible1 #cec2 developing comets' tails1 #cge2 determining the forms of orbits1 #chb1 #chc2 maintaining the stability of the solar system1 #dbf2 mutual relations of1 #dbg4 ,forests1 change produced in the atmosphere by1 #bda1 #bdc2 number of species of trees found in ,american and ,european1 #beb4 ,formentera1 quadrant of the meridian passing through1 furnishing a unit of linear measure1 #hi4 ,fornix1 nebulous patches crossing1 #dag4 ,forster1 ,lieutenant1 conversation carried on by1 across ,port ,bowen ,harbour1 #acf4 ,fossil plants1 an evidence of change in temperature1 #gd4 ,fourier1 mean temperature of space according to1 #aai2 rate of decrease in the earth's central heat computed by1 #bcb4 ,fox1 ,mr41 temperatures in mines tested by1 #bbh1 #bbi2 law of paramagnetic force ascertained by1 #cch2 observations in mines1 proving agency of electro-magnetism1 #cdf4 ,france1 arcs of the meridian measured in1 #dh2 unit of linear measure in1 #hi2 mode of arithmetical computation1 #ij2 atmospheric pressure in1 #abj2 cliffs of1 seen from ,hastings1 #aeg4 ,fraunhofer1 ,m41 discovery of rayless lines in the solar spectrum1 #afb2 comparative refrangibility of rays ascertained by1 #afc2 data furnished by1 to determine the dispersive power of rays1 #afe2 his discovery determining the length of waves independently of refraction1 #bja2 spectrum of an electric spark observed by1 #bhi4 ,freezing1 temperature required for1 under pressure1 #bga2 theory of1 #bgf4 ,fresnel1 ,m41 his testimony in favour of the undulatory theory of light1 #aga2 theory of refraction1 #ahc2 discoveries in polarization of light1 #aia1 #aic4 ,freyberg1 green plants found in mines at1 #bec4 ,friction evolving heat1 #bgd1 #bge2 electricity1 #bhb1 #bhc4 ,fringes of coloured light bordering shadows1 #agd1 #age2 produced by interference of polarized rays1 #aid4 ,fulgorites1 found in ,silesia1 #bic4 ,fundy1 the ,gulf of1 cross tides pouring into1 #id4 ,gage1 ,mr41 experiments of1 on magnetism1 #cae4 ,gales4 ,see ,winds4 ,galileo1 laws affecting music discovered by1 #ade2 his method of finding distances of fixed stars1 #chh4 ,galle1 ,dr41 ,neptune's place communicated to1 by ,le ,verrier1 #fb4 ,galloway1 ,mr41 sun's motion proved by1 #dje4 ,galvani1 ,professor1 peculiar effects of electricity suggested to1 #big4 ,galvanism1 phenomenon suggesting the theory of1 #big2 batteries1 #bih1 #cjj2 heat and light evolved by currents of1 #cjj1 #cjd2 decomposition and composition1 #cjg1 #cjh2 applied to plating and gilding1 #cji2 effect of heat on1 #caj2 effect of1 on the senses1 ib42 fish exhibiting analogous phenomena1 #caj1 #caa2 phenomena exhibited by currents of1 on magnets1 #cab1 #cad3 intensity of a current measured1 #cae2 conditions obtaining a circuit in1 #ccb4 ,galvanometer1 the principle of its construction1 #cae2 experiment by means of1 identifying magnetism and electricity1 #cbb1 #cbc4 ,gambart1 ,m41 parabolic elements of a comet computed by1 #cfg4 ,gamma ,andromeda1 colours of1 #dja4 ---- ,aquarii1 planetary nebula near1 #dab4 ---- ,hydr1 a variable star1 #cia4 ---- ,leonis1 focus of a meteoric shower in1 #dbb4 ---- ,sagittarii1 cluster of the ,milky ,way round1 #chg4 ---- ,virginis1 eclipse in1 #cig2 orbit of the revolving star determined1 #cih4 ,ganges1 the1 tidal wave at the mouths of1 #id4 ,gardner1 ,mr41 extent of diametrically opposite lands estimated by1 #bdd4 ,gases1 conditions retaining matter in the form of1 #ajd1 #aje2 combinations of1 #aaa2 transmission of radiant heat through1 #beh2 expansion of1 #bga2 voltaic spectrum modified by1 #cjc2 effect of heat on the conducting powers of1 #cji4 ,gassiot1 ,mr41 experiments of1 on the electric discharge1 #cjf2 connexion between magnetism and light discovered by1 #cba2 electric apparatus improved1 #cbh4 ,geneva1 the ,lake of1 experiment on the velocity of sound in1 #ace4 ,gensanne1 ,m41 increasing temperature of mines tested by1 #bbh4 ,geographers1 lunar motions important to1 #db4 ,geological changes1 probable cause of1 #gg4 ,geology1 the lessons of1 #cbf4 ,georgia ,island1 ,s41 excess of cold in1 over corresponding latitudes1 #bda4 ,germany1 shooting stars seen from1 #dba4 ,gibraltar1 the ,straits of1 turning aside the tidal wave1 #ih4 ,giromagny1 temperature of the lead-mines of1 #bbh4 ,glass1 effect of cohesion on plates of1 #ajf2 musical notes elicited from rods and plates of1 #add-adg2 transmission of waves of light in1 #agg2 polarizing light1 #ahd1 #ahe2 elliptical polarization produced by1 #aic1 #aid2 used in photography1 #bjg2 impressions on1 from bodies in contact with1 #bbj2 impressions on1 traced by electricity1 #bba2 transmission of radiant heat by1 #bei2 by coloured1 #bfa1 #bfb2 its temper altered by magnetism1 #ceb1 #cec4 ,globular clusters of nebul1 #dac-dae4 ,glow-discharge observed by ,captain ,bonnycastle1 #bie1 #bif4 ,gold1 action of1 on light1 #agc4 ,good ,hope1 the ,cape of1 icebergs drifted to1 #aja4 ,goodricke1 ,mr41 variable stars discovered by1 #cia2 opaque bodies represented as revolving round fixed stars by1 #cid4 ,graham1 ,mrs41 account of an earthquake by1 #bcd4 ,graham's compensation pendulum1 #bgb4 ,gravitating force of the sun1 #cfe1 #dbd1 #dbe4 ,gravitation1 offices of1 in the material creation1 #a1 #b2 process of reasoning in ascertaining the law of1 #c2 law determining its intensity in the solar system1 #e2 complex action of1 by attraction in mass and in particles1 #f2 increase of1 towards the poles of the earth1 #de2 calculations founded on its increase1 #di-ea2 in a mine1 its excess over surface1 #eg2 action of1 modifying tides1 #ib1 #ic2 law1 universally acting on matter1 #aje2 the air subject to1 #aag2 influence of1 in motions of the heavenly bodies1 #chb1 #chc2 double stars revolving by1 #cih2 stellar systems subject to1 #djj2 influence of1 on nebul1 #daf2 a general law of the visible creation1 #dbd2 mode of its action1 #dbe1 #dbf4 ,gravity1 centre of1 in spheres1 effect of impulses passing through1 #g2 of the solar system1 invariable plane passing through1 #bc2 straight line described by1 #bd2 action of1 in determining the figure of the earth1 #dd1 #de2 definition irreconcilable with the conservation of force1 #ced1 #cee2 question of its transmission1 #cee1 #cef4 ,great ,bear1 the nebulous zone passing1 #daf4 ---- ,gobi1 the1 effect of the expansion of air over1 #abd4 ,greeks1 astronomical observations of1 confirming results of analysis1 #ch4 ,greenland1 ocean on the northern coast of1 #id4 ,greenwich1 lunar distances computed for1 #dc2 quadrant of the meridian passing through1 furnishing a unit of linear measure1 #hi2 periodic circuits of winds1 #abe4 ,grimaldi1 coloured fringes bordering shadows described by1 #age4 ,groombridge1 velocity of his proper motion1 #djd4 ,grotthus1 the transmission of voltaic electricity investigated by1 #bih4 ,grove1 ,mr41 copper and zinc plates electrified by1 #bbj2 substances radiating heat of different refrangibilities enumerated by1 #beg2 the transmission of voltaic electricity investigated by1 #bih2 electric heat tested by1 #cja1 #cjb2 remarks of1 on carbon1 #cjb1 #cjc2 on the voltaic arc1 #cjd1 #cje2 remarks of1 on light and heat1 #cai2 electric apparatus improved by1 #cbh2 his definition of the ethereal medium1 #cee4 ,grylli1 supposed delicate sense of hearing in1 #acb4 ,guanaxato1 temperature of the silver-mine of1 #bbh4 ,gulfs separating stars1 #cij4 ,gum-guaiacum1 chemically affected by rays of the solar spectrum1 #bjc2 condition of its sensibility to light1 #bjf2 effect of red rays on1 #bji2 used in experiments on parathermic rays1 #bag1 #bah4 ,gum-lac1 electrical intensity measured by means of1 #bhf1 #bhg4 ,gymnotus electricus1 the1 #caj4 ,haidinger1 ,m41 experiments of1 proving water an essential part of crystals1 #ajg4 ,hail1 formation of1 #bgj4 ,hales1 his calculation of the amount of surface exposed by the leaves of a helianthus1 #bdc4 ,hall1 ,mr41 achromatic telescope constructed by1 #afe4 ,halley1 elements of a comet's orbit computed by1 #cfb2 return of his comet1 #cfc2 changes in its aspect1 #cfc1 #cfd2 records of1 #cfe2 no solid nucleus in1 #cgd2 cause of its luminous sectors1 #cgf2 ,sir ,john ,herschel's observations on1 #cgh4 ,hare1 the1 comet observed near1 #cgb1 #cgc4 ,harmonics of the fundamental note in music1 #adj1 #ada4 ,harmony1 property of sound regulating1 #aca2 definition of1 vibrations producing1 #adb4 ,harris1 ,sir ,william ,snow1 experiments of1 in electricity1 #bhg1 #bhh2 lightning-conductors invented by1 #bic4 ,harrison1 pendulum invented by1 #bgb4 ,hastings1 coast of ,france distinctly seen from1 #aeg4 ,heat affecting the form of crystals1 #ajg2 evolved in chemical combinations1 #aaj2 irregular decrease of1 in the atmosphere1 #aai2 maxima of1 in the solar spectrum1 #bae2 peculiar chemical quality of1 in parathermic rays1 #bah2 impressions traced by1 #bbj-bbb2 periodical variations in the sun's1 #bbe2 different proportions of solar1 reaching the planets1 #bbe1 #bbf2 effect of the terrestrial atmosphere on lunar1 #bbg2 mode of its development in opaque bodies1 ib42 sources of terrestrial1 #bbh-bch2 irregular distribution of1 #bci-bdg2 laws affecting its radiation1 #beg2 its transmission1 #beh-bfb2 polarization of1 #bfd-bfg2 undulatory theory1 #bfg2 absorption and reflection of radiant1 #bfh2 phenomena caused by radiation of1 #bfi2 accumulation of1 producing light1 #bgj2 expansive force of1 #bga1 #bgb2 modes of propagation1 #bgc1 #bgd2 produced by motion and equivalent to it1 #bgd-bgg2 laws regulating the force of artificial1 #bgi1 #bhj2 power evolved by application of1 #bhj2 identical in nature with sound1 #bha2 electrical1 #bhh2 sheet-lightning caused by1 #bid2 phosphorescence1 #bid2 developed by voltaic electricity1 #cja1 #cjb2 effect of1 on electrical conductors1 #cji2 connexion between the production of electricity and1 #caj2 its direct relation to magnetism and electricity1 #cai1 #cbj2 mechanical power and convertible forces1 #cbi2 terrestrial magnetism attributed to the action of1 #ccc2 measured by electric currents1 #ccd2 affecting atmospheric magnetism1 #cdd2 fundamental principle of the dynamic theory1 #ceg4 ,helena1 ,st41 distinct flora of1 #beb4 ,helix1 circular and elliptical1 described in polarization of light1 #aib1 #aic2 electrical experiments by means of1 #cad2 induction of1 increasing electric power1 #cbb1 #cbc4 ,heller1 his observations on the comet of #aeef1 #cgj1 #cga4 ,helmholtz1 ,professor1 power of chemical force estimated by1 #aab2 his calculation of the chemical force developed by combustion1 #bgh2 of the amount of latent force in our system1 #bhj4 ,hemisphere1 cause of excess of cold in the southern1 #bda2 superficial extent of land in northern and southern1 #bdd4 ,henley1 ,mr41 magneto-electric machine constructed by1 #cbe4 ,henderson1 ,professor1 parallax of ,centauri calculated by1 #chg2 of ,sirius1 #chi4 ,henry1 ,professor1 experiments of1 on magnetism1 #cae4 ,herapath1 ,mr41 his view of elastic force preferred to ,sir ,humphry ,davy's1 #bgf4 ,hercules1 eclipse of a double star in1 #cih2 globular nebulous cluster1 #dad4 ,herschel1 ,sir ,william1 observations of ,saturn's and ,uranus's satellites by1 #cb1 #cc2 theory of1 regarding the solar constitution1 #da2 cause of effects of light in eclipses according to1 #db2 rotation of ,jupiter's satellites determined by1 #gj2 mutual independence of light and heat1 #bad1 #bae2 influence of the sun's spots on heat1 #bbe2 point of maximum heat in the solar spectrum1 #bfc2 comet of #ahaa observed by1 #cgd2 its luminous envelopes examined1 #cge2 the ,milky ,way examined by1 #che2 his discovery of the orbital motions of double stars1 #chh2 catalogue of double stars by1 #cie1 #cif2 periodic time of ,virginis determined by1 #cih2 eclipse of a double star observed1 ib42 binary system discovered1 #djj2 remarks on the motions of the stars1 #dje2 nebul resolvable into stars1 #ejg4 ,herschel1 ,sir ,john1 approximate periods of satellites ascertained by1 #cc2 thickness of ,saturn's ring computed1 #fg2 observations of1 on seasons1 #gd2 difficulty of varying time1 in observations at distances1 obviated by1 #hf2 tenuity of atmospheric air demonstrated1 #aaj2 rapid decrease of density in the atmosphere1 #aah2 mean temperature of space computed by1 #aai2 height of ,etna measured1 #abj2 his explanation of anomalies in atmospheric phenomena1 ib42 quotation from1 on the transmission of sound1 #acf2 observations of1 on thunder1 #ach2 remarks on the absorption of light by coloured media1 #age1 #agf2 on polarization of light1 #agi2 experimentalising apparatus1 #ahh2 discovery of epipolic light1 #aig2 discoveries in photography1 #bje1 #bjf2 analysis of the solar spectrum1 discovery of its chemical properties1 #bjg-bai2 his theory of volcanic action1 #bce-bcg2 observations showing the maximum of heating influence of the solar rays1 #bch2 theory of the original distribution of plants1 #bed2 divergent flame of a comet observed by1 #cfd2 remarks on the possible destruction of the solar system1 #cgb2 causes assigned by1 for contraction of diameter in comets1 #cgh2 comparative lustre of stars measured by1 #chd1 #che2 the ,milky ,way described1 #che1 #chf2 number of stars in a group of the ,milky ,way computed1 #chg2 variable star discovered1 #cia2 remarks of1 on the nature of the fixed stars1 #cib2 variable stars discovered by1 #cic2 remarks on variable stars1 #cid2 star missed by1 #cie2 double stars discovered1 #cif2 eclipse of a double star observed1 #cig2 orbits determined1 #cih1 #cii2 observations on colours of double stars1 #dja2 light of ,centauri compared with the moon's by1 #djd2 light of the fixed stars calculated1 ib42 observations on nebul corrected1 #djg2 catalogues of nebul1 #djh2 nebul discovered by1 #dji2 annular nebula described1 #daj2 magnitude of planetary nebul computed1 #dab2 globular nebulous cluster described1 #dac2 law of gravitation ascribed to nebul1 #daf2 nebula round ,argus described1 #dah2 his work on ,nebul1 #dai4 ,herschel1 ,miss1 ,encke's comet seen by1 #cfe2 catalogue of nebul1 #djg4 ,hevelius1 divergent flames of a comet described by1 #cfd2 contraction in diameter of comets observed1 #cgg2 phases in comets observed1 #chj4 ,hieroglyphics interpreted by astronomy1 #hi4 ,himalaya1 the1 inappreciable effect of1 on the globe's surface1 #f2 singular effect of refraction on1 #aef2 cause of greater elevation of the snow-line on the northern side of1 #bda2 flora of1 #bej4 ,hind1 ,mr41 comet's orbit computed by1 #cgj1 #cga2 observations of1 on ,donati's comet1 #cgi2 variable stars discovered by1 #cia2 vanishing star discovered1 #cic2 his belief in planetary systems1 #cid4 ,hindostan1 the tidal wave striking on its coasts1 #id4 ,hipparchus1 precession discovered by1 change of seasons since his age1 #hj2 phenomenon suggesting his catalogue of the stars1 #cib4 ,history corroborated and corrected by astronomy1 #hg1 #hi4 ,hoar-frost1 cause of1 #bfi4 ,holtzmann1 ,m41 opinion of1 with regard to the vibrations of polarized light1 #bbc4 ,hoogly1 the1 bore of1 #id4 ,horizon1 effects produced by the denser stratum of air in1 #aeg1 #aeh4 ,horologium1 nebulous patches in1 #dag4 ,horton coal-mine1 experiments with the pendulum in1 #eg4 ,hours1 cause of their mal-correspondence over the globe1 #hf4 ,hudson's ,bay1 tide in1 #ih4 ,humboldt1 his sufferings from rarity of the atmosphere1 #aah2 his explanation of the apparent greater acuteness of hearing observed at night1 #ace2 observations of1 in mines1 #bbh2 causes of disturbance in the equal diffusion of heat enumerated by1 #bdj2 identical productions of the ,old and ,new ,world found by1 #bea2 his distribution of palms and grasses1 #beb2 green plants found growing in mines by1 #bec4 ,hunt1 ,mr41 coloured image of the solar spectrum obtained by1 #bji2 image obtained in ,england1 #bac2 his experiments in tracing images by juxtaposition of bodies1 #bbj1 #bba2 experiments on the condensing power of rays1 #bbc4 ,hurricanes1 origin and cause of1 #abe1 #abf2 curve described by the axis of1 ib42 their extent and velocity1 #abf1abg2 phenomena resulting from their revolving motion1 #abg2 laws of1 making avoidance possible1 #abh4 ,huygens1 theory originated by1 #afi4 ,hydrogen1 proportion of1 in water and gases1 #aaa2 spectrum from1 #cjc2 separated from water by electricity1 #cjg4 ,hygrometer1 dew-point measured by1 #bfi4 ,hyperbolic motion1 ratio of forces procuring1 #chb4 ,iapetus1 seen by ,mr4 ,lassell1 #cc4 ,ibn ,junis1 progress of science in his time1 #ij4 ,ice1 formation of1 #bga2 force acting in its formation1 #bgf2 stopping the current of voltaic electricity1 #cji4 ,icebergs1 drifting of1 #ajj1 #aja2 farthest range of northern and southern1 #bda2 effect of electricity in collisions1 #bhd4 ,iceland spar1 its property of double refraction1 #aha2 polarized ray analyzed by1 #ahg2 transmission of radiant heat by1 #beh2 electricity elicited from1 #bhd4 ,illumination1 comparative1 of objects1 experiments determining1 #bbg4 ,images1 coloured1 of the solar spectrum1 #bjh-baa2 traced by contact and juxtaposition of bodies1 #bai1 #bbj2 by electricity1 #bba2 by media absorbing hot rays1 #bbb4 ,india1 arcs of the meridian measured in1 #dh2 discovery of ,saturn's ring1 #ff2 ancient monument of astronomical knowledge1 #he2 observations confirming the antiquity of astronomical science in1 #hh4 ,indian ,ocean1 the tidal wave in1 #id2 monsoons blowing over1 #abd4 ,induction1 law of1 in electricity1 #bhe1 #bhf2 magnetic1 #cad1 #cae2 phenomena of1 produced by electric currents1 #cbd2 illustrated by the ,atlantic telegraph1 #cbe1 #cbf2 velocity of electricity modified by power of1 #cbg2 possibility of electro1 furnishing a motive power1 #cbh2 of electricity by rotation of magnets1 #ccj-ccb2 as possessed by magnets1 #ccf2 paramagnetism evolved by1 #ccg2 means of accelerating1 ib42 subject to the laws of mechanics1 #cch2 analogy between electric and magnetic1 #cda2 of heavenly bodies1 affecting terrestrial magnetism1 #cdf1 #cdg2 diamagnetic substances capable of1 #cdh4 ,indus1 comet passing through the constellation of1 #cgi4 ,inequality1 the1 of ,jupiter and ,saturn marking historical epochs1 #hh4 ,insects1 law of their dispersion1 #bee4 ,instruments1 musical1 #adc1 #adi1 #aej2 imitating articulation of letters1 #aea1 #aeb4 ,insulation in electricity1 #bhe4 ,interference1 laws of1 neutralizing undulations1 #ach1 #aci2 the theory of1 referred to a general law1 #afi4 ,iota ,cet1 comet observed near1 #cgb4 ---- ,orionis1 a nebulous star1 #daa4 ,ireland1 progress of the tidal wave towards1 #id4 ,iron1 distilled1 #cje2 rotation of its particles1 ib42 magnetized by electricity1 #cad1 #cae2 magnetic properties of1 #ccb2 rendered paramagnetic1 #ccf1 #ccg2 magnetic and electric properties of1 #cdg2 elasticity of1 affected by magnetism1 #ceb4 ,islands1 character of their floras1 #beb4 ,isogeothermal lines of temperature defined1 #bch1 #bci2 parallel with the isothermal lines1 #bdf4 ,isomorphous crystals1 #aji4 ,isothermal lines of temperature defined1 #bdj2 latitudes of1 deviation from the line of the equator1 #bde2 formula determining1 #bdf2 similarity of vegetation in the same1 #bec4 ,italy1 local attraction1 occasioning inaccuracy in measurement1 #dh4 ,ivory1 ,m41 his method of computing heights1 #abj2 his theoretical investigation of planet forms1 #dd2 deduction from measurement of arcs of the meridian1 #dh4 ,jacob1 ,mr41 discovery of ,saturn's ring by1 #ff2 periodic time of ,centauri determined by1 #cii2 periodic time of #gj ,ophiuchi1 #djj4 ,james1 ,colonel1 measurements of1 in the ,general ,survey of ,great ,britain1 #dg2 density of the earth determined by1 #eh4 ,jamin1 ,m41 remarks of1 on substances producing elliptical polarization1 #aic4 ,january1 epoch of its beginning the year1 #he4 ,jews1 denominations of time in their calendars1 #he4 ,josephstadt1 discovery of a comet from1 #cfg4 ,joule1 ,mr41 heat considered a mechanical force by1 #bge2 his view of elastic force1 #bgf2 amount of latent force in a pound of coal1 computed by1 #bgh2 furnishing data to ,professor ,thomson1 #bgi2 quantity of heat generated in a unit of time by electricity computed by1 #cjb2 powerful magnet obtained by electricity1 #cae2 electric machines constructed by1 #cbh2 experiments proving heat and mechanical power convertible1 #cbi4 ,jovial system1 mass of the whole1 #ee4 ,julian ,calendar1 year of1 the first of our era1 #hf4 ,june1 #ahcc1 reappearance of ,saturn's rings1 #fg2 coincidence of times in1 #hd4 ,juno1 the diameter of1 #ef2 astronomical tables of1 #fc4 ,jupiter1 rotation of1 distinguished from the other planets1 #g2 periodical inequality in his motions1 #ae2 discovery of telescopic planets between ,mars and1 #bj1 #ba2 diameter of1 #ba2 his position with respect to the equator of the solar system1 #bd2 inequalities in the motion of1 apparently anomalous1 #be1 #bf2 his mass proved not homogeneous1 #bi2 eclipses1 #cj1 #ca2 compression of his spheroid computed1 #ci2 eclipsed by ,mars1 #db2 mass of1 compared with the sun1 #ee2 his diameter1 #ef2 increase of density in1 #eh2 astronomical tables of1 #fj2 rapid rotation1 #ff2 period of a year in1 ib42 effect of his disturbing energy1 #ha2 photographic images of1 #bbf2 light reflected by his atmosphere1 #bbg2 action of1 on the comet of #aggj1 #cfa1 #cfb2 on ,halley's comet1 #cfb1 #cfc2 comet revolving between the orbits of the earth and1 #cfg2 future influence of1 on comets1 #cfi2 comet nearly approaching his fourth satellite1 #cgj2 comets having their perihelia in his orbit1 #cha4 ----1 orbit of1 revolutions of its major axis1 source of variation in excentricity1 #ag2 slow revolution of its nodes1 decrease in its inclination to the ecliptic1 #ai4 ---- with his satellites1 an epitome of the solar system1 #bg2 effect of his excessive equatorial diameter on their orbits1 #bh2 satellites1 libration in1 #fi2 rotation of1 #gj4 ,kane1 ,dr41 ,polar ,sea discovered by1 #id2 cold of ,northern ,greenland marked by1 #bdg4 ,kappa ,crucis1 cluster of coloured stars round1 #dai4 ---- ,draconis1 seen in the pole of the equator1 #hh1 #hi4 ,karsten1 ,mr41 impressions made on glass by electricity1 #bba4 ,kasan1 summer and winter mean temperature of1 compared with ,edinburgh1 #bdf1 #bdg4 ,kater1 ,captain1 approximate length of the pendulum1 determined by1 #hi4 ,kempelen1 ,m41 speaking-machine invented by1 #aea4 ,kepler1 paths1 revolutions of planets discovered by1 #e2 his law regarding the mean distances of planets from the sun1 #ai2 law of1 applied to calculating distances1 #ec1 #ed2 rapidity of planetary revolutions determined by his law1 #ff2 his law finding areas described by heavenly bodies1 referred to1 #cfj4 ,kew1 balloon ascent from1 #aai4 ,knoblauch1 position of the magnecrystallic axis proved by1 #cdi4 ,knowledge1 limited nature of human1 #b4 ,kotzebue1 stratum in the ocean discovered by1 #aja4 ,kratzenstein1 ,m41 instrument invented by1 articulating words1 #aea4 ,kupffer1 ,m41 observations of1 on temperature1 #bdf4 ,la ,basilicata1 earthquake in1 #bcd4 ,lacaille1 his globular nebulous cluster1 #dad2 nebula1 #dah4 ,la ,grange1 his investigations into the stability of the solar system1 #bj1 #ba2 greatest discovery of1 #bc4 ,la ,hire1 phases in comets observed by1 #chj4 ,la ,place1 stability of the solar system proved by1 #bj2 principle in astronomical calculations established1 #bc2 angle of inclination fixed1 #bd2 his theory accounting for acceleration in the moon's mean motion1 #cf1 #cg2 result of observations compared with his theory of ,jupiter's satellites1 #ee2 theory of planetary motion1 #fe1 #ff2 universal epoch proposed by1 #hg2 scientific observations complementing historical records1 #hg2 date fixed by1 for the lunar tables of the ,indians1 #hh2 justifies ,newton's theory of tides1 #if2 density of a liquid column estimated by1 #aad2 action of the earth on a comet1 #cei2 change in a comet's orbit1 #cfa2 cause of error in ,clairaut's calculation pointed out by1 #cfc2 opinion of1 as to the comet of #afhb1 #cgh4 8,lake of the ,gazelles0 ascribed to an effect of reflection1 #aeg4 ,lalande1 epochs of conjunctions computed by1 #db4 ,lambda ,herculis1 general motion of the stars determined by1 #dje4 ,land1 dry1 comparative extent of1 on the globe1 #bdb1 #bdd2 extent of1 in diametrical opposition1 #bdd4 ,landscapes in chiaroscuro1 produced by photography1 #bjg4 ,languages1 resemblances and analogies between1 #bee1 #bef4 ,lapland1 arcs of the meridian measured in1 #dh2 transit of ,venus observed in1 #ec4 ,laroche1 ,m41 his experiments on transmission of radiant heat1 #bei1 #bfa4 ,lassell1 ,mr41 satellite of ,saturn discovered by1 #cb2 observations of1 on ,uranus' satellites1 #cc2 his discovery of ,neptune's satellite1 ib42 observations on ,saturn's rings1 #ff4 ,latent heat1 energetic action of1 on matter1 #bge-bgg4 ,latitude1 the1 of a planet defined1 mode of obtaining1 #i1 #aj2 cause of periodical inequalities in1 #ae2 perturbations from action of the perpendicular force1 #ah2 moon's motion in1 disturbed1 #ce2 effects of disturbance1 #ch2 data of1 used in computing a planet's place in the heavens1 #eh-fj2 conditions ensuring the invariability of geographical1 #gf1 #gg2 change effected by nutation in1 #ha2 climate not invariable in the same1 #bci2 degrees of1 where diminution of mean heat is most rapid1 #bdd1 #bde2 the same mean temperature in different1 #bdf1 #bdg2 of wine-growing1 #bej2 magnetic storms varying with1 #cde4 ,layang1 observations made at1 #aajj years before the ,christian era1 #hh4 ,le ,sueur1 specific diversity of marine animals observed by1 #bed4 ,le ,verrier1 ,m41 principle of ,la ,grange applied by1 #ba2 zone of instability found1 ib42 discovery of ,neptune1 #fb2 his observations on atmospheric waves1 #abb2 comets identified by1 #cfb2 his table of comets' orbits1 ib4 ,lenticular nebul1 #dji2 haze surrounding the sun1 #dab4 ,leo1 nebulous system in1 #dag4 ,lon-,faucault1 ,m41 velocity of light in air and water ascertained by1 #bjb4 ,lerius1 banks of alg found by1 #bec4 ,leslie1 ,professor1 compression of air calculated by1 #gh2 experiments on radiation of heat1 #beg4 ,lexel1 observations of1 on the comet of #aggj1 #cfa1 #cfb4 ,libra1 the five great planets in conjunction near1 #db4 ,librations of the moon1 of ,jupiter's satellites1 #fi2 of ,centauri1 #cii4 ,lichen1 red1 growing on snow1 #bdi4 ,light1 rate of its velocity1 #ca2 truth deduced from the uniformity of its velocity1 #cb2 from the aberration of1 ib42 period required to reach the earth from ,centauri1 #ed2 action of the atmosphere on1 #aec2 conditions regulating the transmission and reflection of1 #aef2 loss of1 transmitted by the horizontal stratum1 #aeg2 effects of transmission through the atmosphere1 #aeh2 ,newton's analysis of1 #aei2 ,brewster's1 #afa2 phenomena disproving ,newton's theory1 #afg1 #afh2 undulatory theory1 #afh-agj2 conditions affecting its intensity and colour1 #agj2 experiments testing the mutual relations of colour and1 #aga-age2 law of its absorption identical with a law of motion1 #age-agg2 repeated vibrations producing the sensation of1 #agh2 polarized1 defined1 #agi2 modes of polarization1 substances polarizing1 #agi-ahe2 accidental polarization of1 #aie2 degraded1 or fluorescence1 #aif2 objections to the undulatory theory analyzed and disproved1 #aii-bjb2 comparative velocity of1 in air and water1 #bjb2 pictures produced by reflected1 #bjc-bjg2 rays of1 independent of heat1 #bad1 #bae2 comparative amounts of solar and lunar1 #bbe2 different measures of illumination from1 #bbg2 influence of1 on vegetation1 #bdi2 colour developed without the influence of1 #bec2 separated from heat by ,melloni1 #bfe2 produced by accumulation of heat1 #bgj2 law regulating the force of artificial1 #bgi1 #bhj2 electrical1 #bhh1 #bhi2 produced by voltaic electricity1 #cjb2 stratifications of the electric1 #cjf2 influence of magnetism and electricity on1 #cai1 #cbj2 of comets1 #cgi-cha2 of the fixed stars1 #dja-djd4 ,lightning1 development of heat exhibited by1 #bgf1 #bgg2 experiment showing the velocity of1 #bhi2 theory of1 #bib2 the back stroke1 ib42 force of the direct stroke1 #bic2 sheet1 #bid2 effect of1 on the compass1 #cab4 ,lime1 carbonate of1 variety of form in its crystals1 #ajg2 invariable form ultimately assumed by1 #aji4 ,lines of magnetic force1 #cch1 #cci2 experiment ascertaining the form of1 #cci1 #cdj2 terrestrial1 #cda1 #cdb2 extensive courses of1 #cdd2 a connected system1 #cde2 diamagnetic1 #cdh4 ,lion1 the1 conjunction of planets in1 #db4 ,liquids1 balance of forces constituting1 #ajd1 #aje2 action of capillary attraction on1 #aac-aaf4 ---- possessing the property of circular polarization of light1 #aij1 #aia-aic4 ,liquids1 conditions affecting the transmission of radiant heat by1 #bfc2 evaporation from1 #bfi2 expansion of1 by heat1 #bga2 propagation of heat in1 #bgc2 action of heat as a mechanical force on1 #bge-bgg4 ,london1 retarding of the tidal wave between ,aberdeen and1 #id4 ----1 pendulum vibrating in its latitude1 a standard of measurement1 #hi2 fulgorites exhibited in1 #bic4 ,long1 ,dr41 his attempt to measure distances of fixed stars1 #chh4 ,longitude1 mode of reckoning mean and true1 #i2 of the perihelion and of the epoch defined1 #aj2 cause of periodical perturbations in1 #ad2 calculation from the moon's influence on the sun's1 #ee2 data of1 used in computing a planet's place in the heavens1 #eh-fj2 change effected by precession and nutation in1 #ha4 ,lloyd1 experiments of1 in polarization of heat1 #bfd4 ,lubbock1 ,sir ,john1 theory of planetary motion completed by1 #fd2 his theory of shooting stars1 #dbc4 ,lumire cendr1 definition of1 #bbg4 ,lunar distance1 defined1 #dc4 ---- theory1 mean distances obtained from1 #dc4 ---- tides of the terrestrial atmosphere1 #aba4 ,lundahles1 ,m41 motions of heavenly bodies investigated by1 #dje4 ,lupus1 position of1 #cij4 ,lussac1 ,gay1 ,m41 uniting of gases by volumes discovered by1 #aaa2 ascent of1 in a balloon1 #aah2 course of a lightning flash ascertained by1 #bib4 ,lutetia1 diameter of1 #ef4 ,lyell1 ,sir ,charles1 his theory of changes of temperature in the northern hemisphere1 #ge2 annual number of volcanic eruptions computed by1 #bcc2 volcanic phenomena related by1 #bcd4 ,lyncis #ab1 a triple star1 #cie4 ,lyra1 a variable star in1 #cia2 a double star1 #cie2 nebula1 #daj4 ,machinery1 relations of1 to force1 #cec4 ,mackintosh1 ,sir ,james1 quotation from1 illustrating the essential advantages of study1 #a4 ,maclear1 ,mr41 parallax calculated by1 #chg4 ,madeira1 vegetation of1 #beb4 ,madras1 ,saturn's ring discovered from1 #ff4 ,magellanic clouds1 the1 #dag1 #dah4 ,magnecrystallic action1 #cdi2 temperature affecting1 #ceb4 ,magnetic bodies1 difference in power of1 #cdg4 ---- elements1 the three terrestrial1 #cdc4 ---- equator of the earth1 #cdc4 ---- meridian1 the1 mean action of forces determining1 #cdc4 ---- poles of the earth1 #cdc4 ---- storms1 #cdd2 varying with latitude1 #cde1 #cdf4 ,magnetism1 source of1 #cah2 producing electrical phenomena1 #cbb1 #cbc2 rotatory motion a source of1 #ccj2 classification of substances1 with regard to their susceptibility of1 #ccb2 residing in substances after two manners1 #cce2 experiment illustrating the forces of1 #cch2 antithesis1 its general character1 #cci2 form of its lines of force1 #cci1 #cdj2 analogous properties of electricity and of1 #cdj1 #cda2 terrestrial1 #cdb-cdg2 connexion between solar and terrestrial1 #cdd2 action of1 in crystals1 #cdi-cea2 influence of temperature in1 #ceb2 affecting elasticity of matter1 #ceb1 #cec2 a property of the ethereal medium 7871 #cef1 #ceg4 ----1 electro1 discovery1 importance of the science1 #cab2 rotation effected by1 #cac1 #cad2 electric intensity measured1 #cae2 action of currents in1 defined1 #caf2 ,ampre's theory of1 #cag1 #cah2 causing rotation of polarized rays1 #cai2 action of1 on light1 #cbj2 accidental combinations1 #cdb2 influencing metalliferous deposits1 #cdf4 ,magneto-electricity1 principle suggesting1 #cbb2 machine constructed on the principle of1 #cbe2 relation of heat to1 #cbi4 ,magnets1 influence of1 on electric light1 #cjg2 fish possessing the power of making1 #caa2 effect of an electric stream on1 #cab-cad2 obtained by electricity1 #cae2 power of electro1 measured1 #cae2 cylinders acting as1 #caf1 #cag2 producing electrical effects1 #cbb1 #cbc2 evolving electricity by rotation1 #ccj2 classification of substances in relation to1 #ccb2 polarity a property of1 #ccf2 effect on themselves of imparting paramagnetism1 #ccg2 experiment showing the lines of force of1 #cch2 properties of1 indestructible by subdivision1 #cch1 #cci2 the earth reckoned among1 #cdb2 planets reckoned among1 #cdf2 action of an electro1 on copper1 #cea4 ,maguire1 ,captain1 his observations on magnetic storms1 #cde1 #cdf4 ,malo1 ,st41 rising of the tide at1 #ih4 ,malus1 ,m41 discovery of polarization of light by1 #aie2 attempts of1 to polarize heat1 #bfd4 ,malta1 observations on ,saturn's rings made at1 #ff4 ,manchester1 thunderstorm near1 in #ahce1 #bib4 ,mankind1 distinct tribes of1 #bee2 limited perceptions of1 #bfg4 ,marcet1 ,m41 rate of increase in temperature below the earth's surface calculated by1 #bcj4 ,marco ,polo1 atmospheric effects observed by1 in ascending mountains1 #aah4 ,marine plants1 laws regulating their distribution1 #beb1 #bec2 animals1 specific localities of1 #bed4 ,mariner's compass4 ,see ,compass4 ,mars1 used in illustrating the possible effects of the radial distributing force1 #ai2 telescopic planets between ,jupiter and1 #bj1 #ba2 diameter of1 #ba2 mean distance from the sun1 ib4 note2 eclipse of ,jupiter by1 #db2 parallax found by observing his oppositions1 parallax of1 #ec2 internal structure1 #eh2 astronomical tables of1 #fc2 climate of1 #bbe2 approach of the comet of #aggj to1 #cfb2 comets having their perihelia in his orbit1 #cha4 ,marseilles1 transit of a comet across the sun observed from1 #cgd4 ,masses1 of the sun1 of planets and their satellites1 computations finding1 #ee1 #ef4 ,mathematics1 use of1 in the study of astronomy1 #b4 ,matter1 theory of its constitution1 #ajb2 hypotheses as to forces uniting its particles1 #ajc1 #ajd2 counterbalancing action of elasticity and cohesion1 #aje2 crystallization common to all forms of1 #aji2 indestructibility of its particles1 #aaj2 composition of unorganised bodies1 subject to permanent law1 #aaj1 #aaa2 agent composing or decomposing1 #aab2 mode of ascertaining the magnetism of1 #cce2 increatable1 indestructible1 #cec2 proportion of1 to spare1 #dbd4 ,matteucci1 ,m41 effect of electricity on polished silver observed by1 #bba2 experiment showing polarization by electricity1 #bhf2 doubts of1 on the polarity of diamagnetism1 #cdh note2 experiments on magnetic action in crystals1 #cej2 observation on the action of compression1 #ceb4 ,maury1 ,lieutenant1 calms named by1 #abc4 ,measurement of astronomical distances1 formula assisting1 #dc4 ,mechain1 ,m41 ,encke's comet seen by1 #cfe4 ,mechanical equivalent of heat1 #bge4 ---- engines1 incapable of generating force1 #bgi4 ,mediterranean1 the1 conditions of1 shutting out the tidal wave1 #ih2 hurricane in1 divided into two storms1 #abf2 vegetation of1 #beb4 ,medium1 ethereal1 transmitting magnetism1 #cdd2 density of1 #cef2 probable relations of1 to gravity1 ib42 experiment testing its magnetic properties1 #cef1 #ceg2 functions of1 #ceg2 pervading the visible creation1 #ceh2 unsolved question touching1 #cfe2 a cause of accelerated revolutions of comets1 #cff1 #cfg2 direction of its increase in density1 #cfg4 ,medium occupying space1 #dbd4 ,medusa tribes1 the1 phosphorescent brilliancy of1 #bie4 ,melloni1 ,m41 experiments of1 in photography1 #bad2 his application of the principle of thermo-electricity1 #ccc2 experiments of1 in transmission of heat1 #beh-bfc2 fixing the maximum of heat in the solar spectrum1 #bfd2 in polarization of heat1 #bfd-bff2 light separated from heat by1 #bfe4 ,melville ,island1 height of the thermometer in1 in ,january1 #bdg4 ,mercury1 inclination of his orbit to the plane of the ecliptic1 #ba2 eclipse of1 #db2 cause of his rotation unknown1 #fe2 ellipticity of his orbit compared with the terrestrial1 #gd2 climate of1 #bbf2 comet revolving between the orbits of ,pallas and1 #cfg2 attraction of1 determining a comet's orbit1 #cfi2 comets revolving in his orbit1 #cha2 velocity of1 #djj4 ----1 propagation of heat in1 #bgc2 rotating by electricity1 #cad4 ,meridian1 constant1 of high water1 #ib4 ----1 mode of determining the magnetic1 #cdc4 ,meridians1 size and form of the earth determined from1 #df2 measurement of arcs1 #dg2 anomalies from local attraction1 #dh2 result of the computations1 #dh1 #di2 permanent1 of the moon1 #fi1 #gj4 ----1 magnetic1 influencing the direction of metallic veins1 #cdf4 ,messier1 comet of #aggj observed by1 #cfa2 ,encke's comet seen by1 #cfe2 nebula described by1 #dji4 ,metallic salts1 action of the rays of the solar spectrum on1 #bjc4 ---- springs used in construction of musical instruments1 #adc2 rods giving musical notes1 #add4 ,metallic surfaces1 polarized light reflected from1 #aic2 plates1 impressions on1 from bodies in contact with1 #bbj4 ,metals1 expansion of1 by heat1 #bga2 propagation of heat in1 #bgd2 transmission of electricity by1 #bhd2 electricity developed by oxidation of1 #bih2 determining the appearance of a spectrum of voltaic flame1 #cjc2 distilled in the voltaic arc1 #cjd1 #cje2 electro-plating of1 #cji2 properties of1 modifying electric susceptibility1 #ccc2 magnetism an agent in the formation of1 #cdf4 ,meteor1 the bursting of a1 #aah4 ,meteors1 #dbj2 theory of1 #dba-dbc4 ,meteoric stones1 proofs of their foreign origin1 #dbj1 #dba2 shower of1 #dba1 #dbb4 ,mtre1 adopted by the ,french as their unit of linear measure1 #hi4 ,mica1 polarization by induction effected with1 #bhf4 ,milky ,way1 the1 described1 #che2 ,sir ,john ,herschel's description1 #che1 #chf2 8,coal ,sacks10 #chf2 stars composing1 #bhf1 #bhg2 zone of stars crossing1 #cij2 position of variable stars with regard to1 #cie2 crowding in1 apparent only1 #dje2 orbit in the plane of1 #djf2 relation of1 to the stellar universe1 #djg2 nebula resembling1 #dji2 its quarter of the heavens1 #dad1 #dae2 dividing the nebulous system1 #daf1 #dag2 great nebula in1 #dah2 remote branches of1 #dai4 ,minerals1 possessing the phosphorescent property1 #bid4 ,mines1 cause of increased temperature in1 #bbi2 green plants growing in1 #bec4 ,mira1 periods of its fluctuations in lustre1 #cij4 ,mirage1 supposed cause of1 #aeg4 ,miraldi1 rotation of ,jupiter's satellite determined by1 #gj4 ,mitscherlich1 ,m41 his experiments on crystals1 #ajg2 discoveries1 #ajh2 experiments of1 in expansions of crystals1 #bgb4 ,mocha1 meteors falling at1 #dba4 ,moignot1 ,m41 crystals compressed by1 #ahi4 ,moisture1 an indispensable requisite for vegetation1 #bdh2 transmission of electricity effected by1 #bhd1 #bhh4 ,molecular polarity1 produced by electricity1 #bhb2 attraction1 electricity developed by destruction of1 #bhd4 ---- structure affecting transmission of electricity1 #cjc4 ---- vortices1 hypothesis of1 accounting for the absorption of light1 #agg4 ,molecules1 material1 attraction and repulsion of1 #ajc2 effect of elasticity and cohesion on1 #ajd-ajf2 uniting to form crystals1 #ajg-aji2 extreme minuteness of ultimate1 #aaj2 of ether1 modes of their vibration in natural and polarized light1 #aic2 in fluorescent light1 #aif1 #aig2 images traced by the mutual action of1 #bai-bbb2 arrangement of1 connected with magnetism1 #cej-ceb4 ,mollusks1 distinct species of1 #bed4 ,monocerotis #aa1 a triple star1 #cie4 ,monsoons1 theory of the1 #abc1 #abd4 ,months1 antiquity of1 as a measure of time1 #he4 ,moon1 the1 force restraining1 #d1 #e2 mean distance of1 from the earth1 #d2 results effected by her nearness to the earth1 #g2 annual rate of decrease in her orbit's excentricity1 #ag2 average distance of1 from the earth's centre1 period of her circuit of the heavens1 #cd2 her periodic perturbations1 #ce-ch2 causes assigned for acceleration of her mean motion1 #cf1 #cg2 eclipses of1 #ci1 #dj2 longitudes determined by observations of1 #db1 #dc2 her mean horizontal parallax1 #eb2 sources whence her mass may be determined1 #ee1 #ef2 her diameter1 #ef2 rotation of1 #fh2 librations1 #fi2 mountains1 #gj2 precession resulting from her attraction1 #gi-ha2 influence of1 producing tides1 #ia1 #ib1 #if-ih2 period of her declinations1 #ig2 atmospheric equilibrium disturbed by her attraction1 #aba2 cause of her apparent increased magnitude in the horizon1 #aeh2 photographic image of1 #bad2 comparative amount of light emitted by1 #bbe2 cause of the rarity of her atmosphere1 #bbf2 increased intensity of light at full1 ib42 effect of the terrestrial atmosphere on heat radiated from1 #bbg2 cause of acceleration in the mean motion of1 #cff2 light reaching the earth from1 #djd4 ,moorcroft1 herbarium collected by1 #bej1 #bea4 ,moser1 ,professor1 mutual influence of bodies in contact tested by1 #bai1 #bbj4 ,mossotti1 ,professor1 his analysis to prove the identity of the cohesive force with gravitation1 #ajc1 #ajd2 his definition of gravity1 #cee4 ,motion1 a law of the universe1 #bgd2 perpetual1 impossible1 #bgi4 ,mountains1 anomalies in measurement caused by1 #dh2 rarity of atmosphere on1 #aah2 cause of perpetual snow1 #aai2 modes of determining heights of1 #abj2 becoming new centres of motion in hurricanes1 #abf2 influence of chains on temperature1 #bda1 #bdb2 cause of boulemens in1 #bga2 tops of1 fused by lightning1 #bic4 ----1 lunar1 effect of solar rays passing between1 in eclipses1 #da2 influence of1 on the moon's motions1 #if2 three classes of1 #gj4 ,mu ,herculis1 direction of solar motion with regard to1 #djf4 ,multiple systems of stars1 #cie4 ,mundy1 ,captain1 mirage described by1 #aeg4 ,music1 comparison instituted of sympathetic notes in1 #b2 regulated undulations of sound producing1 #adb2 instruments of1 #adc2 experiments by means of vibrating plates1 #add-adf2 sympathetic vibrations1 #adg1 #adh2 experiments showing1 #adh1 #adi4 ,musical instruments constructed by ,professor ,wheatstone1 #adc4 ,naples1 comet discovered from1 #cgj4 ,nautical ,almanac1 computations for calculating longitudes1 #dc2 time calculated by1 #hd4 ,navigation1 importance of lunar motions in1 #db2 laws of storms to be observed in1 #abg1 #abh4 ,neap-tides1 #if1 #ii4 ,nebul1 number and general aspect of1 #djg2 catalogues1 #djg1 #djh2 classes1 #djh2 irregular1 #djh1 #dji2 of definite form1 #dji2 spiral1 #dji1 #daj2 annular1 #daj1 #daa2 elliptical1 double1 #daa2 distance of a nebulous star discoverable1 #daa1 #dab2 aspect and colour of planetary1 #dab2 elliptical common1 #dac2 globular clusters1 #dac-dae2 resolution of1 #dae2 star clusters1 #dae1 #daf2 probable law of motion1 #daf2 distribution of1 #daf1 #dag2 the ,magellanic clouds1 #dag1 #dah2 round ,args1 #dah1 #dai2 remote systems1 #dai2 invisible solar1 #dba2 meteors falling from1 #dbb4 ,nebulous appearances of a comet1 #cfd2 extent of1 matter surrounding a comet1 #cgc2 its variable brilliancy1 #cgd2 appearances round the sun1 #dab4 ---- stars1 #daa1 #dab4 ,needle1 magnetized1 effect of ,voltaic electricity on a1 #cab1 #cac2 suspended by means of electricity1 #cad2 condition of its deviation by an electric current1 #cag4 ,negative electricity defined1 #bhb2 mode of exciting1 #bhc4 ---- impressions in photography1 #bjd4 ,neptune1 periodical variations in his orbit1 #bb2 revolution of his satellite from east to west1 #cc2 remoteness of1 #ed2 anticipation of discovery1 #fa2 orbit and motions of1 determined1 #fb2 his diameter1 mean distance from the sun1 #fc2 temperature of1 #bbe2 action of1 on ,halley's comet1 #cfc4 ,neutral phosphate of soda1 its crystals1 #aji4 ,new ,mexico1 monsoons occasioned by its deserts1 #abd4 ,newton1 ,sir ,isaac1 steps of his argument for the universal influence of gravitation1 #c2 his discoveries of modes of attraction1 #d2 motions of bodies projected in space1 ascertained by1 #e2 form of a fluid mass in rotation ascertained1 #de2 problem occupying astronomers since1 #fd2 discrepancy between his theory of tides and observations1 #if2 compound nature of white light proved by1 #aei2 his analysis of the solar spectrum disputed1 #afa2 his theory of light disproved1 #afg2 measurements of coloured rays1 #agb1 #agc2 scale of colours1 #agd2 decisive experiment disproving the theory of light1 #bjb2 remarks on the transmission of gravity1 #cee4 ,niagara1 the falls of1 not independent of the influence of astronomy1 #a4 ,nickel1 sulphate of1 change in its crystals1 when exposed to the sun1 #ajg4 ,niepc1 ,m41 photographic pictures rendered permanent by1 #bjd2 discovery in photography suggested1 #bjg2 colours of images of the sun taken1 #bac2 experiments by1 on saturation of substances with light1 #bif4 ,nimes1 discovery of a telescopic planet at1 #ba4 ,nitrogen1 proportion of1 in the atmosphere1 #aag2 spectrum from1 #cjc2 iron volatilized by the ,voltaic arc in1 #cjd2 unaffected by magnetism1 #cdd4 ,nobili1 ,m41 direction of electric currents ascertained by1 #ccc4 ,nodes1 ascending and descending1 of a planet defined1 #i2 movement of their lines in secular disturbances1 #ad2 advance and recession of1 #ah2 supposed recession of1 on the equator of the solar system1 #bd2 of the moon1 period of their sidereal revolution1 #cg2 secular inequality affecting1 #ch2 influence of1 on eclipses1 #ci2 cause of their rapid motion1 #ee2 points of rest on a vibrating string1 #ada2 in the vibrations of an undulating column of air1 #adb2 in vibrations of solids1 #adg4 ,non-conductors of electricity1 #bhd1 #bhe4 ,non-electrics1 #bhe4 ,north ,atlantic1 the1 winds in1 #abd4 ---- ,polar ,ocean1 tide in the1 #id4 ,norway1 course of the tidal wave to1 #id4 ,notes in music1 #adb1 #adc4 ,nubecula1 ,major and ,minor1 #dag1 #dah4 ,nucleus1 of ,halley's comet1 changes in its aspect1 #cfd2 disappearance of1 in ,encke's1 #cfi2 division1 in ,biela's1 #cfi1 #cgj2 diaphanous1 #cgc2 solidity of1 tested1 #cgd2 of a spiral nebula1 #dji4 ,nuremburg1 observations on a comet from1 #cgj4 ,nutations produced by the moon's nearness to the earth1 #g2 in ,jupiter's equator1 #bi2 in the planetary axes1 #ff2 effect of1 on the pole of the equator1 longitudes and latitudes altered by1 #ha4 ,nysa1 nearness of its orbit to the earth1 #ba4 ,oaks1 range of1 near the equator1 #bej4 ,occultation1 central1 by ,halley's comet1 #cfd2 geographical position ascertained by1 #chd2 prospective1 by a sun of ,centauri1 #djj4 ,occultations of stars1 #db1 #dc4 ,ocean1 the1 density and mean depth of1 #ea2 mean density1 compared with the earth's1 #gg2 its form in equilibrio1 when revolving round an axis1 #ib2 solar and lunar attraction disturbing its equilibrium1 ib42 inequalities in periodic motions1 #ic2 motions of the tidal wave in #ie2 stability of its equilibrium1 #ajj2 circulation of currents in1 ib42 stratum of constant temperature in1 #aja2 zones of1 ib42 decrease and increase of temperature with depth1 #bca2 absorption and radiation of heat by1 #bdb2 electricity evolved from1 #bia4 ,oceans of light and heat1 processes producing1 #bbe4 ,ochotzk1 the sea of1 depression of the barometer observed in1 #abj4 ,october1 #ahcb1 position of ,saturn's rings in1 #fg4 ,olbers1 ,m41 computations for a comet by1 #cfg2 period of his comet1 #cgj2 comet of #ahaa observed by1 #cgd4 ,opaque bodies1 mode in which heat is developed in1 #bbg4 ,ophiuchi #gj1 anomalies in the motions of1 #djj4 ,ophiuchus1 clusters of the ,milky ,way between the ,shield and1 #chg2 new star disappearing from1 #cic4 ,optic axis1 the1 of crystals1 #ahc2 phenomena exhibited by transmission of a polarized ray along1 #ahg1 #ahh2 affected by compression1 #ahi4 ,orbit1 the1 of the earth1 attraction intensified by its diminished excentricity1 #cg2 excentricity of1 affecting temperature1 #gd1 #ge2 crossed by comets1 #cfh4 ---- of the moon1 force ruling1 #d2 its excentricity1 #cd2 changes in1 #ce2 its inclination to the plane of the ecliptic1 #gi4 ---- of a nebula1 #dae4 ---- of the solar system1 #dje1 #djf4 ,orbits of comets1 subject to variation1 #cfa2 examples1 #cfa-cfc2 prospective changes in1 #cfi1 #cgj2 of ,donati's1 #cgi2 forces determining their forms1 #chb1 #chc4 ---- of double stars1 #cif-djj4 ---- of planets1 force regulating a planet's velocity in1 #h2 measurement of their excentricity1 #i2 seven elements of1 determining their position in space1 #aj2 unequal movements in1 #ae2 variation from elliptical to circular1 #ag2 secular variations of1 in inclination to the plane of the ecliptic1 #ah1 #ai2 stable and unstable in form1 #ba1 #bb2 influence of the ethereal medium on1 #bb2 principle facilitating observations on secular inequalities1 #bc1 #bd2 revolutions of ,saturn compared with ,jupiter1 #be2 periodic inequality increased by secular variations in their elements1 #bf2 comets revolving in1 #cha1 #chb2 cause of diversity in form of1 #chb4 ,orbits of satellites1 forms of ,jupiter's1 #bg2 their inclinations1 #bh2 inclinations of ,saturn's1 #cb2 positions of ,uranus's1 #cc2 forms of data in computing a planet's place in the heavens1 #ei4 ,orinoco1 the cataracts of the1 heard by day and by night1 #ace2 area occupied by forests on1 #bdc4 ,orion1 the ,milky ,way between ,antinous and1 #che1 #chf2 position of1 #cij2 variable star in1 #cic1 #cid2 multiple system in1 #cie2 nebula in1 #djh4 ,oersted1 ,professor1 discovery of1 suggesting the theory of electro-magnetism1 #cab2 science founding the reputation of1 #caf4 ,oscillations1 wide-spreading1 produced by gravitation1 #b2 mechanical principle affecting small1 #aa2 of the sines and cosines of circular arcs1 #bj2 invariable plane whence they may be estimated1 #bd2 of the pendulum retarded1 #cb2 of the pendulum1 experiments founded on1 #ej1 #ea2 experiments testing the earth's density1 #eg2 a measure of time1 #hc2 produced by tides1 #ie1 #if2 instruments measuring atmospheric1 #aac2 barometer affected by periodic atmospheric1 #abj1 #abb2 of ears of corn1 #abi1 #acj2 producing musical notes1 #adj-adb2 instances of forced sympathetic1 #adh2 causing vicissitudes in climates1 #bdg2 of the pendulum1 disturbed by effects of temperature1 #bgb2 measuring variation of electrical intensity1 #bhg4 ,otto1 ,m41 motions of the heavenly bodies observed by1 #dje4 ,oxidation of metals1 electricity developed by1 #bih2 by the ,voltaic discharge on polished silver1 #cje4 ,oxides decomposed by electricity1 #cjg2 alkalies resolved into metallic1 #cjg4 ,oxygen1 in crystals1 #aji2 proportion of1 in water and carbonic oxide1 #aaa2 in the atmosphere1 #aag2 chemical combination with1 evolving light and heat1 #bgj2 action of electricity on1 #bhd2 electricity afforded by combination of metals with1 #bih2 spectrum from1 #cjc2 separated from water by electricity1 #cjg2 paramagnetic1 #cdd4 ,ozone1 produced by electricity1 #bhd4 ,pacific ,ocean1 mean depth of1 #gg2 course of tidal waves down1 #ic2 mean depth of1 #if2 currents1 #ajj4 ,paderborn1 fulgorites from1 #bic4 ,pallas1 inclination of its orbit to the ecliptic1 #aj2 diameter of1 #ba2 astronomical tables1 #fc2 ellipticity of its orbit compared with the terrestrial1 #gd2 height of its atmosphere1 #bbf2 comet revolving between the orbits of ,mercury and1 #cfg4 ,pan's pipes1 vibrations in the air passing over1 #adb4 ,parabolic motion1 ratio of forces procuring1 #chb4 ,parallax of the sun1 circumstance favourable to its correction1 #ba4 ---- of an object defined1 #dc4 ----1 definition1 mode of ascertaining1 #eb2 distances computed from1 #eb-ed2 calculation from the moon's horizontal1 #ee4 ---- of fixed stars1 #chg-cij4 ---- of meteors1 #dba1 #dbb4 ,paramagnetic substances1 #cce1 #ccf4 ,paramagnetism defined1 #cce2 substances it is resident in1 #ccf2 modes of imparting1 ib42 a dual power1 ib42 imparted by induction1 #ccg2 law of its intensity1 #cch2 a property of oxygen1 #cdd2 in antithesis to diamagnetism1 #cdg2 neutral substances obtained by combinations of diamagnetism and1 ib42 ,dr4 ,tyndall's experiments on polarity of1 #cdh2 dependent on arrangement of molecules1 #cej1 #cea2 affected by compression1 #cea2 truth establishing its identity with diamagnetism1 #cef1 #ceg4 ,parathermic rays1 analyzed by ,sir ,john ,herschel1 #bag-bai4 ,paris1 variation in length of the pendulum at1 #ea2 mean annual temperature1 #bbh2 temperature of an ,artesian well in1 #bcj4 ,paths of comets1 #cei1 #cfj2 secrets disclosed by their excentricities1 #cfe4 ,parry1 ,sir ,edward1 turned back by the ,polar current1 #aja2 mean temperature calculated from observations of1 #bde2 thermometer at ,melville ,island marked by1 #bdg4 ,pauxis1 the ,straits of1 ebb and flow of the sea in1 #ih4 ,peel1 ,sir ,william1 thunderstorm experienced by1 #bic1 #bid4 ,pegasus1 nebulous region of1 #dag4 ,pendulum1 the1 principle equalizing its oscillations1 #ej2 the earth's figure calculated by1 #ej1 #ea2 experiments ascertaining the earth's density1 #eg2 isochronous1 a measure of time1 #hc2 a standard of the measure of extension1 #hi2 the1 a connecting link between time and force1 #id2 inventions to neutralise the effects of temperature1 #bgb4 ,penumbra1 in lunar eclipse1 breadth of space occupied by1 #dj4 ,perigee1 of the lunar orbit1 period of its revolution1 #cg1 #ch2 cause of its rapid motion1 #ee4 ----1 solar1 periods of its coincidence with the equinoxes1 #hf4 ,perihelion of a planet's path defined1 #af4 ---- of the earth's orbit1 its position regulating the length of seasons1 #gd4 ,periodic inequalities of planets1 #ac1 #ad2 law from which they are deduced1 #bd1 #be2 of ,jupiter's satellites1 #bh1 #bi2 lunar1 #ce4 ,perkins1 ,mr41 experiments of1 testing the laws of compression1 #gh4 ,peron1 ,m41 specific diversity of marine animals asserted by1 #bed4 ,perpendicular force1 the source of periodic inequalities1 #ae2 effects produced by1 #ah4 ,perpetual motion1 invariable proportion between heat and force precluding1 #bgi4 ,perseus1 variable star in1 #cij1 #cia4 ,peters1 ,mr41 comet discovered by1 #cgj2 parallax of ,lyr1 #chh1 #chi2 distances of fixed stars calculated1 #chi2 his theory of ,sirius' irregular motions1 #cib2 sun's motion proved by1 #dje4 ,petit1 ,m41 observations of1 on meteoric satellites1 #dbc4 ,peru1 arcs of the meridian measured in1 #dh4 ,phases of the moon1 regulating returns of eclipses1 #ci4 ,phenomena1 of effects of light in eclipses1 #da1 #db2 applied to computing longitudes1 #dc2 caused by tidal oscillation1 #if2 from force of cohesion1 #ajf1 #ajg2 of capillary attraction1 #aae2 produced by refraction and reflection1 #aee-aeg2 by polarization of light1 #ahf-aij2 exhibited in fluorescence of light1 #aif1 #aig2 resulting from interaction of rays and molecules1 analogous to effects of photography1 #bai-bbb2 phosphorescent1 #bie1 #bif2 of galvanism1 #caj2 of magnetism1 #cce1 #cde-cdh2 magnecrystallic1 #cdi1 #cej2 exhibited by comets1 #cfc1 #cfd1 #cfi1 #cgj1 #cgb-cgf2 by the ,milky ,way1 #che-chg2 by variable stars1 #cij-cic2 by double stars1 #cig-dja2 by nebul1 #dji-dae1 #dag-dai2 by meteoric showers1 #dba1 #dbb4 ,phosphorescence1 rays of the solar spectrum exciting1 #baf2 cause of1 in the solar spectrum1 #bag2 excited by electricity1 #bid2 fish possessing the property of1 #bie2 the glow discharge1 #bie1 #bif2 experiments investigating the nature of1 #bif4 ,photo-galvanic engraving1 #cji4 ,photography1 first suggestions1 #bjc2 discoveries and improvements in1 #bjd-bjg2 conditions affecting the chemical properties of rays producing1 #bjg1 #bjh2 images of the solar spectrum obtained by1 #bjh-baj2 coloured copy of an engraving1 #baa2 phenomena in1 suggesting an absorptive action in the solar atmosphere1 #bab1 #bac2 chemical energy producing1 distinct from light and heat1 #bad2 experiments by means of1 testing the properties of rays1 #bah1 #bai2 experiments on action of light1 heat1 electricity1 producing results analogous to effects of1 #bai-bbc4 ,photosphere1 the1 of the sun described1 #bbd4 ,physical ,sciences1 the most extensive example of their connection1 mode of its operation1 #a4 ,pi ,herculis1 direction of solar motion with regard to1 #djf4 ,pisces1 nebulous region of1 #dag4 ,planetary motion1 representation of1 #ad4 ---- nebul1 #dji2 appearance of1 #dab4 ,planets1 paths round the sun described by1 #e2 law determining their revolutions1 ib42 forces adjusting their forms1 #f2 their motions in elliptical orbits1 mean distance from the sun1 #h2 mode of obtaining the place of1 in their orbits1 #i2 computations giving the place of1 in space1 #aj2 disturbances from reciprocal attraction affecting1 compensations1 #ac-ai2 telescopic1 #bj1 #ba2 perturbations in the mean motions of1 #be1 #bf2 influence of1 on lunar motions1 #cf2 eclipses and conjunctions of1 #db2 formula finding their masses1 #ee2 their diameters1 #ef2 mass of the telescopic1 compared with the moon1 ib42 comparative density1 #eh2 method of computing their places1 #eh-fd2 discovery of1 #fa-fc2 exploded theory touching telescopic1 #fc2 periods of their rotations1 #ff2 variation and position of the plane of the ecliptic produced by1 #gi2 its effect on the equinoctial points1 #hj2 climates of1 #bbe1 #bbf2 probably magnets1 #cdf2 constant velocity of their mean motions1 #cff4 ,plants1 distribution of known species over the globe1 #bdi1 #bej4 ,plates1 vibrating1 experiments by means of1 #add-adf4 ,plateau1 ,m41 experiments of1 on colour1 #afe1 #aff4 ,platina1 incandescent1 used as a source of heat1 #bfj4 ,platinum1 experiment producing spontaneous combustion of1 #aab1 #aac4 ,playfair1 ,professor1 quoted in reference to ,la ,grange's discovery1 #bc4 ,pleiades1 the1 nebulous stars1 #dae4 ,plcker1 ,professor1 discoveries of1 in the action of magnetism in crystals1 #cdi4 ,plumb-line1 deviations of1 from local attraction1 #dh2 earth's density calculated from a deviation of1 #eh4 ,poinsot1 ,m41 ,la ,place's discovery extended by1 #bc2 comparison by1 #bd4 ,point1 ready escape of electricity from a1 #bhh4 ,poisson1 ,m41 decisions of1 on the phenomena of capillary attraction1 #aad4 ,polar basin1 probable temperature of1 #bde1 #bdf4 ---- star1 change of position in the1 #ha1 #hb4 ---- vegetation1 contrasted with tropical1 #bdh4 ,polarity1 produced by electricity1 #bhb2 of magnets defined1 #ccf2 induced in iron1 #ccg2 its antithetical manifestations of1 #cci2 invariably dual1 #cda2 of diamagnetic substances1 #cdg1 #cdh4 ,polarization of light1 definition of1 #agi2 refracted by various substances1 #ahj-ahc2 by reflection1 #ahd2 angles of1 #ahe2 phenomena exhibited by transmission through analyzing media1 #ahf-ahh2 circular1 #ahi-aia2 theory of circular and elliptical1 #aib1 #aic2 substances producing1 #aic1 #aid2 theory of coloured images formed by1 #aid2 accidental1 #aie2 discovery of1 ib42 degraded light incapable of1 #aih2 communicating electricity1 #bbj2 plane of motion of vibrations in1 #bbc4 ,polarization of heat1 first attempts1 #bfd2 successful experiments1 #bfe-bfg4 ---- of electricity by induction1 #bhf4 ----1 experiment showing the action of magnetism on1 #cai2 affected by mechanical compression1 #ceb4 ,poldice mine1 the1 temperature of the water pumped from1 #bbi4 ,poles1 the1 cause of the flattening of a spheroidal mass at1 #f2 diameter of ,jupiter at1 #bg2 experiment determining the increase of gravitation towards1 #di1 #ej2 the1 drifting of ice from1 #ajj1 #aja2 of maximum cold1 centres of the isothermal lines1 #bde1 #bdf2 nature of magnetic force distinguished by1 #ccb2 four terrestrial1 of maximum magnetic force1 two magnetic1 #cdc4 ,pollux1 an optically double star1 #dja4 ,port ,bowen ,harbour1 transmission of sound across1 when frozen1 #acf4 ,positive electricity1 defined1 #bhb2 mode of exciting1 #bhc4 ---- impressions in photography1 #bjd4 ,pouillet1 ,m41 his estimate of the mean temperature of space1 #aai2 quantity of solar heat received by the earth computed by1 #bch2 data furnished by1 to ,professor ,thomson1 #bgi2 development of electricity investigated by1 #bia4 ,powell1 ,baden1 substances producing elliptical polarization enumerated by1 #aic2 dispersion of light accounted for by the undulatory theory1 #bjj1 #bja2 experiments in transmission of radiant heat1 #bfb2 attempts to polarise heat1 #bfd4 ,power1 ,mr41 undulations producing fluorescent light computed by1 #aig2 law of solar rays acting on media1 #aih4 ,prsepe1 the1 in ,cancer1 #dae4 ,precession1 a1 in the equinoxes of planets1 its cause1 #ff2 mean1 of the equinoctial points1 defined and calculated1 #hj2 influence of1 on the pole of the equator1 on longitudes1 #ha4 ,pressure1 electricity elicited by1 #bhc1 #bhd2 law of electrical1 #bhh4 ,principato ,citeriore1 earthquake in1 #bcd4 ,prisms1 solar spectrum formed by1 #aei2 neutralizing effects of colour1 #afd2 of brown tourmaline1 light polarized by1 #ahj2 resolution of the pure white sunbeam by1 #bbb2 substance of1 determining the point of maximum heat in the solar spectrum1 #bfc1 #bfd2 electrical light analysed by1 #bhh4 ,problem determining the motions of translation of the celestial bodies1 #aa2 of the three bodies1 #eh2 the hardest astronomical1 #ib4 ,procyon1 light of1 #djb4 ,proportion1 definite1 the law of1 in mixing substances1 #aaa1 #aab4 ,protoxides of metals1 their crystals1 #aji4 ,prussia1 ,eastern1 fulgorites from1 #bic4 ,ptolemy1 decrease in the inclination of ,jupiter's orbit since the age of1 #ai2 discovery of the ,evection by1 #ce2 ,indian lunar tables calculated in his time1 #hh2 horoscope ascribed to the age of1 #hi2 effects of refraction observed by1 #aee2 colour of ,sirius in his time1 #dja4 ,quadratures1 the equation of the centre in1 #i2 lunar orbit augmented in1 #ce2 tides affected by the moon in1 #if4 ,quadrupeds1 distribution of distinct species of1 #bee4 ,quartz1 crystallised1 light polarized circularly by1 #ahi1 #aij2 varieties of polarization exhibited by1 #aic4 ,quebec1 extremes of temperature found in1 #bdg4 ,quinine1 sulphate of1 producing fluorescence of light1 #aig4 ,radial force producing periodical changes in relative positions of the heavenly bodies1 #ae2 effects produced by1 #af1 #ag2 principle neutralising its ultimate result1 #ai1 #bj4 ,radiation of heat1 laws regulating1 #beg2 universal from substances1 #bfh2 natural phenomena resulting from1 #bfi2 slow decrease of the earth's central heat from1 #bcb2 influence of1 on temperature1 #bci2 power of1 in water compared with dry land1 #bdb2 of heat1 a transfer of motion1 #bgg4 ,radii vectores1 signification of1 #h2 areas described by1 #aj2 force disturbing in the direction of1 #ad1 #ae4 ,ragona-,scina1 ,m41 his theory of rayless lines in the spectrum1 #afc4 ,rain1 force shaping drops1 #ajf2 cause of periodic tropical1 #abc2 region of1 #abd2 theory of its formation1 #bgj2 an electric conductor1 #bib4 ,rankine1 ,mr41 his theory of the structure of matter1 #ajd2 his theory of the absorption of light1 #agg4 ,rays1 common nature and common properties of1 #bfh4 ---- of heat1 existing independently of luminous1 #beg2 laws of transmission of1 #beh2 analogy between transmission of luminous rays and1 #bei2 temperature of their source affecting transmission1 #bfj2 varying in nature with their origin1 #bfa2 transmitted through coloured glass1 #bfb2 traversing various media1 ib42 subject to refraction and reflection1 #bfc2 polarized1 #bfe-bfg2 absorption and reflection of1 #bfh2 rotation of polarized1 caused by magnetism1 #cai4 ---- of light1 bent by passing from rare into dense media1 #aec2 partial and total reflection of1 #aef2 loss of1 by obliquity of incidence1 #aeh2 theory of their transmission and absorption1 #aei-afa2 comparative refrangibility of1 #afc2 experiments on dispersion of1 #afd2 principle determining their colour1 #agj1 #aga2 transmission of1 in glass or water1 #agg1 #agh2 conditions of polarized1 #agi2 double refraction1 #aha-ahc2 polarized by reflection1 #ahd1 #ahe2 coloured images produced by interference of1 #aid1 #aie2 internal dispersion of1 #aie-aih2 heat1 light1 chemical action1 independent properties of1 #bad1 #bae2 undulations constituting1 #bbc2 conditions modifying the power of solar1 to produce heat1 #bcg2 transmitted independently of calorific rays1 #beh2 magnetizing of polarized1 #cah1 #cai4 ,rays1 solar1 effect produced by their refraction in lunar eclipse1 #dj2 passing between lunar mountains in solar eclipse1 #da4 ---- of the solar spectrum1 their chemical properties1 #bjc2 varying chemical energy1 #bjg1 #bjh2 varying nature of their action1 #bjh2 peculiar chemical action of the red1 #bji-baa2 deoxydating and oxydating action of1 #baa1 #bab2 experiments detailed1 #bab-bae2 new1 obscure1 detected by ,sir ,john ,herschel1 #bag4 ,red ,sea1 the1 tide in1 #ih4 ,reflection of waves of sound1 #acg1 #ach2 of rays at surfaces of strata differing in density1 phenomena occasioned by1 #aef1 #aeg2 affecting colour1 #afj2 motion of a ray of light in1 #agg2 light polarized by1 #ahd1 #ahe2 elliptical polarization produced by1 #aic2 heat polarized by1 #bff2 of radiant heat from surfaces1 #bfh4 ,refraction of the sun's rays in lunar eclipses1 #dj2 of waves of sound1 #ach2 of light by the atmosphere1 #aec1 #aed2 mode of estimating1 in case of celestial bodies1 #aee2 formul obtaining in case of terrestrial objects1 ib42 phenomena occasioned by1 #aee1 #aef2 colours decomposed by1 #aei1 #afj2 produced without colour1 #afd1 #afe2 power of1 in media affecting the elasticity of the luminous ether1 #agg2 of a polarized ray1 #ahj2 double1 #aha1 #ahb2 ,fresnel's theory of1 #ahc2 diminished capability of producing fluorescence1 #aif2 capability of1 in rays1 affecting their chemical action1 #bji-bab2 effect of1 on the lunar atmosphere1 #bbf2 influence of1 on transmission of heat1 #beh2 of rays of heat1 #bfa-bfd2 heat polarized by1 #bff4 ,refrangibility1 substances diminishing1 of light1 #aif2 affecting the chemical action of rays1 #bji-bab2 affecting radiation of heat1 #beg2 affecting transmission of radiant heat1 #bfa-bfc4 ,reich1 ,professor1 temperature of mines observed by1 #bbh2 mean increase calculated by1 #bcj4 ,reptiles1 distribution of distinct species of1 #bed4 ,repulsion of electricities1 #bhc2 experiments determining the laws of electrical1 #bhf1 #bhg2 modes of1 in static and in ,voltaic electricity1 #cag2 developing comets' tails1 #cge-cgg4 ,resistance1 a cause of accelerated motion1 #cfg4 ,retina1 the1 action of1 in receiving impressions1 #aff2 comparative sensibility of its fibres to light1 #agh4 ,retrograde motion of comets1 #cei1 #cfh1 #cgc1 #cgi4 ,rhodiola rosea1 identical species of1 found in ,tartary and in ,scotland1 #bea4 ,rhombohedrons of carbonate of lime1 #aji4 ,richman1 ,professor1 killed by lightning1 #bic4 ,richter1 variation in length of the pendulum observed by1 #ea4 ,rings of ,saturn1 #ff-fh2 ,saturn's1 diamagnetic1 #cdg2 luminous1 surrounding comets1 #cgd1 #cge2 surrounding ,donati's1 #cgi4 ,ritchie1 ,professor1 electrical experiments of1 #cad4 ,ritter1 ,m41 chemical properties of the solar spectrum observed by1 #bjc2 oxydizing effect of red rays1 #bji4 ,rive1 ,m4 ,auguste de la1 rate of increase of temperature in wells observed by1 #bcj4 ,rivers1 curvature of the land proved by1 #df2 influence of1 on the earth's rotation1 #ga2 rising of tides in1 #ih2 effect of1 in cooling the atmosphere1 #bdc4 ,roget1 ,dr41 phenomena of electro-magnetism explained by1 #cac4 ,rome1 observations on lunar mountains made at1 #gj2 era fixed at1 #he2 comet discovered from1 #cgj4 ,ross1 ,sir ,james1 stratum in the ocean discovered by1 #aja2 depressure of the barometer observed by1 #abj2 volcanic region discovered1 #bcb4 ,rosse1 ,lord1 nebul resolved by his telescope1 #djg1 #djh2 spiral nebula1 #dji1 #daj2 annular nebul discovered by1 #daj2 nebulous star1 #daa2 planetary nebul1 #dab2 nebul resolved by1 #dae4 ,rotation affecting winds1 #abb-abg2 of winds1 #abd1 #abe2 of hurricanes1 #abe1 #abf2 produced by the ,voltaic current acting on iron1 #cje2 of stratifications of electrical light1 #cjg2 caused by electricity1 #cac1 #cad2 of light caused by an electric current1 #cai2 of magnets producing electricity1 #ccj-ccb2 changes produced in comets by1 #cgf4 ,rotations of the solar system1 #g2 of the sun1 #fe2 of the planets1 #ff2 of satellites1 #fh2 of ,jupiter's satellites1 #gj2 of the earth1 a measure of time1 #ga2 influence of temperature on1 #gb2 axis of1 invariable1 #gf1 #gg4 ,rotatory motion1 form indicating1 #fe2 of ,donati's comet1 #cgi4 ,roux1 ,m4 le1 observations on magnetic action in crystals1 #cej4 ,rudberg1 ,m41 refrangibility of substances ascertained by1 #bja1 #bjb4 ,ruhmkorff1 ,m41 improvements on his electro-inductive apparatus1 #cbh4 ,russell1 ,scott1 ,mr41 velocity of the tidal wave estimated by1 #ie4 ,russia1 arc of the meridian measured in1 #dh2 climates of1 #bdd4 ,sabine1 ,general1 variations in the magnetic elements investigated by1 #cdc1 #cdd4 ,sagittarius1 comet traversing the constellation of1 #cgi2 the ,milky ,way in1 #chf2 nebula1 #dad4 ,sahara1 the1 causing monsoons1 #abd4 ---- desert1 extent1 influence of1 on the atmosphere1 #bdc4 ,salt1 ,mr41 papyrus sent from ,egypt by1 #hi4 ,sand1 tubes in1 formed by lightning1 #bic4 ,sandy deserts influencing temperature1 #bdc4 ,sandwich ,land1 excess of cold in1 over corresponding latitudes1 #bda4 ,sargassa1 or grassy sea1 found in the ,atlantic1 #bec4 ,satellites1 intensified action of attraction upon1 #g2 intimate union of1 with their primaries1 #bf2 exceptions to a general law of the solar system1 #fe1 note2 rotations equal to the times of their revolutions1 #fh2 comet passing through1 #fi4 ----1 ,jupiter's1 proportion of their mass to that of their primary1 #bg2 disturbing force of attraction affecting their orbits1 #bh2 periodic and secular inequalities1 #bh1 #bi2 eclipses1 #cj2 rotation1 #gj2 passage of a comet through1 #cei2 comet nearly approaching1 #cgj4 ---- of ,saturn1 #cb2 of ,uranus and ,neptune1 #cc4 ----1 mode of computing their masses1 #ee2 comparative density of1 #eh4 ---- of ,neptune1 #fc4 ---- of the earth1 shooting stars1 #dbc4 ,saturn1 unequally occurring compensations of disturbance in its motions1 #ae2 disturbing influence of1 on ,jupiter1 excentricity of its orbit compared with ,jupiter's1 #ag2 retarding the revolution of ,jupiter's nodes1 #ai2 invariable plane passing between ,jupiter and1 #bd2 observations on the mean motions of ,jupiter and1 #be1 #bf2 eclipse of1 #db2 internal structure1 #eh2 astronomical tables of1 #fj2 period of his year1 #ff2 the rings of1 described1 #ff-fh2 his ring probably diamagnetic1 #cdg2 action of1 on ,halley's comet1 #cfb1 #cfc2 comets having their perihelia in his orbit1 #cha4 ,saurian reptiles1 distinct tribes of1 #bed4 ,saussure1 ,m41 temperature of mines observed by1 #bbh1 #bbi2 lichen discovered by1 #bdi4 ,savart1 ,m41 his researches and experiments in acoustics1 #acb1 #acc2 experiments on vibrations of glass rulers1 #ade-adg2 experiments showing sympathetic undulations1 #adh1 #adi2 discoveries on the nature of voice1 #aeb4 ,savary1 ,m41 orbital elements of a double star determined by1 #cif2 his mode of ascertaining the actual distances of fixed stars1 #djb1 #djc4 ,scheele1 ,m41 chemical changes effected by the solar spectrum observed by1 #bjc4 ,schroter1 height of planetary atmospheres calculated by1 #bbf4 ,schwabe1 ,m41 periodic variation in the solar spots observed by1 #cdd4 ,science1 its value regarded as the pursuit of truth1 #a2 errors of the senses corrected by1 #cb2 evidence of its antiquity1 #hg4 ,sciences1 mutual relations of forces proving the connexion between1 #cai-cba2 analysis proving the whole circle of1 kin1 #dbg1 #dbh4 ,scoresby1 ,captain1 phenomenon occasioned by refraction observed by1 #aef4 ,scorpio1 vacant patch of the ,milky ,way in1 #chf2 position of1 #cij2 a double star in1 #cie2 nebula in1 #dad4 ,scotland1 progress of the tidal wave round1 #id4 ,sea1 the1 inappreciable influence of1 on the direction of gravity1 #gg2 mean height of snow-line above the level of1 #bda2 comparative extent of1 #bdb4 ,seasons1 conditions determining the duration of1 #gd2 cause of their unequal periods1 #hg2 theory of the tropical dry and rainy1 #abc4 ,seaweeds1 photographic impressions of1 #bje1 #bjf2 luxuriance1 deep colours of1 #bec4 ,secchi1 ,professor1 mountains of the moon observed by1 #gj2 photographic image of the moon obtained1 #bad2 temperatures of the sun's surface estimated1 #bbe2 experiments of1 in photographing the moon and ,jupiter1 #bbf1 #bbg4 ,secular inequalities of planets1 #ac1 #ad2 means of discovering1 #bd1 #be2 effect of1 on the mean motion of the moon1 #cf1 #cg4 ---- variations in mean values of the magnetic elements1 #cdc4 ,seebeck1 point of maximum heat in solar spectrum fixed by1 #bfc2 discovery of1 #bfd2 relations of heat to electricity discovered by1 #ccb1 #ccc4 ,seed-lobes1 proportion in the distribution of plants having one or two1 #beb4 ,seleniate of zinc1 crystals of1 #ajg4 ,senarmont1 ,m41 experiments of1 in expansion of crystals1 #bgc4 ,senses1 necessarily inaccurate testimony of the1 #bha4 ,september1 times coinciding in1 #hd4 ,serpentarius1 star in1 vanishing1 #cib4 ,shell-fish1 their mode of clinging to rocks1 #aag4 ,shield1 the1 clusters of the ,milky ,way between ,ophiuchus and1 #chg4 ,shooting stars1 phenomena of1 described1 #dba1 #dbb2 theories of1 #dbc4 ,siberia1 ,eastern1 depression of the barometer observed in1 #abj4 ,sidereal times1 mean1 periods of1 #hc2 measurement of apparent1 ib4 ,sigma ,eridani1 period of revolution in1 #djj4 ,silesia1 fulgorites from1 #bic4 ,silver iodized1 its sensitiveness to impressions1 #bba4 ,sirius1 the ,egyptian year estimated from1 #he2 comet's tail extending from the ,hare to1 #cgc2 rank of1 #chd2 comparative magnitude1 #che2 parallax1 #chi2 cause of his irregular motion1 #cib2 change in colour1 #dja2 light1 #djb2 extent of surface1 #djd4 ,smyth1 ,admiral1 his measurement of ,etna compared with ,sir ,john ,herschel's1 #abj2 eclipse of a double star observed by1 #cig2 its periodic time determined1 #cih4 ----1 ,piazzi1 heat of the moon felt by1 #bbg4 ,snow1 cause of perpetual1 on summits of alpine chains1 #aai2 causes modifying the height of the line of perpetual1 #bda2 protecting vegetation1 #bdi2 radiation of heat by1 #beg4 ,soda1 sulphate of1 change of form in its crystals1 #ajg2 crystals of the neutral phosphate and the arseniate of1 #aji4 ,soil1 the1 dependence of temperature on the nature of its products1 #bdc4 ,solar gravitation1 #dbd1 #dbe4 ---- magnetism1 its connexion with terrestrial1 #cdd4 ---- spectrum1 cause of the point of maximum heat varying in1 #bfc1 #bfd4 ---- system1 the1 gravitation of the bodies composing1 #e2 conditions securing the stability of1 #aa1 #ab2 proof of its stability1 #bj2 equilibrium of1 underanged by the ethereal medium1 #bb2 invariable plane1 forming the equator of1 #bc1 #bd2 question of its revolution round a common centre1 #bd2 properties of its medium1 #cb2 masses of bodies composing1 #ee1 #ef2 their diameters1 #ef2 uniform direction of rotation in1 #fe2 comparative apparent importance of1 in creation1 #bbf2 probably magnetic throughout1 #cdf2 comets forming part of1 #cfe2 possible ultimate destruction of1 #cgb2 computations of comets revolving within1 #cha1 #chb2 paths described by heavenly bodies in1 #chb1 #chc2 position of1 relative to the ,milky ,way1 #che2 direction of its motion1 #dje4 ,soleil1 ,m41 crystals compressed by1 #ahi4 ,solids1 conditions reducing molecular particles to1 #ajd1 #aje2 distinctive forms taken by matter in1 #ajf2 velocity of sound passing through1 #ace2 change of shape in1 accompanying ringing sound1 #adg2 expansion of1 by heat1 #bga4 ,solstices1 the1 solar motion at1 affecting the duration of time1 #hd2 the year estimated from the winter1 #he2 periodical coincidence of the solar perigee and apogee with1 #hf1 #hg4 ,sothaic period1 the1 of the ,egyptians1 #he4 ,sound1 medium conveying1 #abi2 its propagation by undulations illustrated1 #abi1 #acj2 conditions modifying the intensity of1 musical notes1 #aca2 experiments testing the compass of audible1 #acb1 #acc2 media modifying the velocity of1 #acc-acg2 laws of its reflection from surfaces1 #acg1 #ach2 undulations of1 subject to the laws of interference1 #ach1 #aci2 laws of the foundation of musical science1 #adj-adc2 reinforced by resonance of cavities1 #aej1 #aea2 repeated vibrations required to produce1 #agh2 different modes of action in undulations producing light and1 #aii1 #bjj2 identical nature of heat and1 #bhj1 #bha2 measuring velocity1 #bij1 #bia4 ,sounding boards1 intensifying musical vibrations1 #adi2 action of1 in musical instruments1 #aej4 ,south1 ,sir ,james1 positions of stellar systems measured by1 #cif4 ,south pole1 the1 excess of cold at1 #bda4 ---- ,sea islands1 height of tides at1 #ih4 ,southern ,ocean1 rise of the tidal wave in1 #ic2 velocity of the wave1 #id4 ,spain1 meteoric showers off the coast of1 #dba4 ,specific heat defined1 #bge4 ,spectra of gases and flames1 their characteristic peculiarities1 #afc1 #afd2 three superposed1 of the pure white sunbeam1 #bbb4 ,spectrum1 the solar1 decomposed into seven colours1 #aei2 colours of1 modified by thickness of the medium absorbing1 #afj2 decomposed into three colours1 #afa2 rayless lines in1 #afb2 observations and experiments on rayless lines1 #afc1 #afd2 experiment of fluorescent light1 #aig2 obtained independently of prismatic refraction1 #bja2 energetic action of1 on matter1 #bjc2 photographic coloured images of1 #bjh-baj2 analysis1 properties of1 experiments1 #baa-bai2 complex nature of1 #bbb2 produced from diffracted light1 #bbc4 ---- of an electric spark1 #bhi4 ---- of the ,voltaic arc1 #cjc4 ,spheres1 mode of attraction in hollow and solid1 #d2 planets partaking the nature of1 #g2 impulses regulating rotations1 ib42 conditions procuring the figure of1 #dd2 formula finding the density1 #ef2 force giving the form of1 #ajf2 power of retaining electricity1 #bhh4 ,spherical form1 the result of cohesion1 #ajf4 ,spheroids1 influencing attraction differently from spheres1 #d2 force disturbing attraction in1 #bg2 compression of the terrestrial and of ,jupiter's1 computed1 #ch1 #ci2 of elliptical strata1 quantities invariable in1 #df2 of the sun1 #fe2 effect produced by the attraction of an external body on1 #gi2 power of retaining electricity1 #bhh4 ,spiral nebula1 #dji1 #daj4 ,spots on the sun's surface1 periods of their vicissitudes1 #bbd2 amount of heat varying with1 #bbe4 ,spring tides1 #if-ii4 ,springs1 hot1 rising in mines1 #bbi2 mean heat of the earth determined from1 #bch4 ,standards of weights and measures1 whence derived1 #hi1 #ij4 ,stars1 fixed1 the1 the solar system probably not independent of1 #bd2 velocity of light deduced from aberration of1 #ca2 vast distances of1 #ed2 precession affecting their longitudes1 #hj2 computations of their positions furnishing historical data1 #hh1 #hi2 made visible by refraction1 #aed2 peculiar law of light demonstrated by the aberration of1 #bjb2 magnitude of the solar system seen from1 #bbf2 numbers1 classification of1 #chd2 positions1 #che2 the ,milky ,way1 #che-chg2 parallaxes and distances of1 #chg-chi2 variable1 #cij-cie2 missing1 #cie2 systems of multiple1 classified1 ib42 binary1 #cie-djf 7see ,double stars72 nebulous1 #djf-dai 7see ,nebul72 seemingly innumerable1 #dbj2 meteors1 #dbj-dbc4 ,static electricity1 #bhb3 see ,electricity4 ,steam1 formation of1 #bfi2 force converting liquids into1 #bgg2 measure of its elasticity1 #bgh2 question of its being superseded by electricity1 #cbh4 ,steel1 paramagnetism induced in1 #ccf2 conditions of magnetic power remaining permanently in1 #ccg1 #cch2 its elasticity affected by magnetism1 #ceb4 ,stephenson1 ,george1 quotation from1 #bgi-bhj4 ,stokes1 ,professor1 remarks of1 on gradation of colours1 #afa2 experiments on fluorescence of light1 #aig2 his decision with regard to vibrations of polarised light1 #bbc4 ,storms1 magnetic1 #cdd2 varying with latitude1 #cde1 #cdf4 ,strata of the earth1 position and comparative density of1 #gg4 ,stratifications1 experiments showing1 in electric light1 #cjf1 #cjg4 ,struve1 ,m41 measurement by1 #dh2 his observations on ,saturn's rings1 #fh2 occultation by a comet observed by1 #cfd2 comet's nucleus described1 ib42 distance of a fixed star measured by1 #chh1 #chi2 catalogue of double stars1 #cif2 remarks on colour and light of double stars1 #dja2 sun's motion proved by1 #dje4 ,stutgardt1 natural hot springs used in manufactories near1 #bca4 ,submarine telegraph1 #cbe-cbg4 ,sulphate of magnesia1 its crystals boiled in alcohol1 #ajh4 ---- of nickel1 effect of exposure to the sun1 on its crystals1 #ajg4 ---- of soda1 its crystals1 #ajg4 ---- of zinc1 experiment on its crystals1 #ajh4 ,sulphuretted hydrogen gas1 its constituent parts1 #aaa4 ,sumbawa1 volcanic eruption of1 #bcc4 ,summer1 mean temperature of1 varying in the same latitude1 #bdf1 #bdg2 atmospheric electricity in1 #bia4 ,sun1 the1 law regulating his attraction of heavenly bodies1 #e2 effect of his attraction on planetary orbits1 mean distance of planets from1 #h2 importance of his magnitude in the solar system1 #ab2 disturbances in the relative positions of planets and1 #ad2 force modifying his intensity of attraction1 #af2 resistance offered by1 to the power of disturbing forces1 #bj2 periods of conjunctions of ,jupiter1 ,saturn1 and1 #be2 influence of1 on lunar motions1 #cd1 #ce2 action of the planets reflected by1 #cg2 eclipses of1 #dj1 #da2 supposed constitution of1 #da2 his atmosphere1 #db2 mode of finding his parallax1 #eb1 #ec2 mean distance from the earth1 #ec2 mass of1 #ee2 diameter1 #ef2 comparative density1 attractive force1 #ef1 #eg2 astronomical tables of1 #fc2 deductions from his rotation about an axis1 period of1 #fe2 attraction of1 producing a precession of the equinoxes1 #gi1 #ha2 returns of1 a measure of time1 #hc-he2 divisions of time1 dependent on revolutions of the major axis of his orbit1 #hf1 #hg2 action on tides1 #ib1 #ig2 disturbing the equilibrium of the atmosphere1 #aba2 dry and rainy seasons regulated by1 #abc2 cause of decreased light and heat in horizontal rays1 #aeg1 #aeh2 distance of1 falsely estimated1 #aeh2 light polarized by1 #aie2 indications of an absorptive atmosphere surrounding1 #bab1 #bac2 his diameter1 #bbd2 appearance of1 through his atmospheres1 ib42 variations in heat and light emitted from1 #bbe1 #bbf2 amount of heat annually received by the earth from1 #bch2 effect of his brilliancy on the heat emitted by1 #bei2 his position affecting variations in the magnetic elements1 #cdc1 #cdd2 connexion between periodic variation in his spots and in the magnetic elements1 #cdd2 vast sweep of his gravitating force1 #cfe2 increased attraction of1 for comets1 #cgb2 gulfs separating stars from1 #cij2 possibility of change in his lustre1 #cid2 spot on1 measured by ,sir ,john ,herschel1 #cid1 #cie2 proportion of his light to the moon's1 #djd2 rate and orbit of motion with his system1 #dje1 #djf2 a nebulous star1 #dab2 meteoric nebula revolving round1 #dbb2 gravitating force of1 #dbd1 #dbe4 ,sunbeams1 resolved into their component colours1 #aei-afb2 law prevailing in the phenomena of1 #aih2 light a distinct property of1 #bad2 resolved into three spectra1 #bbb2 undulations constituting1 #bbc2 their influence on vegetation1 #bdi4 ,swan1 the1 vanishing star in1 #cic4 ,switzerland1 meteors falling in1 #dba4 ,syene1 arc of the meridian measured between ,alexandria and1 #di4 ,sykes1 ,colonel1 extensive range of cultivation of wheat observed by1 #bej4 ,sympathetic vibrations in musical instruments1 #adg-adi4 ,syren1 the1 an instrument ascertaining the number of musical pulsations in a second1 #adc4 ,syzygies1 tides increased in the1 #if4 ,table-lands1 high1 influence of1 on the atmosphere1 #bda4 ,tahiti1 transit of ,venus observed at1 #ec4 ,tail of comets1 sudden development of1 #cgb2 forces producing1 #cge2 unequal illumination of1 #cge1 #cgf2 change in position of1 #cgf2 divided1 ib42 constitution of1 #cgg4 ,talbot1 ,fox1 his inventions in photography1 #bjd4 ,tangent1 a1 to planetary orbits1 planets impelled in the direction of1 #h2 force1 disturbing1 in the direction of1 #ad1 #ae2 deflection from1 a measurement of centrifugal force1 #di4 ,tangential force1 occasioning secular inequalities1 #ad2 effects produced by1 #ae2 producing the variation of the moon1 #ce2 force acting on the sea1 #ajj4 ---- velocity1 effects produced by modifications of1 #af2 undiminished by the ethereal medium1 #bb4 ,telegraph1 the electric1 discovery leading to the invention of1 #cbc1 #cbd2 the ,atlantic1 #cbe2 principles of its construction1 #cbf1 #cbg2 date of its completion1 #cbg4 ,telegraphs1 land1 principle of their construction1 #cbh4 ,telescope1 the achromatic1 principle of its construction1 #afd4 ----1 the differential1 differences in illumination determined by1 #bbg4 ----1 ,lord ,rosse's1 nebul resolved by1 #djg1 #dae4 ,telescopium1 comet traversing the constellation of1 #cgi2 nebula in1 #dad4 ,temperature1 a decrease in1 affecting the earth's rotation1 #gb2 excentricity of the terrestrial orbit1 a cause of decreasing1 #gc2 law equalising1 #gd2 geological changes affecting1 #ge4 ----1 varying in the terrestrial atmosphere1 zone of constant1 #aai2 affecting atmospheric undulations1 #aba2 modifying the velocity of sound1 #acd2 chemical action of light affected by1 #bah-bbb2 of the ethereal medium1 #bbg1 #bbh2 underground stratum of constant1 #bbh2 rate of increase in1 below the earth's crust1 #bbh1 #bca2 of the ocean1 #bca2 mode of finding annual average1 #bci2 causes of disturbance in regular variation of1 #bdj-bde2 variations in the same latitude1 #bdf1 #bdg2 influence of1 on vegetation1 #bdh2 affecting transmission of heat1 #bei1 #bfj2 of solid bodies1 caused by absorption of rays1 #bfh2 affecting the length of the pendulum1 #bgb2 causes of perpetual variations in1 #bgd2 transmission of electricity affected by1 #bhd2 affecting magnetism1 #ceb4 ,teneriffe1 the ,peak of1 prevailing winds on1 #abd2 lunar heat on1 #bbg2 zones of vegetation1 #bej2 character of its flora1 #beb4 ,terrestrial globe1 the1 a magnet1 #ccf4 ---- magnetism1 #cda-cdc2 the three elements and their variations1 #cdc1 #cdd2 storms1 period of their variation1 #cdd2 its connexion with solar magnetism1 ib42 effect of atmospheric magnetism on1 #cde2 probable cause of1 #cdf2 effect of planetary magnetism on1 #cdf1 #cdg4 ---- meridian1 a1 defined1 #df4 ,tessular system of crystallization1 #ajh4 ,texas1 monsoons occasioned by its deserts1 #abd4 ,thames1 the1 period occupied by the tidal wave in reaching1 #id4 ,thaw1 cause of the sensible chilliness of1 #bgf4 ,theory of probabilities1 use of1 in determining astronomical data1 #fj4 ,thermo-electric currents1 discovery of1 #ccb2 phenomena exhibited by1 #ccc2 principle of1 applied to measuring heat1 #ccc1 #ccd4 ,thermography1 examples of1 #bai-bba4 ,thermometer1 the1 principles applied to the construction of1 #aac2 consulted in determining mountain heights1 #aai1 #abj2 refraction varying with1 #aed2 heat measured by motion in1 #bgd4 ,thermomultiplier1 use of1 in experiments1 #bfd2 principle of its construction1 #ccc1 #ccd4 ,theta ,orionis1 the multiple system of1 #cie4 ,thomas1 ,st41 the island of1 hurricane with pauses at1 #abg4 ,thomson1 ,w41 ,professor1 experiments of1 in freezing water1 #bga2 dynamical theory of heat maintained by1 #bge note2 his calculation of the force exerted in vibrations of light1 #bgi2 investigation into the relations of light and magnetism1 #cbj2 density of the ethereal medium computed by1 #cef2 magnetic property of the ethereal medium pleaded for1 #ceg4 ,thunder1 theory of prolonged peals of1 #ach4 ,tibet1 wheat ripening in1 #bej4 ,tidal wave1 theory of1 #ib2 its birthplace1 #ic2 course of1 #ic1 #id2 velocity1 #id2 effect of depth on its motion1 #ie4 ,tides1 calculation from the moon's action on1 #ee2 theory of forces producing1 #ia1 #ib2 circumstances occasioning irregularities1 #ic2 rising1 progress of1 #ic1 #id2 three kinds of oscillations in1 #ie1 #if2 variations in1 from lunar and solar influence1 #if-ih2 effect of interference of waves on1 #ii2 the sea's equilibrium underanged by1 #ajj4 ----1 lunar and diurnal1 of the terrestrial atmosphere1 #aba2 examples of sympathetic undulation1 #adh4 ,time1 a measure of motion1 #eh2 a measure of angular motion1 #hc2 difference between mean and apparent solar1 #hd2 mean equinoctial1 mode of computing its object1 #hf2 estimation of1 corrected by means of laws of unequal expansion1 #bgb4 ,timocharis1 comparison of his observations with ,hipparchus1 #hj4 ,tomboro1 submerged in a volcanic eruption1 #bcc4 ,toronto1 observations on magnetic storms at1 #cdf4 ,torpedo1 the1 electrical action of1 #caj1 #caa4 ,torricellian vacuum1 experiment on the electric discharge in the1 #cjf2 lines of magnetic force passing through1 #cdd4 ,toucan1 comet approaching the constellation of1 #cgi2 a nebula in1 #dad4 ,toucani1 #dg2 globular nebulous cluster1 #dad4 ,tourmaline1 brown1 light polarized by prisms of1 #ahj2 property qualifying it to analyze polarized light1 #ahb2 coloured images produced by1 #ahf1 #ahg2 changed by compression1 #ahi2 heat polarized by1 #bfe2 electricity communicated to1 #bhd4 ,trade winds1 friction of1 not affecting the earth's velocity1 #gb2 action on the general motion of the sea1 #ajj2 system of1 accounting for atmospheric anomalies1 #abj2 theory of their origin1 phenomena connected with1 #abb1 #abc2 becoming monsoons1 #abd4 ,transits of ,venus1 #eb1 #ec4 ----1 two consecutive1 of any star1 a measure of time1 #hc4 ,transmission of radiant heat1 #beh1 #bfb2 of electricity1 #bhd1 #bhe2 of voltaic electricity1 #bih2 molecular structure affecting1 #cjc2 method of1 determining the influence of electric currents1 #cag2 of gravity1 an unsolved question1 #cee2 probable agent1 #cef2 medium of1 in space1 #dbd4 ,transparent bodies1 temperature of1 unaffected by the sun's rays1 #bbg4 ,trees1 number of species of forest1 found in ,america and ,europe1 #beb4 ,tribes1 apparently distinct1 of the human race1 #bee4 ,triple stars1 #cie2 periods of revolution in1 #djj4 ,tropical year1 change in its length1 #hj2 period of1 #hc2 difficulty of adjusting its estimation1 #he4 ---- revolution of the major axis of the solar ellipse1 its period1 #hf4 ---- vegetation1 the luxuriance of1 #bdh4 ,tuileries1 clock in the1 showing decimal time1 #hd4 ,twilight1 caused by refraction1 #aed2 effect of reflection1 #aeh4 ,tyndall1 ,professor1 his experiments proving diamagnetic polarity1 #cdh2 on magnetic action in crystals1 #cdi4 ,undulations1 theory of1 #ii2 of the atmosphere1 #aba1 #abb2 of the waves of sound1 #abi1 #acj2 intervals produced by interference1 #aci2 giving musical notes1 #adb1 #adc2 sympathetic1 #adg1 #adi2 of the luminous ether1 #afi1 #agj2 in refraction and reflection1 #agg2 producing fluorescence1 #aig2 different1 in light and sound1 #aii1 #bjj2 constituting a sunbeam1 #bbc2 heat propagated by1 #bfg2 of light1 evolution of latent force in extinguished1 #bgi1 #bhj2 of natural forces identical1 #bha4 ,undulatory theory of light1 #afh-agj2 law of motion affecting1 #agf1 #agg2 phenomena proving1 #aih2 objection1 from the different action of light and sound1 refuted1 #aii2 proving the existence of the ethereal medium1 #ceh2 acceleration in comet's motion proving1 #cfg4 ---- theory1 experiments determining in favour of1 #bjj1 #bja2 final and decisive experiment1 #bjb2 of heat1 #bfg4 ,unison1 note in1 #adb4 ,united ,states1 astronomical observations made in1 #cga1 #cgc4 ,uranium1 phosphorescent property of1 #bif2 peculiar luminous properties of1 #bif4 ,uranus1 effect of reciprocal attraction between ,neptune and1 #bb2 periods of the revolutions of his satellites1 #cc2 distance from the sun1 #ed2 astronomical tables of1 #fj2 discovery suggested by his perturbations1 #fa2 observations on1 leading to ,neptune's discovery1 #fb2 sun's influence in1 #bbe2 action of1 on ,halley's comet1 #cfc2 appearance of the sun to1 #chj1 #cha2 comets in his orbit1 #cha1 #chb4 ,ursa ,major1 periodic time of a double star in1 #cih2 nebulous region of1 #dag4 ,utah1 deserts of1 causing monsoons1 #abd4 ,vacuum produced by shell-fish1 #aag2 existing in the air1 #aah4 ,valz1 ,m41 telescopic planet discovered by1 #ba2 comet observed by1 #ceh2 observations on a comet's approach to the sun1 #cfd2 cause assigned by1 for contraction in diameter of comets1 #cgg1 #cgh4 ,vapour1 formation and dispersion of1 #bfi1 #bgj2 force developing1 #bgg4 ,variable stars1 periodic fluctuation of lustre in1 #cij1 #cia2 new1 appearing and vanishing1 #cib1 #cid2 missing1 #cie4 ,variables1 region of the1 #abb4 ,vegetation1 effect of1 in lowering temperature1 #bdc2 the two requisites for1 #bdh2 strength and vitality of1 #bdi2 chemical action of light influencing1 ib42 laws of its distribution1 #bdi-beb2 distribution of marine1 #beb1 #bec2 theories of specific diversity of original distribution of1 #bec1 #bed4 ,venus1 zone of instability between the sun and1 #ba2 perturbation in the mean motion of the earth and1 #bf2 eclipsing ,mercury1 #db2 transits of1 parallaxes calculated from1 #eb1 #ec2 astronomical tables of1 #fc2 climate1 #bbf4 ,vernal equinox1 planetary motions estimated from1 #i4 ,vesta1 astronomical tables of1 #fc2 no atmosphere surrounding1 #bbf4 ,vesuvius1 revived volcanic action of1 #bcd4 ,vibrating plates used in experiments on musical sound1 #add1 #adg4 ,vibrations of the air producing sound1 #abi2 in music1 #aca2 number made by the human voice in a second1 #acb4 ---- of the ether in natural and polarized light1 #aic2 in fluorescence of light1 #aif2 plane of1 in polarized light1 #bbc4 ,vico1 ,padre de1 comet discovered by1 #cgj4 ,vienna1 observations on comets from1 #cgj4 ,vietch1 ,james1 comet with luminous rings discovered by1 #cgd1 #cge4 ,vincent1 ,st41 revival of an extinct volcano in1 #bcd4 ,virginia1 daguerreotyped spectral image obtained in1 #bac4 ,virgo1 planetary conjunction between ,libra and1 #db2 variable star in1 #cib2 star vanished from1 #cie2 nebulous zone passing1 #daf1 #dag4 ,viviers1 transit of a comet across the sun observed from1 #cgd4 ,volcanic regions of the globe1 #bcb2 annual number of eruptions1 #bcc2 celebrated eruptions1 ib42 earthquakes caused by1 #bcd2 supposed causes of action1 #bce2 ,sir ,john ,herschel's theory1 #bce-bcg4 ,volta1 ,professor1 electricity rendered manageable by1 #big2 the world's debt to1 #cbh4 ,voltaic electricity1 first suggestions of1 #big2 theory of the transmission of1 #bih2 construction of the battery1 #bih1 #bii2 theory of its production1 #cjj2 characteristic properties1 #cjj1 #cja2 action of1 generating heat and light1 #cja-cjc2 arc1 experiments1 #cjc-cje2 the1 discharge oxidizing silver1 #cje1 #cjf2 stratified light1 #cjf1 #cjg2 chemical decomposition effected by agency of1 #cjg1 #cjh2 crystallization1 #cjh2 an agent in the fine arts1 #cji2 conductors of1 ib42 relations of heat and1 #caj2 fish producing effects of1 #caj1 #caa2 science suggested by its influence on a magnetized needle1 #cab2 rotation effected by1 #cac1 #cad2 inducing magnetism1 #cad1 #cae2 distinction between static electricity and1 #cag2 unvarying dual force of1 #ccd4 ,voltaic pile1 the1 invention of1 #big2 perfected1 #bih-cjj4 ,vortices1 molecular1 theory of1 #ajd4 ,vosges mountains1 temperature of mines in the1 #bbh4 ,vulpecula1 nebula in1 #dji4 ,wardhus1 transit of ,venus observed at1 #ec4 ,watches1 irregular action of1 corrected by the laws of unequal expansion1 #bgb4 ,water1 constituent parts of1 #aaa2 boiling point of1 an estimate of mountain heights1 #abj2 as a medium for sound1 #ace2 light polarized circularly by1 #aid2 experiment deciding the velocity of light in1 #bjb2 law of expansion of1 #bga2 process of congelation1 #bgf2 boiling points of1 #bgg2 decomposed by electric agency1 #cjg2 as an electric conductor1 #cji2 rotating by electricity1 #cad4 ,waterspouts1 origin and cause of1 #abh4 ,waterstone1 ,mr41 magnetic property of the ethereal medium maintained by1 #ceg4 ,waves neutralized by interference1 #ii4 ----1 atmospheric1 over local districts1 periods1 dimensions of1 #aba1 #abb4 ---- of sound1 #aca2 furnishing an illustration of reflections of sound and light1 #acg2 interference of1 producing calm1 #aci4 ,wedgwood1 ,dr41 attempts of1 to trace objects by means of light1 #bjc1 #bjd4 ,week1 the1 of seven days1 the most ancient and universal division of time1 #he4 ,wells1 increase of temperature in1 #bcj1 #bca4 ,welsh1 ,mr41 observations made by1 in a balloon ascent1 #aai4 ,west ,indies1 the1 cause of hurricanes in1 #abf4 ,wheels invented to test intensity of sound1 #acb1 #acc4 ,wheat1 range of its cultivation1 #bej4 ,wheatstone1 ,professor1 experiments in acoustics of1 #acb2 musical instruments invented by1 #adc2 paper on musical vibrations read by1 #ade2 experiments on sounding boards of1 #aej2 experiments on sound reinforced by resonance1 #aea2 instrument measuring velocities of electricity and light invented by1 #bjb2 spectrum of an electric spark observed1 #bhi2 speed of electricity measured1 #bhi1 #bij2 experiments on the spectrum of ,voltaic flame1 #cjc4 ,willis1 ,mr41 articulating machine invented by1 #aea2 investigations of1 into the mechanism of the larynx1 #aeb4 ,winds1 trade1 #abb1 #abc2 monsoons1 #abd2 extra-tropical1 in the ,north ,atlantic1 ib42 currents above the trade winds1 #abd1 #abe2 phenomena of rotatory motion1 #abe2 hurricanes1 #abe1 #abh2 agency of1 influencing temperature1 #bdd1 #bde4 ,wines1 range of cultivation of the best1 #bej4 ,winter1 atmospheric electricity in1 #bia4 ----1 mean temperature of1 varying in the same latitude1 #bdf1 #bdg4 ,wolf1 ,professor1 periods of variation in solar heat computed by1 #bbe4 ,wollaston1 ,dr41 experiments of1 on sensitiveness to sound1 quotation from1 #acb2 experiment of1 to show the effect of variable media on refraction1 #aef2 discovery of rayless lines in the solar spectrum1 #afb2 observations of1 on the chemical properties of the solar spectrum1 #bjc1 #bji2 magnetic rotation suggested by1 #cac2 light emitted by the heavenly bodies calculated1 #djd4 ,xi ,urs ,majoris1 periodic time of1 #cih2 velocity of the revolving star1 #djj4 ,year1 a1 in ,jupiter and ,saturn1 #ff2 tropical change in its length1 #hj2 length of the sidereal1 ib42 period of the mean1 #hc2 estimation of the ,egyptian1 #he2 first of our era1 #hf2 length of the1 affected by a comet's passage1 #cei4 ,young1 ,dr41 his calculation of the possible compression of solids1 #gh2 date of a horoscope determined by1 #hi2 density of a liquid column estimated by1 #aad2 exception adduced by1 to a general law in acoustics1 #acg2 his theory of the pleasures of harmony1 #adb2 undulatory theory established by1 #afi2 data used by1 to test his theory of light1 #age2 illustration of1 proving sound and heat kindred forces1 #bhj1 #bha4 ,zeta ,cancri1 a triple star1 #cie2 periodic time of1 #cih2 revolution1 #djj2 colours1 #dja4 ,zeta ,herculis1 periodic time1 eclipse of1 #cih2 light1 #djb4 ,zinc1 seleniate of1 effect of temperature on its crystals1 #ajg2 sulphate of1 its crystals boiled in alcohol1 #ajh4 ----1 electricity communicated to plates of1 #bbj4 ,zodiac1 the1 signs of1 change in their positions1 #hj4 ,zone of constant temperature in the atmosphere1 #aai2 laws of storms in the temperate and torrid1 #abg1 #abh2 of spots on the sun's surface1 its breadth1 #bbd2 of constant temperature below the earth's crust1 #bbh2 comparative unequal distribution of land in temperate and torrid1 #bdd2 of fixed stars1 #che2 of stars nearest the sun1 #cij2 nebulous1 #daf2 of nebulous patches1 #dag2 of meteoric nebul1 #dbc4 ,zones of instability of planetary orbits1 #ba4 ---- of temperature in the ocean1 #aja4 ---- of vegetation on the ,peak of ,teneriffe1 #bej4 ,zoophytes1 specific distribution of1 #bed4 ,,the ,,end4 ,,london3 ,,printed ,,by ,w4 ,,clowes ,,and ,,sons1 ,,stamford ,,street1 ,,and ,,charing ,,cross4 ,illustration3 ,,plate #a4 ,illustration3 ,,plate #b4 ,illustration3 ,,plate #c4 ,illustration3 ,,plate #d4 ,illustration3 ,,plate #e4 ,illustration3 ,,plate #f4 ,illustration3 ,,plate #g4 ,these correspond to ,no4 #a1 #f1 and #g of ,faraday's plate in his #bith ,series of ,experimental ,researches in ,electricity4 ,illustration3 ,,plate #h4 ,illustration3 ,,plate #i4 ,illustration3 ,,plate #aj4 ,fig4 #a4 ,spiral nebul of #ea ,messier1 as seen by ,lord ,rosse4 ,fig4 #b4 ,great nebula of ,orion4 ,footnotes ,footnote #a3 ,the mean distance of the earth from the sun is #ie1jjj1jjj miles1 but to avoid the inconvenience of large numbers1 it is assumed to be the unit of distance2 hence the mean distance of ,mars is #aebcfi1 or #ae nearly1 that of the earth being #a4 ,footnote #b3 ,the obliquity given in the text is for the year #aheh4 ,footnote #c3 ,sir ,john ,herschel remarks that there are just as many thousands of feet in a degree of the meridian in our latitude as there are days in the year1 viz4 #cfe1jjj4 ,the ,greenwich ,observatory is in ,n4 lat4 #ea #bhdj4 ,footnote #d3 ,or more correctly #cdbbcbe and #bch1gic miles1 as deduced from ,mr4 ,adams' more accurate calculations4 ,footnote #e3 ,neptune was discovered in the year #ahdf4 ,footnote #f3 ,the satellites of the two great planets on the farthest verge of the solar system form a singular exception to this law4 ,footnote #g3 ,see the chapter on the ,tides and ,currents in the #8,physical ,geography1' by the author1 #dth edition4 ,footnote #h3 ,sir ,john ,herschel on ,meteorology4 ,footnote #i3 ,bakerian ,lecture1 by ,michael ,faraday1 ,esq4 ,phil4 ,trans4 #aheg4 ,footnote #aj3 ,see page #ajd4 ,footnote #aa3 ,m4 ,marbach of ,breslau4 ,footnote #ab3 #8,meteorology1' by ,sir ,j4 ,herschel4 ,footnote #ac3 ,this theory of heat and motion originated with ,mr4 ,joule1 of ,manchester1 who has maintained it with the greatest talent1 both by experiment and analysis2 and it has had an able advocate in ,professor ,w4 ,thomson1 of ,glasgow4 ,footnote #ad3 ,to this remarkable man the world is indebted for the locomotive railway system1 which is rapidly advancing the civilization of mankind4 ,britain may well be proud of its working classes1 which can produce such men2 and ,mr4 ,george ,stephenson is not the only one2 there are many others2 but no man has ever had greater influence by his labours and discoveries on human affairs4 ,footnote #ae3 #8,correlation of the ,physical ,forces1 by ,w4 ,r4 ,grove1 ,esq41' one of the most remarkable and talented works that has appeared1 to which the author with pleasure acknowledges her obligations4 ,footnote #af3 8,eripuit fulmen ,clo1 sceptrumque tyrannis10 is the inscription on a medal struck in honour of ,franklin4 ,footnote #ag3 ,faraday4 ,footnote #ah3 ,professor ,matteucci still expresses doubts on this subject1 but has not yet finished his experiments4 ,footnote #ai3 ,babbage4 ,footnote #bj3 ,phil4 ,mag4 for ,may #aheh4 ,transcriber's ,notes ,some corrections were made to the original text4 ,in particular1 punctuation was corrected without further note4 ,inconsistent spelling and hyphenation was retained unless noted otherwise4 ,there were two ,notes #ahi in the original2 this was retained as printed4 ,spelling of ,index entries was changed to reflect the body text where inconsistencies were found4 ,index page numbers were corrected where errors were found4 ,further corrections are noted below3 p4 #ej #jjjeaddi - #jjjeaddi p4 #fa #bd1jjj - #bdj1jjj p4 #fb ,m4 ,leverrier - ,m4 ,le ,verrier p4 #hd in mean solar day - in a mean solar day p4 #if syzigies - syzygies p4 #aae arrising - arising p4 #abj ,herchel - ,herschel p4 #abc generally know - generally known p4 #aei ,fraunhoffer's - ,fraunhofer's p4 #afh contaary - contrary p4 #bad oxyde - oxide p4 #baf aperature - aperture p4 #bif ,m ,niepce - ,m4 ,niepc p4 #cjf torrecelian - ,torricellian p4 #cjg potass - potash p4 #cej de ,roux - le ,roux p4 #dbc - p4 #ddg areal - arial p4 #def perigree - perigee p4 #dga #ajh - #ahj p4 #dgh ,meissier - ,messier