,,man',s ,,place ,,in ,,the ,,universe ,,man',s ,,place ,,in ,,the ,,universe ,a ,study of the ,results of ,scientific ,research in ,relation to the ,unity or ,plurality of ,worlds ,,by ,,alfred ,r4 ,,wallace ,,ll4,d41 ,d4,c4,l41 ,f4,r4,s41 ,,etc4 ',o1 glittering host6 ,o1 golden line6 ,i would ,i had an angel's ken1 ,your deepest secrets to divine1 ,and read your mysteries to men4' ,,third ,,edition ,,london ,,chapman ,,and ,,hall ,,limited #aijd ',i said unto my inmost heart1 ,shall ,i don corslet1 helm1 and shield1 ,and shall ,i with a ,giant strive1 ,and charge a ,dragon on the field8' ,j4,h4 ,,dell4 ,,preface ,this work has been written in consequence of the great interest excited by my article1 under the same title1 which appeared simultaneously in ,the ,fortnightly ,review and the ,new ,york ,independent4 ,two friends who read the manuscript were of opinion that a volume1 in which the evidence could be given much more fully1 would be desirable1 and the result of the publication of the article confirmed their view4 ,i was led to a study of the subject when writing four new chapters on ,astronomy for a new edition of ,the ,wonderful ,century4 ,i then found that almost all writers on general astronomy1 from ,sir ,john ,herschel to ,professor ,simon ,newcomb and ,sir ,norman ,lockyer1 stated1 as an indisputable fact1 that our sun is situated in the plane of the great ring of the ,milky ,way1 and also very nearly in the centre of that ring4 ,the most recent researches also showed that there was little or no proof of there being any stars or nebul very far beyond the ,milky ,way1 which thus seemed to be the limit1 in that direction1 of the stellar universe4 ,turning to the earth and the other planets of the ,solar ,system1 ,i found that the most recent researches led to the conclusion that no other planet was likely to be the seat of organic life1 unless perhaps of a very low type4 ,for many years ,i had paid special attention to the problem of the measurement of geological time1 and also that of the mild climates and generally uniform conditions that had prevailed throughout all geological epochs2 and on considering the number of concurrent causes and the delicate balance of conditions required to maintain such uniformity1 ,i became still more convinced that the evidence was exceedingly strong against the probability or possibility of any other planet being inhabited4 ,having long been acquainted with most of the works dealing with the question of the supposed ,plurality of ,worlds1 ,i was quite aware of the very superficial treatment the subject had received1 even in the hands of the most able writers1 and this made me the more willing to set forth the whole of the available evidence--astronomical1 physical1 and biological--in such a way as to show both what was proved and what suggested by it4 ,the present work is the result1 and ,i venture to think that those who will read it carefully will admit that it is a book that was worth writing4 ,it is founded almost entirely on the marvellous body of facts and conclusions of the ,new ,astronomy together with those reached by modern physicists1 chemists1 and biologists4 ,its novelty consists in combining the various results of these different branches of science into a connected whole1 so as to show their bearing upon a single problem--a problem which is of very great interest to ourselves4 ,this problem is1 whether or no the logical inferences to be drawn from the various results of modern science lend support to the view that our earth is the only inhabited planet1 not only in the ,solar ,system but in the whole stellar universe4 ,of course it is a point as to which absolute demonstration1 one way or the other1 is impossible4 ,but in the absence of any direct proofs1 it is clearly rational to inquire into probabilities2 and these probabilities must be determined not by our prepossessions for any particular view1 but by an absolutely impartial and unprejudiced examination of the tendency of the evidence4 ,as the book is written for the general1 educated body of readers1 many of whom may not be acquainted with any aspect of the subject or with the wonderful advance of recent knowledge in that department often termed the ,new ,astronomy1 a popular account has been given of all those branches of it which bear upon the special subject here discussed4 ,this part of the work occupies the first six chapters4 ,those who are fairly acquainted with modern astronomical literature1 as given in popular works1 may begin at my seventh chapter1 which marks the commencement of the considerable body of evidence and of argument ,i have been able to adduce4 ,to those of my readers who may have been influenced by any of the adverse criticisms on my views as set forth in the article already referred to1 ,i must again urge1 that throughout the whole of this work1 neither the facts nor the more obvious conclusions from the facts are given on my own authority1 but always on that of the best astronomers1 mathematicians1 and other men of science to whose works ,i have had access1 and whose names1 with exact references1 ,i generally give4 ,what ,i claim to have done is1 to have brought together the various facts and phenomena they have accumulated2 to have set forth the hypotheses by which they account for them1 or the results to which the evidence clearly points2 to have judged between conflicting opinions and theories2 and lastly1 to have combined the results of the various widely-separated departments of science1 and to have shown how they bear upon the great problem which ,i have here endeavoured1 in some slight degree1 to elucidate4 ,as such a large body of facts and arguments from distinct sciences have been here brought together1 ,i have given a rather full summary of the whole argument1 and have stated my final conclusions in six short sentences4 ,i then briefly discuss the two aspects of the whole problem--those from the materialistic and from the spiritualistic points of view2 and ,i conclude with a few general observations on the almost unthinkable problems raised by ideas of ,infinity--problems which some of my critics thought ,i had attempted in some degree to deal with1 but which1 ,i here point out1 are altogether above and beyond the questions ,i have discussed1 and equally above and beyond the highest powers of the human intellect4 ,,broadstone1 ,,dorset1 ,september #aijc4 ',the wilder'd mind is tost and lost1 ,o sea1 in thy eternal tide2 ,the reeling brain essays in vain1 ,o stars1 to grasp the vastness wide6 ,the terrible tremendous scheme ,that glimmers in each glancing light1 ,o night1 ,o stars1 too rudely jars ,the finite with the infinite6' ,j4,h4 ,,dell4 ,,contents ,,chap4 ,,page ,i4 ,,early ,,ideas1 #a ,,ii4 ,,modern ,,ideas1 #g ,,iii4 ,,the ,,new ,,astronomy1 #bd ,,iv4 ,,the ,,distribution ,,of ,,the ,,stars1 #dg ,v4 ,,distances ,,of ,,stars3 ,,the ,,sun',s ,,motion1 #gc ,,vi4 ,,unity ,,and ,,evolution ,,of ,,the ,,star-,,system1 #ii ,,vii4 ,,are ,,the ,,stars ,,infinite8 #ace ,,viii4 ,,our ,,relation ,,to ,,the ,,milky ,,way1 #aef ,,ix4 ,,the ,,uniformity ,,of ,,matter ,,and ,,its ,,laws1 #ahc ,x4 ,,the ,,essential ,,characters ,,of ,,organisms1 #aia ,,xi4 ,,physical ,,conditions ,,essential ,,for ,,life1 #bjf ,,xii4 ,,the ,,earth ,,in ,,relation ,,to ,,life1 #bah ,,xiii4 ,,the ,,atmosphere ,,in ,,relation ,,to ,,life1 #bdc ,,xiv4 ,,the ,,other ,,planets ,,are ,,not ,,habitable1 #bfb ,,xv4 ,,the ,,stars3 ,,have ,,they ,,planets8 ,,are ,,they ,,useful ,,to ,,us8 #bhb ,,xvi4 ,,stability ,,of ,,the ,,star-,,system3 ,,importance ,,of ,,central ,,position3 ,,summary ,,and ,,conclusion1 #bie ,,index1 #cbf ,,eight ,,diagrams ,,in ,,the ,,text ,,and ,,two ,,star ,,charts ,,at ,,end4 ',who is man1 and what his place8 ,anxious asks the heart1 perplext ,in this recklessness of space1 ,worlds with worlds thus intermixt3 ,what has he1 this atom creature1 ,in the infinitude of ,nature8' ,f4,t4 ,,palgrave4 ,,man',s ,,place ,,in ,,the ,,universe ,,chapter ,i ,,early ,,ideas ,,as ,,to ,,the ,,universe ,,and ,,its ,,relation ,,to ,,man ,when men attained to sufficient intelligence for speculations as to their own nature and that of the earth on which they lived1 they must have been profoundly impressed by the nightly pageant of the starry heavens4 ,the intense sparkling brilliancy of ,sirius and ,vega1 the more massive and steady luminosity of ,jupiter and ,venus1 the strange grouping of the brighter stars into constellations to which fantastic names indicating their resemblance to various animals or terrestrial objects seemed appropriate and were soon generally adopted1 together with the apparently innumerable stars of less and less brilliancy scattered broadcast over the sky1 many only being visible on the clearest nights and to the acutest vision1 constituted altogether a scene of marvellous and impressive splendour of which it must have seemed almost impossible to attain any real knowledge1 but which afforded an endless field for the imagination of the observer4 ,the relation of the stars to the sun and moon in their respective motions was one of the earliest problems for the astronomer1 and it was only solved by careful and continuous observation1 which showed that the invisibility of the former during the day was wholly due to the blaze of light1 and this is said to have been proved at an early period by the observed fact that from the bottom of very deep wells stars can be seen while the sun is shining4 ,during total eclipses of the sun also the brighter stars become visible1 and1 taken in connection with the fixity of position of the pole-star1 and the course of those circumpolar stars which never set in the latitudes of ,greece1 ,egypt1 and ,chaldea1 it soon became possible to frame a simple hypothesis which supposed the earth to be suspended in space1 while at an unknown distance from it a crystal sphere revolved upon an axis indicated by the pole-star1 and carried with it the whole host of heavenly bodies4 ,this was the theory of ,anaximander 7#edj ,b4,c471 and it served as the starting-point for the more complex theory which continued to be held in various forms and with endless modifications down to the end of the sixteenth century4 ,it is believed that the early ,greeks obtained some knowledge of astronomy from the ,chaldeans1 who appear to have been the first systematic observers of the heavenly bodies by means of instruments1 and who are said to have discovered the cycle of eighteen years and ten days after which the sun and moon return to the same relative positions as seen from the earth4 ,the ,egyptians perhaps derived their knowledge from the same source1 but there is no proof that they were great observers1 and the accurate orientation1 proportions1 and angles of the ,great ,pyramid and its inner passages may perhaps indicate a ,chaldean architect4 ,the very obvious dependence of the whole life of the earth upon the sun1 as a giver of heat and light1 sufficiently explains the origin of the belief that the latter was a mere appanage of the former2 and as the moon also illuminates the night1 while the stars as a whole also give a very perceptible amount of light1 especially in the dry climate and clear atmosphere of the ,east1 and when compared with the pitchy darkness of cloudy nights when the moon is below the horizon1 it seemed clear that the whole of these grand luminaries--sun1 moon1 stars1 and planets--were but parts of the terrestrial system1 and existed solely for the benefit of its inhabitants4 ,empedocles 7#ddd ,b4,c47 is said to have been the first who separated the planets from the fixed stars1 by observing their very peculiar motions1 while ,pythagoras and his followers determined correctly the order of their succession from ,mercury to ,saturn4 ,no attempt was made to explain these motions till a century later1 when ,eudoxus of ,cnidos1 a contemporary of ,plato and of ,aristotle1 resided for some time in ,egypt1 where he became a skilful astronomer4 ,he was the first who systematically worked out and explained the various motions of the heavenly bodies on the theory of circular and uniform motion round the earth as a centre1 by means of a series of concentric spheres1 each revolving at a different rate and on a different axis1 but so united that all shared in the motion round the polar axis4 ,the moon1 for example1 was supposed to be carried by three spheres1 the first revolved parallel to the equator and accounted for the diurnal motion--the rising and setting--of the moon2 another moved parallel to the ecliptic and explained the monthly changes of the moon2 while the third revolved at the same rate but more obliquely1 and explained the inclination of the moon's orbit to that of the earth4 ,in the same way each of the five planets had four spheres1 two moving like the first two of the moon1 another one also moving in the ecliptic was required to explain the retrograde motion of the planets1 while a fourth oblique to the ecliptic was needed to explain the diverging motions due to the different obliquity of the orbit of each planet to that of the earth4 ,this was the celebrated ,ptolemaic system in the simplest form needed to account for the more obvious motions of the heavenly bodies4 ,but in the course of ages the ,greek and ,arabian astronomical observers discovered small divergences due to the various degrees of excentricity of the orbits of the moon and planets and their consequent varying rates of motion2 and to explain these other spheres were added1 together with smaller circles sometimes revolving excentrically1 so that at length about sixty of these spheres1 epicycles and excentrics were required to account for the various motions observed with the rude instruments1 and the rates of motion determined by the very imperfect time-measurers of those early ages4 ,and although a few great philosophers had at different times rejected this cumbrous system and had endeavoured to promulgate more correct ideas1 their views had no influence on public opinion even among astronomers and mathematicians1 and the ,ptolemaic system held full sway down to the time of ,copernicus1 and was not finally given up till ,kepler's ,laws and ,galileo's ,dialogues compelled the adoption of simpler and more intelligible theories4 ,we are now so accustomed to look upon the main facts of astronomy as mere elementary knowledge that it is difficult for us to picture to ourselves the state of almost complete ignorance which prevailed even among the most civilised nations throughout antiquity and the ,middle ,ages4 ,the rotundity of the earth was held by a few at a very early period1 and was fairly well established in later classical times4 ,the rough determination of the size of our globe followed soon after2 and when instrumental observations became more perfect1 the distance and size of the moon were measured with sufficient accuracy to show that it was very much smaller than the earth4 ,but this was the farthest limit of the determination of astronomical sizes and distances before the discovery of the telescope4 ,of the sun's real distance and size nothing was known except that it was much farther from us and much larger than the moon2 but even in the century before the commencement of the ,christian era ,posidonius determined the circumference of the earth to be #bdj1jjj stadia1 equal to about #bh1fjj miles1 a wonderfully close approximation considering the very imperfect data at his command4 ,he is also said to have calculated the sun's distance1 making it only one-third less than the true amount1 but this must have been a chance coincidence1 since he had no means of measuring angles more accurately than to one degree1 whereas in the determination of the sun's distance instruments are required which measure to a second of arc4 ,before the discovery of the telescope the sizes of the planets were quite unknown1 while the most that could be ascertained about the stars was1 that they were at a very great distance from us4 ,this being the extent of the knowledge of the ancients as to the actual dimensions and constitution of the visible universe1 of which1 be it remembered1 the earth was held to be the centre1 we cannot be surprised at the almost universal belief that this universe existed solely for the earth and its inhabitants4 ,in classical times it was held to be at once the dwelling-place of the gods and their gift to man1 while in ,christian ages this belief was but slightly1 if at all1 changed2 and in both it would have been considered impious to maintain that the planets and stars did not exist for the service and delight of mankind alone but in all probability had their own inhabitants1 who might in some cases be even superior in intellect to man himself4 ,but apparently1 during the whole period of which we are now treating1 no one was so daring as even to suggest that there were other worlds with other inhabitants1 and it was no doubt because of the idea that we occupied the world1 the very centre of the whole surrounding universe which existed solely for us1 that the discoveries of ,copernicus1 ,tycho ,brah1 ,kepler1 and ,galileo excited so much antagonism and were held to be impious and altogether incredible4 ,they seemed to upset the whole accepted order of nature1 and to degrade man by removing his dwelling-place1 the earth1 from the commanding central position it had always before occupied4 ,,chapter ,,ii ,,modern ,,ideas ,,as ,,to ,,man',s ,,relation ,,to ,,the ,,universe ,the beliefs as to the subordinate position held by sun1 moon1 and stars in relation to the earth1 which were almost universal down to the time of ,copernicus1 began to give way when the discoveries of ,kepler and the revelations of the telescope demonstrated that our earth was not specially distinguished from the other planets by any superiority of size or position4 ,the idea at once arose that the other planets might be inhabited2 and when the rapidly increasing power of the telescope1 and of astronomical instruments generally1 revealed the wonders of the solar system and the ever-increasing numbers of the fixed stars1 the belief in other inhabited worlds became as general as the opposite belief had been in all preceding ages1 and it is still held in modified forms to the present day4 ,but it may be truly said that the later like the earlier belief is founded more upon religious ideas than upon a scientific and careful examination of the whole of the facts both astronomical1 physical1 and biological1 and we must agree with the late ,dr4 ,whewell1 that the belief that other planets are inhabited has been generally entertained1 not in consequence of physical reasons but in spite of them4 ,and he adds3--',it was held that ,venus1 or that ,saturn was inhabited1 not because anyone could devise1 with any degree of probability1 any organised structure which would be suitable to animal existence on the surfaces of those planets2 but because it was conceived that the greatness or goodness of the ,creator1 or ,his wisdom1 or some other of ,his attributes1 would be manifestly imperfect1 if these planets were not tenanted by living creatures4' ,those persons who have only heard that many eminent astronomers down to our own day have upheld the belief in a ',plurality of ,worlds' will naturally suppose that there must be some very cogent arguments in its favour1 and that it must be supported by a considerable body of more or less conclusive facts4 ,they will therefore probably be surprised to hear that any direct evidence which may be held to support the view is almost wholly wanting1 and that the greater part of the arguments are weak and flimsy in the extreme4 ,of late years1 it is true1 some few writers have ventured to point out how many difficulties there are in the way of accepting the belief1 but even these have never examined the question from the various points of view which are essential to a proper consideration of it2 while1 so far as it is still upheld1 it is thought sufficient to show1 that in the case of some of the planets1 there seem to be such conditions as to render life possible4 ,in the millions of planetary systems supposed to exist it is held to be incredible that there are not great numbers as well fitted to be inhabited by animals of all grades1 including some as high as man or even higher1 and that we must1 therefore1 believe that they are so inhabited4 ,as in the present work ,i propose to show1 that the probabilities and the weight of direct evidence tend to an exactly opposite conclusion1 it will be well to pass briefly in review the various writers on the subject1 and to give some indication of the arguments they have used and the facts they have set forth4 ,for the earlier upholders of the theory ,i am indebted to ,dr4 ,whewell1 who1 in his ,dialogue on the ,plurality of ,worlds--a ,supplement to his well-known volume on the subject--refers to all writers of importance known to him4 ,the earliest are the great astronomers ,kepler and ,huygens1 and the learned ,bishop ,wilkins1 who all believed that the moon was or might probably be inhabited2 and of these ,whewell considers ,wilkins to have been by far the most thoughtful and earnest in supporting his views4 ,then we have ,sir ,isaac ,newton himself who1 at considerable length1 argued that the sun was probably inhabited4 ,but the first regular work devoted to the subject appears to have been written by ,m4 ,fontenelle1 ,secretary to the ,academy of ,sciences in ,paris1 who in #afhf published his ,conversations on the ,plurality of ,worlds4 ,the book consisted of five chapters1 the first explaining the ,copernican ,theory2 the second maintaining that the moon is a habitable world2 the third gives particulars as to the moon1 and argues that the other planets are also inhabited2 the fourth gives details as to the worlds of the five planets2 while the fifth declares that the fixed stars are suns1 and that each illuminates a world4 ,this work was so well written1 and the subject proved so attractive1 that it was translated into all the chief ,european languages1 while the astronomer ,lalande edited one of the ,french editions4 ,three ,english translations were published1 and one of these went through six editions down to the year #agcg4 ,the influence of this work was very great and no doubt led to that general acceptance of the theory by such men as ,sir ,william ,herschel1 ,sir ,john ,herschel1 ,dr4 ,chalmers1 ,dr4 ,dick1 ,dr4 ,isaac ,taylor1 and ,m4 ,arago1 although it was wholly founded on pure speculation1 and there was nothing that could be called evidence on one side or the other4 ,this was the state of public opinion when an anonymous work appeared 7in #ahec7 under the somewhat misleading title of ,the ,plurality of ,worlds3 ,an ,essay4 ,this was written1 as already stated1 by ,dr4 ,whewell1 who1 for the first time1 ventured to doubt the generally accepted theory1 and showed that all the evidence at our command led to the conclusion that some of the planets were certainly not habitable1 that others were probably not so1 while in none was there that close correspondence with terrestrial conditions which seemed essential for their habitability by the higher animals or by man4 ,the book was ably written and showed considerable knowledge of the science of the time1 but it was very diffuse1 and the larger part of it was devoted to showing that his views were not in any way opposed to religion4 ,one of his best arguments was founded on the proposition that 'the ,earth's ,orbit is the ,temperate ,zone of the ,solar ,system1' that there only is it possible to have those moderate variations of heat and cold1 dryness and moisture1 which are suitable for animal life4 ,he suggested that the outer planets of the system consisted mainly of water1 gases1 and vapour1 as indicated by their low specific gravity1 and were therefore quite unsuitable for terrestrial life2 while those near the sun were equally unsuited1 because1 owing to the great amount of solar heat1 water could not exist on their surfaces4 ,he devotes a great deal of space to the evidence that there is no animal life on the moon1 and taking this as proved1 he uses it as a counter argument against the other side4 ,they always urge that1 the earth being inhabited1 we must suppose the other planets to be so too2 to which he replies3--,we know that the moon is not inhabited though it has all the advantage of proximity to the sun that the earth has2 why then should not other planets be equally uninhabited8 ,he then comes to ,mars and admits that this planet is very like the earth so far as we can judge1 and that it may therefore be inhabited1 or as the author expresses it1 'may have been judged worthy of inhabitants by its ,maker4' ,but he urges the small size of ,mars1 its coldness owing to distance from the sun1 and that the annual melting of its polar ice-caps will keep it cold all through the summer4 ,if there are animals they are probably of a low type like the saurians and iguanodons of our seas during the ,wealden epoch2 but1 he argues1 as even on our earth the long process of preparation for man was carried on for countless millions of years1 we need not discuss whether there are intelligent beings on ,mars till we have some better evidence that there are any living creatures at all4 ,several of the early chapters are devoted to an attempt to minimise the difficulties of those religious persons who feel oppressed by the immensity and complexity of the material universe as revealed by modern astronomy2 and by the almost infinite insignificance of man and his dwelling-place1 the earth1 in comparison with it1 an insignificance vastly increased if not only the planets of the solar system1 but also those which circle around the myriads of suns1 are also theatres of life4 ,and these persons are further disquieted because the very same facts are used by sceptics of various kinds in their attacks upon ,christianity4 ,such writers point out the irrationality and absurdity of supposing that the ,creator of all this unimaginable vastness of suns and systems1 filling for all we know endless space1 should take any special interest in so mean and pitiful a creature as man1 the imperfectly developed inhabitant of one of the smaller worlds attached to a second or third-rate sun1 a being whose whole history is one of war and bloodshed1 of tyranny1 torture1 and death2 whose awful record is pictured by himself in such books as ,josephus' ,history of the ,jews1 the ,decline and ,fall of the ,roman ,empire1 and even more forcibly summarised in that terrible picture of human fiendishness and misery1 ,the ,martyrdom of ,man2 while their character is indicated by one of the kindest and simplest of their poets in the restrained but expressive lines3-- ',man's inhumanity to man ,makes countless thousands mourn4' ,it is for such a being as this1 they say1 that ,god should have specially revealed ,his will some thousands of years ago1 and finding that ,his commands were not obeyed1 ,his will not fulfilled1 yet ordained for their benefit the necessarily unique sacrifice of ,his ,son1 in order to save a small portion of these 'miserable sinners' from the natural and well-deserved consequence of their stupendous follies1 their unimaginable crimes8 ,such a belief they maintain is too absurd1 too incredible1 to be held by any rational being1 and it becomes even less credible and less rational if we maintain that there are countless other inhabited worlds4 ,it is very difficult for the religious man to make any adequate reply to such an attack as this1 and as a result many have felt their position to be untenable and have accordingly lost all faith in the special dogmas of orthodox ,christianity4 ,they feel themselves really to be between the horns of a dilemma4 ,if there are myriads of other worlds1 it seems incredible that they should each be the object of a special revelation and a special sacrifice4 ,if1 on the other hand1 we are the only intelligent beings that exist in the material universe1 and are really the highest creative product of a ,being of infinite wisdom and power1 they cannot but wonder at the vast apparent disproportion between the ,creator and the created1 and are sometimes driven to ,atheism from the hopelessness of comprehending so mean and petty a result as the sole outcome of infinite power4 ,whewell tells us that the great preacher1 ,dr4 ,chalmers1 in his ,astronomical ,discourses1 attempted a reply to these difficulties1 but1 in his opinion1 not a very successful one2 and a large part of his own work is devoted to the same purpose4 ,his main point seems to be that we know too little of the universe to arrive at any definite conclusions on the question at issue1 and that any ideas that we may have as to the purposes of the ,creator in forming the vast system we see around us are almost sure to be erroneous4 ,we must therefore be content to remain ignorant1 and must rest satisfied in the belief that the ,creator had a purpose although we are not yet permitted to know what it was4 ,and to those who urge that in other worlds there may be other laws of nature which may render them quite as habitable by intelligent beings as our world is for us1 he replies1 that if we are to suppose new laws of nature in order to render each planet habitable1 there is an end of all rational inquiry on the subject1 and we may maintain and believe that animals may live on the moon without air or water1 and on the sun exposed to heat which vaporises earths and metals4 ,his concluding argument1 and perhaps one of his strongest1 is that founded upon the dignity of man1 as conferring a pre-eminence upon the planet which has produced him4 ',if1' he says1 'man be not merely capable of ,virtue and ,duty1 of universal ,love and ,self-,devotion1 but be also immortal2 if his being be of infinite duration1 his soul created never to die2 then1 indeed1 we may well say that one soul outweighs the whole unintelligent creation4' ,and then1 addressing the religious world1 he urges that1 if1 as they believe1 ,god has redeemed man by the sacrifice of ,his ,son1 and has given to him a revelation of ,his will1 then indeed no other conception is possible than that he is the sole and highest product of the universe4 ',the elevation of millions of intellectual1 moral1 religious1 spiritual creatures1 to a destiny so prepared1 consummated1 and developed1 is no unworthy occupation of all the capacities of space1 time1 and matter4' ,then with a chapter on ',the ,unity of the ,world1' and one on ',the ,future1' neither of which contains anything which adds to the force of his argument1 the book ends4 ,the publication of this able if rather vague and diffuse work1 contesting popular opinions1 was followed by a burst of indignant criticism on the part of a man of considerable eminence in some branches of physics--,sir ,david ,brewster1 but who was very inferior1 both in general knowledge of science and in literary skill1 to the writer whose views he opposed4 ,the purport of the book in which he set forth his objections is indicated by its title--,more ,worlds than ,one1 the ,creed of the ,philosopher and the ,hope of the ,christian4 ,though written with much force and conviction it appeals mainly to religious prejudices1 and assumes throughout that every planet and star is a special creation1 and that the peculiarities of each were designed for some special purpose4 ',if1' he says1 'the moon had been destined to be merely a lamp to our earth1 there was no occasion to variegate its surface with lofty mountains and extinct volcanoes1 and cover it with large patches of matter that reflect different quantities of light and give its surface the appearance of continents and seas4 ,it would have been a better lamp had it been a smooth piece of lime or of chalk4' ,it is1 therefore1 he thinks1 prepared for inhabitants2 and then he argues that all the other satellites are also inhabited4 ,again he says that 'when it was found that ,venus was about the same size as the ,earth1 with mountains and valleys1 days and nights1 and years analogous to our own1 the absurdity of believing that she had no inhabitants1 when no other rational purpose could be assigned for her creation1 became an argument of a certain amount that she was1 like the ,earth1 the seat of animal and vegetable life4' ,then1 when it was found that ,jupiter was so gigantic 'as to require four moons to give him light1 the argument from analogy that he was inhabited became stronger also1 because it extended to two planets4' ,and thus each successive planet having certain points of analogy with the others becomes an additional argument2 so that when we take account of all the planets1 with atmosphere1 and clouds1 and arctic snows1 and trade-winds1 the argument from analogy becomes1 he urges1 very powerful2--'and the absurdity of the opposite opinion1 that planets should have moons and no inhabitants1 atmospheres with no creatures to breathe in them1 and currents of air without life to be fanned1 became a formidable argument which few minds1 if any1 could resist4' ,the work is full of such weak and fallacious rhetoric and even1 if possible1 still weaker4 ,thus after describing double stars1 he adds--',but no person can believe that two suns could be placed in the heavens for no other purpose than to revolve round their common centre of gravity'2 and he concludes his chapter on the stars thus3--',wherever there is matter there must be ,life2 ,life ,physical to enjoy its beauties--,life ,moral to worship its ,maker1 and ,life ,intellectual to proclaim ,his wisdom and ,his power4' ,and again--',a house without tenants1 a city without citizens1 presents to our minds the same idea as a planet without life1 and a universe without inhabitants4 ,why the house was built1 why the city was founded1 why the planet was made1 and why the universe was created1 it would be difficult even to conjecture4' ,arguments of this kind1 which in almost every case beg the question at issue1 are repeated ad nauseam4 ,but he also appeals to the ,old ,testament to support his views1 by quoting the fine passage in the ,psalms--',when ,i consider ,thy heavens the work of ,thy fingers1 the moon and the stars which ,thou hast ordained2 what is man that ,thou art mindful of him8' on which he remarks--',we cannot doubt that inspiration revealed to him ,david the magnitude1 the distances1 and the final cause1 of the glorious spheres which fixed his admiration4' ,and after quoting various other passages from the prophets1 all as he thinks supporting the same view1 he sets forth the extraordinary idea as a confirmatory argument1 that the planets or some of them are to be the future abode of man4 ,for1 he says--',man in his future state of existence is to consist1 as at present1 of a spiritual nature residing in a corporeal frame4 ,he must live1 therefore1 upon a material planet1 subject to all the laws of matter4' ,and he concludes thus3--',if there is not room1 then1 on our globe for the millions of millions of beings who have lived and died on its surface1 we can scarcely doubt that their future abode must be on some of the primary or secondary planets of the solar system1 whose inhabitants have ceased to exist1 or upon planets which have long been in a state of preparation1 as our earth was1 for the advent of intellectual life4' ,it is pleasant to turn from such weak and trivial arguments to the only other modern works which deal at some length with this subject1 the late ,richard ,a4 ,proctor's ,other ,worlds than ,ours1 and a volume published five years later under the title--,our ,place ,among ,infinities4 ,written as these were by one of the most accomplished astronomers of his day1 remarkable alike for the acuteness of his reasoning and the clearness of his style1 we are always interested and instructed even when we cannot agree with his conclusions4 ,in the first work mentioned above1 he assumes1 like ,sir ,david ,brewster1 the antecedent probability that the planets are inhabited and on much the same theological grounds4 ,so strongly does he feel this that he continually speaks as if the planets must be inhabited unless we can show very good reason that they cannot be so1 thus throwing the burden of proving a negative on his opponents1 while he does not attempt to prove his positive contention that they are inhabited1 except by purely hypothetical considerations as to the ,creator's purpose in bringing them into existence4 ,but starting from this point he endeavours to show how ,whewell's various difficulties may be overcome1 and here he always appeals to astronomical or physical facts1 and reasons well upon them4 ,but he is quite honest2 and1 coming to the conclusion that ,jupiter and ,saturn1 ,uranus and ,neptune1 cannot be habitable1 he adduces the evidence and plainly states the result4 ,but then he thinks that the satellites of ,jupiter and ,saturn may be habitable1 and if they may be1 then he concludes that they must4 ,one great oversight in his whole argument is1 that he is satisfied with showing the possibility that life may exist now1 but never deals with the question of whether life could have been developed from its earliest rudiments up to the production of the higher vertebrates and man2 and this1 as ,i shall show later1 is the crux of the whole problem4 ,with regard to the other planets1 after a careful examination of all that is known about them1 he arrives at the conclusion that if ,mercury is protected by a cloud-laden atmosphere of a peculiar kind it may possibly1 but not probably1 support high forms of animal life4 ,but in the case of ,venus and ,mars he finds so much resemblance to and so many analogies with our earth1 that he concludes that they almost certainly are so4 ,in the case of the fixed stars1 now that we know by spectroscopic observations that they are true suns1 many of which closely resemble our sun and give out light and heat as he does1 ,mr4 ,proctor argues1 that ',the vast supplies of heat thus emitted by the stars not only suggest the conclusion that there must be worlds around these orbs for which these heat-supplies are intended1 but point to the existence of the various forms of force into which heat may be transmuted4 ,we know that the sun's heat poured upon our earth is stored up in vegetable and animal forms of life2 is present in all the phenomena of nature--in winds and clouds and rain1 in thunder and lightning1 storm and hail2 and that even the works of man are performed by virtue of the solar heat-supplies4 ,thus the fact that the stars send forth heat to the worlds which circle around them suggests at once the thought that on those worlds there must exist animal and vegetable forms of life4' ,we may note that in the first part of this passage the presence of worlds or planets is 'suggested1' while later on 'the worlds which circle round them' is spoken of as if it were a proved fact from which the presence of vegetable and animal life may be inferred4 ,a suggestion depending on a preceding suggestion is not a very firm basis for so vast and wide-reaching a conclusion4 ,in the second work referred to above there is one chapter entitled1 ',a ,new ,theory of ,life in other ,worlds1' where the author gives his more matured views of the question1 which are briefly stated in the preface as being 'that the weight of evidence favours my theory of the 7relative7 paucity of worlds4' ,his views are largely founded on the theory of probabilities1 of which subject he had made a special study4 ,taking first our earth1 he shows that the period during which life has existed upon it is very small in comparison with that during which it must have been slowly forming and cooling1 and its atmosphere condensing so as to form land and water on its surface4 ,and if we consider the time the earth has been occupied by man1 that is a very minute part1 perhaps not the thousandth part1 of the period during which it has existed as a planet4 ,it follows that even if we consider only those planets whose physical condition seems to us to be such as to be able to sustain life1 the chances are perhaps hundreds to one against their being at that particular stage when life has begun to be developed1 or if it has begun has reached as high a development as on our earth4 ,with regard to the stars1 the argument is still stronger1 because the epochs required for their formation are altogether unknown1 while as to the conditions required for the formation of planetary systems around them we are totally ignorant4 ,to this ,i would add that we are equally ignorant as to the probability or even possibility of many of these suns producing planets which1 by their position1 size1 atmosphere1 or other physical conditions can possibly become life-producing worlds4 ,and1 as we shall see later1 this point has been overlooked by all writers1 including ,mr4 ,proctor himself4 ,his conclusion is1 then1 that although the worlds which possess life at all approaching that of our earth may be relatively few in number1 yet considering the universe as practically infinite in extent1 they may be really very numerous4 ,it has been necessary to give this sketch of the views of those who have written specially on the question of the ,plurality of ,worlds1 because the works referred to have been very widely read and have influenced educated opinion throughout the world4 ,moreover1 ,mr4 ,proctor1 in his last work on the subject1 speaks of the theory as being 'identified with modern astronomy'2 and in fact popular works still discuss it4 ,but all these follow the same general line of argument as those already referred to1 and the curious thing is that while overlooking many of the most essential conditions they often introduce others which are by no means essential--as1 for instance1 that the atmosphere must have the same proportion of oxygen as our own4 ,they seem to think that if any of our quadrupeds or birds taken to another planet could not live there1 no animals of equally high organisation could inhabit it2 entirely overlooking the very obvious fact that1 supposing1 as is almost certain1 that oxygen is necessary for life1 then1 whatever proportion of oxygen within certain limits was present1 the forms of life that arose would necessarily be organised in adaptation to that proportion1 which might be considerably less or greater than on the earth4 ,the present volume will show how extremely inadequate has been the treatment of this question1 which involves a variety of important considerations hitherto altogether overlooked4 ,these are extremely numerous and very varied in their character1 and the fact that they all point to one conclusion--a conclusion which so far as ,i am aware no previous writer has reached--renders it at least worthy of the careful consideration of all unbiassed thinkers4 ,the whole subject is one as to which no direct evidence is obtainable1 but ,i venture to think that the convergence of so many probabilities and indications towards a single definite theory1 intimately connected with the nature and destiny of man himself1 raises this theory to a very much higher level of probability than the vague possibilities and theological suggestions which are the utmost that have been adduced by previous writers4 ,in order to make every step of my argument clearly intelligible to all educated readers1 it will be necessary to refer continually to the marvellous extension of our knowledge of the universe obtained during the last half-century1 and constituting what is termed the ,new ,astronomy4 ,the next chapter will therefore be devoted to a popular exposition of the new methods of research1 so that the results reached1 which will have to be referred to in succeeding chapters1 may be not only accepted1 but clearly understood4 ,,chapter ,,iii ,,the ,,new ,,astronomy ,during the latter half of the nineteenth century discoveries were made which extended the powers of astronomical research into entirely new and unexpected regions1 comparable to those which were opened up by the discovery of the telescope more than two centuries before4 ,the older astronomy for more than two thousand years was purely mechanical and mathematical1 being limited to observation and measurement of the apparent motions of the heavenly bodies1 and the attempts to deduce1 from these apparent motions1 their real motions1 and thus determine the actual structure of the solar system4 ,this was first done when ,kepler established his three celebrated laws3 and later1 when ,newton showed that these laws were necessary consequences of the one law of gravitation1 and when succeeding observers and mathematicians proved that each fresh irregularity in the motions of the planets was explicable by a more thorough and minute application of the same laws1 this branch of astronomy reached its highest point of efficiency and left very little more to be desired4 ,then1 as the telescope became successively improved1 the centre of interest was shifted to the surfaces of the planets and their satellites1 which were watched and scrutinised with the greatest assiduity in order if possible to attain some amount of knowledge of their physical constitution and past history4 ,a similar minute scrutiny was given to the stars and nebul1 their distribution and grouping1 and the whole heavens were mapped out1 and elaborate catalogues constructed by enthusiastic astronomers in every part of the world4 ,others devoted themselves to the immensely difficult problem of determining the distances of the stars1 and by the middle of the century a few such distances had been satisfactorily measured4 ,thus1 up to the middle of the nineteenth century it appeared likely that the future of astronomy would rest almost entirely on the improvement of the telescope1 and of the various instruments of measurement by means of which more accurate determinations of distances might be obtained4 ,indeed1 the author of the ,positive ,philosophy1 ,auguste ,comte1 felt so sure of this that he deprecated all further attention to the stars as pure waste of time that could never lead to any useful or interesting result4 ,in his ,philosophical ,treatise on ,popular ,astronomy published in #ahdd1 he wrote very strongly on this point4 ,he there tells us that1 as the stars are only accessible to us by sight they must always remain very imperfectly known4 ,we can know little more than their mere existence4 ,even as regards so simple a phenomenon as their temperature this must always be inappreciable to a purely visual examination4 ,our knowledge of the stars is for the most part purely negative1 that is1 we can determine only that they do not belong to our system4 ,outside that system there exists1 in astronomy1 only obscurity and confusion1 for want of indispensable facts2 and he concludes thus3--',it is1 then1 in vain that for half a century it has been endeavoured to distinguish two astronomies1 the one solar the other sidereal4 ,in the eyes of those for whom science consists of real laws and not of incoherent facts1 the second exists only in name1 and the first alone constitutes a true astronomy2 and ,i am not afraid to assert that it will always be so4' ,and he adds that--'all efforts directed to this subject for half a century have only produced an accumulation of incoherent empirical facts which can only interest an irrational curiosity4' ,seldom has a confident assertion of finality in science received so crushing a reply as was given to the above statements of ,comte by the discovery in #ahfj 7only three years after his death7 of the method of spectrum-analysis which1 in its application to the stars1 has revolutionised astronomy1 and has enabled us to obtain that very kind of knowledge which he declared must be for ever beyond our reach4 ,through it we have acquired accurate information as to the physics and chemistry of the stars and nebul1 so that we now know really more of the nature1 constitution1 and temperature of the enormously distant suns which we distinguish by the general term stars1 than we do of most of the planets of our own system4 ,it has also enabled us to ascertain the existence of numerous invisible stars1 and to determine their orbits1 their rate of motion1 and even1 approximately1 their mass4 ,the despised stellar astronomy of the early part of the century has now taken rank as the most profoundly interesting department of that grand science1 and the branch which offers the greatest promise of future discoveries4 ,as the results obtained by means of this powerful instrument will often be referred to1 a short account of its nature and of the principles on which it depends must here be given4 ,the solar spectrum is the band of coloured light seen in the rainbow and1 partially1 in the dew-drop1 but more completely when a ray of sunlight passes through a prism--a piece of glass having a triangular section4 ,the result is1 that instead of a spot of white light we have a narrow band of brilliant colours which succeed each other in regular order1 from violet at one end through blue1 green1 and yellow to red at the other4 ,we thus see that light is not a simple and uniform radiation from the sun1 but is made up of a large number of separate rays1 each of which produces in our eyes the sensation of a distinct colour4 ,light is now explained as being due to vibrations of ether1 that mysterious substance which not only permeates all matter1 but which fills space at least as far as the remotest of the visible stars and nebul4 ,the exceedingly minute waves or vibrations of the ether produce all the phenomena of heat1 light1 and colour1 as well as those chemical actions to which photography owes its wonderful powers4 ,by ingenious experiments the size and rate of vibration of these waves have been measured1 and it is found that they vary considerably1 those forming the red light1 which is least refracted1 having a wave-length of about #a_/cbfjjj of an inch1 while the violet rays at the other end of the spectrum are only about half that length or #a_/fcjjjj of an inch4 ,the rate at which the vibrations succeed each other is from #cjb millions of millions per second for the extreme red rays1 to #gcg millions of millions for those at the violet end of the spectrum4 ,these figures are given to show the wonderful minuteness and rapidity of these heat and light waves on which the whole life of the world1 and all our knowledge of other worlds and other suns1 directly depends4 ,but the mere colours of the spectrum are not the most important part of it4 ,very early in the nineteenth century a close examination showed that it was everywhere crossed by black lines of various thicknesses1 sometimes single1 sometimes grouped together4 ,many observers studied them and made accurate drawings or maps showing their positions and thicknesses1 and by combining several prisms1 so that the beam of sunlight had to pass through them successively1 a spectrum could be produced several feet long1 and more than #cjjj of these dark lines were counted in it4 ,but what they were and how they were caused remained a mystery1 till1 in the year #ahfj1 the ,german physicist ,kirchhoff discovered the secret and gave to chemists and astronomers a new and quite unexpected engine of research4 ,it had already been observed that the chemical elements and various compounds1 when heated to incandescence1 produced spectra consisting of coloured lines or bands which were constant for each element1 so that the elements could at once be recognised by their characteristic spectra2 and it had also been noticed that some of these bands1 especially the yellow band produced by sodium1 corresponded in position with certain black lines in the solar spectrum4 ,kirchhoff's discovery consisted in showing that1 when the light from an incandescent body passes through the same substance in a state of vapour or gas1 so much of the light is absorbed that the coloured lines or bands become black4 ,the mystery of more than half a century was thus solved2 and the thousands of black lines in the solar spectrum were shown to be caused by the light from the incandescent matter of the sun's surface passing through the heated gases or vapours immediately above it1 and thereby having the bright coloured lines of their spectra changed1 by absorption1 to comparative blackness4 ,chemists and physicists immediately set to work examining the spectra of the elements1 fixing the position of the several coloured lines or bands by accurate measurement1 and comparing them with the dark lines of the solar spectrum4 ,the results were in the highest degree satisfactory4 ,in a large proportion of the elements the coloured bands corresponded exactly with a group of dark lines in the spectrum of the sun1 in which1 therefore1 the same terrestrial elements were proved to exist4 ,among the elements first detected in this manner were hydrogen1 sodium1 iron1 copper1 magnesium1 zinc1 calcium1 and many others4 ,nearly forty of the elements have now been found in the sun1 and it seems highly probable that all our elements really exist there1 but as some are very rare and are present in very minute quantities they cannot be detected4 ,some of the dark lines in the sun were found not to correspond to any known element1 and as this was thought to indicate an element peculiar to the sun it was named ,helium2 but quite recently it has been discovered in a rare mineral4 ,many of the elements are represented by a great number of lines1 others by very few4 ,thus iron has more than #bjjj1 while lead and potassium have only one each4 ,the value of the spectroscope both to the chemist in discovering new elements and to the astronomer in determining the constitution of the heavenly bodies1 is so great1 that it became of the highest importance to have the position of all the dark lines in the solar spectrum1 as well as the bright lines of all the elements1 determined with extreme accuracy1 so as to be able to make exact comparisons between different spectra4 ,at first this was done by means of very large-scale drawings showing the exact position of every dark or bright line4 ,but this was found to be both inconvenient and not sufficiently exact2 and it was therefore agreed to adopt the natural scale of the wave-lengths of the different parts of the spectrum1 which by means of what are termed diffraction-gratings can now be measured with great accuracy4 ,diffraction-gratings are formed of a polished surface of hard metal ruled with excessively fine lines1 sometimes as many as #bj1jjj to an inch4 ,when sunlight falls upon one of these gratings it is reflected1 and by interference of the rays from the spaces between the fine grooves1 it is spread out into a beautiful and well-defined spectrum1 which1 when the lines are very close1 is several yards in length4 ,in these diffraction spectra many dark lines are seen which can be shown in no other way1 and they also give a spectrum which is far more uniform than that produced by glass prisms in which minute differences in the composition of the glass cause some rays to be refracted more and others less than the normal amount4 ,the spectra produced by diffraction-gratings are double2 that is1 they are spread out on both sides of the central line of the ray which remains white1 and the several coloured or dark lines are so clearly defined that they can be thrown on a screen at a considerable distance1 giving a great length to the spectrum4 ,the data for obtaining the wave-lengths are the distance apart of the lines1 the distance of the screen1 and the distance apart of the first pair of dark lines on each side of the central bright line4 ,all these can be measured with extreme accuracy by means of telescopes with micrometers and other contrivances1 and the result is an accuracy of determination of wave-lengths which can probably not be equalled in any other kind of measurement4 ,as the wave-lengths are so excessively minute1 it has been found convenient to fix upon a still smaller unit of measurement1 and as the millimetre is the smallest unit of the metric system1 the ten-millionth of a millimetre 7technically termed 'tenth meter'7 is the unit adopted for the measurement of wave-lengths1 which is equal to about the #bej millionth of an inch4 ,thus the wave-lengths of the red and blue lines characteristic of hydrogen are #fefc4jg and #dhfa4ea respectively4 ,this excessively minute scale of wave-lengths1 once determined by the most refined measurement1 is of very great importance4 ,having the wave-lengths of any two lines of a spectrum so determined1 the space between them can be laid down on a diagram of any length1 and all the lines that occur in any other spectrum between these two lines can be marked in their exact relative positions4 ,now1 as the visible spectrum consists of about #cjj1jjj rays of light1 each of different wave-lengths and therefore of different refrangibilities1 if it is laid down on such a scale as to be of a length of #cjjj inches 7#bej feet71 each wave-length will be #a_/ajj of an inch long1 a space easily visible by the naked eye4 ,the possession of an instrument of such wonderful delicacy1 and with powers which enable it to penetrate into the inner constitution of the remotest orbs of space1 rendered it possible1 within the next quarter of a century1 to establish what is practically a new science--,astrophysics--often popularly termed the ,new ,astronomy4 ,a brief outline of the main achievements of this science must now be given4 ,the first great discovery made by ,spectrum analysis1 after the interpretation of the sun's spectrum had been obtained1 was1 the real nature of the fixed stars4 ,it is true they had long been held by astronomers to be suns1 but this was only an opinion of the accuracy of which it did not seem possible to obtain any proof4 ,the opinion was founded on two facts--their enormous distance from us1 so great that the whole diameter of the earth's orbit did not lead to any apparent change of their relative positions1 and their intense brilliancy which at such distances could only be due to an actual size and splendour comparable with our sun4 ,the spectroscope at once proved the correctness of this opinion4 ,as one after another was examined1 they were found to exhibit spectra of the same general type as that of the sun--a band of colours crossed by dark lines4 ,the very first stars examined by ,sir ,william ,huggins showed the existence of nine or ten of our elements4 ,very soon all the chief stars of the heavens were spectroscopically examined1 and it was found that they could be classed in three or four groups4 ,the first and largest group contains more than half the visible stars1 and a still larger proportion of the most brilliant1 such as ,sirius1 ,vega1 ,regulus1 and ,alpha ,crucis in the ,southern ,hemisphere4 ,they are characterised by a white or bluish light1 rich in the ultra-violet rays1 and their spectra are distinguished by the breadth and intensity of the four dark bands due to the absorption of hydrogen1 while the various black lines which indicate metallic vapours are comparatively few1 though hundreds of them can be discovered by careful examination4 ,the next group1 to which ,capella and ,arcturus belong1 is also very numerous1 and forms the solar type of stars4 ,their light is of a yellowish colour1 and their spectra are crossed throughout by innumerable fine dark lines more or less closely corresponding with those in the solar spectrum4 ,the third group consists of red and variable stars1 which are characterised by fluted spectra4 ,such spectra show like a range of ,doric columns seen in perspective1 the red side being that most illuminated4 ,the last group1 consisting of few and comparatively small stars1 has also fluted spectra1 but the light appears to come from the opposite direction4 ,these groups were established by ,father ,secchi1 the ,roman astronomer1 in #ahfg1 and have been adopted with some modifications by ,vogel of the ,astrophysical ,observatory at ,potsdam4 ,the exact interpretation of these different spectra is somewhat uncertain1 but there can be little doubt that they coincide primarily with differences of temperature and with corresponding differences in the composition and extent of the absorptive atmospheres4 ,stars with fluted spectra indicate the presence of vapours of the metalloids or of compound substances1 while the reversed flutings indicate the presence of carbon4 ,these conclusions have been reached by careful laboratory experiments which are now carried on at the same time as the spectral examination of the stars and other heavenly bodies1 so that each peculiarity of their spectra1 however puzzling and apparently unmeaning1 has been usually explained1 by being shown to indicate certain conditions of chemical constitution or of temperature4 ,but whatever difficulty there may be in explaining details1 there remains no doubt whatever of the fundamental fact that all the stars are true suns1 differing no doubt in size1 and their stage of development as indicated by the colour or intensity of their light or heat1 but all alike possessing a photosphere or light-emitting surface1 and absorptive atmospheres of various qualities and density4 ,innumerable other details1 such as the often contrasted colours of double stars1 the occasional variability of their spectra1 their relations to the nebul1 the various stages of their development and other problems of equal interest1 have occupied the continued attention of astronomers1 spectroscopists1 and chemists2 but further reference to these difficult questions would be out of place here4 ,the present sketch of the nature of spectrum-analysis applied to the stars is for the purpose of making its principle and method of observation intelligible to every educated reader1 and to illustrate the marvellous precision and accuracy of the results attained by it4 ,so confident are astronomers of this accuracy that nothing less than perfect correspondence of the various bright lines in the spectrum of an element in the laboratory with the dark lines in the spectrum of the sun or of a star is required before the presence of that element is accepted as proved4 ,as ,miss ,clerke tersely puts it--',spectroscopic coincidences admit of no compromise4 ,either they are absolute or they are worthless4' ,,measurement ,,of ,,motion ,,in ,,the ,,line ,,of ,,sight ,we must now describe another and quite distinct application of the spectroscope1 which is even more marvellous than that already described4 ,it is the method of measuring the rate of motion of any of the visible heavenly bodies in a direction either directly towards us1 or directly away from us1 technically described as 'radial motion1' or by the expression--'in the line of sight4' ,and the extraordinary thing is that this power of measurement is altogether independent of distance1 so that the rate of motion in miles per second of the remotest of the fixed stars1 if sufficiently bright to show a distinct spectrum1 can be measured with as much certainty and accuracy as in the case of a much nearer star or a planet4 ,in order to understand how this is possible we have again to refer to the wave-theory of light2 and the analogy of other wave-motions will enable us better to grasp the principle on which these calculations depend4 ,if on a nearly calm day we count the waves that pass each minute by an anchored steamboat1 and then travel in the direction the waves come from1 we shall find that a larger number pass us in the same time4 ,again1 if we are standing near a railway1 and an engine comes towards us whistling1 we shall notice that it changes its tone as it passes us2 and as it recedes the sound will be in a lower key1 although the engine may be at exactly the same distance from us as when it was approaching4 ,yet the sound does not change to the ear of the engine driver1 the cause of the change being that the sound-waves reach us in quicker succession as the source of the waves is approaching us than when it is retreating from us4 ,now1 just as the pitch of a note depends upon the rapidity with which the successive air-vibrations reach our ear1 so does the colour of a particular part of the spectrum depend upon the rapidity with which the ethereal waves which produce colour reach our eyes2 and as this rapidity is greater when the source of the light is approaching than when it is receding from us1 a slight shifting of the position of the coloured bands1 and therefore of the dark lines1 will occur1 as compared with their position in the spectrum of the sun or of any stationary source of light1 if there is any motion sufficient in amount to produce a perceptible shift4 ,that such a change of colour would occur was pointed out by ,professor ,doppler of ,prague in #ahdb1 and it is hence usually spoken of as the ',doppler principle'2 but as the changes of colour were so minute as to be impossible of measurement it was not at that time of any practical importance in astronomy4 ,but when the dark lines in the spectrum were carefully mapped1 and their positions determined with minute accuracy1 it was seen that a means of measuring the changes produced by motion in the line of sight existed1 since the position of any of the dark or coloured lines in the spectra of the heavenly bodies could be compared with those of the corresponding lines produced artificially in the laboratory4 ,this was first done in #ahfh by ,sir ,william ,huggins1 who1 by the use of a very powerful spectroscope constructed for the purpose1 found that such a change did occur in the case of many stars1 and that their rate of motion towards us or away from us--the radial motion--could be calculated4 ,as the actual distance of some of these stars had been measured1 and their change of position annually 7their proper motion7 determined1 the additional factor of the amount of motion in the direction of our line of sight completed the data required to fix their true line of motion among the other stars4 ,the accuracy of this method under favourable conditions and with the best instruments is very great1 as has been proved by those cases in which we have independent means of calculating the real motion4 ,the motion of ,venus towards or away from us can be calculated with great accuracy for any period1 being a resultant of the combined motions of the planet and of our earth in their respective orbits4 ,the radial motions of ,venus were determined at the ,lick ,observatory in ,august and ,september #ahij1 by spectroscopic observations1 and also by calculation1 to be as follows3-- ,by ,observation4 ,by ,calculation4 ,aug4 #afth4 #g4c miles per second4 #h4a miles per second4 #bbnd4 #h4i #h4b #cjth4 #g4c #h4c ,sep4 #crd4 #h4c #h4c #dth4 #h4b #h4c showing that the maximum error was only one mile per second1 while the mean error was about a quarter of a mile4 ,in the case of the stars the accuracy of the method has been tested by observations of the same star at times when the earth's motion in its orbit is towards or away from the star1 whose apparent radial velocity is1 therefore1 increased or diminished by a known amount4 ,observations of this kind were made by ,dr4 ,vogel1 ,director of the ,astrophysical ,observatory at ,potsdam1 showing1 in the case of three stars1 of which ten observations were taken1 a mean error of about two miles per second2 but as the stellar motions are more rapid than those of the planets1 the proportionate error is no greater than in the example given above4 ,the great importance of this mode of determining the real motion of the stars is1 that it gives us a knowledge of the scale on which such motions are progressing2 and when in the course of time we discover whether any of their paths are rectilinear or curved1 we shall be in a position to learn something of the nature of the changes that are going on and of the laws on which they depend4 ,,invisible ,,stars ,,and ,,imperceptible ,,motions ,but there is another result of this power of determining radial motion which is even more unexpected and marvellous1 and which has extended our knowledge of the stars in quite a new direction4 ,by its means it is possible to determine the existence of invisible stars and to measure the rate of otherwise imperceptible motions2 that is of stars which are invisible in the most powerful modern telescopes1 and whose motions have such a limited range that no telescope can detect them4 ,double or binary stars forming systems which revolve around their common centre of gravity were discovered by ,sir ,william ,herschel1 and very great numbers are known2 but in most cases their periods of revolution are long1 the shortest being about twelve years1 while many extend to several hundred years4 ,these are1 of course1 all visible binaries1 but many are now known of which one star only is visible while the other is either non-luminous or is so close to its companion that they appear as a single star in the most powerful telescopes4 ,many of the variable stars belong to the former class1 a good example of which is ,algol in the constellation ,perseus1 which changes from the second to the fourth magnitude in about four and a half hours1 and in about four and a half hours more regains its brilliancy till its next period of obscuration which occurs regularly every two days and twenty-one hours4 ,the name ,algol is from the ,arabic ,al ,ghoul1 the familiar 'ghoul' of the ,arabian ,nights1 so named--',the ,demon'--from its strange and weird behaviour4 ,it had long been conjectured that this obscuration was due to a dark companion which partially eclipsed the bright star at every revolution1 showing that the plane of the orbit of the pair was almost exactly directed towards us4 ,the application of the spectroscope made this conjecture a certainty4 ,at an equal time before and after the obscuration1 motion in the line of sight was shown1 towards and away from us1 at a rate of twenty-six miles per second4 ,from these scanty data and the laws of gravitation which fix the period of revolution of planets at various distances from their centres of revolution1 ,professor ,pickering of the ,harvard ,observatory was able to arrive at the following figures as highly probable1 and they may be considered to be certainly not far from the truth4 ,diameter of ,algol1 #a1jfa1jjj miles4 ,diameter of dark companion1 #hcj1jjj ,distance between their centres1 #c1bcj1jjj ,orbital speed of ,algol1 #bf4c miles per sec4 ,orbital speed of companion1 #ee4d ,mass of ,algol1 #d_/i mass of our ,sun4 ,mass of companion1 #b_/i ,when it is considered that these figures relate to a pair of stars only one of which has ever been seen1 that the orbital motion even of the visible star cannot be detected in the most powerful telescopes1 when1 further1 we take into account the enormous distance of these objects from us1 the great results of spectroscopic observation will be better appreciated4 ,but besides the marvel of such a discovery by such simple means1 the facts discovered are themselves in the highest degree marvellous4 ,all that we had known of the stars through telescopic observation indicated that they were at very great distances from each other however thickly they may appear scattered over the sky4 ,this is the case even with close telescopic double stars1 owing to their enormous remoteness from us4 ,it is now estimated that even stars of the first magnitude are1 on a general average1 about eighty millions of millions of miles distant2 while the closest double stars that can be distinctly separated by large telescopes are about half a second apart4 ,these1 if at the above distance1 will be about #aejj millions of miles from each other4 ,but in the case of ,algol and its companion1 we have two bodies both larger than our sun1 yet with a distance of only #b-a_/d millions of miles between their surfaces1 a distance not much exceeding their combined diameters4 ,we should not have anticipated that such huge bodies could revolve so closely to each other1 and as we now know that the neighbourhood of our sun--and probably of all suns--is full of meteoric and cometic matter1 it would seem probable that in the case of two suns so near together the quantity of such matter would be very great1 and would lead probably by continued collisions to increase of their bulk1 and perhaps to their final coalescence into a single giant orb4 ,it is said that a ,persian astronomer in the tenth century calls ,algol a red star1 while it is now white or somewhat yellowish4 ,this would imply an increase of temperature caused by collisions or friction1 and increasing proximity of the pair of stars4 ,a considerable number of double stars with dark companions have been discovered by means of the spectroscope1 although their motion is not directly in the line of sight1 and therefore there is no obscuration4 ,in order to discover such pairs the spectra of large numbers of stars are taken on photographic plates every night and for considerable periods--for a year or for several years4 ,these plates are then carefully examined with a high magnifying power to discover any periodical displacement of the lines1 and it is astonishing in how large a number of cases this has been found to exist and the period of revolution of the pair determined4 ,but besides discovering double stars of which one is dark and one bright1 many pairs of bright stars have been discovered by the same means4 ,the method in this case is rather different4 ,each component star1 being luminous1 will give a separate spectrum1 and the best spectroscopes are so powerful that they will separate these spectra when the stars are at their maximum distance although no telescope in existence1 or ever likely to be made1 can separate the component stars4 ,the separation of the spectra is usually shown by the most prominent lines becoming double and then after a time single1 indicating that the plane of revolution is more or less obliquely towards us1 so that the two stars if visible would appear to open out and then get nearer together every revolution4 ,then1 as each star alternately approaches and recedes from us the radial velocity of each can be determined1 and this gives the relative mass4 ,in this way not only doubles1 but triple and multiple systems1 have been discovered4 ,the stars proved to be double by these two methods are so numerous that it has been estimated by one of the best observers that about one star in every thirteen shows inequality in its radial motion and is therefore really a double star4 ,,the ,,nebul ,one other great result of spectrum-analysis1 and in some respects perhaps the greatest1 is its demonstration of the fact that true nebul exist1 and that they are not all star-clusters so remote as to be irresolvable1 as was once supposed4 ,they are shown to have gaseous spectra1 or sometimes gaseous and stellar spectra combined1 and this1 in connection with the fact that nebul are frequently aggregated around nebulous stars or groups of stars1 renders it certain that the nebul are in no way separated in space from the stars1 but that they constitute essential parts of one vast stellar universe4 ,there is1 indeed1 good reason to believe that they are really the material out of which stars are made1 and that in their forms1 aggregations1 and condensations1 we can trace the very process of evolution of stars and suns4 ,,photographic ,,astronomy ,but there is yet another powerful engine of research which the new astronomy possesses1 and which1 either alone or in combination with the spectroscope1 had produced and will yet produce in the future an amount of knowledge of the stellar universe which could never be attained by any other means4 ,it has already been stated how the discovery of new variable and binary stars has been rendered possible by the preservation of the photographic plates on which the spectra are self-recorded1 night after night1 with every line1 whether dark or coloured1 in true position1 so as to bear magnification1 and1 by comparison with others of the series1 enabling the most minute changes to be detected and their amount accurately measured4 ,without the preservation of such comparable records1 which is in no other way possible1 by far the larger portion of spectroscopic discoveries could never have been made4 ,but there are two other uses of photography of quite a different nature which are equally and perhaps in their final outcome may be far more important4 ,the first is1 that by the use of the photographic plate the exact positions of scores1 hundreds1 or even thousands of stars can be self-mapped simultaneously with extreme accuracy1 while any number of copies can be made of these star-maps4 ,this entirely obviates the necessity for the old method of fixing the position of each star by repeated measurement by means of very elaborate instruments1 and their registration in laborious and expensive catalogues4 ,so important is this now seen to be1 that specially constructed cameras are made for stellar photography1 and by means of the best kinds of equatorial mounting are made to revolve slowly so that the image of each star remains stationary upon the plate for several hours4 ,arrangements have been now made among all the chief observatories of the world to carry out a photographic survey of the heavens with identical instruments1 so as to produce maps of the whole star-system on the same scale4 ,these will serve as fixed data for future astronomers1 who will thus be able to determine the movements of stars of all magnitudes with a certainty and accuracy hitherto unattainable4 ,the other important use of photography depends upon the fact that with a longer exposure within certain limits we increase the light-collecting power4 ,it will surprise many persons to learn that an ordinary good portrait-camera with a lens three or four inches in diameter1 if properly mounted so that an exposure of several hours can be made1 will show stars so minute that they are invisible even in the great ,lick telescope4 ,in this way the camera will often reveal double-stars or small groups which can be made visible in no other way4 ,such photographs of the stars are now constantly reproduced in works on ,astronomy and in popular magazine articles1 and although some of them are very striking1 many persons are disappointed with them1 and cannot understand their great value1 because each star is represented by a white circle often of considerable size and with a somewhat undefined outline1 not by a minute point of light as stars appear in a good telescope4 ,but the essential matter in all such photographs is not so much the smallness1 as the roundness1 of the star-images1 as this proves the extreme precision with which the image of every star has been kept by the clockwork motion of the instrument on the same point of the plate during the whole exposure4 ,for example1 in the fine photograph of the ,great ,nebula in ,andromeda1 taken #bith ,december #ahhh1 by ,dr4 ,isaac ,roberts1 with an exposure of four hours1 there are probably over a thousand stars large and small to be seen1 every one represented by an almost exactly circular white dot of a size dependent on the magnitude of the star4 ,these round dots can be bisected by the cross hairs of a micrometer with very great accuracy1 and thus the distance between the centres of any of the pairs1 as well as the direction of the line joining their centres1 can be determined as accurately as if each was represented by a point only4 ,but as a minute white speck would be almost invisible on the maps1 and would convey no information as to the approximate magnitude of the star1 mistakes would be much more easily made1 and it would probably be found necessary to surround each star with a circle to indicate its magnitude1 and to enable it to be easily seen4 ,it is probable1 therefore1 that the supposed defect is really an important advantage4 ,the above-mentioned photograph is beautifully reproduced in ,proctor's ,old and ,new ,astronomy1 published after his greatly lamented death4 ,but besides the amount of altogether new knowledge obtained by the methods of research here briefly explained1 a great deal of light has been thrown on the distribution of the stars as a whole1 and hence on the nature and extent of the stellar universe1 by a careful study of the materials obtained by the old methods1 and by the application of the doctrine of probabilities to the observed facts4 ,in this way alone some very striking results have been reached1 and these have been supported and strengthened by the newer methods1 and also by the use of new instruments in the measurement of stellar distances4 ,some of these results bear so closely and directly upon the special subject of the present volume1 that our next chapter must be devoted to a consideration of them4 ,,chapter ,,iv ,,the ,,distribution ,,of ,,the ,,stars ,if we look at the heavens on a clear1 moonless night in winter1 and from a position embracing the entire horizon1 the scene is an inexpressibly grand one4 ,the intense sparkling brilliancy of ,sirius1 ,capella1 ,vega1 and other stars of the first magnitude2 their striking arrangement in constellations or groups1 of which ,orion1 the ,great ,bear1 ,cassiopei1 and the ,pleiades1 are familiar examples2 and the filling up between these by less and less brilliant points down to the limit of vision1 so as to cover the whole sky with a scintillating tracery of minute points of light1 convey together an idea of such confused scattering and such enormous numbers1 that it seems impossible to count them or to reduce them to systematic order4 ,yet this was done for all except the faintest stars by ,hipparchus1 #acd ,b4,c41 who catalogued and fixed the positions of more than #ajjj stars1 and this is about the number1 down to the fifth magnitude1 visible in the latitude of ,greece4 ,a recent enumeration of all the stars visible to the naked eye1 under the most favourable conditions and by the best eyesight1 has been made by the ,american astronomer1 ,pickering4 ,his numbers are--for the ,northern ,hemisphere #beji1 and for the ,southern ,hemisphere #bhbd1 thus showing a somewhat greater richness in the southern celestial hemisphere4 ,but as this difference is due entirely to a preponderance of stars between magnitudes #e-a_/b and #f1 that is1 just on the limits of vision1 while those down to magnitude #e-a_/b are more numerous by #he in the ,northern ,hemisphere1 ,professor ,newcomb is of opinion that there is no real superiority of numbers of visible stars in one hemisphere over the other4 ,again1 the total number of the visible stars by the above enumeration is #eccc4 ,but this includes stars down to #f4b magnitude1 while it is generally considered that magnitude #f marks the limit of visibility4 ,on a re-examination of all the materials1 the ,italian astronomer ,schiaparelli concludes that the total number of stars down to the sixth magnitude is #dcjc2 and they seem to be about equally divided between the northern and southern skies4 ,,the ,,milky ,,way ,but besides the stars themselves1 a most conspicuous object both in the northern and southern hemisphere is that wonderful irregular belt of faintly diffused light termed the ,milky ,way or ,galaxy4 ,this forms a magnificent arch across the sky1 best seen in the autumn months in our latitude4 ,this arch1 while following the general course of a great circle round the heavens1 is extremely irregular in detail1 sometimes being single1 sometimes double1 sending off occasional branches or offshoots1 and also containing in its very midst dark rifts1 spots1 or patches1 where the black background of almost starless sky can be seen through it4 ,when examined through an opera-glass or small telescope quantities of stars are seen on the luminous background1 and with every increase in the size and power of the telescope more and more stars become visible1 till with the largest and best modern instruments the whole of the ,galaxy seems densely packed with them1 though still full of irregularities1 wavy streams of stars1 and dark rifts and patches1 but always showing a faint nebulous background as if there remained other myriads of stars which a still higher optical power would reveal4 ,the relations of this great belt of telescopic stars to the rest of the star-system have long interested astronomers1 and many have attempted its solution4 ,by a system of gauging1 that is counting all the stars that passed over the field of his telescope in a certain time1 ,sir ,william ,herschel was the first who made a systematic effort to determine the shape of the stellar universe4 ,from the fact that the number of stars increased rapidly as the ,milky ,way was approached from whatever direction1 while in the ,galaxy itself the numbers visible were at once more than doubled1 he formed the idea that the shape of the entire system must be that of a highly compressed very broad mass or ring rather less dense towards the centre where our sun was situated4 ,roughly speaking1 the form was likened to a flat disc or grindstone1 but of irregular thickness1 and split in two on one side where it appears to be double4 ,the immense quantity of the stars which formed it was supposed to be due to the fact that we looked at it edgewise through an immense depth of stars2 while at right angles to its direction when looking towards what is termed the pole of the ,galaxy1 and also in a less degree when looking obliquely1 we see out into space through a much thinner stratum of stars1 which thus seem on the average to be very much farther apart4 ,but1 in the latter part of his life1 ,sir ,william ,herschel realised that this was not the true explanation of the features presented by the ,galaxy4 ,the brilliant spots and patches in it1 the dark rifts and openings1 the narrow streams of light often bounded by equally narrow streams or rifts of darkness1 render it quite impossible to conceive that this complex luminous ring has the form of a compressed disc extending in the direction in which we see it to a distance many times greater than its thickness4 ,in one very luminous cluster ,herschel thought that his telescope had penetrated to regions twenty times as far off as the more brilliant stars forming the nearer portions of the same object4 ,now1 in the case of the ,magellanic clouds1 which are two roundish nebular patches of large size some distance from the ,milky ,way in the ,southern ,hemisphere and looking like detached portions of it1 ,sir ,john ,herschel himself has shown that any such interpretation of its form is impossible2 because it requires us to suppose that in both these cases we see1 not rounded masses of a roughly globular shape1 but immensely long cones or cylinders1 placed in such a direction that we see only the ends of them4 ,he remarks that one such object so situated would be an extraordinary coincidence1 but that there should be two or many such is altogether out of the question4 ,but in the ,milky ,way there are hundreds or even thousands of such spots or masses of exceptional brilliancy or exceptional darkness2 and1 if the form of the ,galaxy is that of a disc many times broader than thick1 and which we see edgewise1 then every one of these patches and clusters1 and all the narrow winding streams of bright light or intense blackness1 must be really excessively long cylinders1 or tunnels1 or deep curving lamin1 or narrow fissures4 ,and every one of these1 which are to be found in every part of this vast circle of luminosity1 must be so arranged as to be exactly turned towards our sun4 ,the weight of this argument1 which has been most forcibly and clearly set forth by the late ,mr4 ,r4,a4 ,proctor1 in his very instructive volume ,our ,place among ,infinities1 is now generally admitted by astronomers1 and the natural conclusion is that the form of the ,milky ,way is that of a vast irregular ring1 of which the section at any part is1 roughly speaking1 circular2 while the many narrow rifts or lanes or openings where we seem to be able to see completely through it to the darkness of outer space beyond1 render it probable that in those directions its thickness is less instead of greater than its apparent width1 that is1 that we see the broader side rather than the narrow edge of it4 ,before entering on the consideration of the relations which the bulk of the stars we see scattered over the entire vault of heaven bear to this great belt of telescopic stars1 it will be advisable to give a somewhat full description of the ,galaxy itself1 both because it is not often delineated on star-maps with sufficient accuracy1 or so as to show its wonderful intricacies of structure1 and also because it constitutes the fundamental phenomenon upon which the argument set forth in this volume primarily rests4 ,for this purpose ,i shall use the description of it given by ,sir ,john ,herschel in his ,outlines of ,astronomy1 both because he1 of all the astronomers of the last century1 had studied it most thoroughly1 in the northern and in the southern hemispheres1 by eye-observation and with the aid of telescopes of great power and admirable quality2 and also because1 amid the throng of modern works and the exciting novelties of the last thirty years1 his instructive volume is1 comparatively speaking1 very little known4 ,this precise and careful description will also be of service to any of my readers who may wish to form a closer personal acquaintance with this magnificent and intensely interesting object1 by examining its peculiarities of form and beauties of structure either with the naked eye1 or with the aid of a good opera-glass1 or with a small telescope of good defining power4 ,a ,,description ,,of ,,the ,,milky ,,way ,sir ,john ,herschel's description is as follows3--',the course of the ,milky ,way as traced through the heavens by the unaided eye1 neglecting occasional deviations and following the line of its greatest brightness as well as its varying breadth and intensity will permit1 conforms1 as nearly as the indefiniteness of its boundary will allow it to be fixed1 to that of a great circle inclined at an angle of about #fc to the equinoctial1 and cutting that circle in ,right ,ascension #f;h4 #dgm4 and #ah;h4 #dgm41 so that its northern and southern poles respectively are situated in ,right ,ascension #ab;h4 #dgm41 ,north ,polar ,distance #fc1 and ,r4,a4 #j;h4 #dgm41 ,,npd4 #aag4 ,throughout the region where it is so remarkably subdivided1 this great circle holds an intermediate situation between the two great streams2 with a nearer approximation however to the brighter and continuous stream than to the fainter and interrupted one4 ,if we trace its course in order of right ascension1 we find it traversing the constellation ,cassiopei1 its brightest part passing about two degrees to the north of the star ,delta of that constellation4 ,passing thence between ,gamma and ,epsilon ,cassiopei1 it sends off a branch to the south-preceding side1 towards ,alpha ,persei1 very conspicuous as far as that star1 prolonged faintly towards ,eta of the same constellation1 and possibly traceable towards the ,hyades and ,pleiades as remote outliers4 ,the main stream1 however 7which is here very faint71 passes on through ,auriga1 over the three remarkable stars1 ,epsilon1 ,zeta1 ,eta1 of that constellation called the ,hdi1 preceding ,capella1 between the feet of ,gemini and the horns of the ,bull 7where it intersects the ecliptic nearly in the ,solstitial ,colure7 and thence over the club of ,orion to the neck of ,monoceros1 intersecting the equinoctial in ,r4,a4 #f;h4 #edm4 ,up to this point1 from the offset in ,perseus1 its light is feeble and indefinite1 but thenceforward it receives a gradual accession of brightness1 and where it passes through the shoulder of ,monoceros and over the head of ,canis ,major it presents a broad1 moderately bright1 very uniform1 and to the naked eye1 starless stream up to the point where it enters the prow of the ship ,argo1 nearly on the southern tropic4 ,here it again subdivides 7about the star m ,puppis71 sending off a narrow and winding branch on the preceding side as far as ,gamma ,args1 where it terminates abruptly4 ,the main stream pursues its southward course to the #abcrd parallel of ,,npd41 where it diffuses itself broadly and again subdivides1 opening out into a wide fan-like expanse1 nearly #bj in breadth1 formed of interlacing branches1 which all terminate abruptly1 in a line drawn nearly through ,lambda and ,gamma ,args4 ',at this place the continuity of the ,milky ,way is interrupted by a wide gap1 and where it recommences on the opposite side it is by a somewhat similar fan-shaped assemblage of branches which converge upon the bright star ,eta ,args4 ,thence it crosses the hind feet of the ,centaur1 forming a curious and sharply-defined semicircular concavity of small radius1 and enters the ,cross by a very bright neck or isthmus of not more than three or four degrees in breadth1 being the narrowest portion of the ,milky ,way4 ,after this it immediately expands into a broad and bright mass1 enclosing the stars ,alpha and ,beta ,crucis and ,beta ,centauri1 and extending almost up to ,alpha of the latter constellation4 ,in the midst of this bright mass1 surrounded by it on all sides1 and occupying about half its breadth1 occurs a singular dark pear-shaped vacancy1 so conspicuous and remarkable as to attract the notice of the most superficial gazer and to have acquired among the early southern navigators the uncouth but expressive appellation of the coal-sack4 ,in this vacancy1 which is about #h in length and #e broad1 only one very small star visible to the naked eye occurs1 though it is far from devoid of telescopic stars1 so that its striking blackness is simply due to the effect of contrast with the brilliant ground with which it is on all sides surrounded4 ,this is the place of nearest approach of the ,milky ,way to the ,south ,pole4 ,throughout all this region its brightness is very striking1 and when compared with that of its more northern course already traced1 conveys strongly the impression of greater proximity1 and would almost lead to a belief that our situation as spectators is separated on all sides by a considerable interval from the dense body of stars composing the ,galaxy1 which in this view of the subject would come to be considered as a flat ring or some other re-entering form of immense and irregular breadth and thickness1 within which we are excentrically situated1 nearer to the southern than to the northern part of its circuit4 ',at ,alpha ,centauri the ,milky ,way again subdivides1 sending off a great branch of nearly half its breadth1 but which thins off rapidly1 at an angle of about #bj with its general direction to ,eta and d ,lupi1 beyond which it loses itself in a narrow and faint streamlet4 ,the main stream passes on increasing in breadth to ,gamma ,norm1 where it makes an abrupt elbow and again subdivides into one principal and continuous stream of very irregular breadth and brightness1 and a complicated system of interlaced streaks and masses1 which covers the tail of ,scorpio1 and terminates in a vast and faint effusion over the whole extensive region occupied by the preceding leg of ,ophiuchus1 extending northward to the parallel of #ajc ,,npd41 beyond which it cannot be traced2 a wide interval of #ad1 free from all appearance of nebulous light1 separating it from the great branch on the north side of the equinoctial of which it is usually represented as a continuation4 ',returning to the point of separation of this great branch from the main stream1 let us now pursue the course of the latter4 ,making an abrupt bend to the following side1 it passes over the stars ,iota ,ar1 ,theta and ,iota ,scorpii1 and ,gamma ,tubi to ,gamma ,sagittarii1 where it suddenly collects into a vivid oval mass about #f in length and #d in breadth1 so excessively rich in stars that a very moderate calculation makes their number exceed #ajj1jjj4 ,northward of this mass1 this stream crosses the ecliptic in longitude about #bgf1 and proceeding along the bow of ,sagittarius into ,antinous has its course rippled by three deep concavities1 separated from each other by remarkable protuberances1 of which the larger and brighter forms the most conspicuous patch in the southern portion of the ,milky ,way visible in our latitudes4 ',crossing the equinoctial at the #aith hour of ,r4,a41 it next runs in an irregular1 patchy1 and winding stream through ,aquila1 ,sagitta1 and ,vulpecula up to ,cygnus2 at ,epsilon of which constellation its continuity is interrupted1 and a very confused and irregular region commences1 marked by a broad dark vacuity1 not unlike the southern coal-sack1 occupying the space between ,epsilon1 ,alpha1 and ,gamma ,cygni1 which serves as a kind of centre for the divergence of three great streams2 one1 which we have already traced2 a second1 the continuation of the first 7across the interval7 from ,alpha northward1 between ,lacerta and the head of ,cepheus to the point in ,cassiopei whence we set out1 and a third branching off from ,gamma ,cygni1 very vivid and conspicuous1 running off in a southern direction through ,beta ,cygni1 and s ,aquil almost to the equinoctial1 where it loses itself in a region thinly sprinkled with stars1 where in some maps the modern constellation ,taurus ,poniatowski is placed4 ,this is the branch which1 if continued across the equinoctial1 might be supposed to unite with the great southern effusion in ,ophiuchus already noticed4 ,a considerable offset1 or protuberant appendage1 is also thrown off by the northern stream from the head of ,cepheus directly towards the pole1 occupying the greater part of the quartile formed by ,alpha1 ,beta1 ,iota1 and ,delta of that constellation4' ,to complete this careful1 detailed description of the ,milky ,way1 it will be well to add a few passages from the same work as to its telescopic appearance and structure4 ',when examined with powerful telescopes1 the constitution of this wonderful zone is found to be no less various than its aspect to the naked eye is irregular4 ,in some regions the stars of which it is composed are scattered with remarkable uniformity over immense tracts1 while in others the irregularity of their distribution is quite as striking1 exhibiting a rapid succession of closely clustering rich patches separated by comparatively poor intervals1 and indeed in some instances by spaces absolutely dark and completely void of any star1 even of the smallest telescopic magnitude4 ,in some places not more than #dj or #ej stars on an average occur in a gauge-field of #ae'1 while in others a similar average gives a result of #djj or #ejj4 ,nor is less variety observable in the character of its different regions in respect of the magnitudes of the stars they exhibit1 and the proportional numbers of the larger and smaller magnitudes associated together1 than in respect of their aggregate numbers4 ,in some1 for instance1 extremely minute stars occur in numbers so moderate as to lead us irresistibly to the conclusion that in these regions we see fairly through the starry stratum1 since it is impossible otherwise that the numbers of the smaller magnitudes should not go on continually increasing ad infinitum4 ,in such cases1 moreover1 the ground of the heavens is for the most part perfectly dark1 which again would not be the case if innumerable multitudes of stars1 too minute to be individually discernible1 existed beyond4 ,in other regions we are presented with the phnomenon of an almost uniform degree of brightness of the individual stars1 accompanied with a very even distribution of them over the ground of the heavens1 both the larger and smaller magnitudes being strikingly deficient4 ,in such cases it is equally impossible not to perceive that we are looking through a sheet of stars nearly of a size1 and of no great thickness compared with the distance which separates them from us4 ,were it otherwise we should be driven to suppose the more distant stars uniformly the larger1 so as to compensate by their greater intrinsic brightness for their greater distance1 a supposition contrary to all probability4444 ',throughout by far the larger portion of the extent of the ,milky ,way in both hemispheres1 the general blackness of the ground of the heavens on which its stars are projected1 and the absence of that innumerable multitude and excessive crowding of the smallest visible magnitudes1 and of glare produced by the aggregate light of multitudes too small to affect the eye singly1 must1 we think1 be considered unequivocal indications that its dimensions in directions where these conditions obtain are not only not infinite1 but that the space-penetrating power of our telescopes suffices fairly to pierce through and beyond it4' ,in the above-quoted passage the italics are those of ,sir ,john ,herschel himself1 and we see that he drew the very same conclusions from the facts he describes1 and for much the same reasons1 as ,mr4 ,proctor has drawn from the observations of ,sir ,william ,herschel2 and1 as we shall see1 the best astronomers to-day have arrived at a similar result1 from the additional facts at their disposal1 and in some cases from fresh lines of argument4 ,,the ,,stars ,,in ,,relation ,,to ,,the ,,milky ,,way ,sir ,john ,herschel was so impressed with the form1 structure1 and immensity of the ,galactic ,circle1 as he sometimes terms it1 that he says 7in a footnote p4 #ege1 #ajth ed471 ',this circle is to sidereal what the invariable ecliptic is to planetary astronomy--a plane of ultimate reference1 the ground-plane of the sidereal system4' ,we have now to consider what are the relations of the whole body of the stars to this ,galactic ,circle--this plane of ultimate reference for the whole stellar universe4 ,if we look at the heavens on a starry night1 the whole vault appears to be thickly strewn with stars of various degrees of brightness1 so that we could hardly say that any extensive region--the north1 east1 south1 or west1 or the portion vertically above us--is very conspicuously deficient or superior in numbers4 ,in every part there are to be found a fair proportion of stars of the first two or three magnitudes1 while where these may seem deficient a crowd of smaller stars takes their place4 ,but an accurate survey of the visible stars shows that there is a large amount of irregularity in their distribution1 and that all magnitudes are really more numerous in or near the ,milky ,way1 than at a distance from it1 though not in so large a degree as to be very conspicuous to the naked eye4 ,the area of the whole of the ,milky ,way cannot be estimated at more than one-seventh of the whole sphere1 while some astronomers reckon it at only one-tenth4 ,if stars of any particular size were uniformly distributed1 at most one-seventh of the whole number should be found within its limits4 ,but ,mr4 ,gore finds that of #cb stars brighter than the second magnitude #ab lie upon the ,milky ,way1 or considerably more than twice as many as there should be if they were uniformly distributed4 ,and in the case of the #ii stars which are brighter than the third magnitude #cc lie upon the ,milky ,way1 or one-third instead of one-seventh4 ,mr4 ,gore also counted all the stars in ,heis's ,atlas which lie upon the ,milky ,way1 and finds there are #aahf out of a total of #ecef1 a proportion of between a fourth and a fifth instead of a seventh4 ,the late ,mr4 ,proctor in #ahga laid down on a chart two feet diameter all the stars down to magnitude #i-a_/b given in ,agrelander's forty large charts of the stars visible in the northern hemisphere4 ,they were #cbd1aih in number1 and they distinctly showed by their greater density not only the whole course of the ,milky ,way but also its more luminous portions and many of the curious dark rifts and vacuities1 which latter are almost wholly avoided by these stars4 ,later on ,professor ,seeliger of ,munich made an investigation of the relation of more than #ace1jjj stars down to the ninth magnitude to the ,milky ,way1 by dividing the whole of the heavens into nine regions1 one and nine being circles of #bj wide 7equal to #dj diameter7 at the two poles of the ,galaxy2 the middle region1 five1 is a zone #bj wide including the ,milky ,way itself1 and the other six intermediate zones are each #bj wide4 ,the following table shows the results as given by ,professor ,newcomb1 who has made some alterations in the last column of ',density of ,stars' in order to correct differences in the estimate of magnitudes by the different authorities4 ,regions4 ,area in ,degree4 ,number of ,stars4 ,density4 ,i4 #a1cih4g #d1bgg #b4gh ,,ii4 #c1adf4i #aj1ahe #c4jc ,,iii4 #e1abf4f #ai1dhh #c4ed ,,iv4 #d1ehi4h #bd1dib #e4cb ,v4 #d1eai4e #cc1bfg #h4ag ,,vi4 #c1iga4e #bc1ehj #f4jg ,,vii4 #b1ied4d #aa1gij #c4ga ,,viii4 #a1gif4f #f1cge #c4ba ,,ix4 #dfh4b #a1fdd #c4ad ,n4,b4--,the inequality of the ,n4 and ,s4 areas is because the enumeration of the stars only went as far as #bd ,s4 ,decl41 and therefore included only a part of ,regions ,,vii41 ,,viii41 and ,,ix4 ,illustration3 ,,diagram ,,of ,,star-,,density ,from ,herschel's ,gauges 7as given by ,professor ,newcomb1 p4 #bea74 ,upon this table of densities ,professor ,newcomb remarks as follows3--',the star-density in the several regions increases continuously from each pole 7regions ,i4 and ,,ix47 to the ,galaxy itself 7region ,v474 ,,if ,,the ,,latter ,,were ,a ,,simple ,,ring ,,of ,,stars ,,surrounding ,a ,,spherical ,,system ,,of ,,stars1 ,,the ,,star-,,density ,,would ,,be ,,about ,,the ,,same ,,in ,,regions ,i41 ,,ii41 and ,,iii41 and also in ,,vii41 ,,viii41 and ,,ix41 but would suddenly increase in ,,iv4 and ,,vi4 as the boundary of the ring was approached4 ,instead of such being the case1 the numbers #b4gh1 #c4jc1 and #c4ed in the north1 and #c4ad1 #c4ba1 and #c4ga in the south1 show a progressive increase from the galactic pole to the ,galaxy itself4 ,the conclusion to be drawn is a fundamental one4 ,the universe1 or at least the denser portion of it1 is really flattened between the galactic poles1 as supposed by ,herschel and ,struve4' ,but looking at the series of figures in the table1 and again as quoted by ,professor ,newcomb1 they seem to me to show in some measure what he says they do not show4 ,i therefore drew out the above diagram from the figures in the table1 and it certainly shows that the density in regions ,i41 ,,ii41 and ,,iii41 and in regions ,,vii41 ,,viii41 and ,,ix41 may be said to be 'about the same1' that is1 they increase very slowly1 and that they do 'suddenly increase' in ,,iv4 and ,,vi4 as the boundary of the ,galaxy is approached4 ,this may be explained either by a flattening towards the poles of the ,galaxy1 or by the thinning out of stars in that direction4 ,in order to show the enormous difference of star-density in the ,galaxy and at the galactic poles1 ,professor ,newcomb gives the following table of the ,herschelian gauges1 on which he only remarks that they show an enormously increased density in the galactic region due to the ,herschels having counted so many more stars there than any other observers4 ---------------------------------------- ------------ ,region1 4 ,i4 ,,ii4,,iii4 ,,iv4 ,v4 ,,vi4 ,,vii4,,viii4 ,,ix4 ,density1 4#ajg #aed #bha #efj #b1jai #fgb #bfa #aed #aaa ---------------------------------------- ------------ ,illustration3 ,,diagram ,,of ,,star-,,density ,from a table in ,the ,stars 7p4 #bdi74 ,but an important characteristic of these figures is1 that the ,herschels alone surveyed the whole of the heavens from the north to the south pole1 that they did this with instruments of the same size and quality1 and that from almost life-long experience in this particular work they were unrivalled in their power of counting rapidly and accurately the stars that passed over each field of view of their telescopes4 ,their results1 therefore1 must be held to have a comparative value far above those of any other observer or combination of observers4 ,i have therefore thought it advisable to draw a diagram from their figures1 and it will be seen how strikingly it agrees with the former diagram in the very slow increase of star-richness in the first three regions north and south1 the sudden increase in regions ,,iv4 and ,,vi4 as we approach the ,galaxy1 while the only marked difference is in the enormously greater richness of the ,galaxy itself1 which is an undoubtedly real phenomenon1 and is brought out here by the unrivalled observing power of the two greatest astronomers in this special department that have ever lived4 ,we shall find later on that ,professor ,newcomb himself1 as the result of a quite different inquiry arrives at a result in accordance with these diagrams which will then be again referred to4 ,as this is a very interesting subject1 it will be well to give another diagram from two tables of star-density in ,sir ,john ,herschel's volume already quoted4 ,the tables are as follows3-- ,zones of ,galactic ,average number of ,star ,north ,polar ,distance4 per ,field of #ae'4 #j to #ae #d4cb #ae to #cj #e4db #cj to #de #h4ba #de to #fj #ac4fa #fj to #ge #bd4ji #ge to #ij #ec4dc ,zones of ,galactic ,average number of ,stars ,south ,polar ,distance4 per ,field of #ae'4 #j to #ae #f4je #ae to #cj #f4fb #cj to #de #i4jh #de to #fj #ac4di #fj to #ge #bf4bi #ge to #ij #ei4jf ,in these tables the ,milky ,way itself is taken as occupying two zones of #ae each1 instead of one of #bj as in ,professor ,newcomb's tables1 so that the excess in the number of stars over the other zones is not so large4 ,they show also a slight preponderance in all the zones of the southern hemisphere1 but this is not great1 and may probably be due to the clearer atmosphere of the ,cape of ,good ,hope as compared with that of ,england4 ,illustration3 ,,diagram ,,of ,,star-,,density4 ,from ,table in ,sir ,j4 ,herschel's ,outlines of ,astronomy 7#ajth ed41 pp4 #egg-egh74 ,it need only be noted here that this diagram shows the same general features as those already given1 of a continuous increase of star-density from the poles of the ,galaxy1 but more rapidly as the ,galaxy itself is more nearly approached4 ,this fact must1 therefore1 be accepted as indisputable4 ,,clusters ,,and ,,nebul ,,in ,,relation ,,to ,,the ,,galaxy ,an important factor in the structure of the heavens is afforded by the distribution of the two classes of objects known as clusters and nebul4 ,although we can form an almost continuous series from double stars which revolve round their common centre of gravity1 through triple and quadruple stars1 to groups and aggregations of indefinite extent--of which the ,pleiades form a good example1 since the six stars visible to the naked eye are increased to hundreds by high telescopic powers1 while photographs with three hours' exposure show more than #bjjj stars--yet none of these correspond to the large class known as clusters1 whether globular or irregular1 which are very numerous1 about #fjj having been recorded by ,sir ,john ,herschel more than fifty years ago4 ,many of these are among the most beautiful and striking objects in the heavens even with a very small telescope or good opera-glass4 ,such is the luminous spot called ,praesepe1 or the ,beehive in the constellation ,cancer1 and another in the sword handle of ,perseus4 ,in the southern hemisphere there is a hazy star of about the fourth magnitude1 ,omega ,centauri1 which with a good telescope is seen to be really a magnificent cluster nearly two-thirds the diameter of the moon1 and described by ,sir ,john ,herschel as very gradually increasing in brightness to the centre1 and composed of innumerable stars of the thirteenth and fifteenth magnitudes1 forming the richest and largest object of the kind in the heavens4 ,he describes it as having rings like lace-work formed of the larger stars4 ,by actual count1 on a good photograph1 there are more than #fjjj stars1 while other observers consider that there are at least #aj1jjj4 ,in the northern hemisphere one of the finest is that in the constellation ,hercules1 known as #ac ,messier4 ,it is just visible to the naked eye or with an opera glass as a hazy star of the sixth magnitude1 but a good telescope shows it to be a globular cluster1 and the great ,lick telescope resolves even the densest central portion into distinct stars1 of which ,sir ,john ,herschel considered there were many thousands4 ,these two fine clusters are figured in many of the modern popular works on astronomy1 and they afford an excellent idea of these beautiful and remarkable objects1 which1 when more thoroughly studied1 will probably aid in elucidating some of the obscure problems connected with the constitution and development of the stellar universe4 ,but for the purpose of the present work the most interesting fact connected with star-clusters is their remarkable distribution in the heavens4 ,their special abundance in and near the ,milky ,way had often been noted1 but the full importance of the fact could not be appreciated till ,mr4 ,proctor and1 later1 ,mr4 ,sidney ,waters marked down1 on maps of the two hemispheres1 all the star-clusters and nebul in the best catalogues4 ,the result is most interesting4 ,the clusters are seen to be thickly strewn over the entire course of the ,milky ,way1 and along its margins1 while in every other part of the heavens they are thinly scattered at very distant intervals1 with the one exception of the ,magellanic clouds of the southern hemisphere where they are again densely grouped2 and if anything were needed to prove the physical connection of these clusters with the ,galaxy it would be their occurrence in these extensive nebulous patches which seem like outlying portions of the ,milky ,way itself4 ,with these two exceptions probably not one-twentieth part of the whole number of star-clusters are found in any part of the heavens remote from the ,milky ,way4 ,nebul were for a long time confounded with star-clusters1 because it was thought that with sufficient telescopic power they could all be resolvable into stars as in the case of the ,milky ,way itself4 ,but when the spectroscope showed that many of the nebul consisted wholly or mainly of glowing gases1 while neither the highest powers of the best telescopes nor the still greater powers of the photographic plate gave any indications of resolvability1 although a few stars were often found to be1 as it were1 entangled in them1 and evidently forming part of them1 it was seen that they constituted a distinct stellar phenomenon1 a view which was enforced and rendered certain by their quite unique mode of distribution4 ,a few of the larger and irregular type1 as in the case of the grand ,orion nebula visible to the naked eye1 the great spiral nebula in ,andromeda1 and the wonderful ,keyhole nebula round ,eta ,args1 are situated in or near the ,milky ,way2 but with these and a few other exceptions the overwhelming majority of the smaller irresolvable nebul appear to avoid it1 there being a space almost wholly free from nebul along its borders1 both in the northern and southern hemispheres2 while the great majority are spread over the sky1 far away from it in the southern hemisphere1 and in the north clustering in a very marked degree around the galactic pole4 ,the distribution of nebul is thus seen to be the exact opposite to that of the star-clusters1 while both are so distinctly related to the position of the ,milky ,way--the ground-plane of the sidereal system1 as ,sir ,john ,herschel termed it--that we are compelled to include them all as connected portions of one grand and1 to some extent1 symmetrical universe1 whose remarkable and opposite mode of distribution over the heavens may probably afford a clue to the mode of development of that universe and to the changes that are even now taking place within it4 ,the maps referred to above are of such great importance1 and are so essential to a clear comprehension of the nature and constitution of the vast sidereal system which surrounds us1 that ,i have1 with the permission of the ,royal ,astronomical ,society1 reproduced them here4 7,see end of volume47 ,a careful examination of them will give a clearer idea of the very remarkable facts of distribution of star-clusters and nebul than can be afforded by any amount of description or of numerical statements4 ,the forms of many of the nebul are very curious4 ,some are quite irregular1 as the ,orion nebula1 the ,keyhole nebula in the southern hemisphere1 and many others4 ,some show a decidedly spiral form1 as those in ,andromeda and ,canes ,venatici2 others again are annular or ring-shaped1 as those in ,lyra and ,cygnus1 while a considerable number are termed planetary nebul1 from their exhibiting a faint circular disc like that of a planet4 ,many have stars or groups of stars evidently forming parts of them1 and this is especially the case with those of the largest size4 ,but all these are comparatively few in number and more or less exceptional in type1 the great majority being minute cloudy specks only visible with good telescopes1 and so faint as to leave much doubt as to their exact shape and nature4 ,sir ,john ,herschel catalogued #ejjj in #ahfd1 and more than #hjjj were discovered up to #ahij2 while the application of the camera has so increased the numbers that it is thought there may really be many hundreds of thousands of them4 ,the spectroscope shows the larger irregular nebul to be gaseous1 as are the annular and planetary nebul as well as many very brilliant white stars2 and all these objects are most frequent in or near the ,milky ,way4 ,their spectra show a green line not produced by any terrestrial element4 ,with the great ,lick telescope several of the planetary nebul have been found to be irregular and sometimes to be formed of compressed or looped rings and other curious forms4 ,many of the smaller nebul are double or triple1 but whether they really form revolving systems is not yet known4 ,the great mass of the small nebul that occupy large tracts of the heavens remote from the ,galaxy are often termed irresolvable nebul1 because the highest powers of the largest telescopes show no indication of their being star-clusters1 while they are too faint to give any definite indications of structure in the spectroscope4 ,but many of them resemble comets in their forms1 and it is thought not impossible that they may be not very dissimilar in constitution4 99 99 99 99 99 ,we have now passed in review the main features presented to us in the heavens outside the solar system1 so far as regards the numbers and distribution of the lucid stars 7those visible to the naked eye7 as well as those brought to view by the telescope2 the form and chief characteristics of the ,milky ,way or ,galaxy2 and lastly1 the numbers and distribution of those interesting objects--star-clusters and nebul in their special relations to the ,milky ,way4 ,this examination has brought clearly before us the unity of the whole visible universe2 that everything we can see1 or obtain any knowledge of1 with all the resources of modern gigantic telescopes1 of the photographic plate1 and of the even more marvellous spectroscope1 forms parts of one vast system which may be shortly and appropriately termed the ,stellar universe4 ,in our next chapter we shall carry the investigation a step further1 by sketching in outline what is known of the motions and distances of the stars1 and thus obtain some important information bearing upon our special subject of inquiry4 ,,chapter ,v ,,distance ,,of ,,the ,,stars--,,the ,,sun',s ,,motion ,,through ,,space ,in early ages1 before any approximate idea was reached of the great distances of the stars from us1 the simple conception of a crystal sphere to which these luminous points were attached and carried round every day on an axis near which our pole-star is situated1 satisfied the demands for an explanation of the phenomena4 ,but when ,copernicus set forth the true arrangement of the heavenly bodies1 earth and planets alike revolving round the sun at distances of many millions of miles1 and when this scheme was enforced by the laws of ,kepler and the telescopic discoveries of ,galileo1 a difficulty arose which astronomers were unable satisfactorily to overcome4 ,if1 said they1 the earth revolves round the sun at a distance which cannot be less 7according to ,kepler's measurement of the distance of ,mars at opposition7 than #ac-a_/b millions of miles1 then how is it that the nearer stars are not seen to shift their apparent places when viewed from opposite sides of this enormous orbit8 ,copernicus1 and after him ,kepler and ,galileo1 stoutly maintained that it was because the stars were at such an enormous distance from us that the earth's orbit was a mere point in comparison4 ,but this seemed wholly incredible1 even to the great observer ,tycho ,brah1 and hence the ,copernican theory was not so generally accepted as it otherwise would have been4 ,galileo always declared that the measurement would some day be made1 and he even suggested the method of effecting it which is now found to be the most trustworthy4 ,but the sun's distance had to be first measured with greater accuracy1 and that was only done in the latter part of the eighteenth century by means of transits of ,venus2 and by later observations with more perfect instruments it is now pretty well fixed at about #ib1ghj1jjj miles1 the limits of error being such that #ib-c_/d millions may perhaps be quite as accurate4 ,with such an enormous base-line as twice this distance1 which is available by making observations at intervals of about six months when the earth is at opposite points in its orbit1 it seemed certain that some parallax or displacement of the nearer stars could be found1 and many astronomers with the best instruments devoted themselves to the work4 ,but the difficulties were enormous1 and very few really satisfactory results were obtained till the latter half of the nineteenth century4 ,about forty stars have now been measured with tolerable certainty1 though of course with a considerable margin of possible or probable error2 and about thirty more1 which are found to have a parallax of one-tenth of a second or less1 must be considered to leave a very large margin of uncertainty4 ,the two nearest fixed stars are ,alpha ,centauri and #fa ,cygni4 ,the former is one of the brightest stars in the southern hemisphere1 and is about #bge1jjj times as far from us as the sun4 ,the light from this star will take #d-a_/d years to reach us1 and this 'light-journey1' as it is termed1 is generally used by astronomers as an easily remembered mode of recording the distances of the fixed stars1 the distance in miles--in this case about #be millions of millions--being very cumbrous4 ,the other star1 #fa ,cygni1 is only of about the fifth magnitude1 yet it is the second nearest to us1 with a light-journey of about #g-a_/d years4 ,if we had no other determinations of distance than these two1 the facts would be of the highest importance4 ,they teach us1 first1 that magnitude or brightness of a star is no proof of nearness to us1 a fact of which there is much other evidence2 and in the second place1 they furnish us with a probable minimum distance of independent suns from one another1 which1 in proportion to their sizes1 some being known to be many times larger than our sun1 is not more than we might expect4 ,this remoteness may be partly due to those which were once nearer together having coalesced under the influence of gravitation4 ,as this measurement of the distance of the nearer stars should be clearly understood by every one who wishes to obtain some real comprehension of the scale of this vast universe of which we form a part1 the method now adopted and found to be most effectual will be briefly explained4 ,everyone who is acquainted with the rudiments of trigonometry or mensuration1 knows that an inaccessible distance can be accurately determined if we can measure a base-line from both ends of which the inaccessible object can be seen1 and if we have a good instrument with which to measure angles4 ,the accuracy will mainly depend upon our base-line being not excessively short in comparison with the distance to be measured4 ,if it is as much as half or even a quarter as long the measurement may be as accurate as if directly performed over the ground1 but if it is only one-hundredth or one-thousandth part as long1 a very small error either in the length of the base or in the amount of the angles will produce a large error in the result4 ,in measuring the distance of the moon1 the earth's diameter1 or a considerable portion of it1 has served as a base-line4 ,either two observers at great distances from each other1 or the same observer after an interval of nine or ten hours1 may examine the moon from positions six or seven thousand miles apart1 and by accurate measurements of its angular distance from a star1 or by the time of its passage over the meridian of the place as observed with a transit instrument1 the angular displacement can be found and the distance determined with very great accuracy1 although that distance is more than thirty times the length of the base4 ,the distance of the planet ,mars when nearest to us has been found in the same way4 ,his distance from us even when at his nearest point during the most favourable oppositions is about #cf million miles1 or more than four thousand times the earth's diameter1 so that it requires the most delicate observations many times repeated and with the finest instruments to obtain a tolerably approximate result4 ,when this is done1 by ,kepler's law of the fixed proportion between the distances of planets from the sun and their times of revolution1 the proportionate distance of all the other planets and that of the sun can be ascertained4 ,this method1 however1 is not sufficiently accurate to satisfy astronomers1 because upon the sun's distance that of every other member of the solar system depends4 ,fortunately there are two other methods by which this important measurement has been made with much greater approach to certainty and precision4 ,illustration3 ,diagram illustrating the transit of ,venus4 ,the first of these methods is by means of the rare occasions when the planet ,venus passes across the sun's disc as seen from the earth4 ,when this takes place1 observations of the transit1 as it is termed1 are made at remote parts of the earth1 the distance between which places can of course easily be calculated from their latitudes and longitudes4 ,the diagram here given illustrates the simplest mode of determining the sun's distance by this observation1 and the following description from ,proctor's ,old and ,new ,astronomy is so clear that ,i copy it verbally3--',v represents ,venus passing between the ,earth ,e and the ,sun ,s2 and we see how an observer at ,e will see ,venus as at v'1 while an observer at ,e' will see her as at v4 ,the measurement of the distance v v'1 as compared with the diameter of the sun's disc1 determines the angle v ,v v' or ,e ,v ,e'2 whence the distance ,e ,v can be calculated from the known length of the base-line ,e ,e'4 ,for instance1 it is known 7from the known proportions of the ,solar ,system as determined from the times of revolution by ,kepler's third law7 that ,e ,v bears to ,v v the proportion #bh to #gb1 or #g to #ah2 whence ,e ,e' bears to v v' the same proportion4 ,suppose1 now1 that the distance between the two stations is known to be #gjjj miles1 so that v v' is #ah1jjj miles2 and that v v' is found by accurate measurement to be #a_/dh part of the sun's diameter4 ,then the sun's diameter1 as determined by this observation1 is #dh times #ah1jjj miles1 or #hfd1jjj miles2 whence from his known apparent size1 which is that of a globe #ajg-a_/c times farther away from us than its own diameter1 his distance is found to be #ib1gcf1jjj miles4' ,of course1 there being two observers1 the proportion of the distance v v' to the diameter of the sun's disc cannot be measured directly1 but each of them can measure the apparent angular distance of the planet from the sun's upper and lower margins as it passes across the disc1 and thus the angular distance between the two lines of transit can be obtained4 ,the distance v v' can also be found by accurately noting the times of the upper and lower passage of ,venus1 which1 as the line of transit is considerably shorter in one than the other1 gives by the known properties of the circle the exact proportion of the distance between them to the sun's diameter2 and as this is found to be the most accurate method1 it is the one generally adopted4 ,for this purpose the stations of the observers are so chosen that the length of the two chords1 v and v'1 may have a considerable difference1 thus rendering the measurement more easy4 ,the other method of determining the sun's distance is by the direct measurement of the velocity of light4 ,this was first done by the ,french physicist1 ,fizeau1 in #ahdi1 by the use of rapidly revolving mirrors1 as described in most works on physics4 ,this method has now been brought to such a decree of perfection that the sun's distance so determined is considered to be equally trustworthy with that derived from the transits of ,venus4 ,the reason that the determination of the velocity of light leads to a determination of the sun's distance is1 because the time taken by light to pass from the sun to the earth is independently known to be #h min4 #ac-a_/c sec4 ,this was discovered so long ago as #afge by means of the eclipses of ,jupiter's satellites4 ,these satellites revolve round the planet in from #a-c_/d to #af days1 and1 owing to their moving very nearly in the plane of the ecliptic and the shadow of ,jupiter being so large1 the three which are nearest to the planet are eclipsed at every revolution4 ,this rapid revolution of the satellites and frequency of the eclipses enabled their periods of recurrence to be determined with extreme accuracy1 especially after many years of careful observation4 ,it was then found that when ,jupiter was at its farthest distance from the earth the eclipses of the satellites took place a little more than eight minutes later than the time calculated from the mean period of revolution1 and when the planet was nearest to us the eclipses occurred the same amount earlier4 ,and when further observation showed that there was no difference between calculation and observation when the planet was at its mean distance from us1 and that the error arose and increased exactly in proportion to our varying distance from it1 then it became clear that the only cause adequate to produce such an effect was1 that light had not an infinite velocity but travelled at a certain fixed rate4 ,this however1 though a highly probable explanation1 was not absolutely proved till nearly two centuries later1 by means of two very difficult measurements--that of the actual distance of the sun from the earth1 and that of the actual speed of light in miles per second2 the latter corresponding almost exactly with the speed deduced from the eclipses of ,jupiter's satellites and the sun's distance as measured by the transits of ,venus4 ,illustration3 7,a7 and the last #b lines of the next paragraph should be replaced with3 one-tenth of an inch long1 and from a point 7,a7 #e-c_/d inches from it 7accurately #e4gbieggie inches7 we draw straight lines to ,b and ,c4 ,then the angle at ,a is one degree47 ,but this problem of measuring the sun's distance1 and through it the dimensions of the orbits of all the planets of our system1 sinks into insignificance when compared with the enormous difficulties in the way of the determination of the distance of the stars4 ,as a great many people1 perhaps the majority of the readers of any popular scientific book1 have little knowledge of mathematics and cannot realise what an angle of a minute or a second really means1 a little explanation and illustration of these terms will not be out of place4 ,an angle of one degree 7#a7 is the #cfjth part of a circle 7viewed from its centre71 the #ijth part of a right angle1 the #fjth part of either of the angles of an equilateral triangle4 ,to see exactly how much is an angle of one degree we draw a short line 7,b ,c7 one-tenth of an inch long1 and from a point we draw straight lines to ,b and ,c4 ,then the angle at ,a is one degree4 ,now1 in all astronomical work1 one degree is considered to be quite a large angle4 ,even before the invention of the telescope the old observers fixed the position of the stars and planets to half or a quarter of a degree1 while ,mr4 ,proctor thinks that ,tycho ,brah's positions of the stars and planets were correct to about one or two minutes of arc4 ,but a minute of arc is obtained by dividing the line ,b ,c into sixty equal parts and seeing the distance between two of these with the naked eye from the point ,a4 ,but as very long-sighted people can see very minute objects at #aj or #ab inches distance1 we may double the distance ,a ,b1 and then making the line ,b ,c one three-hundredth part of an inch long1 we shall have the angle of one minute which ,tycho ,brah was perhaps able to measure4 ,how very large an amount a minute is to the modern astronomer is1 however1 well shown by the fact that the maximum difference between the calculated and observed positions of ,uranus1 which led ,adams and ,leverrier to search for and discover ,neptune1 was only #a-a_/b minutes1 a space so small as to be almost invisible to the average eye1 so that if there had been two planets1 one in the calculated1 the other in the observed place1 they would have appeared as one to unassisted vision4 ,in order now to realise what one second of arc really means1 let us look at the circle here shown1 which is as nearly as possible one-tenth of an inch in diameter--7one-,o-tenth of an inch74 ,if we remove this circle to a distance of #bh feet #h inches it will subtend an angle of one minute1 and we shall have to place it at a distance of nearly #agcj feet--almost one-third of a mile--to reduce the angle to one second4 ,but the very nearest to us of the fixed stars1 ,alpha ,centauri1 has a parallax of only three-fourths of a second2 that is1 the distance of the earth from the sun--about #ib-c_/d millions of miles--would appear no wider1 seen from the nearest star1 than does three-fourths of the above small circle at one-third of a mile distance4 ,to see this circle at all at that distance would require a very good telescope with a power of at least #ajj1 while to see any small part of it and to measure the proportion of that part to the whole would need very brilliant illumination and a large and powerful astronomical telescope4 ,,what ,,is ,a ,,million8 ,but when we have to deal with millions1 and even with hundreds and thousands of millions1 there is another difficulty--that few people can form any clear conception of what a million is4 ,it has been suggested that in every large school the walls of one room or hall should be devoted to showing a million at one view4 ,for this purpose it would be necessary to have a hundred large sheets of paper each about #d feet #f inches square1 ruled in quarter inch squares4 ,in each alternate square a round black wafer or circle should be placed a little over-lapping the square1 thus leaving an equal amount of white space between the black spots4 ,at each tenth spot a double width should be left so as to separate each hundred spots 7#aj #aj74 ,each sheet would then hold ten thousand spots1 which would all be distinctly visible from the middle of a room #bj feet wide1 each horizontal or vertical row containing a thousand4 ,one hundred such sheets would contain a million spots1 and they would occupy a space #dej feet long in one row1 or #ij feet long in five rows1 so that they would entirely cover the walls of a room1 about #cj feet square and #be feet high1 from floor to ceiling1 allowing space for doors but not for windows1 the hall or gallery being lighted from above4 ,such a hall would be in the highest degree educational in a country where millions are spoken of so glibly and wasted so recklessly2 while no one can really appreciate modern science1 dealing as it does with the unimaginably great and little1 unless he is enabled to realise by actual vision1 and summing up1 what a vast number is comprised in one of those millions1 which1 in modern astronomy and physics1 he has to deal with not singly only1 but by hundreds and thousands or even by millions4 ,in every considerable town1 at all events1 a hall or gallery should have a million thus shown upon its walls4 ,it would in no way interfere with the walls being covered when required with maps1 or ornamental hangings1 or pictures2 but when these were removed1 the visible and countable million would remain as a permanent lesson to all visitors2 and ,i believe that it would have widespread beneficial effects in almost every department of human thought and action4 ,on a small scale any one can do this for himself by getting a hundred sheets of engineer's paper ruled in small squares1 and making the spots very small2 and even this would be impressive1 but not so much so as on the larger scale4 ,in order to enable every reader of this volume at once to form some conception of the number of units in a million1 ,i have made an estimate of the number of letters contained in it1 and ,i find them to amount to about #dbj1jjj--;considerably less than half a million4 ,try and realise1 when reading it1 that if every letter were a pound sterling1 we waste as many pounds as there are letters in two such volumes whenever we build a battleship4 ,having thus obtained some real conception of the immensity of a million1 we can better realise what it must be to have every one of the dots above described1 or every one of the letters in two such volumes as this lengthened out so as to be each a mile long1 and even then we should have reached little more than a hundredth part of the distance from our earth to the sun4 ,when1 by careful consideration of these figures1 we have even partially realised this enormous distance1 we may take the next step1 which is1 to compare this distance with that of the nearest fixed star4 ,we have seen that the parallax of that star is three-fourths of a second1 an amount which implies that the star is #bga1djj times as far from us as our sun is4 ,if after seeing what a million is1 and knowing that the sun is #ib-c_/d times this distance from us in miles--a distance which itself is almost inconceivable to us--we find that we have to multiply this almost inconceivable distance #bga1djj times--more than a quarter of a million times--to reach the nearest of the fixed stars1 we shall begin to realise1 however imperfectly1 how vast is the system of suns around us1 and on what a scale of immensity the material universe1 which we see so gloriously displayed in the starry heavens and the mysterious galaxy1 is constructed4 ,this somewhat lengthy preliminary discussion is thought necessary in order that my readers may form some idea of the enormous difficulty of obtaining any measurement whatever of such distances4 ,i now propose to point out what the special difficulties are1 and how they have been overcome2 and thus ,i hope to be able to satisfy them that the figures astronomers give us of the distances of the stars are in no way mere guesses or probabilities1 but are real measurements which1 within certain not very wide limits of error1 may be trusted as giving us correct ideas of the magnitude of the visible universe4 ,,measurement ,,of ,,stellar ,,distances ,the fundamental difficulty of this measurement is1 of course1 that the distances are so vast that the longest available base-line1 the diameter of the earth's orbit1 only subtends an angle of little more than a second from the nearest star1 while for all the rest it is less than one second and often only a small fraction of it4 ,but this difficulty1 great as it is1 is rendered far greater by the fact that there is no fixed point in the heavens from which to measure1 since many of the stars are known to be in motion1 and all are believed to be so in varying degrees1 while the sun itself is now known to be moving among the stars at a rate which is not yet accurately determined1 but in a direction which is fairly well known4 ,as the various motions of the earth while passing round the sun1 though extremely complex1 are very accurately known1 it was first attempted to determine the changed position of stars by observations1 many times repeated at six months' intervals1 of the moment of their passage over the meridian and their distance from the zenith2 and then by allowing for all the known motions of the earth1 such as precession of the equinoxes and nutation of the earth's axis1 as well as for refraction and for the aberration of light1 to determine what residual effect was due to the difference of position from which the star was viewed2 and a result was thus obtained in several cases1 though almost always a larger one than has been found by later observations and by better methods4 ,these earlier observations1 however perfect the instruments and however skilful the observer1 are liable to errors which it seems impossible to avoid4 ,the instruments themselves are subject in all their parts to expansion and contraction by changes of temperature2 and when these changes are sudden1 one part of the instrument may be affected more than another1 and this will often lead to minute errors which may seriously affect the amount to be measured when that is so small4 ,another source of error is due to atmospheric refraction1 which is subject to changes both from hour to hour and at different seasons4 ,but perhaps most important of all are minute changes in level of the foundations of the instruments even when they are carried down to solid rock4 ,both changes of temperature and changes of moisture of the soil produce minute alterations of level2 while earth-tremors and slow movements of elevation or depression are now known to be very frequent4 ,owing to all these causes1 actual measurements of differences of position at different times of the year1 amounting to small fractions of a second1 are found to be too uncertain for the determination of such minute angles with the required accuracy4 ,but there is another method which avoids almost all these sources of error1 and this is now generally preferred and adopted for these measurements4 ,it is1 that of measuring the distance between two stars situated apparently very near each other1 one of which has large proper motion1 while the other has none which is measurable4 ,the proper motions of the stars was first suspected by ,halley in #agag1 from finding that several stars1 whose places had been given by ,hipparchus1 #acj ,b4,c41 were not in the positions where they now ought to be2 and other observations by the old astronomers1 especially those of occultations of stars by the moon1 led to the same result4 ,since the time of ,halley very accurate observations of the stars have been made1 and in many cases it is found that they move perceptibly from year to year1 while others move so slowly that it is only after forty or fifty years that the motion can be detected4 ,the greatest proper motions yet determined amount to between #g and #h in a year1 while other stars require twenty1 or even fifty or a hundred years to show an equal amount of displacement4 ,at first it was thought that the brightest stars would have the largest proper motion1 because it was supposed they were nearest to us1 but it was soon found that many small and quite inconspicuous stars moved as rapidly as the most brilliant1 while in many very bright stars no proper motion at all can be detected4 ,that which moves most rapidly is a small star of less than the sixth magnitude4 ,it is a matter of common observation that the motion of things at a distance cannot be perceived so well as when near1 even though the speed may be the same4 ,if a man is seen on the top of a hill several miles off1 we have to observe him closely for some time before we can be sure whether he is walking or standing still4 ,but objects so enormously distant as we now know that the stars are1 may be moving at the rate of many miles in a second and yet require years of observation to detect any movement at all4 ,the proper motions of nearly a hundred stars have now been ascertained to be more than one second of arc annually1 while a large number have less than this1 and the majority have no perceptible motion1 presumably due to their enormous distance from us4 ,it is therefore not difficult in most cases to find one or two motionless stars sufficiently close to a star having a large proper motion 7anything more than one-tenth of a second is so called7 to serve as fixed points of measurement4 ,all that is then required is1 to measure with extreme accuracy the angular distance of the moving from the fixed stars at intervals of six months4 ,the measurements can be made1 however1 on every fine night1 each one being compared with one at nearly an interval of six months from it4 ,in this way a hundred or more measurements of the same star may be made in a year1 and the mean of the whole1 allowance being made for proper motion in the interval1 will give a much more accurate result than any single measurement4 ,this kind of measurement can be made with extreme accuracy when the two stars can be seen together in the field of the telescope2 either by the use of a micrometer1 or by means of an instrument called a heliometer1 now often constructed for the purpose4 ,this is an astronomical telescope of rather large size1 the object glass of which is cut in two straight across the centre1 and the two halves made to slide upon each other by means of an exceedingly fine and accurate screw-motion1 so adjusted and tested as to measure the angular distance of two objects with extreme accuracy4 ,this is done by the number of turns of the screw required to bring the two stars into contact with each other1 the image of each one being formed by one of the halves of the object glass4 ,but the greatest advantage of this method of determining parallax is1 as ,sir ,john ,herschell points out1 that it gets rid of all the sources of error which render the older methods so uncertain and inaccurate4 ,no corrections are required for precession1 nutation1 or aberration1 since these affect both stars alike1 as is the case also with refraction2 while alterations of level of the instrument have no prejudicial effect1 since the measures of angular distance taken by this method are quite independent of such movements4 ,a test of the accuracy of the determination of parallax by this instrument is the very close agreement of different observers1 and also their agreement with the new and perhaps even superior method by photography4 ,this method was first adopted by ,professor ,pritchard of the ,oxford ,observatory1 with a fine reflector of thirteen inches aperture4 ,its great advantage is1 that all the small stars in the vicinity of the star whose parallax is sought are shown in their exact positions upon the plate1 and the distances of all of them from it can be very accurately measured1 and by comparing plates taken at six months' intervals1 each of these stars gives a determination of parallax1 so that the mean of the whole will lead to a very accurate result4 ,should1 however1 the result from any one of these stars differ considerably from that derived from the rest1 it will be due in all probability to that star having a proper motion of its own1 and it may therefore be rejected4 ,to illustrate the amount of labour bestowed by astronomers on this difficult problem1 it may be mentioned that for the photographic measurement of the star #fa ,cygni1 #ccj separate plates were taken in #ahhf-g1 and on these #cj1jjj measurements of distances of the pairs of star-images were made4 ,the result agreed closely with the best previous determination by ,sir ,robert ,ball1 using the micrometer1 and the method was at once admitted by astronomers as being of the greatest value4 ,although1 as a rule1 stars having large proper motions are found to be comparatively near us1 there is no regular proportion between these quantities1 indicating that the rapidity of the motion of the stars varies greatly4 ,among fifty stars whose distances have been fairly well determined1 the rate of actual motion varies from one or two up to more than a hundred miles per second4 ,among six stars with less than a tenth of a second of annual proper motion there is one with a parallax of nearly half a second1 and another of one-ninth of a second1 so that they are nearer to us than many stars which move several seconds a year4 ,this may be due to actual slowness of motion1 but is almost certainly caused in part by their motion being either towards us or away from us1 and therefore only measurable by the spectroscope2 and this had not been done when the lists of parallaxes and proper motions from which these facts are taken were published4 ,it is evident that the actual direction and rate of motion of a star cannot be known till this radial movement1 as it is termed--that is1 towards or away from us--has been measured2 but as this element always tends to increase the visually observed rate of motion1 we cannot1 through its absence1 exaggerate the actual motions of the stars4 ,,the ,,sun',s ,,movement ,,through ,,space ,but there is yet another important factor which affects the apparent motions of all the stars--the movement of our sun1 which1 being a star itself1 has a proper motion of its own4 ,this motion was suspected and sought for by ,sir ,william ,herschel a century ago1 and he actually determined the direction of its motion towards a point in the constellation ,hercules1 not very far removed from that fixed upon as the average of the best observations since made4 ,the method of determining this motion is very simple1 but at the same time very difficult4 ,when we are travelling in a railway carriage near objects pass rapidly out of sight behind us1 while those farther from us remain longer in view1 and very distant objects appear almost stationary for a considerable time4 ,for the same reason1 if our sun is moving in any direction through space1 the nearer stars will appear to travel in an opposite direction to our movement1 while the more distant will remain quite stationary4 ,this movement of the nearest stars is detected by an examination and comparison of their proper motions1 by which it is found that in one part of the heavens there is a preponderance of the proper motions in one direction and a deficiency in the opposite direction1 while in the directions at right angles to these the proper motions are not on the average greater in one direction than in the opposite4 ,but the proper motions of the stars being themselves so minute1 and also so irregular1 it is only by a most elaborate mathematical investigation of the motions of hundreds or even of thousands of stars1 that the direction of the solar motion can be determined4 ,till quite recently astronomers were agreed that the motion was towards a point in ,hercules near the outstretched arm in the figure of that constellation4 ,but the latest inquiries into this problem1 involving the comparison of the motions of several thousand stars in all parts of the heavens1 have led to the conclusion that the most probable direction of the 'solar apex' 7as the point towards which the sun is moving is termed71 is in the adjacent constellation ,lyra1 and not far from the brilliant star ,vega4 ,this is the position which ,professor ,newcomb of ,washington thinks most probable1 though there is still room for further investigation4 ,to determine the rate of the motion is very much more difficult than to fix its direction1 because the distances of so few stars have been determined1 and very few indeed of these lie in the directions best adapted to give accurate results4 ,the best measurements down to #ahij led to a motion of about #ae miles a second4 ,but more recently the ,american astronomer1 ,campbell1 has determined by the spectroscope the motion in the line of sight of a considerable number of stars towards and away from the solar apex1 and by comparing the average of these motions1 he derives a motion for the sun of about #ab-a_/b miles a second1 and this is probably as near as we can yet reach towards the true amount4 ,,some ,,numerical ,,results ,,of ,,the ,,above ,,measurements ,the measurements of distances and proper motions of a considerable number of the stars1 of the motion of our sun in space 7its proper motion71 together with accurate determinations of the comparative brilliancy of the brightest stars as compared with our sun and with each other1 have led to some very remarkable numerical results which serve as indications of the scale of magnitude of the stellar universe4 ,the parallaxes of about fifty stars have now been repeatedly measured with such consistent results that ,professor ,newcomb considers them to be fairly trustworthy1 and these vary from one-hundredth to three-quarters of a second4 ,three more1 all stars of the first magnitude--,rigel1 ,canopus1 and ,alpha ,cygni--have no measurable parallax1 notwithstanding the long-continued efforts of many astronomers1 affording a striking example of the fact that brilliancy alone is no test of proximity4 ,six more stars have a parallax of only one-fiftieth of a second1 and five of these are either of the first or second magnitudes4 ,of these nine stars having very small parallax or none1 six are situated in or near to the ,milky ,way1 another indication of exceeding remoteness1 which is further shown by the fact that they all have a very small proper motion or none at all4 ,these facts support the conclusion1 which had been already reached by astronomers from a careful study of the distribution of the stars1 that the larger portion of the stars of all magnitudes scattered throughout the ,milky ,way or along its borders really belong to the same great system1 and may be said to form a part of it4 ,this is a conclusion of extreme importance because it teaches us that the grandest of the suns1 such as ,rigel and ,betelgeuse in the constellation ,orion1 ,antares in the ,scorpion1 ,deneb in the ,swan 7,alpha ,cygni71 and ,canopus 7,alpha ,argus71 are in all probability as far removed from us as are the innumerable minute stars which give the nebulous or milky appearance to the ,galaxy4 ,it is well to consider for a moment what these facts mean4 ,professor ,s4 ,newcomb1 one of the highest authorities on these problems1 tells us that the long series of measurements to discover the parallax of ,canopus1 the brightest star in the southern hemisphere1 would have shown a parallax of one-hundredth of a second1 had such existed4 ,yet the results always seemed to converge to a mean of #j4jjj6 ,suppose1 then1 we assume the parallax of this star to be somewhat less than the hundredth of a second--let us say #a_/abe of a second4 ,at the distance this gives1 light would take almost exactly #djj years to reach us1 so that if we suppose this very brilliant star to be situated a little on this side of the ,galaxy1 we must give to that great luminous circle of stars a distance of about #ejj light years4 ,we shall now perceive the advantage of being able to realise what a million really is4 ,a person who had once seen a wall-space more than #ajj feet long and #bj feet high completely covered with quarter-inch spots a quarter of an inch apart2 and then tried to imagine every spot to be a mile long and to be placed end to end in one row1 would form a very different conception of a million miles than those who almost daily read of millions1 but are quite unable to visualise even one of them4 ,having really seen one million1 we can partially realise the velocity of light1 which travels over this million miles in a little less than #e-a_/b seconds2 and yet light takes more than #d-a_/c years at this inconceivable speed to come to us from the very nearest of the stars4 ,to realise this still more impressively1 let us take the distance of this nearest star1 which is #bf millions of millions of miles4 ,let us look in imagination at this large and lofty hall covered from floor to ceiling with quarter-inch spots--only one million4 ,let all these be imagined as miles4 ,then repeat this number of miles in a straight line1 one after the other1 as many times as there are spots in this hall2 and even then you have reached only one twenty-sixth part of the distance to the nearest fixed star6 ,this million times a million miles has to be repeated twenty-six times to reach the nearest fixed star2 and it seems probable that this gives us a good indication of the distance from each other of at least all the stars down to the sixth magnitude1 perhaps even of a large number of the telescopic stars4 ,but as we have found that the bright stars of the ,milky ,way must be at least one hundred times farther from us than these nearest stars1 we have found what may be termed a minimum distance for that vast star-ring4 ,it may be immensely farther1 but it is hardly possible that it should be anything less4 ,,the ,,probable ,,size ,,of ,,the ,,stars ,having thus obtained an inferior limit for the distance of several stars of the first magnitude1 and their actual brilliancy or light-emission as compared with our sun having been carefully measured1 we have afforded us some indication of size though perhaps an uncertain one4 ,by these means it has been found that ,rigel gives out about ten thousand times as much light as our sun1 so that if its surface is of the same brightness1 it must be a hundred times the diameter of the sun4 ,but as it is one of the white or ,sirian type of stars it is probably very much more luminous1 but even if it were twenty times brighter it would still have to be twenty-two and a half times the diameter of the sun2 and as the stars of this type are probably wholly gaseous and much less dense than our sun1 this enormous size may not be far from the truth4 ,it is believed that the ,sirian stars generally have a greater surface brilliancy than our sun4 ,beta ,aurig1 a star of the second magnitude but of the ,sirian type1 is one of the double stars whose distance has been measured1 and this has enabled ,mr4 ,gore to find the mass of the binary system to be five times that of the sun1 and their light one hundred and seventeen times greater4 ,even if the density is much less than the sun's1 the intrinsic brilliancy of the surface will be considerably greater4 ,another double star1 ,gamma ,leonis1 has been found to be three hundred times more brilliant than the sun if of the same density1 but it would require to be seven times rarer than air to have the extent of surface needed to give the same amount of light if its surface emitted no more light than our sun from equal areas4 ,it is clear1 therefore1 that many of the stars are much larger than our sun as well as more luminous2 but there are also large numbers of small stars whose large proper motions1 as well as the actual measurement of some1 prove them to be comparatively near to us which yet are only about one-fiftieth part as bright as the sun4 ,these must1 therefore1 be either comparatively small1 or if large must be but slightly luminous4 ,in the case of some double stars it has been proved that the latter is the case2 but it seems probable that others are very much smaller than the average4 ,up to the present time no means of determining the size of a star by actual measurement has been discovered1 since their distances are so enormous that the most powerful telescopes show only a point of light4 ,but now that we have really measured the distance of a good many stars we are able to determine an upper limit for their actual dimensions4 ,as the nearest fixed star1 ,alpha ,centauri1 has a parallax of #j4ge1 this means that if this star has a diameter as great as our distance from the sun 7which is not much more than a hundred times the sun's diameter7 it would be seen to have a distinct disc about as large as that of ,jupiter's first satellite4 ,if it were even one-tenth of the size supposed it would probably be seen as a disc in our best modern telescopes4 ,the late ,mr4 ,ranyard remarks that if the ,nebular ,hypothesis is true1 and our sun once extended as far as the orbit of ,neptune1 then1 among the millions of visible suns there ought to be some now to be found in every stage of development4 ,but any sun having a diameter at all approaching this size1 and situated as far off as a hundred times the distance of ,alpha ,centauri1 would be seen by the ,lick telescope to have a disc half a second in diameter4 ,hence the fact that there are no stars with visible discs proves that there are no suns of the required size1 and adds another argument1 though not perhaps a strong one1 against the acceptance of the ,nebular ,hypothesis4 ,,chapter ,,vi ,,the ,,unity ,,and ,,evolution ,,of ,,the ,,star ,,system ,the very condensed sketch now given of such of the discoveries of recent ,astronomy as relate to the subject we are discussing will1 it is hoped1 give some idea both of the work already done and of the number of interesting problems yet remaining to be solved4 ,the most eminent astronomers in every part of the world look forward to the solution of these problems not1 perhaps1 as of any great value in themselves1 but as steps towards a more complete knowledge of our universe as a whole4 ,their aim is to do for the star-system what ,darwin did for the organic world1 to discover the processes of change that are at work in the heavens1 and to learn how the mysterious nebul1 the various types of stars1 and the clusters and systems of stars are related to each other4 ,as ,darwin solved the problem of the origin of organic species from other species1 and thus enabled us to understand how the whole of the existing forms of life have been developed out of pre-existing forms1 so astronomers hope to be able to solve the problem of the evolution of suns from some earlier stellar types1 so as to be able1 ultimately1 to form some intelligible conception of how the whole stellar universe has come to be what it is4 ,volumes have already been written on this subject1 and many ingenious suggestions and hypotheses have been advanced4 ,but the difficulties are very great2 the facts to be co-ordinated are excessively numerous1 and they are necessarily only a fragment of an unknown whole4 ,yet certain definite conclusions have been reached2 and the agreement of many independent observers and thinkers on the fundamental principles of stellar evolution seems to assure us that we are progressing1 if slowly yet with some established basis of truth1 towards the solution of this1 the most stupendous scientific problem with which the human intellect has ever attempted to grapple4 ,,the ,,unity ,,of ,,the ,,stellar ,,universe ,during the latter half of the nineteenth century the opinion of astronomers has been tending more and more to the conception that the whole of the visible universe of stars and nebul constitutes one complete and closely-related system2 and during the last thirty years especially the vast body of facts accumulated by stellar research has so firmly established this view that it is now hardly questioned by any competent authority4 ,the idea that the nebul were far more remote from us than the stars long held sway1 even after it had been given up by its chief supporter4 ,when ,sir ,william ,herschel1 by means of his then unapproached telescopic power1 resolved the ,milky ,way more or less completely into stars1 and showed that numerous objects which had been classed as nebul were really clusters of stars1 it was natural to suppose that those which still retained their cloudy appearance under the highest telescopic powers were also clusters or systems of stars1 which only needed still higher powers to show their true nature4 ,this idea was supported by the fact that several nebul were found to be more or less ring-shaped1 thus corresponding on a smaller scale to the form of the ,milky ,way2 so that when ,herschel discovered thousands of telescopic nebul1 he was accustomed to speak of them as so many distinct universes scattered through the immeasurable depths of space4 ,now1 although any real conception of the immensity of the one stellar universe1 of which the ,milky ,way with its associated stars is the fundamental feature1 is1 as ,i have shown1 almost unattainable1 the idea of an unlimited number of other universes1 almost infinitely remote from our own and yet distinctly visible in the heavens1 so seized upon the imagination that it became almost a commonplace of popular astronomy and was not easily given up even by astronomers themselves4 ,and this was in a large part due to the fact that ,sir ,william ,herschel's voluminous writings1 being almost all in the ,philosophical ,transactions of the ,royal ,society1 were very little read1 and that he only indicated his change of view by a few brief sentences which might easily be overlooked4 ,the late ,mr4 ,proctor appears to have been the first astronomer to make a thorough study of the whole of ,herschel's papers1 and he tells us that he read them all over five times before he was able thoroughly to grasp the writer's views at different periods4 ,but the first person to point out the real teaching of the facts as to the distribution of the nebul was not an astronomer1 but our greatest philosophical student of science in general1 ,herbert ,spencer4 ,in a remarkable essay on ',the ,nebular ,hypothesis' in the ,westminster ,review of ,july1 #aheh1 he maintained that the nebul really formed a part of our own ,galaxy and of our own stellar universe4 ,a single passage from his paper will indicate his line of argument1 which1 it may be added1 had already been partially set forth by ,sir ,john ,herschel in his ,outlines of ,astronomy4 ',if there were but one nebula1 it would be a curious coincidence were this one nebula so placed in the distant regions of space as to agree in direction with a starless spot in our own sidereal system4 ,if there were but two nebul1 and both were so placed1 the coincidence would be excessively strange4 ,what1 then1 shall we say on finding that there are thousands of nebul so placed8 ,shall we believe that in thousands of cases these far-removed galaxies happen to agree in their visible positions with the thin places in our own galaxy8 ,such a belief is impossible4' ,he then applies the same argument to the distribution of the nebul as a whole3--',in that zone of celestial space where stars are excessively abundant1 nebul are rare1 while in the two opposite celestial spaces that are farthest removed from this zone1 nebul are abundant4 ,scarcely any nebul lie near the galactic circle 7or plane of the ,milky ,way72 and the great mass of them lie round the galactic poles4 ,can this also be mere coincidence8' ,and he concludes1 from the whole mass of the evidence1 that 'the proofs of a physical connection become overwhelming4' ,nothing could be more clear or more forcible2 but ,spencer not being an astronomer1 and writing in a comparatively little read periodical1 the astronomical world hardly noticed him2 and it was from ten to fifteen years later1 when ,mr4 ,r4,a4 ,proctor1 by his laborious charts and his various papers read before the ,royal and ,royal ,astronomical ,societies from #ahfi to #ahge1 compelled the attention of the scientific world1 and thus did more perhaps than any other man to establish firmly the grand and far-reaching principle of the essential unity of the stellar universe1 which is now accepted by almost every astronomical writer of eminence in the civilised world4 ,,the ,,evolution ,,of ,,the ,,stellar ,,universe ,amid the enormous mass of observations and of suggestive speculation upon this great and most interesting problem1 it is difficult to select what is most important and most trustworthy4 ,but the attempt must be made1 because1 unless my readers have some knowledge of the most important facts bearing upon it 7besides those already set forth71 and also learn something of the difficulties that meet the inquirer into causes at every step of his way1 and of the various ideas and suggestions which have been put forth to account for the facts and to overcome the difficulties1 they will not be in a position to estimate1 however imperfectly1 the grandeur1 the marvel1 and the mystery of the vast and highly complex universe in which we live and of which we are an important1 perhaps the most important1 if not the only permanent outcome4 ,,the ,,sun ,a ,,typical ,,star ,it being now a recognised fact that the stars are suns1 some knowledge of our own sun is an essential preliminary to an inquiry into their nature1 and into the probable changes they have undergone4 ,the fact that the sun's density is only one-fourth that of the earth1 or less than one and a half times that of water1 demonstrates that it cannot be solid1 since the force of gravity at its surface being twenty-six and a half times that at the earth's surface1 the materials of a solid globe would be so compressed that the resulting density would be at least twenty times greater instead of four times less than that of the earth4 ,all the evidence goes to show that the body of the sun is really gaseous1 but so compressed by its gravitative force as to behave more like a liquid4 ,a few figures as to the vast dimensions of the sun and the amount of light and heat emitted by it will enable us better to understand the phenomena it presents1 and the interpretation of those phenomena4 ,proctor estimated that each square inch of the sun's surface emitted as much light as twenty-five electric arcs2 and ,professor ,langley has shown by experiment that the sun is #ecjj times brighter1 and eighty-seven times hotter than the white-hot metal in a ,bessemer converter4 ,the actual amount of solar heat received by the earth is sufficient1 if wholly utilised1 to keep a three-horse-power engine continually at work on every square yard of the surface of our globe4 ,the size of the sun is such1 that if the earth were at its centre1 not only would there be ample space for the moon's orbit1 but sufficient for another satellite #aij1jjj miles beyond the moon1 all revolving inside the sun4 ,the mass of matter in the sun is #gde times greater than that of all the planets combined2 hence the powerful gravitative force by which they are retained in their distant orbits4 ,what we see as the sun's surface is the photosphere or outer layer of gaseous or partially liquid matter kept at a definite level by the power of gravitation4 ,the photosphere has a granular texture implying some diversity of surface or of luminosity2 although the even contour of the sun's margin shows that these irregularities are not on a very large scale4 ,this surface is apparently rent asunder by what are termed sun-spots1 which were long supposed to be cavities1 showing a dark interior2 but are now thought to be due to downpours of cooled materials driven out from the sun1 and forming the prominences seen during solar eclipses4 ,they appear to be black1 but around their margin is a shaded border or penumbra formed of elongated shining patches crossing and over-lapping1 something like heaps of straw4 ,sometimes brilliant portions overhang the dark spots1 and often completely bridge them over2 and similar patches1 called facul1 accompany spots1 and in some cases almost surround them4 ,sun-spots are sometimes numerous on the sun's disc1 sometimes very few1 and they are of such enormous size that when present they can easily be seen with the naked eye1 protected by a piece of smoked glass2 or1 better still1 with an ordinary opera-glass similarly protected4 ,they are found to increase in number for several years1 and then to decrease2 the maxima recurring after an average period of eleven years1 but with no exactness1 since the interval between two maxima or minima is sometimes only nine and sometimes as much as thirteen years2 while the minima do not occur midway between two maxima1 but much nearer to the succeeding than to the preceding one4 ,what is more interesting is1 that variations in terrestrial magnetism follow them with great accuracy2 while violent commotions in the sun1 indicated by the sudden appearance of facul1 sun-spots1 or prominences on the sun's limb1 are always accompanied by magnetic disturbances on the earth4 ,,what ,,surrounds ,,the ,,sun ,it has been well said that what we commonly term the sun is really the bright spherical nucleus of a nebulous body4 ,this nucleus consists of matter in the gaseous state1 but so compressed as to resemble a liquid or even a viscous fluid4 ,about forty of the elements have been detected in the sun by means of the dark lines in its spectrum1 but it is almost certain that all the elements1 in some form or other1 exist there4 ,this semi-liquid glowing surface is termed the photosphere1 since from it are given out the light and heat which reach our earth4 ,immediately above this luminous surface is what is termed the 'reversing layer' or absorbing layer1 consisting of dense metallic vapours only a few hundred miles thick1 and1 though glowing1 somewhat cooler than the surface of the photosphere4 ,its spectrum1 taken1 at the moment when the sun is totally darkened1 through a slit which is directed tangentially to the sun's limb1 shows a mass of bright lines corresponding in a large degree to the dark lines in the ordinary solar spectrum4 ,it is thus shown to be a vaporous stratum which absorbs the special rays emitted by each element and forming its characteristic coloured lines1 changing them into black lines4 ,but as coloured lines are not found in this layer corresponding to all the black lines in the solar spectrum1 it is now held that special absorption must also occur in the chromosphere and perhaps in the corona itself4 ,sir ,norman ,lockyer1 in his volume on ,inorganic ,evolution1 even goes so far as to say1 that the true 'reversing layer' of the sun--that which by its absorption produced the dark lines in the solar spectrum--is now shown to be not the chromosphere itself but a layer above it1 of lower temperature4 ,above the reversing layer comes the chromosphere1 a vast mass of rosy or scarlet emanations surrounding the sun to a depth of about #djjj miles4 ,when seen during eclipses it shows a serrated waving outline1 but subject to great changes of form1 producing the prominences already mentioned4 ,these are of two kinds3 the 'quiescent1' which are something like clouds of enormous extent1 and which keep their forms for a considerable time2 and the 'eruptive1' which shoot out in towering tree-like flames or geyser-like eruptions1 and while doing so have been proved to reach velocities of over #cjj miles a second1 and subside again with almost equal rapidity4 ,the chromosphere and its quiescent prominences appear to be truly gaseous1 consisting of hydrogen1 helium1 and coronium1 while the eruptive prominences always show the presence of metallic vapours1 especially of calcium4 ,prominences increase in size and number in close accordance with the increase of sun-spots4 ,beyond the red chromosphere and prominences is the marvellous white glory of the corona1 which extends to an enormous distance round the sun4 ,like the prominences of the chromosphere1 it is subject to periodical changes in form and size1 corresponding to the sun-spot period1 but in inverse order1 a minimum of sun-spots going with a maximum extension of the corona4 ,at the total eclipse of ,july #ahgh1 when the sun's surface was almost wholly clear1 a pair of enormous equatorial streamers stretched east and west of the sun to a distance of ten millions of miles1 and less extensions of the corona occurred at the poles4 ,at the eclipses of #ahhb and #ahhc1 on the other hand1 when sun-spots were at a maximum1 the corona was regularly stellate with no great extensions1 but of high brilliancy4 ,this correspondence has been noted at every eclipse1 and there is therefore an undoubted connection between the two phenomena4 ,the light of the corona is believed to be derived from three sources--from incandescent solid or liquid particles thrown out from the sun1 from sunlight reflected from these particles1 and from gaseous emissions4 ,its spectrum possesses a green ray1 which is peculiar to it1 and is supposed to indicate a gas named 'coronium'2 in other respects the spectrum is more like that of reflected sunlight4 ,the enormous extensions of the corona into great angular streamers seem to indicate electrical repulsive forces analogous to those which produce the tails of comets4 ,connected with the sun's corona is that strange phenomenon1 the zodiacal light4 ,this is a delicate nebulosity1 which is often seen after sunset in spring and before sunrise in autumn1 tapering upwards from the sun's direction along the plane of the ecliptic4 ,under very favourable conditions it has been traced in the eastern sky in spring to #ahj from the sun's position1 indicating that it extends beyond the earth's orbit4 ,long-continued observations from the summit of the ,pic du ,midi show that this is really the case1 and that it lies almost exactly in the plane of the sun's equator4 ,it is therefore held to be produced by the minute particles thrown off the sun1 through those coronal wings and streamers which are visible only during solar eclipses4 ,the careful study of the solar phenomena has very clearly established the fact that none of the sun's envelopes1 from the reversing layer to the corona itself1 is in any sense an atmosphere4 ,the combination of enormous gravitative force with an amount of heat which turns all the elements into the liquid or gaseous state1 leads to consequences which it is difficult for us to follow or comprehend4 ,there is evidently constant internal movement or circulation in the interior of the sun1 resulting in the facul1 the sun-spots1 the intensely luminous photosphere1 and the chromosphere with its vast flaming coruscations and eruptive protuberances4 ,but it seems impossible that this incessant and violent movement can be kept up without some great and periodical or continuous inrush of fresh materials to renew the heat1 keep up the internal circulation1 and supply the waste4 ,perhaps the movement of the sun through space may bring him into contact with sufficiently large masses of matter to continually excite that internal movement without which the exterior surface would rapidly become cool and all planetary life cease4 ,the various solar envelopes are the result of this internal agitation1 uprushes1 and explosions1 while the vast white corona is probably of little more density than comets' tails1 probably even of less density1 since comets not unfrequently rush through its midst without suffering any loss of velocity4 ,the fact that none of the solar envelopes are visible to us until the light of the photosphere is completely shut off1 and that they all vanish the very instant the first gleam of direct sunlight reaches us1 is another proof of their extreme tenuity1 as is also the sharply defined edge of the sun's disc4 ,the envelopes therefore consist partly of liquid or vaporous matter1 in a very finely divided state1 driven off by explosions or by electrical forces1 and this matter1 rapidly cooling1 becomes solidified into minutest particles1 or even physical molecules4 ,much of this matter continually falls back on the sun's surface1 but a certain quantity of the very finest dust is continually driven away by electrical repulsion1 so as to form the corona and the zodiacal light4 ,the vast coronal streamers and the still more extensive ring of the zodiacal light are therefore in all probability due to the same causes1 and have a similar physical constitution with the tails of comets4 ,as the whole of our sunlight must pass through both the reversing layer and the red chromosphere1 its colour must be somewhat modified by them4 ,hence it is believed that1 if they were absent1 not only would the light and heat of the sun be considerably greater1 but its colour would be a purer white1 tending towards bluish rather than towards the yellowish tinge it actually possesses4 ,,the ,,nebular ,,and ,,meteoritic ,,hypotheses ,as the constitution of the sun1 and its agency in producing magnetism and electricity in the matter and orbs around it1 afford us our best guide to the constitution of the stars and nebul1 and to their possible action on each other1 and even upon our earth1 so the mode of evolution of the sun and solar system1 from some pre-existing condition1 is likely to help us towards gaining some knowledge of the constitution of the stellar universe and the processes of change going on there4 ,at the very commencement of the nineteenth century the great mathematician ,laplace published his ,nebular ,theory of the ,origin of the ,solar ,system2 and although he put it forth merely as a suggestion1 and did not support it with any numerical or physical data1 or by any mathematical processes1 his great reputation1 and its apparent probability and simplicity1 caused it to be almost universally accepted1 and to be extended so as to apply to the evolution of the stellar universe4 ,this theory1 very briefly stated1 is1 that the whole of the matter of the solar system once formed a globular or spheroidal mass of intensely heated gases1 extending beyond the orbit of the outermost planet1 and having a slow motion of revolution about an axis4 ,as it cooled and contracted1 its rate of revolution increased1 and this became so great that at successive epochs it threw off rings1 which1 owing to slight irregularities1 broke up1 and1 gravitating together1 formed the planets4 ,the contraction continuing1 the sun1 as we now see it1 was the result4 ,for about half a century this nebular hypothesis was generally accepted1 but during the last thirty years so many objections and difficulties have been suggested1 that it has been felt impossible to retain it even as a working hypothesis4 ,at the same time another hypothesis has been put forth which seems more in accordance with the facts of nature as we find them in our own solar system1 and which is not open to any of the objections against the nebular theory1 even if it introduces a few new ones4 ,a fundamental objection to ,laplace's theory is1 that in a gas of such extreme tenuity as the solar nebula must have been1 even when it extended only to ,saturn or ,uranus1 it could not possibly have had any cohesion1 and therefore could not have given off whole rings at distant intervals1 but only small fragments continuously as condensation went on1 and these1 rapidly cooling1 would form solid particles1 a kind of meteoric dust1 which might aggregate into numerous small planets1 or might persist for indefinite periods1 like the rings of ,saturn or the great ring of the ,asteroids4 ,another equally vital objection is1 that1 as the nebula when extending beyond the orbit of ,neptune could have had a mean density of only about the two-hundred millionth of our air at sea level1 it must have been many hundred times less dense than this at and near its outer surface1 and would there be exposed to the cold of stellar space--a cold that would solidify hydrogen4 ,it is thus evident that the gases of all the metallic and other solid elements could not possibly exist as such1 but would rapidly1 perhaps almost instantaneously1 become first liquid and then solid1 forming meteoric dust even before contraction had gone far enough to produce such increased rotation as would throw off any portion of the gaseous matter4 ,here we have the foundations of the meteoritic hypothesis which is now steadily making its way4 ,it is supported by the fact that we everywhere find proofs of such solid matter in the planetary spaces around us4 ,it falls continually upon the earth4 ,it can be collected on the ,arctic and ,alpine snows4 ,it occurs everywhere in the deepest abysses of the ocean where there are not sufficient organic deposits to mask it4 ,it constitutes1 as has now been demonstrated1 the rings of ,saturn4 ,thousands of vast rings of solid particles circulate around the sun1 and when our earth crosses any of these rings1 and their particles enter our atmosphere with planetary velocity1 the friction ignites them and we see falling stars4 ,comets' tails1 the sun's corona1 and the zodiacal light are three strange phenomena1 which1 though wholly insoluble on any theory of gaseous formation1 receive their intelligible explanation by means of excessively minute solid particles--microscopic cosmic dust--driven outward by the tremendous electrical repulsions that emanate from the sun4 ,having these and other proofs that solid matter1 ranging in size1 perhaps1 from the majestic orbs of ,jupiter and ,saturn down to the inconceivably minute particles driven millions of miles into space to form a comet's tail1 does actually exist everywhere around us1 and by collisions between the particles or with planetary atmospheres can produce heat and light and gaseous emanations1 we find a basis of fact and observation for the meteoritic hypothesis which ,laplace's nebular1 and essentially gaseous1 theory does not possess4 ,during the latter half of the nineteenth century several writers suggested this idea of the possible formation of the ,solar ,system1 but so far as ,i am aware1 the late ,r4,a4 ,proctor was the first to discuss it in any detail1 and to show that it explained many of the peculiarities in the size and arrangement of the planets and their satellites which the nebular hypothesis did not explain4 ,this he does at some length in the chapter on meteors and comets in his ,other ,worlds than ,ours1 published in #ahgj4 ,he assumed1 instead of the fire-mist of ,laplace1 that the space now occupied by the solar system1 and for an unknown distance around it1 was occupied by vast quantities of solid particles of all the kinds of matter which we now find in the earth1 sun1 and stars4 ,this matter was dispersed somewhat irregularly1 as we see that all the matter of the universe is now distributed2 and he further assumed that it was all in motion1 as we now know that all the stars and other cosmical masses are1 and must be1 in motion towards or around some centre4 ,under these conditions1 wherever the matter was most aggregated1 there would be a centre of attraction through gravitation1 which would necessarily lead to further aggregation1 and the continual impacts of such aggregating matter would produce heat4 ,in course of time1 if the supply of cosmic matter was ample 7as the result shows that it must have been1 whatever theory we adopt71 our sun1 thus formed1 would approximate to its present mass and acquire sufficient heat by collision and gravitation to convert its whole body into the liquid or gaseous condition4 ,while this was going on1 subordinate centres of aggregation might form1 which would capture a certain proportion of the matter flowing in under the attraction of the central mass1 while1 owing to the nearly uniform direction and velocity with which the whole system was revolving1 each subordinate centre would revolve around the central mass1 in somewhat different planes1 but all in the same direction4 ,mr4 ,proctor shows the probability that the largest outside aggregation would be at a great distance from the central mass1 and this having once been formed1 any centres farther away from the sun would be both smaller and very remote1 while those inside the first would1 as a rule1 become smaller as they were nearer the centre4 ,the heated condition of the earth's interior would thus be due1 not to the primitive heat of matter in a gaseous state out of which it was formed--a condition physically impossible--but would be acquired in the process of aggregation by the collisions of meteoric masses falling on it1 and by its own gravitative force producing continuous condensation and heat4 ,on this view ,jupiter would probably be formed first1 and after him at very great distances1 ,saturn1 ,uranus1 and ,neptune2 while the inner aggregations would be smaller1 as the much greater attractive power of the sun would give them comparatively little opportunity of capturing the meteoric matter that was continuously flowing towards him4 ,,the ,,meteoritic ,,nature ,,of ,,the ,,nebul ,having thus reached the conclusion that wherever apparently nebulous matter exists within the limits of the solar system it is not gaseous but consists of solid particles1 or1 if heated gases are associated with the solid matter they can be accounted for by the heat due to collisions either with other solid particles or with accumulations of gases at a low temperature1 as when meteorites enter our atmosphere1 it was an easy step to consider whether the cosmic nebul and stars may not have had a similar origin4 ,from this point of view the nebul are supposed to be vast aggregations of meteorites or cosmic dust1 or of the more persistent gases1 revolving with circular or spiral motions1 or in irregular streams1 and so sparsely scattered that the separate particles of dust may be miles--perhaps hundreds of miles--apart2 yet even those nebul1 only visible by the telescope1 may contain as much matter as the whole solar system4 ,from this simple origin1 by steps which can be observed in the skies1 almost all the forms of suns and systems can be traced by means of the known laws of motion1 of heat-production1 and of chemical action4 ,the chief ,english advocate of this view at the present time is ,sir ,norman ,lockyer1 who1 in numerous papers1 and in his works on ,the ,meteoritic ,hypothesis and ,inorganic ,evolution1 has developed it in detail1 as the result of many years' continuous research1 aided by the contributory work of continental and ,american astronomers4 ,these views are gradually spreading among astronomers and mathematicians1 as will be seen by the very brief outline which will now be given of the explanations they afford of the main groups of phenomena presented by the stellar universe4 ,,dr4 ,,roberts ,,on ,,spiral ,,nebul ,dr4 ,isaac ,roberts1 who possesses one of the finest telescopes constructed for photographing stars and nebul1 has given his views on stellar evolution1 in ,knowledge of ,february #ahig1 illustrated by four beautiful photographs of spiral nebul4 ,these curious forms were at first thought to be rare1 but are now found to be really very numerous when details are brought out by the camera4 ,many of the very large and apparently quite irregular nebul1 like the ,magellanic ,clouds1 are found to have faint indications of spiral structure4 ,as more than ten thousand nebul are now known1 and new ones are continually being discovered1 it will be a long time before these can all be carefully studied and photographed1 but present indications seem to show that a considerable proportion of them will exhibit spiral forms4 ,dr4 ,roberts tells us that all the spiral nebul he has photographed are characterised by having a nucleus surrounded by dense nebulosity1 most of them being also studded with stars4 ,these stars are always arranged more or less symmetrically1 following the curves of the spiral1 while outside the visible nebula are other stars arranged in curves strongly suggesting a former greater extension of the nebulous matter4 ,this is so marked a feature that it at once leads to a possible explanation of the numerous slightly curved lines of stars found in every part of the heavens1 as being the result of their origin from spiral nebul whose material substance has been absorbed by them4 ,dr4 ,roberts proposes several problems in relation to these bodies3 ,of what materials are spiral nebul composed8 ,whence comes the vortical motion which has produced their forms8 ,the material he finds in those faint clouds of nebulous matter1 often of vast extent1 that exist in many parts of the sky1 and these are so numerous that ,sir ,william ,herschel alone recorded the positions of fifty-two such regions1 many of which have been confirmed by recent photographs4 ,dr4 ,roberts considers these to be either gaseous or with discrete solid particles intermixed4 ,he also enumerates smaller nebulous masses undergoing condensation and segregation into more regular forms2 spiral nebul in various stages of condensation and of aggregation2 elliptic nebul2 and globular nebul4 ,in the last three classes there is clear evidence1 on every photograph that has been taken1 that condensation into stars or star like forms is now going on4 ,he adopts ,sir ,norman ,lockyer's view that collisions of meteorites within each swarm or cloud would produce luminous nebulosity2 so also would collisions between separate swarms of meteorites produce the conditions required to account for the vortical motions and the peculiar distribution of the nebulosity in the spiral nebul4 ,almost any collision between unequal masses of diffused matter would1 in the absence of any massive central body round which they would be forced to revolve1 lead to spiral motions4 ,it is to be noted that1 although the stars formed in the spiral convolutions of the nebul follow those curves1 and retain them after the nebulous matter has been all absorbed by them1 yet1 whenever such a nebula is seen by us edgewise1 the convolutions with their enclosed stars will appear as straight lines2 and thus not only numbers of star groups arranged in curves1 but also those which form almost perfect straight lines1 may possibly be traced back to an origin from spiral nebul4 ,motion being a necessary result of gravitation1 we know that every star1 planet1 comet1 or nebula must be in motion through space1 and these motions--except in systems physically connected or which have had a common origin--are1 apparently1 in all directions4 ,how these motions originated and are now regulated we do not know2 but there they are1 and they furnish the motive power of the collisions1 which1 when affecting large bodies or masses of diffused matter1 lead to the formation of the various kinds of permanent stars2 while when smaller masses of matter are concerned those temporary stars are formed which have interested astronomers in all ages4 ,it must be noted that although the motions of the single stars appear to be in straight lines1 yet the spaces through which they have been observed to move are so small that they may really be moving in curved orbits around some central body1 or the centre of gravity of some aggregation of stars bright and dark1 which may itself be comparatively at rest4 ,there may be thousands of such centres around us1 and this may sufficiently explain the apparent motions of stars in all directions4 ,a ,,suggestion ,,as ,,to ,,the ,,formation ,,of ,,spiral ,,nebul ,in a remarkable paper in the ,astrophysical ,journal 7,july #aija71 ,mr4 ,t4,c4 ,chamberlin suggests an origin for the spiral nebul1 as well as of swarms of meteorites and comets1 which seems likely to be a true1 although perhaps not the only one4 ,there is a well-known principle which shows that when two bodies in space1 of stellar size1 pass within a certain distance of each other1 the smaller one will be liable to be torn into fragments by the differential attraction of the larger and denser body4 ,this was originally proved in the case of gaseous and liquid bodies1 and the distance within which the smaller one will be disrupted 7termed the ,roche limit7 is calculated on the supposition that the disrupted body is a liquid mass4 ,mr4 ,chamberlin shows1 however1 that a solid body will also be disrupted at a lesser distance dependent on its size and cohesive strength2 but1 as the size of the two bodies increases1 the distance at which disruption will occur increases also1 till with very large bodies1 such as suns1 it becomes almost as large as in the case of liquids or gases4 ,the disruption occurs from the well-known law of differential gravitation on the two sides of a body leading to tidal deformation in a liquid1 and to unequal strain in a solid4 ,when the changes of gravitative force take place slowly1 and are also small in amount1 the tides in liquids or strains in solids are very small1 as in the case of our earth when acted on by the sun and moon1 the result is a small tide in the ocean and atmosphere1 and no doubt also in the molten interior1 to which the comparatively thin crust may partially adjust itself4 ,but if we suppose two dark or luminous suns whose proper motions are in such a direction as to bring them near each other1 then1 as they approach1 each will be deflected towards the other1 and will pass round their common centre of gravity with immense velocity1 perhaps hundreds of miles in a second4 ,at a considerable distance they will begin to produce tidal elongation towards and away from each other1 but when the disruptive limit is nearly reached1 the gravitative forces will be increasing so rapidly that even a liquid mass could not adjust its shape with sufficient quickness and the tremendous internal strains would produce the effects of an explosion1 tearing the whole mass 7of the smaller of the two7 into fragments and dust4 ,but it is also shown that1 during the entire process1 the two elongated portions of the originally spherical mass would be so acted upon by gravity as to produce increasing rotation1 which as the crisis approached would extend the elongation1 and aid in the explosive result4 ,this rapid rotation of the elongated mass would1 when the disruption occurred1 necessarily give to the fragments a whirling or spiral motion1 and thus initiate a spiral nebula of a size and character dependent on the size and constitution of the two masses1 and on the amount of the explosive forces set up by their approach4 ,there is one very suggestive phenomenon which seems to prove that this is one of the modes of formation of spiral nebul4 ,when the explosive disruption occurs the two protuberances or elongations of the body will fly apart1 and having also a rapid rotatory movement1 the resulting spiral will necessarily be a double one4 ,now1 it is the fact that almost all the well-developed spiral nebul have two such arms opposite to each other1 as beautifully shown in ,m4 #ajj ,com1 ,m4 #ea ,canum1 and others photographed by ,dr4 ,i4 ,roberts4 ,it does not seem likely that any other origin of these nebul should give rise to a double rather than to a single spiral4 ,,the ,,evolution ,,of ,,double ,,stars ,the advance in knowledge of double and multiple stars has been wonderfully rapid1 numerous observers having devoted themselves to this special branch4 ,many thousands were discovered during the first half of the nineteenth century1 and as telescopic power increased new ones continued to flow in by hundreds and thousands1 and there has been recently published by the ,yerkes ,observatory a catalogue of #abij such stars1 discovered between #ahga and #ahii by one observer1 ,mr4 ,s4,w4 ,burnham4 ,all these have been found by the use of the telescope1 but during the last quarter of a century the spectroscope has opened up a new world of double stars of enormous extent and the highest interest4 ,the telescopic binaries which have been observed for a sufficient time to determine their orbits1 range from periods of about eleven years as a minimum up to hundreds and even more than a thousand years4 ,but the spectroscope reveals the fact that the many thousands of telescopic binaries form only a very small part of the binary systems in existence4 ,the overwhelming importance of this discovery is1 that it carries the times of revolution from the minimum of the telescopic doubles downward in unbroken series through periods of a few years1 to those reckoned by months1 by days1 and even by hours4 ,and with this reduction of period there necessarily follows a corresponding reduction of distance1 so that sometimes the two stars must be in contact1 and thus the actual birth or origin of a double star has been observed to occur1 even though not actually seen4 ,this mode of origin was indeed anticipated by ,dr4 ,lee of ,chicago in #ahib1 and it has been confirmed by observation in the short space of ten years4 ,in a remarkable communication to ,nature 7,september #abth1 #aija7 ,mr4 ,alexander ,w4 ,roberts of ,lovedale1 ,south ,africa1 gives some of the main results of this branch of inquiry4 ,of course all the variable stars are to be found among the spectroscopic binaries4 ,they consist of that portion of the class in which the plane of the orbit is directed towards us1 so that during their revolution one of the pair either wholly or partially eclipses the other4 ,in some of these cases there are irregularities1 such as double maxima and minima of unequal lengths1 which may be due to triple systems or to other causes not yet explained1 but as they all have short periods and always appear as one star in the most powerful telescopes1 they form a special division of the spectroscopic binary systems4 ,there are known at present twenty-two variables of the ,algol type1 that is1 stars having each a dark companion very close to it which obscures it either wholly or partially during every revolution4 ,in these cases the density of the systems can be approximately determined1 and they are found to be1 on the average1 only one-fifth that of water1 or one-eighth that of our sun4 ,but as many of them are as large as our sun1 or even considerably larger1 it is evident that they must be wholly gaseous1 and1 even if very hot1 of a less complex constitution than our luminary4 ,mr4 ,a4,w4 ,roberts tells us that five out of these twenty-two variables revolve in absolute contact forming systems of the shape of a dumb-bell4 ,the periods vary from twelve days to less than nine hours2 and1 starting from these1 we now have a continuous series of lengthening periods up to the twin stars of ,castor which require more than a thousand years to complete their revolution4 ,during his observations of the above five stars1 ,mr4 ,roberts states that one1 ,x ,carin1 was found to have parted company1 so that instead of being actually united to its companion the two are now at a distance apart equal to one-tenth of their diameters1 and he may thus be said to have been almost a witness of the birth of a stellar system4 ,a year later we find the record 7in ,knowledge1 ,october #aijb7 of ,professor ,campbell's researches at the ,lick ,observatory4 ,he states that1 out of #cej stars observed spectroscopically1 one in eight is a spectroscopic binary2 and so impressed is he with their abundance that1 as accuracy of measurement increases1 he believes that the star that is not a spectroscopic binary will prove to be the rare exception6 ,professor ,g4 ,darwin had already shown that the 'dumb-bell' was a figure of equilibrium in a rotating mass of fluid2 and we now find proofs that such figures exist1 and that they form the starting-point for the enormous and ever-increasing quantities of spectroscopic binary star-systems that are now known4 ,the origin of these binary stars is also of especial interest as giving support to ,professor ,darwin's well-known explanation of the origin of the moon by disruption from the earth1 owing to the very rapid rotation of the parent planet4 ,it now appears that suns often subdivide in the same manner1 but1 owing perhaps to their intensely heated gaseous state they seem usually to form nearly equal globes4 ,the evolution of this special form of star-system is therefore now an observed fact2 though it by no means follows that all double stars have had the same mode of origin4 ,,clusters ,,of ,,stars ,,and ,,variables ,the clusters of stars1 which are tolerably abundant in the heavens and offer so many strange and beautiful forms to the telescopist1 are yet among the most puzzling phenomena the philosophic astronomer has to deal with4 ,many of these clusters which are not very crowded and of irregular forms1 strongly suggest an origin from the equally irregular and fantastic forms of nebul by a process of aggregation like that which ,dr4 ,roberts describes as developing within the spiral nebul4 ,but the dense globular clusters which form such beautiful telescopic objects1 and in some of which more than six thousand stars have been counted besides considerable numbers so crowded in the centre as to be uncountable1 are more difficult to explain4 ,one of the problems suggested by these clusters is as to their stability4 ,professor ,simon ,newcomb remarks on this point as follows3 ',where thousands of stars are condensed into a space so small1 what prevents them from all falling together into one confused mass8 ,are they really doing so1 and will they ultimately form a single body8 ,these are questions which can be satisfactorily answered only by centuries of observation2 they must therefore be left to the astronomers of the future4' ,there are1 however1 some remarkable features in these clusters which afford possible indications of their origin and essential constitution4 ,when closely examined most of them are seen to be less regular than they at first appear4 ,vacant spaces can be noted in them2 even rifts of definite forms4 ,in some there is a radiated structure2 in others there are curved appendages2 while some have fainter centres4 ,these features are so exactly like what are found1 in a more pronounced form1 in the larger nebul1 that we can hardly help thinking that in these clusters we have the result of the condensation of very large nebul1 which have first aggregated towards numerous centres1 while these agglomerations have been slowly drawn towards the common centre of gravity of the whole mass4 ,it is suggestive of this origin that while the smaller telescopic nebul are far removed from the ,milky ,way1 the larger ones are most abundant near its borders2 while the star-clusters are excessively abundant on and near the ,milky ,way1 but very scarce elsewhere1 except in or near vast nebul like the ,magellanic ,clouds4 ,we thus see that the two phenomena may be complementary to each other1 the condensation of nebul having gone on most rapidly where material was most abundant1 resulting in numerous star-clusters where there are now few nebul4 ,there is one striking feature of the globular clusters which calls for notice2 the presence in some of them of enormous quantities of variable stars1 while in others few or none can be found4 ,the ,harvard ,observatory has for several years devoted much time to this class of observations1 and the results are given in ,professor ,newcomb's recent volume on ',the ,stars4' ,it appears that twenty-three clusters have been observed spectroscopically1 the number of stars examined in each cluster varying from #ade up to #cjjj1 the total number of stars thus minutely tested being #ai1jej4 ,out of this total number #eji were found to be variable2 but the curious fact is1 the extreme divergence in the proportion of variables to the whole number examined in the several clusters4 ,in two clusters1 though #abgi stars were examined1 not a single variable was found4 ,in three others the proportion was from one in #ajej to one in #ejj4 ,five more ranged up to one in #ajj1 and the remainder showed from that proportion up to one in seven1 #ijj stars being examined in the last mentioned cluster of which #acb were variable6 ,when we consider that variable stars form only a portion1 and necessarily a very small proportion1 of binary systems of stars1 it follows that in all the clusters which show a large proportion of variables1 a very much larger proportion--in some cases perhaps all1 must be double or multiple stars revolving round each other4 ,with this remarkable evidence1 in addition to that adduced for the prevalence of double stars and variables among the stars in general1 we can understand ,professor ,newcomb adding his testimony to that of ,professor ,campbell already quoted1 that 'it is probable that among the stars in general1 single stars are the exception rather than the rule4 ,if such be the case1 the rule should hold yet more strongly among the stars of a condensed cluster4' ,,the ,,evolution ,,of ,,the ,,stars ,so long as astronomers were limited to the use of the telescope only1 or even the still greater powers of the photographic plate1 nothing could be learnt of the actual constitution of the stars or of the process of their evolution4 ,their apparent magnitudes1 their movements1 and even the distances of a few could be determined2 while the diversity of their colours offered the only clue 7a very imperfect one7 even to their temperature4 ,but the discovery of spectrum analysis has furnished the means of obtaining some definite knowledge of the physics and chemistry of the stars1 and has thus established a new branch ofscience--,astrophysics--which has already attained large proportions1 and which furnishes the materials for a periodical and some important volumes4 ,this branch of the subject is very complex1 and as it is not directly connected with our present inquiry1 it is only referred to again in order to introduce such of its results as bear upon the question of the classification and evolution of the stars4 ,by a long series of laboratory experiments it has been shown that numerous changes occur in the spectra of the elements when subjected to different temperatures1 ranging upwards to the highest attainable by means of a battery producing an electric spark several feet long4 ,these changes are not in the relative position of the bands or dark lines1 but in their number1 breadth1 and intensity4 ,other changes are due to the density of the medium in which the elements are heated1 and to their chemical condition as to purity2 and from these various modifications and their comparison with the solar spectrum and those of its appendages1 it has become possible to determine1 from the spectrum of a star1 not only its temperature as compared with that of the electric spark and of the sun1 but also its place in a developmental series4 ,the first general result obtained by this research is1 that the bluish white or pure white stars1 having a spectrum extending far towards the violet end1 and which exhibits the coloured bands of gases only1 usually hydrogen and helium1 are the hottest4 ,next come those with a shorter spectrum not extending so far towards the violet end1 and whose light is therefore more yellow in tint4 ,to this group our sun belongs2 and they are all characterised like it by dark lines due to absorption1 and by the presence of metals1 especially iron1 in a gaseous state4 ,the third group have the shortest spectra and are of a red colour1 while their spectra contain lines denoting the presence of carbon4 ,these three groups are often spoken of as 'gaseous stars1' 'metallic stars1' and 'carbon stars4' ,other astronomers call the first group ',sirian stars1' because ,sirius1 though not the hottest1 is a characteristic type2 the second being termed 'solar stars'2 others again speak of them as stars of ,class ,i41 ,class ,,ii41 etc41 according to the system of classification they have adopted4 ,it was soon perceived1 however1 that neither the colour nor the temperature of stars gave much information as to their nature and state of development1 because1 unless we supposed the stars to begin their lives already intensely hot 7and all the evidence is against this71 there must be a period during which heat increases1 then one of maximum heat1 followed by one of cooling and final loss of light altogether4 ,the meteoritic theory of the origin of all luminous bodies in the heavens1 now very widely adopted1 has been used1 as we have seen1 to explain the development of stars from nebul1 and its chief exponent in this country1 ,sir ,norman ,lockyer1 has propounded a complete scheme of stellar evolution and decay which may be here briefly outlined3 ,beginning with nebul1 we pass on to stars having banded or fluted spectra1 indicating comparatively low temperatures and showing bands or lines of iron1 manganese1 calcium1 and other metals4 ,they are more or less red in colour1 ,antares in the ,scorpion being one of the most brilliant red stars known4 ,these stars are supposed to be in the process of aggregation1 to be continually increasing in size and heat1 and thus to be subject to great disturbances4 ,alpha ,cygni has a similar spectrum but with more hydrogen1 and is much hotter4 ,the increase of heat goes on through ,rigel and ,beta ,crucis1 in which we find mainly hydrogen1 helium1 oxygen1 nitrogen1 and also carbon1 but only faint traces of metals4 ,reaching the hottest of all--,epsilon ,orionis and two stars in ,argo--hydrogen is predominant1 with traces of a few metals and carbon4 ,the cooling series is indicated by thicker lines of hydrogen and thinner lines of the metallic elements1 through ,sirius1 to ,arcturus and our sun1 thence to #ai ,piscium1 which shows chiefly flutings of carbon1 with a few faint metallic lines4 ,the process of further cooling brings us to the dark stars4 ,we have here a complete scheme of evolution1 carrying us from those ill-defined but enormously diffused masses of gas and cosmic dust we know as nebul1 through planetary nebul1 nebulous stars1 variable and double-stars1 to red and white stars and on to those exhibiting the most intense blue-white lustre4 ,we must remember1 however1 that the most brilliant of these stars1 showing a gaseous spectrum and forming the culminating point of the ascending series1 are not necessarily hotter than1 or even so hot as1 some of those far down on the descending scale2 since it is one of the apparent paradoxes of physics that a body may become hotter during the very process of contraction through loss of heat4 ,the reason is that by cooling it contracts and thus becomes denser1 that a portion of its mass falls towards its centre1 and in doing so produces an amount of heat which1 though absolutely less than the heat lost in cooling1 will under certain conditions cause the reduced surface to become hotter4 ,the essential point is1 that the body in question must be wholly gaseous1 allowing of free circulation from surface to centre4 ,the law1 as given by ,professor ,s4 ,newcomb1 is as follows3-- ',when a spherical mass of incandescent gas contracts through the loss of its heat by radiation into space1 its temperature continually becomes higher as long as the gaseous condition is retained4' ,to put it in another way3 if the compression was caused by external force and no heat was lost1 the globe would get hotter by a calculable amount for each unit of contraction4 ,but the heat lost in causing a similar amount of contraction is so little more than the increase of heat produced by contraction1 that the slightly diminished total heat in a smaller bulk causes the temperature of the mass to increase4 ,but if1 as there is reason to believe1 the various types of stars differ also in chemical constitution1 some consisting mainly of the more permanent gases1 while in others the various metallic and non-metallic elements are present in very different proportions1 there should really be a classification by constitution as well as by temperature1 and the course of evolution of the differently constituted groups may be to some extent dissimilar4 ,with this limitation the process of evolution and decay of sun through a cycle of increasing and decreasing temperature1 as suggested by ,sir ,norman ,lockyer1 is clear and suggestive4 ,during the ascending series the star is growing both in mass and heat1 by the continual accretion of meteoritic matter either drawn to it by gravitation or falling towards it through the proper motions of independent masses4 ,this goes on till all the matter for some distance around the star has been utilised1 and a maximum of size1 heat1 and brilliancy attained4 ,then the loss of heat by radiation is no longer compensated by the influx of fresh matter1 and a slow contraction occurs accompanied by a slightly increased temperature4 ,but owing to the more stable conditions continuous envelopes of metals in the gaseous state are formed1 which check the loss of heat and reduce the brilliancy of colour2 whence it follows that bodies like our sun may be really hotter than the most brilliant white stars1 though not giving out quite so much heat4 ,the loss of heat is therefore reduced2 and this may serve to account for the undoubted fact that during the enormous epochs of geological time there has been very little diminution in the amount of heat we have received from the sun4 ,on the general question of the meteoritic hypothesis one of our first mathematicians1 ,professor ,george ,darwin1 has thus expressed his views3 ',the conception of the growth of the planetary bodies by the aggregation of meteorites is a good one1 and perhaps seems more probable than the hypothesis that the whole solar system was gaseous4' ,i may add1 that one of the chief objections made to it1 that meteorites are too complex to be supposed to be the primitive matter out of which suns and worlds have been made1 does not seem to me valid4 ,the primitive matter1 whatever it was1 may have been used up again and again1 and if collisions of large solid globes ever occur--and it is assumed by most astronomers that they must sometimes occur--then meteoric particles of all sizes would be produced which might exhibit any complexity of mineral constitution4 ,the material universe has probably been in existence long enough for all the primitive elements to have been again and again combined into the minerals found upon the earth and many others4 ,it cannot be too often repeated that no explanation--no theory--can ever take us to the beginning of things1 but only one or two steps at a time into the dim past1 which may enable us to comprehend1 however imperfectly1 the processes by which the world1 or the universe1 as it is1 has been developed out of some earlier and simpler condition4 ,,chapter ,,vii ,,are ,,the ,,stars ,,infinite ,,in ,,number8 ,most of the critics of my first short discussion of this subject laid great stress upon the impossibility of proving that the universe1 a part of which we see1 is not infinite2 and a well-known astronomer declared that unless it can be demonstrated that our universe is finite the entire argument founded upon our position within it fall to the ground4 ,i had laid myself open to this objection by rather incautiously admitting that if the preponderance of evidence pointed in this direction any inquiry as to our place in the universe would be useless1 because as regards infinity there can be no difference of position4 ,but this statement is by no means exact1 and even in an infinite universe of matter containing an infinite number of stars1 such as those we see1 there might well be such infinite diversities of distribution and arrangement as would give to certain positions all the advantages which ,i submit we actually possess4 ,supposing1 for example1 that beyond the vast ring of the ,milky ,way the stars rapidly decrease in number in all directions for a distance of a hundred or a thousand times the diameter of that ring1 and that then for an equal distance they slowly increase again and become aggregated into systems or universes totally distinct from ours in form and structure1 and so remote that they can influence us in no way whatever4 ,then1 ,i maintain1 our position within our own stellar universe might have exactly the same importance1 and be equally suggestive1 as if ours were the only material universe in existence--as if the apparent diminution in the number of stars 7which is an observed fact7 indicated a continuous diminution1 leading at some unknown distance to entire absence of luminous--that is1 of active1 energy-emitting aggregations of matter4#a ,as to whether there are such other material universes or not ,i offer no opinion1 and have no belief one way or the other4 ,i consider all speculations as to what may or may not exist in infinite space to be utterly valueless4 ,i have limited my inquiries strictly to the evidence accumulated by modern astronomers1 and to direct inferences and logical deductions from that evidence4 ,yet1 to my great surprise1 my chief critic declares that ',dr4 ,wallace's underlying error is1 indeed1 that he has reasoned from the area which we can embrace with our limited perceptions to the infinite beyond our mental or intellectual grasp4' ,i have distinctly not done this1 but many astronomers have done so4 ,the late ,richard ,proctor not only continually discussed the question of infinite matter as well as infinite space1 but also argued1 from the supposed attributes of the ,deity1 for the necessity of holding this material universe to be infinite1 and the last chapter of his ,other ,worlds than ,ours is mainly devoted to such speculations4 ,in a later work1 ,our ,place among ,infinities1 he says that 'the teachings of science bring us into the presence of the unquestionable infinities of time and of space1 and the presumable infinities of matter and of operation--hence therefore into the presence of infinity of energy4 ,but science teaches us nothing about these infinities as such4 ,they remain none the less inconceivable1 however clearly we may be taught to recognise their reality4' ,all this is very reasonable1 and the last sentence is particularly important4 ,nevertheless1 many writers allow their reasonings from facts to be influenced by these ideas of infinity4 ,in ,proctor's posthumous work1 ,old and ,new ,astronomy1 the late ,mr4 ,ranyard1 who edited it1 writes3 ',if we reject as abhorrent to our minds the supposition that the universe is not infinite1 we are thrown back on one of two alternatives--either the ether which transmits the light of the stars to us is not perfectly elastic1 or a large proportion of the light of the stars is obliterated by dark bodies4' ,here we have a well-informed astronomer allowing his abhorrence of the idea of a finite universe to affect his reasoning on the actual phenomena we can observe--doing in fact exactly what my critic erroneously accuses me of doing4 ,but setting aside all ideas and prepossessions of the kind here indicated1 let us see what are the actual facts revealed by the best instruments of modern astronomy1 and what are the natural and logical inferences from those facts4 ,,are ,,the ,,stars ,,infinite ,,in ,,number8 ,the views of those astronomers who have paid attention to this subject are1 on the whole1 in favour of the view that the stellar universe is limited in extent and the stars therefore limited in number4 ,a few quotations will best exhibit their opinions on this question1 with some of the facts and observations on which they are founded4 ,miss ,a4,m4 ,clerke1 in her admirable volume1 ,the ,system of the ,stars1 says3 ',the sidereal world presents us1 to all appearance1 with a finite system4444 ,the probability amounts almost to certainty that star-strewn space is of measurable dimensions4 ,for from innumerable stars a limitless sum-total of radiations should be derived1 by which darkness would be banished from our skies2 and the intense inane1 glowing with the mingled beams of suns individually indistinguishable1 would bewilder our feeble senses with its monotonous splendour4444 ,unless1 that is to say1 light suffer some degree of enfeeblement in space4444 ,but there is not a particle of evidence that any such toll is exacted2 contrary indications are strong2 and the assertion that its payment is inevitable depends upon analogies which may be wholly visionary4 ,we are then1 for the present1 entitled to disregard the problematical effect of a more than dubious cause4' ,professor ,simon ,newcomb1 one of the first of ,american mathematicians and astronomers1 arrives at a similar conclusion in his most recent volume1 ,the ,stars 7#aijb74 ,he says1 in his conclusions at the end of the work3 ',that collection of stars which we call the universe is limited in extent4 ,the smallest stars that we see with the most powerful telescopes are not1 for the most part1 more distant than those a grade brighter1 but are mostly stars of less luminosity situate in the same regions' 7p4 #cai74 ,and on page #bbi of the same work he gives reasons for this conclusion1 as follows3 ',there is a law of optics which throws some light on the question4 ,suppose the stars to be scattered through infinite space so that every great portion of space is1 in the general average1 equally rich in stars4 ,then at some great distance we describe a sphere having its centre in our sun4 ,outside this sphere describe another one of a greater radius1 and beyond this other spheres at equal distances apart indefinitely4 ,thus we shall have an endless succession of spherical shells1 each of the same thickness4 ,the volume of each of these shells will be nearly proportional to the squares of the diameters of the spheres which bound it4 ,hence each of the regions will contain a number of stars increasing as the square of the radius of the region4 ,since the amount of light we receive from each star is as the inverse square of its distance1 it follows that the sum total of the light received from each of these spherical shells will be equal4 ,thus as we add sphere after sphere we add equal amounts of light without limit4 ,the result would be that if the system of stars extended out indefinitely the whole heavens would be filled with a blaze of light as bright as the sun4' ,but the whole light given us by the stars is variously estimated at from one-fortieth to one-twentieth or1 as an extreme limit1 to one-tenth of moonlight1 while the sun gives as much light as #cjj1jjj full moons1 so that starlight is only equivalent at a fair estimate to the six-millionth part of sunlight4 ,keeping this in mind1 the possible causes of the extinction of almost the whole of the light of the stars 7if they are infinite in number and distributed1 on the average1 as thickly beyond the ,milky ,way as they are up to its outer boundary7 are absurdly inadequate4 ,these causes are 7#a7 the loss of light in passing through the ether1 and 7#b7 the stoppage of light by dark stars or diffused meteoritic dust4 ,as to the first1 it is generally admitted that there is not a particle of evidence of its existence4 ,there is1 however1 some distinct evidence that1 if it exists1 it is so very small in amount that it would not produce a perceptible effect for any distances less remote than hundreds or perhaps thousands of times as far as the farthest limits of the ,milky ,way are from us4 ,this is indicated by the fact that the brightest stars are not always1 or even generally1 the nearest to us1 as is shown both by their small proper motions and the absence of measurable parallax4 ,mr4 ,gore states that out of twenty-five stars1 with proper motions of more than two seconds annually1 only two are above the third magnitude4 ,many first magnitude stars1 including ,canopus1 the second brightest star in the heavens1 are so remote that no parallax can be found1 notwithstanding repeated efforts4 ,they must therefore be much farther off than many small and telescopic stars1 and perhaps as far as the ,milky ,way1 in which so many brilliant stars are found2 whereas if any considerable amount of light were lost in passing that distance we should find but few stars of the first two or three magnitudes that were very remote from us4 ,of the twenty-three stars of the first magnitude1 only ten have been found to have parallaxes of more than one-twentieth of a second1 while five range from that small amount down to one or two hundredths of a second1 and there are two with no ascertainable parallax4 ,again1 there are #cji stars brighter than magnitude #c4e1 yet only thirty-one of these have proper motions of more than #ajj a century1 and of these only eighteen have parallaxes of more than one-twentieth of a second4 ,these figures are from tables given in ,professor ,newcomb's book1 and they have very great significance1 since they indicate that the brightest stars are not the nearest to us4 ,more than this1 they show that out of the seventy-two stars whose distance has been measured with some approach to certainty1 only twenty-three 7having a parallax of more than one-fiftieth of a second7 are of greater magnitudes than #c4e1 while no less than forty-nine are smaller stars down to the eighth or ninth magnitude1 and these are on the average much nearer to us than the brighter stars6 ,taking the whole of the stars whose parallaxes are given by ,professor ,newcomb1 we find that the average parallax of the thirty-one bright stars 7from #c4e magnitude up to ,sirius7 is #j4aa seconds2 while that of the forty-one stars below #c4e magnitude down to about #i4e1 is #j4ba seconds1 showing that they are1 on the average1 only half as far from us as the brighter stars4 ,the same conclusion was reached by ,mr4 ,thomas ,lewis of the ,greenwich ,observatory in #ahie1 namely1 that the stars from #b4gj magnitude down to about #h4dj magnitude have1 on the average1 double the parallaxes of the brighter stars4 ,this very curious and unexpected fact1 however it may be accounted for1 is directly opposed to the idea of there being any loss of light by the more distant as compared with the nearer stars2 for if there should be such a loss it would render the above phenomenon still more difficult of explanation1 because it would tend to exaggerate it4 ,the bright stars being on the whole farther away from us than the less bright down to the eighth and ninth magnitudes1 it follows1 if there is any loss of light1 that the bright stars are really brighter than they appear to us1 because1 owing to their enormous distance some of their light has been lost before it reached us4 ,of course it may be said that this does not demonstrate that no light is lost in passing through space2 but1 on the other hand1 it is exactly the opposite of what we should expect if the more distant stars were perceptibly dimmed by this cause1 and it may be considered to prove that if there is any loss it is exceedingly small1 and will not affect the question of the limits of our stellar system1 which is all that we are dealing with4 ,this remarkable fact of the enormous remoteness of the majority of the brighter stars is equally effective as an argument against the loss of light by dark stars or cosmic dust1 because1 if the light is not appreciably diminished for stars which have less than the fiftieth of a second of parallax1 it cannot greatly interfere with our estimates of the limits of our universe4 ,both ,mr4 ,e4,w4 ,maunder of the ,greenwich ,observatory and ,professor ,w4,w4 ,turner of ,oxford lay great stress on these dark bodies1 and the former quotes ,sir ,robert ,ball as saying1 'the dark stars are incomparably more numerous than those that we can see 444 and to attempt to number the stars of our universe by those whose transitory brightness we can perceive would be like estimating the number of horseshoes in ,england by those which are red-hot4' ,but the proportion of dark stars 7or nebul7 to bright ones cannot be determined a priori1 since it must depend upon the causes that heat the stars1 and how frequently those causes come into action as compared with the life of a bright star4 ,we do know1 both from the stability of the light of the stars during the historic period and much more precisely by the enormous epochs during which our sun has supported life upon this earth--yet which must have been 'incomparably' less than its whole existence as a light-giver--that the life of most stars must be counted by hundreds or perhaps by thousands of millions of years4 ,but we have no knowledge whatever of the rate at which true stars are born4 ,the so-called 'new stars' which occasionally appear evidently belong to a different category4 ,they blaze out suddenly and almost as suddenly fade away into obscurity or total invisibility4 ,but the true stars probably go through their stages of origin1 growth1 maturity1 and decay1 with extreme slowness1 so that it is not as yet possible for us to determine by observation when they are born or when they die4 ,in this respect they correspond to species in the organic world4 ,they would probably first be known to us as stars or minute nebul3 at the extreme limit of telescopic vision or of photographic sensitiveness1 and the growth of their luminosity might be so gradual as to require hundreds1 perhaps thousands of years to be distinctly recognisable4 ,hence the argument derived from the fact that we have never witnessed the birth of a true permanent star1 and that1 therefore1 such occurrences are very rare1 is valueless4 ,new stars may arise every year or every day without our recognising them2 and if this is the case1 the reservoir of dark bodies1 whether in the form of large masses or of clouds of cosmic dust1 so far from being incomparably greater than the whole of the visible stars and nebul1 may quite possibly be only equal to it1 or at most a few times greater2 and in that case1 considering the enormous distances that separate the stars 7or star-systems7 from each other1 they would have no appreciable effect in shutting out from our view any considerable proportion of the luminous bodies constituting our stellar universe4 ,it follows1 that ,professor ,newcomb's argument as to the very small total light given by the stars has not been even weakened by any of the facts or arguments adduced against it4 ,mr4 ,w4,h4,s4 ,monck1 in a letter to ,knowledge 7,may #aijc71 puts the case very strongly so as to support my view4 ,he says3--',the highest estimate that ,i have seen of the total light of the full moon is #a_/cjjjjj of that of the sun4 ,suppose that the dark bodies were a hundred and fifty thousand times as numerous as the bright ones4 ,then the whole sky ought to be as bright as the illuminated portion of the moon4 ,every one knows that this is not so4 ,but it is said that the stars1 though infinite1 may only extend to infinity in particular directions1 e4g4 in that of the ,galaxy4 ,be it so4 ,where1 in the very brightest portion of the ,galaxy1 will we find a part equal in angular magnitude to the moon which affords us the same quantity of light8 ,in the very brightest spot1 the light probably does not amount to one hundredth part that of the full moon4' ,it follows that1 even if dark stars were fifteen million times as numerous as the bright ones1 ,professor ,newcomb's argument would still apply against an infinite universe of stars of the same average density as the portion we see4 ,,telescopic ,,evidence ,,as ,,to ,,the ,,limits ,,of ,,the ,,star ,,system ,throughout the earlier portion of the nineteenth century every increase of power and of light-giving qualities of telescopes added so greatly to the number of the stars which became visible1 that it was generally assumed that this increase would go on indefinitely1 and that the stars were really infinite in number and could not be exhausted4 ,but of late years it has been found that the increase in the number of stars visible in the larger telescopes was not so great as might be expected1 while in many parts of the heavens a longer exposure of the photographic plate adds comparatively little to the number of stars obtained by a shorter exposure with the same instrument4 ,mr4 ,j4,e4 ,gore's testimony on this point is very clear4 ,he says3--',those who do not give the subject sufficient consideration1 seem to think that the number of the stars is practically infinite1 or at least1 that the number is so great that it cannot be estimated4 ,but this idea is totally incorrect1 and due to complete ignorance of telescopic revelations4 ,it is certainly true that1 to a certain extent1 the larger the telescope used in the examination of the heavens1 the more the number of the stars seems to increase2 but we now know that there is a limit to this increase of telescopic vision4 ,and the evidence clearly shows that we are rapidly approaching this limit4 ,although the number of stars visible in the ,pleiades rapidly increases at first with increase in the size of the telescope used1 and although photography has still further increased the number of stars in this remarkable cluster1 it has recently been found that an increased length of exposure--beyond three hours--adds very few stars to the number visible on the photograph taken at the ,paris ,observatory in #ahhe1 on which over two thousand stars can be counted4 ,even with this great number on so small an area of the heavens1 comparatively large vacant places are visible between the stars1 and a glance at the original photograph is sufficient to show that there would be ample room for many times the number actually visible4 ,i find that if the whole heavens were as rich in stars as the ,pleiades1 there would be only thirty-three millions in both hemispheres4' ,again1 referring to the fact that ,celoria1 with a telescope showing stars down to the eleventh magnitude1 could see almost exactly the same number of stars near the north pole of the ,galaxy as ,sir ,william ,herschel found with his much larger and more powerful telescope1 he remarks3 ',their absence1 therefore1 seems certain proof that very faint stars do not exist in that direction1 and that here1 at least1 the sidereal universe is limited in extent4' ,sir ,john ,herschel notes the same phenomena1 stating that even in the ,milky ,way there are found 'spaces absolutely dark and completely void of any star1 even of the smallest telescopic magnitude'2 while in other parts 'extremely minute stars1 though never altogether wanting1 occur in numbers so moderate as to lead us irresistibly to the conclusion that in these regions we see fairly through the starry stratum1 since it is impossible otherwise 7supposing their light not intercepted7 that the numbers of the smaller magnitudes should not go on continually increasing ad infinitum4 ,in such cases1 moreover1 the ground of the heavens1 as seen between the stars1 is for the most part perfectly dark1 which again would not be the case if innumerable multitudes of stars1 too minute to be individually discernible1 existed beyond4' ,and again he sums up as follows3--',throughout by far the larger portion of the extent of the ,milky ,way in both hemispheres1 the general blackness of the ground of the heavens on which its stars are projected1 and the absence of that innumerable multitude and excessive crowding of the smallest visible magnitudes1 and of glare produced by the aggregate light of multitudes too small to affect the eye singly1 which the contrary supposition would appear to necessitate1 must1 we think1 be considered unequivocal indications that its dimensions in directions where these conditions obtain1 are not only not infinite1 but that the space-penetrating power of our telescopes suffices fairly to pierce through and beyond it4'#b ,this expression of opinion by the astronomer who1 probably beyond any now living1 was the most competent authority on this question1 to which he devoted a long life of observation and study extending over the whole heavens1 cannot be lightly set aside by the opinions or conjectures of those who seem to assume that we must believe in an infinity of stars if the contrary cannot be absolutely proved4 ,but as not a particle of evidence can be adduced to prove infinity1 and as all the facts and indications point1 as here shown1 in a directly opposite direction1 we must1 if we are to trust to evidence at all in this matter1 arrive at the conclusion that the universe of stars is limited in extent4 ,dr4 ,isaac ,roberts gives similar evidence as regards the use of photographic plates4 ,he writes3--',eleven years ago photographs of the ,great ,nebula in ,andromeda were taken with the #bj-;inch reflector1 and exposures of the plates during intervals up to four hours2 and upon some of them were depicted stars to the faintness of #agth to #ahth magnitude1 and nebulosity to an equal degree of faintness4 ,the films of the plates obtainable in those days were less sensitive than those which have been available during the past five years1 and during this period photographs of the nebula with exposures up to four hours have been taken with the #bj-;inch reflector4 ,no extensions of the nebulosity1 however1 nor increase in the number of the stars can be seen on the later rapid plates than were depicted upon the earlier slower ones1 though the star-images and the nebulosity have greater density on the later plates4' ,exactly similar facts are recorded in the cases of the ,great ,nebula in ,orion1 and the group of the ,pleiades4 ,in the case of the ,milky ,way in ,cygnus photographs have been taken with the same instrument1 but with exposures varying from one hour to two hours and a half1 but no fainter stars could be found on one than on the other2 and this fact has been confirmed by similar photographs of other areas in the sky4 ,,the ,,law ,,of ,,diminishing ,,numbers ,,of ,,stars ,we will now consider another kind of evidence equally weighty with the two already adduced4 ,this is what may be termed the law of diminishing numbers beyond a certain magnitude1 as observed by larger and larger telescopes4 ,for some years past star-magnitudes have been determined very accurately by means of careful photometric comparisons4 ,down to the sixth magnitude stars are visible to the naked eye1 and are hence termed lucid stars4 ,all fainter stars are telescopic1 and continuing the magnitudes in a series in which the difference in luminosity between each successive magnitude is equal1 the seventeenth magnitude is reached and indicates the range of visibility in the largest telescopes now in existence4 ,by the scale now used a star of any magnitude gives nearly two and a half times as much light as one of the next lower magnitude1 and for accurate comparison the apparent brightness of each star is given to the tenth of a magnitude which can easily be observed4 ,of course1 owing to differences in the colour of stars1 these determinations cannot be made with perfect accuracy1 but no important error is due to this cause4 ,according to this scale a sixth magnitude star gives about one-hundredth part of the light of an average first magnitude star4 ,sirius is so exceptionally bright that it gives nine times as much light as a standard or average first magnitude star4 ,now it is found that from the first to the sixth magnitude the stars increase in number at the rate of about three and a half times those of the preceding magnitudes4 ,the total number of stars down to the sixth magnitude is given by ,professor ,newcomb as #gfdg4 ,for higher magnitudes the numbers are so great that precision and uniformity are more difficult of attainment2 yet there is a wonderful continuance of the same law of increase down to the tenth magnitude1 which is estimated to include #b1caa1jjj stars1 thus conforming very nearly with the ratio of #c4e as determined by the lucid stars4 ,but when we pass beyond the tenth magnitude to those vast numbers of faint stars only to be seen in the best or the largest telescopes1 there appears to be a sudden change in the ratio of increased numbers per magnitude4 ,the numbers of these stars are so great that it is impossible to count the whole as with the higher magnitude stars1 but numerous counts have been made by many astronomers in small measured areas in different parts of the heavens1 so that a fair average has been obtained1 and it is possible to make a near approximation to the total number visible down to the seventeenth magnitude4 ,the estimate of these by astronomers who have made a special study of this subject is1 that the total number of visible stars does not exceed one hundred millions4#c ,but if we take the number of stars down to the ninth magnitude1 which are known with considerable accuracy1 and find the numbers in each succeeding magnitude down to the seventeenth1 according to the same ratio of increase which has been found to correspond very nearly in the case of the higher magnitudes1 ,mr4 ,j4,e4 ,gore finds that the total number should be about #adjj millions4 ,of course neither of these estimates makes any pretence to exact accuracy1 but they are founded on all the facts at present available1 and are generally accepted by astronomers as being the nearest approach that can be made to the true numbers4 ,the discrepancy is1 however1 so enormous that probably no careful observer of the heavens with very large telescopes doubts that there is a very real and very rapid diminution in the numbers of the fainter as compared with the brighter stars4 ,there is1 however1 yet one more indication of the decreasing numbers of the faint telescopic stars1 which is almost conclusive on this question1 and1 so far as ,i am aware1 has not yet been used in this relation4 ,i will therefore briefly state it4 ,,the ,,light ,,ratio ,,as ,,indicating ,,the ,,number ,,of ,,faint ,,stars ,professor ,newcomb points out a remarkable result depending on the fact that1 while the average light of successively lower magnitudes diminishes in a ratio of #b4e1 their numbers increase at nearly a ratio of #c4e4 ,from this it follows that1 so long as this law of increase continues1 the total of starlight goes on increasing by about forty per cent4 for each successive magnitude1 and he gives the following table to illustrate it3-- ,mag4 #a ,total ,light #a #b #a4d #c #b4j #d #b4h #e #d4j #f #e4g #g #h4j #h #aa4c #i #af4j #aj #bb4f -------- ,total light to ,mag4 #aj #gd4h -------- ,thus the total amount of the light given by all stars down to the tenth magnitude is seventy-four times as great as that from the few first magnitude stars4 ,we also see that the light given by the stars of any magnitude is twice as much as that of the stars two magnitudes higher in the scale1 so that we can easily calculate what additional light we ought to receive from each additional magnitude if they continue to increase in numbers below the tenth as they do above that magnitude4 ,now it has been calculated as the result of careful observations1 that the total light given by stars down to nine and a half magnitude is one-eightieth of full moonlight1 though some make it much more4 ,but if we continue the table of light-ratios from this low starting-point down to magnitude seventeen and a half1 we shall find1 if the numbers of the stars go on increasing at the same rate as before1 that the light of all combined should be at least seven times as great as moonlight2 whereas the photometric measurements make it actually about one-twentieth4 ,and as the calculation from light-ratios only includes stars just visible in the largest telescopes1 and does not include all those proved to exist by photography1 we have in this case a demonstration that the numbers of the stars below the tenth and down to the seventeenth magnitude diminish rapidly4 ,we must remember that the minuter telescopic stars preponderate enormously in and near the ,milky ,way4 ,at a distance from it they diminish rapidly1 till near its poles they are almost entirely absent4 ,this is shown by the fact 7already referred to at p4 #adf7 that ,professor ,celoria of ,milan1 with a telescope of less than three inches aperture1 counted almost as many stars in that region as did ,herschel with his eighteen-inch reflector4 ,but if the stellar universe extends without limit we can hardly suppose it to do so in one plane only2 hence the absence of the minuter stars and of diffused milky light over the larger part of the heavens is now held to prove that the myriads of very minute stars in the ,milky ,way really belong to it1 and not to the depths of space far beyond4 ,it seems to me that here we have a fairly direct proof that the stars of our universe are really limited in number4 ,there are thus four distinct lines of argument all pointing with more or less force to the conclusion that the stellar universe we see around us1 so far from being infinite1 is strictly limited in extent and of a definite form and constitution4 ,they may be briefly summarised as follows3-- 7#a7 ,professor ,newcomb shows that1 if the stars were infinite in number1 and if those we see were approximately a fair sample of the whole1 and further1 if there were not sufficient dark bodies to shut out almost the whole of their light1 then we should receive from them an amount of light theoretically greater than that of sunlight4 ,i have shown1 at some length1 that neither of these causes of loss of light will account for the enormous disproportion between the theoretical and the actual light received from the stars2 and therefore ,professor ,newcomb's argument must be held to be a valid one against the infinite extent of our universe4 ,of course1 this does not imply that there may not be any number of other universes in space1 but as we know absolutely nothing of them--even whether they are material or non-material--all speculation as to their existence is worse than useless4 7#b7 ,the next argument depends on the fact that all over the heavens1 even in the ,milky ,way itself1 there are areas of considerable extent1 besides rifts1 lanes1 and circular patches1 where stars are either quite absent or very faint and few in number4 ,in many of these areas the largest telescopes show no more stars than those of moderate size1 while the few stars seen are projected on an intensely dark background4 ,sir ,william ,herschel1 ,humboldt1 ,sir ,john ,herschel1 ,r4,a4 ,proctor1 and many living astronomers hold that1 in these dark areas1 rifts1 and patches1 we see completely through our stellar universe into the starless depths of space beyond4 7#c7 ,then we have the remarkable fact that the steady increase in the number of stars1 down to the ninth or tenth magnitudes1 following one constant ratio either gradually or suddenly changes1 so that the total number from the tenth down to the seventeenth magnitudes is only about one-tenth of what it would have been had the same ratio of increase continued4 ,the conclusion to be drawn from this fact clearly is1 that these faint stars are becoming more and more thinly scattered in space1 while the dark background on which they are usually seen shows that1 except in the region of the ,milky ,way1 there are not multitudes of still smaller invisible stars beyond them4 7#d7 ,the last indication of a limited stellar universe--the estimate of numbers by the light-ratio of each successive magnitude--powerfully supports the three preceding arguments4 ,the four distinct classes of evidence now adduced must be held to constitute1 as nearly as the circumstances permit1 a satisfactory proof that the stellar universe1 of which our solar system forms a part1 has definite limits2 and that a full knowledge of its form1 structure1 and extent1 is not beyond the possibility of attainment by the astronomers of the future4 ,,footnotes3 #a ,in a letter to ,knowledge1 ,june #aijc1 ,mr4 ,w4,h4,t4 ,monck puts the same point in a mathematical form4 #b ,outlines of ,astronomy1 pp4 #egh-i4 ,in the passages quoted the italics are ,sor ,john ,herschel's4 #c ,mr4 ,j4,e4 ,gore in ,concise ,knowledge ,astronomy1 pp4 #eda-b4 ,,chapter ,,viii ,,our ,,relation ,,to ,,the ,,milky ,,way ,we now approach what may be termed the very heart of the subject of our inquiry1 the determination of how we are actually situated within this vast but finite universe1 and how that position is likely to affect our globe as being the theatre of the development of life up to its highest forms4 ,we begin with our relation to the ,milky ,way 7which we have fully described in our fourth chapter71 because it is by far the most important feature in the whole heavens4 ,sir ,john ,herschel termed it 'the ground-plane of the sidereal system'2 and the more it is studied the more we become convinced that the whole of the stellar universe--stars1 clusters of stars1 and nebul--are in some way connected with it1 and are probably dependent on it or controlled by it4 ,not only does it contain a greater number of stars of the higher magnitudes than any other part of the heavens of equal extent1 but it also comprises a great preponderance of star-clusters1 and a great extent of diffused nebulous matter1 besides the innumerable myriads of minute stars which produce its characteristic cloud-like appearance4 ,it is also the region of those strange outbursts forming new stars2 while gaseous stars of enormous bulk--some probably a thousand or even ten thousand times that of our sun1 and of intense heat and brilliancy--are more abundant there than in any other part of the heavens4 ,it is now almost certain that these enormous stars and the myriads of minute stars just visible with the largest telescopes1 are actually intermingled1 and together constitute its essential features2 in which case the fainter stars are really small and cannot be far apart1 forming1 as it were1 the first aggregations of the nebulous substratum1 and perhaps supplying the fuel which keeps up the intense brilliancy of the giant suns4 ,if this is so1 then the ,galaxy must be the theatre of operation of vast forces1 and of continuous combinations of matter1 which escape our notice owing to its enormous distance from us4 ,among its millions of minute telescopic stars1 hundreds or thousands may appear or disappear yearly without being perceived by us1 till the photographic charts are completed and can be minutely scrutinised at short intervals4 ,as undoubted changes have occurred in many of the larger nebul during the last fifty years1 we may anticipate that analogous changes will soon be noted in the stars and the nebulous masses of the ,milky ,way4 ,dr4 ,isaac ,roberts has even observed changes in nebul after such a short interval as eight years4 ,,the ,,milky ,,way ,a ,,great ,,circle ,notwithstanding all its irregularities1 its divisions1 and its diverging branches1 astronomers are generally agreed that the ,milky ,way forms a great circle in the heavens4 ,sir ,john ,herschel1 whose knowledge of it was unrivalled1 stated that its course 'conforms1 as nearly as the indefiniteness of its boundary will allow it to be fixed1 to that of a great circle'2 and he gives the ,right ,ascension and ,declination of the points where it crosses the equinoctial1 in figures which define those points as being exactly opposite each other4 ,he also defines its northern and southern poles by other figures1 so as to show that they are the poles of a great circle4 ,and after referring to ,struve's view that it was not a great circle1 he says1 ',i retain my own opinion4' ,professor ,newcomb says that its position 'is nearly always near a great circle of the sphere'2 and again he says3 'that we are in the galactic plane itself seems to be shown in two ways3 7#a7 the equality in the counts of stars on the two sides of this plane all the way to its poles2 and 7#b7 the fact that the central line of the ,galaxy is a great circle1 which it would not be if we viewed it from one side of its central plane' 7,the ,stars1 p4 #cag74 ,miss ,clerke1 in her ,history of ,astronomy1 speaks of 'our situation in the galactic plane' as one of the undisputed facts of astronomy2 while ,sir ,norman ,lockyer1 in a lecture delivered in #ahii1 said1 'the middle line of the ,milky ,way is really not distinguishable from a great circle1' and again in the same lecture--'but the recent work1 chiefly of ,gould in ,argentina1 has shown that it practically is a great circle4'#d ,about this fact1 then1 there can be no dispute4 ,a great circle is a circle dividing the celestial sphere into two equal portions1 as seen from the earth1 and therefore the plane of this circle must pass through the earth4 ,of course the whole thing is on such a vast scale1 the ,milky ,way varying from ten to thirty degrees wide1 that the plane of its circular course cannot be determined with minute accuracy4 ,but this is of little importance4 ,when carefully laid down on a chart1 as in that of ,mr4 ,sidney ,waters 7see end of volume71 we can see that its central line does follow a very even circular course1 conforming 'as nearly as may be' to a great circle4 ,we are therefore certainly well within the space that would be enclosed if its northern and southern margins were connected together across the vast intervening abyss1 and in all probability not far from the central plane of that enclosed space4 ,,the ,,form ,,of ,,the ,,milky ,,way ,,and ,,our ,,position ,,on ,,its ,,plane ,although the ,galaxy forms a great circle in the heavens from our point of view1 it by no means follows that it is circular in plan4 ,being unequal in width and irregular in outline1 it might be elliptic or even angular in shape without being at all obviously so to us4 ,if we were standing in an open plain or field two or three miles in diameter1 and bounded in every direction by woods of very irregular height and density and great diversity of tint1 we should find it difficult to judge of the shape of the field1 which might be either a true circle1 an oval1 a hexagon1 or quite irregular in outline1 without our being able to detect the exact shape unless some parts were very much nearer to us than others4 ,again1 just as the woods bounding the field might be either a narrow belt of nearly uniform width1 or might in some places be only a few yards wide and in others stretch out for miles1 so there have been many opinions as to the width of the ,milky ,way in the direction of its plane1 that is1 in the direction in which we look towards it4 ,lately1 however1 as the result of long-continued observation and study1 astronomers are fairly well agreed as to its general form and extent1 as will be seen by the following statements of fact and reasoning4 ,miss ,clerke1 after giving the various views of many astronomers--and as the historian of modern astronomy her opinion has much weight--considers that the most probable view of it is1 that it is really very much what it seems to us--an immense ring with streaming appendages extending from the main body in all directions1 producing the very complex effect we see4 ,the belief seems to be now spreading that the whole universe of stars is spherical or spheroidal1 the ,milky ,way being its equator1 and therefore in all probability circular or nearly so in plan2 and it is also held that it must be rotating--perhaps very slowly--as nothing else can be supposed to have led to the formation of such a vast ring1 or can preserve it when formed4 ,professor ,newcomb considers1 from the numbers of the stars in all directions towards the ,milky ,way being approximately equal1 that there cannot be much difference in our distance from it in various directions4 ,it would follow that its plan is approximately circular or broadly elliptic4 ,the existence of ring-nebul may be held to render such a form probable4 ,sir ,norman ,lockyer gives facts which tend in the same direction4 ,in an article in ,nature of ,november #hth1 #aijj1 he says3 ',we find that the gaseous stars are not only confined to the ,milky ,way1 but they are the most remote in every direction1 in every galactic longitude2 all of them have the smallest proper motion4' ,and again1 referring to the hottest stars being equally remote on all sides of us1 he says3 ',it is because we are in the centre1 because the solar system is in the centre1 that the observed effect arises4' ,he also considers that the ring-nebula in ,lyra nearly represents the form of our whole system2 and he adds3 ',we practically know that in our system the centre is the region of least disturbance1 and therefore cooler conditions4' ,these various facts and conclusions of some of the most eminent astronomers all point to one definite inference1 that our position1 or that of the solar system1 is not very far from the centre of the vast ring of stars constituting the ,milky ,way1 while the same facts imply a nearly circular form to this ring4 ,here1 more than as regards our position in the plane of the ,galaxy1 there is no possibility of precise determination2 but it is quite certain that if we were situated very far away from the centre1 say1 for instance1 one-fourth of its diameter from one side of it and three-fourths from the other1 the appearances would not be what they are1 and we should easily detect the excentricity of our position4 ,even if we were one-third the diameter from one side and two-thirds from the other1 it will1 ,i think1 be admitted that this also would have been ascertained by the various methods of research now available4 ,we must1 therefore1 be somewhere between the actual centre and a circle whose radius is one-third of the distance to the ,milky ,way4 ,but if we are about midway between these two positions1 we shall only be one-sixth of the radius or one-twelfth of the diameter of the ,milky ,way from its exact centre2 and if we form part of a cluster or group of stars slowly revolving around that centre1 we should probably obtain all the advantages1 if any1 that may arise from a nearly central position in the entire star-system4 ,this question of our situation within the great circle of the ,milky ,way is of considerable importance from the point of view ,i am here suggesting1 so that every fact bearing upon it should be noted2 and there is one which has not1 ,i think1 been given the full weight due to it4 ,it is generally admitted that the greater brilliancy of some parts of the ,milky ,way is no indication of nearness1 because surfaces possess equal brilliancy from whatever distance they are seen4 ,thus each planet has its special brilliancy or reflective power1 technically termed its 'albedo1' and this remains the same at all distances if the other conditions are similar4 ,but notwithstanding this well-known fact1 ,sir ,john ,herschel's remark that the greater brightness of the southern ,milky ,way 'conveys strongly the impression of greater proximity1' and therefore1 that we are excentrically placed in its plane1 has been adopted by many writers as if it were the statement of a fact1 or at least a clearly expressed opinion1 instead of being a mere 'impression1' and really a misleading one4 ,i therefore wish to adduce a phenomenon which has a real bearing on the question4 ,it is evident that1 if the ,milky ,way were actually of uniform width throughout1 then differences of apparent width would indicate differences of distance4 ,in the parts nearer to us it would appear wider1 where more remote1 narrower2 but in these opposite directions there would not necessarily be any differences in brightness4 ,we should1 however1 expect that in the parts nearer to us the lucid stars1 as well as those within any definite limits of magnitude1 would be either more numerous or more wide apart on the average4 ,no such difference as this1 however1 has been recorded2 but there is a peculiar correspondence in the opposite portions of the ,galaxy which is very suggestive4 ,in the beautiful charts of the ,nebul and ,star ,clusters by the late ,mr4 ,sidney ,waters1 published by the ,royal ,astronomical ,society and here reproduced by their permission 7see end of volume71 the ,milky ,way is delineated in its whole extent with great detail and from the best authorities4 ,these charts show us that1 in both hemispheres1 it reaches its maximum extension on the right and left margins of the charts1 where it is almost equal in extent2 while in the centre of each chart1 that is at its nearest points to the north and south poles respectively1 it is at its narrowest portion2 and1 although this part in the southern hemisphere is brightest and most strongly defined1 yet the actual extent1 including the fainter portions1 is1 again1 not very unequal in the opposite segments4 ,here we have a remarkable and significant symmetry in the proportions of the ,milky ,way1 which1 taken in connection with the nearly symmetrical scattering of the stars in all parts of the vast ring1 is strongly suggestive of a nearly circular form and of our nearly central position within its plane4 ,there is one other feature in this delineation of the ,milky ,way which is worthy of notice4 ,it has been the universal practice to speak of it as being double through a considerable portion of its extent1 and all the usual star-maps show the division greatly exaggerated1 especially in the northern hemisphere2 and this division was considered so important as to lead to the cloven-disc theory of its form1 or that it consisted of two separate irregular rings1 the nearer one partly hiding the more distant2 while various spiral combinations were held by others to be the best way of explaining its complex appearance4 ,but this newer map1 reduced from a large one by ,lord ,rosse's astronomer1 ,dr4 ,boeddicker1 who devoted five years to its delineation1 shows us that there is no actual division in any portion of it in the northern hemisphere1 but that everywhere1 throughout its whole width1 it consists of numerous intermingled streams and branches1 varying greatly in luminosity1 and with many faint or barely distinguishable extensions along its margins1 yet forming one unmistakable nebulous belt2 and the same general character applies to it in the southern hemisphere as delineated by ,dr4 ,gould4 ,another feature1 which is well shown to the eye by these more accurate maps1 is the regular curvature of the central line of the ,milky ,way4 ,we can judge of this almost sufficiently by the eye2 but if1 with a pair of compasses1 we find the proper radius and centre of curvature1 we shall see that the true circular curve is always in the very centre of the nebulous mass1 and the same radius applied in the same manner to the opposite hemisphere gives a similar result4 ,it will be noted that as the ,milky ,way is obliquely situated on these charts1 the centre of the curve will be about in ,r4,a4 #j;h4 #djm4 in the map of the southern hemisphere1 and in ,r4,a4 #ab;h4 #djm4 in that of the northern hemisphere2 while the radius of curvature will be about the length of the chord of eight hours of ,r4,a4 as measured on the margin of the maps4 ,this great regularity of curve of the central line of the ,galaxy strongly suggests rotation as the only means by which it could have originated and be maintained4 ,,the ,,solar ,,cluster ,astronomers are now generally agreed that there is a cluster of stars of which our sun forms a part1 though its exact dimensions1 form1 and limits are still under discussion4 ,sir ,william ,herschel long ago arrived at the conclusion that the ,milky ,way 'consists of stars very differently scattered from those immediately around us4' ,dr4 ,gould believed that there were about five hundred bright stars much nearer to us than the ,milky ,way1 which he termed the solar cluster4 ,and ,miss ,clerke observes that the actual existence of such a cluster is indicated by the fact that 'an enumeration of the stars in photometric order discloses a systematic excess of stars brighter than the #dth magnitude1 making it certain that there is an actual condensation in the neighbourhood of the sun--that the average allowance of cubical space per star is smaller within a sphere enclosing him with a radius1 say1 of #adj light-years1 than further away4'#e ,but the most interesting inquiry into this subject is that by ,professor ,kapteyn of ,grningen1 one of the most painstaking students of the distribution of the stars4 ,he founds his conclusions mainly on the proper motions of the stars1 this being the best general indication of distance in the absence of actual determination of parallax4 ,he made use of the proper motions and the spectra of more than two thousand stars1 and he finds that a considerable body of stars having large proper motions1 and also presenting the solar type of spectra1 surround our sun in all directions1 and show no increased density1 as the more distant stars do1 towards the ,milky ,way4 ,he finds also that towards the centre of this cluster stars are far closer together than near its outer limits 7he says there are ninety-eight times as many71 that it is roughly spherical in shape1 and that the maximum compression is1 as nearly as can be ascertained1 at the centre of the circle of the ,milky ,way1 while the sun is at some distance away from this central point4#f ,it is a very suggestive fact that most of the stars belonging to this cluster have spectra of the solar type1 which indicates that they are of the same general chemical constitution as our sun1 and are also at about the same stage of evolution2 and this may well have arisen from their origin in a great nebulous mass situated at or near the centre of the galactic plane1 and probably revolving round their common centre of gravity4 ,as ,kapteyn's result was based on materials which were not so full or reliable as those now available1 ,professor ,s4 ,newcomb has examined the question himself1 using two recent lists of stars1 one limited to those having proper motions of #aj a century1 of which there are #bie1 and the other of nearly #aejj stars with 'appreciable proper motions4' ,they are situated in two zones1 each about #e in breadth and cutting across the ,milky ,way in different parts of its course4 ,they afford1 therefore1 a good test of the distribution of these nearer stars with regard to the ,galaxy4 ,the result is1 that on the average these stars are not more numerous in or near the ,milky ,way than elsewhere2 and ,professor ,newcomb expresses himself on this point as follows3--',the conclusion is interesting and important4 ,if we should blot out from the sky all the stars having no proper motion large enough to be detected1 we should find remaining stars of all magnitudes2 but they would be scattered almost uniformly over the sky1 and show little or no tendency to crowd towards the ,galaxy1 unless1 perhaps1 in the region near #ai;h4 of ,right ,ascension4'#g ,a little consideration will show that1 as the stars of all magnitudes which are1 on the average1 nearest to us are spread over the sky in 'all directions' and 'almost uniformly1' this necessarily implies that they form a cluster or group1 and that our sun is somewhere not very far from the centre of this group4 ,again1 ,professor ,newcomb refers to 'the remarkable equality in the number of stars in opposite directions from us4 ,we do not detect any marked difference between the numbers lying round the opposite poles of the ,galaxy1 nor1 so far as known1 between the star-density in different regions at equal distances from the ,milky ,way' 7,the ,stars1 p4 #cae74 ,and again he refers to the same question at p4 #cag1 where he says3 ',so far as we can judge from the enumeration of the stars in all directions1 and from the aspect of the ,milky ,way1 our system is near the centre of the stellar universe4' ,it will1 ,i think1 now be clear to my readers that the four main astronomical propositions stated in my article which appeared in the ,new ,york ,independent and in the ,fortnightly ,review1 and which were either denied or declared to be unproved by my astronomical critics1 have been shown to be supported by so many converging lines of evidence1 that it is no longer possible to deny that they are1 at least provisionally1 fairly well established4 ,these facts are1 7#a7 that the stellar universe is not of infinite extent2 7#b7 that our sun is situated in the central plane of the ,milky ,way2 7#c7 that it is also situated near to the centre of that plane2 7#d7 that we are surrounded by a group or cluster of stars of unknown extent1 which occupy a place not far removed from the centre of the galactic plane1 and therefore1 near to the centre of our universe of stars4 ,not only are these four propositions each supported by converging lines of evidence1 including some which ,i believe have not before been adduced in their support1 but a number of astronomers1 admittedly of the first rank1 have arrived at the same conclusions as to the bearing of the evidence1 and have expressed their convictions in the clearest manner1 as quoted by me4 ,it is their conclusions which ,i appeal to and adopt2 yet my two chief astronomical critics positively deny that there is any valid evidence of the finiteness of the stellar universe1 which one of them terms 'a myth1' and he even accuses me of having started it4 ,both of them1 however1 agree in stating very strongly one objection to my main thesis--that our central position 7not necessarily at the precise centre7 in the stellar universe has a meaning and a purpose1 in connection with the development of life and of man upon this earth1 and1 so far as we know1 here only4 ,with this one objection1 the only one that in my opinion has the slightest weight1 ,i will now proceed to deal4 ,,the ,,sun',s ,,motion ,,through ,,space ,the two astronomers who did me the honour to criticise my original article laid the greatest stress on the fact1 that even if ,i had proved that the sun now occupied a nearly central position in the great star-system1 it was really of no importance whatever1 because1 at the rate the sun was travelling1 'five million years ago we were deep in the actual stream of the ,milky ,way2 five million years hence we shall have completely crossed the gulf which it encircles1 and again be a member of one of its constituent groups1 but on the opposite side4 ,and ten million years are regarded by geologists and biologists as but a trifle on account to meet their demands upon the bank of ,time4' ,thus speaks one of my critics4 ,the other is equally crushing4 ,he says3--',if there is a centre to the visible universe1 and if we occupy it to-day1 we certainly did not do so yesterday1 and shall not do so to-morrow4 ,the ,solar ,system is known to be moving among the stars with a velocity which would carry us to ,sirius within #ajj1jjj years1 if we happened to be travelling in his direction1 as we are not4 ,in the #ej or #ajj million years during which1 according to geologists1 this earth has been a habitable globe1 we must have passed by thousands of stars on the right hand and on the left4444 ,in his eagerness to limit the universe in space1 ,dr4 ,wallace has surely forgotten that it is equally important1 for his purpose1 to limit it in time2 but incomparably more difficult in the face of ascertained facts4444 ,indeed1 so far from our having tranquilly enjoyed a central position in unbroken continuity for scores or perhaps hundreds of millions of years1 we should in that time have traversed the universe from boundary to boundary4'#h ,now the average reader of these two criticisms1 taking account of the high official position of both writers1 would accept their statements of the case as being demonstrated facts1 requiring no qualification whatever1 and would conclude that my whole argument had been thereby rendered worthless1 and all that ,i founded upon it a fantastic dream4 ,but if1 on the other hand1 ,i can show that their stated facts as to the sun's motion are by no means demonstrated1 because founded upon assumptions which may be quite erroneous2 and further1 that if the facts should turn out to be substantially correct1 they have both omitted to state well-known and admitted qualifications which render the conclusions they derive from the facts very doubtful1 then the average reader will learn the valuable lesson that official advocacy1 whether in medicine1 law1 or science is never to be accepted till the other side of the case has been heard4 ,let us see1 therefore1 what the facts really are4 ,professor ,simon ,newcomb calculates that1 if there are one hundred million stars in the stellar universe each five times the mass of our sun1 and spread over a space which light would require thirty thousand years to cross1 then any mass traversing such a system with a velocity of more than twenty-five miles a second1 would fly off into infinite space never to return4 ,now as there are many stars which have1 apparently1 very much more than this velocity1 it would follow that the visible universe is unstable4 ,it also implies that these great velocities were not acquired in the system itself1 but that the bodies which possess them must have entered it from without1 thus requiring other universes as the feeders of our universe4 ,for the accuracy of the above statement the authority of ,professor ,newcomb is an ample guarantee2 but there may be modifications required in the data on which it is founded1 and these may greatly alter the result4 ,if ,i do not mistake1 the estimate of a hundred million stars is founded on actual counts or estimates of stars of successive magnitudes in different parts of the heavens1 and it does not include either those of the denser star clusters nor the countless millions just beyond the reach of telescopes in the ,milky ,way4 ,neither does it make allowance for the dark stars supposed by some astronomers to be many times more numerous than the bright ones1 nor for the vast number of the nebul1 great and small1 in calculating the total mass of the stellar system4#i ,in his latest work ,professor ,newcomb says1 ',the total number of stars is to be counted by hundreds of millions'2 and hence the controlling power of the system on bodies within it will be many times greater than that given above1 and might even be ample to retain within its bounds such a rapidly moving star as ,arcturus1 which is believed to be travelling at the rate of more than three hundred miles a second4 ,but there is another very important limitation to the conclusions to be drawn from ,professor ,newcomb's calculation4 ,it assumes the stars to be nearly uniformly distributed through the whole of the space to which the system extends4 ,but the facts are very different4 ,the existence of clusters1 some of which comprise many thousands of stars1 is one example of irregularity of distribution1 and any one of these larger clusters would probably be able to change the course of even the swiftest stars passing near it4 ,the larger nebul might have the same effect1 since the late ,mr4 ,ranyard1 taking all his data so as to produce a minimum result1 calculated the probable mass of the ,orion nebula to be four and a half million times that of the sun1 and there may be many other nebul equally large4 ,but far more important is the fact of the vast ring of the ,milky ,way1 which is now universally held by astronomers to be1 not only apparently but really1 more densely crowded with stars and also with vast masses of nebulous matter than any other part of the heavens1 so that it may possibly comprise within itself a very large proportion of the whole of the matter of the visible universe4 ,this is rendered more probable by the fact that the great majority of star-clusters lie along its course1 most of the huge gaseous stars belong to it1 while the occurrence there only of 'new stars' is evidence of a superabundance of matter in various forms leading to frequent heat-producing collisions1 just as the frequent occurrence of meteoric showers on our earth is evidence of the superabundance of meteoric matter in the solar system4 ,it is recognised by mathematicians that within any great system of bodies subject to the law of gravitation there can be no such thing as motion of any of them in a straight line2 neither can any amount of motion arise within such a system through the action of gravitation alone capable of carrying any of its masses out of the system4 ,the ultimate tendency must be towards concentration rather than towards dispersal4 ,it seems1 therefore1 only reasonable to consider whatever motions and whatever velocities we find among the stars1 as having been produced by the gravitative power of the larger aggregations1 modified perhaps by electrical repulsive forces1 by collisions1 and by the results of those collisions2 and we may look to the changes now visibly going on in some of the nebul and clusters as indications of the forces that have probably brought about the actual condition of the whole stellar universe4 ,if we examine the beautiful photographs of nebul by ,dr4 ,roberts and other observers1 we find that they are of many forms4 ,some are extremely irregular and almost like patches of cirrus clouds1 but a large number are either distinctly spiral in form1 or show indications of becoming spiral1 and this has been found to be the case even with some of the large irregular nebula4 ,then again we have numerous ring-formed nebul1 usually with a star involved in dense nebulosity in the centre1 separated by a dark space of various widths from the outer ring4 ,all these kinds of nebul have stars involved in them1 and apparently forming part of their structure1 while others which do not differ in appearance from ordinary stars are believed by ,dr4 ,roberts to lie between us and the nebula4 ,in the case of many of the spiral nebul1 stars are often strung along the coils of the spiral1 while other curved lines of stars are seen just outside the nebula1 so that it is impossible to avoid the conclusion that both are really connected with it1 the outer lines of stars indicating a former greater extension of the nebula whose material has been used up in the growth of these stars4 ,some of these spiral nebul show beautifully regular convolutions1 and these usually have a large central star like mass1 as in ,m4 #ajj ,com and ,i4 #hd ,com1 in ,vol4 ,,ii4 ,pl4 #ad of ,dr4 ,roberts's photographs4 ,the straight white streaks across the nebula of the ,pleiades and some others are believed by ,dr4 ,roberts to be indications of spiral nebul seen edgewise4 ,in other cases1 clusters of stars are more or less nebulous1 and the arrangement of the stars seems to indicate their development from a spiral nebula4 ,it is to be noted that many of the objects classed as planetary nebul by ,sir ,john ,herschel are shown by the best photographs to be really of the ring-type1 though often with a very narrow division between the ring and the central mass4 ,this form may therefore be of frequent occurrence4 ,but if this annular form with some kind of central nucleus1 often very large1 is produced under certain conditions by the action of the ordinary laws of motion upon more or less extensive masses of discrete matter1 why may not the same laws acting upon similar matter once dispersed over the whole extent of the existing stellar universe1 or even beyond what are now its farthest limits1 have led to the aggregation of the vast annular formation of the ,milky ,way1 with all the subordinate centres of concentration or dispersal to be found within or around it8 ,and if this is a reasonable conception1 may we not hope that by a concentration of attention upon a few of the best marked and most favourably situated annular and spiral systems1 sufficient knowledge of their internal motions may be obtained which may serve as a guide to the kind of motion we may expect to find in the great galactic ring and its subordinate stars8 ,we may then perhaps discover which now seem so erratic1 are really all parts of a series of orbital movements limited and controlled by the forces of the great system to which they belong1 so that1 if not mathematically stable1 they may yet be sufficiently so to endure for some thousand millions of years4 ,it is a suggestive fact that the calculated position of the 'solar apex'--the point towards which our sun appears to move--is now found to be much more nearly in the plane of the ,milky ,way than the position first assigned to it1 and ,professor ,newcomb adopts1 as most likely to be accurate1 a point near the bright star ,vega in the constellation ,lyra4 ,other calculators have placed it still farther east1 while ,rancken and ,otto ,stumpe assign it a position actually in the ,milky ,way2 and ,mr4 ,g4,c4 ,bompas concludes that the sun's plane of motion nearly coincides with that of the ,galaxy4 ,m4 ,rancken found that #ajf stars near the ,milky ,way showed1 in their very small proper motions1 a drift along it in a direction from ,cassiopei towards ,orion1 and this1 it is supposed1 may be partly due to our sun's motion in an opposite direction4 ,in many other parts of the heavens there are groups of stars which have almost identical proper motions--a phenomenon which the late ,r4,a4 ,proctor termed 'star-drift'2 and he especially pointed out that five of the stars of the ,great ,bear were all drifting in the same direction2 and although this has been denied by later writers1 ,professor ,newcomb1 in his recent book on ,the ,stars1 declares that ,proctor was right1 and explains that the error of his critics was due to not making allowance for the divergence of the circles of right ascension4 ,the ,pleiades are another group1 the stars of which drift in the same direction1 and it is a most suggestive fact that photographs now show this cluster to be embedded in a vast nebula1 which1 therefore1 has also a proper motion2 but some of the smaller stars do not partake of it4 ,three stars in ,cassiopei also move together1 and no doubt many other similarly connected groups remain to be discovered4 ,these facts have a very important bearing on the question of the motion of our sun in space4 ,for this motion has been determined by comparing the motions of large numbers of stars which are assumed to be wholly independent of each other1 and to move1 as it were1 at random4 ,miss ,a4,m4 ,clerke1 in her ,system of the ,stars1 puts this point very clearly1 as follows3 ',for the assumption that the absolute movements of the stars have no preference for one direction over another1 forms the basis of all investigations hitherto conducted into the translatory advance of the solar system4 ,the little fabric of laboriously acquired knowledge regarding it at once crumbles if that basis has to be removed4 ,in all investigations of the sun's movement1 the movements of the stars have been regarded as casual irregularities2 should they prove to be in any visible degree systematic1 the mode of treatment adopted 7and there is no other at present open to us7 becomes invalid1 and its results null and void4 ,the point is then of singular interest1 and the evidence bearing upon it deserves our utmost attention4' ,mr4 ,w4,h4,s4 ,monck1 a well-known astronomer1 takes the same view4 ,he says3 ',the proof of this motion rests on the assumption that if we take a sufficient number of stars1 their real motions in all directions will be equal1 and that therefore the apparent preponderances which we observe in particular directions result from the real motion of the sun4 ,but there is no impossibility in a systematic motion of the majority of the stars used in these researches which might reconcile the observed facts with a motionless sun4 ,and1 in the second place1 if the sun is not in the exact centre of gravity of the universe1 we might expect him to be moving in an orbit around this centre of gravity1 and our observations on his actual motion are not sufficiently numerous or accurate to enable us to affirm that he is moving in a right line rather than such an orbit4' ,now this 'systematic motion1' which would render all calculations as to the sun's motion inaccurate or even altogether worthless1 is by many astronomers held to be an observed reality4 ,the star-drift1 first pointed out by ,proctor1 has been shown to exist in many other groups of stars1 while the curious arrangements of stars all over the heavens in straight lines1 or regular curves1 or spirals1 strongly suggests a wide extension of the same kind of relation4 ,but even more extensive systematic movements have been observed or suggested by astronomers4 ,sir ,d4 ,gill1 by an extensive research1 believes that he has found indications of a rotation of the brighter fixed stars as a whole in regard to the fainter fixed stars as a whole4 ,mr4 ,maxwell ,hall has also found indications of a movement of a large group of stars1 including our sun1 around a common centre1 situated in a direction towards ,epsilon ,andromed1 and at a distance of about #dij years of light-travel4 ,these last two motions are not yet established2 but they seem to prove two important facts--7a7 that eminent astronomers believe that some systematic motions must exist among the stars1 or they would not devote so much labour to the search for them2 and 7b7 that extensive systematic motions of some kind do exist1 or even these results would not have been obtained4 ,mr4 ,w4,w4 ,campbell1 of the ,lick ,observatory1 thus remarks on the uncertainty of determinations of the sun's motions3 ',the motion of the solar system is a purely relative quantity4 ,it refers to specified groups of stars4 ,the results for various groups may differ widely1 and all be correct4 ,it would be easy to select a group of stars with reference to which the solar motion would be reversed #ahj from the values assigned above' 7,astrophysical ,journal1 vol4 xiii4 p4 #hg4 #aija74 ,it must be remembered that1 within a uniform cluster of stars1 each moving round the common centre of gravity of the whole cluster1 ,kepler's laws do not prevail1 the law being that the angular velocities are all identical1 so that the more distant stars move faster than those nearer the centre1 subject to modifications1 however1 due to the varying density of the cluster4 ,but if the cluster is nearly globular1 there must be stars moving round the centre in every plane1 and this would lead to apparent motions in many directions as viewed by us1 although those which were moving in the same plane as ourselves would1 when compared with remote stars outside the cluster1 appear to be all moving in the same direction and at the same rate1 forming1 in fact1 one of those drifting systems of stars already referred to4 ,again1 if in the process of formation of our cluster1 smaller aggregations already having a rotatory motion were drawn into it1 this might lead to their revolving in an opposite direction to those which were formed from the original nebula1 thus increasing the diversities of apparent motion4 ,the evidence now briefly set forth fully justifies1 ,i submit1 the remarks as to the statements of my astronomical critics at the beginning of this section4 ,they have both given the accepted views as to direction and rate of movement of our sun without any qualification whatever1 as if they were astronomical facts of the same certainty and the same degree of accuracy as the sun's distance from the earth2 and they will assuredly have been so understood by the great body of non-mathematical readers4 ,it appears1 however1 if the authorities ,i have quoted are right1 that the whole calculation rests upon certain assumptions1 which are certainly to some extent1 and may be to a very large extent1 erroneous4 ,this is my reply to one part of their criticism4 ,in the next place1 they both assert1 or imply1 not only that the sun's motion is now in a straight line1 but that it has been in a straight line from some enormously remote period when it first entered the stellar system on one side1 and will so continue to move till it reaches the utmost bounds of that system on the other side4 ,and this is stated by them both1 not as a possibility1 but as a certainty4 ,they use such terms as 'must' and 'will be1' leaving no room for any doubt whatever4 ,but such a result implies the abrogation of the law of gravitation1 since under its action motion in a straight line in the midst of thousands or millions of suns of various sizes is an absolute impossibility2 while it also implies that the sun must have been started on its course from some other system outside the ,milky ,way1 with such a precise determination of direction as not to collide with1 or even make a near approach to1 any one of the suns or clusters of suns1 or vast nebulous masses1 during its passage through the very midst of the stellar universe4 ,this is my reply to the main point of their criticism1 and ,i think ,i am justified in saying that nothing in my whole article is so demonstrably baseless as the statements ,i have now examined4 99 99 99 99 99 ,considering then the whole bearing of the evidence1 ,i refuse to accept the unsupported dicta of those who would have us believe that our admitted position not far from the centre of the stellar universe is a mere temporary coincidence of no significance whatever2 or that our sun and hosts of other similar orbs near to us have come together by an accident1 and are being dispersed into surrounding space1 never to meet again4 ,until this is proved by indisputable evidence1 it seems to me far more probable that we are moving in an orbit of some kind around the centre of gravity of a vast cluster1 as determined by the investigations of ,kapteyn1 ,newcomb1 and other astronomers2 and1 consequently1 that the nearly central position we now occupy may be a permanent one4 ,for even if our sun's orbit should have a diameter a thousand times that of ,neptune1 it would be but a small fraction of the diameter of the ,milky ,way2 while so vast is the scale of our universe1 that it might be even a hundred thousand times as great and still leave us deeply immersed in the solar cluster1 and very much nearer to the dense central portion than to its more diffused outer regions4 ,here the subject may be left for the present4 ,after having studied the evidence afforded by the essential conditions of life-development on the earth1 and the numerous indications that these conditions do not exist on any of the other planets of the solar system1 it may be again touched upon in a general review of the conclusions arrived at4 ,,footnotes3 #d ,nature1 ,october #bf1 #ahii4 #e ,the ,system of the ,stars1 p4 #che4 #f ,this account of ,professor ,kapteyn's research is taken from an article by ,miss ,a4,m4 ,clerke in ,knowledge1 ,april #ahic4 #g ,the ,stars1 p4 #bef4 ,the region here referred to is that where the ,milky ,way has its greatest width 7though nearly as wide in the part exactly opposite71 and where it may perhaps extend somewhat in our direction4 ,miss ,a4,m4 ,clerke informs me that in ,april #aija ,kapteyn withdrew the conclusions arrived at in #ahic1 as being founded on illegitimate reasoning as to the relation of parallaxes to proper motions4 ,but as this relation is still accepted1 under certain limitations1 by ,professor ,newcomb and other astronomers1 who have arrived independently at very similar results1 it seems not improbable that1 after all1 ,professor ,kapteyn's conclusions may not require very much modification4 ,professor ,newcomb also tells us 7,the ,stars1 p4 #bad1 footnote7 that he has seen the latest of ,professor ,kapteyn's papers1 down to #aija2 but he does not therefore express any doubt as to his own conclusions as here referred to4 #h ,see ,knowledge and ,the ,fortnightly ,review of ,april #aijc4 #i ,sir ,r4 ,ball in an article in ,good ,words 7,april #aijc7 says that luminosity is an exceptional phenomenon in nature1 and that luminous stars are but the glow-worms and fire-flies of the universe1 as compared with the myriads of other animals4 ,,chapter ,,ix ,,the ,,uniformity ,,of ,,matter ,,and ,,its ,,laws ,,throughout ,,the ,,stellar ,,universe ,i have shown in the second chapter of this work that none of the previous writers on the question of the habitability of the other planets have really dealt with the subject in any adequate manner1 since not only do they appear to be quite unaware of the delicate balance of conditions which alone renders organic life possible on any planet1 but they have altogether omitted any reference to the fact that not only must the conditions be such as to render life possible now1 but these conditions must have persisted during the long geological epochs needed for the slow development of life from its most rudimentary forms4 ,it will therefore be necessary to enter into some details both as to the physical and chemical essentials for a continuous development of organic life1 and also into the combination of mechanical and physical conditions which are required on any planet to render such life possible4 ,,the ,,uniformity ,,of ,,matter ,one of the most important and far-reaching of the discoveries due to the spectroscope is that of the wonderful identity of the elements and material compounds in earth and sun1 stars and nebul1 and also of the identity of the physical and chemical laws that determine the states and forms assumed by matter4 ,more than half the total number of the known elements have been already detected in the sun1 including all those which compose the bulk of the earth's solid material1 with the one exception of oxygen4 ,this is a very large proportion when we consider the very peculiar conditions which enable us to detect them4 ,for we can only recognise an element in the sun when it exists at its surface in an incandescent state1 and also above its surface in the form of a somewhat cooler gas4 ,many of the elements may rarely or never be brought to the surface of so vast a body1 or if they do sometimes appear there1 it may not be in sufficient quantity or in sufficient purity to produce any bands in the spectroscope1 while the cooler gas or vapour may either not be present1 or be so dispersed as not to produce sufficient absorption to render its spectral lines visible4 ,again1 it is believed that many elements are dissociated by the intense heat of the sun1 and may not be recognisable by us1 or they may only exist at its surface in a compound form unknown on the earth2 and in some such way those lines of the solar spectrum which remain still unrecognised may have been produced4 ,one of these unknown lines was that of ,helium1 a gas found soon afterwards in the rare mineral ',cleveite1' and since detected frequently in many stars4 ,some of the stars have spectra very closely resembling that of the sun4 ,the dark lines are almost as numerous1 and most of them correspond accurately with solar lines1 so that we cannot doubt their having almost exactly the same chemical constitution1 and being also in the same condition as regards heat and stage of development4 ,other stars1 as we have already stated1 exhibit mainly lines of hydrogen1 sometimes combined with fine metallic lines4 ,of the spectra of the nebul comparatively little is known1 but many are decidedly gaseous1 while others show a continuous spectrum indicating a more complex constitution4 ,but we also obtain considerable knowledge of the matter of non-terrestrial bodies by the analysis of the numerous meteorites which fall upon the earth4 ,most of these belong to some of the many meteoric streams which circulate round the sun1 and which may be supposed to give us samples of planetary matter4 ,but as it is now believed that many of them are produced by the debris of comets1 and the orbits of some of these indicate that they have come from stellar space and have been drawn into our system by the attractive power of the larger planets1 it is almost certain that the meteoric stones not infrequently bring us matter from the remoter regions of space1 and probably afford us samples of the solid constituents of nebula2 or the cooler stars4 ,it is1 therefore1 a most suggestive fact that none of these meteorites have been found to contain a single non-terrestrial element1 although no less than twenty-four elements have been found in them1 and it will be of interest to give the list of these1 as follows3--,oxygen1 ,hydrogen1 ,chlorine1 ,sulphur1 ,phosphorus1 ,carbon1 ,silicon1 ,iron1 ,nickel1 ,cobalt1 ,magnesium1 ,chromium1 ,manganese1 ,copper1 ,tin1 ,antimony1 ,aluminium1 ,calcium1 ,potassium1 ,sodium1 ,lithium1 ,titanium1 ,arsenic1 and ,vanadium4 ,seven of the above1 printed in italics1 have not yet been found in the sun1 such as ,oxygen1 ,chlorine1 ,sulphur1 and ,phosphorus1 which form the constituents of many widespread minerals1 and they supply important gaps in the series of solar and stellar elements4 ,it may be noted that although meteorites have supplied no new elements1 they have furnished examples of some new combinations of these elements forming minerals distinct from any found in our rocks4 ,the fact of the occurrence in meteorites not only of minerals which are peculiar to them or are found on the earth1 but also of structures resembling our breccias1 veins1 and even slicken-side surfaces1 has been held to be opposed to the meteoritic theory of the origin of suns and planets1 because meteorites seem to be thus proved to be the fragments of suns or worlds1 not their primary constituents4 ,but these cases are exceptional1 and ,mr4 ,sorby1 who made a special study of meteorites1 concluded that their materials have usually been in a state of fusion or even of vapour1 as they now exist in the sun1 and that they became condensed into minute globular particles1 which afterwards collected into larger masses1 and may have been broken up by mutual impact1 and again and again become aggregated together--thus presenting features which are completely in accordance with the meteoritic theory4 ,but1 quite recently1 ,mr4 ,t4,c4 ,chamberlin has applied the theory of tidal distortion to showing how solid bodies in space1 without ever coming into actual contact1 must sometimes be torn apart or disrupted into numerous fragments by passing near to each other4 ,especially when a small body passes near a much larger one1 there is a certain distance of approach 7termed the ,roche limit7 when the increasing differential force of gravity will be sufficient to tear asunder the smaller body and cause the fragments either to circulate around it or to be dispersed in space4#aj ,in this way1 therefore1 those larger meteorites which exhibit planetary structure may have been produced4 ,of course they would rarely have been true planets attached to a sun1 but more frequently some of the smaller dark suns1 which may possess many of the physical characteristics of planets1 and of which there may be myriads in the stellar spaces4 ,on the whole1 then1 we have positive knowledge of the existence1 in the sun1 stars1 and planetary and stellar spaces1 of such a large proportion of the elements of our globe1 and so few indications of any not forming part of it1 that we are justified in the statement1 that the whole stellar universe is1 broadly speaking1 constructed of the same series of elementary substances as those we can study upon our earth1 and of which the whole realm of nature1 animal1 vegetable1 and mineral1 is composed4 ,the evidence of this identity of substance is really far more complete than we could expect1 considering the very limited means of inquiry that we possess2 and we shall1 therefore1 not be justified in assuming that any important difference exists4 ,when we pass from the elements of matter to the laws which govern it1 we also find the clearest proofs of identity4 ,that the fundamental law of gravitation extends to the whole physical universe is rendered almost certain by the fact that double stars move round their common centre of gravity in elliptical orbits which correspond well with both observation and calculation4 ,that the laws of light are the same both here and in inter-planetary space is indicated by the fact that the actual measurement of the velocity of light on the earth's surface gives a result so completely identical with that prevailing to the limits of the solar system1 that the measurement of the sun's distance1 by means of the eclipses of ,jupiter's satellites combined with the measured velocity of light1 agrees almost exactly with that obtained by means of the transits of ,venus1 or through our nearest approach to the planets ,mars or ,eros4 ,again1 the more recondite laws of light are found to be identical in sun and stars with those observed within the narrow bounds of laboratory experiments4 ,the minute change of position of spectral lines caused by the source of light moving towards or away from us enables us to determine this kind of motion in the most distant stars1 in the planets1 or in the moon1 and these results can be tested by the motion of the earth either in its orbit or in its rotation2 and these latter tests agree with the theoretical determination of what must occur1 dependent on the wave-lengths of the different dark lines of the solar spectrum determined by measurements in the laboratory4 ,in like manner1 minute changes in the widening or narrowing of spectral lines1 their splitting up1 their increase or decrease in number1 and their arrangement so as to form flutings1 can all be interpreted by experiments in the laboratory1 showing that such phenomena are due to alterations of temperature1 of pressure1 or of the magnetic field1 thus proving that the very same physical and chemical laws act in the same way here and in the remotest depths of space4 ,these various discoveries give us the certain conviction that the whole material universe is essentially one1 both as regards the action of physical and chemical laws1 and also in its mechanical relations of form and structure4 ,it consists throughout of the very same elements with which we are so familiar on our earth2 the same ether whose vibrations bring us light and heat1 electricity and magnetism1 and a whole host of other mysterious and as yet imperfectly known forces2 gravitation acts throughout its vast extent2 and in whatever direction and by whatever means we obtain a knowledge of the stellar universe1 we find the same mechanical1 physical1 and chemical laws prevailing as upon our earth1 so that we have in some cases been actually enabled to reproduce in our laboratories phenomena with which we had first become acquainted in the sun or among the stars4 ,we may therefore feel it to be an almost certain conclusion that--the elements being the same1 the laws which act upon1 and combine1 and modify those elements being the same--organised living beings wherever they may exist in this universe must be1 fundamentally1 and in essential nature1 the same also4 ,the outward forms of life1 if they exist elsewhere1 may vary almost infinitely1 as they do vary on the earth2 but1 throughout all this variety of form--from fungus or moss to rose-bush1 palm or oak2 from mollusc1 worm1 or butterfly to humming-bird1 elephant1 or man--the biologist recognises a fundamental unity of substance and of structure1 dependent on the absolute requirements of the growing1 moving1 developing1 living organism1 built up of the same elements1 combined in the same proportions1 and subject to the same laws4 ,we do not say that organic life could not exist under altogether diverse conditions from those which we know or can conceive1 conditions which may prevail in other universes constructed quite differently from ours1 where other substances replace the matter and ether of our universe1 and where other laws prevail4 ,but1 within the universe we know1 there is not the slightest reason to suppose organic life to be possible1 except under the same general conditions and laws which prevail here4 ,we will1 therefore1 now proceed to describe1 very generally1 what are the conditions essential to the existence and the continuous development of vegetable and animal life4 ,,footnote3 #aj ,the ,astrophysical ,journal1 vol4 xiv41 ,july #aija1 p4 #ag4 ,,chapter ,x ,,the ,,essential ,,characters ,,of ,,the ,,living ,,organism ,before trying to comprehend the physical conditions on any planet which are essential for the development and maintenance of a varied and complex system of organic life comparable to that of our earth1 we must obtain some knowledge of what life is1 and of the fundamental nature and properties of the living organism4 ,physiologists and philosophers have made many attempts to define 'life1' but in most cases in aiming at absolute generality they have been vague and uninstructive4 ,thus ,de ,blainville defined it as ',the twofold internal movement of composition and decomposition1 at once general and continuous'2 while ,herbert ,spencer's latest definition was ',life is the continuous adjustment of internal relations to external relations4' ,but neither of these is sufficiently precise1 explanatory1 or distinctive1 and they might almost be applied to the changes occurring in a sun or planet1 or to the elevation and gradual formation of a continent4 ,one of the oldest definitions1 that of ,aristotle1 seems to come nearer the mark3 ',life is the assemblage of the operations of nutrition1 growth1 and destruction4' ,but these definitions of 'life' are unsatisfactory1 because they apply to an abstract idea rather than to the actual living organism4 ,the marvel and mystery of life1 as we know it1 resides in the body which manifests it1 and this living body the definitions ignore4 ,the essential points in the living body1 as seen in its higher developments1 are1 first1 that it consists throughout of highly complex but very unstable forms of matter1 every particle of which is in a continual state of growth or decay2 that it absorbs or appropriates dead matter from without2 takes this matter into the interior of its body2 acts upon it mechanically and chemically1 rejecting what is useless or hurtful2 and so transforming the remainder as to renew every atom of its own structure internal and external1 at the same time throwing off1 particle by particle1 all the worn-out or dead portions of its own substance4 ,secondly1 in order to be able to do all this1 its whole body is permeated throughout by branching vessels or porous tissues1 by which liquids and gases can reach every part and carry on the various processes of nutrition and excretion above referred to4 ,as ,professor ,burdon ,sanderson well puts it3 ',the most distinctive peculiarity of living matter as compared with non-living is1 that it is ever changing while ever the same4' ,and these changes are the more remarkable because they are accompanied1 and even produced1 by a very large amount of mechanical work--in animals by means of their normal activities in search of food1 in assimilating that food1 in continually renewing and building up their whole organism1 and in many other ways2 in plants by building up their structure1 which often involves raising tons of material high into the air1 as in forest trees4 ,as a recent writer puts it3 ',the most prominent1 and perhaps the most fundamental1 phenomenon of life is what may be described as the ,energy ,traffic or the function of trading in energy4 ,the chief physical function of living matter seems to consist in absorbing energy1 storing it in a higher potential state1 and afterwards partially expending it in the kinetic or active form4'#aa ,thirdly--and perhaps most marvellous of all--all living organisms have the power of reproduction or increase1 in the lowest forms by a process of self-division or 'fission1' as it is termed1 in the higher by means of reproductive cells1 which1 though in their earliest stage quite indistinguishable physically or chemically in very different species1 yet possess the mysterious power of developing a perfect organism1 identical with its parents in all its parts1 shapes1 and organs1 and so wonderfully resembling them1 that the minutest distinctive details of size1 form1 and colour1 in hair or feathers1 in teeth or claws1 in scales1 spines1 or crests1 are reproduced with very close accuracy1 though often involving metamorphic changes during growth of so strange a nature that1 if they were not familiar to us but were narrated as occurring only in some distant and almost inaccessible region1 would be treated as travellers' tales1 incredible and impossible as those of ,sindbad the ,sailor4 ,in order that the substance of living bodies should be able to undergo these constant changes while preserving the same form and structure in minute details--that they should be1 as it were1 in a constant state of flux while remaining sensibly unchanged1 it is necessary that the molecules of which they are built up should be so combined as to be easily separated and as easily united--be1 as it is termed1 labile or flowing2 and this is brought about by their chemical composition1 which1 while consisting of few elements1 is yet highly complex in structure1 a large number of chemical atoms being combined in an endless variety of ways4 ,the physical basis of life1 as ,huxley termed it1 is protoplasm1 a substance which consists essentially of only four common elements1 the three gases1 nitrogen1 hydrogen1 and oxygen1 with the non-metallic solid1 carbon2 hence all the special products of plants and animals are termed carbon-compounds1 and their study constitutes one of the most extensive and intricate branches of modern chemistry4 ,their complexity is indicated by the fact that the molecule of sugar contains #de1 and that of stearine no less than #agc1 constituent atoms4 ,the chemical compounds of carbon are far more numerous than those of all the other chemical elements combined2 and it is this wonderful variety and the complexity of its possible combinations which explain the fact1 that all the various animal tissues--skin1 horn1 hair1 nails1 teeth1 muscle1 nerve1 etc41 consist of the same four elements 7with occasionally minute quantities of sulphur1 phosphorus1 lime1 or silica1 in some of them71 as proved by the marvellous fact that these tissues are all produced as well by the grass-eating sheep or ox as by the fish or flesh-eating seal or tiger4 ,and the marvel is still further increased when we consider that the innumerable diverse substances produced by plants and animals are all formed out of the same three or four elements4 ,such are the endless variety of organic acids1 from prussic acid to those of the various fruits2 the many kinds of sugars1 gums1 and starches2 the number of different kinds of oil1 wax1 etc42 the variety of essential oils which are mostly forms of turpentines1 with such substances as camphor1 resins1 caoutchouc1 and gutta-percha2 and the extensive series of vegetable alkaloids1 such as nicotine from tobacco1 morphine from opium1 strychnine1 curarine1 and other poisons2 quinine1 belladonna1 and similar medicinal alkaloids2 together with the essential principles of our refreshing drinks1 tea1 coffee1 and cocoa1 and others too numerous to be named here--all alike consisting solely of the four common elements from which almost our whole organism is built up4 ,if this were not indisputably proved1 it would scarcely be credited4 ,professor ,f4,j4 ,allen considers that the most important element in protoplasm1 and that which confers upon it its most essential properties in the living organism--its extreme mobility and transposibility--is nitrogen4 ,this element1 though inert in itself1 readily enters into compounds when energy is supplied to it1 the most striking illustration of which is the formation of ammonia1 a compound of nitrogen and hydrogen1 produced by electric discharges through the atmosphere4 ,ammonia1 and certain oxides of nitrogen produced in the atmosphere in the same way1 are the chief sources of the nitrogen assimilated by plants1 and through them by animals2 for although plants are continually in contact with the free nitrogen of the atmosphere1 they are unable to absorb it4 ,by their leaves they absorb oxygen and carbon-dioxide to build up their woody tissues1 while by their roots they absorb water in which ammonia and oxides of nitrogen are dissolved1 and from these they produce the protoplasm which builds up the whole substance of the animal world4 ,the energy required to produce these nitrogen-compounds is given up by them when undergoing further changes1 and thus the production of ammonia by electricity in the atmosphere1 and its being carried by rain into the soil1 constitute the first steps in that long series of operations which culminates in the production of the higher forms of life4 ,but the remarkable transformations and combinations continually going on in every living body1 which are1 in fact1 the essential conditions of its life1 are themselves dependent on certain physical conditions which must be always present4 ,professor ,allen remarks3 ',the sensitiveness of nitrogen1 its proneness to change its state of combination and energy1 appear to depend on certain conditions of temperature1 pressure1 etc41 which exist at the surface of this earth4 ,most vital phenomena occur between the temperature of freezing water and #ajd ,f4 ,if the general temperature of the earth's surface rose or fell #gb ,f4 7a small amount relatively71 the whole course of life would be changed1 even perchance to extinction4' ,another important1 and even more essential fact1 in connection with life1 is the existence in the atmosphere of a small but nearly constant proportion of carbonic acid gas1 this being the source from which the whole of the carbon in the vegetable and animal kingdoms is primarily derived4 ,the leaves of plants absorb carbonic acid gas from the atmosphere1 and the peculiar substance1 chlorophyll1 from which they derive their green colour1 has the power1 under the influence of sunlight1 to decompose it1 using the carbon to build up its own structure and giving out the oxygen4 ,in the laboratory the carbon can only be separated from the oxygen by the application of heat1 under which certain metals burn by combining with the oxygen1 thus setting free the carbon4 ,chlorophyll has a highly complex chemical structure very imperfectly known1 but it is said to be only produced when there is iron in the soil4 ,the leaves of plants1 so often looked upon as mere ornamental appendages1 are among the most marvellous structures in living organisms1 since in decomposing carbonic acid at ordinary temperatures they do what no other agency in nature can perform4 ,in doing this they utilise a special group of ether-waves which alone appear to have this power4 ,the complexity of the processes going on in leaves is well indicated in the following quotation3-- ',we have seen how green leaves are supplied with gases1 water1 and dissolved salts1 and how they can trap special ether-waves4 ,the active energy of these waves is used to transmute the simple inorganic compounds into complex organic ones1 which in the process of respiration are reduced to simpler substances again1 and the potential energy transformed into kinetic4 ,these metabolic changes take place in living cells full of intense activities4 ,currents course through the protoplasm and cell-sap in every direction1 and between the cells which are also united by strands of protoplasm4 ,the gases used and given off in respiration and assimilation are floated in and out1 and each protoplasm particle burned or unburned is the centre of an area of disturbance4 ,pure protoplasm is influenced equally by all rays3 that associated with chlorophyll is affected by certain red and violet rays in particular4 ,these1 especially the red ones1 bring about the dissociation of the elements of the carbonic acid1 the assimilation of the carbon1 and the excretion of the oxygen4'#ab ,it is this vigorous life-activity ever at work in the leaves1 the roots1 and the sap-cells1 that builds up the plant1 in all its wondrous beauty of bud and foliage1 flower and fruit2 and at the same time produces1 either as useful or waste-products1 all that wealth of odours and flavours1 of colours and textures1 of fibres and varied woods1 of roots and tubers1 of gums and oils and resins innumerable1 that1 taken altogether1 render the world of vegetable life perhaps more varied1 more beautiful1 more enjoyable1 more indispensable to our higher nature than even that of animals4 ,but there is really no comparison between them4 ,we could have plants without animals2 we could not have animals without plants4 ,and all this marvel and mystery of vegetable life1 a mystery which we rarely ponder over because its effects are so familiar1 is usually held to be sufficiently explained by the statement that it is all due to the special properties of protoplasm4 ,well might ,huxley say1 that protoplasm is not only a substance but a structure or mechanism1 a mechanism kept at work by solar heat and light1 and capable of producing a thousand times more varied and marvellous results than all the human mechanism ever invented4 ,but besides absorbing carbonic acid from the atmosphere1 separating and utilising the carbon and giving out the oxygen1 plants as well as animals continually absorb oxygen from the atmosphere1 and this is so universally the case that oxygen is said to be the food of protoplasm1 without which it cannot continue to live2 and it is the peculiar but quite invisible structure of the protoplasm which enables it to do this1 and also in plants to absorb an enormous amount of water as well4 ,but although protoplasm is so complex chemically as to defy exact analysis1 being an elaborate structure of atoms built up into a molecule in which each atom must occupy its true place 7like every carved stone in a ,gothic cathedral71 yet it is1 as it were1 only the starting-point or material out of which the infinitely varied structures of living bodies are formed4 ,the extreme mobility and changeability of the structure of these molecules enables the protoplasm to be continually modified both in constitution and form1 and1 by the substitution or addition of other elements1 to serve special purposes4 ,thus when sulphur in small quantities is absorbed and built into the molecular structure1 proteids are formed4 ,these are most abundant in animal structures1 and give the nourishing properties to meat1 cheese1 eggs1 and other animal foods2 but they are also found in the vegetable kingdom1 especially in nuts and seeds such as grain1 peas1 etc4 ,these are generally known as nitrogenous foods1 and are very nutritious1 but not so easily digestible as meat4 ,proteids exist in very varied forms and often contain phosphorus as well as sulphur1 but their main characteristic is the large proportion of nitrogen they contain1 while many other animal and vegetable products1 as most roots1 tubers1 and grains1 and even fats and oils1 are mainly composed of starch and sugar4 ,in its chemical and physiological aspects protein is thus described by ,professor ,w4,d4 ,haliburton3--',proteids are produced only in the living laboratory of animals and plants2 proteid matter is the all-important material present in protoplasm4 ,this molecule is the most complex that is known2 it always contains five and often six or even seven elements4 ,the task of thoroughly understanding its composition is necessarily vast1 and advance slow4 ,but1 little by little1 the puzzle is being solved1 and this final conquest of organic chemistry1 when it does arrive1 will furnish physiologists with new light on many of the dark places of physiological science4'#ac ,what makes protoplasm and its modifications still more marvellous is the power it possesses of absorbing and moulding a number of other elements in various parts of living organisms for special uses4 ,such are silica in the stems of the grass family1 lime and magnesia in the bones of animals1 iron in blood1 and many others4 ,besides the four elements constituting protoplasm1 most animals and plants contain also in some parts of their structure sulphur1 phosphorus1 chlorine1 silicon1 sodium1 potassium1 calcium1 magnesium1 and iron2 while1 less frequently1 fluorine1 iodine1 bromine1 lithium1 copper1 manganese1 and aluminium are also found in special organs or structures2 and the molecules of all these are carried by the protoplasmic fluids to the places where they are required and built into the living structure1 with the same precision and for similar ends as brick and stone1 iron1 slate1 wood1 and glass are each utilised in their proper places in any large building4#ad ,the organism1 however1 is not built1 but grows4 ,every organ1 every fibre1 cell1 or tissue is formed from diverse materials1 which are first decomposed into their elementary molecules1 transformed by the protoplasm or by special solvents formed from it1 carried to the places where they are needed by the vital fluids1 and there built up atom by atom or molecule by molecule into the special structures of which they are to form a part4 ,but even this marvel of growth and repair of every individual organism is far surpassed by the greater marvel of reproduction4 ,every living thing of the higher orders arises from a single microscopic cell1 when fertilised1 as it is termed1 by the absorption of another microscopic cell derived from a different individual4 ,these cells are often1 even under the highest powers of the microscope1 hardly distinguishable from other cells which occur in all animals and plants and of which their structure is built up2 yet these special cells begin to grow in a totally different manner1 and instead of forming one particular part of the organism1 develop inevitably into a complete living thing with all the organs1 powers1 and peculiarities of its parents1 so as to be recognisably of the same species4 ,if the simple growth of the fully formed organism is a mystery1 what of this growth of thousands of complex organisms each with all its special peculiarities1 yet all arising from minute germs or cells the diverse natures of which are wholly indistinguishable by the highest powers of the microscope8 ,this1 too1 is said to be the work of protoplasm under the influence of heat and moisture1 and modern physiologists hope some day to learn 'how it is done4' ,it may be well here to give the views of a modern writer on this point4 ,referring to a difficulty which had been stated by ,clerk-,maxwell twenty-five years ago1 that there was not room in the reproductive cell for the millions of molecules needed to serve as the units of growth for all the different structures in the body of the higher animals1 ,professor ,m',kendrick says3--',but to-day1 it is reasonable from existing data to suppose that the germinal vesicle might contain a million of millions of organic molecules4 ,complex arrangements of these molecules suited for the development of all the parts of a highly complicated organism1 might satisfy all the demands of the theory of heredity4 ,doubtless the germ was a material system through and through4 ,the conception of the physicist was1 that molecules were in various states of movement2 and the thinkers were striving toward a kinetic theory of molecules and of atoms of solid matter1 which might be as fruitful as the kinetic theory of gases4 ,there were motions atomic and molecular4 ,it was conceivable that the peculiarities of vital action might be determined by the kind of motion that took place in the molecules of what we call living matter4 ,it might be different in kind from some of the motions dealt with by physicists4 ,life is continually being created from non-living material--such1 at least1 is the existing view of growth by the assimilation of food4 ,the creation of living matter out of non-living may be the transmission to the dead matter of molecular motions which are sui generis in form4' ,this is the modern physiological view of 'how it may be done1' and it seems hardly more intelligible than the very old theory of the origin of stone axes1 given by ,adrianus ,tollius in #afdi1 and quoted by ,mr4 ,e4,b4 ,tylor1 who says3--',he gives drawings of some ordinary stone axes and hammers and tells how naturalists say that they are generated in the sky by a fulgureous exhalation conglobed in a cloud by the circumfixed humour1 and are1 as it were1 baked hard by intense heat1 and the weapon becomes pointed by the damp mixed with it flying from the dry part1 and leaving the other end denser1 but the exhalations press it so hard that it breaks through the cloud and makes thunder and lightning4 ,but--he says--if this is really the way in which they are generated1 it is odd they are not round1 and that they have holes through them4 ,it is hardly to be believed1 he thinks4'#ae ,and so1 when the physiologists1 determined to avoid the assumption of anything beyond matter and motion in the germ1 impute the whole development and growth of the elephant or of man from minute cells internally alike1 by means of 'kinds of motion' and the 'transmission of motions which are sui generis in form1' many of us will be inclined to say with the old author--',it is hardly to be believed1 ,i think4' ,this brief statement of the conclusions arrived at by chemists and physiologists as to the composition and structure of organised living things has been thought advisable1 because the non-scientific reader has often no conception of the incomparable marvel and mystery of the life-processes he has always seen going on1 silently and almost unnoticed1 in the world around him4 ,and this is still more the case now that two-thirds of our population are crowded into cities where1 removed from all the occupations1 the charms1 and the interests of country life1 they are driven to seek occupation and excitement in the theatre1 the music-hall1 or the tavern4 ,how little do these know what they lose by being thus shut out from all quiet intercourse with nature2 its soothing sights and sounds2 its exquisite beauties of form and colour2 its endless mysteries of birth1 and life1 and death4 ,most people give scientific men credit for much greater knowledge than they possess in these matters2 and many educated readers will1 ,i feel sure1 be surprised to find that even such apparently simple phenomena as the rise of the sap in trees are not yet completely explained4 ,as to the deeper problems of life1 and growth1 and reproduction1 though our physiologists have learned an infinite amount of curious or instructive facts1 they can give us no intelligible explanation of them4 ,the endless complexities and confusing amount of detail in all treatises on the physiology of animals and plants are such1 that the average reader is overwhelmed with the mass of knowledge presented to him1 and concludes that after such elaborate researches everything must be known1 and that the almost universal protests against the need of any causes but the mechanical1 physical1 and chemical laws and forces are well founded4 ,i have1 therefore1 thought it advisable to present a kind of bird's-eye view of the subject1 and to show1 in the words of the greatest living authorities on these matters1 both how complex are the phenomena and how far our teachers are from being able to give us any adequate explanation of them4 ,i venture to hope that the very brief sketch of the subject ,i have been able to give will enable my readers to form some faint general conception of the infinite complexity of life and the various problems connected with it2 and that they will thus be the better enabled to appreciate the extreme delicacy of those adjustments1 those forces1 and those complex conditions of the environment1 that alone render life1 and above all the grand age-long panorama of the development of life1 in any way possible4 ,it is to these conditions1 as they prevail in the world around us1 that we will now direct our attention4 ,,footnotes3 #aa ,professor ,f4,j4 ,allen3 ,what is ,life8 #ab ,art4 ',vegetable ,physiology' in ,chambers's ,encyclopdia4 #ac ,address to the ,british ,association1 #aijb1 ,section ,physiology4 #ad ,this enumeration of the elements that enter into the structure of plants and animals is taken from ,professor ,f4,j4 ,allen's paper already referred to4 #ae ,early ,history of ,mankind1 #bnd ed4 p4 #bbg4 ,,chapter ,,xi ,,the ,,physical ,,conditions ,,essential ,,for ,,organic ,,life ,the physical conditions on the surface of our earth which appear to be necessary for the development and maintenance of living organisms may be dealt with under the following headings3-- #a4 ,regularity of heat-supply1 resulting in a limited range of temperature4 #b4 ,a sufficient amount of solar light and heat4 #c4 ,water in great abundance1 and universally distributed4 #d4 ,an atmosphere of sufficient density1 and consisting of the gases which are essential for vegetable and animal life4 ,these are ,oxygen1 ,carbonic-acid gas1 ,aqueous vapour1 ,nitrogen1 and ,ammonia4 ,these must all be present in suitable proportions4 #e4 ,alternations of day and night4 ,,small ,,range ,,of ,,temperature ,,required ,,for ,,growth ,,and ,,development ,vital phenomena for the most part occur between the temperatures of freezing water and #ajd ,fahr41 and this is supposed to be due mainly to the properties of nitrogen and its compounds1 which between these temperatures only can maintain those peculiarities which are essential to life--extreme sensitiveness and lability2 facility of change as regards chemical combination and energy2 and other properties which alone render nutrition1 growth1 and continual repair possible4 ,a very small increase or decrease of temperature beyond these limits1 if continued for any considerable time1 would certainly destroy most existing forms of life1 and would not improbably render any further development of life impossible except in some of its lowest forms4 ,as one example of the direct effects of increased temperature1 we may adduce the coagulation of albumen4 ,this substance is one of the proteids1 and plays an important part in the vital phenomena of both plants and animals1 and its fluidity and power of easy combination and change of form are lost by any degree of coagulation which takes place at about #afj ,fahr4 ,the extreme importance to all the higher organisms of a moderate temperature is strikingly shown by the complex and successful arrangements for maintaining a uniform degree of heat in the interior of the body4 ,the normal blood-heat in a man is #ih ,fahr41 and this is constantly maintained within one or two degrees though the external temperature may be more than fifty degrees below the freezing-point4 ,high temperatures upon the earth's surface do not range so far from the mean as do the low4 ,in the greater part of the tropics the air-temperature seldom reaches #if ,fahr41 though in arid districts and deserts1 which occur chiefly along the margins of the northern and southern tropics1 it not unfrequently surpasses #aaj ,fahr41 and even occasionally rises to #aae or #abj in ,australia and ,central ,india4 ,yet with suitable food and moderate care the blood-temperature of a healthy man would not rise or fall more than one or at most two degrees4 ,the great importance of this uniformity of temperature in all the vital organs is distinctly shown by the fact that when1 during fevers1 the temperature of the patient rises six degrees above the normal amount1 his condition is critical1 while an increase of seven or eight degrees is an almost certain indication of a fatal result4 ,even in the vegetable kingdom seeds will not germinate under a temperature of four or five degrees above the freezing-point4 ,now this extreme sensibility to variations of internal temperature is quite intelligible when we consider the complexity and instability of protoplasm1 and of all the proteids in the living organism1 and how important it is that the processes of nutrition and growth1 involving constant motion of fluids and incessant molecular decompositions and recombinations1 should be effected with the greatest regularity4 ,and though a few of the higher animals1 including man1 are so perfectly organised that they can adapt or protect themselves so as to be able to live under very extreme conditions as regards temperature1 yet this is not the case with the great majority1 or with the lower types1 as evidenced by the almost complete absence of reptiles from the arctic regions4 ,it must also be remembered that extreme cold and extreme heat are nowhere perpetual4 ,there is always some diversity of seasons1 and there is no land animal which passes its whole life where the temperature never rises above the freezing point4 ,,the ,,necessity ,,of ,,solar ,,light ,whether the higher animals and man could have been developed upon the earth without solar light1 even if all the other essential conditions were present1 is doubtful4 ,that1 however1 is not the point ,i am at present considering1 but one that is much more fundamental4 ,without plant life land animals at all events could never have come into existence1 because they have not the power of making protoplasm out of inorganic matter4 ,the plant alone can take the carbon out of the small proportion of carbonic acid in the atmosphere1 and with it1 and the other necessary elements1 as already described1 build up those wonderful carbon compounds which are the very foundation of animal life4 ,but it does this solely by the agency of solar light1 and even uses a special portion of that light4 ,not only1 therefore1 is a sun needed to give light and heat1 but it is quite possible that any sun would not answer the purpose4 ,a sun is required whose light possesses those special rays which are effective for this operation1 and as we know that the stars differ greatly in their spectra1 and therefore in the nature of their light1 all might not be able to effect this great transformation1 which is one of the very first steps in rendering animal life possible on our earth1 and therefore probably on all earths4 ,,water ,a ,,first ,,essential ,,of ,,organic ,,life ,it is hardly necessary to point out the absolute necessity of water1 since it actually constitutes a very large proportion of the material of every living organism1 and about three-fourths of our own bodies4 ,water1 therefore1 must be present everywhere1 in one form or another1 on any globe where life is possible4 ,neither animal nor plant can exist without it4 ,it must also be present in such quantity and so distributed as to be constantly available on every part of a globe where life is to be maintained2 and it is equally necessary that it should have persisted in equal profusion throughout those enormous geological epochs during which life has been developing4 ,we shall see later on how very special are the conditions that have secured this continuous distribution of water on our earth1 and we shall also learn that this large amount of water1 its wide distribution1 and its arrangement with regard to the land-surface1 is an essential factor in producing that limited range of temperature which1 as we have seen1 is a primary condition for the development and maintenance of life4 ,,the ,,atmosphere ,,must ,,be ,,of ,,sufficient ,,density ,,and ,,composed ,,of ,,suitable ,,gases ,the atmosphere of any planet on which life can be developed must have several qualities which are unconnected with each other1 and the coincidence of which may be a rare phenomenon in the universe4 ,the first of these is a sufficient density1 which is required for two purposes--as a storer of heat1 and in order to supply the oxygen1 carbonic acid1 and aqueous vapour in sufficient quantities for the requirements of vegetable and animal life4 ,as a reservoir of heat and a regulator of temperature1 a rather dense atmosphere is a first necessity1 in co-operation with the large quantity and wide distribution of water referred to in the last section4 ,the very different character of our south-west from our north-east winds is a good illustration of its power of distributing heat and moisture4 ,this it does owing to the peculiar property it possesses of allowing the sun's rays to pass freely through it to the earth which it warms1 but acting like a blanket in preventing the rapid escape of the non-luminous heat so produced4 ,but the heat stored up during the day is given out at night1 and thus secures a uniformity of temperature which would not otherwise exist4 ,this effect is strikingly seen at high altitudes1 where the temperature becomes lower and lower1 till at a not very great elevation1 even in the tropics1 snow lies on the ground all the year round4 ,this is almost wholly due to the rarity of the air1 which1 on that account1 has not so much capacity for heat4 ,it also allows the heat it acquires to radiate more freely than denser air1 so that the nights are much colder4 ,at about #ah1jjj feet high our atmosphere is exactly half its density at the sea-level4 ,this is considerably higher than the usual snow-line1 even under the equator1 whence it follows that if our atmosphere was only half its present density it would render the earth unsuitable for the higher forms of animal life4 ,it is not easy to say exactly what would be the result as regards climate2 but it seems likely that1 except perhaps in limited areas in the tropics1 where conditions were very favourable1 the whole land-surface would become buried in snow and ice4 ,this appears inevitable1 because evaporation from the oceans by direct sun-heat would be more rapid than now2 but as the vapour rose in the rare atmosphere it would rapidly become frozen1 and snow would fall almost perpetually1 although it might not lie permanently on the ground in the equatorial lowlands4 ,it appears certain1 therefore1 that with half our present bulk of atmosphere life would be hardly possible on the earth on account of lowered temperature alone4 ,and as there would certainly be an added difficulty in the needful supply of oxygen to animals and carbonic acid to plants1 it seems highly probable that a reduction of density of even one-fourth might be sufficient to render a large portion of the globe a snow and ice-clad waste1 and the remainder liable to such extremes of climate that only low forms of life could have arisen and been permanently maintained4 ,,the ,,gases ,,of ,,the ,,atmosphere ,coming now to consider the constituent gases of the atmosphere1 there is reason to believe that they form a mixture as nicely balanced in regard to animal and vegetable life as are the density and the temperature4 ,at a first view of the subject we might conclude that oxygen is the one great essential for animal life1 and that all else is of little importance4 ,but further consideration shows us that nitrogen1 although merely a diluent of the oxygen as regards the respiration of animals1 is of the first importance to plants1 which obtain it from the ammonia formed in the atmosphere and carried down into the soil by the rain4 ,although there is only one part of ammonia to a million of air1 yet upon this minute proportion the very existence of the animal world depends1 because neither animals nor plants can assimilate the free nitrogen of the air into their tissues4 ,another fundamentally important gas in the atmosphere is carbonic acid1 which forms about four parts in ten thousand parts of air1 and1 as already stated1 is the source from which plants build up the great bulk of their tissues1 as well as those protoplasms and proteids so absolutely necessary as food for animals4 ,an important fact to notice here is1 that carbonic acid1 so essential to plants1 and to animals through plants1 is yet a poison to animals4 ,when present in much more than the normal quantity1 as it often is in cities and in badly ventilated buildings1 it becomes highly prejudicial to health2 but this is believed to be partly due to the various corporeal emanations and other impurities associated with it4 ,pure carbonic acid gas to the amount of even one per cent4 in otherwise pure air can1 it is said1 be breathed for a time without bad effects1 but anything more than this proportion will soon produce suffocation4 ,it is probable1 therefore1 that a very much smaller proportion than one per cent41 if constantly present1 would be dangerous to life2 though no doubt1 if this had always been the proportion1 life might have been developed in adaptation to it4 ,considering1 however1 that this poisonous gas is largely given out by the higher animals as a product of respiration1 it would evidently be dangerous to the permanence of life if the quantity forming a constant constituent of the atmosphere were much greater than it is4 ,,aqueous ,,vapour ,,in ,,the ,,atmosphere ,this water-gas1 although it occurs in the atmosphere in largely varying quantities1 is yet1 in two distinct ways1 essential to organic life4 ,it prevents the too rapid loss of moisture from the leaves of plants when exposed to the sun1 and it is also absorbed by the upper surface of the leaf and by the young shoots1 which thus obtain both water and minute quantities of ammonia when the supply by the roots is insufficient4 ,but it is of even more vital importance in supplying the hydrogen which1 when united with the nitrogen of the atmosphere by electrical discharges1 produces the ammonia1 which is the main source of all the proteids of the plant1 which proteids are the very foundation of animal life4 ,from this brief statement of the purposes served by the various gases forming our atmosphere1 we see that they are to some extent antagonistic1 and that any considerable increase of one or the other would lead to results that might be injurious either directly or in their ultimate results4 ,and as the elements which constitute the bulk of all living matter possess properties which render them alone suitable for the purpose1 we may conclude that the proportions in which they exist in our atmosphere cannot be very widely departed from wherever organic forms are developed4 ,,the ,,alternation ,,of ,,day ,,and ,,night ,although it is difficult to decide positively whether alternations of light and darkness at short intervals are absolutely essential for the development of the various higher forms of life1 or whether a world in which light was constant might do as well1 yet on the whole it seems probable that day and night are really important factors4 ,all nature is full of rhythmic movements of infinitely varied kinds1 degrees1 and durations4 ,all the motions and functions of living things are periodic2 growth and repair1 assimilation and waste1 go on alternately4 ,all our organs are subject to fatigue and require rest4 ,all kinds of stimulus must be of short duration or injurious results follow4 ,hence the advantage of darkness1 when the stimuli of light and heat are partially removed1 and we welcome 'tired nature's sweet restorer1 balmy sleep'--giving rest to all the senses and faculties of body and mind1 and endowing us with renewed vigour for another period of activity and enjoyment of life4 ,plants as well as animals are invigorated by this nightly repose2 and all alike benefit by these longer periods of greater and less amounts of work caused by summer and winter1 dry and wet seasons4 ,it is a suggestive fact1 that where the influence of heat and light is greatest--within the tropics--the days and nights are of equal length1 giving equal periods of activity and rest4 ,but in cold and ,arctic regions where1 during the short summer1 light is nearly perpetual1 and all the functions of life1 in vegetation especially1 go on with extreme rapidity1 this is followed by the long rest of winter1 with its short days and greatly lengthened periods of darkness4 ,of course1 all this is rather suggestion than proof4 ,it is possible that in a world of perpetual day or in one of perpetual night1 life might have been developed4 ,but on the other hand1 considering the great variety of physical conditions which are seen to be necessary for the development and preservation of life in its endless varieties1 any prejudicial influences1 however slight1 might turn the scale1 and prevent that harmonious and continuous evolution which we know must have occurred4 ,so far ,i have only considered the question of day and night as regards the presence or absence of light4 ,but it is probably far more important in its heat aspect2 and here its period becomes of great1 perhaps vital1 importance4 ,with its present duration of twelve hours day and twelve night on the average1 there is not time1 even between the tropics1 for the earth to become so excessively heated as to be inimical to life2 while a considerable portion of the heat1 stored up in the soil1 the water1 and the atmosphere1 is given out at night1 and thus prevents a too sudden and injurious contrast of heat and cold4 ,if the day and night were each very much longer--say #ej or #ajj hours--it is quite certain that1 during a day of that duration1 the heat would become so great as to be inimical1 perhaps prohibitive1 to most forms of life2 while the absence of all sun-heat for an equally long period would result in a temperature far below the freezing point of water4 ,it is doubtful whether any high forms of animal life could have arisen under such great and continual contrasts of temperature4 ,we will now proceed to point out the special features which1 in our earth1 have combined to bring about and to maintain the various and complex conditions we have seen to be essential for life as it exists around us4 ,,chapter ,,xii ,,the ,,earth ,,in ,,its ,,relation ,,to ,,the ,,development ,,and ,,maintenance ,,of ,,life ,the first circumstance to be considered in relation to the habitability of a planet is its distance from the sun4 ,we know that the heating power of the sun upon our earth is ample for the development of life in an almost infinite variety of forms2 and we have a large amount of evidence to show that1 were it not for the equalising power of air and water1 distributed as they are with us1 the heat received from the sun would be sometimes too great and sometimes too little4 ,in some parts of ,africa1 ,australia1 and ,india1 the sandy soil becomes so hot that an egg can be cooked by placing it just below the surface4 ,on the other hand1 at an elevation of about #ab1jjj feet in lat4 #dj it freezes every night1 and throughout the day in all places sheltered from the sun4 ,now1 both these temperatures are adverse to life1 and if either of them persisted over a considerable portion of the earth1 the development of life would have been impossible4 ,but the heat derived from the sun is inversely as the square of the distance1 so that at half the distance we should have four times as much heat1 and at twice the distance only one-fourth of the heat4 ,even at two-thirds of the distance we should receive more than twice as much heat2 and1 considering the facts as to the extreme sensitiveness of protoplasm and the coagulation of albumen1 it seems certain that we are situated in what has been termed the temperate zone of the solar system1 and that we could not be removed far from our present position without endangering a considerable portion of the life now existing upon the earth1 and in all probability rendering the actual development of life1 through all its phases and gradations1 impossible4 ,,the ,,obliquity ,,of ,,the ,,ecliptic ,the effect of the obliquity of the earth's equator to its path round the sun1 upon which depend our varying seasons and the inequality of day and night throughout all the temperate zones1 is very generally known4 ,but it is not usually considered that this obliquity is of any great importance as regards the suitability of the earth for the development and maintenance of life2 and it seems to have been passed over as an accident hardly worth notice1 because almost any other obliquity or none at all would have been equally advantageous4 ,but if we consider what the direction of the earth's axis might possibly have been1 we shall find that it is really a matter of great importance from our present point of view4 ,let us suppose1 first1 that the earth's axis was1 like that of ,uranus1 almost exactly in the plane of its orbit or directed towards the sun4 ,there can be little doubt that such a position would have rendered our world unfitted for the development of life4 ,for the result would be the most tremendous contrasts of the seasons2 at mid-winter1 on one half the globe1 arctic night and more than arctic cold would prevail2 while on the other half there would be a midsummer of continuous day with a vertical sun and such an amount of heat as nowhere exists with us4 ,at the two equinoxes the whole globe would enjoy equal day and night1 all our present tropics and part of the sub-tropical zone having the sun at noon so near to the zenith as to have the essential of a tropical climate4 ,but the change to about a month of constant sunshine or a month of continuous night would be so rapid1 that it seems almost impossible that either vegetable or animal life would ever have developed under such terrible conditions4 ,the other extreme direction of the earth's axis1 exactly at right angles to the plane of the orbit1 would be much more favourable1 but would still have its disadvantages4 ,the whole surface from equator to poles would enjoy equal day and night1 and every part would receive the same amount of sun-heat all the year round1 so that there would be no change of seasons2 but the heat received would vary with the latitude4 ,in our latitude the sun's altitude at noon all the year would be less than #dj1 the same as now occurs at the equinoxes1 and we might therefore have a perpetual spring as regards temperature4 ,but the constancy of the heat in the equatorial and tropical regions and of cold towards the poles would lead to a more constant and more rapid circulation of air1 and we should probably experience such continuous north-westerly winds as to render our climate always cold and probably very damp4 ,near the poles the sun would always be on1 or close to1 the horizon1 and would give so little heat that the sea might be perpetually frozen and the land deeply snow-buried2 and these conditions would probably extend into the temperate zone1 and possibly so far south as to render life impossible in our latitudes1 since whatever results arose would be due to permanent causes1 and we know how powerful are snow and ice to extend their sway over adjacent areas if not counteracted by summer heat or warm moist winds4 ,on the whole1 therefore1 it seems probable that this position of the earth's axis would result in a much smaller portion of its surface being capable of supporting a luxuriant and varied vegetable and animal life than is now the case2 while the extreme uniformity of conditions everywhere present might be so antagonistic to the great law of rhythm that seems to pervade the universe1 and be in other ways so unfavourable1 that life-development would probably have taken quite a different course from that which it has taken4 ,it appears almost certain1 therefore1 that some intermediate position of the axis would be the most favourable2 and that which actually exists seems to combine the advantage of change of seasons with good climatical conditions over the largest possible area4 ,we know that during the greater part of the epoch of life-development this area was much greater than at present1 since a luxuriant vegetation of deciduous and evergreen trees and shrubs extended up to and within the ,arctic ,circle1 leading to the formation of coal-beds both in palozoic and tertiary times2 the extremely favourable conditions for organic life which then prevailed over so large a portion of the globe's surface1 and which persisted down to a comparatively recent epoch1 lead to the conclusion that no more favourable degree of obliquity was possible than that which we actually possess4 ,a short account of the evidence on this interesting subject will now be given4 ,,persistence ,,of ,,mild ,,climates ,,through ,,geologic ,,time ,the whole of the geological evidence goes to show that in remote ages the climate of the earth was generally more uniform1 though perhaps not warmer1 than it is now1 and this can be best explained by a slightly different distribution of sea and land1 which allowed the warm waters of the tropical oceans to penetrate into various parts of the continents 7which were then more broken up than they are now71 and also to extend more freely into the ,arctic regions4 ,so soon as we go back into the tertiary period1 we find indications of a warmer climate in the north temperate zone2 and when we reach the middle of that period1 we find abundant indications1 both in plant and animal remains1 of mild climates near to the ,arctic ,circle1 or actually within it4 ,on the west coast of ,greenland1 in #gj ,n4 lat41 there are found abundance of fossil plants very beautifully preserved1 among which are many different species of oaks1 beeches1 poplars1 plane-trees1 vines1 walnuts1 plums1 chestnuts1 sequoias1 and numerous shrubs--#acg species in all1 indicating a vegetation such as now grows in the north temperate parts of ,america and ,eastern ,asia4 ,and even further north1 in ,spitzbergen1 in ,n4 lat4 #gh and #gi1 a somewhat similar flora is found1 not quite so varied1 but with oaks1 poplars1 birches1 planes1 limes1 hazels1 pines1 and many aquatic plants such as may now be found in ,west ,norway and in ,alaska1 nearly twenty degrees further south4 ,still more remote1 in the ,cretaceous period1 fossil plants have been found in ,greenland1 consisting of ferns1 cycads1 conifers1 and such trees and shrubs as poplars1 sassafras1 andromedas1 magnolias1 myrtles1 and many others1 similar in character and often identical in species with fossils of the same period found in ,central ,europe and the ,united ,states1 indicating a widespread uniformity of climate1 such as would be brought about by the great ocean-currents carrying the warm waters of the tropics into the ,arctic seas4 ,still further back1 in the ,jurassic period1 we have proofs of a mild climate in ,east ,siberia and at ,and in ,norway just within the ,arctic ,circle1 in numerous plant remains1 and also remains of great reptiles allied to those found in the same strata in all parts of the world4 ,similar phenomena occur in the still earlier ,triassic period2 but we will pass on to the much more remote ,carboniferous period1 during which most of the great coal-beds of the world were formed from a luxuriant vegetation1 consisting mostly of ferns1 giant horse-tails1 and primitive conifers4 ,the luxuriance of these plants1 which are often found beautifully preserved and in immense quantities1 is supposed to indicate an atmosphere in which carbonic acid gas was much more abundant than now2 and this is rendered probable by the small number and low type of terrestrial animals1 consisting of a few insects and amphibia4 ,but the interesting point is1 that true coal-beds1 with similar fossils to those of our own coal-measures1 are found at ,spitzbergen and at ,bear ,island in ,east ,siberia1 both far within the ,arctic ,circle1 again indicating a great uniformity of climate1 and probably a denser and more vapour-laden atmosphere1 which would act as a blanket over the earth and preserve the heat brought to the ,arctic seas by the ocean currents from the warmer regions4 ,the still earlier ,silurian rocks are also found abundantly in the ,arctic regions1 but their fossils are entirely of marine animals4 ,yet they show the same phenomena as regards climate1 since the corals and cephalopodous mollusca found in the ,arctic beds closely resemble those of all other parts of the earth4#af ,many other facts indicate that throughout the enormous periods required for the development of the varied forms of life upon the earth1 the great phenomena of nature were but little different from those that prevail in our own times4 ,the slow and gentle processes by which the various vegetable and animal remains were preserved are shown by the perfect state in which many of the fossils exist4 ,often trunks of trees1 cycads1 and tree-ferns are found standing erect1 with their roots still imbedded in the soil they grew in4 ,large leaves of poplars1 maples1 oaks1 and other trees are often preserved in as perfect a state as if gathered by a botanist and dried between paper for his herbarium1 and the same is especially the case with the beautiful ferns of the ,permian and ,carboniferous periods4 ,throughout these and most other formations well-preserved ripple-marks are found in the solidified mud or sand of old seashores1 differing in no respect from similar marks to be found on almost every coast to-day4 ,equally interesting are the marks of rain-drops preserved in the rocks of almost all ages4 ,sir ,charles ,lyell has given illustrations of recent impressions of rain-drops on the extensive mud-flats of ,nova ,scotia1 and also an illustration of rain-drops on a slab of shale from the carboniferous formation of the same country2 and the two are as much alike as the prints of two different showers a few days apart4 ,the general size and form of the drops are almost identical1 and imply a great similarity in the general atmospheric conditions4 ,we must not forget that this presence of rain throughout geological time implies1 as we have seen in our last chapter1 a constant and universal distribution of atmospheric dust4 ,the two chief sources of this dust--the total quantity of which in the atmosphere must be enormous--are volcanoes and deserts1 and we are therefore sure that these two great natural phenomena have always been present4 ,of volcanoes we have ample independent evidence in the presence of lavas and volcanic ashes1 as well as actual stumps or cores of old volcanoes1 through all geological formations2 and we can have little doubt that deserts also were present1 though perhaps not always so extensive as they are now4 ,it is a very suggestive fact that these two phenomena1 usually held to be blots on the fair face of nature1 and even to be opposed to belief in a beneficent ,creator1 should now be proved to be really essential to the earth's habitability4 ,notwithstanding this prevalence of warm and uniform conditions1 there is also evidence of considerable changes of climate2 and at two periods--in the ,eocene and in the remote ,permian--there are even indications of ice-action1 so that some geologists believe that there were then actual glacial epochs4 ,but it seems more probable that they imply only local glaciation1 owing to there having been high land and other suitable conditions for the production of glaciers in certain areas4 ,the whole bearing of the geological evidence indicates the wonderful continuity of conditions favourable for life1 and for the most part of climatal conditions more favourable than those now prevailing1 since a larger extent of land towards the ,north ,pole was available for an abundant vegetation1 and in all probability for an equally abundant animal life4 ,we know1 too1 that there was never any total break in life-development2 no epoch of such lowering or raising of temperature as to destroy all life2 no such general subsidence as to submerge the whole land-surface4 ,although the geological record is in parts very imperfect1 yet it is1 on the whole1 wonderfully complete2 and it presents to our view a continuous progress1 from simple to complex1 from lower to higher4 ,type after type becomes highly specialised in adaptation to local or climatal conditions1 and then dies out1 giving room for some other type to arise and be specialised in harmony with the changed conditions4 ,the general character of the inorganic change appears to have been from more insular to more continental conditions1 accompanied by a change from more uniform to less uniform climates1 from an almost sub-tropical warmth and moisture1 extending up to the ,arctic ,circle1 to that diversity of tropical1 temperate1 and cold areas1 capable of supporting the greatest possible variety in the forms of life1 and which seems especially adapted to stimulate mankind to civilisation and social development by means of the necessary struggle against1 and utilisation of1 the various forces of nature4 ,,water1 ,,its ,,amount ,,and ,,distribution ,,on ,,the ,,earth ,although it is generally known that the oceans occupy more than two-thirds of the whole surface of the globe1 the enormous bulk of the water in proportion to the land that rises above its surface is hardly ever appreciated4 ,but as this is a matter of the greatest importance1 both as regards the geological history of the globe and the special subject we are here discussing1 it will be necessary to enter into some details in regard to it4 ,according to the best recent estimates1 the land area of the globe is #j4bh of the whole surface1 and the water area #j4gb4 ,but the mean height of the land above the sea-level is found to be #bbej feet1 while the mean depth of the seas and oceans is #ac1hfj feet2 so that though the water area is two and a half times that of the land1 the mean depth of the water is more than six times the mean height of the land4 ,this is1 of course1 due to the fact that lowlands occupy most of the land-area1 the plateaus and high mountains a comparatively small portion of it2 while1 though the greatest depths of the oceans about equal the greatest heights of the mountains1 yet over enormous areas the oceans are deep enough to submerge all the mountains of ,europe and temperate ,north ,america1 except the extreme summits of one or two of them4 ,hence it follows that the bulk of the oceans1 even omitting all the shallow seas1 is more than thirteen times that of the land above sea-level2 and if all the land-surface and ocean-floors were reduced to one level1 that is1 if the solid mass of the globe were a true oblate spheroid1 the whole would be covered with water about two miles deep4 ,the diagram here given will render this more intelligible and will serve to illustrate what follows4 ,illustration3 ,diagram of proportionate mean height of ,land and depth of ,oceans4 ,land ,area4 4#bh of area of ,globe4 ,ocean ,area 4#gb of area of ,globe4 ,in this diagram the lengths of the sections representing land and ocean are proportionate to their areas1 while the thickness of each is proportionate to their mean height and mean depth respectively4 ,hence the two sections are in correct proportion to their cubic contents4 ,a mere inspection of this diagram is sufficient to disprove the old idea1 still held by a few geologists and by many biologists1 that oceans and continents have repeatedly changed places during geological times1 or that the great oceans have again and again been bridged over to facilitate the distribution of beetles or birds1 reptiles or mammals4 ,we must remember that although the diagram shows the continents and oceans as a whole1 yet it also shows1 with quite sufficient accuracy1 the proportions of each of the great continents to the oceans which are adjacent to them4 ,it must also be borne in mind that there can be no elevation on a large scale without a corresponding subsidence elsewhere2 because if there were not a vast unsupported hollow would be left beneath the rising land or in some part adjacent to it4 ,now1 looking at the diagram and at a chart or globe1 try to imagine the ocean-bottom rising gradually1 to form a continent joining ,africa with ,south ,america or with ,australia 7both of which are demanded by many biologists73 it is clear that1 while such an elevation was going on1 either some continental land or some other part of the ocean-bed must sink to a corresponding amount4 ,we shall then see1 that if such changes of elevation on a continental scale have taken place again and again at different periods1 it would have been almost impossible1 on every occasion1 to avoid a whole continent being submerged 7or even all the continents7 in order to equalise subsidence with elevation while new continents were being raised up from the abyssal depths of the ocean4 ,we conclude1 therefore1 that with the exception of a comparatively narrow belt around the continents1 which may be roughly indicated by the thousand fathom line of soundings1 the great ocean depths are permanent features of the earth's surface4 ,it is this stability of the general distribution of land and water that has secured the continuity of life upon the earth4 ,had the great oceanic basins1 on the other hand1 been unstable1 changing places with the land at various periods of geological time1 they would1 almost certainly1 again and again have swallowed up the land in their vast abysses1 and have thus destroyed all the organic life of the world4 ,there are many confirmatory proofs of this view 7which is now widely accepted by geologists and physicists71 and a few of them may be briefly stated4 #a4 ,none of the continents present us with marine deposits of any one geological age and occupying a large part of the surface of each1 as must have been the case had they ever been sunk deep beneath the ocean and again elevated2 neither do any of them contain extensive formations corresponding to the deep oceanic clays and oozes1 which again they must have done had they been at any time raised up from the ocean depths4 #b4 ,all the continents present an almost complete and continuous series of rocks of all geological ages1 and in each of the great geological periods there are found fresh water and estuarine deposits1 and even old land-surfaces1 demonstrating continuity of continental or insular conditions4 #c4 ,all the great oceans possess1 scattered over them1 a few or many islands termed 'oceanic1' and characterised by a volcanic or coralline structure1 with no ancient stratified rocks in anyone of them2 and in none of these is there found a single indigenous land mammal or amphibian4 ,it is incredible that1 if these oceans had ever contained extensive continents1 and if these oceanic islands are--as even now they are often alleged to be--parts of these now submerged continents1 not one fragment of any of the old stratified rocks1 which characterise all existing continents1 should remain to show their origin4 ,in the ,atlantic we find the ,azores1 ,madeira1 and ,st4 ,helena2 in the ,indian ,ocean1 ,mauritius1 ,bourbon1 and ,kerguelen ,island2 in the ,pacific1 the ,fiji1 ,samoan1 ,society1 ,sandwich1 and ,galapagos ,islands1 all without exception telling us the same tale1 that they have been built up from the ocean depths by submarine volcanoes and coralline growths1 but have never formed part of continental areas4 #d4 ,the contours of the floors of all the great oceans1 now fairly well known through the soundings of exploring vessels and for submarine telegraph lines1 also give confirmatory evidence that they have never been continental land4 ,for if any part of them were a sunken continent1 that part must have retained some impress of its origin4 ,some of the numerous mountain ranges which characterise every continent would have remained4 ,we should find slopes of from #bj to #ej not uncommon1 while valleys bordered by rocky precipices1 as in ,lake ,lucerne and a hundred others1 or isolated rock-walled mountains like ,roraima1 or ranges of precipices as in the ,ghts of ,india or the ,fiords of ,norway1 would frequently be met with4 ,but not a single feature of this kind has ever been found in the ocean abysses4 ,instead of these we have vast plains1 which1 if the water were removed1 would appear almost exactly level1 with no abrupt slopes anywhere4 ,when we consider that deposits from the land never reach these remote ocean depths1 and that there is no wave-action below a few hundred feet1 these continental features once submerged would be indestructible2 and their total absence is1 therefore1 itself a demonstration that none of the great oceans are on the sites of submerged continents4 ,,how ,,ocean ,,depths ,,were ,,produced ,it is a very difficult problem to determine how the vast basins which are filled by the great oceans1 especially that of the ,pacific1 were first produced4 ,when the earth's surface was still in a molten state1 it would necessarily take the form of a true oblate spheroid1 with a compression at the poles due to its speed of rotation1 which is supposed to have been very great4 ,the crust formed by the gradual cooling of such a globe would be of the same general form1 and1 being thin1 would easily be fractured or bent so as to accommodate itself to any unequal stresses from the interior4 ,as the crust thickened and the whole mass slowly cooled and contracted1 fissures and crumpling would occur1 the former serving as outlets for volcanic activities whose results are found throughout all geological ages2 the latter producing mountain chains in which the rocks are almost always curved1 folded1 or even thrust over each other1 indicating the mighty forces due to the adjustments of a solid crust upon a shrinking fluid or semi-fluid interior4 ,but during this whole process there seem to be no forces at work that could lead to the production of such a feature as the ,pacific1 a vast depression covering nearly one-third of the whole surface of the globe4 ,the ,atlantic ,ocean1 being smaller and nearly opposite to the ,pacific1 but approximately of equal depth1 may be looked upon as a complementary phenomenon which will be probably explained as a result of the same causes as the vaster cavity4 ,so far as ,i am aware1 there is only one suggested cause of the formation of these great oceans that seems adequate2 and as that cause is to some extent supported by quite independent astronomical evidence1 and also directly bears upon the main subject of the present volume1 it must be briefly considered4 ,a few years ago1 ,professor ,george ,darwin1 of ,cambridge1 arrived at a certain conclusion as to the origin of the moon1 which is now comparatively well known by ,sir ,robert ,ball's popular account of it in his small volume1 ,time and ,tide4 ,briefly stated1 it is as follows4 ,the tides produce friction on the earth and very slowly increase the length of our day1 and also cause the moon to recede further from us4 ,the day is lengthened only by a small fraction of a second in a thousand years1 and the moon is receding at an equally imperceptible rate4 ,but as these forces are constant1 and have always acted on the earth and moon1 as we go back and back into the almost infinite past we come to a time when the rotation of the earth was so rapid that gravity at the equator could hardly retain its outer portion1 which was spread out so that the form of the whole mass was something like a cheese with rounded edges4 ,and about the same epoch the distance of the moon is found to have been so small that it was actually touching the earth4 ,all this is the result of mathematical calculation from the known laws of gravitation and tidal effects2 and as it is difficult to see how so large a body as the moon could have originated in any other way1 it is supposed that at a still earlier period the moon and earth were one1 and that the moon separated from the parent mass owing to centrifugal force generated by the earth's rapid rotation4 ,whether the earth was liquid or solid at this epoch1 and exactly how the separation occurred1 is not explained either by ,professor ,darwin or ,sir ,robert ,ball2 but it is a very suggestive fact that1 quite recently1 it has been shown1 by means of the spectroscope1 that double stars of short period do originate in this way from a single star1 as already described in our sixth chapter2 but in these cases it seems probable that the parent star is in a gaseous state4 ,these investigations of ,professor ,g4 ,darwin have been made use of by the ,rev4 ,osmond ,fisher 7in his very interesting and important work1 ,physics of the ,earth's ,crust7 to account for the basins of the great oceans1 the ,pacific being the chasm left when the larger portion of the mass of the moon parted from the earth4 ,adopting1 as ,i do1 the theory of the origin of the earth by meteoric accretion of solid matter1 we must consider our planet as having been produced from one of those vast rings of meteorites which in great numbers still circulate round the sun1 but which at the much earlier period now contemplated were both more numerous and much more extensive4 ,owing to irregularities of distribution in such a ring and through disturbance by other bodies1 aggregations of various size would inevitably occur1 and the largest of these would in time draw in to itself all the rest1 and thus form a planet4 ,during the early stages of this process the particles would be so small and would come together so gradually1 that little heat would be produced1 and there would result merely a loose aggregation of cold matter4 ,but as the process went on and the mass of the incipient planet became considerable--perhaps half that of the earth--the rest of the ring would fall in with greater and greater velocity2 and this1 added to the gravitative compression of the growing mass might1 when nearly its present size1 have produced sufficient heat to liquefy the outer layers1 while the central portion remained solid and to some extent incoherent1 with probably large quantities of heavy gases in the interstices4 ,when the amount of the meteoric accretions became so reduced as to be insufficient to keep up the heat to the melting-point1 a crust would form1 and might have reached about half or three-fourths of its present thickness when the moon became separated4 ,let us now try to picture to ourselves what happened4 ,we should have a globe somewhat larger than our earth is now1 both because it then contained the material of the moon and also because it was hotter1 revolving so rapidly as to be very greatly flattened at the poles2 while the equatorial belt bulged out enormously1 and would probably have separated in the form of a ring with a very slight increase of the time of rotation1 which is supposed to have been about four hours4 ,this globe would have a comparatively thin crust1 beneath which there was molten rock to an unknown depth1 perhaps a few hundreds1 perhaps more than a thousand miles4 ,at this time the attraction of the sun acting on the molten interior produced tides in it1 causing the thin crust to rise and fall every two hours1 but to so small an extent--only about a foot or so--as not necessarily to fracture it2 but it is calculated that this slight rhythmic undulation coincided with the normal period of undulation due to such a large mass of heavy liquid1 and so tended to increase the instability due to rapid rotation4 ,the bulk of the moon is about one-fiftieth part that of the earth1 and an easy calculation shows us that1 taking the area of the ,pacific1 ,atlantic1 and ,indian ,oceans combined as about two-thirds that of the globe1 it would require a thickness 7or depth7 of about forty miles to furnish the material for the moon4 ,we must1 of course1 assume that there were some inequalities in the thickness of the crust and in its comparative rigidity1 so that when the critical moment came and the earth could no longer retain its equatorial protuberance against the centrifugal force due to rotation combined with the tidal undulations caused by the sun1 instead of a continuous ring slowly detaching itself1 the crust gave way in two or more great masses where it was weakest1 and as the tidal wave passed under it and a quantity of the liquid substratum rose with it1 the whole would break up and collect into a sub-globular mass a short distance from the earth1 and continue revolving with it for some time at about the same rate as the surface had rotated4 ,but as tidal action is always equal on opposite sides of a globe1 there would be a similar disruption there1 forming1 it may be supposed1 the ,atlantic basin1 which1 as may be seen on a small globe1 is almost exactly opposite a part of the ,central ,pacific4 ,so soon as these two great masses had separated from the earth1 the latter would gradually settle down into a state of equilibrium1 and the molten matter of the interior1 which would now fill the great oceanic basins up to a level of a few mile below the general surface would soon cool enough to form a thin crust4 ,the larger portion of the nascent moon would gradually attract to itself the one or more smaller portions and form our satellite2 and from that time tidal friction by both moon and sun would begin to operate and would gradually lengthen our day and1 more rapidly1 our month in the way explained in ,sir ,robert ,ball's volume4 ,a very interesting point may now be referred to1 because it seems confirmatory of this origin of the great ocean basins4 ,in ,mr4 ,osmond ,fisher's work it is explained how the variations in the force of gravity1 at numerous points all over the world1 have been determined by observations with the pendulum1 and also how these variations afford a measure of the thickness of the solid crust1 which is of less specific gravity than the molten interior on which it rests4 ,by this means a very interesting result was obtained4 ,the observations on numerous oceanic islands proved that the sub-oceanic crust was considerably more dense than the crust under the continents1 but also thinner1 the result being to bring the average mass of the sub-oceanic crust and oceans to an equality with that of the continental crust1 and this causes the whirling earth to be in a state of balance1 or equilibrium4 ,now1 both the thinness and the increased density of the crust seem to be well explained by this theory of the origin of the oceanic basins4 ,the new crust would necessarily for a long time be thinner than the older portion1 because formed so much later1 but it would very soon become cool enough to allow the aqueous vapour of the atmosphere and that given off through fissures from the molten interior to collect in the ocean basins1 which would thenceforth be cooled more rapidly and kept at a uniform temperature and also under a uniform pressure1 and these conditions would lead to the steady and continuous increase of thickness1 with a greater compactness of structure than in the continental areas4 ,it is no doubt to this uniformity of conditions1 with a lowering of the bottom temperature throughout the greater part of geological time1 till it has become only a few degrees above the freezing-point1 that we owe the remarkable persistence of the vast and deep ocean basins on which1 as we have seen1 the continuity of life on the earth has largely depended4 ,there is one other fact which lends some support to this theory of the origin of the ocean basins--their almost complete symmetry with regard to the equator4 ,both the ,atlantic and ,pacific basins extend to an equal distance north and south of the equator1 an equality which could hardly have been produced by any cause not directly connected with the earth's rotation4 ,the polar seas which are coterminous with the two great oceans are very much shallower1 and cannot1 therefore1 be considered as forming part of the true oceanic basins4 ,,water ,,as ,,an ,,equaliser ,,of ,,temperature ,the importance of water in regulating the temperature of the earth is so great that1 even if we had enough water on the land for all the wants of plants and animals1 but had no great oceans1 it is almost certain that the earth could not have produced and sustained the various forms of life which it now possesses4 ,the effect of the oceans is twofold4 ,owing to the great specific heat of water1 that is1 its property of absorbing heat slowly but to a large amount1 and giving it out with equal slowness1 the surface-waters of the oceans and seas are heated by the sun so that by the evening of a bright day they have become quite warm to a depth of several feet4 ,but air has much less specific heat than water1 a pound of water in cooling one degree being capable of warming four pounds of air one degree2 but as air is #ggj times as light as water1 it follows that the heat from one cubic foot of water will warm more than #cjjj cubic feet of air as much as it cools itself4 ,hence the enormous surface of the seas and oceans1 the larger part of which is within the tropics1 warms the whole of the lower and denser portions of the air1 especially during the night1 and this warmth is carried to all parts of the earth by the winds1 and thus ameliorates the climate4 ,another quite distinct effect is due to the great ocean currents1 like the ,gulf ,stream and the ,japan ,current1 which carry the warm water of the tropics to temperate and arctic regions1 and thus render many countries habitable which would otherwise suffer the rigour of an almost arctic winter4 ,these currents are1 however1 directly due to the winds1 and properly belong to the section on the atmosphere4 ,the other equalising action1 due primarily to the great area of the seas and oceans1 is a result of the vast evaporating surface from which the land derives almost all its water in the form of rain and rivers2 and it is quite evident that if there were not sufficient water-surface to produce an ample supply of vapour for this purpose1 arid districts would occupy more and more of the earth's surface4 ,how much water-surface is necessary for life we do not know2 but if the proportions of water and land-surfaces were reversed1 it seems probable that the larger proportion of the earth might be uninhabitable4 ,the vapour thus produced has also a very great effect in equalising temperature2 but this also is a point which will come better under our next chapter on the atmosphere4 99 99 99 99 99 ,there are1 however1 some matters connected with the water-supply of the earth1 and its relation to the development of life1 that call for a few remarks here4 ,what has determined the total quantity of water on the earth or on other planets does not appear to be known2 but presumably it would depend1 partially or wholly1 on the mass of the planet being sufficient to enable it to retain by its gravitative force the oxygen and hydrogen of which water is composed4 ,as the two gases are so easily combined to form water1 but can only be separated under special conditions1 its quantity would be dependent on the supply of hydrogen1 which is but rarely found on the earth in a free state4 ,the important fact1 however1 is1 that we do possess so great a quantity of water1 that if the whole surface of the globe was as regularly contoured as are the continents1 and merely wrinkled with mountain chains1 then the existing water would cover the whole globe nearly two miles deep1 leaving only the tops of high mountains above its surface as rows of small islands1 with a few larger islands formed by what are now the high plateaus of ,tibet and the ,southern ,andes4 ,now there seems no reason why this distribution of the water should not have occurred--in fact it seems probable that it would have occurred1 had it not been for the fortunate coincidence of the formation of enormously deep ocean basins4 ,so far as ,i am aware1 no sufficient explanation of the formation of these basins has been given but that of ,mr4 ,osmond ,fisher1 as here described1 and that depends upon three unique circumstances3 7#a7 the formation of a satellite at a very late period of the planet's development when there was already a rather thick crust2 7#b7 the satellite being far larger in proportion to its primary than any other in the solar system2 and 7#c7 its having been produced by fission from its primary on account of extremely rapid rotation1 combined with solar tides in its molten interior1 and a rate of oscillation of that molten interior coinciding with the tidal period4#ag ,whether this very remarkable theory of the origin of our moon is the true one1 and if so1 whether the explanation it seems to afford of the great oceanic basins is correct1 ,i am not mathematician enough to judge4 ,the tidal theory of the origin of the moon1 as worked out mathematically by ,professor ,g4,h4 ,darwin1 has been supported by ,sir ,robert ,ball and accepted by many other astronomers2 while the researches of the ,rev4 ,osmond ,fisher into the ,physics of the ,earth's ,crust1 together with his mathematical abilities and his practical work as a geologist1 entitle his opinion on the question of the mode of origin of the ocean basins to the highest respect4 ,and1 as we have seen1 the existence of these vast and deep ocean basins1 produced by the agency of a series of events so remarkable as to be quite unique in the solar system1 played an important part in rendering the earth fit for the development of the higher forms of animal life1 while without them it seems not improbable that the conditions would have been such as to render any varied forms of terrestrial life hardly possible4 ,,footnotes3 #af ,for a fuller account of this ,arctic fauna and flora see the works of ,sir ,c4 ,lyell1 ,sir ,a4 ,geikie1 and other geologists4 ,a full summary of it is also given in the author's ,island ,life4 #ag ,professor ,g4,h4 ,darwin states that it is nearly certain that no other satellite nor any of the planets originated in the same way as the moon4 ,,chapter ,,xiii ,,the ,,earth ,,in ,,relation ,,to ,,life3 ,,atmospheric ,,conditions ,we have seen in our tenth chapter that the physical basis of life--protoplasm--consists of the four elements1 oxygen1 nitrogen1 hydrogen1 and carbon1 and that both plants and animals depend largely upon the free oxygen in the air to carry on their vital processes2 while the carbonic acid and ammonia in the atmosphere seem to be absolutely essential to plants4 ,whether life could have arisen and have been highly developed with an atmosphere composed of different elements from ours it is1 of course1 impossible to say2 but there are certain physical conditions which seem absolutely essential whatever may be the elements which compose it4 ,the first of these essentials is an atmosphere which shall be of such density at the surface of the planet1 and of so great a bulk1 as to be not too rare to fulfil its various functions at all altitudes where there is a considerable area of land4 ,what determines the total quantity of gaseous matter on the surface of a planet will be1 mainly1 its mass1 together with the average temperature of its surface4 ,the molecules of gases are in a state of rapid motion in all directions1 and the lighter gases have the most rapid motions4 ,the average speed of the motion of the molecules has been roughly determined under varying conditions of pressure and temperature1 and also the probable maximum and minimum rates1 and from these data1 and certain known facts as to planetary atmospheres1 ,mr4 ,g4 ,johnstone ,stoney1 ,f4,r4,s41 has calculated what gases will escape from the atmospheres of the earth and the other planets4 ,he finds that all the gases which are constituents of air have such comparatively low molecular rates of motion that the force of gravity at the upper limits of the earth's atmosphere is amply sufficient to retain them2 hence the stability in its composition4 ,but there are two other gases1 hydrogen and helium1 which are both known to enter the atmosphere1 but never accumulate so as to form any measurable portion of it1 and these are found to have sufficient molecular motion to escape from it4 ,with regard to hydrogen1 if the earth were much larger and more massive than it is1 so as to retain the hydrogen1 disastrous consequences might ensue1 because1 whenever a sufficient quantity of this gas accumulated1 it would form an explosive mixture with the oxygen of the atmosphere1 and a flash of lightning or even the smallest flame would lead to explosions so violent and destructive as perhaps to render such a planet unsuited for the development of life4 ,we appear1 therefore1 to be just at the major limit of mass to secure habitability1 except in such planets as may have no continuous supply of free hydrogen4 99 99 99 99 99 ,perhaps the most important mechanical functions of the atmosphere dependent on its density are3 7#a7 the production of winds1 which in many ways bring about an equalisation of temperature1 and which also produce surface-currents on the ocean2 and 7#b7 the distribution of moisture over the earth by means of clouds which also have other important functions4 ,winds depend primarily on the local distribution of heat in the air1 especially on the great amount of heat constantly present in the equatorial zone1 due to the sun being always nearly vertical at noon1 and to its being similarly vertical at each tropic once a year1 with a longer day1 leading to even higher temperatures than at the equator1 and producing also that continuous belt of arid lands or deserts which almost encircle the globe in the region of the tropics4 ,heated air being lighter1 the colder air from the temperate zones continually flows towards it1 lifting it up and causing it to flow over1 as it were1 to the north and south4 ,but as the inflow comes from an area of less rapid to one of more rapid rotation1 the course of the air is diverted1 and produces the north-east and south-east trades2 while the overflow from the equator going to an area of less rapid rotation1 turns westward and produces the south-west winds so prevalent over the north ,atlantic and the north temperate zone generally1 and the north-west in the southern hemisphere4 ,it is outside the zone of the equable trade-winds1 and in a region a few degrees on each side of the tropics1 that destructive hurricanes and typhoons prevail4 ,these are really enormous whirlwinds due to the intensely heated atmosphere over the arid regions already mentioned1 causing an inrush of cool air from various directions1 thus setting up a rotatory motion which increases in rapidity till equilibrium is restored4 ,the hurricanes of the ,west ,indies and ,mauritius1 and the typhoons of the ,eastern seas1 are thus caused4 ,some of these storms are so violent that no human structures can resist them1 while the largest and most vigorous trees are torn to pieces or overturned by them4 ,but if our atmosphere were much denser than it is1 its increased weight would give it still greater destructive force2 and if to this were added a somewhat greater amount of sun-heat--which might be due either to our greater proximity to the sun or to the sun's greater size or greater heat-intensity1 these tempests might be so increased in frequency and violence as to render considerable portions of the earth uninhabitable4 ,the constant and equable trade-winds have a very important function in initiating those far-reaching ocean-currents which are of the greatest importance in equalising temperature4 ,the well known ,gulf ,stream is to us the most important of these currents1 because it plays the chief part in giving us the mild climate we enjoy in common with the whole of ,western ,europe1 a mildness which is felt to a considerable distance within the ,arctic ,circle2 and1 in conjunction with the ,japan current1 which does the same for the whole of the temperate regions of the ,north ,pacific1 renders a large portion of the globe better adapted for life than it would be without these beneficial influences4 ,these equalising currents1 however1 are almost entirely due to the form and position of the continents1 and especially to the fact that they are so situated as to leave vast expanses of ocean along the equatorial zone1 and extending north and south to the arctic and antarctic regions4 ,if with the same amount of land the continents had been so grouped as to occupy a considerable portion of the equatorial oceans--such as would have been the case had ,africa been turned so as to join ,south ,america1 and ,asia been brought to the south-east so as to take the place of part of the equatorial ,pacific1 then the great ocean-currents could have been but feeble or have hardly existed4 ,without these currents much of the north and south temperate lands would have been buried in ice1 while the largest portion of the continents would have been so intensely heated as perhaps to be unsuited for the development of the higher forms of animal life1 since we have shown 7in chapters ,x4 and ,,xi47 how delicate is the balance and how narrow the limits of temperature which are required4 ,there seems to be no reason whatever why some such distribution of the sea and land should not have existed1 had it not been for the admittedly exceptional conditions which led to the production of our satellite1 thus necessarily forming vast chasms along the region of the equator where centrifugal force as well as the internal solar tides were most powerful1 and where the thin crust was thus compelled to give way4 ,and as the highest authorities declare that there are no indications of such an origin of satellites in the case of any other planet1 the whole series of conditions favourable to life on the earth become all the more remarkable4 ,,clouds1 ,,their ,,importance ,,and ,,their ,,causes ,few persons have any adequate conception of the real nature of clouds and of the important part they take in rendering our world a habitable and an enjoyable one4 ,on the average1 the rainfall over the oceans is much less than over the land1 the whole region of the trade-winds having usually a cloudless sky and very little rain2 but in the intervening belt of calms1 near to the equator1 a cloudy sky and heavy rains are frequent4 ,this arises from the fact that the warm1 moist air over the ocean is raised upwards1 by the cold and heavy air from north and south1 into a cooler region where it cannot hold so much aqueous vapour1 which is there condensed and falls as rain4 ,generally1 wherever the winds blow over extensive areas of water on to the land1 especially if there are mountains or elevated plateaus which cause the moisture-laden air to rise to heights where the temperature is lower1 clouds are formed and more or less rain falls4 ,but if the land is of an arid nature and much heated by the sun1 the air becomes capable of holding still more aqueous vapour1 and even dense rain-clouds disperse without producing any rainfall4 ,from these simple causes1 with the large area of sea as compared with the land upon our earth1 by far the larger portion of the surface is well supplied with rain1 which1 falling most abundantly in the elevated and therefore cooler regions1 percolates the soil1 and gives rise to those innumerable springs and rivulets which moisten and beautify the earth1 and which1 uniting together1 form streams and rivers1 which return to the seas and oceans whence they were originally derived4 ,,clouds ,,and ,,rain ,,depend ,,upon ,,atmospheric ,,dust ,the beautiful system of aqueous circulation by means of the atmosphere as sketched above was long thought to explain the whole process1 and to require no further elucidation2 but about a quarter of a century back a curious experiment was made which indicated that there was another factor in the process which had been entirely overlooked4 ,if a small jet of steam is sent into two large glass receivers1 one filled with ordinary air1 the other with air which has been filtered by passing through a thick layer of cotton wool so as to keep back all particles of solid matter1 the first vessel will be instantly filled with condensed cloudy-looking vapour1 while in the other vessel the air and vapour will remain perfectly transparent and invisible4 ,another experiment was then made to imitate more nearly what occurs in nature4 ,the two vessels were prepared as before1 but a small quantity of water was placed in each vessel and allowed to evaporate till the air was nearly saturated with vapour1 which remained invisible in both4 ,both vessels were then slightly cooled1 when instantly a dense cloud was formed in that filled with unfiltered air1 while the other remained quite clear4 ,these experiments proved that the mere cooling of air below the dew point will not cause the aqueous vapour in it to condense into drops so as to form mist1 fog1 or cloud1 unless small particles of solid or liquid matter are present to act as nuclei upon which condensation begins4 ,the density of a cloud will therefore depend not only on the quantity of vapour in the air1 but on the presence of an abundance of minute dust-particles on which condensation can begin4 ,that such dust exists everywhere in the air1 even up to great heights1 is not a supposition but a proved fact4 ,by exposing glass plates covered with glycerine in different places and at different altitudes the number of these particles in each cubic foot of air has been determined2 and it is found that not only are they present everywhere at low levels1 but that there are a considerable number even at the tops of the highest mountains4 ,these solid particles also act in another way4 ,by radiation in the higher atmosphere they become very cold1 and thus condense the vapour by contact1 just as the points of grass-blades condense it to form dew4 ,when steam is escaping from an engine we see a mass of dense white vapour1 a miniature cloud2 and if we are near it in cold1 damp weather1 we feel little drops of rain produced from it4 ,but on a fine1 warm day it rises quickly and soon melts away1 and entirely disappears4 ,exactly the same thing happens on a larger scale in nature4 ,in fine weather we may have abundant clouds continually passing high overhead1 but they never produce rain1 because as the minute globules of water slowly fall towards the earth1 the warm dry air again turns them into invisible vapour4 ,again1 in fine weather1 we often see a small cloud on a mountain top which remains there a considerable time1 even though a brisk wind is blowing4 ,the mountain top is colder than the surrounding air1 and the invisible vapour becomes condensed into cloud by passing over it1 but the moment these cloud particles are carried past the summit into the warmer and drier air they are again evaporated and disappear4 ,on ,table ,mountain1 near ,cape ,town1 this phenomenon occurs on a large scale1 and is termed the table-cloth1 the mass of white fleecy cloud seeming to hang over the flat mountain top to some distance down where it remains for several months1 while all around there is bright sunshine4 ,another phenomenon that indicates the universal presence of dust to enormous heights in the atmosphere is the blue colour of the sky4 ,this is caused by the presence of such excessively minute particles of dust through an enormous thickness of the higher atmosphere--probably up to a height of twenty or thirty miles1 or more--that they reflect only the light of short wave-length from the blue end of the spectrum4 ,this also has been proved by experiment4 ,if a glass cylinder1 several feet long1 is filled with pure air from which all solid particles have been removed by filtering and passing over red-hot platinum wires1 and a ray of electric light is passed through it1 the interior1 when viewed laterally1 appears quite dark1 the light passing through in a straight line and not illuminating the air4 ,but if a little more air is passed through the filter so rapidly as to allow only the minutest particles of dust to enter with it1 the vessel becomes gradually filled with a blue haze1 which gradually deepens into a beautiful blue1 comparable with that of the sky4 ,if now some of the unfiltered air is admitted1 the blue fades away into the ordinary tint of daylight4 ,since it has been known that liquid oxygen is blue1 many people have concluded that this explains the blue colour of the sky4 ,but it has really nothing to do with the point at issue4 ,the blue of the liquid oxygen becomes so excessively faint in the gas1 further attenuated as it is by the colourless nitrogen1 that it would have no perceptible colour in the whole thickness of our atmosphere4 ,again1 if it had a perceptible blue tint we could not see it against the blackness of space behind it2 but white objects seen through it1 such as the moon and clouds1 should all appear blue1 which they do not do4 ,the blue we see is from the whole sky1 and is therefore reflected light2 and as pure air is quite transparent1 there must be solid or liquid particles so minute as to reflect blue light only4 ,in the lower atmosphere the rain-producing particles are larger1 and reflect all the rays1 thus diluting the blue colour near the horizon1 and1 by refraction and reflection combined1 producing the various beautiful hues of sunrise and sunset4 ,this production of exquisite colours by the dust in the atmosphere1 though adding greatly to the enjoyment of life1 cannot be considered essential to it2 but there is another circumstance connected with atmospheric dust which1 though little appreciated1 might have effects which can hardly be calculated4 ,if there were no dust in the atmosphere1 the sky would appear black even at noon1 except in the actual direction of the sun2 and the stars would be visible in the day as well as at night4 ,this would follow because air does not reflect light1 and is not visible4 ,we should therefore receive no light from the sky itself as we do now1 and the north side of every hill1 house1 and other solid objects1 would be totally dark1 unless there were any surfaces in that direction to reflect the light4 ,the surface of the ground at a little distance would be in sunshine1 and this would be the only source of light wherever direct sunlight was cut off4 ,to get a good amount of pleasant light in houses it would be necessary to have them built on nearly level ground1 or on ground rising to the north1 and with walls of glass all round and down to the floor line1 to receive as much as possible of the reflected light from the ground4 ,what effect this kind of light would have on vegetation it is difficult to say1 but trees and shrubs would probably grow laterally towards the south1 east1 and west1 so as to get as much direct sunshine as possible4 ,a more important result would be that1 as sunshine would be perpetual during the day1 so much evaporation would take place that the soil would become arid and almost bare in places that are now covered with vegetation1 and plants like the cactuses of ,arizona and the euphorbias of ,south ,africa would occupy a large portion of the surface4 ,returning now from this collateral subject of light and colour to the more important aspect of the question--the absence of cloud and rain--we have to consider what would happen1 and in what way the enormous quantity of water which would be evaporated under continual sunshine would be returned to the earth4 ,the first and most obvious means would be by abnormally abundant dews1 which would be deposited almost every night on every form of leafy vegetation4 ,not only would all grass and herbage1 but all the outer leaves of shrubs and trees1 condense so much moisture as to take the place of rain so far as the needs of such vegetation were concerned4 ,but without arrangements for irrigation cultivation would be almost impossible1 because the bare soil would become intensely heated during the day1 and would retain so much of its heat through the night so as to prevent any dew forming upon it4 ,some more effective mode1 therefore1 of returning the aqueous vapour of the atmosphere to the earth and ocean1 would be required1 and this1 ,i believe1 would be done by means of hills and mountains of sufficient height to become decidedly colder than the lowlands4 ,the air from over the oceans would be constantly loaded with moisture1 and whenever the winds blew on to the land the air would be carried up the slopes of the hills into the colder regions1 and there be rapidly condensed upon the vegetation1 and also on the bare earth and rocks of northern slopes1 and wherever they cooled sufficiently during the afternoon or night to be below the temperature of the air4 ,the quantity of vapour thus condensed would reduce the atmospheric pressure1 which would lead to an inrush of air from below1 bringing with it more vapour1 and this might give rise to perpetual torrents1 especially on northern and eastern slopes4 ,but as the evaporation would be much greater than at the present time1 owing to perpetual sunshine1 so the water returned to the earth would be greater1 and as it would not be so uniformly distributed over the land as it is now1 the result would perhaps be that extensive mountain sides would become devastated by violent torrents1 rendering permanent vegetation almost impossible2 while other and more extensive areas1 in the absence of rain1 would become arid wastes that would support only the few peculiar types of vegetation that are characteristic of such regions4 ,whether such conditions as here supposed would prevent the development of the higher forms of life it is impossible to say1 but it is certain that they would be very unfavourable1 and might have much more disastrous consequences than any we have here suggested4 ,we can hardly suppose that1 with winds and rock-formations at all like what they are now1 any world could be wholly free from atmospheric dust4 ,if1 however1 the atmosphere itself were much less dense than it is1 say one-half1 which might very easily have been the case1 then the winds would have less carrying power1 and at the elevations at which clouds are usually formed there would not be enough dust-particles to assist in their formation4 ,hence fogs close to the earth's surface would largely take the place of clouds floating far above it1 and these would certainly be less favourable to human life and to that of many of the higher animals than existing conditions4 ,the world-wide distribution of atmospheric dust is a remarkable phenomenon4 ,as the blue colour of the sky is universal1 the whole of the higher atmosphere must be pervaded by myriads of ultra-microscopical particles1 which1 by reflecting the blue rays only1 give us not only the azure vault of heaven1 but in combination with the coarser dust of lower altitudes1 diffused daylight1 the grand forms and motions of the fleecy clouds1 and the 'gentle rain from heaven' to refresh the parched earth and make it beautiful with foliage and flowers4 ,over every part of the vast ,pacific ,ocean1 whose islands must produce a minimum of dust1 the sky is always blue1 and its thousand isles do not suffer for want of rain4 ,over the great forest-plain of the ,amazon valley1 where the production of dust must be very small1 there is yet abundance of rain-clouds and of rain4 ,this is due primarily to the two great natural sources of dust--the active volcanoes1 together with the deserts and more arid regions of the world2 and1 in the second place1 to the density and wonderful mobility of the atmosphere1 which not only carries the finest dust-particles to an enormous height1 but distributes them through its whole extent with such wonderful uniformity4 ,every dust-particle is of course much heavier than air1 and in a comparatively short time1 if the atmosphere were still1 would fall to the ground4 ,tyndall found that the air of a cellar under the ,royal ,institution in ,albemarle ,street1 which had not been opened for several months1 was so pure that the path of a beam of electric light sent through it was quite invisible4 ,but careful experiments show that not only is the air in continual motion1 but the motion is excessively irregular1 being hardly ever quite horizontal1 but upwards and downwards and in every intermediate direction1 as well as in countless whirls and eddies2 and this complexity of motion must extend to a vast height1 probably to fifty miles or more1 in order to provide a sufficient thickness of those minutest particles which produce the blue of the sky4 ,all this complexity of motion is due to the action of the sun in heating the surface of the earth1 and the extreme irregularity of that surface both as regards contour and its capacity for heat-absorption4 ,in one area we have sand or rock or bare clay1 which1 when exposed to bright sunshine1 become scorching hot2 in another area we have dense vegetation1 which1 owing to evaporation caused by the sunshine1 remains comparatively cool1 and also the still cooler surfaces of rivers and ,alpine lakes4 ,but if the air were much less dense than it is1 these movements would be less energetic1 while all the dust that was raised to any considerable height would1 by its own weight1 fall back again to the earth much more rapidly than it does now4 ,there would thus be much less dust permanently in the atmosphere1 and this would inevitably lead to diminished rainfall and1 partially1 to the other injurious effects already described4 ,,atmospheric ,,electricity ,we have already seen that vegetable organisms obtain the chief part of the nitrogen in their tissues from ammonia produced in the atmosphere and carried into the earth by rain4 ,this substance can only be thus produced by the agency of electrical discharges1 or lightning1 which cause the combination of the hydrogen in the aqueous vapour with the free nitrogen of the air4 ,but clouds are important agents in the accumulation of electricity in sufficient amount to produce the violent discharges we know as lightning1 and it is doubtful whether without them there would be any discharges through the atmosphere capable of decomposing the aqueous vapour in it4 ,not only are clouds beneficial in the production of rain1 and also in moderating the intensity of continuous sun-heat1 but they are also requisite for the formation of chemical compounds in vegetables which are of the highest importance to the whole animal kingdom4 ,so far as we know1 animal life could not exist on the earth's surface without this source of nitrogen1 and therefore without clouds and lightning2 and these1 we have just seen1 depend primarily on a due proportion of dust in the atmosphere4 ,but this due proportion of dust is mainly supplied by volcanoes and deserts1 and its distribution and constant presence in the air depends upon the density of the atmosphere4 ,this again depends on two other factors3 the force of gravity due to the mass of the planet1 and the absolute quantity of the free gases constituting the atmosphere4 ,we thus find that the vast1 invisible ocean of air in which we live1 and which is so important to us that deprivation of it for a few minutes is destructive of life1 produces also many other beneficial effects of which we usually take little account1 except at times when storm or tempest1 or excessive heat or cold1 remind us how delicate is the balance of conditions on which our comfort1 and even our lives1 depend4 ,but the sketch ,i have here attempted to give of its varied functions shows us that it is really a most complex structure1 a wonderful piece of machinery1 as it were1 which in its various component gases1 its actions and reactions upon the water and the land1 its production of electrical discharges1 and its furnishing the elements from which the whole fabric of organic life is composed and perpetually renewed1 may be truly considered to be the very source and foundation of life itself4 ,this is seen1 not only in the fact of our absolute dependence upon it every minute of our lives1 but in the terrible effects produced by even a slight degree of impurity in this vital element4 ,yet it is among those nations that claim to be the most civilised1 those that profess to be guided by a knowledge of the laws of nature1 those that most glory in the advance of science1 that we find the greatest apathy1 the greatest recklessness1 in continually rendering impure this all-important necessary of life1 to such a degree that the health of the larger portion of their populations is injured and their vitality lowered1 by conditions which compel them to breathe more or less foul and impure air for the greater part of their lives4 ,the huge and ever-increasing cities1 the vast manufacturing towns belching forth smoke and poisonous gases1 with the crowded dwellings1 where millions are forced to live under the most terrible insanitary conditions1 are the witnesses to this criminal apathy1 this incredible recklessness and inhumanity4 ,for the last fifty years and more the inevitable results of such conditions have been fully known2 yet to this day nothing of importance has been done1 nothing is being done4 ,in this beautiful land there is ample space and a superabundance of pure air for every individual4 ,yet our wealthy and our learned classes1 our rulers and law-makers1 our religious teachers and our men of science1 all alike devote their lives and energies to anything or everything but this4 ,yet this is the one great and primary essential of a people's health and well-being1 to which everything should1 for the time1 be subordinate4 ,till this is done1 and done thoroughly and completely1 our civilisation is naught1 our science is naught1 our religion is naught1 and our politics are less than naught--are utterly despicable2 are below contempt4 ,it has been the consideration of our wonderful atmosphere in its various relations to human life1 and to all life1 which has compelled me to this cry for the children and for outraged humanity4 ,will no body of humane men and women band themselves together1 and take no rest till this crying evil is abolished1 and with it nine-tenths of all the other evils that now afflict us8 ,let everything give way to this4 ,as in a war of conquest or aggression nothing is allowed to stand in the way of victory1 and all private rights are subordinated to the alleged public weal1 so1 in this war against filth1 disease1 and misery let nothing stand in the way--neither private interests nor vested rights--and we shall certainly conquer4 ,this is the gospel that should be preached1 in season and out of season1 till the nation listens and is convinced4 ,let this be our claim3 ,pure air and pure water for every inhabitant of the ,british ,isles4 ,vote for no one who says ',it can't be done4' ,vote only for those who declare ',it shall be done4' ,it may take five or ten or twenty years1 but all petty ameliorations1 all piecemeal reforms1 must wait till this fundamental reform is effected4 ,then1 when we have enabled our people to breathe pure air1 and drink pure water1 and live upon simple food1 and work and play and rest under healthy conditions1 they will be in a position to decide 7for the first time7 what other reforms are really needed4 ,remember6 ,we claim to be a people of high civilisation1 of advanced science1 of great humanity1 of enormous wealth6 ,for very shame do not let us say ',we cannot arrange matters so that our people may all breathe unpolluted1 unpoisoned air6' ,,chapter ,,xiv ,,the ,,earth ,,is ,,the ,,only ,,habitable ,,planet ,,in ,,the ,,solar ,,system ,having shown in the last three chapters how numerous and how complex are the conditions which alone render life possible on our earth1 how nicely balanced are opposing forces1 and how curious and delicate are the means by which the essential combinations of the elements are brought about1 it will be a comparatively easy task to show how totally unfitted are all the other planets either to develop or to preserve the higher forms of life1 and1 in most cases1 any forms above the lowest and most rudimentary4 ,in order to make this clear we will take the most important of the conditions in order1 and see how the various planets fulfil them4 ,,mass ,,of ,a ,,planet ,,and ,,its ,,atmosphere ,the height and density of the atmosphere of a planet is important as regards life in several ways4 ,on its density depends its power of carrying moisture2 of holding a sufficient supply of dust-particles for the formation of clouds2 of carrying ultra-microscopic particles to such a height and in such quantity as to diffuse the light of the sun by reflection from the whole sky2 of raising waves in the ocean and thus aerating its waters1 and of producing the ocean currents which so greatly equalise temperature4 ,now this density depends on two factors3 the mass of the planet and the quantity of the atmospheric gases4 ,but there is good reason to think that the latter depends directly upon the former1 because it is only when a certain mass is attained that any of the lighter permanent gases can be held on the surface of a planet4 ,thus1 according to ,dr4 ,g4 ,johnstone ,stoney1 who has specially studied this subject1 the moon cannot retain even such a heavy gas as carbonic acid1 or the still heavier carbon disulphide2 while no particle of oxygen1 nitrogen1 or water-vapour can possibly remain on it1 owing to the fact of its mass being only about one-eightieth that of the earth4 ,it is believed that there are considerable quantities of gases in the stellar spaces1 and probably also within the solar system1 but perhaps in the liquid or solid form4 ,in that state they might be attracted by any small mass such as the moon1 but the heat of its surface when exposed to the solar rays would quickly restore them to the gaseous condition1 when they would at once escape4 ,it is only when a planet attains a mass at least a quarter that of the earth that it is capable of retaining water-vapour1 one of the most essential of the gases2 but with so small a mass as this1 its whole atmosphere would probably be so limited in amount and so rare at the planet's surface that it would be quite unable to fulfil the various purposes for which an atmosphere is required in order to support life4 ,for their adequate fulfilment the mass of a planet cannot be much less than that of the earth4 ,here we come to one of those nice adjustments of which so many have been already pointed out4 ,dr4 ,johnstone ,stoney arrives at the conclusion that hydrogen escapes from the earth4 ,it is continually produced in small quantities by submarine volcanoes1 by fissures in volcanic regions1 from decaying vegetation1 and from some other sources2 yet1 though sometimes found in minute quantities1 it forms no regular constituent of our atmosphere4#ah ,the quantity of hydrogen combined with oxygen to form the mass of water in our vast and deep oceans is enormous4 ,yet if it had been only one-tenth more than it actually is1 the present land-surface would have been almost all submerged4 ,how the adjustments occurred so that there was exactly enough hydrogen to fill the vast ocean basins with water to such a depth as to leave enough land-surface for the ample development of vegetable and animal life1 and yet not so much as to be injurious to climate1 it is difficult to imagine4 ,yet the adjustment stares us in the face4 ,first1 we have a satellite unique in size as compared with its primary1 and apparently in lateness of origin2 then we have a mode of origin for that satellite said to be certainly unique in the solar system2 as a consequence of this origin1 it is believed1 we have enormously deep ocean basins symmetrically placed with regard to the equator--an arrangement which is very important for ocean circulation2 then we must have had the right quantity of hydrogen1 obtained in some unknown way1 which formed water enough to fill these chasms1 so as to leave an ample area of dry land1 but which one-tenth more water would have ingulfed2 and1 lastly1 we have oxygen enough left to form an atmosphere of sufficient density for all the requirements of life4 ,it could not be that the surplus hydrogen escaped when the water had been produced1 because it escapes very slowly1 and it combines so easily with free oxygen by means of even a spark1 as to make it certain that all the available hydrogen was used up in the oceanic waters1 and that the supply from the earth's interior has been since comparatively small in amount4 ,there is yet one more adjustment to be noticed4 ,all the facts now referred to show that the earth's mass is sufficient to bring about the conditions favourable for life4 ,but if our globe had been a little larger1 and proportionately denser1 in all probability no life would have been possible4 ,between a planet of #hjjj and one of #iejj miles diameter is not a large difference1 when compared with the enormous range of size of the other planets4 ,yet this slight increase in diameter would give two-thirds increase in bulk1 and1 with a corresponding increase of density due to the greater gravitative force1 the mass would be about double what it is4 ,but with double the mass the quantity of gases of all sorts attracted and retained by gravity would probably have been double2 and in that case there would have been double the quantity of water produced1 as no hydrogen could then escape4 ,but the surface of the globe would only be one half greater than at present1 in which case the water would have sufficed to cover the whole surface several miles deep4 ,,habitability ,,of ,,other ,,planets ,when we look to the other planets of our system we see everywhere illustrations of the relation of size and mass to habitability4 ,the smaller planets1 ,mercury and ,mars1 have not sufficient mass to retain water-vapour1 and1 without it1 they cannot be habitable4 ,all the larger planets can have very little solid matter1 as indicated by their very low density notwithstanding their enormous mass4 ,there is1 therefore1 very good reason for the belief that the adaptability of a planet for a full development of life is primarily dependent1 within very narrow limits1 on its size and1 more directly1 on its mass4 ,but if the earth owes its specially constituted atmosphere and its nicely adjusted quantity of water to such general causes as here indicated1 and the same causes apply to the other planets of the solar system1 then the only planet on which life can be possible is ,venus4 ,as1 however1 it may be urged that exceptional causes may have given other planets an equal advantage in the matter of air and water1 we will briefly consider some of the other conditions which we have found to be essential in the case of the earth1 but which it is almost impossible to conceive as existing1 to the required extent1 on any of the other planets of the solar system4 ,a ,,small ,,and ,,definite ,,range ,,of ,,temperature ,we have already seen within what narrow limits the temperature on a planet's surface must be maintained in order to develop and support life4 ,we have also seen how numerous and how delicate are the conditions1 such as density of atmosphere1 extent and permanence of oceans1 and distribution of sea and land1 which are requisite1 even with us1 in order to render possible the continuous preservation of a sufficiently uniform temperature4 ,slight alterations one way or another might render the earth almost uninhabitable1 through its being liable to alternations of too great heat or excessive cold4 ,how then can we suppose that any other of the planets1 which have either very much more or very much less sun-heat than we receive1 could1 by any possible modification of conditions1 be rendered capable of producing and supporting a full and varied life-development8 ,mars receives less than half the amount of sun-heat per unit of surface that we do4 ,and as it is almost certain that it contains no water 7its polar snows being caused by carbonic acid or some other heavy gas7 it follows that1 although it may produce vegetable life of some low kinds1 it must be quite unsuited for that of the higher animals4 ,its small size and mass1 the latter only one-ninth that of the earth1 may probably allow it to possess a very rare atmosphere of oxygen and nitrogen1 if those gases exist there1 and this lack of density would render it unable to retain during the night the very moderate amount of heat it might absorb during the day4 ,this conclusion is supported by its low reflecting power1 showing that it has hardly any clouds in its scanty atmosphere4 ,during the greater part of the twenty-four hours1 therefore1 its surface-temperature would probably be much below the freezing point of water2 and this1 taken in conjunction with the total absence of aqueous vapour or liquid water1 would add still further to its unsuitability for animal life4 ,in ,venus the conditions are equally adverse in the other direction4 ,it receives from the sun almost double the amount of heat that we receive1 and this alone would render necessary some extraordinary combination of modifying agencies in order to reduce and render uniform the excessively high temperature4 ,but it is now known that ,venus has one peculiarity which is in itself almost prohibitive of animal life1 and probably of even the lowest forms of vegetable life4 ,this peculiarity is1 that through tidal action caused by the sun1 its day has been made to coincide with its year1 or1 more properly1 that it rotates on its axis in the same time that it revolves round the sun4 ,hence it always presents the same face to the sun2 and while one half has a perpetual day1 the other half has perpetual night1 with perpetual twilight through refraction in a narrow belt adjoining the illuminated half4 ,but the side that never receives the direct rays of the sun must be intensely cold1 approximating1 in the central portions1 to the zero of temperature1 while the half exposed to perpetual sunshine of double intensity to ours must almost certainly rise to a temperature far too great for the existence of protoplasm1 and probably1 therefore1 of any form of animal life4 ,venus appears to have a dense atmosphere1 and its brilliancy suggests that we see the upper surface of a cloud-canopy1 and this would no doubt greatly reduce the excessive solar heat4 ,its mass1 being a little more than three-fourths that of the earth1 would enable it to retain the same gases as we possess4 ,but under the extraordinary conditions that prevail on the surface of this planet1 it is hardly possible that the temperature of the illuminated side can be preserved in a sufficient state of uniformity for the development of life in any of its higher forms4 ,mercury possesses the same peculiarity of keeping one face always towards the sun1 and as it is so much smaller and so much nearer the sun its contrasts of heat and cold must be still more excessive1 and we need hardly discuss the possibility of this planet being habitable4 ,its mass being only one-thirtieth that of the earth1 water-vapour will certainly escape from it1 and1 most probably1 nitrogen and oxygen also1 so that it can possess very little atmosphere2 and this is indicated by its low reflecting power1 no less than #hc per cent4 of the sun's light being absorbed1 and only #ag per cent4 reflected1 whereas clouds reflect #gb per cent4 ,this planet is therefore intensely heated on one side and frozen on the other2 it has no water and hardly any atmosphere1 and is therefore1 from every point of view1 totally unfitted for supporting living organisms4 ,even if it is supposed that1 in the case of ,venus1 its perpetual cloud-canopy may keep down the surface temperature within the limits necessary for animal life1 the extraordinary turmoil in its atmosphere caused by the excessively contrasted temperatures of its dark and light hemispheres must be extremely inimical to life1 if not absolutely prohibitive of it4 ,for on the greater part of the hemisphere that never receives a ray of light or heat from the sun all the water and aqueous vapour must be turned into ice or snow1 and it seems almost impossible that the air itself can escape congelation4 ,it could only do so by a very rapid circulation of the whole atmosphere1 and this would certainly be produced by the enormous and permanent difference of temperature between the two hemispheres4 ,indications of refraction by a dense atmosphere are visible during the planet's transit over the sun's disc1 and also when it is in conjunction with the sun1 and the refraction is so great that ,venus is believed to have an atmosphere much higher than ours4 ,but during the rapid circulation of such an atmosphere1 heated on one half the planet and cooled on the other1 most of the aqueous vapour must be taken out of it on the dark side as fast as it is produced on the heated side1 though sufficient may remain to produce a canopy of very lofty clouds analogous to our cirri4 ,the occasional visibility of the dark side of ,venus may be caused by an electrical glow due to the friction of the perpetually overflowing and inflowing atmosphere1 this being increased by reflection from a vast surface of perpetual snow4 ,if we consider all the exceptional features of this planet1 it appears certain that the conditions as regards climate cannot now be such as to maintain a temperature within the narrow limits essential for life1 while there is little probability that at any earlier period it can have possessed and maintained the necessary stability during the long epochs which are requisite for its development4 ,before considering the condition of the larger planets1 it will be well to refer to an argument which has been supposed to minimise the difficulties already stated as to those planets which approach nearest to the earth in size and distance from the sun4 ,,the ,,argument ,,from ,,extreme ,,conditions ,,on ,,the ,,earth ,in reply to the evidence showing how nice are the adaptations required for life-development1 it is often objected that life does now exist under very extreme conditions--under tropic heat and arctic snows2 in the burnt-up desert as well as in the moist tropical forest2 in the air as well as in the water2 on lofty mountains as well as on the level lowlands4 ,this is no doubt true1 but it does not prove that life could have been developed in a world where any of these extremes of climate characterised the whole surface4 ,the deserts are inhabited because there are oases where water is attainable1 as well as in the surrounding fertile areas4 ,the arctic regions are inhabited because there is a summer1 and during that summer there is vegetation4 ,if the surface of the ground were always frozen1 there would be no vegetation and no animal life4 ,the late ,mr4 ,r4,a4 ,proctor put this argument of the diversity of conditions under which life actually does exist on the earth as well probably as it can be put4 ,he says3 ',when we consider the various conditions under which life is found to prevail1 that no difference of climatic relations1 or of elevation1 of land1 or of air1 or of water1 of soil in land1 of freshness or saltness in water1 of density in air1 appears 7so far as our researches have extended7 to render life impossible1 we are compelled to infer that the power of supporting life is a quality which has an exceedingly wide range in nature4' ,this is true1 but with certain reservations4 ,the only species of animal which does really exist under the most varied conditions of climate is man1 and he does so because his intellect renders him to some extent the ruler of nature4 ,none of the lower animals have such a wide range1 and the diversity of conditions is not really so great as it appears to be4 ,the strict limits are nowhere permanently overpassed1 and there is always the change from winter to summer1 and the possibility of migration to less inhospitable areas4 ,,the ,,great ,,planets ,,all ,,uninhabitable ,having already shown that the condition of ,mars1 both as regards water1 atmosphere1 and temperature1 is quite unfitted to maintain life1 a view in which both general principles and telescopic examination perfectly agree1 we may pass on to the outer planets1 which1 however1 have long been given up as adapted for life even by the most ardent advocates for 'life in other worlds4' ,their remoteness from the sun--even ,jupiter being five times as far as the earth1 and therefore receiving only one twenty-fifth of the light and heat that we receive per unit of surface--renders it almost impossible1 even if other conditions were favourable1 that they should possess surface-temperatures adequate to the necessities of organic life4 ,but their very low densities1 combined with very large size1 renders it certain that they none of them have a solidified surface1 or even the elements from which such a surface could be formed4 ,it is supposed that ,jupiter and ,saturn1 as well as ,uranus and ,neptune1 retain a considerable amount of internal heat1 but certainly not sufficient to keep the metallic and other elements of which the sun and earth consist in a state of vapour1 for if so they would be planetary stars and would shine by their own light4 ,and if any considerable portion of their bulk consisted of these elements1 whether in a solid or a liquid state1 their densities would necessarily be much greater than that of the earth instead of very much less--,jupiter is under one-fourth the density of the earth1 ,saturn under an eighth1 while ,uranus and ,neptune are of intermediate densities1 though much less in bulk even than ,saturn4 ,it thus appears that the solar system consists of two groups of planets which differ widely from each other4 ,the outer group of four very large planets are almost wholly gaseous1 and probably consist of the permanent gases--those which can only be liquefied or solidified at a very low temperature4 ,in no other way can their small density combined with enormous bulk be accounted for4 ,the inner group also of four planets are totally unlike the preceding4 ,they are all of small size1 the earth being the largest4 ,they are all of a density roughly proportionate to their bulk4 ,the earth is both the largest and the densest of the group2 not only is it situated at that distance from the sun which1 through solar heat alone1 allows water to remain in the liquid state over almost the whole of its surface1 but it possesses numerous characteristics which secure a very equable temperature1 and which have secured to it very nearly the same temperature during those enormous geological periods in which terrestrial life has existed4 ,we have already shown that no other planet possesses these characteristics now1 and it is almost equally certain that they never have possessed them in the past1 and never will possess them in the future4 ,a ,,last ,,argument ,,for ,,habitability ,,of ,,the ,,planets ,although it has been admitted by the late ,mr4 ,proctor and some other astronomers that most of the planets are not now habitable1 yet1 it is often urged1 they may have been so in the past or may become so in the future4 ,some are now too hot1 others are now too cold2 some have now no water1 others have too much2 but all go through their appointed series of stages1 and during some of these stages life may be or may have been possible4 ,this argument1 although vague1 will appeal to some readers1 and it may1 therefore1 be necessary to reply to it4 ,this is the more necessary as it is still made use of by astronomers4 ,in a criticism of my article in ,the ,fortnightly ,review1 ,m4 ,camille ,flammarion1 of the ,paris ,observatory1 dramatically remarks3 ',yes1 life is universal1 and eternal1 for time is one of its factors4 ,yesterday the moon1 to-day the earth1 to-morrow ,jupiter4 ,in space there are both cradles and tombs4'#ai ,it is thus suggested that the moon was once inhabited and that ,jupiter will be inhabited in some remote future2 but no attempt is made to deal with the essential physical conditions of these very diverse objects1 rendering them not only now1 but always1 unfitted to develop and to maintain terrestrial or aerial life4 ,this vague supposition--it can hardly be termed an argument--as regards past or future adaptability for life1 of all the planets and some of the satellites in the solar system1 is1 however1 rendered invalid by an equally general objection to which its upholders appear never to have given a moment's consideration2 and as it is an objection which still further enforces the view as to the unique position of the earth in the solar system1 it will be well to submit it to the judgment of our readers4 ,,limitation ,,of ,,the ,,sun',s ,,heat ,it is well known that there is1 and has been for nearly half a century1 a profound difference of opinion between geologists and physicists as to the actual or possible duration in years of life upon the earth4 ,the geologists1 being greatly impressed with the vast results produced by the slow processes of the wearing away of the rocks and the deposit of the material in seas or lakes1 to be again upheaved to form dry land1 and to be again carved out by rain and wind1 by heat and cold1 by snow and ice1 into hills and valleys and grand mountain ranges2 and further1 by the fact that the highest mountains in every part of the globe very often exhibit on their loftiest summits stratified rocks which contain marine organisms1 and were therefore originally laid down beneath the sea2 and1 yet again1 by the fact that the loftiest mountains are often the most recent1 and that these grand features of the earth's surface are but the latest examples of the action of forces that have been at work throughout all geological time--studying all their lives the detailed evidences of all these changes1 have come to the conclusion that they imply enormous periods only to be measured by scores or hundreds of millions of years4 ,and the collateral study of fossil remains in the long series of rock-formations enforces this view4 ,in the whole epoch of human history1 and far back into prehistoric times during which man existed on the earth1 although several animals have become extinct1 yet there is no proof that any new one has been developed4 ,but this human era1 so far as yet known1 going back certainly to the glacial epoch and almost certainly to pre-glacial times1 cannot be estimated at less than a million1 some think even several million years2 and as there have certainly been some considerable alterations of level1 excavation of valleys1 deposits of great beds of gravel1 and other superficial changes during this period1 some kind of a scale of measurement of geological time has been obtained1 by comparison with the very minute changes that have occurred during the historical period4 ,this scale is admittedly a very imperfect one1 but it is better than none at all2 and it is by comparing these small changes with the far greater ones which have occurred during every successive step backward in geological history that these estimates of geological time have been arrived at4 ,they are also supported by the palontologists1 to whom the vast panorama of successive forms of life is an ever-present reality4 ,directly they pass into the latest stage of the ,tertiary period--the ,pliocene of ,sir ,charles ,lyell--all over the world new forms of life appear which are evidently the forerunners of many of our still existing species2 and as they go a little further back1 into the ,miocene1 there are indications of a warmer climate in ,europe1 and large numbers of mammals resembling those which now inhabit the tropics1 but of quite distinct species and often of distinct genera and families4 ,and here1 though we have only reached to about the middle of the ,tertiary period1 the changes in the forms of life1 in the climate1 and in the land-surfaces are so great when compared with the very minute changes during the human epoch1 as to require us to multiply the time elapsed many times over4 ,yet the whole of the ,tertiary period1 during which all the great groups of the higher animals were developed from a comparatively few generalised ancestral forms1 is yet the shortest by far of the three great geological periods--the ,mesozoic or ,secondary1 having been much longer1 with still vaster changes both in the earth's crust and in the forms of life2 while the ,palozoic or ,primary1 which carries us back to the earliest forms of life as represented by fossilised remains1 is always estimated by geologists to be at least as long as the other two combined1 and probably very much longer4 ,from these various considerations most geologists who have made any estimates of geological time from the period of the earliest fossiliferous rocks1 have arrived at the conclusion that about #bjj millions of years are required4 ,but from the variety of the forms of life at this early period it is concluded that a very much greater duration is needed for the whole epoch of life4 ,speaking of the varied marine fauna of the ,cambrian period1 the late ,professor ,ramsay says3--',in this earliest known varied life we find no evidence of its having lived near the beginning of the zoological series4 ,in a broad sense1 compared with what must have gone before1 both biologically and physically1 all the phenomena connected with this old period seem1 to my mind1 to be of quite a recent description2 and the climates of seas and lands were of the very same kind as those the world enjoys at the present day4' ,and ,professor ,huxley held very similar views when he declared3 ',if the very small differences which are observable between the crocodiles of the older ,secondary formations and those of the present day furnish any sort of an approximation towards an estimate of the average rate of change among reptiles1 it is almost appalling to reflect how far back in ,palozoic times we must go before we can hope to arrive at that common stock from which the crocodiles1 lizards1 ,ornithoscelida1 and ,plesiosauria1 which had attained so great a development in the ,triassic epoch1 must have been derived4' ,now1 in opposition to these demands of the geologists1 in which they are almost unanimous1 the most celebrated physicists1 after full consideration of all possible sources of the heat of the sun1 and knowing the rate at which it is now expending heat1 declare1 with complete conviction1 that our sun cannot have existed as a heat-giving body for so long a period1 and they would therefore reduce the time during which life can possibly have existed on the earth to about one-fourth of that demanded by geologists4 ,in one of his latest articles1 ,lord ,kelvin says3--',now we have irrefragable dynamics proving that the whole life of our sun as a luminary is a very moderate number of million years1 probably less than #ej million1 possibly between #ej and #ajj' 7,phil4 ,mag41 vol4 ii41 ,sixth ,ser41 p4 #age1 ,aug4 #aija74 ,in my ,island ,life 7chap4 ,x47 ,i have myself given reasons for thinking that both the stratigraphical and biological changes may have gone on more quickly than has been supposed1 and that geological time 7meaning thereby the time during which the development of life upon the earth has been going on7 may be reduced so as possibly to be brought within the maximum period allowed by physicists2 but there will certainly be no time to spare1 and any planets dependent on our sun whose period of habitability is either past or to come1 cannot possibly have1 or have had1 sufficient time for the necessarily slow evolution of the higher life-forms4 ,again1 all physicists hold that the sun is now cooling1 and that its future life will be much less than its past4 ,in a lecture at the ,royal ,institution 7published in ,nature ,series1 in #ahhi71 ,lord ,kelvin says3--',it would1 ,i think1 be exceedingly rash to assume as probable anything more than twenty million years of the sun's light in the past history of the earth1 or to reckon more than five or six million years of sunlight for time to come4' ,these extracts serve to show that1 unless either geologists or physicists are very far from any approach to accuracy in their estimates of past or future age of the sun1 there is very great difficulty in bringing them into harmony or in accounting for the actual facts of the geological history of the earth and of the whole course of life-development upon it4 ,we are1 therefore1 again brought to the conclusion that there has been1 and is1 no time to spare2 that the whole of the available past life-period of the sun has been utilised for life-development on the earth1 and that the future will be not much more than may be needed for the completion of the grand drama of human history1 and the development of the full possibilities of the mental and moral nature of man4 ,we have here1 then1 a very powerful argument1 from a different point of view than any previously considered1 for the conclusion that man's place in the solar system is altogether unique1 and that no other planet either has developed or can develop such a full and complete life-series as that which the earth has actually developed4 ,even if the conditions had been more favourable than they are seen to be on other planets1 ,mercury1 ,venus1 and ,mars could not possibly have preserved equability of conditions long enough for life-development1 since for unknown ages they must have been passing slowly towards their present wholly unsuitable conditions2 while ,jupiter and the planets beyond him1 whose epoch of life-development is supposed to be in the remote future when they shall have slowly cooled down to habitability1 will then be still more faintly illuminated and scantily warmed by a rapidly cooling sun1 and may thus become1 at the best1 globes of solid ice4 ,this is the teaching of science--of the best science of the twentieth century4 ,yet we find even astronomers who1 more than any other exponents of science1 should give heed to the teachings of the sister sciences to which they owe so much1 indulging in such rhapsodies as the following3--',in our solar system1 this little earth has not obtained any special privileges from ,nature1 and it is strange to wish to confine life within the circle of terrestrial chemistry4' ,and again3 ',infinity encompasses us on all sides1 life asserts itself1 universal and eternal1 our existence is but a fleeting moment1 the vibration of an atom in a ray of the sun1 and our planet is but an island floating in the celestial archipelago1 to which no thought will ever place any bounds4'#bj ,in place of such 'wild and whirling words1' ,i have endeavoured to state the sober conclusions of the best workers and thinkers as to the nature and origin of the world in which we live1 and of the universe which on all sides surrounds us4 ,i leave it to my readers to decide which is the more trustworthy guide4 ,,footnotes3 #ah ,transactions of ,royal ,dublin ,society1 vol4 vi4 7ser4 ii471 part xiii4 ',of ,atmospheres upon ,planets and ,satellites4' ,by ,g4 ,johnstone ,stoney1 ,f4,r4,s41 etc4 etc4 #ai ,knowledge1 ,june #aijc4 #bj ,m4 ,camille ,flammarion1 in ,knowledge1 ,june #aijc4 ,,chapter ,,xv ,,the ,,stars--,,have ,,they ,,planetary ,,systems8 ,,are ,,they ,,beneficial ,,to ,,us8 ,most of the writers on the ,plurality of ,worlds1 from ,fontenelle to ,proctor1 taking into consideration the enormous number of the stars and their apparent uselessness to our world1 have assumed that many of them must have systems of planets circling round them1 and that some of these planets1 at all events1 must possess inhabitants1 some1 perhaps1 lower1 but others no doubt higher than ourselves4 ,one of our well-known modern astronomers1 writing only ten years ago1 adopts the same view4 ,he says3 ',the suns which we call stars were clearly not created for our benefit4 ,they are of very little practical use to the earth's inhabitants4 ,they give us very little light2 an additional small satellite--one considerably smaller than the moon--would have been much more useful in this respect than the millions of stars revealed by the telescope4 ,they must therefore have been formed for some other purpose4444 ,we may therefore conclude1 with a high degree of probability1 that the stars--at least those with spectra of the solar type--form centres of planetary systems somewhat similar to our own4'#ba ,the author then discusses the conditions necessary for life analogous to that of our earth1 as regards temperature1 rotation1 mass1 atmosphere1 water1 etc41 and he is the only writer ,i have met with who has considered these conditions2 but he touches on them very briefly1 and he arrives at the conclusion that1 in the case of the stars of solar type1 it is probable that one planet1 situated at a proper distance1 would be fitted to support life4 ,he estimates roughly that there are about ten million stars of this type1 that is1 closely resembling our sun1 and that if only one in ten of these has a planet at the proper distance and properly constituted in other respects1 there will be one million worlds fitted for the support of animal life4 ,he therefore concludes that there are probably many stars having life-bearing planets revolving round them4 ,there are1 however1 many considerations not taken account of by this writer which tend to reduce very considerably the above estimate4 ,it is now known that immense numbers of the stars of smaller magnitudes are nearer to us than are the majority of the stars of the first and second magnitudes1 so that it is probable that these1 as well as a considerable proportion of the very faint telescopic stars1 are really of small dimensions4 ,we have evidence that many of the brightest stars are much larger than our sun1 but there are probably ten times as many that are much smaller4 ,we have seen that the whole of the past light and heat-giving duration of our sun has1 according to the best authorities1 been only just sufficient for the development of life upon the earth4 ,but the duration of a sun's heat-giving power will depend mainly upon its mass1 together with its constituent elements4 ,suns which are much smaller than ours are1 therefore1 from that cause alone1 unsuited to give adequate light and heat for a sufficient time1 and with sufficient uniformity1 for life-development on planets1 even if they possess any at the right distance1 and with the extensive series of nicely adjusted conditions which ,i have shown to be necessary4 ,again1 we must1 probably1 rule out as unfitted for life-development the whole region of the ,milky ,way1 on account of the excessive forces there in action1 as shown by the immense size of many of the stars1 their enormous heat-giving power1 the crowding of stars and nebulous matter1 the great number of star-clusters1 and1 especially1 because it is the region of 'new stars1' which imply collisions of masses of matter sufficiently large to become visible from the immense distance we are from them1 but yet excessively small as compared with suns the duration of whose light is to be measured by millions of years4 ,hence the ,milky ,way is the theatre of extreme activity and motion2 it is comparatively crowded with matter undergoing continual change1 and is therefore not sufficiently stable for long periods to be at all likely to possess habitable worlds4 ,we must1 therefore1 limit our possible planetary systems suitable for life-development1 to stars situated inside the circle of the ,milky ,way and far removed from it--that is1 to those composing the solar cluster4 ,these have been variously estimated to consist of a few hundred or many thousand stars--at all events to a very small number as compared with the 'hundreds of millions' in the whole stellar universe4 ,but even here we find that only a portion are probably suitable4 ,professor ,newcomb arrives at the conclusion--as have some other astronomers--that the stars in general have a much smaller mass in proportion to the light they give than our sun has2 and1 after an elaborate discussion1 he finally concludes that the brighter stars are1 on the average1 much less dense than our sun4 ,in all probability1 therefore1 they cannot give light and heat for so long a period1 and as this period in the case of our sun has only been just sufficient1 the number of suns of the solar type and of a sufficient mass may be very limited4 ,yet further1 even among stars having a similar physical constitution to our sun and of an equal or greater mass1 only a portion of their period of luminosity would be suitable for the support of planetary life4 ,while they are in process of formation by accretions of solid or gaseous masses1 they would be subject to such fluctuations of temperature1 and to such catastrophic outbursts when any larger mass than usual was drawn towards them1 that the whole of this period--perhaps by far the longest portion of their existence--must be left out of the account of planet-producing suns4 ,yet all these are to us stars of various degrees of brilliancy4 ,it is almost certain that it is only when the growth of a sun is nearly completed1 and its heat has attained a maximum1 that the epoch of life-development is likely to begin upon any planets it may possess at the most suitable distance1 and upon which all the requisite conditions should be present4 ,it may be said that there are great numbers of stars beyond our solar cluster and yet within the circle of the ,milky ,way1 as well as others towards the poles of the ,milky ,way1 which ,i have not here referred to4 ,but of these regions very little is known1 because it is impossible to tell whether stars in these directions are situated in the outer portion of the solar cluster or in the regions beyond it4 ,some astronomers appear to think that these regions may be nearly empty of stars1 and ,i have endeavoured to represent what seems to be the general view on this very difficult subject in the two diagrams of the stellar universe at pp4 #cjj1 #cja4 ,the regions beyond our cluster and above or below the plane of the ,milky ,way are those where the small irresolvable nebul abound1 and these may indicate that sun-formation is not yet active in those regions4 ,the two charts of ,nebul and ,clusters at the end of the volume illustrate1 and perhaps tend to support this view4 ,,double ,,and ,,multiple ,,star ,,systems ,we have already seen1 in our sixth chapter1 how rapid and extraordinary has been the discovery of what are termed spectroscopic binaries--pairs of stars so close together as to appear like a single star in the most powerful telescopes4 ,the systematic search for such stars has only been carried on for a few years1 yet so many have been already found1 and their numbers are increasing so rapidly1 as to quite startle astronomers4 ,one of the chief workers in this field1 ,professor ,campbell of the ,lick ,observatory1 has stated his opinion that1 as accuracy of measurement increases1 these discoveries will go on till--'the star that is not a spectroscopic binary will prove to be the rare exception1'--and other astronomers of eminence have expressed similar views4 ,but these close revolving star-systems are generally admitted to be out of the category of life-producing suns4 ,the tidal disturbances mutually produced must be enormous1 and this must be inimical to the development of planets1 unless they were very close to each sun1 and thus in the most unfavourable position for life4 ,we thus see that the result of the most recent researches among the stars is entirely opposed to the old idea that the countless myriads of stars all had planets circulating round them1 and that the ultimate purpose of their existence was1 that they should be supporters of life1 as our sun is the supporter of life upon the earth4 ,so far is this from being the case1 that vast numbers of stars have to be put aside as wholly unfitted for such a purpose2 and when by successive eliminations of this nature we have reduced the numbers which may possibly be available to a few millions1 or even to a few thousands1 there comes the last startling discovery1 that the entire host of stars is found to contain binary systems in such rapidly increasing numbers1 as to lead some of the very first astronomers of the day to the conclusion that single stars may someday be found to be the rare exception6 ,but this tremendous generalisation would1 at one stroke1 sweep away a large proportion of the stars which other successive disqualifications had spared1 and thus leave our sun1 which is certainly single1 and perhaps two or three companion orbs1 alone among the starry host as possible supporters of life on some one of the planets which circulate around them4 ,but we do not really know that any such suns exist4 ,if they exist we do not know that they possess planets4 ,if any do possess planets these may not be at the proper distance1 or be of the proper mass1 to render life possible4 ,if these primary conditions should be fulfilled1 and if there should possibly be not only one or two1 but a dozen or more that so far fulfil the first few conditions which are essential1 what probability is there that all the other conditions1 all the other nice adaptations1 all the delicate balance of opposing forces that we have found to prevail upon the earth1 and whose combination here is due to exceptional conditions which exist in the case of no other known planet--should all be again combined in some of the possible planets of these possibly existing suns8 ,i submit that the probability is now all the other way4 ,so long as we could assume that all the stars might be1 in all essentials1 like our sun1 it seemed almost ludicrous to suppose that our sun alone should be in a position to support life4 ,but when we find that enormous classes like the gaseous stars of small density1 the solar stars while increasing in size and temperature1 the stars which are much smaller than our sun1 the nebulous stars1 probably all the stars of the ,milky ,way1 and lastly that enormous class of spectroscopic doubles--veritable ,aaron's rods which threaten to swallow up all the rest--that all these are for various reasons unlikely to have attendant planets adapted to develop life1 then the probabilities seem to be enormously against there being any considerable number of suns possessing attendant habitable earths4 ,just as the habitability of all the planets and larger satellites1 once assumed as so extremely probable as to amount almost to a certainty1 is now generally given up1 so that in speculating on life in stellar systems ,mr4 ,gore assumes that only one planet to each sun can be habitable2 in like manner it may1 and ,i believe will1 turn out1 that of all the myriad stars1 the more we learn about them1 the smaller and smaller will become the scanty residue which1 with any probability1 we can suppose to illuminate and vivify habitable earths4 ,and when with this scanty probability we combine the still scantier probability that any such planet will possess simultaneously1 and for a sufficiently long period1 all the highly complex and delicately balanced conditions known to be essential for a full life-development1 the conception that on this earth alone has such development been completed will not seem so wildly improbable a conjecture as it has hitherto been held to be4 ,,are ,,the ,,stars ,,beneficial ,,to ,,us8 ,when ,i suggested in my first publication on this subject that some emanations from the stars might be beneficial or injurious1 and that a central position might be essential in order to render these emanations equable1 one of my astronomical critics laughed the idea to scorn1 and declared that 'we might wander into outer space without losing anything more serious than we lose when the night is cloudy and we cannot see the stars4'#bb ,how my critic knows that this is so he does not tell us4 ,he states it positively1 with no qualification1 as if it were an established fact4 ,it may be as well to inquire1 therefore1 if there is any evidence bearing upon the point at issue4 ,astronomers are so fully occupied with the vast number and variety of the phenomena presented by the stellar universe and the various difficult problems arising therefrom1 that many lesser but still interesting inquiries have necessarily received little attention4 ,such a minor problem is the determination of how much heat or other active radiation we receive from the stars2 yet a few observations have been made with results that are of considerable interest4 ,in the years #aijj and #aija ,mr4 ,e4,f4 ,nichols of the ,yerkes ,observatory made a series of experiments with a radiometer of special construction1 to determine the heat emitted by certain stars4 ,the result arrived at was1 that ,vega gave about #a_/bjjjjjjjj of the heat of a candle at one metre distance1 and ,arcturus about #b4b times as much4 ,in #ahie and #ahif ,mr4 ,g4,m4 ,minchin made a series of experiments on the ,electrical ,measurement of ,starlight1 by means of a photo-electric cell of peculiar construction which is sensitive to the whole of the rays in the spectrum1 and also to some of the ultra-red and ultra-violet rays4 ,combined with this was a very delicate electrometer4 ,the telescope employed to concentrate the light was a reflector of two feet aperture4 ,mr4 ,minchin was assisted in the experiments by the late ,professor ,g4,f4 ,fitzgerald1 ,f4,r4,s41 of ,trinity ,college1 ,dublin1 which may be considered guarantee of the accuracy of the observations4 ,the following are the chief results obtained3-- ---------------------------------------- ------------------------------- ---------------------------------------- ---------------------------- ,deflection ,light ,source of ,light4 in in ,e4 ,m4 ,f4 ,millimetres ,candles4 ,volts4 ---------------------------------------- ---------------------------- #ahif ,candle at #aj feet distance #ah4gj ,betelgeuse 7#j4i mag47 #ab4hj #j4fhe #j4jbf ,aldebaran 7#a4a mag47 #e4ba #j4bgi #j4jab ,procyon 7#j4e mag7 #d4hi #j4bfa #j4jaa ,alpha ,cygni 7#a4c mag47 #d4ij #j4bfb #j4jaa ,polaris 7#b4a mag47 #c4aj #j4aff #j4jjg ---------------------------------------- ---------------------------- #a volt4 #dcb4jj #ahie ,arcturus 7#j4c mag47 #h4b #a4ja #j4jai ,vega 7#j4a mag7 #aa4e #a4db #j4jbf ,candle at #aj feet #h4a ---------------------------------------- ---------------------------- ,n4,b4--,the standard candle shone directly on the cell1 whereas the star's light was concentrated by a #b-;foot mirror4 ,the sensitive surface on which the light of the stars was concentrated was #a_/bj inch in diameter4 ,we must therefore diminish the amount of candle light in this table in the proportion of the square of the diameter of the mirror 7in #a_/bj of an inch7 to one1 equal to #a_/bcjdjj4 ,if we make the necessary reduction in the case of ,vega1 and also equalise the distance at which the candle was placed1 we find the following result3-- ,observer4 ,star4 ,candle power at #aj ft4 ,minchin4 ,vega #a_/afbbej ,nichols4 #a_/bbjjjjjj ,this enormous difference in the result is no doubt largely due to the fact that ,mr4 ,nichols's apparatus measured heat alone1 whereas ,mr4 ,minchin's cell measured almost all the rays4 ,and this is further shown by the fact that1 whereas ,mr4 ,nichols found ,arcturus a red star1 hotter than ,vega a white one1 ,mr4 ,minchin1 measuring also the light-giving and some of the chemical rays1 found ,vega considerably more energetic than ,arcturus4 ,these comparisons also suggest that other modes of measurement might give yet higher results1 but it will no doubt be urged that such minute effects must necessarily be quite inoperative upon the organic world4 ,there are1 however1 some considerations which tend the other way4 ,mr4 ,minchin remarks on the unexpected fact that ,betelgeuse produces more than double the electrical energy of ,procyon1 a much brighter star4 ,this indicates that many of the stars of smaller visual magnitudes may give out a large amount of energy1 and it is this energy1 which we now know can take many strange and varied forms1 that would be likely to influence organic life4 ,and as to the quantity being too minute to have any effect1 we know that the excessively minute amount of light from the very smallest telescopic stars produces such chemical changes on a photographic plate as to form distinct images1 with comparatively small lenses or reflectors and with an exposure of two or three hours4 ,and if it were not that the diffused light of the surrounding sky also acts upon the plate and blurs the faint images1 much smaller stars could be photographed4 ,we know that not all the rays1 but a portion only1 are capable of producing these effects2 we know also that there are many kinds of radiation from the stars1 and probably some yet undiscovered comparable with the ,x rays and other new forms of radiation4 ,we must also remember the endless variety and the extreme instability of the protoplasmic products in the living organism1 many of which are perhaps as sensitive to special rays as is the photographic plate4 ,and we are not here limited to action for a few minutes or a few hours1 but throughout the whole night and day1 and continued whenever the sky is clear for months or years4 ,thus the cumulative effect of these very weak radiations may become important4 ,it is probable that their action would be most influential on plants1 and here we find all the conditions requisite for its accumulation and utilisation in the large amount of leaf-surface exposed to it4 ,a large tree must present some hundreds of superficial feet of receptive surface1 while even shrubs and herbs often have a leaf-area of greater superficial extent than the object-glasses of our largest telescopes4 ,some of the highly complex chemical processes that go on in plants may be helped by these radiations1 and their action would be increased by the fact that1 coming from every direction over the whole surface of the heavens1 the rays from the stars would be able to reach and act upon every leaf of the densest masses of foliage4 ,the large amount of growth that takes place at night may be in part due to this agency4 ,of course all this is highly speculative2 but ,i submit1 in view of the fact that the light of the very faintest stars does produce distinct chemical changes1 that even the very minute heat-effects are measureable1 as well as the electro-motive forces caused by them2 and further1 that when we consider the millions1 perhaps hundreds of millions of stars1 all acting simultaneously on any organism which may be sensitive to them1 the supposition that they do produce some effect1 and possibly a very important effect1 is not one to be summarily rejected as altogether absurd and not worth inquiring into4 ,it is not1 however1 these possible direct actions of the stars upon living organisms to which ,i attach much weight as regards our central position in the stellar universe4 ,further consideration of the subject has convinced me that the fundamental importance of that position is a physical one1 as has already been suggested by ,sir ,norman ,lockyer and some other astronomers4 ,briefly1 the central position appears to be the only one where suns can be sufficiently stable and long-lived to be capable of maintaining the long process of life-development in any of the planets they may possess4 ,this point will be further developed in the next 7and concluding7 chapter4 ,,footnotes3 #ba ,the ,worlds of ,space1 by ,j4,e4 ,gore1 chapter iii4 #bb ,the ,fortnightly ,review1 ,april #aijc1 p4 #fj4 ,,chapter ,,xvi ,,stability ,,of ,,the ,,star-,,system3 ,,importance ,,of ,,our ,,central ,,position3 ,,summary ,,and ,,conclusion ,one of the greatest difficulties with regard to the vast system of stars around us is the question of its permanence and stability1 if not absolutely and indefinitely1 yet for periods sufficiently long to allow for the many millions of years that have certainly been required for our terrestrial life-development4 ,this period1 in the case of the earth1 as ,i have sufficiently shown1 has been characterised throughout by extreme uniformity1 while a continuance of that uniformity for a few millions of years in the future is almost equally certain4 ,but our mathematical astronomers can find no indications of such stability of the stellar universe as a whole1 if subject to the law of gravitation alone4 ,in reply to some questions on this point1 my friend ,professor ,george ,darwin writes as follows3--',a symmetrical annular system of bodies might revolve in a circle with or without a central body4 ,such a system would be unstable4 ,if the bodies are of unequal masses and not symmetrically disposed1 the break-up of the system would probably be more rapid than in the ideal case of symmetry4' ,this would imply that the great annular system of the ,milky ,way is unstable4 ,but if so1 its existence at all is a greater mystery than ever4 ,although in detail its structure is very irregular1 as a whole it is wonderfully symmetrical2 and it seems quite impossible that its generally circular ring-like form can be the result of the chance aggregation of matter from any pre-existing different form4 ,star-clusters are equally unstable1 or1 rather1 nothing is known or can be predicated about their stability or instability1 according to ,professors ,newcomb and ,darwin4 ,mr4 ,e4,t4 ,whittaker 7,secretary to the ,royal ,astronomical ,society71 to whom ,professor ,g4 ,darwin sent my questions1 writes3--',i doubt whether the principal phenomena of the stellar universe are consequences of the law of gravitation at all4 ,i have been working myself at spiral nebul1 and have got a first approximation to an explanation--but it is electro-dynamical and not gravitational4 ,in fact1 it may be questioned whether1 for bodies of such tremendous extent as the ,milky ,way or nebul1 the effect which we call gravitation is given by ,newton's law2 just as the ordinary formul of electrostatic attraction break down when we consider charges moving with very great velocities4' ,accepting these statements and opinions of two mathematicians who have given special attention to similar problems1 we need not limit ourselves to the laws of gravitation as having determined the present form of the stellar universe2 and this is the more important because we may thus escape from a conclusion which many astronomers seem to think inevitable1 viz4 that the observed proper motions of the stars cannot be explained by the gravitative forces of the system itself4 ,in chapter ,,viii4 of this work ,i have quoted ,professor ,newcomb's calculation as to the effect of gravitation in a universe of #ajj million stars1 each five times the mass of our sun1 and spread over a sphere which it would take light #cj1jjj years to cross2 then1 a body falling from its outer limits to the centre could at the utmost acquire a velocity of twenty-five miles a second2 and therefore1 any body in any part of such a universe having a greater velocity would pass away into infinite space4 ,now1 as several stars have1 it is believed1 much more than this velocity1 it follows not only that they will inevitably escape from our universe1 but that they do not belong to it1 as their great velocity must have been acquired elsewhere4 ,this seems to have been the idea of the astronomer who stated that1 even at the very moderate speed of our sun1 we should in five million years be deep in the actual stream of the ,milky ,way4 ,to this ,i have already sufficiently replied2 but ,i now wish to bring before my readers an excellent illustration of the importance of the late ,professor ,huxley's remark1 that the results you got out of the 'mathematical mill' depend entirely on what you put into it4 ,in the ,philosophical ,magazine 7,january #aijb7 is a remarkable article by ,lord ,kelvin1 in which he discusses the very same problem as that which ,professor ,newcomb had discussed at a much earlier date1 but1 starting from different assumptions1 equally based on ascertained facts and probabilities deduced from them1 brings out a very different result4 ,lord ,kelvin postulates a sphere of such a radius that a star at its confines would have a parallax of one-thousandth part of a second 7#j4jja71 equivalent to #cbae light-years4 ,uniformly distributed through this sphere there is matter equal in mass to #ajjj million suns like ours4 ,if this matter becomes subject to gravitation1 it all begins to move at first with almost infinite slowness1 especially near its centre2 but nevertheless1 in twenty-five million years many of these suns would have acquired velocities of from twelve to twenty miles a second1 while some would have less and some probably more than seventy miles a second4 ,now such velocities as these agree generally with the measured velocities of the stars1 hence ,lord ,kelvin thinks there may be as much matter as #ajjj million suns within the above-named distance4 ,he then states that if we suppose there to be #aj1jjj million suns within the same sphere1 velocities would be produced very much greater than the known star-velocities2 hence it is probable that there is very much less matter than #aj1jjj million times the sun's mass4 ,he also states that if the matter were not uniformly distributed within the sphere1 then1 whatever was the irregularity1 the acquired motions would be greater2 again indicating that the #ajjj million suns would be ample to produce the observed effects of stellar motion4 ,he then calculates the average distance apart of each of the #ajjj million stars1 which he finds to be about #cjj millions of millions of miles4 ,now the nearest star to our sun is about twenty-six million million of miles distant1 and1 as the evidence shows1 is situated in the denser part of the solar cluster4 ,this gives ample allowance for the comparative emptiness of the space between our cluster and the ,milky ,way1 as well as of the whole region towards the poles of the ,milky ,way 7as shown by the diagrams in chapter ,,iv471 while the comparative density of extensive portions of the ,galaxy itself may serve to make up the average4 ,now1 previous writers have come to a different conclusion from the same general line of argument1 because they have started with different assumptions4 ,professor ,newcomb1 whose statement made some years back is usually followed1 assumed #ajj million stars each five times as large as our sun1 equal to #ejj million suns in all1 and he distributed them equally throughout a sphere #cj1jjj light-years in diameter4 ,thus he has half the amount of matter assumed by ,lord ,kelvin1 but nearly five times the extent1 the result being that gravity could only produce a maximum speed of twenty-five miles a second2 whereas on ,lord ,kelvin's assumption a maximum speed of seventy miles a second would be produced1 or even more4 ,by this latter calculation we find no insuperable difficulty in the speed of any of the stars being beyond the power of gravitation to produce1 because the rates here given are the direct results of gravitation acting on bodies almost uniformly distributed through space4 ,irregular distribution1 such as we see everywhere in the universe1 might lead to both greater and less velocities2 and if we further take account of collisions and near approaches of large masses resulting in explosive disruptions1 we might have almost any amount of motion as the result1 but as this motion would be produced by gravitation within the system1 it could equally well be controlled by gravitation4 ,illustration3 ,,diagram ,,of ,,stellar ,,universe 7,plan74 #a4 ,central part of ,solar ,cluster4 #b4 ,sun's ,orbit 7,black spot74 #c4 ,outer limit of ,solar ,cluster4 #d4 ,milky ,way4 ,in order that my readers may better understand the calculations of ,lord ,kelvin1 and also the general conclusions of astronomers as to the form and dimensions of the stellar universe1 ,i have drawn two diagrams1 one showing a plan on the central plane of the ,milky ,way1 the other a section through its poles4 ,both are on the same scale1 and they show the total diameter across the ,milky ,way as being #cfjj light-years1 or about half that postulated by ,lord ,kelvin for his hypothetical universe4 ,i do this because the dimensions given by him are those which are sufficient to lead to motions near the centre such as the stars now possess in a minimum period of twenty-five million years after the initial arrangement he supposes1 at which later epoch which we are now supposed to have reached1 the whole system would of course be greatly reduced in extent by aggregations towards and near the centre4 ,these dimensions also seem to accord sufficiently with the actual distances of stars as yet measured4 ,the smallest parallax which has been determined with any certainty1 according to ,professor ,newcomb's list1 is that of ,gamma ,cassiopei1 which is one-hundredth of a second 7#j4ja71 while ,lord ,kelvin gives none smaller than #j4jb1 and these will all be included within the solar cluster as ,i have shown it4 ,illustration3 ,,diagram ,,of ,,stellar ,,universe 7,section74 ,section through ,poles of ,milky ,way4 ,it must be clearly understood that these two illustrations are merely diagrams to show the main features of the stellar universe according to the best information available1 with the proportionate dimensions of these features1 so far as the facts of the distribution of the stars and the views of those astronomers who have paid most attention to the subject can be harmonised4 ,of course it is not suggested that the whole arrangement is so regular as here shown1 but an attempt has been made by means of the dotted shading to represent the comparative densities of the different portions of space around us1 and a few remarks on this point may be needed4 ,the solar cluster is shown very dense at the central portion1 occupying one-tenth of its diameter1 and it is near the outside of this dense centre that our sun is supposed to be situated4 ,beyond this there seems to be almost a vacuity1 beyond which again is the outer portion of the cluster consisting of comparatively thinly scattered stars1 thus forming a kind of ring-cluster1 resembling in shape the beautiful ring-nebula in ,lyra1 as has been suggested by several astronomers4 ,there is some direct evidence for this ring-form4 ,professor ,newcomb in his recent book on ,the ,stars gives a list of all stars of which the parallax is fairly well known4 ,these are sixty-nine in number2 and on arranging them in the order of the amount of their parallax1 ,i find that no less than thirty-five of them have parallaxes between #j4a and #j4d of a second1 thus showing that they constitute part of the dense central mass2 while three others1 from #j4d to #j4ge1 indicate those which are our closest companions at the present time1 but still at an enormous distance4 ,those which have parallaxes of less than the tenth and down to one-hundredth of a second are only thirty-one in all2 but as they are spread over a sphere ten times the diameter1 and therefore a thousand times the cubic content of the sphere containing those above one-tenth of a second1 they ought to be immensely more numerous even if very much more thinly scattered4 ,the interesting point1 however1 is1 that till we get down to a parallax of #j4jf1 there are only three stars as yet measured1 whereas those between #j4jb and #j4jf1 an equal range of parallax1 are twenty-six in number1 and as these are scattered in all directions they indicate an almost vacant space followed by a moderately dense outer ring4 ,in the enormous space between our cluster and the ,milky ,way1 and also above and below its plane to the poles of the ,galaxy1 stars appear to be very thinly scattered1 perhaps more densely in the plane of the ,milky ,way than above and below it where the irresolvable nebul are so numerous2 and there may not improbably be an almost vacant space beyond our cluster for a considerable distance1 as has been supposed1 but this cannot be known till some means are discovered of measuring parallaxes of from one-hundredth to one five-hundredth of a second4 ,these diagrams also serve to indicate another point of considerable importance to the view here advocated4 ,by placing the solar system towards the outer margin of the dense central portion of the solar cluster 7which may very possibly include a large proportion of dark stars and thus be much more dense towards the centre than it appears to us71 it may very well be supposed to revolve1 with the other stars composing it1 around the centre of gravity of the cluster1 as the force of gravity towards that centre might be perhaps twenty or a hundred times greater than towards the very much less dense and more remote outer portions of the cluster4 ,the sun1 as indicated on the diagrams1 is about thirty light-years from that centre1 corresponding to a parallax of a little more than one-tenth of a second1 and an actual distance of #aij millions of millions of miles1 equal to about #gj1jjj times the distance of the sun from ,neptune4 ,yet we see that this position is so little removed from the exact centre of the whole stellar universe1 that if any beneficial influences are due to that central position in regard to the ,galaxy1 it will receive them perhaps to as full an extent as if situated at the actual centre4 ,but if it is situated as here shown1 there is no further difficulty as to its proper motion carrying it from one side to the other of the ,milky ,way in less time than has been required for the development of life upon the earth4 ,and if the solar cluster is really sub-globular1 and sufficiently condensed to serve as a centre of gravity for the whole of the stars of the cluster to revolve around1 all the component stars which are not situated in the plane of its equator 7and that of the ,milky ,way7 must revolve obliquely at various angles up to an angle of #ij4 ,these numerous diverging motions1 together with the motions of the nearer stars outside the cluster1 some of which may revolve round other centres of gravity made up largely of dark bodies1 would perhaps sufficiently account for the apparent random motions of so many of the stars4 ,,uniform ,,heat-,,supply ,,due ,,to ,,central ,,position ,we now come to a point of the greatest interest as regards the problem we are investigating4 ,we have seen how great is the difference in the estimates of geologists and those of physicists as to the time that has elapsed during the whole development of life4 ,but the position we have now found for the sun1 in the outer portion of the central star-cluster1 may afford a clue to this problem4 ,what we require is1 some mode of keeping up the sun's heat during the enormous geological periods in which we have evidence of a wonderful uniformity in the earth's temperature1 and therefore in the sun's heat-emission4 ,the great central ring-cluster with its condensed central mass1 which presumably has been forming for a much longer period than our sun has been giving heat to the earth1 must during all this time have been exerting a powerful attraction on the diffused matter in the spaces around it1 now apparently almost void as compared with what they may have been4 ,some scanty remnants of that matter we see in the numerous meteoric swarms which have been drawn into our system4 ,a position towards the outside of this central aggregation of suns would evidently be very favourable for the growth by accretion of any considerable mass4 ,the enormous distance apart of the outer components 7the outer ring7 of the cluster would allow a large amount of the inflowing meteoritic matter to escape them1 and the larger suns situated near the surface of the inner dense cluster would draw to themselves the greater part of this matter4#bc ,the various planets of our system were no doubt built up from a portion of the matter that flowed in near the plane of the ecliptic1 but much of that which came from all other directions would be drawn towards the sun itself or to its neighbouring suns4 ,some of this would fall directly into it2 other masses coming from different directions and colliding with each other would have their motion checked1 and thus again fall into the sun2 and so long as the matter falling in were not in too large masses1 the slow additions to the sun's bulk and increase of its heat would be sufficiently gradual to be in no way prejudicial to a planet at the earth's distance4 ,the main point ,i wish to suggest here is1 that by far the greater portion of the matter of the whole stellar universe has1 either through gravitation or in combination with electrical forces1 as suggested by ,mr4 ,whittaker1 become drawn together into the vast ring-formed system of the ,milky ,way1 which is1 presumably1 slowly revolving1 and has thus been checked in its original inflow toward the centre of mass of the stellar universe4 ,it has also probably drawn towards itself the adjacent portions of the scattered material in the spaces around it in all directions4 ,had the vast mass of matter postulated by ,lord ,kelvin acquired no motion of revolution1 but have fallen continuously towards the centre of mass1 the motions developed when the more distant bodies approached that centre would have been extremely rapid2 while1 as they must have fallen in from every direction1 they would have become more and more densely aggregated1 and collisions of the most catastrophic nature would frequently have occurred1 and this would have rendered the central portion of the universe the least stable and the least fitted to develop life4 ,but1 under the conditions that actually prevail1 the very reverse is the case4 ,the quantity of matter remaining between our cluster and the ,milky ,way being comparatively small1 the aggregation into suns has gone on more regularly and more slowly4 ,the motions acquired by our sun and its neighbours have been rendered moderate by two causes3 7#a7 their nearness to the centre of the very slowly aggregating cluster where the motion due to gravitation is least in amount2 and 7#b7 the slight differential attraction away from the centre by the ,milky ,way on the side nearest to us4 ,again1 this protective action of the ,milky ,way has been repeated1 on a smaller scale1 by the formation of the outer ring of the solar cluster1 which has thus preserved the inner central cluster itself from a too abundant direct inflow of large masses of matter4 ,but although the matter composing the outer portion of the original universe has been to a large extent aggregated into the vast system of the ,milky ,way1 it seems probable1 perhaps even certain1 that some portion would escape its attractive forces and would pass through its numerous open spaces--indicated by the dark rifts1 channels1 and patches1 as already described--and thus flow on unchecked towards the centre of mass of the whole system4 ,the quantity of matter thus reaching the central cluster from the enormously remote spaces beyond the ,milky ,way might be very small in comparison with what was retained to build up that wonderful star-system2 but it might yet be so large in total amount as to play an important part in the formation of the central group of suns4 ,it would probably flow inwards almost continuously1 and when it ultimately reached the solar cluster1 it would have attained a very high velocity4 ,if1 therefore1 it were widely diffused1 and consisted of masses of small or moderate size as compared with planets or stars1 it would furnish the energy requisite for bringing these slowly aggregating stars to the required intensity of heat for forming luminous suns4 ,here1 then1 ,i think1 we have found an adequate explanation of the very long-continued light and heat-emitting capacity of our sun1 and probably of many others in about the same position in the solar cluster4 ,these would at first gradually aggregate into considerable masses from the slowly moving diffused matter of the central portions of the original universe2 but at a later period they would be reinforced by a constant and steady inrush of matter from its very outer regions1 and therefore possessing such high velocities as to materially aid in producing and maintaining the requisite temperature of a sun such as ours1 during the long periods demanded for continuous life-development4 ,the enormous extension and mass of the original universe of diffused matter 7as postulated by ,lord ,kelvin7 is thus seen to be of the greatest importance as regards this ultimate product of evolution1 because1 without it1 the comparatively slow-moving and cool central regions might not have been able to produce and maintain the requisite energy in the form of heat2 while the aggregation of by far the larger portion of its matter in the great revolving ring of the galaxy was equally important1 in order to prevent the too great and too rapid inflow of matter to these favoured regions4 ,it appears1 then1 that if we admit as probable some such process of development as ,i have here indicated1 we can dimly see the bearing of all the great features of the stellar universe upon the successful development of life4 ,these are1 its vast dimensions2 the form it has acquired in the mighty ring of the ,milky ,way2 and our position near to1 but not exactly in1 its centre4 ,we know that the star-system has acquired these forms1 presumably from some simple and more diffused condition4 ,we know that we are situated near the centre of this vast system4 ,we know that our sun has emitted light and heat1 almost uniformly1 for periods incompatible with rapid aggregation and the equally rapid cooling which physicists consider inevitable4 ,i have here suggested a mode of development which would lead to a very slow but continuous growth of the more central suns2 to an excessively long period of nearly stationary heat-giving power2 and lastly1 an equally long period of very gradual cooling--a period the commencement of which our sun may have just entered upon4 ,descending now to terrestrial physics1 ,i have shown that1 owing to the highly complex nature of the adjustments required to render a world habitable and to retain its habitability during the ons of time requisite for life-development1 it is in the highest degree improbable that the required conditions and adaptations should have occurred in any other planets of any other suns1 which might occupy an equally favourable position with our own1 and which were of the requisite size and heat-giving power4 ,lastly1 ,i submit that the whole of the evidence ,i have here brought together leads to the conclusion that our earth is almost certainly the only inhabited planet in our solar system2 and1 further1 that there is no inconceivability--no improbability even--in the conception that1 in order to produce a world that should be precisely adapted in every detail for the orderly development of organic life culminating in man1 such a vast and complex universe as that which we know exists around us1 may have been absolutely required4 ,,summary ,,of ,,argument ,as the last ten chapters of this volume embody a connected argument leading to the conclusion above stated1 it may be useful to my readers to summarise rather fully the successive steps of this argument1 the facts on which it rests1 and the various subsidiary conclusions arrived at4 7#a7 ,one of the most important results of modern astronomy is to have established the unity of the vast stellar universe which we see around us4 ,this rests upon a great mass of observations1 which demonstrate the wonderful complexity in detail of the arrangement and distribution of stars and nebul1 combined with a no less remarkable general symmetry1 indicating throughout a single inter-dependent system1 not a number of totally distinct systems so far apart as to have no physical relations with each other1 as was once supposed4 7#b7 ,this view is supported by numerous converging lines of evidence1 all tending to show that the stars are not infinite in number1 as was once generally believed1 and which view is even now advocated by some astronomers4 ,the very remarkable calculations of ,lord ,kelvin1 referred to in the early part of this chapter1 give a further support to this view1 since they show that if the stars extended much beyond those we see or can obtain direct knowledge of1 and with no very great change in their average distance apart1 then the force of gravitation towards the centre would have produced on the average more rapid motions than the stars generally possess4 7#c7 ,an overwhelming consensus of opinion among the best astronomers establishes the fact of our nearly central position in the stellar universe4 ,they all agree that the ,milky ,way is nearly circular in form4 ,they all agree that our sun is situated almost exactly in its medial plane4 ,they all agree that our sun1 although not situated at the exact centre of the galactic circle1 is yet not very far from it1 because there are no unmistakable signs of our being nearer to it at any one point and farther away from the opposite point4 ,thus the nearly central position of our sun in the great star-system is almost universally admitted4 ,on the question of the solar-cluster there is more difference of opinion2 though here1 again1 all are agreed that there is such a cluster4 ,its size1 form1 density1 and exact position are somewhat uncertain1 but ,i have1 as far as possible1 been guided by the best available evidence4 ,if we adopt ,lord ,kelvin's general idea of the gradual condensation of an enormous diffused mass of matter towards its common centre of gravity1 that centre would be approximately the centre of this cluster4 ,also1 as gravitational force at and near this centre would be comparatively small1 the motions produced there would be slow1 and collisions1 being due only to differential motions1 when they did occur would be very gentle4 ,we might therefore expect many dark aggregations of matter here1 which may explain why we do not find any special crowding of visible stars in the direction of this centre2 while1 as no star has a sensible disc1 the dark stars if at great distances would hardly ever be seen to occult the bright ones4 ,thus1 it seems to me1 the controlling force may be explained which has retained our sun in approximately the same orbit around the centre of gravity of this central cluster during the whole period of its existence as a sun and our existence as a planet2 and has thus saved us from the possibility--perhaps even the certainty--of disastrous collisions or disruptive approaches to which suns1 in or near the ,milky ,way1 and to a less extent elsewhere1 are or have been exposed4 ,it seems quite probable that in that region of more rapid and less controlled motions and more crowded masses of matter1 no star can remain in a nearly stable condition as regards temperature for sufficiently long periods to allow of a complete system of life-development on any planet it may possess4 7#d7 ,the various proofs are next stated that assure us of the almost complete uniformity of matter1 and of material physical and chemical laws1 throughout our universe4 ,this ,i believe no one seriously disputes2 and it is a point of the greatest importance when we come to consider the conditions required for the development and maintenance of life1 since it assures us that very similar1 if not identical1 conditions must prevail wherever organic life is or can be developed4 7#e7 ,this leads us on to the consideration of the essential characteristics of the living organism1 consisting as it does of some of the most abundant and most widely distributed of these material elements1 and being always subject to the general laws of matter4 ,the best authorities in physiology are quoted1 as to the extreme complexity of the chemical compounds which constitute the physical basis for the manifestation of life2 as to their great instability2 their wonderful mobility combined with permanence of form and structure2 and the altogether marvellous powers they possess of bringing about unique chemical transformations and of building up the most complicated structures from simple elements4 ,i have endeavoured to put the broad phenomena of vegetable and animal life in a way that will enable my readers to form some faint conception of the intricacy1 the delicacy1 and the mystery of the myriad living forms they see everywhere around them4 ,such a conception will enable them to realise how supremely grand is organic life1 and to appreciate better1 perhaps1 the absolute necessity for the numerous1 complex and delicate adaptations of inorganic nature1 without which it is impossible for life either to exist now1 or to have been developed during the immeasurable past4 7#f7 ,the general conditions which are absolutely essential for life thus manifested on our planet are then discussed1 such as1 solar light and heat2 water universally distributed on the planet's surface and in the atmosphere2 an atmosphere of sufficient density1 and composed of the several gases from which alone protoplasm can be formed2 some alternations of light and darkness1 and a few others4 7#g7 ,having treated these conditions broadly1 and explained why they are important and even indispensable for life1 we next proceed to show how they are fulfilled upon the earth1 and how numerous1 how complex1 and often how exact are the adjustments needed to bring them about1 and maintain them almost unchanged throughout the vast ons of time occupied in the development of life4 ,two chapters are devoted to this subject2 and it is believed that they contain facts that will be new to many of my readers4 ,the combinations of causes which lead to this result are so varied1 and in several cases dependent on such exceptional peculiarities of physical constitution1 that it seems in the highest degree improbable that they can all be found again combined either in the solar system or even in the stellar universe4 ,it will be well here just to enumerate these conditions1 which are all essential within more or less narrow limits3-- ,distance of planet from the sun4 ,mass of planet4 ,obliquity of its ecliptic4 ,amount of water as compared with land4 ,surface distribution of land and water4 ,permanence of this distribution1 dependent probably on the unique origin of our moon4 ,an atmosphere of sufficient density1 and of suitable component gases4 ,an adequate amount of dust in the atmosphere4 ,atmospheric electricity4 ,many of these act and react on each other1 and lead to results of great complexity4 7#h7 ,passing on to other planets of the solar system1 it is shown that none of them combine all the complex conditions which are found to work harmoniously together on the earth2 while in most cases there is some one defect which alone removes them from the category of possible life-producing and life-supporting planets4 ,among these are the small size and mass of ,mars1 being such that it cannot retain aqueous vapour2 and the fact that ,venus rotates on its axis in the same time as it takes to revolve round the sun4 ,neither of these facts was known when ,proctor wrote upon the question of the habitability of the planets4 ,all the other planets are now given up--and were given up by ,proctor himself--as possible life-bearers in their present stage2 but he and others have held that1 if not suitable now1 some of them may have been the scene of life-development in the past1 while others will be so in the future4 ,in order to show the futility of this supposition1 the problem of the duration of the sun as a stable heat-giver is discussed2 and it is shown that it is only by reducing the periods claimed by geologists and biologists for life-development upon the earth1 and by extending the time allowed by physicists to its utmost limits1 that the two claims can be harmonised4 ,it follows that the whole period of the sun's duration as a light and heat-giver has been required for the development of life upon the earth2 and that it is only upon planets whose phases of development synchronise with that of the earth that the evolution of life is possible4 ,for those whose material evolution has gone on quicker or slower1 there has not been1 or will not be1 time enough for the development of life4 7#i7 ,the problem of the stars as possibly having life-supporting planets is next dealt with1 and reasons are given why in only a minute portion of the whole is this possible4 ,even in that minute portion1 reduced probably to a few of the component suns of the solar-cluster1 a large proportion seems likely to be ruled out by being close binary systems1 and another large portion by being in process of aggregation4 ,in those remaining1 whether they may be reckoned by tens or by hundreds we cannot say1 the chances against the same complex combination of conditions as those which we find on the earth occurring on any planet of any other sun are enormously great4 7#aj7 ,i then refer1 briefly1 to some recent measurements of star-radiation1 and suggest that they may thus possibly have important effects on the development of vegetable and animal life2 and1 finally1 ,i discuss the problem of the stability of the stellar universe and the special advantage we derive from our central position1 suggested by some of the latest researches of our great mathematician and physicist--,lord ,kelvin4 ,,conclusions ,having thus brought together the whole of the available evidence bearing upon the questions treated in this volume1 ,i claim that certain definite conclusions have been reached and proved1 and that certain other conclusions have enormous probabilities in their favour4 ,the conclusions reached by modern astronomers are3 7#a7 ,that the stellar universe forms one connected whole2 and1 though of enormous extent1 is yet finite1 and its extent determinable4 7#b7 ,that the solar system is situated in the plane of the ,milky ,way1 and not far removed from the centre of that plane4 ,the earth is therefore nearly in the centre of the stellar universe4 7#c7 ,that this universe consists throughout of the same kinds of matter1 and is subjected to the same physical and chemical laws4 ,the conclusions which ,i claim to have shown to have enormous probabilities in their favour are-- 7#d7 ,that no other planet in the solar system than our earth is inhabited or habitable4 7#e7 ,that the probabilities are almost as great against any other sun possessing inhabited planets4 7#f7 ,that the nearly central position of our sun is probably a permanent one1 and has been specially favourable1 perhaps absolutely essential1 to life-development on the earth4 99 99 99 99 99 ,these latter conclusions depend upon the combination of a large number of special conditions1 each of which must be in definite relation to many of the others1 and must all have persisted simultaneously during enormous periods of time4 ,the weight to be given to this kind of reasoning depends upon a full and fair consideration of the whole evidence as ,i have endeavoured to present it in the last seven chapters of this book4 ,to this evidence ,i appeal4 99 99 99 99 99 ,this completes my work as a connected argument1 founded wholly on the facts and principles accumulated by modern science2 and it leads1 if my facts are substantially correct and my reasoning sound1 to one great and definite conclusion--that man1 the culmination of conscious organic life1 has been developed here only in the whole vast material universe we see around us4 ,i claim that this is the logical outcome of the evidence1 if we consider and weigh this evidence without any prepossessions whatever4 ,i maintain that it is a question as to which we have no right to form a priori opinions not founded upon evidence4 ,and evidence opposed to this conclusion1 or even as to its improbability1 we have absolutely none whatever4 ,but1 if we admit the conclusion1 nothing that need alarm either the scientific or the religious mind necessarily follows1 because it can be explained or accounted for in either of two distinct ways4 ,one considerable body1 including probably the majority of men of science1 will admit that the evidence does apparently lead to this conclusion1 but will explain it as due to a fortunate coincidence4 ,there might have been a hundred or a thousand life-bearing planets1 had the course of evolution of the universe been a little different1 or there might have been none at all4 ,they would probably add1 that1 as life and man have been produced1 that shows that their production was possible2 and therefore1 if not now then at some other time1 if not here then in some other planet of some other sun1 we should be sure to have come into existence2 or if not precisely the same as we are1 then something a little better or a little worse4 ,the other body1 and probably much the largest1 would be represented by those who1 holding that mind is essentially superior to matter and distinct from it1 cannot believe that life1 consciousness1 mind1 are products of matter4 ,they hold that the marvellous complexity of forces which appear to control matter1 if not actually to constitute it1 are and must be mind-products2 and when they see life and mind apparently rising out of matter and giving to its myriad forms an added complexity and unfathomable mystery1 they see in this development an additional proof of the supremacy of mind4 ,such persons would be inclined to the belief of the great eighteenth century scholar1 ,dr4 ,bentley1 that the soul of one virtuous man is of greater worth and excellency than the sun and all his planets and all the stars in the heavens2 and when they are shown that there are strong reasons for thinking that man is the unique and supreme product of this vast universe1 they will see no difficulty in going a little further1 and believing that the universe was actually brought into existence for this very purpose4 ,with infinite space around us and infinite time before and behind us1 there is no incongruity in this conception4 ,a universe as large as ours for the purpose of bringing into existence many myriads of living1 intellectual1 moral1 and spiritual beings1 with unlimited possibilities of life and happiness1 is surely not more out of proportion than is the complex machinery1 the life-long labour1 the ingenuity and invention which we have bestowed upon the production of the humble1 the trivial1 pin4 ,neither is the apparent waste of energy so great in such a universe1 comparatively1 as the millions of acorns1 produced during its life by an oak1 every one of which might grow to be a tree1 but of which only one does actually1 after several hundred years1 produce the one tree which is to replace the parent4 ,and if it is said that the acorns are food for bird and beast1 yet the spores of ferns and the seeds of orchids are not so1 and countless millions of these go to waste for every one which reproduces the parent form4 ,and all through the animal world1 especially among the lower types1 the same thing is seen4 ,for the great majority of these entities we can see no use whatever1 either of the enormous variety of the species1 or the vast hordes of individuals4 ,of beetles alone there are at least a hundred thousand distinct species now living1 while in some parts of sub-arctic ,america mosquitoes are sometimes so excessively abundant that they obscure the sun4 ,and when we think of the myriads that have existed through the vast ages of geological time1 the mind reels under the immensity of1 to us1 apparently useless life4 ,all nature tells us the same strange1 mysterious story1 of the exuberance of life1 of endless variety1 of unimaginable quantity4 ,all this life upon our earth has led up to and culminated in that of man4 ,it has been1 ,i believe1 a common and not unpopular idea that during the whole process of the rise and growth and extinction of past forms1 the earth has been preparing for the ultimate--,man4 ,much of the wealth and luxuriance of living things1 the infinite variety of form and structure1 the exquisite grace and beauty in bird and insect1 in foliage and flower1 may have been mere by-products of the grand mechanism we call nature--the one and only method of developing humanity4 ,and is it not in perfect harmony with this grandeur of design 7if it be design71 this vastness of scale1 this marvellous process of development through all the ages1 that the material universe needed to produce this cradle of organic life1 and of a being destined to a higher and a permanent existence1 should be on a corresponding scale of vastness1 of complexity1 of beauty8 ,even if there were no such evidence as ,i have here adduced for the unique position and the exceptional characteristics which distinguish the earth1 the old idea that all the planets were inhabited1 and that all the stars existed for the sake of other planets1 which planets existed to develop life1 would1 in the light of our present knowledge1 seem utterly improbable and incredible4 ,it would introduce monotony into a universe whose grand character and teaching is endless diversity4 ,it would imply that to produce the living soul in the marvellous and glorious body of man--man with his faculties1 his aspirations1 his powers for good and evil--that this was an easy matter which could be brought about anywhere1 in any world4 ,it would imply that man is an animal and nothing more1 is of no importance in the universe1 needed no great preparations for his advent1 only1 perhaps1 a second-rate demon1 and a third or fourth-rate earth4 ,looking at the long and slow and complex growth of nature that preceded his appearance1 the immensity of the stellar universe with its thousand million suns1 and the vast ons of time during which it has been developing--all these seem only the appropriate and harmonious surroundings1 the necessary supply of material1 the sufficiently spacious workshop for the production of that planet which was to produce first1 the organic world1 and then1 ,man4 ,in one of his finest passages our great world-poet gives us his conception of the grandeur of human nature--',what a piece of work is man6 ,how noble in reason6 ,how infinite in faculty6 ,in form and moving1 how express and admirable6 ,in action how like an angel6 ,in apprehension how like a god6' ,and for the development of such a being what is a universe such as ours8 ,however vast it may seem to our faculties1 it is as a mere nothing in the ocean of the infinite4 ,in infinite space there may be infinite universes1 but ,i hardly think they would be all universes of matter4 ,that would indeed be a low conception of infinite power6 ,here1 on earth1 we see millions of distinct species of animals1 millions of different species of plants1 and each and every species consisting often of many millions of individuals1 no two individuals exactly alike2 and when we turn to the heavens1 no two planets1 no two satellites alike2 and outside our system we see the same law prevailing--no two stars1 no two clusters1 no two nebul alike4 ,why then should there be other universes of the same matter and subject to the same laws--as is implied by the conception that the stars are infinite in number1 and extend through infinite space8 ,of course there may be1 and probably are1 other universes1 perhaps of other kinds of matter and subject to other laws1 perhaps more like our conceptions of the ether1 perhaps wholly non-material1 and what we can only conceive of as spiritual4 ,but1 unless these universes1 even though each of them were a million times vaster than our stellar universe1 were also infinite in number1 they could not fill infinite space1 which would extend on all sides beyond them1 so that even a million million such universes would shrink to imperceptibility when compared with the vast beyond6 ,of infinity in any of its aspects we can really know nothing1 but that it exists and is inconceivable4 ,it is a thought that oppresses and overwhelms4 ,yet many speak of it glibly as if they knew what it contains1 and even use that assumed knowledge as an argument against views that are unacceptable to themselves4 ,to me its existence is absolute but unthinkable--that way madness lies4 ',o night6 ,o stars1 too rudely jars ,the finite with the infinite6' ,i will conclude with one of the finest passages relating to the infinite that ,i am acquainted with1 from the pen of the late ,r4,a4 ,proctor3 ',inconceivable1 doubtless1 are these infinities of time and space1 of matter1 of motion1 and of life4 ,inconceivable that the whole universe can be for all time the scene of the operation of infinite power1 omnipresent1 all-knowing4 ,utterly incomprehensible how ,infinite ,purpose can be associated with endless material evolution4 ,but it is no new thought1 no modern discovery1 that we are thus utterly powerless to conceive or comprehend the idea of an ,infinite ,being1 ,almighty1 ,all-knowing1 ,omnipresent1 and ,eternal1 of whose inscrutable purpose the material universe is the unexplained manifestation4 ,science is in presence of the old1 old mystery2 the old1 old questions are asked of her--,canst thou by searching find out ,god8 ,canst thou find out the ,almighty unto perfection8 ,it is as high as heaven2 what canst thou do8 deeper than hell2 what canst thou know8 ,and science answers these questions as they were answered of old--,as touching the ,almighty we cannot find ,him out4' 99 99 99 99 99 ,the following beautiful lines--among the latest products of ,tennyson's genius--so completely harmonise with the subject-matter of the present volume1 that no apology is needed for quoting them here3-- 7,the ,question7 ,will my tiny spark of being ,wholly vanish in your deeps and heights8 ,must my day be dark by reason1 ,o ye ,heavens1 of your boundless nights1 ,rush of ,suns and roll of systems1 ,and your fiery clash of meteorites8 7,the ,answer7 ',spirit1 nearing yon dark portal ,at the limit of thy human state1 ,fear not thou the hidden purpose ,of that ,power which alone is great1 ,nor the myriad world1 ,his shadow1 ,nor the silent ,opener of the ,gate4' ,,footnote3 #bc ,since writing this chapter ,i have seen a paper by ,luigi d',auria dealing mathematically with ',stellar ,motion1 etc41' and am pleased to see that1 from quite different considerations1 he has found it necessary to place the solar system at a distance from the centre not very much more remote than the position ,i have given it4 ,he says3 ',we have good reasons to suppose that the solar system is rather near the centre of the ,milky ,way1 and as this centre would1 according to our hypothesis1 coincide with the centre of the ,universe1 the distance of #aei light years assumed is not too great1 nor can it be very much smaller4'--,journal of the ,franklin ,institute1 ,march #aijc4 ,,index ,adrianus ,tollius on stone axes1 #bjc4 ,air criminally poisoned by us1 #bfj4 ,albedo explained1 #afb4 ,algol and its companion1 #ci2 change of colour of1 #da4 ,allen1 ,prof4 ,f4,j41 on living matter1 #aic2 on importance of nitrogen1 #aie2 on physical conditions essential for life1 #aif4 ,alpha ,centauri1 nearest star1 #gd4 ,ammonia1 importance of1 to life1 #aie4 ,anaximander's cosmic theory1 #b4 ,angles of a minute and second1 #hj4 ,arcturus1 rapid motion of1 #agb4 ,argument of book1 summary of1 #caj4 ,astronomers1 the first1 #b4 ,astronomy1 the new1 #bd4 ,astrophysics1 a new science1 #cb4 ,atmosphere1 qualities requisite for life1 #baj2 requisite composition of1 #bab2 aqueous vapour in1 #bad2 and life1 #bdc2 effects of density of1 #bde2 a complex structure1 #bei2 its vital importance to us1 #bfj4 ,ball1 ,sir ,r41 on dark stars1 #adc2 ,time and ,tide1 #bcc4 ,barnham1 ,s4,w41 on double stars1 #abc4 ,blue of sky due to dust1 #bea4 ,boeddicker's map of ,milky ,way1 #afd4 ,brewster1 ,sir ,d41 against ,whewell1 #ae4 ,campbell1 ,prof41 on spectroscopic binaries1 #abe2 on uncertainty of sun's motions1 #agi2 on number of binary systems1 #bhf4 ,carbon compounds1 vast numbers of1 #aid4 ,carbonic acid gas essential for life1 #aif4 ,central position of sun1 importance of1 #cje4 ,chaldeans the first astronomers1 #b4 ,chalmers ,dr41 on plurality of worlds1 #ac4 ,chamberlin1 ,t4,c41 origin of nebul1 #abj2 on stellar disruption1 #ahf4 ,chromosphere1 the sun's1 #ajg4 ,clerke1 ,miss ,a4,m41 on limits of star system1 #ach2 on ,milky ,way1 #aeh1 #afj2 on solar cluster1 #afe2 on uncertainty of the sun's motion1 #agg4 ,climate1 persistence of mild1 #bbb4 ,clouds1 importance of1 to life1 #bdh4 ,clusters in relation to ,galaxy1 #fg4 ,comte1 on impossibility of real knowledge of the stars1 #be4 ,conclusions of the book1 #cag2 bearing of1 on science and on religion1 #cai4 ,corona of sun1 #ajh4 ,criticisms of article in ,fortnightly ,review1 #afh1 #ahj4 ,darwin1 ,prof4 ,g41 on meteoritic hypothesis1 #acc2 on origin of moon1 #bcc2 on instability of annular systems1 #bie4 ,day and night1 uses of1 #bae4 ,diagrams of star-distribution1 #fb1 #ff4 ,diffraction-gratings1 #cj4 ,disruption of stellar bodies1 #ahg4 ,doppler principle1 the1 #cg4 ,double stars1 evolution of1 #abc2 not fitted for life1 #bhf4 ,dust1 importance of1 #bdi4 ,dust-free air1 results of1 #bed4 ,earth1 first measured1 #e2 in relation to life1 #bah2 the only habitable planet1 #bfb2 cannot retain hydrogen1 #bfd2 supposed extreme conditions of1 #bga4 ,earth's mass1 how related to life1 #bfe4 ,ecliptic1 obliquity of1 in relation to life1 #bai4 ,electricity1 effects of atmospheric1 #beg2 atmospheric1 how caused1 #beh4 ,elements1 change in spectra of1 #abi2 in the sun1 #ahd2 in meteorites1 #ahe2 in organic structures1 #bja4 ,empedocles an early astronomer1 #c4 ,eudoxus on motions of planets1 #c4 ,evolution of the stars1 #abh4 ,explanations of life-processes1 #bjb4 ,facul of sun1 #aje4 ,fisher1 ,rev4 ,o41 on oceanic basins1 #bcd2 on thin sub-oceanic crust1 #bcg4 ,fizeau measures speed of light1 #gi4 ,flammarion1 ,c41 on universality of life1 #bgd1 #bha4 ,fontenelle on plurality of worlds1 #i4 ,galileo on star measurement1 #gd4 ,geological climates1 #bbb4 ,geologists on duration of sun's heat1 #bge4 ,germinal vesicle1 ,m',kendrick on1 #bjb4 ,gill1 ,sir ,d41 on systematic star-motions1 #agh4 ,globular clusters1 stability of1 #abf2 and variables1 #abg4 ,gore1 ,mr4 ,j4,e41 on stars in ,galaxy1 #fj2 on mass of binary stars1 #ig2 on remoteness of bright stars1 #adj2 on limits of star system1 #ade2 on limited number of stars1 #aea2 on life on planets of other suns1 #bhb1 #bhi4 ,gould on solar cluster1 #afe4 ,gould's map of ,milky ,way1 #afd4 ,gravitation1 motions produced by1 on ,lord ,kelvin's hypothesis1 #bih4 ,haliburton1 ,professor ,w4,d41 on proteids1 #bjj4 ,heat and cold on earth's surface1 #bjg4 ,heat-supply1 our long-continued1 accounted for1 #cje4 ,herschel1 ,sir ,j41 on ,milky ,way1 #ej2 on limits of the star-system1 #adg4 ,heliometer1 description of1 #hi4 ,huggins1 ,sir ,w41 on spectra of stars1 #cb2 measures radial motion1 #cg4 ,huxley1 ,prof41 on protoplasm1 #aih2 on duration of life1 #bgh4 ,hydrogen1 why not in atmosphere1 #bdj2 escapes from earth1 #bfd4 ,infinity1 unknowable1 #cbc2 ,proctor on1 #cbd4 ,jupiter's satellites show speed of light1 #gi4 ,kapteyn on solar cluster1 #aff4 ,kelvin1 ,lord1 on the sun's age1 #bgi2 on a suggested primitive form of star-system1 #bih4 ,kirchhoff1 discovers spectrum-analysis1 #bh4 ,laws of matter uniform throughout universe1 #ahg4 ,leaves1 importance of1 #aig4 ,lee1 ,dr41 on origin of double stars1 #abc4 ,lewis1 on remote bright stars1 #ada4 ,life1 unity of organic1 #ahi2 definitions of1 #aia2 conditions essential for1 #bjf2 water essential for1 #baj2 atmosphere for1 #baj2 dependent on temperature1 #bah2 now improbable in stars1 #bhh2 conditions essential for1 summarised1 #cad4 ,life-processes1 explanations of1 #bjb4 ,light1 velocity of measured1 #gi2 necessity of solar1 #bji2 from sky due to dust1 #beb4 ,light-journey explained1 #ge4 ,light-ratio shows stars to be limited1 #aeb4 ,living bodies1 essential points in1 #aib4 ,lockyer1 ,sir4 ,n41 on inorganic evolution1 #aag2 on evolution of stars1 #acj2 on ,milky ,way1 #aei2 on position of solar system1 #afa4 ,luigi d',auria on stellar motion1 #cjf4 ,m',kendrick1 ,prof41 on germinal vesicle1 #bjb4 ,magnetism and sun-spots1 #ajf4 ,man1 ,shakespeare on1 #cbb4 ,mars1 has no water1 #bff2 excessive temperatures on1 #bfg4 ,matter of universe uniform1 #ahc4 ,maunder on dark stars1 #adc4 ,maxwell ,hall1 ,mr41 on star-motions1 #agh4 ,measurement of star-distances1 #he2 difficulty of1 #hf4 ,mercury not habitable1 #bff4 ,meteorites1 elements in1 #ahe2 not primitive bodies1 #ahf4 ,meteoritic hypothesis1 #aac2 ,proctor on1 #aad2 explains nebul1 #aaf2 ,dr4 ,roberts on1 #aai4 ,milky ,way1 the1 #dh2 form of1 #ea1 #aei2 description of1 #eb2 telescopic view of1 #eg2 stars in relation to1 #ei2 ,mr4 ,gore on1 #fj2 density of stars in1 #fa2 clusters and nebul in relation to1 #fg2 probable distance of1 #if2 forms a great circle1 #aeg1 #afb2 ,prof4 ,newcomb on1 #aeh2 probably no life in1 #bhd2 diagrams of1 #cjj2 revolution of1 important to us1 #cjg4 ,million1 how to appreciate a1 #hb4 ,minchin1 ,g4,m41 on radiation from stars1 #bij4 ,monck1 ,mr4 ,w4,h4,s41 on non-infinity of stars1 #add2 on uncertainty of sun's motion1 #agg4 ,moon1 why no atmosphere1 #bfc4 ,moon's supposed origin1 #bcc4 ,motion1 in line of sight1 #ce4 ,motions1 imperceptible1 #ci4 ,nebul1 with gaseous spectra1 #dc2 in relation to ,galaxy1 #ff2 distribution of1 #fi2 many forms of1 #gj2 gaseous1 #ga2 meteoritic theory of1 #aaf2 planetary and annular1 #age2 ,dr4 ,roberts on spiral1 #aag1 #agd2 ,chamberlin on origin of1 #abj4 ,nebular hypothesis1 #ih1 #aaa2 objection to1 #aab4 ,newcomb1 ,prof4 ,s41 on star distribution1 #fa2 on parallax of stars1 #id2 on stability of star clusters1 #abf2 on scarcity of single stars1 #abh2 on limits of star system1 #ach2 on ,milky ,way1 #aeh1 #afj2 on solar cluster1 #afg2 on star velocities1 #aga2 on average small mass of stars1 #bhe2 on star-motions1 #big4 ,newton1 ,sir ,isaac1 on sun's habitability1 #i4 ,nichols1 ,e4,f41 on heat of stars1 #bij4 ,nitrogen1 its importance to life1 #aie4 ,non-habitability of great planets1 #bgb4 ,ocean and land1 diagram of1 #bbh4 ---- basins1 permanence of1 #bbi4 ---- ---- symmetry of1 #bch4 ---- depths1 how produced1 #bcb4 ,oceans1 effect of1 on temperature1 #bci2 curious relations of1 #bfd4 ,organic products1 diversity of1 #aie4 ,photographic astronomy1 #dc2 measures of star-distances1 #hi4 ,photosphere1 the1 #aje4 ,physicists on sun's duration1 #bgh4 ,pickering's measurements of ,algol1 #dj4 ,planets1 supposed habitability of1 #bff1 #bfi2 the great1 uninhabitable1 #bgb2 internal heat of great1 #bgc2 a last argument for habitability of1 #bgd2 have probably no life1 #cae4 ,planets' motions first explained1 #c2 mass and atmosphere1 #bfb4 ,pleiades1 number of stars in1 #fg2 a drifting cluster1 #agg4 ,plurality of worlds1 early writers on1 #i2 ,proctor on1 #ah4 ,posidonius measures the earth1 #e4 ,pritchard's photographic measures of star-distance1 #hi4 ,proctor1 ,r4,a41 on other worlds1 #ah2 on form of ,galaxy1 #ea2 on ,herschel's views1 #aja2 on stellar universe1 #ajc2 on meteoritic theory1 #aad2 on infinities1 #acf2 on star-drift1 #agf2 on life under varied conditions1 #bga2 on infinity1 #cbd4 ,proctor's ,old and ,new ,astronomy1 #df2 chart of stars1 #fj4 ,prominences of sun1 #ajg4 ,proteids1 formation of1 #aii2 ,prof4 ,haliburton on1 #bjj4 ,protoplasm1 complexity of1 #aid2 a mechanism1 #aih2 sensibility of1 to heat1 #bjh4 ,ptolemaic system of the heavens1 #d4 ,radial motion1 #ce4 ,radiation from stars1 #bij4 ,rain in the ,carboniferous age1 #bbe2 dependent on dust1 #bdi4 ,ramsay1 ,prof41 on geological climates1 #bgh4 ,ranyard1 on star-discs1 #ih2 on infinite universe1 #acg2 on mass of ,orion nebula1 #agc4 ,religious bearing of my conclusions1 #cai4 ,reproduction1 marvel of1 #bja4 ,reversing layer of sun1 #ajg4 ,roberts1 ,a4,w41 on birth of double stars1 #abc4 ---- ,dr4 ,i41 on limits of star-system1 #adh2 on spiral nebul1 #aag2 on meteoritic theory1 #aai2 photographs of nebul1 #de1 #agd4 ,roche limit explained1 #abj1 #ahg4 ,sanderson1 ,prof4 ,burdon1 on living matter1 #aib4 ,scientific and agnostic opinion on my conclusions1 #cah4 ,secchi's classification of stars1 #cc4 ,single stars perhaps rare1 #abh4 ,solar apex1 position of1 #agf4 ,solar cluster1 the1 #afe2 diagram showing1 #cjj2 evidence for1 #cjb2 importance to us1 #cjf-g1 #cab4 ,solar system1 position of1 #cjd4 ,sorby on constitution of meteorites1 #ahf4 ,spectra1 varieties of1 #cd2 of elements1 changes in1 #abi4 ,spectroscopic binaries1 abundance of1 #abe2 great numbers of1 #bhf4 ,spectrum analysis1 discovery of1 #bf4 ,spencer1 ,h41 on status of nebul1 #ajb4 ,spiral nebul1 origin of1 #abj4 ,stars1 proved to be suns1 #cb2 invisible1 #ci2 classification of1 #cc2 spectroscopic double1 #db2 distribution of the1 #dg2 number of visible1 #dh2 description of ,milky ,way1 #eb2 in relation to ,milky ,way1 #ei2 distances of1 #gd2 measurement of distance of1 #he2 mass of binary1 #ig2 evolution of double1 #abb2 spectroscopic double1 #abc2 clusters of1 #abe2 evolution of the1 #abh2 classification of1 #acj2 the hottest1 #aca2 when cooling give more heat1 #acb2 cycle of evolution and decay1 #acc2 supposed infinite number of1 #ace2 not infinite1 #ach2 law of diminishing numbers of1 #adi2 systematic motions of1 #agh2 in relation to life1 #bhb1 #bhg2 possible use of their emanations1 #bhi4 ,star-clusters and variables1 #abg4 ,star-density1 diagram of1 #ff4 ,star-drift1 ,proctor on1 #agf4 ,starlight1 electrical measure of1 #bij2 possible uses of1 #bib4 ,star-motions1 ,prof4 ,newcomb on1 #big4 ,star-system1 limited1 #ade2 stability of1 #bie2 supposed primitive form of1 #big4 ,stellar motion1 ,luigi d',auria on1 #cjf4 ---- universe1 shape of1 #di2 unity of1 #ajj2 evolution of1 #ajc2 diagrams of1 #cjj4 ,stoney1 ,dr41 on atmospheres and gravity1 #bfc4 ,sun1 a typical star1 #ajd2 brightness of1 #ajd2 heat of1 #ajd2 surface of1 #aje2 surroundings of1 #ajf-aaj2 corona of1 #ajh2 colour of1 #aaa2 elements in1 #ahd4 ,sun's distance1 measure of1 #gf4 ---- heat1 supposed limits of1 #bge4 ---- life1 all required to develop earth-life1 #bhj4 ---- motion through space1 #ia1 #afi4 ---- ---- uncertain1 #agg4 ,sun-spots1 nature of1 #aje4 ,symmetry of oceans1 cause of1 #bch4 ,temperature1 essential for life1 #bjf2 equalised by water1 #bci2 as regards life on planets1 #bfg4 ,tennyson on man and the universe1 #cbe4 ,uniformity of matter1 #ahc4 ,unity of stellar universe1 #ajj4 ,universe of stars1 how its form has affected our sun and earth1 #cjh4 ,universe not disproportionate if man is its sole product1 #cbj4 ,,venus1 radial motions of1 #ch2 diagram of transit of1 #gg2 life barely possible on1 #bff2 adverse climatic conditions of1 #bfh4 ,,water1 an essential for life1 #baj2 its amount and distribution1 #bbg2 an equaliser of temperature1 #bci4 ,wave-lengths1 how measured1 #ca4 ,whewell1 on plurality of worlds1 #h1 #ae2 on man as the highest product of the universe1 #ad4 ,whittaker1 ,mr4 ,e4,t41 on gravitative and electro-dynamical forces1 #bif4 ,winds1 importance of1 to life1 #bdf4 ,,zodiacal light1 #aji4 ,printed by ,t4 and ,a4 ,,constable1 ,printers to ,his ,majesty1 at the ,edinburgh ,university ,press ,illustration3 ,,the ,,nebul ,,and ,,clusters ,,of ,,the ,,northern ,,heavens4 ,drawn upon an equal surface projection from ,dr4 ,,dreyer',s catalogue of #ahhh ,the ,milky ,way from a ,dr4 ,,boeddicker',s ,drawing by ,,sidney ,,waters ,illustration3 ,,the ,,nebul ,,and ,,clusters ,,of ,,the ,,southern ,,heavens4 ,drawn upon an equal surface projection from ,dr4 ,,dreyer',s catalogue of #ahhh ,the ,milky ,way from ,,uranometria ,,argentina by ,,sidney ,,waters ,,works ,,by ,,the ,,same ,,author4 ,,travels ,,on ,,the ,,amazon ,,and ,,rio-,,negro4 ,,palm-,,trees ,,of ,,the ,,amazon1 ,,and ,,their ,,uses 7out of print74 ,,the ,,malay ,,archipelago4 ,,natural ,,selection ,,and ,,tropical ,,nature4 ,,the ,,geographical ,,distribution ,,of ,,animals4 #b vols4 ,,island ,,life1 ,,or ,,insular ,,faunas ,,and ,,floras4 ,,darwinism4 ,,australasia4 #b vols4 ,,bad ,,times--,,causes ,,of ,,depression ,,of ,,trade 7out of print74 ,,land ,,nationalisation1 ,,its ,,necessity ,,and ,,its ,,aims4 ,,vaccination ,a ,,delusion4 ,,miracles ,,and ,,modern ,,spiritualism4 ,,the ,,wonderful ,,century 7,new and ,illustrated ,edition74 ,,the ,,wonderful ,,century ,,reader4 ,,studies ,,scientific ,,and ,,social4 99 99 99 99 99 ,transcriber's ,notes3 ,obvious punctuation and spelling errors repaired4 ,inconsistent hyphenation has been repaired4 ,the oe and ae ligatures in the text has been left as it appears in the original book4 ,italic text is denoted by underscores and bold text by equal signs4 ,in ambiguous cases1 the text has been left as it appears in the original book4 ,in particular1 many mismatched quotation marks have not been changed4 ,the remaining corrections made3 ,page v1 ,fort-nightly changed to ,fortnightly4 ,page #ajf1 terrestial changed to terrestrial4