,transcriber's ,note ,the punctuation and spelling from the original text have been faithfully preserved4 ,only obvious typographical errors have been corrected4 ,subscripts are represented as ,x#a4 ,superscripts are represented by ,x#a4 ,,how ,,it ,,works ,,author',s ,,note4 ,i beg to thank the following gentlemen and firms for the help they have given me in connection with the letterpress and illustrations of ,how ,it ,works-- ,messrs4 ,f4,j4,c4 ,pole and ,m4,g4 ,tweedie 7for revision of ,,ms472 ,w4 ,lineham2 ,j4,f4 ,kendall2 ,e4 ,edser2 ,a4,d4 ,helps2 ,j4 ,limb2 ,the ,edison ,bell ,phonograph ,co42 ,messrs4 ,holmes and ,co42 ,the ,pelton ,wheel ,co42 ,messrs4 ,babcock and ,wilcox2 ,messrs4 ,siebe1 ,gorman1 and ,co42 ,messrs4 ,negretti and ,zambra2 ,messrs4 ,chubb2 ,the ,yale ,lock ,co42 ,the ,micrometer ,engineering ,co42 ,messrs4 ,marshall and ,sons2 ,the ,maignen ,filter ,co42 ,messrs4 ,broadwood and ,co4 ,illustration3 ,,on ,,the ,,footplate ,,of ,a ,,locomotive4 ,how ,it ,works ,dealing in ,simple ,language with ,steam1 ,electricity1 ,light1 ,heat1 ,sound1 ,hydraulics1 ,optics1 etc4 and with their applications to ,apparatus in ,common ,use ,by ,,archibald ,,williams ,author of ,the ,romance of ,modern ,invention1 ,the ,romance of ,mining1 etc41 etc4 ,,thomas ,,nelson ,,and ,,sons ,london1 ,edinburgh1 ,dublin1 and ,new ,york ,,preface4 ,how does it work8 ,this question has been put to me so often by persons young and old that ,i have at last decided to answer it in such a manner that a much larger public than that with which ,i have personal acquaintance may be able to satisfy themselves as to the principles underlying many of the mechanisms met with in everyday life4 ,in order to include steam1 electricity1 optics1 hydraulics1 thermics1 light1 and a variety of detached mechanisms which cannot be classified under any one of these heads1 within the compass of about #dej pages1 ,i have to be content with a comparatively brief treatment of each subject4 ,this brevity has in turn compelled me to deal with principles rather than with detailed descriptions of individual devices--though in several cases recognized types are examined4 ,the reader will look in vain for accounts of the ,yerkes telescope1 of the latest thing in motor cars1 and of the largest locomotive4 ,but he will be put in the way of understanding the essential nature of all telescopes1 motors1 and steam-engines so far as they are at present developed1 which ,i think may be of greater ultimate profit to the uninitiated4 ,while careful to avoid puzzling the reader by the use of mysterious phraseology ,i consider that the parts of a machine should be given their technical names wherever possible4 ,to prevent misconception1 many of the diagrams accompanying the letterpress have words as well as letters written on them4 ,this course also obviates the wearisome reference from text to diagram necessitated by the use of solitary letters or figures4 ,i may add1 with regard to the diagrams of this book1 that they are purposely somewhat unconventional1 not being drawn to scale nor conforming to the canons of professional draughtsmanship4 ,where advisable1 a part of a machine has been exaggerated to show its details4 ,as a rule solid black has been preferred to fine shading in sectional drawings1 and all unnecessary lines are omitted4 ,i would here acknowledge my indebtedness to my draughtsman1 ,mr4 ,frank ,hodgson1 for his care and industry in preparing the two hundred or more diagrams for which he was responsible4 ,four organs of the body--the eye1 the ear1 the larynx1 and the heart--are noticed in appropriate places4 ,the eye is compared with the camera1 the larynx with a reed pipe1 the heart with a pump1 while the ear fitly opens the chapter on acoustics4 ,the reader who is unacquainted with physiology will thus be enabled to appreciate the better these marvellous devices1 far more marvellous1 by reason of their absolutely automatic action1 than any creation of human hands4 ,a4,w4 ,,uplands1 ,,stoke ,,poges1 ,,bucks4 ,,contents4 ,chapter ,i4--,,the ,,steam-,,engine4 ,what is steam8--,the mechanical energy of steam--,the boiler--,the circulation of water in a boiler--,the enclosed furnace--,the multitubular boiler--,fire-tube boilers--,other types of boilers--,aids to combustion--,boiler fittings--,the safety-valve--,the water-gauge--,the steam-gauge--,the water supply to a boiler #ac ,chapter ,,ii4--,,the ,,conversion ,,of ,,heat ,,energy ,,into ,,mechanical ,,motion4 ,reciprocating engines--,double-cylinder engines--,the function of the fly-wheel--,the cylinder--,the slide-valve--,the eccentric--,lap of the valve3 expansion of steam--,how the cut-off is managed--,limit of expansive working--,compound engines--,arrangement of expansion engines--,compound locomotives--,reversing gears--,linking-up--,piston-valves--,spe ed governors--,marine-speed governors--,the condenser #dd ,chapter ,,iii4--,,the ,,steam ,,turbine4 ,how a turbine works--,the ,de ,laval turbine--,the ,parsons turbine--,description of the ,parsons turbine--,the expansive action of steam in a ,parsons turbine--,balancing the thrust--,advantages of the marine turbine #gd ,chapter ,,iv4--,,the ,,internal-,,combustion ,,engine4 ,the meaning of the term--,action of the internal-combustion engine--,the motor car--,the starting-handle--,the engine--,the carburetter--,ignition of the charge--,advancing the spark--,governing the engine--,the clutch--,the gear-box--,the compensating gear--,the silencer--,the brakes--,speed of cars #hg ,chapter ,v4--,,electrical ,,apparatus4 ,what is electricity8--,forms of electricity--,magnetism--,the permanent magnet--,lines of force--,electro-magnets--,the electric bell--,the induction coil--,the condenser--,transformation of current--,uses of the induction coil #aab ,chapter ,,vi4--,,the ,,electric ,,telegraph4 ,needle instruments--,influence of current on the magnetic needle--,method of reversing the current--,sounding instruments--,telegraphic relays--,recording telegraphs--,high-speed telegraphy #abg ,chapter ,,vii4--,,wireless ,,telegraphy4 ,the transmitting apparatus--,the receiving apparatus--,syntonic transmission--,the advance of wireless telegraphy #acg ,chapter ,,viii4--,,the ,,telephone4 ,the ,bell telephone--,the ,edison transmitter--,the granular carbon transmitter--,general arrangement of a telephone circuit--,double-line circuits--,telephone exchanges--,submarine telephony #adg ,chapter ,,ix4--,,dynamos ,,and ,,electric ,,motors4 ,a simple dynamo--,continuous-current dynamos--,multipolar dynamos--,exciting the field magnets--,alternating current dynamos--,the transmission of power--,the electric motor--,electric lighting--,the incandescent lamp--,arc lamps--,series and parallel arrangement of lamps--,current for electric lamps--,electroplating #aei ,chapter ,x4--,,railway ,,brakes4 ,the ,vacuum ,automatic brake--,the ,westinghouse air-brake #ahg ,chapter ,,xi4--,,railway ,,signalling4 ,the block system--,position of signals--,interlocking the signals--,locking gear--,points--,points and signals in combination--,working the block system--,series of signalling operations--,single line signals--,the train staff--,train staff and ticket--,electric train staff system--,interlocking--,signalling operations--,power signalling--,pneumatic signalling--,automatic signalling #bjj ,chapter ,,xii4--,,optics4 ,lenses--,the image cast by a convex lens--,focus--,relative position of object and lens--,correction of lenses for colour--,spherical aberration--,distortion of image--,the human eye--,the use of spectacles--,the blind spot #bcj ,chapter ,,xiii4--,,the ,,microscope1 ,,the ,,telescope1 ,,and ,,the ,,magic-,,lantern4 ,the simple microscope--,use of the simple microscope in the telescope--,the terrestrial telescope--,the ,galilean telescope--,the prismatic telescope--,the reflecting telescope--,the parabolic mirror--,the compound microscope--,the magic-lantern--,the bioscope--,the plane mirror #bec ,chapter ,,xiv4--,,sound ,,and ,,musical ,,instruments4 ,nature of sound--,the ear--,musical instruments--,the vibration of strings--,the sounding-board and the frame of a piano--,the strings--,the striking mechanism--,the quality of a note #bgj ,chapter ,,xv4--,,wind ,,instruments4 ,longitudinal vibration--,columns of air--,resonance of columns of air--,length and tone--,the open pipe--,the overtones of an open pipe--,where overtones are used--,the arrangement of the pipes and pedals--,separate sound-boards--,varieties of stops--,tuning pipes and reeds--,the bellows--,electric and pneumatic actions--,the largest organ in the world--,human reeds #bhg ,chapter ,,xvi4--,,talking-,,machines4 ,the phonograph--,the recorder--,the reproducer--,the gramophone--,the making of records--,cylinder records--,gramophone records #caj ,chapter ,,xvii4--,,why ,,the ,,wind ,,blows4 ,why the wind blows--,land and sea breezes--,light air and moisture--,the barometer--,the column barometer--,the wheel barometer--,a very simple barometer--,the aneroid barometer--,barometers and weather--,the diving-bell--,the diving-dress--,air-pumps--,pneumatic tyres--,the air-gun--,the self-closing door-stop--,the action of wind on oblique surfaces--,the balloon--,the flying-machine #cbb ,chapter ,,xviii4--,,hydraulic ,,machinery4 ,the siphon--,the bucket pump--,the force-pump--,the most marvellous pump--,the blood channels--,the course of the blood--,the hydraulic press--,household water-supply fittings--,the ball-cock--,the water-meter--,water-supply systems--,the household filter--,gas traps--,water engines--,the cream separator--,the hydro #cej ,chapter ,,xix4--,,heating ,,and ,,lighting4 ,the hot-water supply--,the tank system--,the cylinder system--,how a lamp works--,gas and gasworks--,automatic stoking--,a gas governor--,the gas meter--,incandescent gas lighting #chf ,chapter ,,xx4--,,various ,,mechanisms4 ,,clocks ,,and ,,watches3--,a short history of timepieces--,the construction of timepieces--,the driving power--,the escapement--,compensating pendulums--,the spring balance--,the cylinder escapement--,the lever escapement--,compensated balance-wheels--,keyless winding mechanism for watches--,the hour hand train4 ,,locks3--,the ,chubb lock--,the ,yale lock4 ,,the ,,cycle3--,the gearing of a cycle--,the free wheel--,the change-speed gear4 ,,agricultural ,,machines3--,the threshing-machine--,mowing-machines4 ,,some ,,natural ,,phenomena3--,why sun-heat varies in intensity--,the tides--,why high tide varies daily #daj ,,how ,,it ,,works4 ,chapter ,i4 ,,the ,,steam-,,engine4 ,what is steam8--,the mechanical energy of steam--,the boiler--,the circulation of water in a boiler--,the enclosed furnace--,the multitubular boiler--,fire-tube boilers--,other types of boilers--,aids to combustion--,boiler fittings--,the safety-valve--,the water-gauge--,the steam-gauge--,the water supply to a boiler4 ,,what ,,is ,,steam8 ,if ice be heated above #cb ,fahrenheit1 its molecules lose their cohesion1 and move freely round one another--the ice is turned into water4 ,heat water above #bab ,fahrenheit1 and the molecules exhibit a violent mutual repulsion1 and1 like dormant bees revived by spring sunshine1 separate and dart to and fro4 ,if confined in an air-tight vessel1 the molecules have their flights curtailed1 and beat more and more violently against their prison walls1 so that every square inch of the vessel is subjected to a rising pressure4 ,we may compare the action of the steam molecules to that of bullets fired from a machine-gun at a plate mounted on a spring4 ,the faster the bullets came1 the greater would be the continuous compression of the spring4 ,,the ,,mechanical ,,energy ,,of ,,steam4 ,if steam is let into one end of a cylinder behind an air-tight but freely-moving piston1 it will bombard the walls of the cylinder and the piston2 and if the united push of the molecules on the one side of the latter is greater than the resistance on the other side opposing its motion1 the piston must move4 ,having thus partly got their liberty1 the molecules become less active1 and do not rush about so vigorously4 ,the pressure on the piston decreases as it moves4 ,but if the piston were driven back to its original position against the force of the steam1 the molecular activity--that is1 pressure--would be restored4 ,we are here assuming that no heat has passed through the cylinder or piston and been radiated into the air2 for any loss of heat means loss of energy1 since heat is energy4 ,,the ,,boiler4 ,the combustion of fuel in a furnace causes the walls of the furnace to become hot1 which means that the molecules of the substance forming the walls are thrown into violent agitation4 ,if the walls are what are called good conductors of heat1 they will transmit the agitation through them to any surrounding substance4 ,in the case of the ordinary house stove this is the air1 which itself is agitated1 or grows warm4 ,a steam-boiler has the furnace walls surrounded by water1 and its function is to transmit molecular movement 7heat1 or energy7 through the furnace plates to the water until the point is reached when steam generates4 ,at atmospheric pressure--that is1 if not confined in any way--steam would fill #a1faj times the space which its molecules occupied in their watery formation4 ,if we seal up the boiler so that no escape is possible for the steam molecules1 their motion becomes more and more rapid1 and pressure is developed by their beating on the walls of the boiler4 ,there is theoretically no limit to which the pressure may be raised1 provided that sufficient fuel-combustion energy is transmitted to the vaporizing water4 ,to raise steam in large quantities we must employ a fuel which develops great heat in proportion to its weight1 is readily procured1 and cheap4 ,coal fulfils all these conditions4 ,of the #hjj million tons mined annually throughout the world1 #djj million tons are burnt in the furnaces of steam-boilers4 ,a good boiler must be--7#a7 ,strong enough to withstand much higher pressures than that at which it is worked2 7#b7 so designed as to burn its fuel to the greatest advantage4 ,even in the best-designed boilers a large part of the combustion heat passes through the chimney1 while a further proportion is radiated from the boiler4 ,professor ,john ,perry#a considers that this waste amounts1 under the best conditions at present obtainable1 to eleven-twelfths of the whole4 ,we have to burn a shillingsworth of coal to capture the energy stored in a pennyworth4 ,yet the steam-engine of to-day is three or four times as efficient as the engine of fifty years ago4 ,this is due to radical improvements in the design of boilers and of the machinery which converts the heat energy of steam into mechanical motion4 ,,circulation ,,of ,,water ,,in ,a ,,boiler4 ,if you place a pot filled with water on an open fire1 and watch it when it boils1 you will notice that the water heaves up at the sides and plunges down at the centre4 ,this is due to the water being heated most at the sides1 and therefore being lightest there4 ,the rising steam-bubbles also carry it up4 ,on reaching the surface1 the bubbles burst1 the steam escapes1 and the water loses some of its heat1 and rushes down again to take the place of steam-laden water rising4 ,illustration3 ,,fig4 #a4 ,illustration3 ,,fig4 #b4 ,if the fire is very fierce1 steam-bubbles may rise from all points at the bottom1 and impede downward currents 7,fig4 #a74 ,the pot then boils over4 ,fig4 #b shows a method of preventing this trouble4 ,we lower into our pot a vessel of somewhat smaller diameter1 with a hole in the bottom1 arranged in such a manner as to leave a space between it and the pot all round4 ,the upward currents are then separated entirely from the downward1 and the fire can be forced to a very much greater extent than before without the water boiling over4 ,this very simple arrangement is the basis of many devices for producing free circulation of the water in steam-boilers4 ,we can easily follow out the process of development4 ,in ,fig4 #c we see a simple ,u-tube depending from a vessel of water4 ,heat is applied to the left leg1 and a steady circulation at once commences4 ,in order to increase the heating surface we can extend the heated leg into a long incline 7,fig4 #d71 beneath which three lamps instead of only one are placed4 ,the direction of the circulation is the same1 but its rate is increased4 ,illustration3 ,,fig4 #c4 ,a further improvement results from increasing the number of tubes 7,fig4 #e71 keeping them all on the slant1 so that the heated water and steam may rise freely4 ,,the ,,enclosed ,,furnace4 ,illustration3 ,,fig4 #d4 ,illustration3 ,,fig4 #e4 ,still1 a lot of the heat gets away4 ,in a steam-boiler the burning fuel is enclosed either by fire-brick or a water-jacket1 forming part of the boiler4 ,a water-jacket signifies a double coating of metal plates with a space between1 which is filled with water 7see ,fig4 #f74 ,the fire is now enclosed much as it is in a kitchen range4 ,but our boiler must not be so wasteful of the heat as is that useful household fixture4 ,on their way to the funnel the flames and hot gases should act on a very large metal or other surface in contact with the water of the boiler1 in order to give up a due proportion of their heat4 ,illustration3 ,,fig4 #f4--;,diagrammatic sketch of a locomotive type of boiler4 ,water indicated by dotted lines4 ,the arrows show the direction taken by the air and hot gases from the air-door to the funnel4 ,,the ,,multitubular ,,boiler4 ,illustration3 ,,fig4 #g4--,the ,babcock and ,wilcox water-tube boiler4 ,one side of the brick seating has been removed to show the arrangement of the water-tubes and furnace4 ,to save room1 boilers which have to make steam very quickly and at high pressures are largely composed of pipes4 ,such boilers we call multitubular4 ,they are of two kinds--7#a7 ,water-tube boilers2 in which the water circulates through tubes exposed to the furnace heat4 ,the ,babcock and ,wilcox boiler 7,fig4 #g7 is typical of this variety4 7#b7 ,fire-tube boilers2 in which the hot gases pass through tubes surrounded by water4 ,the ordinary locomotive boiler 7,fig4 #f7 illustrates this form4 ,the ,babcock and ,wilcox boiler is widely used in mines1 power stations1 and1 in a modified form1 on shipboard4 ,it consists of two main parts--7#a7 ,a drum1 ,h1 in the upper part of which the steam collects2 7#b7 a group of pipes arranged on the principle illustrated by ,fig4 #e4 ,the boiler is seated on a rectangular frame of fire-bricks4 ,at one end is the furnace door2 at the other the exit to the chimney4 ,from the furnace ,f the flames and hot gases rise round the upper end of the sloping tubes ,,tt into the space ,a1 where they play upon the under surface of ,h before plunging downward again among the tubes into the space ,b4 ,here the temperature is lower4 ,the arrows indicate further journeys upwards into the space ,c on the right of a fire-brick division1 and past the down tubes ,,ss into ,d1 whence the hot gases find an escape into the chimney through the opening ,e4 ,it will be noticed that the greatest heat is brought to bear on ,,tt near their junction with ,,uu1 the uptake tubes2 and that every succeeding passage of the pipes brings the gradually cooling gases nearer to the downtake tubes ,,ss4 ,the pipes ,,tt are easily brushed and scraped after the removal of plugs from the headers into which the tube ends are expanded4 ,other well-known water-tube boilers are the ,yarrow1 ,belleville1 ,stirling1 and ,thorneycroft1 all used for driving marine engines4 ,,fire-,,tube ,,boilers4 ,fig4 #f shows a locomotive boiler in section4 ,to the right is the fire-box1 surrounded on all sides by a water-jacket in direct communication with the barrel of the boiler4 ,the inner shell of the fire-box is often made of copper1 which withstands the fierce heat better than steel2 the outer1 like the rest of the boiler1 is of steel plates from #a_/b to #c_/d inch thick4 ,the shells of the jacket are braced together by a large number of rivets1 ,,rr2 and the top1 or crown1 is strengthened by heavy longitudinal girders riveted to it1 or is braced to the top of the boiler by long bolts4 ,a large number of fire-tubes 7only three are shown in the diagram for the sake of simplicity7 extend from the fire-box to the smoke-box4 ,the most powerful mammoth ,american locomotives have #cej or more tubes1 which1 with the fire-box1 give #d1jjj square feet of surface for the furnace heat to act upon4 ,these tubes are expanded at their ends by a special tool into the tube-plates of the fire-box and boiler front4 ,george ,stephenson and his predecessors experienced great difficulty in rendering the tube-end joints quite water-tight1 but the invention of the expander has removed this trouble4 ,the fire-brick arch shown 7,fig4 #f7 in the fire-box is used to deflect the flames towards the back of the fire-box1 so that the hot gases may be retarded somewhat1 and their combustion rendered more perfect4 ,it also helps to distribute the heat more evenly over the whole of the inside of the box1 and prevents cold air from flying directly from the firing door to the tubes4 ,in some ,american and ,continental locomotives the fire-brick arch is replaced by a water bridge1 which serves the same purpose1 while giving additional heating surface4 ,the water circulation in a locomotive boiler is--upwards at the fire-box end1 where the heat is most intense2 forward along the surface2 downwards at the smoke-box end2 backwards along the bottom of the barrel4 ,,other ,,types ,,of ,,boilers4 ,for small stationary land engines the vertical boiler is much used4 ,in ,fig4 #h we have three forms of this type--,a and ,b with cross water-tubes2 ,c with vertical fire-tubes4 ,the furnace in every case is surrounded by water1 and fed through a door at one side4 ,illustration3 ,,fig4 #h4--;,diagrammatic representation of three types of vertical boilers4 ,the ,lancashire boiler is of large size4 ,it has a cylindrical shell1 measuring up to #cj feet in length and #g feet in diameter1 traversed from end to end by two large flues1 in the rear part of which are situated the furnaces4 ,the boiler is fixed on a seating of fire-bricks1 so built up as to form three flues1 ,a and ,,bb1 shown in cross section in ,fig4 #i4 ,the furnace gases1 after leaving the two furnace flues1 are deflected downwards into the channel ,a1 by which they pass underneath the boiler to a point almost under the furnace1 where they divide right and left and travel through cross passages into the side channels ,,bb1 to be led along the boiler's flanks to the chimney exit ,c4 ,by this arrangement the effective heating surface is greatly increased2 and the passages being large1 natural draught generally suffices to maintain proper combustion4 ,the ,lancashire boiler is much used in factories and 7in a modified form7 on ships1 since it is a steady steamer and is easily kept in order4 ,illustration3 ,,fig4 #i4--;,cross and longitudinal sections of a ,lancashire boiler4 ,in marine boilers of cylindrical shape cross water-tubes and fire-tubes are often employed to increase the heating surface4 ,return tubes are also led through the water to the funnels1 situated at the same end as the furnace4 ,,aids ,,to ,,combustion4 ,we may now turn our attention more particularly to the chemical process called combustion1 upon which a boiler depends for its heat4 ,ordinary steam coal contains about #he per cent4 of carbon1 #g per cent4 of oxygen1 and #d per cent4 of hydrogen1 besides traces of nitrogen and sulphur and a small incombustible residue4 ,when the coal burns1 the nitrogen is released and passes away without combining with any of the other elements4 ,the sulphur unites with hydrogen and forms sulphuretted hydrogen 7also named sulphurous acid71 which is injurious to steel plates1 and is largely responsible for the decay of tubes and funnels4 ,more of the hydrogen unites with the oxygen as steam4 ,the most important element in coal is the carbon 7known chemically by the symbol ,c74 ,its combination with oxygen1 called combustion1 is the act which heats the boiler4 ,only when the carbon present has combined with the greatest possible amount of oxygen that it will take into partnership is the combustion complete and the full heat-value 7fixed by scientific experiment at #ad1ejj thermal units per pound of carbon7 developed4 ,now1 carbon may unite with oxygen1 atom for atom1 and form carbon monoxide 7,,co72 or in the proportion of one atom of carbon to two of oxygen1 and form carbon dioxide 7,,co#b74 ,the former gas is combustible--that is1 will admit another atom of carbon to the molecule--but the latter is saturated with oxygen1 and will not burn1 or1 to put it otherwise1 is the product of perfect combustion4 ,a properly designed furnace1 supplied with a due amount of air1 will cause nearly all the carbon in the coal burnt to combine with the full amount of oxygen4 ,on the other hand1 if the oxygen supply is inefficient1 ,,co as well as ,,co#b will form1 and there will be a heat loss1 equal in extreme cases to two-thirds of the whole4 ,it is therefore necessary that a furnace which has to eat up fuel at a great pace should be artificially fed with air in the proportion of from #ab to #bj pounds of air for every pound of fuel4 ,there are two methods of creating a violent draught through the furnace4 ,the first is-- ,the forced draught2 very simply exemplified by the ordinary bellows used in every house4 ,on a ship 7,fig4 #aj7 the principle is developed as follows3--,the boilers are situated in a compartment or compartments having no communication with the outer air1 except for the passages down which air is forced by powerful fans at a pressure considerably greater than that of the atmosphere4 ,there is only one way out--namely1 through the furnace and tubes 7or gas-ways7 of the boiler1 and the funnel4 ,so through these it rushes1 raising the fuel to white heat4 ,as may easily be imagined1 the temperature of a stokehold1 especially in the tropics1 is far from pleasant4 ,in the ,red ,sea the thermometer sometimes rises to #agj ,fahrenheit or more1 and the poor stokers have a very bad time of it4 ,illustration3 ,,fig4 #aj4--,sketch showing how the forced draught is produced in a stokehold and how it affects the furnaces4 ,illustration3 ,,scene ,,in ,,the ,,stokehold ,,of ,a ,,battle-,,ship4 ,the second system is that of the induced draught4 ,here air is sucked through the furnace by creating a vacuum in the funnel and in a chamber opening into it4 ,turning to ,fig4 #f1 we see a pipe through which the exhaust steam from the locomotive's cylinders is shot upwards into the funnel1 in which1 and in the smoke-box beneath it1 a strong vacuum is formed while the engine is running4 ,now1 nature abhors a vacuum1 so air will get into the smoke-box if there be a way open4 ,there is--through the air-doors at the bottom of the furnace1 the furnace itself1 and the fire-tubes2 and on the way oxygen combines with the carbon of the fuel1 to form carbon dioxide4 ,the power of the draught is so great that1 as one often notices when a train passes during the night1 red-hot cinders1 plucked from the fire-box1 and dragged through the tubes1 are hurled far into the air4 ,it might be mentioned in parenthesis that the so-called smoke which pours from the funnel of a moving engine is mainly condensing steam4 ,a steamship1 on the other hand1 belches smoke only from its funnels1 as fresh water is far too precious to waste as steam4 ,we shall refer to this later on 7p4 #gb74 ,,boiler ,,fittings4 ,the most important fittings on a boiler are3--7#a7 the safety-valve2 7#b7 the water-gauge2 7#c7 the steam-gauge2 7#d7 the mechanisms for feeding it with water4 ,,the ,,safety-,,valve4 ,professor ,thurston1 an eminent authority on the steam-engine1 has estimated that a plain cylindrical boiler carrying #ajj lbs4 pressure to the square inch contains sufficient stored energy to project it into the air a vertical distance of #c-a_/b miles4 ,in the case of a ,lancashire boiler at equal pressure the distance would be #b-a_/b miles2 of a locomotive boiler1 at #abe lbs41 #a-a_/b miles2 of a steam tubular boiler1 at #ge lbs41 #a mile4 ,according to the same writer1 a cubic foot of heated water under a pressure of from #fj to #gj lbs4 per square inch has about the same energy as one pound of gunpowder4 ,steam is a good servant1 but a terrible master4 ,it must be kept under strict control4 ,however strong a boiler may be1 it will burst if the steam pressure in it be raised to a certain point2 and some device must therefore be fitted on it which will give the steam free egress before that point is reached4 ,a device of this kind is called a safety-valve4 ,it usually blows off at less than half the greatest pressure that the boiler has been proved by experiment to be capable of withstanding4 ,in principle the safety-valve denotes an orifice closed by an accurately-fitting plug1 which is pressed against its seat on the boiler top by a weighted lever1 or by a spring4 ,as soon as the steam pressure on the face of the plug exceeds the counteracting force of the weight or spring1 the plug rises1 and steam escapes until equilibrium of the opposing forces is restored4 ,on stationary engines a lever safety-valve is commonly employed 7,fig4 #aa74 ,the blowing-off point can be varied by shifting the weight along the arm so as to give it a greater or less leverage4 ,on locomotive and marine boilers1 where shocks and movements have to be reckoned with1 weights are replaced by springs1 set to a certain tension1 and locked up so that they cannot be tampered with4 ,illustration3 ,,fig4 #aa4--;,a ,,lever ,,safety-,,valve4 ,v1 valve2 ,s1 seating2 ,p1 pin2 ,l1 lever2 ,f1 fulcrum2 ,w1 weight4 ,the figures indicate the positions at which the weight should be placed for the valve to act when the pressure rises to that number of pounds per square inch4 ,boilers are tested by filling the boilers quite full and 7#a7 by heating the water1 which expands slightly1 but with great pressure2 7#b7 by forcing in additional water with a powerful pump4 ,in either case a rupture would not be attended by an explosion1 as water is very inelastic4 ,the days when an engineer could sit on the valves--that is1 screw them down--to obtain greater pressure1 are now past1 and with them a considerable proportion of the dangers of high-pressure steam4 ,the ,factory ,act of #ahie1 in force throughout the ,british ,isles1 provides that every boiler for generating steam in a factory or workshop where the ,act applies must have a proper safety-valve1 steam-gauge1 and water-gauge2 and that boilers and fittings must be examined by a competent person at least once in every fourteen months4 ,neglect of these provisions renders the owner of a boiler liable to heavy penalties if an explosion occurs4 ,one of the most disastrous explosions on record took place at the ,redcar ,iron ,works1 ,yorkshire1 in ,june #ahie4 ,in this case1 twelve out of fifteen boilers ranged side by side burst1 through one proving too weak for its work4 ,the flying fragments of this boiler1 striking the sides of other boilers1 exploded them1 and so the damage was transmitted down the line4 ,twenty men were killed and injured2 while masses of metal1 weighing several tons each1 were hurled #bej yards1 and caused widespread damage4 ,the following is taken from a journal1 dated ,december #bb1 #ahie3 ,providence 7,rhode ,island74--,a recent prophecy that a boiler would explode between ,december #af and #bd in a store has seriously affected the ,christmas trade4 ,shoppers are incredibly nervous4 ,one store advertises1 ',no boilers are being used2 lifts running electrically4' ,all stores have had their boilers inspected4 ,,the ,,water-,,gauge4 ,no fitting of a boiler is more important than the water-gauge1 which shows the level at which the water stands4 ,the engineer must continually consult his gauge1 for if the water gets too low1 pipes and other surfaces exposed to the furnace flames may burn through1 with disastrous results2 while1 on the other hand1 too much water will cause bad steaming4 ,a section of an ordinary gauge is seen in ,fig4 #ab4 ,it consists of two parts1 each furnished with a gland1 ,g1 to make a steam-tight joint round the glass tube1 which is inserted through the hole covered by the plug ,p#a4 ,the cocks ,t#a ,t#b are normally open1 allowing the ingress of steam and water respectively to the tube4 ,cock ,t#c is kept closed unless for any reason it is necessary to blow steam or water through the gauge4 ,the holes ,c ,c can be cleaned out if the plugs ,p#b ,p#c are removed4 ,most gauges on high-pressure boilers have a thick glass screen in front1 so that in the event of the tube breaking1 the steam and water may not blow directly on to the attendants4 ,a further precaution is to include two ball-valves near the ends of the gauge-glass4 ,under ordinary conditions the balls lie in depressions clear of the ways2 but when a rush of steam or water occurs they are sucked into their seatings and block all egress4 ,illustration3 ,,fig4 #ab4--,section of a water-gauge4 ,on many boilers two water-gauges are fitted1 since any gauge may work badly at times4 ,the glasses are tested to a pressure of #c1jjj lbs4 or more to the square inch before use4 ,,the ,,steam-,,gauge4 ,it is of the utmost importance that a person in charge of a boiler should know what pressure the steam has reached4 ,every boiler is therefore fitted with one steam-gauge2 many with two1 lest one might be unreliable4 ,there are two principal types of steam-gauge3--7#a7 ,the ,bourdon2 7#b7 the ,schffer-,budenberg4 ,the principle of the ,bourdon is illustrated by ,fig4 #ac1 in which ,a is a piece of rubber tubing closed at one end1 and at the other drawn over the nozzle of a cycle tyre inflator4 ,if bent in a curve1 as shown1 the section of the tube is an oval4 ,when air is pumped in1 the rubber walls endeavour to assume a circular section1 because this shape encloses a larger area than an oval of equal circumference1 and therefore makes room for a larger volume of air4 ,in doing so the tube straightens itself1 and assumes the position indicated by the dotted lines4 ,hang an empty inner tube of a pneumatic tyre over a nail and inflate it1 and you will get a good illustration of the principle4 ,illustration3 ,,fig4 #ac4--,showing the principle of the steam-gauge4 ,illustration3 ,,fig4 #ad4--;,bourdon steam-gauge4 ,part of dial removed to show mechanism4 ,in ,fig4 #ad we have a ,bourdon gauge1 with part of the dial face broken away to show the internal mechanism4 ,t is a flattened metal tube soldered at one end into a hollow casting1 into which screws a tap connected with the boiler4 ,the other end 7closed7 is attached to a link1 ,l1 which works an arm of a quadrant rack1 ,r1 engaging with a small pinion1 ,p1 actuating the pointer4 ,as the steam pressure rises1 the tube ,t moves its free end outwards towards the position shown by the dotted lines1 and traverses the arm of the rack1 so shifting the pointer round the scale4 ,as the pressure falls1 the tube gradually returns to its zero position4 ,the ,schffer-,budenberg gauge depends for its action on the elasticity of a thin corrugated metal plate1 on one side of which steam presses4 ,as the plate bulges upwards it pushes up a small rod resting on it1 which operates a quadrant and rack similar to that of the ,bourdon gauge4 ,the principle is employed in another form for the aneroid barometer 7p4 #cbi74 ,,the ,,water ,,supply ,,to ,a ,,boiler4 ,the water inside a boiler is kept at a proper level by 7#a7 pumps or 7#b7 injectors4 ,the former are most commonly used on stationary and marine boilers4 ,as their mechanism is much the same as that of ordinary force pumps1 which will be described in a later chapter1 we may pass at once to the injector1 now almost universally used on locomotive1 and sometimes on stationary boilers4 ,at first sight the injector is a mechanical paradox1 since it employs the steam from a boiler to blow water into the boiler4 ,in ,fig4 #ae we have an illustration of the principle of an injector4 ,steam is led from the boiler through pipe ,a1 which terminates in a nozzle surrounded by a cone1 ,e1 connected by the pipe ,b with the water tank4 ,when steam is turned on it rushes with immense velocity from the nozzle1 and creates a partial vacuum in cone ,e1 which soon fills with water4 ,on meeting the water the steam condenses1 but not before it has imparted some of its velocity to the water1 which thus gains sufficient momentum to force down the valve and find its way to the boiler4 ,the overflow space ,o ,o between ,e and ,c allows steam and water to escape until the water has gathered the requisite momentum4 ,illustration3 ,,fig4 #ae4--;,diagram illustrating the principle of a steam-injector4 ,illustration3 ,,fig4 #af4--,the ,giffard injector4 ,a form of injector very commonly used is ,giffard's 7,fig4 #af74 ,steam is allowed to enter by screwing up the valve ,v4 ,as it rushes through the nozzle of the cone ,a it takes up water and projects it into the mixing cone ,b1 which can be raised or lowered by the pinion ,d 7worked by the hand-wheel wheel shown7 so as to regulate the amount of water admitted to ,b4 ,at the centre of ,b is an aperture1 ,o1 communicating with the overflow4 ,the water passes to the boiler through the valve on the left4 ,it will be noticed that the cone ,a and the part of ,b above the orifice ,o contract downward4 ,this is to convert the pressure of the steam into velocity4 ,below ,o is a cone1 the diameter of which increases downwards4 ,here the velocity of the water is converted back into pressure in obedience to a well-known hydromechanic law4 ,an injector does not work well if the feed-water be too hot to condense the steam quickly2 and it may be taken as a rule that the warmer the water1 the smaller is the amount of it injected by a given weight of steam4#b ,some injectors have flap-valves covering the overflow orifice1 to prevent air being sucked in and carried to the boiler4 ,when an injector receives a sudden shock1 such as that produced by the passing of a locomotive over points1 it is liable to fly off--that is1 stop momentarily--and then send the steam and water through the overflow4 ,if this happens1 both steam and water must be turned off1 and the injector be restarted2 unless it be of the self-starting variety1 which automatically controls the admission of water to the mixing-cone1 and allows the injector to pick up of itself4 ,for economy's sake part of the steam expelled from the cylinders of a locomotive is sometimes used to work an injector1 which passes the water on1 at a pressure of #gj lbs4 to the square inch1 to a second injector operated by high-pressure steam coming direct from the boiler1 which increases its velocity sufficiently to overcome the boiler pressure4 ,in this case only a fraction of the weight of high-pressure steam is required to inject a given weight of water1 as compared with that used in a single-stage injector4 #a ,the ,steam-,engine1 p4 #c4 #b ,by weight of steam is meant the steam produced by boiling a certain weight of water4 ,a pound of steam1 if condensed1 would form a pound of water4 ,chapter ,,ii4 ,,the ,,conversion ,,of ,,heat ,,energy ,,into ,,mechanical ,,motion4 ,reciprocating engines--,double-cylinder engines--,the function of the fly-wheel--,the cylinder--,the slide-valve--,the eccentric--,lap of the valve3 expansion of steam--,how the cut-off is managed--,limit of expansive working--,compound engines--,arrangement of expansion engines--,compound locomotives--,reversing gears--,linking-up--,piston-valves--,spe ed governors--,marine-speed governors--,the condenser4 ,having treated at some length the apparatus used for converting water into high-pressure steam1 we may pass at once to a consideration of the mechanisms which convert the energy of steam into mechanical motion1 or work4 ,steam-engines are of two kinds3--7#a7 reciprocating1 employing cylinders and cranks2 7#b7 rotary1 called turbines4 ,,reciprocating ,,engines4 ,illustration3 ,,fig4 #ag4--,sketch showing parts of a horizontal steam-engine4 ,fig4 #ag is a skeleton diagram of the simplest form of reciprocating engine4 ,c is a cylinder to which steam is admitted through the steam-ways#c ,w ,w1 first on one side of the piston ,p1 then on the other4 ,the pressure on the piston pushes it along the cylinder1 and the force is transmitted through the piston rod ,p ,r to the connecting rod ,c ,r1 which causes the crank ,k to revolve4 ,at the point where the two rods meet there is a crosshead1 ,h1 running to and fro in a guide to prevent the piston rod being broken or bent by the oblique thrusts and pulls which it imparts through ,c ,r to the crank ,k4 ,the latter is keyed to a shaft ,s carrying the fly-wheel1 or1 in the case of a locomotive1 the driving-wheels4 ,the crank shaft revolves in bearings4 ,the internal diameter of a cylinder is called its bore4 ,the travel of the piston is called its stroke4 ,the distance from the centre of the shaft to the centre of the crank pin is called the crank's throw1 which is half of the piston's stroke4 ,an engine of this type is called double-acting1 as the piston is pushed alternately backwards and forwards by the steam4 ,when piston rod1 connecting rod1 and crank lie in a straight line--that is1 when the piston is fully out1 or fully in--the crank is said to be at a dead point2 for1 were the crank turned to such a position1 the admission of steam would not produce motion1 since the thrust or pull would be entirely absorbed by the bearings4 ,illustration3 ,,fig4 #ah4--,sectional plan of a horizontal engine4 ,,double-,,cylinder ,,engines4 ,illustration3 ,,fig4 #ai4 ,illustration3 ,,fig4 #bj4 ,locomotive1 marine1 and all other engines which must be started in any position have at least two cylinders1 and as many cranks set at an angle to one another4 ,fig4 #ai demonstrates that when one crank1 ,c#a1 of a double-cylinder engine is at a dead point1 the other1 ,c#b1 has reached a position at which the piston exerts the maximum of turning power4 ,in ,fig4 #bj each crank is at #de with the horizontal1 and both pistons are able to do work4 ,the power of one piston is constantly increasing while that of the other is decreasing4 ,if single-action cylinders are used1 at least three of these are needed to produce a perpetual turning movement1 independently of a fly-wheel4 ,,the ,,function ,,of ,,the ,,fly-,,wheel4 ,a fly-wheel acts as a reservoir of energy1 to carry the crank of a single-cylinder engine past the dead points4 ,it is useful in all reciprocating engines to produce steady running1 as a heavy wheel acts as a drag on the effects of a sudden increase or decrease of steam pressure4 ,in a pump1 mangold-slicer1 cake-crusher1 or chaff-cutter1 the fly-wheel helps the operator to pass his dead points--that is1 those parts of the circle described by the handle in which he can do little work4 ,,the ,,cylinder4 ,illustration3 ,,fig4 #ba4--;,diagrammatic section of a cylinder and its slide-valve4 ,the cylinders of an engine take the place of the muscular system of the human body4 ,in ,fig4 #ba we have a cylinder and its slide-valve shown in section4 ,first of all1 look at ,p1 the piston4 ,round it are white grooves1 ,r ,r1 in which rings are fitted to prevent the passage of steam past the piston4 ,the rings are cut through at one point in their circumference1 and slightly opened1 so that when in position they press all round against the walls of the cylinder4 ,after a little use they settle down to their work--that is1 wear to a true fit in the cylinder4 ,each end of the cylinder is closed by a cover1 one of which has a boss cast on it1 pierced by a hole for the piston rod to work through4 ,to prevent the escape of steam the boss is hollowed out true to accommodate a gland1 ,g#a1 which is threaded on the rod and screwed up against the boss2 the internal space between them being filled with packing4 ,steam from the boiler enters the steam-chest1 and would have access to both sides of the piston simultaneously through the steam-ways1 ,w ,w1 were it not for the ,,slide-,,valve1 a hollow box open at the bottom1 and long enough for its edges to cover both steam-ways at once4 ,between ,w ,w is ,e1 the passage for the exhaust steam to escape by4 ,the edges of the slide-valve are perfectly flat1 as is the face over which the valve moves1 so that no steam may pass under the edges4 ,in our illustration the piston has just begun to move towards the right4 ,steam enters by the left steam-way1 which the valve is just commencing to uncover4 ,as the piston moves1 the valve moves in the same direction until the port is fully uncovered1 when it begins to move back again2 and just before the piston has finished its stroke the steam-way on the right begins to open4 ,the steam-way on the left is now in communication with the exhaust port ,e1 so that the steam that has done its duty is released and pressed from the cylinder by the piston4 ,reciprocation is this backward and forward motion of the piston3 hence the term reciprocating engines4 ,the linear motion of the piston rod is converted into rotatory motion by the connecting rod and crank4 ,illustration3 ,,fig4 #bb4--,perspective section of cylinder4 ,the use of a crank appears to be so obvious a method of producing this conversion that it is interesting to learn that1 when ,james ,watt produced his rotative engine in #aghj he was unable to use the crank because it had already been patented by one ,matthew ,wasborough4 ,watt was not easily daunted1 however1 and within a twelvemonth had himself patented five other devices for obtaining rotatory motion from a piston rod4 ,before passing on1 it may be mentioned that ,watt was the father of the modern--that is1 the high-pressure--steam-engine2 and that1 owing to the imperfection of the existing machinery1 the difficulties he had to overcome were enormous4 ,on one occasion he congratulated himself because one of his steam-cylinders was only three-eighths of an inch out of truth in the bore4 ,nowadays a good firm would reject a cylinder #a_/ejj of an inch out of truth2 and in small petrol-engines #a_/ejjj of an inch is sometimes the greatest limit of error allowed4 ,illustration3 ,,fig4 #bc4--,the eccentric and its rod4 ,,the ,,eccentric is used to move the slide-valve to and fro over the steam ports 7,fig4 #bc74 ,it consists of three main parts--the sheave1 or circular plate ,s1 mounted on the crank shaft2 and the two straps which encircle it1 and in which it revolves4 ,to one strap is bolted the big end of the eccentric rod1 which engages at its other end with the valve rod4 ,the straps are semicircular and held together by strong bolts1 ,b ,b1 passing through lugs1 or thickenings at the ends of the semicircles4 ,the sheave has a deep groove all round the edges1 in which the straps ride4 ,the eccentricity or throw of an eccentric is the distance between ,c#b1 the centre of the shaft1 and ,c#a1 the centre of the sheave4 ,the throw must equal half of the distance which the slide-valve has to travel over the steam ports4 ,a tapering steel wedge or key1 ,k1 sunk half in the eccentric and half in a slot in the shaft1 holds the eccentric steady and prevents it slipping4 ,some eccentric sheaves are made in two parts1 bolted together1 so that they may be removed easily without dismounting the shaft4 ,the eccentric is in principle nothing more than a crank pin so exaggerated as to be larger than the shaft of the crank4 ,its convenience lies in the fact that it may be mounted at any point on a shaft1 whereas a crank can be situated at an end only1 if it is not actually a ,v-shaped bend in the shaft itself--in which case its position is of course permanent4 ,,setting ,,of ,,the ,,slide-,,valve ,,and ,,eccentric4 ,the subject of valve-setting is so extensive that a full exposition might weary the reader1 even if space permitted its inclusion4 ,but inasmuch as the effectiveness of a reciprocating engine depends largely on the nature and arrangement of the valves1 we will glance at some of the more elementary principles4 ,illustration3 ,,fig4 #bd4 ,illustration3 ,,fig4 #be4 ,in ,fig4 #bd we see in section the slide-valve1 the ports of the cylinder1 and part of the piston4 ,to the right are two lines at right angles--the thicker1 ,c1 representing the position of the crank2 the thinner1 ,e1 that of the eccentric4 7,the position of an eccentric is denoted diagrammatically by a line drawn from the centre of the crank shaft through the centre of the sheave47 ,the edges of the valve are in this case only broad enough to just cover the ports--that is1 they have no lap4 ,the piston is about to commence its stroke towards the left2 and the eccentric1 which is set at an angle of #ij in advance of the crank1 is about to begin opening the left-hand port4 ,by the time that ,c has got to the position originally occupied by ,e1 ,e will be horizontal 7,fig4 #be7--that is1 the eccentric will have finished its stroke towards the left2 and while ,c passes through the next right angle the valve will be closing the left port1 which will cease to admit steam when the piston has come to the end of its travel4 ,the operation is repeated on the right-hand side while the piston returns4 ,illustration3 ,,fig4 #bf4 ,it must be noticed here--7#a7 that steam is admitted at full pressure all through the stroke2 7#b7 that admission begins and ends simultaneously with the stroke4 ,now1 in actual practice it is necessary to admit steam before the piston has ended its travel1 so as to cushion the violence of the sudden change of direction of the piston1 its rod1 and other moving parts4 ,to effect this1 the eccentric is set more than #ij in advance--that is1 more than what the engineers call square4 ,fig4 #bf shows such an arrangement4 ,the angle between ,e and ,e#a is called the angle of advance4 ,referring to the valve1 you will see that it has opened an appreciable amount1 though the piston has not yet started on its rightwards journey4 ,,lap ,,of ,,the ,,valve--,,expansion ,,of ,,steam4 ,in the simple form of valve that appears in ,fig4 #bd1 the valve faces are just wide enough to cover the steam ports4 ,if the eccentric is not square with the crank1 the admission of steam lasts until the very end of the stroke2 if set a little in advance--that is1 given lead--the steam is cut off before the piston has travelled quite along the cylinder1 and readmitted before the back stroke is accomplished4 ,even with this lead the working is very uneconomical1 as the steam goes to the exhaust at practically the same pressure as that at which it entered the cylinder4 ,its property of expansion has been neglected4 ,but supposing that steam at #ajj lbs4 pressure were admitted till half-stroke1 and then suddenly cut off1 the expansive nature of the steam would then continue to push the piston out until the pressure had decreased to #ej lbs4 per square inch1 at which pressure it would go to the exhaust4 ,now1 observe that all the work done by the steam after the cut-off is so much power saved4 ,the average pressure on the piston is not so high as in the first case2 still1 from a given volume of #ajj lbs4 pressure steam we get much more work4 ,,how ,,the ,,cut-,,off ,,is ,,managed4 ,illustration3 ,,fig4 #bg4--;,a slide-valve with lap4 ,illustration3 ,,fig4 #bh4 ,look at ,fig4 #bg4 ,here we have a slide-valve1 with faces much wider than the steam ports4 ,the parts marked black1 ,p ,p1 are those corresponding to the faces of the valves shown in previous diagrams 7p4 #ed74 ,the shaded parts1 ,l ,l1 are called the lap4 ,by increasing the length of the lap we increase the range of expansive working4 ,fig4 #bh shows the piston full to the left2 the valve is just on the point of opening to admit steam behind the piston4 ,the eccentric has a throw equal to the breadth of a port the lap of the valve4 ,that this must be so is obvious from a consideration of ,fig4 #bg1 where the valve is at its central position4 ,hence the very simple formula3--,travel of valve #b 7lap breadth of port74 ,the path of the eccentric's centre round the centre of the shaft is indicated by the usual dotted line 7,fig4 #bh74 ,you will notice that the angle of advance1 denoted by the arrow ,a1 is now very considerable4 ,by the time that the crank ,c has assumed the position of the line ,s1 the eccentric has passed its dead point1 and the valve begins to travel backwards1 eventually returning to the position shown in ,fig4 #bh1 and cutting off the steam supply while the piston has still a considerable part of its stroke to make4 ,the steam then begins to work expansively1 and continues to do so until the valve assumes the position shown in ,fig4 #bg4 ,if the valve has to have lead to admit steam before the end of the stroke to the other side of the piston1 the angle of advance must be increased1 and the eccentric centre line would lie on the line ,e#b4 ,therefore--total angle of advance angle for lap and angle for lead4 ,,limit ,,of ,,expansive ,,working4 ,theoretically1 by increasing the lap and cutting off the steam earlier and earlier in the stroke1 we should economize our power more and more4 ,but in practice a great difficulty is met with--namely1 that as the steam expands its temperature falls4 ,if the cut-off occurs early1 say at one-third stroke1 the great expansion will reduce the temperature of the metal walls of the cylinder to such an extent1 that when the next spirt of steam enters from the other end a considerable proportion of the steam's energy will be lost by cooling4 ,in such a case1 the difference in temperature between admitted steam and exhausted steam is too great for economy4 ,yet we want to utilize as much energy as possible4 ,how are we to do it8 ,,compound ,,engines4 ,in the year #ahec1 ,john ,elder1 founder of the shipping firm of ,elder and ,co41 ,glasgow1 introduced the compound engine for use on ships4 ,the steam1 when exhausted from the high-pressure cylinder1 passed into another cylinder of equal stroke but larger diameter1 where the expansion continued4 ,in modern engines the expansion is extended to three and even four stages1 according to the boiler pressure2 for it is a rule that the higher the initial pressure is1 the larger is the number of stages of expansion consistent with economical working4 ,illustration3 ,,fig4 #bi4--,sketch of the arrangement of a triple-expansion marine engine4 ,no valve gear or supports1 etc41 shown4 ,in ,fig4 #bi we have a triple-expansion marine engine4 ,steam enters the high-pressure cylinder#d at1 say1 #bjj lbs4 per square inch4 ,it exhausts at #ge lbs4 into the large pipe #b1 and passes to the intermediate cylinder1 whence it is exhausted at #be lbs4 or so through pipe #c to the low-pressure cylinder4 ,finally1 it is ejected at about #h lbs4 per square inch to the condenser1 and is suddenly converted into water2 an act which produces a vacuum1 and diminishes the back-pressure of the exhaust from cylinder ,c4 ,in fact1 the condenser exerts a sucking power on the exhaust side of ,c's piston4 ,,arrangement ,,of ,,expansion ,,engines4 ,in the illustration the cranks are set at angles of #abj1 or a third of a circle1 so that one or other is always at or near the position of maximum turning power4 ,where only two stages are used the cylinders are often arranged tandem1 both pistons having a common piston rod and crank4 ,in order to get a constant turning movement they must be mounted separately1 and work cranks set at right angles to one another4 ,,compound ,,locomotives4 ,in #ahgf ,mr4 ,a4 ,mallet introduced compounding in locomotives2 and the practice has been largely adopted4 ,the various types of compounds may be classified as follows3--7#a7 ,one low-pressure and one high-pressure cylinder2 7#b7 one high-pressure and two low-pressure2 7#c7 one low-pressure and two high-pressure2 7#d7 two high-pressure and two low-pressure4 ,the last class is very widely used in ,france1 ,america1 and ,russia1 and seems to give the best results4 ,where only two cylinders are used 7and sometimes in the case of three and four71 a valve arrangement permits the admission of high-pressure steam to both high and low-pressure cylinders for starting a train1 or moving it up heavy grades4 ,,reversing ,,gears4 ,illustration3 ,,figs4 #cj1 #ca1 #cb4--,showing how a reversing gear alters the position of the slide-valve4 ,the engines of a locomotive or steamship must be reversible--that is1 when steam is admitted to the cylinders1 the engineer must be able to so direct it through the steam-ways that the cranks may turn in the desired direction4 ,the commonest form of reversing device 7invented by ,george ,stephenson7 is known as ,stephenson's ,link ,gear4 ,in ,fig4 #cj we have a diagrammatic presentment of this gear4 ,e#a and ,e#b are two eccentrics set square with the crank at opposite ends of a diameter4 ,their rods are connected to the ends of a link1 ,l1 which can be raised and lowered by means of levers 7not shown74 ,b is a block which can partly revolve on a pin projecting from the valve rod1 working through a guide1 ,g4 ,in ,fig4 #ca the link is half raised1 or in mid-gear1 as drivers say4 ,eccentric ,e#a has pushed the lower end of the link fully back2 ,e#b has pulled it fully forward2 and since any movement of the one eccentric is counterbalanced by the opposite movement of the other1 rotation of the eccentrics would not cause the valve to move at all1 and no steam could be admitted to the cylinder4 ,let us suppose that ,fig4 #cj denotes one cylinder1 crank1 rods1 etc41 of a locomotive4 ,the crank has come to rest at its half-stroke2 the reversing lever is at the mid-gear notch4 ,if the engineer desires to turn his cranks in an anti-clockwise direction1 he raises the link1 which brings the rod of ,e#a into line with the valve rod and presses the block backwards till the right-hand port is uncovered 7,fig4 #ca74 ,if steam be now admitted1 the piston will be pushed towards the left1 and the engine will continue to run in an anti-clockwise direction4 ,if1 on the other hand1 he wants to run the engine the other way1 he would drop the link1 bringing the rod of ,e#b into line with the valve rod1 and drawing ,v forward to uncover the rear port 7,fig4 #cb74 ,in either case the eccentric working the end of the link remote from ,b has no effect1 since it merely causes that end to describe arcs of circles of which ,b is the centre4 ,,linking ,,up4 ,if the link is only partly lowered or raised from the central position it still causes the engine to run accordingly1 but the movement of the valve is decreased4 ,when running at high speed the engineer links up his reversing gear1 causing his valves to cut off early in the stroke1 and the steam to work more expansively than it could with the lever at full1 or end1 gear2 so that this device not only renders an engine reversible1 but also gives the engineer an absolute command over the expansion ratio of the steam admitted to the cylinder1 and furnishes a method of cutting off the steam altogether4 ,in ,figs4 #cj1 #ca1 #cb1 the valve has no lap and the eccentrics are set square4 ,in actual practice the valve faces would have lap and the eccentric lead to correspond2 but for the sake of simplicity neither is shown4 ,,other ,,gears4 ,in the ,gooch gear for reversing locomotives the link does not shift1 but the valve rod and its block is raised or lowered4 ,the ,allan gear is so arranged that when the link is raised the block is lowered1 and vice vers4 ,these are really only modifications of ,stephenson's principle--namely1 the employment of two eccentrics set at equal angles to and on opposite sides of the crank4 ,there are three other forms of link-reversing gear1 and nearly a dozen types of radial reversing devices2 but as we have already described the three most commonly used on locomotives and ships1 there is no need to give particulars of these4 ,before the introduction of ,stephenson's gear a single eccentric was used for each cylinder1 and to reverse the engine this eccentric had to be loose on the axle4 ,a lever and gear worked by a treadle on the footplate controlled the position of the eccentrics4 ,when starting the engine1 the driver put the eccentrics out of gear by the treadle2 then1 by means of a lever he raised the small-ends#e of the eccentric rods1 and1 noting the position of the cranks1 or1 if more convenient1 the balance weight in the wheels1 he1 by means of another handle1 moved the valves to open the necessary ports to steam and worked them by hand until the engine was moving2 then1 with the treadle1 he threw the eccentrics over to engage the studs1 at the same time dropping the small-ends of the rods to engage pins upon the valve spindles1 so that they continued to keep up the movement of the valve4#f ,one would imagine that in modern shunting yards such a device would somewhat delay operations6 ,,piston ,,valves4 ,in marine engines1 and on many locomotives and some stationary engines1 the ,d-valve 7shown in ,figs4 #cj-cb7 is replaced by a piston valve1 or circular valve1 working up and down in a tubular seating4 ,it may best be described as a rod carrying two pistons which correspond to the faces of a ,d-valve4 ,instead of rectangular ports there are openings in the tube in which the piston valve moves1 communicating with the steam-ways into the cylinder and with the exhaust pipe4 ,in the case of the ,d-valve the pressure above it is much greater than that below1 and considerable friction arises if the rubbing faces are not kept well lubricated4 ,the piston valve gets over this difficulty1 since such steam as may leak past it presses on its circumference at all points equally4 ,,speed ,,governors4 ,illustration3 ,,fig4 #cc4--;,a speed governor4 ,practically all engines except locomotives and those known as donkey-engines--used on cranes--are fitted with some device for keeping the rotatory speed of the crank constant within very narrow limits4 ,perhaps you have seen a pair of balls moving round on a seating over the boiler of a threshing-engine4 ,they form part of the governor1 or speed-controller1 shown in principle in ,fig4 #cc4 ,a belt driven by a pulley on the crank shaft turns a small pulley1 ,p1 at the foot of the governor4 ,this transmits motion through two bevel-wheels1 ,g1 to a vertical shaft1 from the top of which hang two heavy balls on links1 ,k ,k4 ,two more links1 ,l ,l1 connect the balls with a weight1 ,w1 which has a deep groove cut round it at the bottom4 ,when the shaft revolves1 the balls fly outwards by centrifugal force1 and as their velocity increases the quadrilateral figure contained by the four links expands laterally and shortens vertically4 ,the angles between ,k ,k and ,l ,l become less and less obtuse1 and the weight ,w is drawn upwards1 bringing with it the fork ,c of the rod ,a1 which has ends engaging with the groove4 ,as ,c rises1 the other end of the rod is depressed1 and the rod ,b depresses rod ,o1 which is attached to the spindle operating a sort of shutter in the steam-pipe4 ,consequently the supply of steam is throttled more and more as the speed increases1 until it has been so reduced that the engine slows1 and the balls fall1 opening the valve again4 ,fig4 #cd shows the valve fully closed4 ,this form of governor was invented by ,james ,watt4 ,a spring is often used instead of a weight1 and the governor is arranged horizontally so that it may be driven direct from the crank shaft without the intervention of bevel gearing4 ,illustration3 ,,fig4 #cd4 ,the ,hartwell governor employs a link motion4 ,you must here picture the balls raising and lowering the free end of the valve rod1 which carries a block moving in a link connected with the eccentric rod4 ,the link is pivoted at the upper end1 and the eccentric rod is attached to the lower4 ,when the engine is at rest the end of the valve rod and its block are dropped till in a line with the eccentric rod2 but when the machinery begins to work the block is gradually drawn up by the governor1 diminishing the movement of the valve1 and so shortening the period of steam admission to the cylinder4 ,governors are of special importance where the load of an engine is constantly varying1 as in the case of a sawmill4 ,a good governor will limit variation of speed within two per cent4--that is1 if the engine is set to run at #ajj revolutions a minute1 it will not allow it to exceed #aja or fall below #ii4 ,in very high-speed engines the governing will prevent variation of less than one per cent41 even when the load is at one instant full on1 and the next taken completely off4 ,,marine ,,governors4 ,these must be more quick-acting than those used on engines provided with fly-wheels1 which prevent very sudden variations of speed4 ,the screw is light in proportion to the engine power1 and when it is suddenly raised from the water by the pitching of the vessel1 the engine would race till the screw took the water again1 unless some regulating mechanism were provided4 ,many types of marine governors have been tried4 ,the most successful seems to be one in which water is being constantly forced by a pump driven off the engine shaft into a cylinder controlling a throttle-valve in the main steam-pipe4 ,the water escapes through a leak1 which is adjustable4 ,as long as the speed of the engine is normal1 the water escapes from the cylinder as fast as it is pumped in1 and no movement of the piston results2 but when the screw begins to race1 the pump overcomes the leak1 and the piston is driven out1 causing a throttling of the steam supply4 ,,condensers4 ,the condenser serves two purposes3--7#a7 ,it makes it possible to use the same water over and over again in the boilers4 ,on the sea1 where fresh water is not obtainable in large quantities1 this is a matter of the greatest importance4 7#b7 ,it adds to the power of a compound engine by exerting a back pull on the piston of the low-pressure cylinder while the steam is being exhausted4 ,illustration3 ,,fig4 #ce4--,the marine condenser4 ,fig4 #ce is a sectional illustration of a marine condenser4 ,steam enters the condenser through the large pipe ,e1 and passes among a number of very thin copper tubes1 through which sea-water is kept circulating by a pump4 ,the path of the water is shown by the featherless arrows4 ,it comes from the pump through pipe ,a into the lower part of a large cap covering one end of the condenser and divided transversely by a diaphragm1 ,d4 ,passing through the pipes1 it reaches the cap attached to the other end1 and flows back through the upper tubes to the outlet ,c4 ,this arrangement ensures that1 as the steam condenses1 it shall meet colder and colder tubes1 and finally be turned to water1 which passes to the well through the outlet ,f4 ,in some condensers the positions of steam and water are reversed1 steam going through the tubes outside which cold water circulates4 #c ,also called ports4 #d ,the bores of the cylinders are in the proportion of #d3 #f3 #i4 ,the stroke of all three is the same4 #e ,the ends furthest from the eccentric4 #f ,the ,locomotive of ,to-day1 p4 #hg4 ,chapter ,,iii4 ,,the ,,steam ,,turbine4 ,how a turbine works--,the ,de ,laval turbine--,the ,parsons turbine--,description of the ,parsons turbine--,the expansive action of steam in a ,parsons turbine--,balancing the thrust--,advantages of the marine turbine4 ,more than two thousand years ago ,hero of ,alexandria produced the first apparatus to which the name of steam-engine could rightly be given4 ,its principle was practically the same as that of the revolving jet used to sprinkle lawns during dry weather1 steam being used in the place of water4 ,from the top of a closed cauldron rose two vertical pipes1 which at their upper ends had short1 right-angle bends4 ,between them was hung a hollow globe1 pivoted on two short tubes projecting from its sides into the upright tubes4 ,two little ,l-shaped pipes projected from opposite sides of the globe1 at the ends of a diameter1 in a plane perpendicular to the axis4 ,on fire being applied to the cauldron1 steam was generated4 ,it passed up through the upright1 through the pivots1 and into the globe1 from which it escaped by the two ,l-shaped nozzles1 causing rapid revolution of the ball4 ,in short1 the first steam-engine was a turbine4 ,curiously enough1 we have reverted to this primitive type 7scientifically developed1 of course7 in the most modern engineering practice4 ,,how ,a ,,turbine ,,works4 ,in reciprocating--that is1 cylinder--engines steam is admitted into a chamber and the door shut behind it1 as it were4 ,as it struggles to expand1 it forces out one of the confining walls--that is1 the piston--and presently the door opens again1 and allows it to escape when it has done its work4 ,in ,hero's toy the impact of the issuing molecules against other molecules that have already emerged from the pipes was used4 ,one may compare the reaction to that exerted by a thrown stone on the thrower4 ,if the thrower is standing on skates1 the reaction of the stone will cause him to glide backwards1 just as if he had pushed off from some fixed object4 ,in the case of the reaction--namely1 the ,hero-type--turbine the nozzle from which the steam or water issues moves1 along with bodies to which it may be attached4 ,in action turbines steam is led through fixed nozzles or steam-ways1 and the momentum of the steam is brought to bear on the surfaces of movable bodies connected with the shaft4 ,,the ,,de ,,laval ,,turbine4 ,in its earliest form this turbine was a modification of ,hero's4 ,the wheel was merely a pipe bent in ,s form1 attached at its centre to a hollow vertical shaft supplied with steam through a stuffing-box at one extremity4 ,the steam blew out tangentially from the ends of the ,s1 causing the shaft to revolve rapidly and work the machinery 7usually a cream separator7 mounted on it4 ,this motor proved very suitable for dairy work1 but was too wasteful of steam to be useful where high power was needed4 ,illustration3 ,,fig4 #cf4--,the wheel and nozzles of a ,de ,laval turbine4 ,in the ,de ,laval turbine as now constructed the steam is blown from stationary nozzles against vanes mounted on a revolving wheel4 ,fig4 #cf shows the nozzles and a turbine wheel4 ,the wheel is made as a solid disc1 to the circumference of which the vanes are dovetailed separately in a single row4 ,each vane is of curved section1 the concave side directed towards the nozzles1 which1 as will be gathered from the transparent specimen on the right of our illustration1 gradually expand towards the mouth4 ,this is to allow the expansion of the steam1 and a consequent gain of velocity4 ,as it issues1 each molecule strikes against the concave face of a vane1 and1 while changing its direction1 is robbed of its kinetic energy1 which passes to the wheel4 ,to turn once more to a stone-throwing comparison1 it is as if a boy were pelting the wheel with an enormous number of tiny stones4 ,now1 escaping high-pressure steam moves very fast indeed4 ,to give figures1 if it enters the small end of a ,de ,laval nozzle at #bjj lbs4 per square inch1 it will leave the big end at a velocity of #dh miles per minute--that is1 at a speed which would take it right round the world in #h-a_/b hours6 ,the wheel itself would not move at more than about one-third of this speed as a maximum4#g ,but even so1 it may make as many as #cj1jjj revolutions per minute4 ,a mechanical difficulty is now encountered--namely1 that arising from vibration4 ,no matter how carefully the turbine wheel may be balanced1 it is practically impossible to make its centre of gravity coincide exactly with the central point of the shaft2 in other words1 the wheel will be a bit--perhaps only a tiny fraction of an ounce--heavier on one side than the other4 ,this want of truth causes vibration1 which1 at the high speed mentioned1 would cause the shaft to knock the bearings in which it revolves to pieces1 if--and this is the point--those bearings were close to the wheel ,m4 de ,laval mounted the wheel on a shaft long enough between the bearings to whip1 or bend a little1 and the difficulty was surmounted4 ,the normal speed of the turbine wheel is too high for direct driving of some machinery1 so it is reduced by means of gearing4 ,to dynamos1 pumps1 and air-fans it is often coupled direct4 ,,the ,,parsons ,,turbine4 ,at the grand naval review held in #ahig in honour of ,queen ,victoria's diamond jubilee1 one of the most noteworthy sights was the little ,turbinia of #dd-a_/b tons burthen1 which darted about among the floating forts at a speed much surpassing that of the fastest destroyer4 ,inside the nimble little craft were engines developing #b1jjj horse power1 without any of the clank and vibration which usually reigns in the engine-room of a high-speed vessel4 ,the ,turbinia was the first turbine-driven boat1 and as such1 even apart from her extraordinary pace1 she attracted great attention4 ,since #ahig the ,parsons turbine has been installed on many ships1 including several men-of-war1 and it seems probable that the time is not far distant when reciprocating engines will be abandoned on all high-speed craft4 ,,description ,,of ,,the ,,parsons ,,turbine4 ,illustration3 ,,fig4 #cg4--,section of a ,parsons turbine4 ,the essential parts of a ,parsons turbine are3--7#a7 ,the shaft1 on which is mounted 7#b7 the drum2 7#c7 the cylindrical casing inside which the drum revolves2 7#d7 the vanes on the drum and casing2 7#e7 the balance pistons4 ,fig4 #cg shows a diagrammatic turbine in section4 ,the drum1 it will be noticed1 increases its diameter in three stages1 ,d#a1 ,d#b1 ,d#c1 towards the right4 ,from end to end it is studded with little vanes1 ,m ,m1 set in parallel rings small distances apart4 ,each vane has a curved section 7see ,fig4 #ch71 the hollow side facing towards the left4 ,the vanes stick out from the drum like short spokes1 and their outer ends almost touch the casing4 ,to the latter are attached equally-spaced rings of fixed vanes1 ,f ,f1 pointing inwards towards the drum1 and occupying the intervals between the rings of moving vanes4 ,their concave sides also face towards the left1 but1 as seen in ,fig4 #ch1 their line of curve lies the reverse way to that of ,m ,m4 ,steam enters the casing at ,a1 and at once rushes through the vanes towards the outlet at ,b4 ,it meets the first row of fixed vanes1 and has its path so deflected that it strikes the ring of moving 7or drum7 vanes at the most effective angle1 and pushes them round4 ,it then has its direction changed by the ring of ,f ,f1 so that it may treat the next row of ,m ,m in a similar fashion4 ,illustration3 ,,fig4 #ch4--;,blades or vanes of a ,parsons turbine4 ,illustration3 ,one of the low-pressure turbines of the ,carmania1 in casing4 ,its size will be inferred from comparison with the man standing near the end of the casing4 ,,the ,,expansive ,,action ,,of ,,steam ,,in ,a ,,turbine4 ,on reaching the end of ,d#a it enters the second1 or intermediate1 set of vanes4 ,the drum here is of a greater diameter1 and the blades are longer and set somewhat farther apart1 to give a freer passage to the now partly expanded steam1 which has lost pressure but gained velocity4 ,the process of movement is repeated through this stage2 and again in ,d#c1 the low-pressure drum4 ,the steam then escapes to the condenser through ,b1 having by this time expanded very many times2 and it is found advisable1 for reasons explained in connection with compound steam-engines1 to have a separate turbine in an independent casing for the extreme stages of expansion4 ,the vanes are made of brass4 ,in the turbines of the ,carmania1 the huge ,cunard liner1 #a1aae1jjj vanes are used4 ,the largest diameter of the drums is #aa feet1 and each low-pressure turbine weighs #cej tons4 ,,balancing ,,of ,,thrust4 ,the push exerted by the steam on the blades not only turns the drum1 but presses it in the direction in which the steam flows4 ,this end thrust is counterbalanced by means of the dummy pistons1 ,p#a1 ,p#b1 ,p#c4 ,each dummy consists of a number of discs revolving between rings projecting from the casing1 the distance between discs and rings being so small that but little steam can pass4 ,in the high-pressure compartment the steam pushes ,p#a to the left with the same pressure as it pushes the blades of ,d#a to the right4 ,after completing the first stage it fills the passage ,c1 which communicates with the second piston1 ,p#b1 and the pressure on that piston negatives the thrust on ,d#b4 ,similarly1 the passage ,e causes the steam to press equally on ,p#c and the vanes of ,d#c4 ,so that the bearings in which the shaft revolves have but little thrust to take4 ,this form of compensation is necessary in marine as well as in stationary turbines4 ,in the former the dummy pistons are so proportioned that the forward thrust given by them and the screw combined is almost equal to the thrust aft of the moving vanes4 ,illustration3 ,one of the turbine drums of the ,carmania4 ,note the rows of vanes4 ,the drum is here being tested for perfect balance on two absolutely level supports4 ,,advantages ,,of ,,the ,,marine ,,turbine4 7#a47 ,absence of vibration4 ,reciprocating engines1 however well balanced1 cause a shaking of the whole ship which is very unpleasant to passengers4 ,the turbine1 on the other hand1 being almost perfectly balanced1 runs so smoothly at the highest speeds that1 if the hand be laid on the covering1 it is sometimes almost impossible to tell whether the machinery is in motion4 ,as a consequence of this smooth running there is little noise in the engine-room--a pleasant contrast to the deafening roar of reciprocating engines4 7#b47 ,turbines occupy less room4 7#c47 ,they are more easily tended4 7#d47 ,they require fewer repairs1 since the rubbing surfaces are very small as compared to those of reciprocating engines4 7#e47 ,they are more economical at high speeds4 ,it must be remembered that a turbine is essentially meant for high speeds4 ,if run slowly1 the steam will escape through the many passages without doing much work4 ,owing to its construction1 a turbine cannot be reversed like a cylinder engine4 ,it therefore becomes necessary to fit special astern turbines to one or more of the screw shafts1 for use when the ship has to be stopped or moved astern4 ,under ordinary conditions these turbines revolve idly in their cases4 ,the highest speed ever attained on the sea was the forty-two miles per hour of the unfortunate ,viper1 a turbine destroyer which developed #aa1ejj horse power1 though displacing only #cgj tons4 ,this velocity would compare favourably with that of a good many expresses on certain railways that we could name4 ,in the future thirty miles an hour will certainly be attained by turbine-driven liners4 #g ,even at this speed the wheel has a circumferential velocity of two-thirds that of a bullet shot from a ,lee-,metford rifle4 ,a vane weighing only #bej grains 7about #a_/b oz47 exerts under these conditions a centrifugal pull of #ae cwt4 on the wheel6 ,chapter ,,iv4 ,,the ,,internal-,,combustion ,,engine4 ,the meaning of the term--,action of the internal-combustion engine--,the motor car--,the starting-handle--,the engine--,the carburetter--,ignition of the charge--,advancing the spark--,governing the engine--,the clutch--,the gear-box--,the compensating gear--,the silencer--,the brakes--,speed of cars4 ,,the ,,meaning ,,of ,,the ,,term ,,internal-,,combustion ,,engine4 ,in the case of a steam-boiler the energy of combustion is transmitted to water inside an air-tight vessel4 ,the fuel does not actually touch the working fluid4 ,in the gas or oil engine the fuel is brought into contact and mixed with the working fluid1 which is air4 ,it combines suddenly with it in the cylinder1 and heat energy is developed so rapidly that the act is called an explosion4 ,coal gas1 mineral oils1 alcohol1 petrol1 etc41 all contain hydrogen and carbon4 ,if air1 which contributes oxygen1 be added to any of these in due proportion1 the mixture becomes highly explosive4 ,on a light being applied1 oxygen and carbon unite1 also hydrogen and oxygen1 and violent heat is generated1 causing a violent molecular bombardment of the sides of the vessel containing the mixture4 ,now1 if the mixture be compressed it becomes hotter and hotter1 until a point is reached at which it ignites spontaneously4 ,early gas-engines did not compress the charge before ignition4 ,alphonse ,beau de ,rochas1 a ,frenchman1 first thought of making the piston of the engine squeeze the mixture before ignition2 and from the year #ahfb1 when he proposed this innovation1 the success of the internal-combustion engine may be said to date4 ,illustration ,illustration3 ,,fig4 #ci4--,showing the four strokes that the piston of a gas-engine makes during one cycle4 ,,action ,,of ,,the ,,engine4 ,the gas-engine1 the oil-engine1 and the motor-car engine are similar in general principles4 ,the cylinder has1 instead of a slide-valve1 two1 or sometimes three1 mushroom valves1 which may be described as small and thick round plates1 with bevelled edges1 mounted on the ends of short rods1 called stems4 ,these valves open into the cylinder1 upwards1 downwards1 or horizontally1 as the case may be2 being pushed in by cams projecting from a shaft rotated by the engine4 ,for the present we will confine our attention to the series of operations which causes the engine to work4 ,this series is called the ,beau de ,rochas1 or ,otto1 cycle1 and includes four movements of the piston4 ,reference to ,fig4 #ci will show exactly what happens in a gas-engine--7#a7 ,the piston moves from left to right1 and just as the movement commences valves ,g 7gas7 and ,a 7air7 open to admit the explosive mixture4 ,by the time that ,p has reached the end of its travel these valves have closed again4 7#b7 ,the piston returns to the left1 compressing the mixture1 which has no way of escape open to it4 ,at the end of the stroke the charge is ignited by an incandescent tube ,i 7in motor car and some stationary engines by an electric spark71 and 7#c7 the piston flies out again on the explosion stroke4 ,before it reaches the limit position1 valve ,e 7exhaust7 opens1 and 7#d7 the piston flies back under the momentum of the fly-wheel1 driving out the burnt gases through the still open ,e4 ,the cycle is now complete4 ,there has been suction1 compression 7including ignition71 combustion1 and exhaustion4 ,it is evident that a heavy fly-wheel must be attached to the crank shaft1 because the energy of one stroke 7the explosion7 has to serve for the whole cycle2 in other words1 for two complete revolutions of the crank4 ,a single-cylinder steam-engine develops an impulse every half-turn--that is1 four times as often4 ,in order to get a more constant turning effect1 motor cars have two1 three1 four1 six1 and even eight cylinders4 ,four-cylinder engines are at present the most popular type for powerful cars4 ,,the ,,motor ,,car4 ,illustration3 ,,fig4 #dj4--,plan of the chassis of a motor car4 ,we will now proceed to an examination of the motor car1 which1 in addition to mechanical apparatus for the transmission of motion to the driving-wheels1 includes all the fundamental adjuncts of the internal-combustion engine4#h ,fig4 #dj is a bird's-eye view of the chassis 7or works and wheels7 of a car1 from which the body has been removed4 ,starting at the left1 we have the handle for setting the engine in motion2 the engine 7a two-cylinder in this case72 the fly-wheel1 inside which is the clutch2 the gear-box1 containing the cogs for altering the speed of revolution of the driving-wheels relatively to that of the engine2 the propeller shaft2 the silencer1 for deadening the noise of the exhaust2 and the bevel-gear1 for turning the driving-wheels4 ,in the particular type of car here considered you will notice that a direct1 or shaft1 drive is used4 ,the shaft has at each end a flexible1 or universal1 joint1 which allows the shaft to turn freely1 even though it may not be in a line with the shaft projecting from the gear-box4 ,it must be remembered that the engine and gear-box are mounted on the frame1 between which and the axles are springs1 so that when the car bumps up and down1 the shaft describes part of a circle1 of which the gear-box end is the centre4 ,an alternative method of driving is by means of chains1 which run round sprocket 7cog7 wheels on the ends of a shaft crossing the frame just behind the gear-box1 and round larger sprockets attached to the hubs of the driving-wheels4 ,in such a case the axles of the driving-wheel are fixed to the springs1 and the wheels revolve round them4 ,where a ,cardan 7shaft7 drive is used the axles are attached rigidly to the wheels at one end1 and extend1 through tubes fixed to the springs1 to bevel-wheels in a central compensating-gear box 7of which more presently74 ,several parts--the carburetter1 tanks1 governor1 and pump--are not shown in the general plan4 ,these will be referred to in the more detailed account that follows4 ,,the ,,starting-,,handle4 ,illustration3 ,,fig4 #da4--,the starting-handle4 ,fig4 #da gives the starting-handle in part section4 ,the handle ,h is attached to a tube which terminates in a clutch1 ,c4 ,a powerful spring keeps ,c normally apart from a second clutch1 ,c#a1 keyed to the engine shaft4 ,when the driver wishes to start the engine he presses the handle towards the right1 brings the clutches together1 and turns the handle in a clockwise direction4 ,as soon as the engine begins to fire1 the faces of the clutches slip over one another4 ,,the ,,engine4 ,illustration3 ,,fig4 #db4--;,end and cross sections of a two-cylinder motor4 ,we next examine the two-cylinder engine 7,fig4 #db74 ,each cylinder is surrounded by a water-jacket1 through which water is circulated by a pump#i 7,fig4 #dc74 ,the heat generated by combustion is so great that the walls of the cylinder would soon become red-hot unless some of the heat were quickly carried away4 ,the pistons are of trunk form--that is1 long enough to act as guides and absorb the oblique thrust of the piston rods4 ,three or more piston rings lying in slots 7not shown7 prevent the escape of gas past the piston4 ,it is interesting to notice that the efficiency of an internal-combustion engine depends so largely on the good fit of these moving parts1 that cylinders1 pistons1 and rings must be exceedingly true4 ,a good firm will turn out standard parts which are well within #a_/ejjj of an inch of perfect truth4 ,it is also a wonderful testimony to the quality of the materials used that1 if properly looked after1 an engine which has made many millions of revolutions1 at the rate of #a1jjj to #b1jjj per minute1 often shows no appreciable signs of wear4 ,in one particular test an engine was run continuously for several months1 and at the end of the trial was in absolutely perfect condition4 ,the cranks revolve in an oil-tight case 7generally made of aluminium71 and dip in oil1 which they splash up into the cylinder to keep the piston well lubricated4 ,the plate1 ,p ,p1 through a slot in which the piston rod works1 prevents an excess of oil being flung up4 ,channels are provided for leading oil into the bearings4 ,the cranks are #ahj apart4 ,while one piston is being driven out by an explosion1 the other is compressing its charge prior to ignition1 so that the one action deadens the other4 ,therefore two explosions occur in one revolution of the cranks1 and none during the next revolution4 ,if both cranks were in line1 the pistons would move together1 giving one explosion each revolution4 ,illustration3 ,,fig4 #dc4--,showing how the water which cools the cylinders is circulated4 ,the valve seats1 and the inlet and exhaust pipes1 are seen in section4 ,the inlet valve here works automatically1 being pulled in by suction2 but on many engines--on all powerful engines--the inlet1 like the exhaust valve1 is lifted by a cam1 lest it should stick or work irregularly4 ,three dotted circles show ,a1 a cog on the crank shaft2 ,b1 a lay cog1 which transmits motion to ,c1 on a short shaft rotating the cam that lifts the exhaust valve4 ,c1 having twice as many teeth as ,a1 revolves at half its rate4 ,this ensures that the valve shall be lifted only once in two revolutions of the crank shaft to which it is geared4 ,the cogs are timed1 or arranged1 so that the cam begins to lift the valve when the piston has made about seven-eighths of its explosion stroke1 and closes the valve at the end of the exhaust stroke4 ,,the ,,carburetter4 ,a motor car generally uses petrol as its fuel4 ,petrol is one of the more volatile products of petroleum1 and has a specific gravity of about #fhj--that is1 volume for volume1 its weight is to that of water in the proportion of #fhj to #a1jjj4 ,it is extremely dangerous1 as it gives off an inflammable gas at ordinary temperatures4 ,benzine1 which we use to clean clothes1 is practically the same as petrol1 and should be treated with equal care4 ,the function of a carburetter is to reduce petrol to a very fine spray and mix it with a due quantity of air4 ,the device consists of two main parts 7,fig4 #dd7--the float chamber and the jet chamber4 ,in the former is a contrivance for regulating the petrol supply4 ,a float--a cork1 or air-tight metal box--is arranged to move freely up and down the stem of a needle-valve1 which closes the inlet from the tank4 ,at the bottom of the chamber are two pivoted levers1 ,w ,w1 which1 when the float rests on them1 tip up and lift the valve4 ,petrol flows in and raises the float4 ,this allows the valve to sink and cut off the supply4 ,if the valve is a good fit and the float is of the correct weight1 the petrol will never rise higher than the tip of the jet ,g4 ,illustration3 ,,fig4 #dd4--,section of a carburetter4 ,the suction of the engine makes petrol spirt through the jet 7which has a very small hole in its end7 and atomize itself against a spraying-cone1 ,a4 ,it then passes to the engine inlet pipe through a number of openings1 after mixing with air entering from below4 ,an extra air inlet1 controllable by the driver1 is generally added1 unless the carburetter be of a type which automatically maintains constant proportions of air and vapour4 ,the jet chamber is often surrounded by a jacket1 through which part of the hot exhaust gases circulate4 ,in cold weather especially this is a valuable aid to vaporization4 ,illustration3 ,,fig4 #de4--,sketch of the electrical ignition arrangements on a motor car4 ,,ignition ,,of ,,the ,,charge4 ,all petrol-cars now use electrical ignition4 ,there are two main systems--7#a7 by an accumulator and induction coil2 7#b7 magneto ignition1 by means of a small dynamo driven by the engine4 ,a general arrangement of the first is shown in ,fig4 #de4 ,a disc1 ,d1 of some insulating material--fibre or vulcanite--is mounted on the cam1 or half-speed1 shaft4 ,into the circumference is let a piece of brass1 called the contact-piece1 through which a screw passes to the cam shaft4 ,a movable plate1 ,m ,p1 which can be rotated concentrically with ,d through part of a circle1 carries a wipe block at the end of a spring1 which presses it against ,d4 ,the spring itself is attached to an insulated plate4 ,when the revolution of ,d brings the wipe and contact together1 current flows from the accumulator through switch ,s to the wipe2 through the contact-piece to ,c2 from ,c to ,m ,p and the induction coil2 and back to the accumulator4 ,this is the primary1 or low-tension1 circuit4 ,a high-tension current is induced by the coil in the secondary circuit1 indicated by dotted lines4#aj ,in this circuit is the sparking-plug 7see ,fig4 #df71 having a central insulated rod in connection with one terminal of the secondary coil4 ,between it and a bent wire projecting from the iron casing of the plug 7in contact with the other terminal of the secondary coil through the metal of the engine1 to which one wire of the circuit is attached7 is a small gap1 across which the secondary current leaps when the primary current is broken by the wipe and contact parting company4 ,the spark is intensely hot1 and suffices to ignite the compressed charge in the cylinder4 ,illustration3 ,,fig4 #df4--,section of a sparking-plug4 ,,advancing ,,the ,,spark4 ,we will assume that the position of ,w 7in ,fig4 #de7 is such that the contact touches ,w at the moment when the piston has just completed the compression stroke4 ,now1 the actual combustion of the charge occupies an appreciable time1 and with the engine running at high speed the piston would have travelled some way down the cylinder before the full force of the explosion was developed4 ,but by raising lever ,l1 the position of ,w may be so altered that contact is made slightly before the compression stroke is complete1 so that the charge is fairly alight by the time the piston has altered its direction4 ,this is called advancing the spark4 ,,governing ,,the ,,engine4 ,there are several methods of controlling the speed of internal-combustion engines4 ,the operating mechanism in most cases is a centrifugal ball-governor4 ,when the speed has reached the fixed limit it either 7#a7 raises the exhaust valve1 so that no fresh charges are drawn in2 7#b7 prevents the opening of the inlet valve2 or 7#c7 throttles the gas supply4 ,the last is now most commonly used on motor cars1 in conjunction with some device for putting it out of action when the driver wishes to exceed the highest speed that it normally permits4 ,illustration3 ,,fig4 #dg4--,one form of governor used on motor cars4 ,a sketch of a neat governor1 with regulating attachment1 is given in ,fig4 #dg4 ,the governor shaft is driven from the engine4 ,as the balls1 ,b ,b1 increase their velocity1 they fly away from the shaft and move the arms1 ,a ,a1 and a sliding tube1 ,c1 towards the right4 ,this rocks the lever ,r1 and allows the valves in the inlet pipe to close and reduce the supply of air and gas4 ,a wedge1 ,w1 which can be raised or lowered by lever ,l1 intervenes between the end of ,r and the valve stem4 ,if this lever be lifted to its highest position1 the governing commences at a lower speed1 as the valve then has but a short distance to travel before closing completely4 ,for high speeds the driver depresses ,l1 forces the wedge down1 and so minimizes the effect of the governor4 ,,the ,,clutch4 ,the engine shaft has on its rear end the fly-wheel1 which has a broad and heavy rim1 turned to a conical shape inside4 ,close to this1 revolving loosely on the shaft1 is the clutch plate1 a heavy disc with a broad edge so shaped as to fit the inside of a fly-wheel4 ,it is generally faced with leather4 ,a very strong spring presses the plate into the fly-wheel1 and the resulting friction is sufficient to prevent any slip4 ,projections on the rear of the clutch engage with the gear-box shaft4 ,the driver throws out the clutch by depressing a lever with his foot4 ,some clutches dispense with the leather lining4 ,these are termed metal to metal clutches4 ,,the ,,gear-,,box4 ,we now come to a very interesting detail of the motor car1 the gear-box4 ,the steam-engine has its speed increased by admitting more steam to the cylinders4 ,but an explosion engine must be run at a high speed to develop its full power1 and when heavier work has to be done on a hill it becomes necessary to alter the speed ratio of engine to driving-wheels4 ,our illustration 7,fig4 #dh7 gives a section of a gear-box1 which will serve as a typical example4 ,it provides three forward speeds and one reverse4 ,to understand how it works1 we must study the illustration carefully4 ,pinion #a is mounted on a hollow shaft turned by the clutch4 ,into the hollow shaft projects the end of another shaft carrying pinions #f and #d4 ,pinion #f slides up and down this shaft1 which is square at this point1 but round inside the loose pinion #d4 ,pinions #b and #c are keyed to a square secondary shaft1 and are respectively always in gear with #a and #d2 but #e can be slid backwards and forwards so as to engage or disengage with #f4 ,in the illustration no gear is in4 ,if the engine is working1 #a revolves #b1 #b turns #c1 and #c revolves #d idly on its shaft4 ,illustration3 ,,fig4 #dh4--,the gear-box of a motor car4 ,to get the lowest1 or first1 speed the driver moves his lever and slides #e into gear with #f4 ,the transmission then is3 #a turns #b1 #b turns #e1 #e turns #f1 #f turns the propeller shaft through the universal joint4 ,for the second speed1 #e and #f are disengaged1 and #f is moved up the page1 as it were1 till projections on it interlock with slots in #d2 thus driving #a1 #b1 #c1 #d1 shaft4 ,for the third1 or solid1 speed1 #f is pulled down into connection with #a1 and couples the engine shaft direct to the propeller shaft4 ,the reverse is accomplished by raising a long pinion1 #g1 which lies in the gear-box under #e and #f4 ,the drive then is #a1 #b1 #e1 #g1 #f4 ,there being an odd number of pinions now engaged1 the propeller shaft turns in the reverse direction to that of the engine shaft4 ,illustration3 ,,fig4 #di4 ,,the ,,compensating ,,gear4 ,every axle of a railway train carries a wheel at each end1 rigidly attached to it4 ,when rounding a corner the outside wheel has further to travel than the other1 and consequently one or both wheels must slip4 ,the curves are made so gentle1 however1 that the amount of slip is very small4 ,but with a traction-engine1 motor car1 or tricycle the case is different1 for all have to describe circles of very small diameter in proportion to the length of the vehicle4 ,therefore in every case a compensating gear is fitted1 to allow the wheels to turn at different speeds1 while permitting them both to drive4 ,fig4 #di is an exaggerated sketch of the gear4 ,the axles of the moving wheels turn inside tubes attached to the springs and a central casing 7not shown71 and terminate in large bevel-wheels1 ,c and ,d4 ,between these are small bevels mounted on a shaft supported by the driving drum4 ,if the latter be rotated1 the bevels would turn ,c and ,d at equal speeds1 assuming that both axles revolve without friction in their bearings4 ,we will suppose that the drum is turned #ej times a minute4 ,now1 if one wheel be held1 the other will revolve #ajj times a minute2 or1 if one be slowed1 the other will increase its speed by a corresponding amount4 ,the average speed remains #ej4 ,it should be mentioned that drum ,a has incorporated with it on the outside a bevel-wheel 7not shown7 rotated by a smaller bevel on the end of the propeller shaft4 ,,the ,,silencer4 ,the petrol-engine1 as now used1 emits the products of combustion at a high pressure4 ,if unchecked1 they expand violently1 and cause a partial vacuum in the exhaust pipe1 into which the air rushes back with such violence as to cause a loud noise4 ,devices called silencers are therefore fitted1 to render the escape more gradual1 and split it up among a number of small apertures4 ,the simplest form of silencer is a cylindrical box1 with a number of finely perforated tubes passing from end to end of it4 ,the exhaust gases pouring into the box maintain a constant pressure somewhat higher than that of the atmosphere1 but as the gases are escaping from it in a fairly steady stream the noise becomes a gentle hiss rather than a pop4 ,there are numerous types of silencers1 but all employ this principle in one form or another4 ,,the ,,brakes4 ,every car carries at least two brakes of band pattern--one1 usually worked by a side hand-lever1 acting on the axle or hubs of the driving-wheel2 the other1 operated by the foot1 acting on the transmission gear 7see ,fig4 #dh74 ,the latter brake is generally arranged to withdraw the clutch simultaneously4 ,tests have proved that even heavy cars can be pulled up in astonishingly short distances1 considering their rate of travel4 ,trials made in the ,united ,states with a touring car and a four-in-hand coach gave #be-a_/c and #gj feet respectively for the distance in which the speed could be reduced from sixteen miles per hour to zero4 ,,speed ,,of ,,cars4 ,as regards speed1 motor cars can rival the fastest express trains1 even on long journeys4 ,in fact1 feats performed during the ,gordon-,bennett and other races have equalled railway performances over equal distances4 ,when we come to record speeds1 we find a car1 specially built for the purpose1 covering a mile in less than half a minute4 ,a speed of over #abj miles an hour has actually been reached4 ,engines of #aej h4p4 can now be packed into a vehicle scaling less than #a-a_/b tons4 ,even on touring cars are often found engines developing #dj to #fj h4p41 which force the car up steep hills at a pace nothing less than astonishing4 ,in the future the motor car will revolutionize our modes of life to an extent comparable to the changes effected by the advent of the steam-engine4 ,even since #ahif1 when the man-with-the-flag law was abolished in the ,british ,isles1 the motor has reduced distances1 opened up country districts1 and generally quickened the pulses of the community in a manner which makes it hazardous to prophesy how the next generation will live4 ,note4--,the author is much indebted to ,mr4 ,wilfrid ,j4 ,lineham1 ,m4 ,inst4 ,c4,e41 for several of the illustrations which appear in the above chapter4 #h ,steam-driven cars are not considered in this chapter1 as their principle is much the same as that of the ordinary locomotive4 #i ,on some cars natural circulation is used1 the hot water flowing from the top of the cylinder to the tank1 from which it returns1 after being cooled1 to the bottom of the cylinder4 #aj ,for explanation of the induction coil1 see p4 #abb ,chapter ,v4 ,,electrical ,,apparatus4 ,what is electricity8--,forms of electricity--,magnetism--,the permanent magnet--,lines of force--,electro-magnets--,the electric bell--,the induction coil--,the condenser--,transformation of current--,uses of the induction coil4 ,,what ,,is ,,electricity8 ,of the ultimate nature of electricity1 as of that of heat and light1 we are at present ignorant4 ,but it has been clearly established that all three phenomena are but manifestations of the energy pervading the universe4 ,by means of suitable apparatus one form can be converted into another form4 ,the heat of fuel burnt in a boiler furnace develops mechanical energy in the engine which the boiler feeds with steam4 ,the engine revolves a dynamo1 and the electric current thereby generated can be passed through wires to produce mechanical motion1 heat1 or light4 ,we must remain content1 therefore1 with assuming that electricity is energy or motion transmitted through the ether from molecule to molecule1 or from atom to atom1 of matter4 ,scientific investigation has taught us how to produce it at will1 how to harness it to our uses1 and how to measure it2 but not what it is4 ,that question may1 perhaps1 remain unanswered till the end of human history4 ,a great difficulty attending the explanation of electrical action is this--that1 except in one or two cases1 no comparison can be established between it and the operation of gases and fluids4 ,when dealing with the steam-engine1 any ordinary intelligence soon grasps the principles which govern the use of steam in cylinders or turbines4 ,the diagrams show1 it is hoped1 quite plainly how it works4 ,but electricity is elusive1 invisible2 and the greatest authorities cannot say what goes on at the poles of a magnet or on the surface of an electrified body4 ,even the existence of negative and positive electricity is problematical4 ,however1 we see the effects1 and we know that if one thing is done another thing happens2 so that we are at least able to use terms which1 while convenient1 are not at present controverted by scientific progress4 ,,forms ,,of ,,electricity4 ,rub a vulcanite rod and hold one end near some tiny pieces of paper4 ,they fly to it1 stick to it for a time1 and then fall off4 ,the rod was electrified--that is1 its surface was affected in such a way as to be in a state of molecular strain which the contact of the paper fragments alleviated4 ,by rubbing large surfaces and collecting the electricity in suitable receivers the strain can be made to relieve itself in the form of a violent discharge accompanied by a bright flash4 ,this form of electricity is known as static4 ,next1 place a copper plate and a zinc plate into a jar full of diluted sulphuric acid4 ,if a wire be attached to them a current of electricity is said to flow along the wire4 ,we must not1 however1 imagine that anything actually moves along inside the wire1 as water1 steam1 or air1 passes through a pipe4 ,professor ,trowbridge says1#aa ,no other agency for transmitting power can be stopped by such slight obstacles as electricity4 ,a thin sheet of paper placed across a tube conveying compressed air would be instantly ruptured4 ,it would take a wall of steel at least an inch thick to stand the pressure of steam which is driving a #aj1jjj horse-power engine4 ,a thin layer of dirt beneath the wheels of an electric car can prevent the current which propels the car from passing to the rail1 and then back to the power-house4 ,there would1 indeed1 be a puncture of the paper if the current had a sufficient voltage1 or pressure2 yet the fact remains that current electricity can be very easily confined to its conductor by means of some insulating or nonconducting envelope4 ,,magnetism4 ,the most familiar form of electricity is that known as magnetism4 ,when a bar of steel or iron is magnetized1 it is supposed that the molecules in it turn and arrange themselves with all their north-seeking poles towards the one end of the bar1 and their south-seeking poles towards the other4 ,if the bar is balanced freely on a pivot1 it comes to rest pointing north and south2 for1 the earth being a huge magnet1 its north pole attracts all the north-seeking poles of the molecules1 and its south poles the south-seeking poles4 7,the north-seeking pole of a magnet is marked ,n41 though it is in reality the south pole2 for unlike poles are mutually attractive1 and like poles repellent47 ,there are two forms of magnet--permanent and temporary4 ,if steel is magnetized1 it remains so2 but soft iron loses practically all its magnetism as soon as the cause of magnetization is withdrawn4 ,this is what we should expect2 for steel is more closely compacted than iron1 and the molecules therefore would be able to turn about more easily4#ab ,it is fortunate for us that this is so1 since on the rapid magnetization and demagnetization of soft iron depends the action of many of our electrical mechanisms4 ,,the ,,permanent ,,magnet4 ,magnets are either 7#a7 straight1 in which case they are called bar magnets2 or 7#b7 of horseshoe form1 as in ,figs4 #ej and #ea4 ,by bending the magnet the two poles are brought close together1 and the attraction of both may be exercised simultaneously on a bar of steel or iron4 ,,lines ,,of ,,force4 ,in ,fig4 #ej are seen a number of dotted lines4 ,these are called lines of magnetic force4 ,if you lay a sheet of paper on a horseshoe magnet and sprinkle it with iron dust1 you will at once notice how the particles arrange themselves in curves similar in shape to those shown in the illustration4 ,it is supposed 7it cannot be proved7 that magnetic force streams away from the ,n4 pole and describes a circular course through the air back to the ,s4 pole4 ,the same remark applies to the bar magnet4 ,,electrical ,,magnets4 ,illustration3 ,,fig4 #ej4--,permanent magnet1 and the lines of force emanating from it4 ,if an insulated wire is wound round and round a steel or iron bar from end to end1 and has its ends connected to the terminals of an electric battery1 current rotates round the bar1 and the bar is magnetized4 ,by increasing the strength and volume of the current1 and multiplying the number of turns of wire1 the attractive force of the magnet is increased4 ,now disconnect the wires from the battery4 ,if of iron1 the magnet at once loses its attractive force2 but if of steel1 it retains it in part4 ,instead of a simple horseshoe-shaped bar1 two shorter bars riveted into a plate are generally used for electromagnets of this type4 ,coils of wire are wound round each bar1 and connected so as to form one continuous whole2 but the wire of one coil is wound in the direction opposite to that of the other4 ,the free end of each goes to a battery terminal4 ,in ,fig4 #ea you will notice that some of the lines of force are deflected through the iron bar ,a4 ,they pass more easily through iron than through air2 and will choose iron by preference4 ,the attraction exercised by a magnet on iron may be due to the effort of the lines of force to shorten their paths4 ,it is evident that the closer ,a comes to the poles of the magnet the less will be the distance to be travelled from one pole to the bar1 along it1 and back to the other pole4 ,illustration3 ,,fig4 #ea4--;,electro-magnet3 ,a1 armature2 ,b1 battery4 ,having now considered electricity in three of its forms--static1 current1 and rotatory--we will pass to some of its applications4 ,,the ,,electric ,,bell4 ,a fit device to begin with is the ,electric ,bell1 which has so largely replaced wire-pulled bells4 ,these last cause a great deal of trouble sometimes1 since if a wire snaps it may be necessary to take up carpets and floor-boards to put things right4 ,their installation is not simple1 for at every corner must be put a crank to alter the direction of the pull1 and the cranks mean increased friction4 ,but when electric wires have once been properly installed1 there should be no need for touching them for an indefinite period4 ,they can be taken round as many corners as you wish without losing any of their conductivity1 and be placed wherever is most convenient for examination4 ,one bell may serve a large number of rooms if an indicator be used to show where the call was made from1 by a card appearing in one of a number of small windows4 ,before answering a call1 the attendant presses in a button to return the card to its normal position4 ,in ,fig4 #eb we have a diagrammatic view of an electric bell and current4 ,when the bell-push is pressed in1 current flows from the battery to terminal ,t#a1 round the electro-magnet ,m1 through the pillar ,p and flat steel springs ,s and ,b1 through the platinum-pointed screw1 and back to the battery through the push4 ,the circulation of current magnetizes ,m1 which attracts the iron armature ,a attached to the spring ,s1 and draws the hammer ,h towards the gong4 ,just before the stroke occurs1 the spring ,b leaves the tip of the screw1 and the circuit is broken1 so that the magnet no longer attracts4 ,h is carried by its momentum against the gong1 and is withdrawn by the spring1 until ,b once more makes contact1 and the magnet is re-excited4 ,the hammer vibrations recur many times a second as long as the push is pressed in4 ,illustration3 ,,fig4 #eb4--,sketch of an electric-bell circuit4 ,the electric bell is used for so many purposes that they cannot all be noted4 ,it plays an especially important part in telephonic installations to draw the attention of the subscribers1 forms an item in automatic fire and burglar alarms1 and is a necessary adjunct of railway signalling cabins4 ,,the ,,induction ,,or ,,ruhmkorff ,,coil4 ,reference was made in connection with the electrical ignition of internal-combustion engines 7p4 #aja7 to the induction coil4 ,this is a device for increasing the voltage1 or pressure1 of a current4 ,the two-cell accumulator carried in a motor car gives a voltage 7otherwise called electro-motive force ,e4,m4,f47 of #dd volts4 ,if you attach a wire to one terminal of the accumulator and brush the loose end rapidly across the other terminal1 you will notice that a bright spark passes between the wire and the terminal4 ,in reality there are two sparks1 one when they touch1 and another when they separate1 but they occur so closely together that the eye cannot separate the two impressions4 ,a spark of this kind would not be sufficiently hot to ignite a charge in a motor cylinder1 and a spark from the induction coil is therefore used4 ,illustration3 ,,fig4 #ec4--,sketch of an induction coil4 ,we give a sketch of the induction coil in ,fig4 #ec4 ,it consists of a core of soft iron wires round which is wound a layer of coarse insulated wire1 denoted by the thick line4 ,one end of the winding of this primary coil is attached to the battery1 the other to the base of a hammer1 ,h1 vibrating between the end of the core and a screw1 ,s1 passing through an upright1 ,t1 connected with the other terminal of the battery4 ,the action of the hammer is precisely the same as that of the armature of an electric bell4 ,outside the primary coil are wound many turns of a much finer wire completely insulated from the primary coil4 ,the ends of this secondary coil are attached to the objects 7in the case of a motor car1 the insulated wire of the sparking-plug and a wire projecting from its outer iron casing7 between which a spark has to pass4 ,as soon as ,h touches ,s the circuit is completed4 ,the core becomes a powerful magnet with external lines of force passing from one pole to the other over and among the turns of the secondary coil4 ,h is almost instantaneously attracted by the core1 and the break occurs4 ,the lines of force now 7at least so it is supposed7 sink into the core1 cutting through the turns of the secondary1 and causing a powerful current to flow through them4 ,the greater the number of turns1 the greater the number of times the lines of force are cut1 and the stronger is the current4 ,if sufficiently intense1 it jumps any gap in the secondary circuit1 heating the intermediate air to a state of incandescence4 ,,the ,,condenser4 ,the sudden parting of ,h and ,s would produce strong sparking across the gap between them if it were not for the condenser1 which consists of a number of tinfoil sheets separated by layers of paraffined paper4 ,all the odd sheets are connected with ,t1 all the even with ,t#a4 ,now1 the more rapid the extinction of magnetism in the core after break of the primary circuit1 the more rapidly will the lines of force collapse1 and the more intense will be the induced current in the secondary coil4 ,the condenser diminishes the period of extinction very greatly1 while lengthening the period of magnetization after the make of the primary current1 and so decreasing the strength of the reverse current4 ,,transformation ,,of ,,current4 ,the difference in the voltage of the primary and secondary currents depends on the length of the windings4 ,if there are #ajj turns of wire in the primary1 and #ajj1jjj turns in the secondary1 the voltage will be increased #a1jjj times2 so that a #d-volt current is stepped up to #d1jjj volts4 ,in the largest induction coils the secondary winding absorbs #bjj-cjj miles of wire1 and the spark given may be anything up to four feet in length4 ,such a spark would pierce a glass plate two inches thick4 ,it must not be supposed that an induction coil increases the amount of current given off by a battery4 ,it merely increases its pressure at the expense of its volume--stores up its energy1 as it were1 until there is enough to do what a low-tension flow could not effect4 ,a fair comparison would be to picture the energy of the low-tension current as the momentum of a number of small pebbles thrown in succession at a door1 say #ajj a minute4 ,if you went on pelting the door for hours you might make no impression on it1 but if you could knead every #ajj pebbles into a single stone1 and throw these stones one per minute1 you would soon break the door in4 ,any intermittent current can be transformed as regards its intensity4 ,you may either increase its pressure while decreasing its rate of flow1 or amperage2 or decrease its pressure and increase its flow4 ,in the case that we have considered1 a continuous battery current is rendered intermittent by a mechanical contrivance4 ,but if the current comes from an alternating dynamo--that is1 is already intermittent--the contact-breaker is not needed4 ,there will be more to say about transformation of current in later paragraphs4 ,,uses ,,of ,,the ,,induction ,,coil4 ,the induction coil is used--7#a47 ,for passing currents through glass tubes almost exhausted of air or containing highly rarefied gases4 ,the luminous effects of these ,geissler tubes are very beautiful4 7#b47 ,for producing the now famous ,x or ,rntgen rays4 ,these rays accompany the light rays given off at the negative terminal 7cathode7 of a vacuum tube1 and are invisible to the eye unless caught on a fluorescent screen1 which reduces their rate of vibration sufficiently for the eye to be sensitive to them4 ,the ,rntgen rays have the peculiar property of penetrating many substances quite opaque to light1 such as metals1 stone1 wood1 etc41 and as a consequence have proved of great use to the surgeon in localizing or determining the nature of an internal injury4 ,they also have a deterrent effect upon cancerous growths4 7#c47 ,in wireless telegraphy1 to cause powerful electric oscillations in the ether4 7#d47 ,on motor cars1 for igniting the cylinder charges4 7#e47 ,for electrical massage of the body4 #aa ,what is ,electricity8 p4 #df4 #ab ,if a magnetized bar be heated to white heat and tapped with a hammer it loses its magnetism1 because the distance between the molecules has increased1 and the molecules can easily return to their original positions4 ,chapter ,,vi4 ,,the ,,electric ,,telegraph4 ,needle instruments--,influence of current on the magnetic needle--,method of reversing the current--,sounding instruments--,telegraphic relays--,recording telegraphs--,high-speed telegraphy4 ,take a small pocket compass and wind several turns of fine insulated wire round the case1 over the top and under the bottom4 ,now lay the compass on a table1 and turn it about until the coil is on a line with the needle--in fact1 covers it4 ,next touch the terminals of a battery with the ends of the wire4 ,the needle at once shifts either to right or left1 and remains in that position as long as the current flows4 ,if you change the wires over1 so reversing the direction of the current1 the needle at once points in the other direction4 ,it is to this conduct on the part of a magnetic needle when in a magnetic field that we owe the existence of the needle telegraph instrument4 ,,needle ,,instruments4 ,illustration3 ,,fig4 #ed4--,sketch of the side elevation of a ,wheatstone needle instrument4 ,probably the best-known needle instrument is the ,cooke-,wheatstone1 largely used in signal-boxes and in some post-offices4 ,a vertical section of it is shown in ,fig4 #ed4 ,it consists of a base1 ,b1 and an upright front1 ,a1 to the back of which are attached two hollow coils on either side of a magnetic needle mounted on the same shaft as a second dial needle1 ,n1 outside the front4 ,the wires ,w ,w are connected to the telegraph line and to the commutator1 a device which1 when the operator moves the handle ,h to right and left1 keeps reversing the direction of the current4 ,the needles on both receiving and transmitting instruments wag in accordance with the movements of the handle4 ,one or more movements form an alphabetical letter of the ,morse code4 ,thus1 if the needle points first to left1 and then to right1 and comes to rest in a normal position for a moment1 the letter ,a is signified2 right-left-left-left in quick succession ,b2 right-left-right-left ,c1 and so on4 ,where a marking instrument is used1 a dot signifies a left1 and a dash a right2 and if a sounder is employed1 the operator judges by the length of the intervals between the clicks4 ,,influence ,,of ,,current ,,on ,a ,,magnetic ,,needle4 ,illustration3 ,,figs4 #ee1 #ef4--,the coils of a needle instrument4 ,the arrows show the direction taken by the current4 ,figs4 #ee and #ef are two views of the coils and magnetic needle of the ,wheatstone instrument as they appear from behind4 ,in ,fig4 #ee the current enters the left-hand coil from the left1 and travels round and round it in a clockwise direction to the other end1 whence it passes to the other coil and away to the battery4 ,now1 a coil through which a current passes becomes a magnet4 ,its polarity depends on the direction in which the current flows4 ,suppose that you are looking through the coil1 and that the current enters it from your end4 ,if the wire is wound in a clockwise direction1 the ,s4 pole will be nearest you2 if in an anti-clockwise direction1 the ,n4 pole4 ,in ,fig4 #ee the ,n4 poles are at the right end of the coils1 the ,s4 poles at the left end2 so the ,n4 pole of the needle is attracted to the right1 and the ,s4 pole to the left4 ,when the current is reversed1 as in ,fig4 #ef1 the needle moves over4 ,if no current passes1 it remains vertical4 ,,method ,,of ,,reversing ,,the ,,current4 ,illustration3 ,,fig4 #eg4--;,general arrangement of needle-instrument circuit4 ,the shaded plates on the left 7,b and ,r7 are in contact4 ,a simple method of changing the direction of the current in a two-instrument circuit is shown diagrammatically in ,fig4 #eg4 ,the principle is used in the ,wheatstone needle instrument4 ,the battery terminals at each station are attached to two brass plates1 ,a ,b1 ,a#a ,b#a4 ,crossing these at right angles 7under ,a ,a#a and over ,b ,b#a7 are the flat brass springs1 ,l ,r1 ,l#a ,r#a1 having buttons at their lower ends1 and fixed at their upper ends to baseboards4 ,when at rest they all press upwards against the plates ,a and ,a#a respectively4 ,r and ,l#a are connected with the line circuit1 in which are the coils of dials #a and #b1 one at each station4 ,l and ,r#a are connected with the earth-plates ,e ,e#a4 ,an operator at station #a depresses ,r so as to touch ,b4 ,current now flows from the battery to ,b1 thence through ,r to the line circuit1 round the coils of both dials through ,l#a ,a#a and ,r to earth-plate ,e#a1 through the earth to ,e1 and then back to the battery through ,l and ,a4 ,the needles assume the position shown4 ,to reverse the current the operator allows ,r to rise into contact with ,a1 and depresses ,l to touch ,b4 ,the course can be traced out easily4 ,in the ,wheatstone drop-handle instrument 7,fig4 #ed7 the commutator may be described as an insulated core on which are two short lengths of brass tubing4 ,one of these has rubbing against it a spring connected with the terminal of the battery2 the other has similar communication with the - terminal4 ,projecting from each tube is a spike1 and rising from the baseboard are four upright brass strips not quite touching the commutator4 ,those on one side lead to the line circuit1 those on the other to the earth-plate4 ,when the handle is turned one way1 the spikes touch the forward line strip and the rear earth strip1 and vice vers when moved in the opposite direction4 ,,sounding ,,instruments4 ,sometimes little brass strips are attached to the dial plate of a needle instrument for the needle to strike against4 ,as these give different notes1 the operator can comprehend the message by ear alone4 ,but the most widely used sounding instrument is the ,morse sounder1 named after its inventor4 ,for this a reversible current is not needed4 ,the receiver is merely an electro-magnet 7connected with the line circuit and an earth-plate7 which1 when a current passes1 attracts a little iron bar attached to the middle of a pivoted lever4 ,the free end of the lever works between two stops4 ,every time the circuit is closed by the transmitting key at the sending station the lever flies down against the lower stop1 to rise again when the circuit is broken4 ,the duration of its stay decides whether a long or short is meant4 ,,telegraphic ,,relays4 ,illustration3 ,,fig4 #eh4--,section of a telegraph wire insulator on its arm4 ,the shaded circle is the line wire1 the two blank circles indicate the wire which ties the line wire to the insulator4 ,when an electric current has travelled for a long distance through a wire its strength is much reduced on account of the resistance of the wire1 and may be insufficient to cause the electro-magnet of the sounder to move the heavy lever4 ,instead1 therefore1 of the current acting directly on the sounder magnet1 it is used to energize a small magnet1 or relay1 which pulls down a light bar and closes a second local circuit--that is1 one at the receiver end--worked by a separate battery1 which has sufficient power to operate the sounder4 ,,recording ,,telegraphs4 ,by attaching a small wheel to the end of a ,morse-sounder lever1 by arranging an ink-well for the wheel to dip into when the end falls1 and by moving a paper ribbon slowly along for the wheel to press against when it rises1 a self-recording ,morse inker is produced4 ,the ribbon-feeding apparatus is set in motion automatically by the current1 and continues to pull the ribbon along until the message is completed4 ,the ,hughes type-printer covers a sheet of paper with printed characters in bold ,roman type4 ,the transmitter has a keyboard1 on which are marked letters1 signs1 and numbers2 also a type-wheel1 with the characters on its circumference1 rotated by electricity4 ,the receiver contains mechanisms for rotating another type-wheel synchronously--that is1 in time--with the first2 for shifting the wheel across the paper2 for pressing the paper against the wheel2 and for moving the paper when a fresh line is needed4 ,these are too complicated to be described here in detail4 ,by means of relays one transmitter may be made to work five hundred receivers4 ,in ,london a single operator1 controlling a keyboard in the central dispatching office1 causes typewritten messages to spell themselves out simultaneously in machines distributed all over the metropolis4 ,the tape machine resembles that just described in many details4 ,the main difference is that it prints on a continuous ribbon instead of on sheets4 ,automatic electric printers of some kind or other are to be found in the vestibules of all the principal hotels and clubs of our large cities1 and in the offices of bankers1 stockbrokers1 and newspaper editors4 ,in ,london alone over #ejj million words are printed by the receivers in a year4 ,,high-,,speed ,,telegraphy4 ,at certain seasons1 or when important political events are taking place1 the telegraph service would become congested with news were there not some means of transmitting messages at a much greater speed than is possible by hand signalling4 ,fifty words a minute is about the limit speed that a good operator can maintain4 ,by means of ,wheatstone's automatic transmitter the rate can be increased to #djj words per minute4 ,paper ribbons are punched in special machines by a number of clerks with a series of holes which by their position indicate a dot or a dash4 ,the ribbons are passed through a special transmitter1 over little electric brushes1 which make contact through the holes with surfaces connected to the line circuit4 ,at the receiver end the message is printed by a ,morse inker4 ,it has been found possible to send several messages simultaneously over a single line4 ,to effect this a distributer is used to put a number of transmitters at one end of the line in communication with an equal number of receivers at the other end1 fed by a second distributer keeping perfect time with the first4 ,instead of a signal coming as a whole to any one instrument it arrives in little bits1 but these follow one another so closely as to be practically continuous4 ,by working a number of automatic transmitters through a distributer1 a thousand words or more per minute are easily dispatched over a single wire4 ,the ,pollak ,virag system employs a punched ribbon1 and the receiver traces out the message in alphabetical characters on a moving strip of sensitized photographic paper4 ,a mirror attached to a vibrating diaphragm reflects light from a lamp on to the strip1 which is automatically developed and fixed in chemical baths4 ,the method of moving the mirror so as to make the rays trace out words is extremely ingenious4 ,messages have been transmitted by this system at the rate of #ahj1jjj words per hour4 ,chapter ,,vii4 ,,wireless ,,telegraphy4 ,the transmitting apparatus--,the receiving apparatus--,syntonic transmission--,the advance of wireless telegraphy4 ,in our last chapter we reviewed briefly some systems of sending telegraphic messages from one point of the earth's surface to another through a circuit consisting partly of an insulated wire and partly of the earth itself4 ,the metallic portion of a long circuit1 especially if it be a submarine cable1 is costly to install1 so that in quite the early days of telegraphy efforts were made to use the ether in the place of wire as one conductor4 ,when a hammer strikes an anvil the air around is violently disturbed4 ,this disturbance spreads through the molecules of the air in much the same way as ripples spread from the splash of a stone thrown into a pond4 ,when the sound waves reach the ear they agitate the tympanum1 or drum membrane1 and we hear a noise4 ,the hammer is here the transmitter1 the air the conductor1 the ear the receiver4 ,in wireless telegraphy we use the ether as the conductor of electrical disturbances4#ac ,marconi1 ,slaby1 ,branly1 ,lodge1 ,de ,forest1 ,popoff1 and others have invented apparatus for causing disturbances of the requisite kind1 and for detecting their presence4 ,the main features of a wireless telegraphy outfit are shown in ,figs4 #ei and #fa4 ,,the ,,transmitter ,,apparatus4 ,we will first consider the transmitting outfit 7,fig4 #ei74 ,it includes a battery1 dispatching key1 and an induction coil having its secondary circuit terminals connected with two wires1 the one leading to an earth-plate1 the other carried aloft on poles or suspended from a kite4 ,in the large station at ,poldhu1 ,cornwall1 for transatlantic signalling1 there are special wooden towers #bae feet high1 between which the arial wires hang4 ,at their upper and lower ends respectively the earth and arial wires terminate in brass balls separated by a gap4 ,when the operator depresses the key the induction coil charges these balls and the wires attached thereto with high-tension electricity4 ,as soon as the quantity collected exceeds the resistance of the air-gap1 a discharge takes place between the balls1 and the ether round the arial wire is violently disturbed1 and waves of electrical energy are propagated through it4 ,the rapidity with which the discharges follow one another1 and their travelling power1 depends on the strength of the induction coil1 the length of the air-gap1 and the capacity of the wires4#ad ,illustration3 ,,fig4 #ei4--,sketch of the transmitter of a wireless telegraphy outfit4 ,illustration3 ,,fig4 #fj4--;,a ,marconi coherer4 ,,receiving ,,apparatus4 ,the human body is quite insensitive to these etheric waves4 ,we cannot feel1 hear1 or see them4 ,but at the receiving station there is what may be called an electric eye4 ,technically it is named a coherer4 ,a ,marconi coherer is seen in ,fig4 #fj4 ,inside a small glass tube exhausted of air are two silver plugs1 ,p ,p1 carrying terminals1 ,t ,t1 projecting through the glass at both ends4 ,a small gap separates the plugs at the centre1 and this gap is partly filled with nickel-silver powder4 ,if the terminals of the coherer are attached to those of a battery1 practically no current will pass under ordinary conditions1 as the particles of nickel-silver touch each other very lightly and make a bad contact4 ,but if the coherer is also attached to wires leading into the earth and air1 and ether waves strike those wires1 at every impact the particles will cohere--that is1 pack tightly together--and allow battery current to pass4 ,the property of cohesion of small conductive bodies when influenced by ,hertzian waves was first noticed in #ahgd by ,professor ,d4,e4 ,hughes while experimenting with a telephone4 ,illustration3 ,,fig4 #fa4--,sketch of the receiving apparatus in a wireless telegraphy outfit4 ,we are now in a position to examine the apparatus of which a coherer forms part 7,fig4 #fa74 ,first1 we notice the arial and earth wires1 to which are attached other wires from battery ,a4 ,this battery circuit passes round the relay magnet ,r and through two choking coils1 whose function is to prevent the ,hertzian waves entering the battery4 ,the relay1 when energized1 brings contact ,d against ,e and closes the circuit of battery ,b1 which is much more powerful than battery ,a1 and operates the magnet ,m as well as the tapper1 which is practically an electric bell minus the gong4 7,the tapper circuit is indicated by the dotted lines47 ,we will suppose the transmitter of a distant station to be at work4 ,the electric waves strike the arial wire of the receiving station1 and cause the coherer to cohere and pass current4 ,the relay is closed1 and both tapper and ,morse inker begin to work4 ,the tapper keeps striking the coherer and shakes the particles loose after every cohesion4 ,if this were not done the current of ,a would pass continuously after cohesion had once taken place4 ,when the key of the transmitter is pressed down1 the waves follow one another very quickly1 and the acquired conductivity of the coherer is only momentarily destroyed by the tap of the hammer4 ,during the impression of a dot by the ,morse inker1 contact is made and broken repeatedly2 but as the armature of the inker is heavy and slow to move it does not vibrate in time with the relay and tapper4 ,therefore the ,morse instrument reproduces in dots and dashes the short and long depressions of the key at the transmitting station1 while the tapper works rapidly in time with the relay4 ,the ,morse inker is shown diagrammatically4 ,while current passes through ,m the armature is pulled towards it1 the end ,p1 carrying an inked wheel1 rises1 and a mark is made on the tape ,w1 which is moved continuously being drawn forward off reel ,r by the clockwork--or electrically-driven rollers ,r#a ,r#b4 ,,syntonic ,,transmission4 ,if a number of transmitting stations are sending out messages simultaneously1 a jumble of signals would affect all the receivers round1 unless some method were employed for rendering a receiver sensitive only to the waves intended to influence it4 ,also1 if distinction were impossible1 even with one transmitter in action its message might go to undesired stations4 ,there are various ways of tuning receivers and transmitters1 but the principle underlying them all is analogous to that of mechanical vibration4 ,if a weight is suspended from the end of a spiral spring1 and given an upward blow1 it bobs up and down a certain number of times per minute1 every movement from start to finish having exactly the same duration as the rest4 ,the resistance of the air and the internal friction of the spring gradually lessen the amplitude of the movements1 and the weight finally comes to rest4 ,suppose that the weight scales #cj lbs41 and that it naturally bobs twenty times a minute4 ,if you now take a feather and give it a push every three seconds you can coax it into vigorous motion1 assuming that every push catches it exactly on the rebound4 ,the same effect would be produced more slowly if #f or #i second intervals were substituted4 ,but if you strike it at #d1 #e1 or #g second intervals it will gradually cease to oscillate1 as the effect of one blow neutralizes that of another4 ,the same phenomenon is witnessed when two tuning-forks of equal pitch are mounted near one another1 and one is struck4 ,the other soon picks up the note4 ,but a fork of unequal pitch would remain dumb4 ,now1 every electrical circuit has a natural period of oscillation in which its electric charge vibrates4 ,it is found possible to tune1 or syntonize1 the arial rod or wire of a receiving station with a transmitter4 ,a vertical wire about #bjj feet in length1 says ,professor ,j4,a4 ,fleming1#ae has a natural time period of electrical oscillation of about one-millionth of a second4 ,therefore if waves strike this wire a million times a second they will reinforce one another and influence the coherer2 whereas a less or greater frequency will leave it practically unaffected4 ,by adjusting the receiving circuit to the transmitter1 or vice vers1 selective wireless telegraphy becomes possible4 ,,advance ,,of ,,wireless ,,telegraphy4 ,the history of wireless telegraphy may be summed up as follows3-- #ahdb4--,professor ,morse sent arial messages across the ,susquehanna ,river4 ,a line containing a battery and transmitter was carried on posts along one bank and earthed in the river at each end4 ,on the other bank was a second wire attached to a receiver and similarly earthed4 ,whenever contact was made and broken on the battery side1 the receiver on the other was affected4 ,distance about #a mile4 #ahei4--;,james ,bowman ,lindsay transmitted messages across the ,tay at ,glencarse in a somewhat similar way4 ,distance about #a_/b mile4 #ahhe4--,sir ,william ,preece signalled from ,lavernock ,point1 near ,cardiff1 to ,steep ,holm1 an island in the ,bristol ,channel4 ,distance about #e-a_/b miles4 ,in all these electrical induction of current was employed4 #ahhf4--;,hertzian waves discovered4 #ahie4--,professor ,a4 ,popoff sent ,hertzian wave messages over a distance of #c miles4 #ahig4--,marconi signalled from the ,needles ,hotel1 ,isle of ,wight1 to ,swanage2 #ag-a_/b miles4 #aija4--,messages sent at sea for #chj miles4 #aija1 ,dec4 #ag4--,messages transmitted from ,poldhu1 ,cornwall1 to ,hospital ,point1 ,newfoundland2 #b1jii miles4 ,mr4 ,marconi has so perfected tuning devices that his transatlantic messages do not affect receivers placed on board ships crossing the ocean1 unless they are purposely tuned4 ,atlantic liners now publish daily small newspapers containing the latest news1 flashed through space from land stations4 ,in the ,united ,states the ,de ,forest and ,fessenden systems are being rapidly extended to embrace the most out-of-the-way districts4 ,every navy of importance has adopted wireless telegraphy1 which1 as was proved during the ,russo-,japanese ,war1 can be of the greatest help in directing operations4 #ac ,named after their first discoverer1 ,dr4 ,hertz of ,carlsruhe1 ,hertzian waves4 #ad ,for long-distance transmission powerful dynamos take the place of the induction coil and battery4 #ae ,technics1 vol4 ii4 p4 #eff4 ,chapter ,,viii4 ,,the ,,telephone4 ,the ,bell telephone--,the ,edison transmitter--,the granular carbon transmitter--,general arrangement of a telephone circuit--,double-line circuits--,telephone exchanges--,submarine telephony4 ,for the purposes of everyday life the telephone is even more useful than the telegraph4 ,telephones now connect one room of a building with another1 house with house1 town with town1 country with country4 ,an infinitely greater number of words pass over the telephonic circuits of the world in a year than are transmitted by telegraph operators4 ,the telephone has become an important adjunct to the transaction of business of all sorts4 ,its wires penetrate everywhere4 ,without moving from his desk1 the ,london citizen may hold easy converse with a ,parisian1 a ,new ,yorker with a dweller in ,chicago4 ,wonderful as the transmission of signals over great distances is1 the transmission of human speech so clearly that individual voices may be distinguished hundreds of miles away is even more so4 ,yet the instrument which works the miracle is essentially simple in its principles4 ,,the ,,bell ,,telephone4 ,illustration3 ,,fig4 #fb4--,section of a ,bell telephone4 ,the first telephone that came into general use was that of ,bell1 shown in ,fig4 #fb4 ,in a central hole of an ebonite casing is fixed a permanent magnet1 ,m4 ,the casing expands at one end to accommodate a coil of insulated wire wound about one extremity of a magnet4 ,the coil ends are attached to wires passing through small channels to terminals at the rear4 ,a circular diaphragm1 ,d1 of very thin iron plate1 clamped between the concave mouthpiece and the casing1 almost touches the end of the magnet4 ,we will suppose that two ,bell telephones1 ,a and ,b1 are connected up by wires1 so that the wires and the coils form a complete circuit4 ,words are spoken into ,a4 ,the air vibrations1 passing through the central hole in the cover1 make the diaphragm vibrate towards and away from the magnet4 ,the distances through which the diaphragm moves have been measured1 and found not to exceed in some cases more than #a_/aj1jjj1jjj of an inch6 ,its movements distort the shape of the lines of force 7see p4 #aah7 emanating from the magnet1 and these1 cutting through the turns of the coil1 induce a current in the line circuit4 ,as the diaphragm approaches the magnet a circuit is sent in one direction2 as it leaves it1 in the other4 ,consequently speech produces rapidly alternating currents in the circuit1 their duration and intensity depending on the nature of the sound4 ,now consider telephone ,b4 ,the currents passing through its coil increase or diminish the magnetism of the magnet1 and cause it to attract its diaphragm with varying force4 ,the vibration of the diaphragm disturbs the air in exact accordance with the vibrations of ,a's diaphragm1 and speech is reproduced4 ,,the ,,edison ,,transmitter4 ,the ,bell telephone may be used both as a transmitter and a receiver1 and the permanent magnetism of the cores renders it independent of an electric battery4 ,but currents generated by it are so minute that they cannot overcome the resistance of a long circuit2 therefore a battery is now always used1 and with it a special device as transmitter4 ,if in a circuit containing a telephone and a battery there be a loose contact1 and this be shaken1 the varying resistance of the contact will cause electrical currents of varying force to pass through the circuit4 ,edison introduced the first successful microphone transmitter1 in which a small platinum disc connected to the diaphragm pressed with varying force against a disc of carbon1 each disc forming part of the circuit4 ,vibrations of the diaphragm caused current to flow in a series of rapid pulsations4 ,illustration3 ,,fig4 #fc4--,section of a granular carbon transmitter4 ,,the ,,granular ,,carbon ,,transmitter4 ,in ,fig4 #fc we have a section of a microphone transmitter now very widely used4 ,it was invented1 in its original form1 by an ,english clergyman named ,hunnings4 ,resting in a central cavity of an ebonite seating is a carbon block1 ,c1 with a face moulded into a number of pyramidal projections1 ,p ,p4 ,the space between ,c and a carbon diaphragm1 ,d1 is packed with carbon granules1 ,g ,g4 ,c has direct contact with line terminal ,t1 which screws into it2 ,d with ,t#a through the brass casing1 screw ,s1 and a small plate at the back of the transmitter4 ,voice vibrations compress ,g ,g1 and allow current to pass more freely from ,d to ,c4 ,this form of microphone is very delicate1 and unequalled for long-distance transmission4 ,illustration3 ,,fig4 #fd4--;,a diagrammatic representation of a telephonic circuit4 ,,general ,,arrangement ,,of ,a ,,telephone ,,circuit4 ,in many forms of subscriber's instruments both receiver and transmitter are mounted on a single handle in such a way as to be conveniently placed for ear and mouth4 ,for the sake of clearness the diagrammatic sketch of a complete installation 7,fig4 #fd7 shows them separated4 ,the transmitters1 it will be noticed1 are located in battery circuits1 including the primary windings ,p ,p#b of induction coils4 ,the transmitters are in the line circuit1 which includes the secondary windings ,s ,s#b of the coils4 ,we will assume that the transmitters are1 in the first instance1 both hung on the hooks of the metallic switches1 which their weight depresses to the position indicated by the dotted lines4 ,the handle of the magneto-generator at the left-end station is turned1 and current passes through the closed circuit3--,line ,a1 ,e ,b#b1 contact #aj1 the switch #i2 line ,b1 #d1 the other switch1 contact #e1 and ,e ,b4 ,both bells ring4 ,both parties now lift their receivers from the switch hooks4 ,the switches rise against contacts #a1 #b1 #c and #f1 #g1 #h respectively4 ,both primary and both secondary circuits are now completed1 while the bells are disconnected from the line wires4 ,the pulsations set up by transmitter ,t in primary coil ,p are magnified by secondary coil ,s for transmission through the line circuit1 and affect both receivers4 ,the same thing happens when ,t#b is used4 ,at the end of the conversation the receivers are hung on their hooks again1 and the bell circuit is remade1 ready for the next call4 ,illustration3 ,a ,,telephone ,,exchange4 ,,double-,,line ,,circuits4 ,the currents used in telephones pulsate very rapidly1 but are very feeble4 ,electric disturbances caused by the proximity of telegraph or tram wires would much interfere with them if the earth were used for the return circuit4 ,it has been found that a complete metallic circuit 7two wires7 is practically free from interference1 though where a number of wires are hung on the same poles1 speech-sounds may be faintly induced in one circuit from another4 ,this defect is1 however1 minimized by crossing the wires about among themselves1 so that any one line does not pass round the corresponding insulator on every pole4 ,,telephone ,,exchanges4 ,in a district where a number of telephones are used the subscribers are put into connection with one another through an exchange1 to which all the wires lead4 ,one wire of each subscriber runs to a common earth2 the other terminates at a switchboard presided over by an operator4 ,in an exchange used by many subscribers the terminals are distributed over a number of switchboards1 each containing #hj to #ajj terminals1 and attended to by an operator1 usually a girl4 ,when a subscriber wishes to be connected to another subscriber1 he either turns the handle of a magneto generator1 which causes a shutter to fall and expose his number at the exchange1 or simply depresses a key which works a relay at the exchange and lights a tiny electric lamp4 ,the operator1 seeing the signal1 connects her telephone with the subscriber's circuit and asks the number wanted4 ,this given1 she rings up the other subscriber1 and connects the two circuits by means of an insulated wire cord having a spike at each end to fit the jack sockets of the switchboard terminals4 ,the two subscribers are now in communication4 ,illustration3 ,,fig4 #fe4--,the headdress of an operator at a telephone exchange4 ,the receiver is fastened over one ear1 and the transmitter to the chest4 ,if a number on switchboard ,a calls for a number on switchboard ,c1 the operator at ,a connects her subscriber by a jack cord to a trunk line running to ,c1 where the operator similarly connects the trunk line with the number asked for1 after ringing up the subscriber4 ,the central exchange of one town is connected with that of another by one or more trunk lines1 so that a subscriber may speak through an indefinite number of exchanges4 ,so perfect is the modern telephone that the writer remembers on one occasion hearing the door-bell ring in a house more than a hundred miles away1 with which he was at the moment in telephonic connection1 though three exchanges were in the circuit4 ,,submarine ,,telephony4 ,though telegraphic messages are transmitted easily through thousands of miles of cable1#af submarine telephony is at present restricted to comparatively short distances4 ,when a current passes through a cable1 electricity of opposite polarity induced on the outside of the cable damps the vibration in the conductor4 ,in the ,atlantic cable1 strong currents of electricity are poured periodically into one end1 and though much enfeebled when they reach the other they are sufficiently strong to work a very delicate mirror galvanometer 7invented by ,lord ,kelvin71 which moves a reflected ray up and down a screen1 the direction of the movements indicating a dot or a dash4 ,reversible currents are used in transmarine telegraphy4 ,the galvanometer is affected like the coils and small magnet in ,wheatstone's needle instrument 7p4 #abh74 ,telephonic currents are too feeble to penetrate many miles of cable4 ,there is telephonic communication between ,england and ,france1 and ,england and ,ireland4 ,but transatlantic telephony is still a thing of the future4 ,it is hoped1 however1 that by inserting induction coils at intervals along the cables the currents may be stepped up from point to point1 and so get across4 ,turning to ,fig4 #fd1 we may suppose ,s to be on shore at the ,english end1 and ,s#b to be the primary winding of an induction coil a hundred miles away in the sea1 which magnifies the enfeebled vibrations for a journey to ,s#c1 where they are again revived2 and so on1 till the ,new ,world is reached4 ,the difficulty is to devise induction coils of great power though of small size4 ,yet science advances nowadays so fast that we may live to hear words spoken at the ,antipodes4 #af ,in #ahif the late ,li ,hung ,chang sent a cablegram from ,china to ,england 7#ab1fjh miles71 and received a reply1 in seven minutes4 ,chapter ,,ix4 ,,dynamos ,,and ,,electric ,,motors4 ,a simple dynamo--,continuous-current dynamos--,multipolar dynamos--,exciting the field magnets--,alternating current dynamos--,the transmission of power--,the electric motor--,electric lighting--,the incandescent lamp--,arc lamps--,series and parallel arrangement of lamps--,current for electric lamps--,electroplating4 ,in previous chapters we have incidentally referred to the conversion of mechanical work into electrical energy4 ,in this we shall examine how it is done--how the silently spinning dynamo develops power1 and why the motor spins when current is passed through it4 ,we must begin by returning to our first electrical diagram 7,fig4 #ej71 and calling to mind the invisible lines of force which permeate the ether in the immediate neighbourhood of a magnet's poles1 called the magnetic field of the magnet4 ,many years ago 7#ahca7 the great ,michael ,faraday discovered that if a loop of wire were moved up and down between the poles of an electro-magnet 7,fig4 #ff7 a current was induced in the loop1 its direction depending upon that in which the loop was moved4 ,the energy required to cut the lines of force passed in some mysterious way into the wire4 ,why this is so we cannot say1 but1 taking advantage of the fact1 electricians have gradually developed the enormous machines which now send vehicles spinning over metal tracks1 light our streets and houses1 and supply energy to innumerable factories4 ,illustration3 ,,fig4 #ff4 ,the strength of the current induced in a circuit cutting the lines of force of a magnet is called its pressure1 voltage1 or electro-motive force 7expressed shortly ,e4,m4,f474 ,it may be compared with the pounds-to-the-square-inch of steam4 ,in order to produce an ,e4,m4,f4 of one volt it is calculated that #ajj1jjj1jjj lines of force must be cut every second4 ,the voltage depends on three things3--7#a47 ,the strength of the magnet3 the stronger it is1 the greater the number of lines of force coming from it4 7#b47 ,the length of the conductor cutting the lines of force3 the longer it is1 the more lines it will cut4 7#c47 ,the speed at which the conductor moves3 the faster it travels1 the more lines it will cut in a given time4 ,it follows that a powerful dynamo1 or mechanical producer of current1 must have strong magnets and a long conductor2 and the latter must be moved at a high speed across the lines of force4 ,a ,,simple ,,dynamo4 ,in ,fig4 #fg we have the simplest possible form of dynamo--a single turn of wire1 w x y z1 mounted on a spindle1 and having one end attached to an insulated ring ,c1 the other to an insulated ring ,c#a4 ,two small brushes1 ,b ,b#a1 of wire gauze or carbon1 rubbing continuously against these collecting rings1 connect them with a wire which completes the circuit4 ,the armature1 as the revolving coil is called1 is mounted between the poles of a magnet1 where the lines of force are thickest4 ,these lines are supposed to stream from the ,n4 to the ,s4 pole4 ,in ,fig4 #fg the armature has reached a position in which y z and w x are cutting no1 or very few1 lines of force1 as they move practically parallel to the lines4 ,this is called the zero position4 ,illustration3 ,,fig4 #fg4 ,illustration3 ,,fig4 #fh4 ,in ,fig4 #fh the armature1 moving at right angles to the lines of force1 cuts a maximum number in a given time1 and the current induced in the coil is therefore now most intense4 ,here we must stop a moment to consider how to decide in which direction the current flows4 ,the armature is revolving in a clockwise direction1 and y z1 therefore1 is moving downwards4 ,now1 suppose that you rest your left hand on the ,n4 pole of the magnet so that the arm lies in a line with the magnet4 ,point your forefinger towards the ,s4 pole4 ,it will indicate the direction of the lines of force4 ,bend your other three fingers downwards over the edge of the ,n4 pole4 ,they will indicate the direction in which the conductor is moving across the magnetic field4 ,stick out the thumb at right angles to the forefinger4 ,it points in the direction in which the induced current is moving through the nearer half of the coil4 ,therefore lines of force1 conductor1 and induced current travel in planes which1 like the top and two adjacent sides of a box1 are at right angles to one another4 ,while current travels from z to y--that is1 from the ring ,c#a to y--it also travels from x to w1 because w x rises while y z descends4 ,so that a current circulates through the coil and the exterior part of the circuit1 including the lamp4 ,after z y has passed the lowest possible point of the circle it begins to ascend1 w x to descend4 ,the direction of the current is therefore reversed2 and as the change is repeated every half-revolution this form of dynamo is called an alternator or creator of alternating currents4 ,a well-known type of alternator is the magneto machine which sends shocks through any one who completes the external circuit by holding the brass handles connected by wires to the brushes4 ,the faster the handle of the machine is turned the more frequent is the alternation1 and the stronger the current4 ,illustration3 ,,fig4 #fi4 ,,continuous-,,current ,,dynamos4 ,an alternating current is not so convenient for some purposes as a continuous current4 ,it is therefore sometimes desirable 7even necessary7 to convert the alternating into a uni-directional or continuous current4 ,how this is done is shown in ,figs4 #fi and #gj4 ,in place of the two collecting rings ,c ,c#a1 we now have a single ring split longitudinally into two portions1 one of which is connected to each end of the coil w x y z4 ,in ,fig4 #fi brush ,b has just passed the gap on to segment ,c1 brush ,b#a on to segment ,c#a4 ,for half a revolution these remain respectively in contact2 then1 just as y z begins to rise and w x to descend1 the brushes cross the gaps again and exchange segments1 so that the current is perpetually flowing one way through the circuit4 ,the effect of the commutator#ag is1 in fact1 equivalent to transposing the brushes of the collecting rings of the alternator every time the coil reaches a zero position4 ,figs4 #ga and #gb give end views in section of the coil and the commutator1 with the coil in the position of minimum and maximum efficiency4 ,the arrow denotes the direction of movement2 the double dotted lines the commutator end of the revolving coil4 ,illustration3 ,,fig4 #gj4 ,,practical ,,continuous-,,current ,,dynamos4 ,the electrical output of our simple dynamo would be increased if1 instead of a single turn of wire1 we used a coil of many turns4 ,a further improvement would result from mounting on the shaft1 inside the coil1 a core or drum of iron1 to entice the lines of force within reach of the revolving coil4 ,it is evident that any lines which pass through the air outside the circle described by the coil cannot be cut1 and are wasted4 ,illustration3 ,,fig4 #ga4 ,illustration3 ,,fig4 #gb4 ,the core is not a solid mass of iron1 but built up of a number of very thin iron discs threaded on the shaft and insulated from one another to prevent electric eddies1 which would interfere with the induced current in the conductor4#ah ,sometimes there are openings through the core from end to end to ventilate and cool it4 ,illustration3 ,,fig4 #gc4 ,we have already noticed that in the case of a single coil the current rises and falls in a series of pulsations4 ,such a form of armature would be unsuitable for large dynamos1 which accordingly have a number of coils wound over their drums1 at equal distances round the circumference1 and a commutator divided into an equal number of segments4 ,the subject of drum winding is too complicated for brief treatment1 and we must therefore be content with noticing that the coils are so connected to their respective commutator segments and to one another that they mutually assist one another4 ,a glance at ,fig4 #gc will help to explain this4 ,here we have in section a number of conductors on the right of the drum 7marked with a cross to show that current is moving1 as it were1 into the page71 connected with conductors on the left 7marked with a dot to signify current coming out of the page74 ,if the crossed and dotted conductors were respectively the up and down turns of a single coil terminating in a simple split commutator 7,fig4 #fi71 when the coil had been revolved through an angle of #ij some of the up turns would be ascending and some descending1 so that conflicting currents would arise4 ,yet we want to utilize the whole surface of the drum2 and by winding a number of coils in the manner hinted at1 each coil1 as it passes the zero point1 top or bottom1 at once generates a current in the desired direction and reinforces that in all the other turns of its own and of other coils on the same side of a line drawn vertically through the centre4 ,there is thus practically no fluctuation in the pressure of the current generated4 ,the action of single and multiple coil windings may be compared to that of single and multiple pumps4 ,water is ejected by a single pump in gulps2 whereas the flow from a pipe fed by several pumps arranged to deliver consecutively is much more constant4 ,,multipolar ,,dynamos4 ,hitherto we have considered the magnetic field produced by one bi-polar magnet only4 ,large dynamos have four1 six1 eight1 or more field magnets set inside a casing1 from which their cores project towards the armature so as almost to touch it 7,fig4 #gd74 ,the magnet coils are wound to give ,n4 and ,s4 poles alternately at their armature ends round the field2 and the lines of force from each ,n4 pole stream each way to the two adjacent ,s4 poles across the path of the armature coils4 ,in dynamos of this kind several pairs of collecting brushes pick current off the commutator at equidistant points on its circumference4 ,illustration3 ,,fig4 #gd4--;,a ,holmes continuous current dynamo3 ,a1 armature2 ,c1 commutator2 ,m1 field magnets4 ,,exciting ,,the ,,field ,,magnets4 ,until current passes through the field magnet coils1 no magnetic field can be created4 ,how are the coils supplied with current8 ,a dynamo1 starting for the first time1 is excited by a current from an outside source2 but when it has once begun to generate current it feeds its magnets itself1 and ever afterwards will be self-exciting1#ai owing to the residual magnetism left in the magnet cores4 ,illustration3 ,,fig4 #ge4--,partly finished commutator4 ,look carefully at ,figs4 #gg and #gh4 ,in the first of these you will observe that part of the wire forming the external circuit is wound round the arms of the field magnet4 ,this is called a series winding4 ,in this case all the current generated helps to excite the dynamo4 ,at the start the residual magnetism of the magnet cores gives a weak field4 ,the armature coils cut this and pass a current through the circuit4 ,the magnets are further excited1 and the field becomes stronger2 and so on till the dynamo is developing full power4 ,series winding is used where the current in the external circuit is required to be very constant4 ,illustration3 ,,fig4 #gf4--,the brushes of a ,holmes dynamo4 ,fig4 #gh shows another method of winding--the shunt4 ,most of the current generated passes through the external circuit #b1 #b2 but a part is switched through a separate winding for the magnets1 denoted by the fine wire #a1 #a4 ,here the strength of the magnetism does not vary directly with the current1 as only a small part of the current serves the magnets4 ,the shunt winding is therefore used where the voltage 7or pressure7 must be constant4 ,illustration3 ,,fig4 #gg4--,sketch showing a series winding4 ,illustration3 ,,fig4 #gh4--,shunt winding4 ,a third method is a combination of the two already named4 ,a winding of fine wire passes from brush to brush round the magnets2 and there is also a series winding as in ,fig4 #gg4 ,this compound method is adapted more especially for electric traction4 ,,alternating ,,dynamos4 ,these have their field magnets excited by a separate continuous current dynamo of small size4 ,the field magnets usually revolve inside a fixed armature 7the reverse of the arrangement in a direct-current generator72 or there may be a fixed central armature and field magnets revolving outside it4 ,this latter arrangement is found in the great power stations at ,niagara ,falls1 where the enormous field-rings are mounted on the top ends of vertical shafts1 driven by water-turbines at the bottom of pits #agh feet deep1 down which water is led to the turbines through great pipes1 or penstocks4 ,the weight of each shaft and the field-ring attached totals about thirty-five tons4 ,this mass revolves #bej times a minute1 and #e1jjj horse power is constantly developed by the dynamo4 ,similar dynamos of #aj1jjj horse power each have been installed on the ,canadian side of the ,falls4 ,illustration3 ,,fig4 #gi4 ,,transmission ,,of ,,power4 ,alternating current is used where power has to be transmitted for long distances1 because such a current can be intensified1 or stepped up1 by a transformer somewhat similar in principle to a ,ruhmkorff coil minus a contact-breaker 7see p4 #abb74 ,a typical example of transformation is seen in ,fig4 #gi4 ,alternating current of #e1jjj volts pressure is produced in the generating station and sent through conductors to a distant station1 where a transformer1 ,b1 reduces the pressure to #ejj volts to drive an alternating motor1 ,c1 which in turn operates a direct current dynamo1 ,d4 ,this dynamo has its terminal connected with the insulated or live rail of an electric railway1 and its - terminal with the wheel rails1 which are metallically united at the joints to act as a return4 ,on its way from the live rail to the return the current passes through the motors4 ,in the case of trams the conductor is either a cable carried overhead on standards1 from which it passes to the motor through a trolley arm1 or a rail laid underground in a conduit between the rails4 ,in the top of the conduit is a slit through which an arm carrying a contact shoe on the end projects from the car4 ,the shoe rubs continuously on the live rail as the car moves4 ,to return for a moment to the question of transformation of current4 ,why1 it may be asked1 should we not send low-pressure direct current to a distant station straight from the dynamo1 instead of altering its nature and pressure8 ,or1 at any rate1 why not use high-pressure direct current1 and transform that8 ,the answer is1 that to transmit a large amount of electrical energy at low pressure 7or voltage7 would necessitate large volume 7or amperage7 and a big and expensive copper conductor to carry it4 ,high-pressure direct current is not easily generated1 since the sparking at the collecting brushes as they pass over the commutator segments gives trouble4 ,so engineers prefer high-pressure alternating current1 which is easily produced1 and can be sent through a small and inexpensive conductor with little loss4 ,also its voltage can be transformed by apparatus having no revolving parts4 ,,the ,,electric ,,motor4 ,anybody who understands the dynamo will also be able to understand the electric motor1 which is merely a reversed dynamo4 ,imagine in ,fig4 #gj a dynamo taking the place of the lamp and passing current through the brushes and commutator into the coil w x y z4 ,now1 any coil through which current passes becomes a magnet with ,n4 and ,s4 poles at either end4 7,in ,fig4 #gj we will assume that the ,n4 pole is below and the ,s4 pole above the coil47 ,the coil poles therefore try to seek the contrary poles of the permanent magnet1 and the coil revolves until its ,s4 pole faces the ,n4 of the magnet1 and vice vers4 ,the lines of force of the coil and the magnet are now parallel4 ,but the momentum of revolution carries the coil on1 and suddenly the commutator reverses its polarity1 and a further half-revolution takes place4 ,then comes a further reversal1 and so on ad infinitum4 ,the rotation of the motor is therefore merely a question of repulsion and attraction of like and unlike poles4 ,an ordinary compass needle may be converted into a tiny motor by presenting the ,n4 and ,s4 poles of a magnet to its ,s4 and ,n4 poles alternately every half-revolution4 ,in construction and winding a motor is practically the same as a dynamo4 ,in fact1 either machine can perform either function1 though perhaps not equally well adapted for both4 ,motors may be run with direct or alternating current1 according to their construction4 ,on electric cars the motor is generally suspended from the wheel truck1 and a small pinion on the armature shaft gears with a large pinion on a wheel axle4 ,one great advantage of electric traction is that every vehicle of a train can carry its own motor1 so that the whole weight of the train may be used to get a grip on the rails when starting4 ,where a single steam locomotive is used1 the adhesion of its driving-wheels only is available for overcoming the inertia of the load2 and the whole strain of starting is thrown on to the foremost couplings4 ,other advantages may be summed up as follows3--7#a7 ,ease of starting and rapid acceleration2 7#b7 absence of waste of energy 7in the shape of burning fuel7 when the vehicles are at rest2 7#c7 absence of smoke and smell4 ,,electric ,,lighting4 ,dynamos are used to generate current for two main purposes--7#a7 ,to supply power to motors of all kinds2 7#b7 to light our houses1 factories1 and streets4 ,in private houses and theatres incandescent lamps are generally used2 in the open air1 in shops1 and in larger buildings1 such as railway stations1 the arc lamp is more often found4 ,,incandescent ,,lamp4 ,if you take a piece of very fine iron wire and lay it across the terminals of an accumulator1 it becomes white hot and melts1 owing to the heat generated by its resistance to the current4 ,a piece of fine platinum wire would become white hot without melting1 and would give out an intense light4 ,here we have the principle of the glow or incandescent lamp--namely1 the interposition in an electric circuit of a conductor which at once offers a high resistance to the current1 but is not destroyed by the resulting heat4 ,in ,fig4 #hj is shown a fan propelling liquid constantly through a pipe4 ,let us assume that the liquid is one which develops great friction on the inside of the pipe4 ,at the contraction1 where the speed of travel is much greater than elsewhere in the circuit1 most heat will be produced4 ,illustration3 ,,fig4 #hj4--;,diagram to show circulation of water through a pipe4 ,in quite the early days of the glow-lamp platinum wire was found to be unreliable as regards melting1 and filaments of carbon are now used4 ,to prevent the wasting away of the carbon by combination with oxygen the filament is enclosed in a glass bulb from which practically all air has been sucked by a mercury pump before sealing4 ,illustration3 ,,fig4 #ha4--,the electrical counterpart of ,fig4 #hj4 ,the filament takes the place of the contraction in the pipe4 ,the manufacture of glow-lamps is now an important industry4 ,one brand of lamp#bj is made as follows3--,first1 cotton-wool is dissolved in chloride of zinc1 and forms a treacly solution1 which is squirted through a fine nozzle into a settling solution which hardens it and makes it coil up like a very fine violin string4 ,after being washed and dried1 it is wound on a plumbago rod and baked in a furnace until only the carbon element remains4 ,this is the filament in the rough4 ,it is next removed from the rod and tipped with two short pieces of fine platinum wire4 ,to make the junction electrically perfect the filament is plunged in benzine and heated to whiteness by the passage of a strong current1 which deposits the carbon of the benzine on the joints4 ,the filament is now placed under the glass receiver of an air-pump1 the air is exhausted1 hydro-carbon vapour is introduced1 and the filament has a current passed through it to make it white hot4 ,carbon from the vapour is deposited all over the filament until the required electrical resistance is attained4 ,the filament is now ready for enclosure in the bulb4 ,when the bulb has been exhausted and sealed1 the lamp is tested1 and1 if passed1 goes to the finishing department1 where the two platinum wires 7projecting through the glass7 are soldered to a couple of brass plates1 which make contact with two terminals in a lamp socket4 ,finally1 brass caps are affixed with a special water-tight and hard cement4 ,,arc ,,lamps4 ,in arc lighting1 instead of a contraction at a point in the circuit1 there is an actual break of very small extent4 ,suppose that to the ends of the wires leading from a dynamo's terminals we attach two carbon rods1 and touch the end of the rods together4 ,the tips become white hot1 and if they are separated slightly1 atoms of incandescent carbon leap from the positive to the negative rod in a continuous and intensely luminous stream1 which is called an arc because the path of the particles is curved4 ,no arc would be formed unless the carbons were first touched to start incandescence4 ,if they are separated too far for the strength of the current to bridge the gap the light will flicker or go out4 ,the arc lamp is therefore provided with a mechanism which1 when the current is cut off1 causes the carbons to fall together1 gradually separates them when it is turned on1 and keeps them apart4 ,the principle employed is the effort of a coil through which a current passes to draw an iron rod into its centre4 ,some of the current feeding the lamp is shunted through a coil1 into which projects one end of an iron bar connected with one carbon point4 ,a spring normally presses the points together when no current flows4 ,as soon as current circulates through the coil the bar is drawn upwards against the spring4 ,,series ,,and ,,parallel ,,arrangement ,,of ,,lamps4 ,when current passes from one lamp to another1 as in ,fig4 #hb1 the lamps are said to be in series4 ,should one lamp fail1 all in the circuit would go out4 ,but where arc lamps are thus arranged a special mechanism on each lamp short-circuits it in case of failure1 so that current may pass uninterruptedly to the next4 ,illustration3 ,,fig4 #hb4--;,incandescent lamps connected in series4 ,fig4 #hc shows a number of lamps set in parallel4 ,one terminal of each is attached to the positive conductor1 the other to the negative conductor4 ,each lamp therefore forms an independent bridge1 and does not affect the efficiency of the rest4 ,parallel series signifies a combination of the two systems1 and would be illustrated if1 in ,fig4 #hc1 two or more lamps were connected in series groups from one conductor to the other4 ,this arrangement is often used in arc lighting4 ,illustration3 ,,fig4 #hc4--;,incandescent lamps connected in parallel4 ,,current ,,for ,,electric ,,lamps4 ,this may be either direct or alternating4 ,the former is commonly used for arc lamps1 the latter for incandescent1 as it is easily stepped-down from the high-pressure mains for use in a house4 ,glow-lamps usually take current of #aaj or #bej volts pressure4 ,in arc lamps fed with direct current the tip of the positive carbon has a bowl-shaped depression worn in it1 while the negative tip is pointed4 ,most of the illumination comes from the inner surface of the bowl1 and the positive carbon is therefore placed uppermost to throw the light downwards4 ,an alternating current1 of course1 affects both carbons in the same manner1 and there is no bowl4 ,the carbons need frequent renewal4 ,a powerful lamp uses about #gj feet of rod in #a1jjj hours if the arc is exposed to the air4 ,some lamps have partly enclosed arcs--that is1 are surrounded by globes perforated by a single small hole1 which renders combustion very slow1 though preventing a vacuum4 ,,electroplating4 ,electroplating is the art of coating metals with metals by means of electricity4 ,silver1 copper1 and nickel are the metals most generally deposited4 ,the article to be coated is suspended in a chemical solution of the metal to be deposited4 ,fig4 #hd shows a very simple plating outfit4 ,a is a battery2 ,b a vessel containing1 say1 an acidulated solution of sulphate of copper4 ,a spoon1 ,s1 hanging in this from a glass rod1 ,r1 is connected with the zinc or negative element1 ,z1 of the battery1 and a plate of copper1 ,p1 with the positive element1 ,c4 ,current flows in the direction shown by the arrows1 from ,z to ,c1 ,c to ,p1 ,p to ,s1 ,s to ,z4 ,the copper deposited from the solution on the spoon is replaced by gradual dissolution of the plate1 so that the latter serves a double purpose4 ,illustration3 ,,fig4 #hd4--;,an electroplating outfit4 ,in silver plating1 ,p is of silver1 and the solution one of cyanide of potassium and silver salts4 ,where nickel or silver has to be deposited on iron1 the article is often given a preliminary coating of copper1 as iron does not make a good junction with either of the first two metals1 but has an affinity for copper4 #ag ,from the ,latin commuto1 ,i exchange4 #ah ,only the drum type of armature is treated here4 #ai ,this refers to continuous-current dynamos only4 #bj ,the ,robertson4 ,chapter ,x4 ,,railway ,,brakes4 ,the ,vacuum ,automatic brake--,the ,westinghouse air-brake4 ,in the early days of the railway1 the pulling up of a train necessitated the shutting off of steam while the stopping-place was still a great distance away4 ,the train gradually lost its velocity1 the process being hastened to a comparatively small degree by the screw-down brakes on the engine and guard's van4 ,the goods train of to-day in many cases still observes this practice1 long obsolete in passenger traffic4 ,an advance was made when a chain1 running along the entire length of the train1 was arranged so as to pull on subsidiary chains branching off under each carriage and operating levers connected with brake blocks pressing on every pair of wheels4 ,the guard strained the main chain by means of a wheel gear in his van4 ,this system was1 however1 radically defective1 since1 if any one branch chain was shorter than the rest1 it alone would get the strain4 ,furthermore1 it is obvious that the snapping of the main chain would render the whole arrangement powerless4 ,accordingly1 brakes operated by steam were tried4 ,under every carriage was placed a cylinder1 in connection with a main steam-pipe running under the train4 ,when the engineer wished to apply the brakes1 he turned high-pressure steam into the train pipe1 and the steam1 passing into the brake cylinders1 drove out in each a piston operating the brake gear4 ,unfortunately1 the steam1 during its passage along the pipe1 was condensed1 and in cold weather failed to reach the rear carriages4 ,water formed in the pipes1 and this was liable to freeze4 ,if the train parted accidentally1 the apparatus of course broke down4 ,hydraulic brakes have been tried2 but these are open to several objections2 and railway engineers now make use of air-pressure as the most suitable form of power4 ,whatever air system be adopted1 experience has shown that three features are essential3--7#a47 ,the brakes must be kept off artificially4 7#b47 ,in case of the train parting accidentally1 the brakes must be applied automatically1 and quickly bring all the vehicles of the train to a standstill4 7#c47 ,it must be possible to apply the brakes with greater or less force1 according to the needs of the case4 ,at the present day one or other of two systems is used on practically all automatically-braked cars and coaches4 ,these are known as--7#a7 ,the vacuum automatic1 using the pressure of the atmosphere on a piston from the other side of which air has been mechanically exhausted2 and 7#b7 the ,westinghouse automatic1 using compressed air4 ,the action of these brakes will now be explained as simply as possible4 ,,the ,,vacuum ,,automatic ,,brake4 ,under each carriage is a vacuum chamber 7,fig4 #he7 riding on trunnions1 ,e ,e1 so that it may swing a little when the brakes are applied4 ,inside the chamber is a cylinder1 the piston of which is rendered air-tight by a rubber ring rolling between it and the cylinder walls4 ,the piston rod works through an air-tight stuffing-box in the bottom of the casing1 and when it rises operates the brake rods4 ,it is obvious that if air is exhausted from both sides of the piston at once1 the piston will sink by reason of its own weight and that of its attachments4 ,if air is now admitted below the piston1 the latter will be pushed upwards with a maximum pressure of #ae lbs4 to the square inch4 ,the ball-valve ensures that while air can be sucked from both sides of the piston1 it can be admitted to the lower side only4 ,illustration3 ,,fig4 #he4--,vacuum brake off4 ,illustration3 ,,fig4 #hf4--,vacuum brake on4 ,let us imagine that a train has been standing in a siding1 and that air has gradually filled the vacuum chamber by leakage4 ,the engine is coupled on1 and the driver at once turns on the steam ejector1#ba which sucks all the air out of the pipes and chambers throughout the train4 ,the air is sucked directly from the under side of the piston through pipe ,d2 and from the space ,a ,a and the cylinder 7open at the top7 through the channel ,c1 lifting the ball1 which1 as soon as exhaustion is complete1 or when the pressure on both sides of the piston is equal1 falls back on its seat4 ,on air being admitted to the train pipe1 it rushes through ,d and into the space ,b 7,fig4 #hf7 below the piston1 but is unable to pass the ball1 so that a strong upward pressure is exerted on the piston1 and the brakes go on4 ,to throw them off1 the space below the piston must be exhausted4 ,this is to be noted3 ,if there is a leak1 as in the case of the train parting1 the brakes go on at once1 since the vacuum below the piston is automatically broken4 ,illustration3 ,,fig4 #hg4--;,guard's valve for applying the ,vacuum brake4 ,for ordinary stops the vacuum is only partially broken--that is1 an air-pressure of but from #e to #aj lbs4 per square inch is admitted4 ,for emergency stops full atmospheric pressure is used4 ,in this case it is advisable that air should enter at both ends of the train2 so in the guard's van there is installed an ingenious automatic valve1 which can at any time be opened by the guard pressing down a lever1 but which opens of itself when the train-pipe vacuum is rapidly destroyed4 ,fig4 #hg shows this device in section4 ,seated on the top of an upright pipe is a valve1 ,a1 connected by a bolt1 ,b1 to an elastic diaphragm1 ,c1 sealing the bottom of the chamber ,d4 ,the bolt ,b has a very small hole bored through it from end to end4 ,when the vacuum is broken slowly1 the pressure falls in ,d as fast as in the pipe2 but a sudden inrush of air causes the valve ,a to be pulled off its seat by the diaphragm ,c1 as the vacuum in ,d has not been broken to any appreciable extent4 ,air then rushes into the train pipe through the valve4 ,it is thus evident that the driver controls this valve as effectively as if it were on the engine4 ,these emergency valves are sometimes fitted to every vehicle of a train4 ,when a carriage is slipped1 taps on each side of the coupling joint of the train pipe are turned off by the guard in the slip2 and when he wishes to stop he merely depresses the lever ,e1 gradually opening the valve4 ,under the van is an auxiliary vacuum chamber1 from which the air is exhausted by the train pipe4 ,if the guard1 after the slip has parted from the train1 finds that he has applied his brakes too hard1 he can put this chamber into communication with the brake cylinder1 and restore the vacuum sufficiently to pull the brakes off again4 ,when a train has come to rest1 the brakes must be sucked off by the ejector4 ,until this has been done the train cannot be moved1 so that it is impossible for it to leave the station unprepared to make a sudden stop if necessary4 ,,the ,,westinghouse ,,air-,,brake4 ,this system is somewhat more complicated than the vacuum1 though equally reliable and powerful4 ,owing to the complexity of certain parts1 such as the steam air-pump and the triple-valve1 it is impossible to explain the system in detail2 we therefore have recourse to simple diagrammatic sketches1 which will help to make clear the general principles employed4 ,the air-brake1 as first evolved by ,mr4 ,george ,westinghouse1 was a very simple affair--an air-pump and reservoir on the engine2 a long pipe running along the train2 and a cylinder under every vehicle to work the brakes4 ,to stop the train1 the high-pressure air collected in the reservoir was turned into the train pipe to force out the pistons in the coach cylinders1 connected to it by short branch pipes4 ,one defect of this straight system was that the brakes at the rear of a long train did not come into action until a considerable time after the driver turned on the air2 and since1 when danger is imminent1 a very few seconds are of great importance1 this slowness of operation was a serious fault4 ,also1 it was found that the brakes on coaches near the engine went on long before those more distant1 so that during a quick stop there was a danger of the forward coaches being bumped by those behind4 ,it goes without saying that any coaches which might break loose were uncontrollable4 ,mr4 ,westinghouse therefore patented his automatic brake1 now so largely used all over the world4 ,the brake ensures practically instantaneous and simultaneous action on all the vehicles of a train of any length4 ,illustration3 ,,fig4 #hh4--;,diagrammatic sketch of the details of the ,westinghouse air-brake4 ,brake off4 ,the principle of the brake will be gathered from ,figs4 #hh and #hi4 ,p is a steam-driven air-pump on the engine1 which compresses air into a reservoir1 ,a1 situated below the engine or tender1 and maintains a pressure of from #hj to #ij lbs4 per square inch4 ,a three-way cock1 ,c1 puts the train pipe into communication with ,a or the open air at the wish of the driver4 ,under each coach is a triple-valve1 ,t1 an auxiliary reservoir1 ,b1 and a brake cylinder1 ,d4 ,the triple-valve is the most noteworthy feature of the whole system4 ,the reader must remember that the valve shown in the section is only diagrammatic4 ,now for the operation of the brake4 ,when the engine is coupled to the train1 the compressed air in the main reservoir is turned into the train pipe1 from which it passes through the triple-valve into the auxiliary reservoir1 and fills it till it has a pressure of1 say1 #hj lbs4 per square inch4 ,until the brakes are required1 the pressure in the train pipe must be maintained4 ,if accidentally1 or purposely 7by turning the cock ,c to the position shown in ,fig4 #hi71 the train-pipe pressure is reduced1 the triple-valve at once shifts1 putting ,b in connection with the brake cylinder ,d1 and cutting off the connection between ,d and the air1 and the brakes go on4 ,to get them off1 the pressure in the train pipe must be made equal to that in ,b1 when the valve will assume its original position1 allowing the air in ,d to escape4 ,the force with which the brake is applied depends upon the reduction of pressure in the train pipe4 ,a slight reduction would admit air very slowly from ,b to ,d1 whereas a full escape from the train pipe would open the valve to its utmost4 ,we have not represented the means whereby the valve is rendered sensitive to these changes1 for the reason given above4 ,illustration3 ,,fig4 #hi4--;,brake on4 ,the latest form of triple-valve includes a device which1 when air is rapidly discharged from the train pipe1 as in an emergency application of the brake1 opens a port through which compressed air is also admitted from the train pipe directly into ,d4 ,it will easily be understood that a double advantage is hereby gained--first1 in utilizing a considerable portion of the air in the train pipe to increase the available brake force in cases of emergency2 and1 secondly1 in producing a quick reduction of pressure in the whole length of the pipe1 which accelerates the action of the brakes with extraordinary rapidity4 ,it may be added that this secondary communication is kept open only until the pressure in ,d is equal to that in the train pipe4 ,then it is cut off1 to prevent a return of air from ,b to the pipe4 ,an interesting detail of the system is the automatic regulation of air-pressure in the main reservoir by the air-pump governor 7,fig4 #ij74 ,the governor is attached to the steam-pipe leading from the locomotive boiler to the air-pump4 ,steam from the boiler1 entering at ,f1 flows through valve #ad and passes by ,d into the pump1 which is thus brought into operation1 and continues to work until the pressure in the main reservoir1 acting on the under side of the diaphragm #i1 exceeds the tension to which the regulating spring #g is set4 ,any excess of pressure forces the diaphragm upwards1 lifting valve #aa1 and allowing compressed air from the main reservoir to flow into the chamber ,c4 ,the air-pressure forces piston #ab downwards and closes steam-valve #ad1 thus cutting off the supply of steam to the pump4 ,as soon as the pressure in the reservoir is reduced 7by leakage or use7 below the normal1 spring #g returns diaphragm #i to the position shown in ,fig4 #ij1 and pin-valve #aa closes4 ,the compressed air previously admitted to the chamber ,c escapes through the small port a to the atmosphere4 ,the steam1 acting on the lower surface of valve #ad1 lifts it and its piston to the position shown1 and again flows to the pump1 which works until the required air-pressure is again obtained in the reservoir4 ,illustration3 ,,fig4 #ij4--;,air-pump of ,westinghouse brake4 #ba ,this resembles the upper part of the rudimentary water injector shown in ,fig4 #ae4 ,the reader need only imagine pipe ,b to be connected with the train pipe4 ,a rush of steam through pipe ,a creates a partial vacuum in the cone ,e1 causing air from the train pipe to rush into it and be expelled by the steam blast4 ,chapter ,,xi4 ,,railway ,,signalling4 ,the block system--,position of signals--,interlocking the signals--,locking gear--,points--,points and signals in combination--,working the block system--,series of signalling operations--,single line signals--,the train staff--,train staff and ticket--,electric train staff system--,interlocking--,signalling operations--,power signalling--,pneumatic signalling--,automatic signalling4 ,under certain conditions--namely1 at sharp curves or in darkness--the most powerful brakes might not avail to prevent a train running into the rear of another1 if trains were allowed to follow each other closely over the line4 ,it is therefore necessary to introduce an effective system of keeping trains running in the same direction a sufficient distance apart1 and this is done by giving visible and easily understood orders to the driver while a train is in motion4 ,in the early days of the railway it was customary to allow a time interval between the passings of trains1 a train not being permitted to leave a station until at least five minutes after the start of a preceding train4 ,this method did not1 of course1 prevent collisions1 as the first train sometimes broke down soon after leaving the station2 and in the absence of effective brakes1 its successor ran into it4 ,the advent of the electric telegraph1 which put stations in rapid communication with one another1 proved of the utmost value to the safe working of railways4 ,,the ,,block ,,system4 ,time limits were abolished and distance limits substituted4 ,a line was divided into blocks1 or lengths1 and two trains going in the same direction were never allowed on any one block at the same time4 ,the signal-posts carrying the movable arms1 or semaphores1 by means of which the signalman communicates with the engine-driver1 are well known to us4 ,they are usually placed on the left-hand side of the line of rails to which they apply1 with their arms pointing away from the rails4 ,the side of the arms which faces the direction from which a train approaches has a white stripe painted on a red background1 the other side has a black stripe on a white background4 ,the distant and other signal arms vary slightly in shape 7,fig4 #ia74 ,a distant signal has a forked end and a ,v-shaped stripe2 the home and starting signals are square-ended1 with straight stripes4 ,when the arm stands horizontally1 the signal is on1 or at danger2 when dropped1 it is off1 and indicates ,all right2 proceed4 ,at the end nearest the post it carries a spectacle frame glazed with panes of red and green glass4 ,when the arm is at danger1 the red pane is opposite a lamp attached to the signal post2 when the arm drops1 the green pane rises to that position--so that a driver is kept as fully informed at night as during the day1 provided the lamp remains alight4 ,illustration3 ,,fig4 #ia4--;,distant and home signals4 ,,position ,,of ,,signals4 ,on double lines each set of rails has its own separate signals1 and drivers travelling on the up line take no notice of signals meant for the down line4 ,each signal-box usually controls three signals on each set of rails--the distant1 the home1 and the starting4 ,their respective positions will be gathered from ,fig4 #ib1 which shows a station on a double line4 ,between the distant and the home an interval is allowed of #hjj yards on the level1 #a1jjj yards on a falling gradient1 and #fjj yards on a rising gradient4 ,the home stands near the approach end of the station1 and the starting at the departure end of the platform4 ,the last is sometimes reinforced by an advance starting signal some distance farther on4 ,it should be noted that the distant is only a caution signal1 whereas both home and starting are stop signals4 ,this means that when the driver sees the distant on1 he does not stop his train1 but slackens speed1 and prepares to stop at the home signal4 ,he must1 however1 on no account pass either home or starting if they are at danger4 ,in short1 the distant merely warns the driver of what he may expect at the home4 ,to prevent damage if a driver should overrun the home1 it has been laid down that no train shall be allowed to pass the starting signal of one box unless the line is clear to a point at least a quarter of a mile beyond the home of the next box4 ,that point is called the standard clearing point4 ,technically described1 a block is a length of line between the last stop signal worked from one signal-box and the first stop signal worked from the next signal-box in advance4 ,illustration3 ,,fig4 #ib4--,showing position of signals4 ,those at the top are off4 ,,interlocking ,,signals4 ,a signalman cannot lower or restore his signals to their normal positions in any order he likes4 ,he is compelled to lower them as follows3--,starting and home2 then distant4 ,and restore them--distant2 then starting and home4 ,if a signalman were quite independent1 he might1 after the passage of a train1 restore the home or starting1 but forget all about the distant1 so that the next train1 which he wants to stop1 would dash past the distant without warning and have to pull up suddenly when the home came in sight4 ,but by a mechanical arrangement he is prevented from restoring the home or starting until the distant is at danger2 and1 vice vers1 he cannot lower the last until the other two are off4 ,this mechanism is called locking gear4 ,,looking ,,gear4 ,there are many different types of locking gear in use4 ,it is impossible to describe them all1 or even to give particulars of an elaborate locking-frame of any one type4 ,but if we confine ourselves to the simplest combination of a stud-locking apparatus1 such as is used in small boxes on the ,great ,western ,railway1 the reader will get an insight into the general principles of these safety devices1 as the same principles underlie them all4 ,illustration3 ,,fig4 #ic4--;,a signal lever and its connections4 ,to move the lever1 ,c is pressed towards ,b raising the catch-rod from its nick in the rack1 ,g ,g ,g1 guides2 ,r ,r1 anti-friction rollers2 ,s1 sockets for catch-rod to work in4 ,the levers in the particular type of locking gear which we are considering have each a tailpiece or tappet arm attached to it1 which moves backwards and forwards with the lever 7,fig4 #ic74 ,running at right angles to this tappet1 and close to it1 either under or above1 are the lock bars1 or stud bars4 ,refer now to ,fig4 #id1 which shows the ends of the three tappet arms1 ,d1 ,h1 and ,s1 crossed by a bar1 ,b1 from which project these studs4 ,the levers are all forward and the signals all on4 ,if the signalman tried to pull the lever attached to ,d down the page1 as it were1 he would fail to move it on account of the stud a1 which engages with a notch in ,d4 ,before this stud can be got free of the notch the tappets ,h and ,s must be pulled over1 so as to bring their notches in line with studs b and c 7,fig4 #ie74 ,the signalman can now move ,d1 since the notch easily pushes the stud a to the left 7,fig4 #if74 ,the signals must be restored to danger4 ,as ,h and ,s are back-locked by ,d--that is1 prevented by ,d from being put back into their normal positions--,d must be moved first4 ,the interlocking of the three signals described is merely repeated in the interlocking of a large number of signals4 ,illustration3 ,,fig4 #id4 ,illustration3 ,,fig4 #ie4 ,on entering a signal-box a visitor will notice that the levers have different colours3--,green1 signifying distant signals2 red1 signifying home and starting signals2 blue1 signifying facing points2 black1 signifying trailing points2 white1 signifying spare levers4 ,these different colours help the signalman to pick out the right levers easily4 ,to the front of each lever is attached a small brass tablet bearing certain numbers2 one in large figures on the top1 then a line1 and other numbers in small figures beneath4 ,the large number is that of the lever itself2 the others1 called leads1 refer to levers which must be pulled before that particular lever can be released4 ,illustration3 ,,fig4 #if4 ,illustration3 ,,fig4 #ig4--,model signal equipment in a signalling school4 7,by permission of the ,g4,w4,r4 ,magazine74 ,,points4 ,mention was made1 in connection with the lever1 of points4 ,before going further we will glance at the action of these devices for enabling a train to run from one set of rails to another4 ,figs4 #ih and #ii show the points at a simple junction4 ,it will be noticed that the rails of the line to the left of the points are continued as the outer rails of the main and branch lines4 ,the inner rails come to a sharp ,v-point1 and to the left of this are the two short rails which1 by means of shifting portions1 decide the direction of a train's travel4 ,in ,fig4 #ih the main line is open2 in ,fig4 #ii1 the branch4 ,the shifting parts are kept properly spaced by cross bars 7or tie-rods71 ,a ,a4 ,illustration3 ,,fig4 #ih4--,points open to main line4 ,illustration3 ,,fig4 #ii4--,points open to branch line4 ,it might be thought that the wheels would bump badly when they reach the point ,b1 where there is a gap4 ,this is prevented1 however1 by the bent ends ,e ,e 7,fig4 #ih71 on which the tread of the wheel rests until it has reached some distance along the point of ,v4 ,the safety rails ,s ,r keep the outer wheel up against its rail until the ,v has been passed4 ,,points ,,and ,,signals ,,in ,,combination4 ,let us suppose that a train is approaching the junction shown in ,figs4 #ih and #ii from the left4 ,it is not enough that the driver should know that the tracks are clear4 ,he must also be assured that the track1 main or branch1 as the case may be1 along which he has to go1 is open2 and on the other hand1 if he were approaching from the right1 he would want to be certain that no train on the other line was converging on his4 ,danger is avoided and assurance given by interlocking the points and signals4 ,to the left of the junction the home and distant signals are doubled1 there being two semaphore arms on each post4 ,these are interlocked with the points in such a manner that the signals referring to either line can be pulled off only when the points are set to open the way to that line4 ,moreover1 before any shifting of points can be made1 the signals behind must be put to danger4 ,the convergence of trains is prevented by interlocking1 which renders it impossible to have both sets of distant and home signals at ,all right simultaneously4 ,,working ,,of ,,block ,,system4 ,we may now pass to the working of the block system of signalling trains from station to station on one line of a double track4 ,each signal-box 7except1 of course1 those at termini7 has electric communication with the next box in both directions4 ,the instruments used vary on different systems1 but the principle is the same2 so we will concentrate our attention on those most commonly employed on the ,great ,western ,railway4 ,they are3--7#a47 ,two tapper-bell instruments1 connected with similar instruments in the adjacent boxes on both sides4 ,each of these rings one beat in the corresponding box every time its key is depressed4 7#b47 ,two ,spagnoletti disc instruments--one1 having two keys1 communicating with the box in the rear2 and the other1 in connection with the forward box1 having no keys4 ,their respective functions are to give signals and receive them4 ,in the centre of the face of each is a square opening1 behind which moves a disc carrying two flags--,train on line in white letters on red ground1 and ,line clear in black letters on a white ground4 ,the keyed instrument has a red and a white key4 ,when the red key is depressed1 ,train on line appears at the opening2 also in that of a keyless disc at the adjacent signal-box4 ,a depression of the white key similarly gives ,line clear4 ,a piece of wire with the ends turned over and passed through two eyes slides over the keys1 and can be made to hold either down4 ,in addition to these1 telephonic and telegraphic instruments are provided to enable the signalmen to converse4 ,,series ,,of ,,signalling ,,operations4 ,illustration3 ,,fig4 #ajj4--,the signaling instruments in three adjacent cabins4 ,the featherless arrows show the connection of the instruments4 ,we may now watch the doings of signalmen in four successive boxes1 ,a1 ,b1 ,c1 and ,d1 during the passage of an express train4 ,signalman ,a calls signalman ,b's attention by one beat on the tapper-bell4 ,b answers by repeating it to show that he is attending4 ,a asks1 ,is line clear for passenger express8--four beats on the bell4 ,b1 seeing that the line is clear to his clearing point1 sends back four beats1 and pins down the white key of his instrument4 ,line clear appears on the opening1 and also at that of ,a's keyless disc4 ,a lowers starting signal4 ,train moves off4 ,a gives two beats on the tapper ,train entering section4 ,b pins indicator at ,train on line1 which also appears on ,a's instrument4 ,a places signals at danger4 ,b asks ,c1 ,is line clear8 ,c repeats the bell code1 and pins indicator at ,line clear1 shown on ,b's keyless disc also4 ,b lowers all signals4 ,train passes4 ,b signals to ,c1 ,train entering section4 ,b signals to ,a1 ,train out of section1 and releases indicator1 which returns to normal position with half of each flag showing at the window4 ,b signals to ,c1 ,train on line1 and sets all his signals to danger4 ,c pins indicator to ,train on line4 ,c asks1 ,is line clear8 ,but there is a train at station ,d1 and signalman ,d therefore gives no reply1 which is equivalent to a negative4 ,the driver1 on approaching ,c's distant1 sees it at danger1 and slows down1 stopping at the home4 ,c lowers home1 and allows train to proceed to his starting signal4 ,d1 when the line is clear to his clearing point1 signals ,line clear1 and pins indicator at ,line clear4 ,c lowers starting signals1 and train proceeds4 ,c signals to ,d1 ,train entering section1 and ,d pins indicator at ,train on line4 ,c signals to ,b1 ,train out of section1 sets indicator at normal1 and puts signals at danger4 ,and so the process is repeated from station to station4 ,where1 however1 sections are short1 the signalman is advised one section ahead of the approach of a train by an additional signal signifying1 ,fast train approaching4 ,the block indicator reminds the signalman of the whereabouts of the train4 ,unless his keyless indicator is at normal1 he may not ask1 ,is line clear8 ,and until he signals back ,line clear to the box behind1 a train is not allowed to enter his section4 ,in this way a section of line with a full complement of signals is always interposed between any two trains4 ,,the ,,working ,,of ,,single ,,lines4 ,we have dealt with the signalling arrangements pertaining to double lines of railway1 showing that a system of signals is necessary to prevent a train running into the back of its predecessor4 ,where trains in both directions pass over a single line1 not only has this element of danger to be dealt with1 but also the possibility of a train being allowed to enter a section of line from each end at the same time4 ,this is effected in several ways1 the essence of each being that the engine-driver shall have in his possession visible evidence of the permission accorded him by the signalman to enter a section of single line4 ,a ,,single ,,train ,,staff4 ,the simplest form of working is to allocate to the length of line a train staff--a piece of wood about #ad inches long1 bearing the names of the stations at either end4 ,this is adopted where only one engine is used for working a section1 such as a short branch line4 ,in a case like this there is obviously no danger of two trains meeting1 and the train staff is merely the authority to the driver to start a journey4 ,no telegraphic communication is necessary with such a system1 and signals are placed only at the ends of the line4 ,,train ,,staff ,,and ,,ticket4 ,on long lengths of single line where more than one train has to be considered1 the line is divided into blocks in the way already described for double lines1 and a staff is assigned to each1 the staffs for the various blocks differing from each other in shape and colour4 ,the usual signals are provided at each station1 and block telegraph instruments are employed1 the only difference being that one disc1 of the key pattern1 is used for trains in both directions4 ,on such a line it is1 of course1 possible that two or more trains may require to follow each other without any travelling intermediately in the opposite direction4 ,this would be impossible if the staff passed uniformly to and fro in the block section2 but it is arranged by the introduction of a train staff ticket used in conjunction with the staff4 ,no train is permitted to leave a staff station unless the staff for the section of line to be traversed is at the station2 and the driver has the strictest possible instructions that he must see the staff4 ,if a second train is required to follow1 the staff is shown to the driver1 and a train staff ticket handed him as his authority to proceed4 ,if1 however1 the next train over the section will enter from the opposite end1 the staff is handed to the driver4 ,to render this system as safe as possible1 train staff tickets are of the same colour and shape as the staff for the section to which they apply1 and are kept in a special box at the stations1 the key being attached to the staff and the lock so arranged that the key cannot be withdrawn unless the box has been locked4 ,,electric ,,train ,,staff ,,and ,,tablet ,,systems4 ,these systems of working are developments of the last mentioned1 by which are secured greater safety and ease in working the line4 ,on some sections of single line circumstances often necessitate the running of several trains in one direction without a return train4 ,for such cases the train staff ticket was introduced2 but even on the best regulated lines it is not always possible to secure that the staff shall be at the station where it is required at the right time1 and cases have arisen where1 no train being available at the station where the staff was1 it had to be taken to the other station by a man on foot1 causing much delay to traffic4 ,the electric train staff and tablet systems overcome this difficulty4 ,both work on much the same principle1 and we will therefore describe the former4 ,illustration3 ,,fig4 #aja4--;,an electric train staff holder3 ,s ,s1 staffs in the slot of the instrument4 ,leaning against the side of the cabin is a staff showing the key ,k at the end for unlocking a siding points between two stations4 ,the engine driver cannot remove the staff until the points have been locked again4 ,at each end of a block section a train staff instrument 7,fig4 #aja7 is provided4 ,in the base of these instruments are a number of train staffs1 any one of which would be accepted by an engine-driver as permission to travel over the single line4 ,the instruments are electrically connected1 the mechanism securing that a staff can be withdrawn only by the co-operation of the signalman at each end of the section2 that1 when all the staffs are in the instruments1 a staff may be withdrawn at either end2 that1 when a staff has been withdrawn1 another cannot be obtained until the one out has been restored to one or other of the instruments4 ,the safety of such a system is obvious1 as also the assistance to the working by having a staff available for a train no matter from which end it is to enter the section4 ,the mechanism of the instruments is quite simple4 ,a double-poled electro-magnet is energized by the depression of a key by the signalman at the further end of the block into which the train is to run1 and by the turning of a handle by the signalman who requires to withdraw a staff4 ,the magnet1 being energized1 is able to lift a mechanical lock1 and permits the withdrawal of a staff4 ,in its passage through the instrument the staff revolves a number of iron discs1 which in turn raise or lower a switch controlling the electrical connections4 ,this causes the electric currents actuating the electro-magnet to oppose each other1 the magnetism to cease1 and the lock to fall back1 preventing another staff being withdrawn4 ,it will naturally be asked1 ,how is the electrical system restored8 ,we have said that there were a number of staffs in each instrument--in other words1 a given number of staffs1 usually twenty1 is assigned to the section4 ,assume that there are ten in each instrument1 and that the switch in each is in its lower position4 ,now withdraw a staff1 and one instrument has an odd1 the other an even1 number of staffs1 and similarly one switch is raised while the other remains lowered1 therefore the electrical circuit is out of phase--that is1 the currents in the magnets of each staff instrument are opposed to one another1 and cannot release the lock4 ,the staff travels through the section and is placed in the instrument at the other end1 bringing the number of staffs to eleven--an odd number1 and1 what is more important1 raising the switch4 ,both switches are now raised1 consequently the electric currents will support each other1 so that a staff may be withdrawn4 ,briefly1 then1 when there is an odd number of staffs in one instrument and an even number in the other1 as when a staff is in use1 the signalmen are unable to obtain a staff1 and consequently cannot give authority for a train to enter the section2 but when there is either an odd or an even number of staffs in each instrument a staff may be withdrawn at either end on the co-operation of the signalmen4 ,we may add that1 where two instruments are in the same signal-box1 one for working to the box in advance1 the other to the rear1 it is arranged that the staffs pertaining to one section shall not fit the instrument for the other1 and must be of different colours4 ,this prevents the driver accidentally accepting a staff belonging to one section as authority to travel over the other4 ,,interlocking4 ,the remarks made on the interlocking of points and signals on double lines apply also to the working of single lines1 with the addition that not only are the distant1 home1 and starting signals interlocked with each other1 but with the signals and points governing the approach of a train from the opposite direction--in other words1 the signals for the approach of a train to a station from one direction cannot be lowered unless those for the approach to the station of a train from the opposite direction are at danger1 and the points correctly set4 ,,signalling ,,operations4 ,in the working of single lines1 as of double1 the signalman at the station from which a train is to proceed has to obtain the consent of the signalman ahead1 the series of questions to be signalled being very similar to those detailed for double lines4 ,there is1 however1 one notable exception4 ,on long lengths of single line it is necessary to make arrangements for trains to pass each other4 ,this is done by providing loop lines at intervals1 a second pair of rails being laid for the accommodation of one train while another in the opposite direction passes it4 ,to secure that more than one train shall not be on a section of single line between two crossing-places it is laid down that1 when a signalman at a non-crossing station is asked to allow a train to approach his station1 he must not give permission until he has notified the signalman ahead of him1 thus securing that he is not asking permission for trains to approach from both directions at the same time4 ,both for single and double line working a number of rules designed to deal with cases of emergency are laid down1 the guiding principle being safety2 but we have now dealt with all the conditions of everyday working1 and must pass to the consideration of ,illustration3 ,,fig4 #ajb4--;,an electric lever-frame in a signalling cabin at ,didcot4 ,,power ,,signalling4 ,in a power system of signalling the signalman is provided with some auxiliary means--electricity1 compressed air1 etc4--of moving the signals or points under his control4 ,it is still necessary to have a locking-frame in the signal-box1 with levers interlocked with each other1 and connections between the box and the various points and signals4 ,but the frame is much smaller than an ordinary manual frame1 and but little force is needed to move the little levers which make or break an electric circuit1 or open an air-valve1 according to the power-agent used4 ,,electric ,,signalling4 ,fig4 #ajb represents the locking-frame of a cabin at ,didcot1 ,england1 where an all-electric system has been installed4 ,wires lead from the cabin to motors situated at the points and signals1 which they operate through worm gearing4 ,when a lever is moved it closes a circuit and sets the current flowing through a motor1 the direction of the flow 7and consequently of the motor's revolution7 depending on whether the lever has been moved forward or backward4 ,indicators arranged under the levers tell the signalman when the desired movements at the points and signals have been completed4 ,if any motion is not carried through1 owing to failure of the current or obstruction of the working parts1 an electric lock prevents him continuing operations4 ,thus1 suppose he has to open the main line to an express1 he is obliged by the mechanical locking-frame to set all the points correctly before the signals can be lowered4 ,he might move all the necessary levers in due order1 yet one set of points might remain open1 and1 were the signals lowered1 an accident would result4 ,but this cannot happen1 as the electric locks worked by the points in question block the signal levers1 and until the failure has been set right1 the signals must remain at danger4 ,the point motors are connected direct to the points2 but between a signal motor and its arm there is an electric slot1 consisting of a powerful electro-magnet which forms a link in the rod work4 ,to lower a signal it is necessary that the motor shall revolve and a control current pass round the magnet to give it the requisite attractive force4 ,if no control current flows1 as would happen were any pair of points not in their proper position1 the motor can have no effect on the signal arm to lower it1 owing to the magnet letting go its grip4 ,furthermore1 if the signal had been already lowered when the control current failed1 it would rise to danger automatically1 as all signals are weighted to assume the danger position by gravity4 ,the signal control currents can be broken by the signalman moving a switch1 so that in case of emergency all signals may be thrown simultaneously to danger4 ,,pneumatic ,,signalling4 ,in ,england and the ,united ,states compressed air is also used to do the hard labour of the signalman for him4 ,instead of closing a circuit1 the signalman1 by moving a lever half-way over1 admits air to a pipe running along the track to an air reservoir placed beside the points or signal to which the lever relates4 ,the air opens a valve and puts the reservoir in connection with a piston operating the points or signal-arm1 as the case may be4 ,this movement having been performed1 another valve in the reservoir is opened1 and air passes back through a second pipe to the signal-box1 where it opens a third valve controlling a piston which completes the movement of the lever1 so showing the signalman that the operation is complete4 ,with compressed air1 as with electricity1 a mechanical locking-frame is of course used4 ,,automatic ,,signalling4 ,to reduce expense1 and increase the running speed on lines where the sections are short1 the train is sometimes made to act as its own signalman4 ,the rails of each section are all bonded together so as to be in metallic contact1 and each section is insulated from the two neighbouring sections4 ,at the further end of a section is installed an electric battery1 connected to the rails1 which lead the current back to a magnet operating a signal stationed some distance back on the preceding section4 ,as long as current flows the signal is held at ,all right4 ,when a train enters the section the wheels and axles short-circuit the current1 so that it does not reach the signal magnet1 and the signal rises to danger1 and stays there until the last pair of wheels has passed out of the section4 ,should the current fail or a vehicle break loose and remain on the section1 the same thing would happen4 ,the human element can thus be practically eliminated from signalling4 ,to make things absolutely safe1 a train should have positive control over a train following1 to prevent the driver overrunning the signals4 ,on electric railways this has been effected by means of contacts working in combination with the signals1 which either cut the current off from the section preceding that on which a train may be1 or raise a trigger to strike an arm on the train following and apply its brakes4 ,chapter ,,xii4 ,,optics4 ,lenses--,the image cast by a convex lens--,focus--,relative position of object and lens--,correction of lenses for colour--,spherical aberration--,distortion of image--,the human eye--,the use of spectacles--,the blind spot4 ,light is a third form of that energy of which we have already treated two manifestations--heat and electricity4 ,the distinguishing characteristic of ether light-waves is their extreme rapidity of vibration1 which has been calculated to range from #gjj billion movements per second for violet rays to #djj billion for red rays4 ,if a beam of white light be passed through a prism it is resolved into the seven visible colours of the spectrum--violet1 indigo1 blue1 green1 yellow1 orange1 and red--in this order4 ,the human eye is most sensitive to the yellow-red rays1 a photographic plate to the green-violet rays4 ,all bodies fall into one of two classes--7#a7 ,luminous--that is1 those which are a source of light1 such as the sun1 a candle flame1 or a red-hot coal2 and 7#b7 non-luminous1 which become visible only by virtue of light which they receive from other bodies and reflect to our eyes4 ,,the ,,propagation ,,of ,,light4 ,light naturally travels in a straight line4 ,it is deflected only when it passes from one transparent medium into another--for example1 from air to water--and the mediums are of different densities4 ,we may regard the surface of a visible object as made up of countless points1 from each of which a diverging pencil of rays is sent off through the ether4 ,,lenses4 ,if a beam of light encounters a transparent glass body with non-parallel sides1 the rays are deflected4 ,the direction they take depends on the shape of the body1 but it may be laid down as a rule that they are bent toward the thicker part of the glass4 ,the common burning-glass is well known to us4 ,we hold it up facing the sun to concentrate all the heat rays that fall upon it into one intensely brilliant spot1 which speedily ignites any inflammable substance on which it may fall 7,fig4 #ajc74 ,we may imagine that one ray passes from the centre of the sun through the centre of the glass4 ,this is undeflected2 but all the others are bent towards it1 as they pass through the thinner parts of the lens4 ,illustration3 ,,fig4 #ajc4--,showing how a burning-glass concentrates the heat rays which fall upon it4 ,it should be noted here that sunlight1 as we call it1 is accompanied by heat4 ,a burning-glass is used to concentrate the heat rays1 not the light rays1 which1 though they are collected too1 have no igniting effect4 ,in photography we use a lens to concentrate light rays only4 ,such heat rays as may pass through the lens with them are not wanted1 and as they have no practical effect are not taken any notice of4 ,to be of real value1 a lens must be quite symmetrical--that is1 the curve from the centre to the circumference must be the same in all directions4 ,there are six forms of simple lenses1 as given in ,fig4 #ajd4 ,nos4 #a and #b have one flat and one spherical surface4 ,nos4 #c1 #d1 #e1 #f have two spherical surfaces4 ,when a lens is thicker at the middle than at the sides it is called a convex lens2 when thinner1 a concave lens4 ,the names of the various shapes are as follows3--,no4 #a1 plano-convex2 ,no4 #b1 plano-concave2 ,no4 #c1 double convex2 ,no4 #d1 double concave2 ,no4 #e1 meniscus2 ,no4 #f1 concavo-convex4 ,the thick-centre lenses1 as we may term them 7,nos4 #a1 #c1 #e71 concentrate a pencil of rays passing through them2 while the thin-centre lenses 7,nos4 #b1 #d1 #f7 scatter the rays 7see ,fig4 #aje74 ,illustration3 ,,fig4 #ajd4--,six forms of lenses4 ,,the ,,camera4 ,illustration3 ,,fig4 #aje4 ,illustration3 ,,fig4 #ajf4 ,we said above that light is propagated in straight lines4 ,to prove this is easy4 ,get a piece of cardboard and prick a hole in it4 ,set this up some distance away from a candle flame1 and hold behind it a piece of tissue paper4 ,you will at once perceive a faint1 upside-down image of the flame on the tissue4 ,why is this8 ,turn for a moment to ,fig4 #ajf1 which shows a pinhole camera in section4 ,at the rear is a ground-glass screen1 ,b1 to catch the image4 ,suppose that ,a is the lowest point of the flame4 ,a pencil of rays diverging from it strikes the front of the camera1 which stops them all except the one which passes through the hole and makes a tiny luminous spot on ,b1 above the centre of the screen1 though ,a is below the axis of the camera4 ,similarly the tip of the flame 7above the axis7 would be represented by a dot on the screen below its centre4 ,and so on for all the millions of points of the flame4 ,if we were to enlarge the hole we should get a brighter image1 but it would have less sharp outlines1 because a number of rays from every point of the candle would reach the screen and be jumbled up with the rays of neighbouring pencils4 ,now1 though a good1 sharp photograph may be taken through a pinhole1 the time required is so long that photography of this sort has little practical value4 ,what we want is a large hole for the light to enter the camera by1 and yet to secure a distinct image4 ,if we place a lens in the hole we can fulfil our wish4 ,fig4 #ajg shows a lens in position1 gathering up a number of rays from a point1 ,a1 and focussing them on a point1 ,b4 ,if the lens has #a1jjj times the area of the pinhole1 it will pass #a1jjj times as many rays1 and the image of ,a will be impressed on a sensitized photographic plate #a1jjj times more quickly4 ,illustration3 ,,fig4 #ajg4 ,,the ,,image ,,cast ,,by ,a ,,convex ,,lens4 ,fig4 #ajh shows diagrammatically how a convex lens forms an image4 ,from ,a and ,b1 the extremities of the object1 a simple ray is considered to pass through the centre of the lens4 ,this is not deflected at all4 ,two other rays from the same points strike the lens above and below the centre respectively4 ,these are bent inwards and meet the central rays1 or come to a focus with them at ,a#a and ,b#a4 ,in reality a countless number of rays would be transmitted from every point of the object and collected to form the image4 ,illustration3 ,,fig4 #ajh4--,showing how an image is cast by a convex lens4 ,,focus4 ,we must now take special notice of that word heard so often in photographic talk--focus4 ,what is meant by the focus or focal length of a lens8 ,well1 it merely signifies the distance between the optical centre of the lens and the plane in which the image is formed4 ,illustration3 ,,fig4 #aji4 ,we must here digress a moment to draw attention to the three simple diagrams of ,fig4 #aji4 ,the object1 ,o1 in each case is assumed to be to the right of the lens4 ,in the topmost diagram the object is so far away from the lens that all rays coming from a single point in it are practically parallel4 ,these converge to a focus at ,f4 ,if the distance between ,f and the centre of the lens is six inches1 we say that the lens has a six-inch focal length4 ,the focal length of a lens is judged by the distance between lens and image when the object is far away4 ,to avoid confusion1 this focal length is known as the principal focus1 and is denoted by the symbol f4 ,in the middle diagram the object is quite near the lens1 which has to deal with rays striking its nearer surface at an acuter angle than before 7reckoning from the centre74 ,as the lens can only deflect their path to a fixed degree1 they will not1 after passing the lens1 come together until they have reached a point1 ,f#a1 further from the lens than ,f4 ,the nearer we approach ,o to the lens1 the further away on the other side is the focal point1 until a distance equal to that of ,f from the lens is reached1 when the rays emerge from the glass in a parallel pencil4 ,the rays now come to a focus no longer1 and there can be no image4 ,if ,o be brought nearer than the focal distance1 the rays would diverge after passing through the lens4 ,,relative ,,positions ,,of ,,object ,,and ,,image4 ,illustration3 ,,fig4 #aaj4--,showing how the position of the image alters relatively to the position of the object4 ,from what has been said above we deduce two main conclusions--7#a47 ,the nearer an object is brought to the lens1 the further away from the lens will the image be4 7#b47 ,if the object approaches within the principal focal distance of the lens1 no image will be cast by the lens4 ,to make this plainer we append a diagram 7,fig4 #aaj71 which shows five positions of an object and the relative positions of the image 7in dotted lines74 ,first1 we note that the line ,a ,b1 or ,a ,b#a1 denotes the principal focal length of the lens1 and ,a ,c1 or ,a ,c#a1 denotes twice the focal length4 ,we will take the positions in order3-- ,position ,i4 ,object further away than #b;f4 ,inverted image smaller than object1 at distance somewhat exceeding f4 ,position ,,ii4 ,object at distance #b;f4 ,inverted image at distance #b;f1 and of size equal to that of object4 ,position ,,iii ,object nearer than #b;f4 ,inverted image further away than #b;f2 larger than the object4 ,position ,,iv4 ,object at distance f4 ,as rays are parallel after passing the lens no image is cast4 ,position ,v4 ,object at distance less than f4 ,no real image--that is1 one that can be caught on a focussing screen--is now given by the lens1 but a magnified1 erect1 virtual image exists on the same side of the lens as the object4 ,we shall refer to virtual images at greater length presently4 ,it is hoped that any reader who practises photography will now understand why it is necessary to rack his camera out beyond the ordinary focal distance when taking objects at close quarters4 ,from ,fig4 #aaj he may gather one practically useful hint--namely1 that to copy a diagram1 etc41 full size1 both it and the plate must be exactly #b;f from the optical centre of the lens4 ,and it follows from this that the further he can rack his camera out beyond #b;f the greater will be the possible enlargement of the original4 ,,correction ,,of ,,lenses ,,for ,,colour4 ,we have referred to the separation of the spectrum colours of white light by a prism4 ,now1 a lens is one form of prism1 and therefore sorts out the colours4 ,in ,fig4 #aaa we assume that two parallel red rays and two parallel violet rays from a distant object pass through a lens4 ,a lens has most bending effect on violet rays and least on red1 and the other colours of the spectrum are intermediately influenced4 ,for the sake of simplicity we have taken the two extremes only4 ,you observe that the point ,r1 in which the red rays meet1 is much further from the lens than is ,v1 the meeting-point of the violet rays4 ,a photographer very seldom has to take a subject in which there are not objects of several different colours1 and it is obvious that if he used a simple lens like that in ,fig4 #aaa and got his red objects in good focus1 the blue and green portions of his picture would necessarily be more or less out of focus4 ,illustration3 ,,fig4 #aaa4 ,illustration3 ,,fig4 #aab4 ,this defect can fortunately be corrected by the method shown in ,fig4 #aab4 ,a compound lens is needed1 made up of a crown glass convex element1 ,b1 and a concave element1 ,a1 of flint glass4 ,for the sake of illustration the two parts are shown separated2 in practice they would be cemented together1 forming one optical body1 thicker in the centre than at the edges--a meniscus lens in fact1 since ,a is not so concave as ,b is convex4 ,now1 it was discovered by a ,mr4 ,hall many years ago that if white light passed through two similar prisms1 one of flint glass the other of crown glass1 the former had the greater effect in separating the spectrum colours--that is1 violet rays were bent aside more suddenly compared with the red rays than happened with the crown-glass prism4 ,look at ,fig4 #aab4 ,the red rays passing through the flint glass are but little deflected1 while the violet rays turn suddenly outwards4 ,this is just what is wanted1 for it counteracts the unequal inward refraction by ,b1 and both sets of rays come to a focus in the same plane4 ,such a lens is called achromatic1 or colourless4 ,if you hold a common reading-glass some distance away from large print you will see that the letters are edged with coloured bands1 proving that the lens is not achromatic4 ,a properly corrected photographic lens would not show these pretty edgings4 ,colour correction is necessary also for lenses used in telescopes and microscopes4 ,,spherical ,,aberration4 ,a lens which has been corrected for colour is still imperfect4 ,if rays pass through all parts of it1 those which strike it near the edge will be refracted more than those near the centre1 and a blurred focus results4 ,this is termed spherical aberration4 ,you will be able to understand the reason from ,figs4 #aac and #aad4 ,two rays1 ,a1 are parallel to the axis and enter the lens near the centre 7,fig4 #aac74 ,these meet in one plane4 ,two other rays1 ,b1 strike the lens very obliquely near the edge1 and on that account are both turned sharply upwards1 coming to a focus in a plane nearer the lens than ,a4 ,if this happened in a camera the results would be very bad4 ,either ,a or ,b would be out of focus4 ,the trouble is minimized by placing in front of the lens a plate with a central circular opening in it 7denoted by the thick1 dark line in ,fig4 #aad74 ,the rays ,b of ,fig4 #aac are stopped by this plate1 which is therefore called a stop4 ,but other rays from the same point pass through the hole4 ,these1 however1 strike the lens much more squarely above the centre1 and are not unduly refracted1 so that they are brought to a focus in the same plane as rays ,a4 ,illustration3 ,,fig4 #aac4 ,illustration3 ,,fig4 #aad4 ,,distortion ,,of ,,image4 ,illustration3 ,,fig4 #aae4--,section of a rectilinear lens4 ,the lens we have been considering is a single meniscus1 such as is used in landscape photography1 mounted with the convex side turned towards the inside of the camera1 and having the stop in front of it4 ,if you possess a lens of this sort1 try the following experiment with it4 ,draw a large square on a sheet of white paper and focus it on the screen4 ,the sides instead of being straight bow outwards3 this is called barrel distortion4 ,now turn the lens mount round so that the lens is outwards and the stop inwards4 ,the sides of the square will appear to bow towards the centre3 this is pin-cushion distortion4 ,for a long time opticians were unable to find a remedy4 ,then ,mr4 ,george ,s4 ,cundell suggested that two meniscus lenses should be used in combination1 one on either side of the stop1 as in ,fig #aae4 ,each produces distortion1 but it is counteracted by the opposite distortion of the other1 and a square is represented as a square4 ,lenses of this kind are called rectilinear1 or straight-line producing4 ,we have now reviewed the three chief defects of a lens--chromatic aberration1 spherical aberration1 and distortion--and have seen how they may be remedied4 ,so we will now pass on to the most perfect of cameras1 ,,the ,,human ,,eye4 ,the eye 7,fig4 #aaf7 is nearly spherical in form1 and is surrounded outside1 except in front1 by a hard1 horny coat called the sclerotica 7,s74 ,in front is the cornea 7,a71 which bulges outwards1 and acts as a transparent window to admit light to the lens of the eye 7,c74 ,inside the sclerotica1 and next to it1 comes the choroid coat2 and inside that again is the retina1 or curved focussing screen of the eye1 which may best be described as a network of fibres ramifying from the optic nerve1 which carries sight sensations to the brain4 ,the hollow of the ball is full of a jelly-like substance called the vitreous humour2 and the cavity between the lens and the cornea is full of water4 ,we have already seen that1 in focussing1 the distance between lens and image depends on the distance between object and lens4 ,now1 the retina cannot be pushed nearer to or pulled further away from its lens1 like the focussing screen of a camera4 ,how1 then1 is the eye able to focus sharply objects at distances varying from a foot to many miles8 ,illustration3 ,,fig4 #aaf4--,section of the human eye4 ,as a preliminary to the answer we must observe that the more convex a lens is1 the shorter is its focus4 ,we will suppose that we have a box camera with a lens of six-inch focus fixed rigidly in the position necessary for obtaining a sharp image of distant objects4 ,it so happens that we want to take with it a portrait of a person only a few feet from the lens4 ,if it were a bellows camera1 we should rack out the back or front4 ,but we cannot do this here4 ,so we place in front of our lens a second convex lens which shortens its principal focus2 so that in effect the box has been racked out sufficiently4 ,nature1 however1 employs a much more perfect method than this4 ,the eye lens is plastic1 like a piece of india-rubber4 ,its edges are attached to ligaments 7,l ,l71 which pull outwards and tend to flatten the curve of its surfaces4 ,the normal focus is for distant objects4 ,when we read a book the eye adapts itself to the work4 ,the ligaments relax and the lens decreases in diameter while thickening at the centre1 until its curvature is such as to focus all rays from the book sharply on the retina4 ,if we suddenly look through the window at something outside1 the ligaments pull on the lens envelope and flatten the curves4 ,this wonderful lens is achromatic1 and free from spherical aberration and distortion of image4 ,nor must we forget that it is aided by an automatic stop1 the iris1 the central hole of which is named the pupil4 ,we say that a person has black1 blue1 or gray eyes according to the colour of the iris4 ,like the lens1 the iris adapts itself to all conditions1 contracting when the light is strong1 and opening when the light is weak1 so that as uniform an amount of light as conditions allow may be admitted to the eye4 ,most modern camera lenses are fitted with adjustable stops which can be made larger or smaller by twisting a ring on the mount1 and are named iris stops4 ,the image of anything seen is thrown on the retina upside down1 and the brain reverses the position again1 so that we get a correct impression of things4 ,,the ,,use ,,of ,,spectacles4 ,illustration3 ,,fig4 #aag;a4 ,illustration3 ,,fig4 #aag;b4 ,illustration3 ,,fig4 #aah;a4 ,illustration3 ,,fig4 #aah;b4 ,the reader will now be able to understand without much trouble the function of a pair of spectacles4 ,a great many people of all ages suffer from short-sight4 ,for one reason or another the distance between lens and retina becomes too great for a person to distinguish distant objects clearly4 ,the lens1 as shown in ,fig #aag;a1 is too convex--has its minimum focus too short--and the rays meet and cross before they reach the retina1 causing general confusion of outline4 ,this defect is simply remedied by placing in front of the eye 7,fig4 #aag;b7 a concave lens1 to disperse the rays somewhat before they enter the eye1 so that they come to a focus on the retina4 ,if a person's sight is thus corrected for distant objects1 he can still see near objects quite plainly1 as the lens will accommodate its convexity for them4 ,the scientific term for short-sight is myopia4 ,long-sight1 or hypermetropia1 signifies that the eyeball is too short or the lens too flat4 ,fig4 #aah;a represents the normal condition of a long-sighted eye4 ,when looking at a distant object the eye thickens slightly and brings the focus forward into the retina4 ,but its thickening power in such an eye is very limited1 and consequently the rays from a near object focus behind the retina4 ,it is therefore necessary for a long-sighted person to use convex spectacles for reading the newspaper4 ,as seen in ,fig4 #aah;b1 the spectacle lens concentrates the rays before they enter the eye1 and so does part of the eye's work for it4 ,returning for a moment to the diagram of the eye 7,fig4 #aaf71 we notice a black patch on the retina near the optic nerve4 ,this is the yellow spot4 ,vision is most distinct when the image of the object looked at is formed on this part of the retina4 ,the blind spot is that point at which the optic nerve enters the retina1 being so called from the fact that it is quite insensitive to light4 ,the finding of the blind spot is an interesting little experiment4 ,on a card make a large and a small spot three inches apart1 the one an eighth1 the other half an inch in diameter4 ,bring the card near the face so that an eye is exactly opposite to each spot1 and close the eye opposite to the smaller4 ,now direct the other eye to this spot and you will find1 if the card be moved backwards and forwards1 that at a certain distance the large spot1 though many times larger than its fellow1 has completely vanished1 because the rays from it enter the open eye obliquely and fall on the blind spot4 ,chapter ,,xiii4 ,,the ,,microscope1 ,,the ,,telescope1 ,,and ,,the ,,magic-,,lantern4 ,the simple microscope--,use of the simple microscope in the telescope--,the terrestrial telescope--,the ,galilean telescope--,the prismatic telescope--,the reflecting telescope--,the parabolic mirror--,the compound microscope--,the magic-lantern--,the bioscope--,the plane mirror4 ,in ,fig4 #aai is represented an eye looking at a vase1 three inches high1 situated at ,a1 a foot away4 ,if we were to place another vase1 ,b1 six inches high1 at a distance of two feet2 or ,c1 nine inches high1 at three feet2 or ,d1 a foot high1 at four feet1 the image on the retina would in every case be of the same size as that cast by ,a4 ,we can therefore lay down the rule that the apparent size of an object depends on the angle that it subtends at the eye4 ,illustration3 ,,fig4 #aai4 ,to see a thing more plainly1 we go nearer to it2 and if it be very small1 we hold it close to the eye4 ,there is1 however1 a limit to the nearness to which it can be brought with advantage4 ,the normal eye is unable to adapt its focus to an object less than about ten inches away1 termed the least distance of distinct vision4 ,,the ,,simple ,,microscope4 ,illustration3 ,,fig4 #abj4 ,a magnifying glass comes in useful when we want to examine an object very closely4 ,the glass is a lens of short focus1 held at a distance somewhat less than its principal focal length1 ,f 7see ,fig4 #abj71 from the object4 ,the rays from the head and tip of the pin which enter the eye are denoted by continuous lines4 ,as they are deflected by the glass the eye gets the impression that a much longer pin is situated a considerable distance behind the real object in the plane in which the refracted rays would meet if produced backwards 7shown by the dotted lines74 ,the effect of the glass1 practically1 is to remove it 7the object7 to beyond the least distance of distinct vision1 and at the same time to retain undiminished the angle it subtends at the eye1 or1 what amounts to the same thing1 the actual size of the image formed on the retina4#bb ,it follows1 therefore1 that if a lens be of such short focus that it allows us to see an object clearly at a distance of two inches--that is1 one-fifth of the least distance of distinct vision--we shall get an image on the retina five times larger in diameter than would be possible without the lens4 ,the two simple diagrams 7,figs4 #aba and #abb7 show why the image to be magnified should be nearer to the lens than the principal focus1 ,f4 ,we have already seen 7,fig4 #aji7 that rays coming from a point in the principal focal plane emerge as a parallel pencil4 ,these the eye can bring to a focus1 because it normally has a curvature for focussing parallel rays4 ,but1 owing to the power of accommodation1 it can also focus diverging rays 7,fig4 #aba71 the eye lens thickening the necessary amount1 and we therefore put our magnifying glass a bit nearer than ,f to get full advantage of proximity4 ,if we had the object outside the principal focus1 as in ,fig4 #abb1 the rays from it would converge1 and these could not be gathered to a sharp point by the eye lens1 as it cannot flatten more than is required for focussing parallel rays4 ,illustration3 ,,fig4 #aba4 ,illustration3 ,,fig4 #abb4 ,,use ,,of ,,the ,,simple ,,microscope ,,in ,,the ,,telescope4 ,illustration3 ,,fig4 #abc4 ,let us now turn to ,fig4 #abc4 ,at ,a is a distant object1 say1 a hundred yards away4 ,b is a double convex lens1 which has a focal length of twenty inches4 ,we may suppose that it is a lens in a camera4 ,an inverted image of the object is cast by the lens at ,c4 ,if the eye were placed at ,c1 it would distinguish nothing4 ,but if withdrawn to ,d1 the least distance of distinct vision1#bc behind ,c1 the image is seen clearly4 ,that the image really is at ,c is proved by letting down the focussing screen1 which at once catches it4 ,now1 as the focus of the lens is twice d1 the image will be twice as large as the object would appear if viewed directly without the lens4 ,we may put this into a very simple formula3-- ,magnification focal length of lens -------------------- d ,illustration3 ,,fig4 #abd4 ,in ,fig4 #abd we have interposed between the eye and the object a small magnifying glass of #b-a_/b-;inch focus1 so that the eye can now clearly see the image when one-quarter d away from it4 ,b already magnifies the image twice2 the eye-piece again magnifies it four times2 so that the total magnification is #b #d #h times4 ,this result is arrived at quickly by dividing the focus of ,b 7which corresponds to the object-glass of a telescope7 by the focus of the eye-piece1 thus3-- #bj #h #b-a_/b ,the ordinary astronomical telescope has a very long focus object-glass at one end of the tube1 and a very short focus eye-piece at the other4 ,to see an object clearly one merely has to push in or pull out the eye-piece until its focus exactly corresponds with that of the object-glass4 ,,the ,,terrestrial ,,telescope4 ,an astronomical telescope inverts images4 ,this inversion is inconvenient for other purposes4 ,so the terrestrial telescope 7such as is commonly used by sailors7 has an eye-piece compounded of four convex lenses which erect as well as magnify the image4 ,fig4 #abe shows the simplest form of compound erecting eye-piece4 ,illustration3 ,,fig4 #abe4 ,,the ,,galilean ,,telescope4 ,illustration3 ,,fig4 #abf4 ,a third form of telescope is that invented by the great ,italian astronomer1 ,galileo1#bd in #afji4 ,its principle is shown in ,fig4 #abf4 ,the rays transmitted by the object-glass are caught1 before coming to a focus1 on a concave lens which separates them so that they appear to meet in the paths of convergence denoted by the dotted lines4 ,the image is erect4 ,opera-glasses are constructed on the ,galilean principle4 ,,the ,,prismatic ,,telescope4 ,in order to be able to use a long-focus object-glass without a long focussing-tube1 a system of glass reflecting prisms is sometimes employed1 as in ,fig4 #abg4 ,a ray passing through the object-glass is reflected from one posterior surface of prism ,a on to the other posterior surface1 and by it out through the front on to a second prism arranged at right angles to it1 which passes the ray on to the compound eye-piece4 ,the distance between object-glass and eye-piece is thus practically trebled4 ,the best-known prismatic telescopes are the ,zeiss field-glasses4 ,illustration3 ,,fig4 #abg4 ,,the ,,reflecting ,,telescope4 ,we must not omit reference to the reflecting telescope1 so largely used by astronomers4 ,the front end of the telescope is open1 there being no object-glass4 ,rays from the object fall on a parabolic mirror situated in the rear end of the tube4 ,this reflects them forwards to a focus4 ,in the ,newtonian reflector a plane mirror or prism is situated in the axis of the tube1 at the focus1 to reflect the rays through an eye-piece projecting through the side of the tube4 ,herschel's form of reflector has the mirror set at an angle to the axis1 so that the rays are reflected direct into an eye-piece pointing through the side of the tube towards the mirror4 ,,the ,,parabolic ,,mirror4 ,this mirror 7,fig4 #abh7 is of such a shape that all rays parallel to the axis are reflected to a common point4 ,in the marine searchlight a powerful arc lamp is arranged with the arc at the focus of a parabolic reflector1 which sends all reflected light forward in a pencil of parallel rays4 ,the most powerful searchlight in existence gives a light equal to that of #cej million candles4 ,illustration3 ,,fig4 #abh4--;,a parabolic reflector4 ,,the ,,compound ,,microscope4 ,we have already observed 7,fig4 #aaj7 that the nearer an object approaches a lens the further off behind it is the real image formed1 until the object has reached the focal distance1 when no image at all is cast1 as it is an infinite distance behind the lens4 ,we will assume that a certain lens has a focus of six inches4 ,we place a lighted candle four feet in front of it1 and find that a sharp diminished image is cast on a ground-glass screen held seven inches behind it4 ,if we now exchange the positions of the candle and the screen1 we shall get an enlarged image of the candle4 ,this is a simple demonstration of the law of conjugate foci--namely1 that the distance between the lens and an object on one side and that between the lens and the corresponding image on the other bear a definite relation to each other2 and an object placed at either focus will cast an image at the other4 ,whether the image is larger or smaller than the object depends on which focus it occupies4 ,in the case of the object-glass of a telescope the image was at what we may call the short focus4 ,illustration3 ,,fig4 #abi4--;,diagram to explain the compound microscope4 ,now1 a compound microscope is practically a telescope with the object at the long focus1 very close to a short-focus lens4 ,a greatly enlarged image is thrown 7see ,fig4 #abi7 at the conjugate focus1 and this is caught and still further magnified by the eye-piece4 ,we may add that the object-glass1 or objective1 of a microscope is usually compounded of several lenses1 as is also the eye-piece4 ,,the ,,magic-,,lantern4 ,the most essential features of a magic-lantern are3--7#a7 ,the source of light2 7#b7 the condenser for concentrating the light rays on to the slide2 7#c7 the lens for projecting a magnified image on to a screen4 ,fig4 #acj shows these diagrammatically4 ,the illuminant is most commonly an oil-lamp1 or an acetylene gas jet1 or a cylinder of lime heated to intense luminosity by an oxy-hydrogen flame4 ,the natural combustion of hydrogen is attended by a great heat1 and when the supply of oxygen is artificially increased the temperature of the flame rises enormously4 ,the nozzle of an oxy-hydrogen jet has an interior pipe connected with the cylinder holding one gas1 and an exterior1 and somewhat larger1 pipe leading from that containing the other1 the two being arranged concentrically at the nozzle4 ,by means of valves the proportions of the gases can be regulated to give the best results4 ,illustration3 ,,fig4 #acj4--,sketch of the elements of a magic-lantern4 ,the condenser is set somewhat further from the illuminant than the principal focal length of the lenses1 so that the rays falling on them are bent inwards1 or to the slide4 ,the objective1 or object lens1 stands in front of the slide4 ,its position is adjustable by means of a rack and a draw-tube4 ,the nearer it is brought to the slide the further away is the conjugate focus 7see p4 #bci71 and consequently the image4 ,the exhibitor first sets up his screen and lantern1 and then finds the conjugate foci of slide and image by racking the lens in or out4 ,if a very short focus objective be used1 subjects of microscopic proportions can be projected on the screen enormously magnified4 ,during the siege of ,paris in #ahgj-ga the ,parisians established a balloon and pigeon post to carry letters which had been copied in a minute size by photography4 ,these copies could be enclosed in a quill and attached to a pigeon's wing4 ,on receipt1 the copies were placed in a special lantern and thrown as large writing on the screen4 ,micro-photography has since then made great strides1 and is now widely used for scientific purposes1 one of the most important being the study of the crystalline formations of metals under different conditions4 ,,the ,,bioscope4 ,living pictures are the most recent improvement in magic-lantern entertainments4 ,the negatives from which the lantern films are printed are made by passing a ribbon of sensitized celluloid through a special form of camera1 which feeds the ribbon past the lens in a series of jerks1 an exposure being made automatically by a revolving shutter during each rest4 ,the positive film is placed in a lantern1 and the intermittent movement is repeated2 but now the source of illumination is behind the film1 and light passes outwards through the shutter to the screen4 ,in the ,urban bioscope the film travels at the rate of fifteen miles an hour1 upwards of one hundred exposures being made every second4 ,the impression of continuous movement arises from the fact that the eye cannot get rid of a visual impression in less than one-tenth of a second4 ,so that if a series of impressions follow one another more rapidly than the eye can rid itself of them the impressions will overlap1 and give one of motion1 if the position of some of the objects1 or parts of the objects1 varies slightly in each succeeding picture4#be ,,the ,,plane ,,mirror4 ,illustration3 ,,fig4 #aca4 ,this chapter may conclude with a glance at the common looking-glass4 ,why do we see a reflection in it8 ,the answer is given graphically by ,fig4 #aca4 ,two rays1 ,a b1 ,a c1 from a point ,a strike the mirror ,m at the points b and c4 ,lines b ,n1 c ,o1 drawn from these points perpendicular to the mirror are called their normals4 ,the angles ,a b ,n1 ,a c ,o are the angles of incidence of rays ,a b1 ,a c4 ,the paths which the rays take after reflection must make angles with b ,n and c ,o respectively equal to ,a b ,n1 ,a c ,o4 ,these are the angles of reflection4 ,if the eye is so situated that the rays enter it as in our illustration1 an image of the point ,a is seen at the point ,a#a1 in which the lines ,d b1 ,e c meet when produced backwards4 ,illustration3 ,,fig4 #acb4 ,when the vertical mirror is replaced by a horizontal reflecting surface1 such as a pond 7,fig4 #acb71 the same thing happens4 ,the point at which the ray from the reflection of the spire's tip to the eye appears to pass through the surface of the water must be so situated that if a line were drawn perpendicular to it from the surface the angles made by lines drawn from the real spire tip and from the observer's eye to the base of the perpendicular would be equal4 #bb ,glazebrook1 ,light1 p4 #aeg4 #bc ,glazebrook1 ,light1 p4 #aeg4 #bd ,galileo was severely censured and imprisoned for daring to maintain that the earth moved round the sun1 and revolved on its axis4 #be ,for a full account of ,animated ,pictures the reader might advantageously consult ,the ,romance of ,modern ,invention1 pp4 #aff foll4 ,chapter ,,xiv4 ,,sound ,,and ,,musical ,,instruments4 ,nature of sound--,the ear--,musical instruments--,the vibration of strings--,the sounding-board and the frame of a piano--,the strings--,the striking mechanism--,the quality of a note4 ,sound differs from light1 heat1 and electricity in that it can be propagated through matter only4 ,sound-waves are matter-waves1 not ether-waves4 ,this can be proved by placing an electric bell under the bell-glass of an air-pump and exhausting all the air4 ,ether still remains inside the glass1 but if the bell be set in motion no sound is audible4 ,admit air1 and the clang of the gong is heard quite plainly4 ,sound resembles light and heat1 however1 thus far1 that it can be concentrated by means of suitable lenses and curved surfaces4 ,an echo is a proof of its reflection from a surface4 ,before dealing with the various appliances used for producing sound-waves of a definite character1 let us examine that wonderful natural apparatus ,,the ,,ear1 through which we receive those sensations which we call sound4 ,illustration3 ,,fig4 #acc4--;,diagrammatic sketch of the parts of the ear4 ,fig4 #acc is a purely diagrammatic section of the ear1 showing the various parts distorted and out of proportion4 ,beginning at the left1 we have the outer ear1 the lobe1 to gather in the sound-waves on to the membrane of the tympanum1 or drum1 to which is attached the first of a series of ossicles1 or small bones4 ,the last of these presses against an opening in the inner ear1 a cavity surrounded by the bones of the head4 ,inside the inner ear is a watery fluid1 ,p1 called perilymph 7surrounding water71 immersed in which is a membranic envelope1 ,m1 containing endolymph 7inside water71 also full of fluid4 ,into this fluid project ,e ,e ,e1 the terminations of the auditory nerve1 leading to the brain4 ,when sound-waves strike the tympanum1 they cause it to move inwards and outwards in a series of rapid movements4 ,the ossicles operated by the tympanum press on the little opening ,o1 covered by a membrane1 and every time they push it in they slightly squeeze the perilymph1 which in turn compresses the endolymph1 which affects the nerve-ends1 and telegraphs a sensation of sound to the brain4 ,in ,fig4 #acd we have a more developed sketch1 giving in fuller detail1 though still not in their actual proportions1 the components of the ear4 ,the ossicles ,m1 ,i1 and ,s are respectively the malleus 7hammer71 incus 7anvil71 and stapes 7stirrup74 ,each is attached by ligaments to the walls of the middle ear4 ,the tympanum moves the malleus1 the malleus the incus1 and the incus the stapes1 the last pressing into the opening ,o of ,fig4 #acc1 which is scientifically known as the fenestra ovalis1 or oval window4 ,as liquids are practically incompressible1 nature has made allowance for the squeezing in of the oval window membrane1 by providing a second opening1 the round window1 also covered with a membrane4 ,when the stapes pushes the oval membrane in1 the round membrane bulges out1 its elasticity sufficing to put a certain pressure on the perilymph 7indicated by the dotted portion of the inner ear74 ,illustration3 ,,fig4 #acd4--;,diagrammatic section of the ear1 showing the various parts4 ,the inner ear consists of two main parts1 the cochlea--so called from its resemblance in shape to a snail's shell--and the semicircular canals4 ,each portion has its perilymph and endolymph1 and contains a number of the nerve-ends1 which are1 however1 most numerous in the cochlea4 ,we do not know for certain what the functions of the canals and the cochlea are2 but it is probable that the former enables us to distinguish between the intensity or loudness of sounds and the direction from which they come1 while the latter enables us to determine the pitch of a note4 ,in the cochlea are about #b1hjj tiny nerve-ends1 called the rods of ,corti4 ,the normal ear has such a range as to give about #cc rods to the semitone4 ,the great scientist ,helmholtz has advanced the theory that these little rods are like tiny tuning-forks1 each responding to a note of a certain pitch2 so that when a string of a piano is sounded and the air vibrations are transmitted to the inner ear1 they affect only one of these rods and the part of the brain which it serves1 and we have the impression of one particular note4 ,it has been proved by experiment that a very sensitive ear can distinguish between sounds varying in pitch by only #a_/fdth of a semitone1 or but half the range of any one ,corti fibre4 ,this difficulty ,helmholtz gets over by suggesting that in such an ear two adjacent fibres are affected1 but one more than the other4 ,a person who has a good ear for music is presumably one whose ,corti rods are very perfect4 ,unlucky people like the gentleman who could only recognize one tune1 and that because people took off their hats when it commenced1 are physically deficient4 ,their ,corti rods cannot be properly developed4 ,what applies to one single note applies also to the elements of a musical chord4 ,a dozen notes may sound simultaneously1 but the ear is able to assimilate each and blend it with its fellows2 yet it requires a very sensitive and well-trained ear to pick out any one part of a harmony and concentrate the brain's attention on that part4 ,the ear has a much larger range than the eye4 ,while the former ranges over eleven octaves1 but little more than a single octave is possible to the latter4 ,the quickest vibrations which strike the eye1 as light1 have only about twice the rapidity of the slowest2 whereas the quickest vibrations which strike the ear1 as a musical sound1 have more than two thousand times the rapidity of the slowest4#bf ,to come to actual figures1 the ordinary ear is sensitive to vibrations ranging from #af to #ch1jjj per second4 ,the bottom and top notes of a piano make respectively about #dj and #d1jjj vibrations a second4 ,of course1 some ears1 like some eyes1 cannot comprehend the whole scale4 ,the squeak of bats and the chirrup of crickets are inaudible to some people2 and dogs are able to hear sounds far too shrill to affect the human auditory apparatus4 ,not the least interesting part of this wonderful organ is the tympanic membrane1 which is provided with muscles for altering its tension automatically4 ,if we are straining our ears to catch a shrill sound1 we tighten the membrane2 while if we are getting ready for a deep1 loud report like that of a gun1 we allow the drum to slacken4 ,the ,eustachian tube 7,fig4 #acd7 communicates with the mouth4 ,its function is probably to keep the air-pressure equal on both sides of the drum4 ,when one catches cold the tube is apt to become blocked by mucus1 causing unequal pressure and consequent partial deafness4 ,before leaving this subject1 it will be well to remind our more youthful readers that the ear is delicately as well as wonderfully made1 and must be treated with respect4 ,sudden shouting into the ear1 or a playful blow1 may have most serious effects1 by bursting the tympanum or injuring the arrangement of the tiny bones putting it in communication with the inner ear4 ,,musical ,,instruments4 ,these are contrivances for producing sonorous shocks following each other rapidly at regular intervals4 ,musical sounds are distinguished from mere noises by their regularity4 ,if we shake a number of nails in a tin box1 we get only a series of superimposed and chaotic sensations4 ,on the other hand1 if we strike a tuning-fork1 the air is agitated a certain number of times a second1 with a pleasant result which we call a note4 ,we will begin our excursion into the region of musical instruments with an examination of that very familiar piece of furniture1 ,,the ,,pianoforte1 which means literally the soft-strong4 ,by many children the piano is regarded as a great nuisance1 the swallower-up of time which could be much more agreeably occupied1 and is accordingly shown much less respect than is given to a phonograph or a musical-box4 ,yet the modern piano is a very clever piece of work1 admirably adapted for the production of sweet melody--if properly handled4 ,the two forms of piano now generally used are the upright1 with vertical sound-board and wires1 and the grand1 with horizontal sound-board4#bg ,,the ,,vibration ,,of ,,strings4 ,as the pianoforte is a stringed instrument1 some attention should be given to the subject of the vibration of strings4 ,a string in a state of tension emits a note when plucked and allowed to vibrate freely4 ,the pitch of the note depends on several conditions3--7#a7 ,the diameter of the string2 7#b7 the tension of the string2 7#c7 the length of the string2 7#d7 the substance of the string4 ,taking them in order3--7#a47 ,the number of vibrations per second is inversely proportional to the diameter of the string3 thus1 a string one-quarter of an inch in diameter would vibrate only half as often in a given time as a string one-eighth of an inch in diameter4 7#b47 ,the length remaining the same1 the number of vibrations is directly proportional to the square root of the tension3 thus1 a string strained by a #af-lb4 weight would vibrate four times as fast as it would if strained by a #a-lb4 weight4 7#c47 ,the number of vibrations is inversely proportional to the length of the string3 thus1 a one-foot string would vibrate twice as fast as a two-foot string1 strained to the same tension1 and of equal diameter and weight4 7#d47 ,other things being equal1 the rate of vibration is inversely proportional to the square root of the density of the substance3 so that a steel wire would vibrate more rapidly than a platinum wire of equal diameter1 length1 and tension4 ,these facts are important to remember as the underlying principles of stringed instruments4 ,now1 if you hang a wire from a cord1 and hang a heavy weight from the wire1 the wire will be in a state of high tension1 and yield a distinct note if struck4 ,but the volume of sound will be very small1 much too small for a practical instrument4 ,the surface of the string itself is so limited that it sets up but feeble motions in the surrounding air4 ,now hang the wire from a large board and strike it again4 ,the volume of sound has greatly increased1 because the string has transmitted its vibrations to the large surface of the board4 ,to get the full sound-value of the vibrations of a string1 we evidently ought to so mount the string that it may influence a large sounding surface4 ,in a violin this is effected by straining the strings over a bridge resting on a hollow box made of perfectly elastic wood4 ,draw the bow across a string4 ,the loud sound heard proceeds not from the string only1 but also from the whole surface of the box4 ,,the ,,sounding-,,board ,,and ,,frame ,,of ,a ,,piano4 ,a piano has its strings strained across a frame of wood or steel1 from a row of hooks in the top of the frame to a row of tapering square-ended pins in the bottom1 the wires passing over sharp edges near both ends4 ,the tuner is able1 on turning a pin1 to tension its strings till it gives any desired note4 ,readers may be interested to learn that the average tension of a string is #bge lbs41 so that the total strain on the frame of a grand piano is anything between #bj and #cj tons4 ,to the back of the frame is attached the sounding-board1 made of spruce fir 7the familiar ,christmas tree74 ,this is obtained from ,central and ,eastern ,europe1 where it is carefully selected and prepared1 as it is essential that the timber should be sawn in such a way that the grain of the wood runs in the proper direction4 ,,the ,,strings4 ,these are made of extremely strong steel wire of the best quality4 ,if you examine the wires of your piano1 you will see that they vary in thickness1 the thinnest being at the treble end of the frame4 ,it is found impracticable to use wires of the same gauge and the same tension throughout4 ,the makers therefore use highly-tensioned thick wires for the bass1 and finer1 shorter wires for the treble1 taking advantage of the three factors--weight1 tension1 and length--which we have noticed above4 ,the wires for the deepest notes are wrapped round with fine copper wire to add to their weight without increasing their diameter at the tuning-pins4 ,there are about #fjj yards 7roughly one-third of a mile7 of wire in a grand piano4 ,,the ,,striking ,,mechanism4 ,we now pass to the apparatus for putting the strings in a state of vibration4 ,the grand piano mechanism shown in ,fig4 #ace may be taken as typical of the latest improvements4 ,the essentials of an effective mechanism are3--7#a7 ,that the blow delivered shall be sharp and certain2 7#b7 that the string shall be immediately damped1 or have its vibration checked if required1 so as not to interfere with the succeeding notes of other strings2 7#c7 that the hammer shall be able to repeat the blows in quick succession4 ,the hammer has a head of mahogany covered with felt1 the thickness of which tapers gradually and regularly from an inch and a quarter at the bass end to three-sixteenths of an inch at the extreme treble notes4 ,the entire eighty-five hammers for the piano are covered all together in one piece1 and then they are cut apart from each other4 ,the consistency of the covering is very important4 ,if too hard1 it yields a harsh note1 and must be reduced to the right degree by pricking with a needle4 ,in the diagram the felt is indicated by the dotted part4 ,illustration3 ,,fig4 #ace4--,the striking mechanism of a grand piano4 ,the action carriage which operates the hammer is somewhat complicated4 ,when the key is depressed1 the left end rises1 and pushes up the whole carriage1 which is pivoted at one end4 ,the hammer shank is raised by the jack ,b pressing upon a knob1 ,n1 called the notch1 attached to the under side of the shank4 ,when the jack has risen to a certain point1 its arm1 ,b#a1 catches against the button ,c and jerks it from under the notch at the very moment when the hammer strikes1 so that it may not be blocked against the string4 ,as it rebounds1 the hammer is caught on the repetition lever ,r1 which lifts it to allow of perfect repetition4 ,the check catches the tail of the hammer head during its descent when the key is raised1 and prevents it coming back violently on the carriage and rest4 ,the tail is curved so as to wedge against the check without jamming in any way4 ,the moment the carriage begins to rise1 the rear end of the key lifts a lever connected with the damper by a vertical wire1 and raises the damper of the string4 ,if the key is held down1 the vibrations continue for a long time after the blow2 but if released at once1 the damper stifles them as the hammer regains its seat4 ,a bar1 ,l1 passing along under all the damper lifters1 is raised by depressing the loud pedal4 ,the soft pedal slides the whole keyboard along such a distance that the hammers strike two only out of the three strings allotted to all except the bass notes1 which have only one string apiece1 or two1 according to their depth or length4 ,in some pianos the soft pedal presses a special damper against the strings2 and a third kind of device moves the hammers nearer the strings so that they deliver a lighter blow4 ,these two methods of damping are confined to upright pianos4 ,a high-class piano is the result of very careful workmanship4 ,the mechanism of each note must be accurately regulated by its tiny screws to a minute fraction of an inch4 ,it must be ensured that every hammer strikes its blow at exactly the right place on the string1 since on this depends the musical value of the note4 ,the adjustment of the dampers requires equal care1 and the whole work calls for a sensitive ear combined with skilled mechanical knowledge1 so that the instrument may have a light touch1 strength1 and certainty of action throughout the whole keyboard4 ,,the ,,quality ,,of ,a ,,note4 ,if two strings1 alike in all respects and equally tensioned1 are plucked1 both will give the same note1 but both will not necessarily have the same quality of tone4 ,the quality1 or timbre1 as musicians call it1 is influenced by the presence of overtones1 or harmonics1 in combination with the fundamental1 or deepest1 tone of the string4 ,the fact is1 that while a vibrating string vibrates as a whole1 it also vibrates in parts4 ,there are1 as it were1 small waves superimposed on the big fundamental waves4 ,points of least motion1 called nodes1 form on the string1 dividing it into two1 three1 four1 five1 etc41 parts1 which may be further divided by subsidiary nodes4 ,the string1 considered as halved by one node1 gives the first overtone1 or octave of the fundamental4 ,it may also vibrate as three parts1 and give the second overtone1 or twelfth of the fundamental2#bh and as four parts1 and give the third overtone1 the double octave4 ,now1 if a string be struck at a point corresponding to a node1 the overtones which require that point for a node will be killed1 on account of the excessive motion imparted to the string at that spot4 ,thus to hit it at the middle kills the octave1 the double octave1 etc42 while to hit it at a point one-third of the length from one end stifles the twelfth and all its sub-multiples4 ,a fundamental note robbed of all its harmonics is hard to obtain1 which is not a matter for regret1 as it is a most uninteresting sound4 ,to get a rich tone we must keep as many useful harmonics as possible1 and therefore a piano hammer is so placed as to strike the string at a point which does not interfere with the best harmonics1 but kills those which are objectionable4 ,pianoforte makers have discovered by experiment that the most pleasing tone is excited when the point against which the hammer strikes is one-seventh to one-ninth of the length of the wire from one end4 ,the nature of the material which does the actual striking is also of importance4 ,the harder the substance1 and the sharper the blow1 the more prominent do the harmonics become2 so that the worker has to regulate carefully both the duration of the blow and the hardness of the hammer covering4 #bf ,tyndall1 ,on ,sound1 p4 #ge4 #bg ,a ,broadwood grand is made up of #aj1gjj separate pieces1 and in its manufacture forty separate trades are concerned4 #bh ,twelve notes higher up the scale4 ,chapter ,,xv4 ,,wind ,,instruments4 ,longitudinal vibration--,columns of air--,resonance of columns of air--,length and tone--,the open pipe--,the overtones of an open pipe--,where overtones are used--,the arrangement of the pipes and pedals--,separate sound-boards--,varieties of stops--,tuning pipes and reeds--,the bellows--,electric and pneumatic actions--,the largest organ in the world--,human reeds4 ,,longitudinal ,,vibration4 ,in stringed instruments we are concerned only with the transverse vibrations of a string--that is1 its movements in a direction at right angles to the axis of the string4 ,a string can also vibrate longitudinally--that is1 in the direction of its axis--as may be proved by drawing a piece of resined leather along a violin string4 ,in this case the harmonics step up at the same rate as when the movements were transverse4 ,let us substitute for a wire a stout bar of metal fixed at one end only4 ,the longitudinal vibrations of this rod contain overtones of a different ratio4 ,the first harmonic is not an octave1 but a twelfth4 ,while a tensioned string is divided by nodes into two1 three1 four1 five1 six1 etc41 parts1 a rod fixed at one end only is capable of producing only those harmonics which correspond to division into three1 five1 seven1 nine1 etc41 parts4 ,therefore a free-end rod and a wire of the same fundamental note would not have the same timbre1 or quality1 owing to the difference in the harmonics4 ,,columns ,,of ,,air4 ,in wind instruments we employ1 instead of rods or wires1 columns of air as the vibrating medium4 ,the note of the column depends on its length4 ,in the penny whistle1 flute1 clarionet1 and piccolo the length of the column is altered by closing or opening apertures in the substance encircling the column4 ,,resonance ,,of ,,columns ,,of ,,air4 ,why does a tube closed at one end1 such as the shank of a key1 emit a note when we blow across the open end8 ,the act of blowing drives a thin sheet of air against the edge of the tube and causes it to vibrate4 ,the vibrations are confused1 some pulses occurring more frequently than others4 ,if we blew against the edge of a knife or a piece of wood1 we should hear nothing but a hiss4 ,but when1 as in the case which we are considering1 there is a partly-enclosed column of air close to the pulses1 this selects those pulses which correspond to its natural period of vibration1 and augments them to a sustained and very audible musical sound4 ,illustration3 ,,fig #acf4--,showing how the harmonics of a stopped pipe are formed4 ,in ,fig4 #acf1 #a is a pipe1 closed at the bottom and open at the top4 ,a tuning-fork of the same note as the pipe is struck and held over it so that the prongs vibrate upwards and downwards4 ,at the commencement of an outward movement of the prongs the air in front of them is compressed4 ,this impulse1 imparted to the air in the pipe1 runs down the column1 strikes the bottom1 and returns4 ,just as it reaches the top the prong is beginning to move inwards1 causing a rarefaction of the air behind it4 ,this effect also travels down and back up the column of air in the pipe1 reaching the prong just as it arrives at the furthest point of the inward motion4 ,the process is repeated1 and the column of air in the pipe1 striking on the surrounding atmosphere at regular intervals1 greatly increases the volume of sound4 ,we must observe that if the tuning-fork were of too high or too low a note for the column of air to move in perfect sympathy with it1 this increase of sound would not result4 ,now1 when we blow across the end1 we present1 as it were1 a number of vibrating tuning-forks to the pipe1 which picks out those air-pulses with which it sympathizes4 ,,length ,,and ,,tone4 ,the rate of vibration is found to be inversely proportional to the length of the pipe4 ,thus1 the vibrations of a two-foot pipe are twice as rapid as those of a four-foot pipe1 and the note emitted by the former is an octave higher than that of the latter4 ,a one-foot pipe gives a note an octave higher still4 ,we are here speaking of the fundamental tones of the pipes4 ,with them1 as in the case of strings1 are associated the overtones1 or harmonics1 which can be brought into prominence by increasing the pressure of the blast at the top of the pipe4 ,blow very hard on your key1 and the note suddenly changes to one much shriller4 ,it is the twelfth of the fundamental1 of which it has completely got the upper hand4 ,we must now put on our thinking-caps and try to understand how this comes about4 ,first1 let us note that the vibration of a body 7in this case a column of air7 means a motion from a point of rest to a point of rest1 or from node to node4 ,in the air-column in ,fig4 #acf1 #a1 there is only one point of rest for an impulse--namely1 at the bottom of the pipe4 ,so that to pass from node to node the impulse must pass up the pipe and down again4 ,the distance from node to node in a vibrating body is called a ventral segment4 ,remember this term4 ,therefore the pipe represents a semi-ventral segment when the fundamental note is sounding4 ,when the first overtone is sounded the column divides itself into two vibrating parts4 ,where will the node between them be8 ,we might naturally say1 ,half-way up4 ,but this cannot be so2 for if the node were so situated1 an impulse going down the pipe would only have to travel to the bottom to find another node1 while an impulse going up would have to travel to the top and back again--that is1 go twice as far4 ,so the node forms itself one-third of the distance down the pipe4 ,from ,b to ,a 7,fig4 #acf1 #b7 and back is now equal to from ,b to ,c4 ,when the second overtone is blown 7,fig4 #acf1 #c7 a third node forms4 ,the pipe is now divided into five semi-ventral segments4 ,and with each succeeding overtone another node and ventral segment are added4 ,the law of vibration of a column of air is that the number of vibrations is directly proportional to the number of semi-ventral segments into which the column of air inside the pipe is divided4#bi ,if the fundamental tone gives #ajj vibrations per second1 the first overtone in a closed pipe must give #cjj1 and the second #ejj vibrations4 ,,the ,,open ,,pipe4 ,a pipe open at both ends is capable of emitting a note4 ,but we shall find1 if we experiment1 that the note of a stopped pipe is an octave lower than that of an open pipe of equal length4 ,this is explained by ,fig4 #acg1 #a4 ,the air-column in the pipe 7of the same length as that in ,fig4 #acf7 divides itself1 when an end is blown across1 into two equal portions at the node ,b1 the natural point to obtain equilibrium4 ,a pulse will pass from ,a or ,a#a to ,b and back again in half the time required to pass from ,a to ,b and back in ,fig4 #acf1 #a2 therefore the note is an octave higher4 ,illustration3 ,,fig4 #acg4--,showing how harmonics of an open pipe are formed1 ,b1 ,b#a1 and ,c are nodes4 ,the arrows indicate the distance travelled by a sound impulse from a node to a node4 ,,the ,,overtones ,,of ,,an ,,open ,,pipe4 ,the first overtone results when nodes form as in ,fig4 #acg1 #b1 at points one-quarter of the length of the pipe from the ends1 giving one complete ventral segment and two semi-ventral segments4 ,the vibrations now are twice as rapid as before4 ,the second overtone requires three nodes1 as in ,fig4 #acg1 #c4 ,the rate has now trebled4 ,so that1 while the overtones of a closed pipe rise in the ratio #a1 #c1 #e1 #g1 etc41 those of an open pipe rise in the proportion #a1 #b1 #c1 #d1 etc4 ,,where ,,overtones ,,are ,,used4 ,in the flute1 piccolo1 and clarionet1 as well as in the horn class of instrument1 the overtones are as important as the fundamental notes4 ,by artificially altering the length of the column of air1 the fundamental notes are also altered1 while the harmonics of each fundamental are produced at will by varying the blowing pressure2 so that a continuous chromatic1 or semitonal1 scale is possible throughout the compass of the instrument4 ,,the ,,organ4 ,from the theory of acoustics#cj we pass to the practical application1 and concentrate our attention upon the grandest of all wind instruments1 the pipe organ4 ,this mechanism has a separate pipe for every note1 properly proportioned4 ,a section of an ordinary wooden pipe is given in ,fig4 #ach4 ,wind rushes up through the foot of the pipe into a little chamber1 closed by a block of wood or a plate except for a narrow slit1 which directs it against the sharp lip ,a1 and causes a fluttering1 the proper pulse of which is converted by the air-column above into a musical sound4 ,illustration3 ,,fig4 #ach4--,section of an ordinary wooden flue pipe4 ,in even the smallest organs more than one pipe is actuated by one key on the keyboard1 for not only do pipes of different shapes give different qualities of tone1 but it is found desirable to have ranks of pipes with their bottom note of different pitches4 ,the length of an open pipe is measured from the edge of the lip to the top of the pipe2 of a stopped pipe1 from the lip to the top and back again4 ,when we speak of a #af or #h foot rank1 or stop1 we mean one of which the lowest note in the rank is that produced by a #af or #h foot open pipe1 or their stopped equivalents 7#h or #d foot74 ,in a big organ we find #cb1 #af1 #h1 #d1 and #b foot stops1 and some of these repeated a number of times in pipes of different shape and construction4 ,,the ,,arrangement ,,of ,,the ,,pipes4 ,we will now study briefly the mechanism of a very simple single-keyboard organ1 with five ranks of pipes1 or stops4 ,illustration3 ,,fig4 #aci4--,the table of a sound-board4 ,it is necessary to arrange matters so that the pressing down of one key may make all five of the pipes belonging to it speak1 or only four1 three1 two1 or one1 as we may desire4 ,the pipes are mounted in rows on a sound-board1 which is built up in several layers4 ,at the top is the upper board2 below it come the sliders1 one for each stop2 and underneath that the table4 ,in ,fig4 #aci we see part of the table from below4 ,across the under side are fastened parallel bars with spaces 7shown black7 left between them4 ,two other bars are fastened across the ends1 so that each groove is enclosed by wood at the top and on all sides4 ,the under side of the table has sheets of leather glued or otherwise attached to it in such a manner that no air can leak from one groove to the next4 ,upper board1 sliders1 and table are pierced with rows of holes1 to permit the passage of wind from the grooves to the pipes4 ,the grooves under the big pipes are wider than those under the small pipes1 as they have to pass more air4 ,the bars between the grooves also vary in width according to the weight of the pipes which they have to carry4 ,the sliders can be moved in and out a short distance in the direction of the axis of the rows of pipes4 ,there is one slider under each row4 ,when a slider is in1 the holes in it do not correspond with those in the table and upper board1 so that no wind can get from the grooves to the rank over that particular slider4 ,fig4 #adj shows the manner in which the sliders are operated by the little knobs 7also called stops7 projecting from the casing of the organ within convenient reach of the performer's hands4 ,one stop is in1 the other drawn out4 ,illustration3 ,,fig4 #adj4 ,in ,fig4 #ada we see the table1 etc41 in cross section1 with a slider out1 putting the pipes of its rank in communication with the grooves4 ,the same diagram shows us in section the little triangular pallets which admit air from the wind-chest to the grooves2 and ,fig4 #adb gives us an end section of table1 sliders1 and wind-chest1 together with the rods1 etc41 connecting the key to its pallet4 ,when the key is depressed1 the sticker 7a slight wooden rod7 is pushed up4 ,this rocks a backfall1 or pivoted lever1 to which is attached the pulldown1 a wire penetrating the bottom of the wind-chest to the pallet4 ,as soon as the pallet opens1 wind rushes into the groove above through the aperture in the leather bottom1 and thence to any one of the pipes of which the slider has been drawn out4 7,the sliders in ,fig4 #adb are solid black47 ,it is evident that if the sound-board is sufficiently deep from back to front1 any number of rows of pipes may be placed on it4 ,illustration3 ,,fig4 #ada4 ,,pedals4 ,the organ pedals are connected to the pallets by an action similar to that of the keys4 ,the pedal stops are generally of deep tone1 #cb-;foot and #af-;foot1 as they have to sustain the bass part of the musical harmonies4 ,by means of couplers one or more of the keyboard stops may be linked to the pedals4 ,,separate ,,sound-,,boards4 ,the keyboard of a very large organ has as many as five manuals1 or rows of keys4 ,each manual operates what is practically a separate organ mounted on its own sound-board4 ,illustration3 ,,fig4 #adb4 ,illustration3 ,,fig4 #adc4--;,general section of a two-manual organ4 ,the manuals are arranged in steps1 each slightly overhanging that below4 ,taken in order from the top1 they are3--7#a47 ,echo organ1 of stops of small scale and very soft tone1 enclosed in a swell-box4 7#b47 ,solo organ1 of stops imitating orchestral instruments4 ,the wonderful vox humana stop also belongs to this manual4 7#c47 ,swell organ1 contained in a swell-box1 the front and sides of which have shutters which can be opened and closed by the pressure of the foot on a lever1 so as to regulate the amount of sound proceeding from the pipes inside4 7#d47 ,great organ1 including pipes of powerful tone4 7#e47 ,choir organ1 of soft1 mellow stops1 often enclosed in a swell-box4 ,we may add to these the pedal organ1 which can be coupled to any but the echo manual4 ,,varieties ,,of ,,stops4 ,we have already remarked that the quality of a stop depends on the shape and construction of the pipe4 ,some pipes are of wood1 others of metal4 ,some are rectangular1 others circular4 ,some have parallel sides1 others taper or expand towards the top4 ,some are open1 others stopped4 ,the two main classes into which organ pipes may be divided are3--7#a47 ,flue pipes1 in which the wind is directed against a lip1 as in ,fig4 #ach4 7#b47 ,reed pipes--that is1 pipes used in combination with a simple device for admitting air into the bottom of the pipe in a series of gusts4 ,fig4 #add shows a striking reed1 such as is found in the ordinary motor horn4 ,the elastic metal tongue when at rest stands a very short distance away from the orifice in the reed4 ,when wind is blown through the reed the tongue is sucked against the reed1 blocks the current1 and springs away again4 ,a free reed has a tongue which vibrates in a slot without actually touching the sides4 ,harmonium and concertina reeds are of this type4 ,in the organ the reed admits air to a pipe of the correct length to sympathize with the rate of the puffs of air which the reed passes4 ,reed pipes expand towards the top4 ,,tuning ,,pipes ,,and ,,reeds4 ,illustration3 ,,fig4 #add4--;,a reed pipe4 ,pipes are tuned by adjusting their length4 ,the plug at the top of a stopped pipe is pulled out or pushed in a trifle to flatten or sharpen the note respectively4 ,an open pipe1 if large1 has a tongue cut in the side at the top1 which can be pressed inwards or outwards for the purpose of correcting the tone4 ,small metal pipes are flattened by contracting the tops inwards with a metal cone like a candle-extinguisher placed over the top and tapped2 and sharpened by having the top splayed by a cone pushed in point downwards4 ,reeds of the striking variety 7see ,fig4 #add7 have a tuning-wire pressing on the tongue near the fixed end4 ,the end of this wire projects through the casing4 ,by moving it1 the length of the vibrating part of the tongue is adjusted to correctness4 ,,bellows4 ,different stops require different wind-pressures1 ranging from #a_/aj lb4 to #a lb4 to the square inch1 the reeds taking the heaviest pressures4 ,there must therefore be as many sets of bellows and wind-chests as there are different pressures wanted4 ,a very large organ consumes immense quantities of air when all the stops are out1 and the pumping has to be done by a powerful gas1 water1 or electric engine4 ,every bellows has a reservoir 7see ,fig4 #adc7 above it4 ,the top of this is weighted to give the pressure required4 ,a valve in the top opens automatically as soon as the reservoir has expanded to a certain fixed limit1 so that there is no possibility of bursting the leather sides4 ,illustration3 ,,fig4 #ade4--,the keyboard and part of the pneumatic mechanism of the ,hereford ,cathedral organ4 ,c1 composition pedals for pushing out groups of stops2 ,p 7at bottom71 pedals2 ,p ,p 7at top71 pipes carrying compressed air2 ,m1 manuals 7#d72 ,s ,s1 stops4 ,,electric ,,and ,,pneumatic ,,actions4 ,we have mentioned in connection with railway signalling that the signalman is sometimes relieved of the hard manual labour of moving signals and points by the employment of electric and pneumatic auxiliaries4 ,the same is true of organs and organists4 ,the touch of the keys has been greatly lightened by making the keys open air-valves or complete electric circuits which actuate the mechanism for pulling down the pallets4 ,the stops1 pedals1 and couplers also employ power4 ,not only are the performer's muscles spared a lot of heavy work when compressed air and electricity aid him1 but he is able to have the console1 or keyboard1 far away from the pipes4 ,from the console1 the player1 sitting with the singers1 or in any desirable part of the choir or chancel1 would be able to command the working of the whole of the largest organ situated afar at the western end of the nave2 would draw each stop in complete reliance on the sliders and the sound-board fulfilling their office2 444 and--marvel of it all--the player1 using the swell pedal in his ordinary manner1 would obtain crescendo and diminuendo with a more perfect effect than by the old way4#ca ,in cathedrals it is no uncommon thing for the different sound-boards to be placed in positions far apart1 so that to the uninitiated there may appear to be several independent organs scattered about4 ,yet all are absolutely under the control of a man who is sitting away from them all1 but connected with them by a number of tubes or wires4 ,the largest organ in the world is that in the ,town ,hall1 ,sydney4 ,it has a hundred and twenty-six speaking stops1 five manuals1 fourteen couplers1 and forty-six combination studs4 ,the pipes1 about #h1jjj in number1 range from the enormous #fd-;foot contra-trombone to some only a fraction of an inch in length4 ,the organ occupies a space #he feet long and #bf feet deep4 ,,human ,,reeds4 ,the most wonderful of all musical reeds is found in the human throat1 in the anatomical part called the larynx1 situated at the top of the trachea1 or windpipe4 ,slip a piece of rubber tubing over the end of a pipe1 allowing an inch or so to project4 ,take the free part of the tube by two opposite points between the first fingers and thumbs and pull it until the edges are stretched tight4 ,now blow through it4 ,the wind1 forcing its way between the two rubber edges1 causes them and the air inside the tube to vibrate1 and a musical note results4 ,the more you strain the rubber the higher is the note4 ,the larynx works on this principle4 ,the windpipe takes the place of the glass pipe2 the two vocal cords represent the rubber edges2 and the arytenoid muscles stand instead of the hands4 ,when contracted1 these muscles bring the edges of the cords nearer to one another1 stretch the cords1 and shorten the cords4 ,a person gifted with a very good ear can1 it has been calculated1 adjust the length of the vocal cords to #a_/agjjjth of an inch6 ,simultaneously with the adjustment of the cords is effected the adjustment of the length of the windpipe1 so that the column of air in it may be of the right length to vibrate in unison4 ,here again is seen a wonderful provision of nature4 ,the resonance of the mouth cavity is also of great importance4 ,by altering the shape of the mouth the various harmonics of any fundamental note produced by the larynx are rendered prominent1 and so we get the different vocal sounds4 ,helmholtz has shown that the fundamental tone of any note is represented by the sound oo4 ,if the mouth is adjusted to bring out the octave of the fundamental1 o results4 a is produced by accentuating the second harmonic1 the twelfth2 ee by developing the second and fourth harmonics2 while for ah the fifth and seventh must be prominent4 ,when we whistle we transform the lips into a reed and the mouth into a pipe4 ,the tension of the lips and the shape of the mouth cavity decide the note4 ,the lips are also used as a reed for blowing the flute1 piccolo1 and all the brass band instruments of the cornet order4 ,in blowing a coach-horn the various harmonics of the fundamental note are brought out by altering the lip tension and the wind pressure4 ,a cornet is practically a coach-horn rolled up into a convenient shape and furnished with three keys1 the depression of which puts extra lengths of tubing in connection with the main tube--in fact1 makes it longer4 ,one key lowers the fundamental note of the horn half a tone2 the second1 a full tone2 the third1 a tone and a half4 ,if the first and third are pressed down together1 the note sinks two tones2 if the second and third1 two and a half tones2 and simultaneous depression of all three gives a drop of three tones4 ,the performer thus has seven possible fundamental notes1 and several harmonics of each of these at his command2 so that by a proper manipulation of the keys he can run up the chromatic scale4 ,we should add that the cornet tube is an open pipe4 ,so is that of the flute4 ,the clarionet is a stopped pipe4 #bi ,it is obvious that in ,fig4 #acf1 #b1 a pulse will pass from ,a to ,b and back in one-third the time required for it to pass from ,a to ,b and back in ,fig4 #acf1 #a4 #cj ,the science of hearing2 from the ,greek verb1 ,greek3 akouein1 to hear4 #ca ,organs and ,tuning1 p4 #bde4 ,chapter ,,xvi4 ,,talking-,,machines4 ,the phonograph--,the recorder--,the reproducer--,the gramophone--,the making of records--,cylinder records--,gramophone records4 ,in the ,patent ,office ,museum at ,south ,kensington is a curious little piece of machinery--a metal cylinder mounted on a long axle1 which has at one end a screw thread chased along it4 ,the screw end rotates in a socket with a thread of equal pitch cut in it4 ,to the other end is attached a handle4 ,on an upright near the cylinder is mounted a sort of drum4 ,the membrane of the drum carries a needle1 which1 when the membrane is agitated by the air-waves set up by human speech1 digs into a sheet of tinfoil wrapped round the cylinder1 pressing it into a helical groove turned on the cylinder from end to end4 ,this construction is the first phonograph ever made4 ,thomas ,edison1 the wizard of the ,west1 devised it in #ahgf2 and from this rude parent have descended the beautiful machines which record and reproduce human speech and musical sounds with startling accuracy4 ,illustration3 ,,fig4 #adf4--,the governor of a phonograph4 ,we do not propose to trace here the development of the talking-machine2 nor will it be necessary to describe in detail its mechanism1 which is probably well known to most readers1 or could be mastered in a very short time on personal examination4 ,we will content ourselves with saying that the wax cylinder of the phonograph1 or the ebonite disc of the gramophone1 is generally rotated by clockwork concealed in the body of the machine4 ,the speed of rotation has to be very carefully governed1 in order that the record may revolve under the reproducing point at a uniform speed4 ,the principle of the governor commonly used appears in ,fig4 #adf4 ,the last pinion of the clockwork train is mounted on a shaft carrying two triangular plates1 ,a and ,c1 to which are attached three short lengths of flat steel spring with a heavy ball attached to the centre of each4 ,a is fixed2 ,c moves up the shaft as the balls fly out1 and pulls with it the disc ,d1 which rubs against the pad ,p 7on the end of a spring7 and sets up sufficient friction to slow the clockwork4 ,the limit rate is regulated by screw ,s4 ,,the ,,phonograph4 ,though the recording and reproducing apparatus of a phonograph gives very wonderful results1 its construction is quite simple4 ,at the same time1 it must be borne in mind that an immense amount of experimenting has been devoted to finding out the most suitable materials and forms for the parts4 ,illustration3 ,,fig4 #adg4--,section of an ,edison ,bell phonograph recorder4 ,the recorder 7,fig4 #adg7 is a little circular box about one and a half inches in diameter4#cb ,from the top a tube leads to the horn4 ,the bottom is a circular plate1 ,c ,c1 hinged at one side4 ,this plate supports a glass disc1 ,d1 about #a_/aejth of an inch thick1 to which is attached the cutting stylus--a tiny sapphire rod with a cup-shaped end having very sharp edges4 ,sound-waves enter the box through the horn tube2 but instead of being allowed to fill the whole box1 they are concentrated by the shifting nozzle ,n on to the centre of the glass disc through the hole in ,c ,c4 ,you will notice that ,n has a ball end1 and ,c ,c a socket to fit ,n exactly1 so that1 though ,c ,c and ,n move up and down very rapidly1 they still make perfect contact4 ,the disc is vibrated by the sound-impulses1 and drives the cutting point down into the surface of the wax cylinder1 turning below it in a clockwork direction4 ,the only dead weight pressing on ,s is that of ,n1 ,c ,c1 and the glass diaphragm4 ,illustration3 ,,fig4 #adh4--,perspective view of a phonograph recorder4 ,as the cylinder revolves1 the recorder is shifted continuously along by a leading screw having one hundred or more threads to the inch cut on it1 so that it traces a continuous helical groove from one end of the wax cylinder to the other4 ,this groove is really a series of very minute indentations1 not exceeding #a_/ajjjth of an inch in depth4#cc ,seen under a microscope1 the surface of the record is a succession of hills and valleys1 some much larger than others 7,fig4 #aea1 a74 ,a loud sound causes the stylus to give a vigorous dig1 while low sounds scarcely move it at all4 ,the wonderful thing about this sound-recording is1 that not only are the fundamental tones of musical notes impressed1 but also the harmonics1 which enable us to decide at once whether the record is one of a cornet1 violin1 or banjo performance4 ,furthermore1 if several instruments are playing simultaneously near the recorder's horn1 the stylus catches all the different shades of tone of every note of a chord4 ,there are1 so to speak1 minor hills and valleys cut in the slopes of the main hills and valleys4 ,illustration3 ,,fig4 #adi4--,section of the reproducer of an ,edison ,bell phonograph4 ,illustration3 ,,fig4 #aej4--,perspective view of a phonograph reproducer4 ,the reproducer 7,fig4 #adi7 is somewhat more complicated than the recorder4 ,as before1 we have a circular box communicating with the horn of the instrument4 ,a thin glass disc forms a bottom to the box4 ,it is held in position between rubber rings1 ,r ,r1 by a screw collar1 ,c4 ,to the centre is attached a little eye1 from which hangs a link1 ,l4 ,pivoted at ,p from one edge of the box is a floating weight1 having a circular opening immediately under the eye4 ,the link passes through this to the left end of a tiny lever1 which rocks on a pivot projecting from the weight4 ,to the right end of the lever is affixed a sapphire bar1 or stylus1 with a ball end of a diameter equal to that of the cutting point of the recorder4 ,the floating weight presses the stylus against the record1 and also keeps the link between the rocking lever of the glass diaphragm in a state of tension4 ,every blow given to the stylus is therefore transmitted by the link to the diaphragm1 which vibrates and sends an air-impulse into the horn4 ,as the impulses are given at the same rate as those which agitated the diaphragm of the recorder1 the sounds which they represent are accurately reproduced1 even to the harmonics of a musical note4 ,,the ,,gramophone4 ,this effects the same purpose as the phonograph1 but in a somewhat different manner4 ,the phonograph recorder digs vertically downwards into the surface of the record1 whereas the stylus of the gramophone wags from side to side and describes a snaky course 7,fig4 #aea;b74 ,it makes no difference in talking-machines whether the reproducing stylus be moved sideways or vertically by the record1 provided that motion is imparted by it to the diaphragm4 ,illustration3 ,,fig4 #aea;a4 ,illustration3 ,,fig4 #aea;b4 ,illustration3 ,,fig4 #aea;c4--,section of a gramophone reproducer4 ,in ,fig4 #aea;c the construction of the gramophone reproducer is shown in section4 ,a is the cover which screws on to the bottom ,b1 and confines the diaphragm ,d between itself and a rubber ring4 ,the portion ,b is elongated into a tubular shape for connection with the horn1 an arm of which slides over the tube and presses against the rubber ring ,c to make an air-tight joint4 ,the needle-carrier ,n is attached at its upper end to the centre of the diaphragm4 ,at a point indicated by the white dot a pin passes through it and the cover4 ,the lower end is tubular to accommodate the steel points1 which have to be replaced after passing once over a record4 ,a screw1 ,s1 working in a socket projecting from the carrier1 holds the point fast4 ,the record moves horizontally under the point in a plane perpendicular to the page4 ,the groove being zigzag1 the needle vibrates right and left1 and rotating the carrier a minute fraction of an inch on the pivot1 shakes the glass diaphragm and sends waves of air into the horn4 ,the gramophone is a reproducing instrument only4 ,the records are made on a special machine1 fitted with a device for causing the recorder point to describe a spiral course from the circumference to the centre of the record disc4 ,some gramophone records have as many as #bej turns to the inch4 ,the total length of the tracing on a ten-inch concert record is about #a1jjj feet4 ,,the ,,making ,,of ,,records4 ,for commercial purposes it would not pay to make every record separately in a recording machine4 ,the expense of employing good singers and instrumentalists renders such a method impracticable4 ,all the records we buy are made from moulds1 the preparation of which we will now briefly describe4 ,,cylinder1 ,,or ,,phonograph ,,records4 ,first of all1 a wax record is made in the ordinary way on a recording machine4 ,after being tested and approved1 it is hung vertically and centrally from a rotating table pivoted on a vertical metal spike passing up through the record4 ,on one side of the table is a piece of iron4 ,on each side of the record1 and a small distance away1 rises a brass rod enclosed in a glass tube4 ,the top of the rods are hooked1 so that pieces of gold leaf may be suspended from them4 ,a bell-glass is now placed over the record1 table1 and rods1 and the air is sucked out by a pump4 ,as soon as a good vacuum has been obtained1 the current from the secondary circuit of an induction coil is sent into the rods supporting the gold leaves1 which are volatilized by the current jumping from one to the other4 ,a magnet1 whirled outside the bell-glass1 draws round the iron armature on the pivoted table1 and consequently revolves the record1 on the surface of which a very thin coating of gold is deposited4 ,the record is next placed in an electroplating bath until a copper shell one-sixteenth of an inch thick has formed all over the outside4 ,this is trued up on a lathe and encased in a brass tube4 ,the master1 or original wax record1 is removed by cooling it till it contracts sufficiently to fall out of the copper mould1 on the inside surface of which are reproduced1 in relief1 the indentations of the wax master4 ,copies are made from the mould by immersing it in a tank of melted wax4 ,the cold metal chills the wax that touches it1 so that the mould soon has a thick waxen lining4 ,the mould and copy are removed from the tank and mounted on a lathe1 which shapes and smooths the inside of the record4 ,the record is loosened from the mould by cooling4 ,after inspection for flaws1 it is1 if found satisfactory1 packed in cotton-wool and added to the saleable stock4 ,gramophone master records are made on a circular disc of zinc1 coated over with a very thin film of acid-proof fat4 ,when the disc is revolved in the recording machine1 the sharp stylus cuts through the fat and exposes the zinc beneath4 ,on immersion in a bath of chromic acid the bared surfaces are bitten into1 while the unexposed parts remain unaffected4 ,when the etching is considered complete1 the plate is carefully cleaned and tested4 ,a negative copper copy is made from it by electrotyping4 ,this constitutes the mould4 ,from it as many as #a1jjj copies may be made on ebonite plates by combined pressure and heating4 #cb ,the ,edison ,bell phonograph is here referred to4 #cc ,some of the sibilant or hissing sounds of the voice are computed to be represented by depressions less than a millionth of an inch in depth4 ,yet these are reproduced very clearly6 ,chapter ,,xvii4 ,,why ,,the ,,wind ,,blows4 ,why the wind blows--,land and sea breezes--,light air and moisture--,the barometer--,the column barometer--,the wheel barometer--,a very simple barometer--,the aneroid barometer--,barometers and weather--,the diving-bell--,the diving-dress--,air-pumps--,pneumatic tyres--,the air-gun--,the self-closing door-stop--,the action of wind on oblique surfaces--,the balloon--,the flying-machine4 ,when a child's rubber ball gets slack through a slight leakage of air1 and loses some of its bounce1 it is a common practice to hold it for a few minutes in front of the fire till it becomes temporarily taut again4 ,why does the heat have this effect on the ball8 ,no more air has been forced into the ball4 ,after perusing the chapter on the steam-engine the reader will be able to supply the answer4 ,because the molecules of air dash about more vigorously among one another when the air is heated1 and by striking the inside of the ball with greater force put it in a state of greater tension4 ,if we heat an open jar there is no pressure developed1 since the air simply expands and flows out of the neck4 ,but the air that remains in the jar1 being less in quantity than when it was not yet heated1 weighs less1 though occupying the same space as before4 ,if we took a very thin bladder and filled it with hot air it would therefore float in colder air1 proving that heated air1 as we should expect1 tends to rise4 ,the fire-balloon employs this principle1 the air inside the bag being kept artificially warm by a fire burning in some vessel attached below the open neck of the bag4 ,now1 the sun shines with different degrees of heating power at different parts of the world4 ,where its effect is greatest the air there is hottest4 ,we will suppose1 for the sake of argument1 that1 at a certain moment1 the air envelope all round the globe is of equal temperature4 ,suddenly the sun shines out and heats the air at a point1 ,a1 till it is many degrees warmer than the surrounding air4 ,the heated air expands1 rises1 and spreads out above the cold air4 ,but1 as a given depth of warm air has less weight than an equal depth of cold air1 the cold air at once begins to rush towards ,b and squeeze the rest of the warm air out4 ,we may therefore picture the atmosphere as made up of a number of colder currents passing along the surface of the earth to replace warm currents rising and spreading over the upper surface of the cold air4 ,a similar circulation takes place in a vessel of heated water 7see p4 #ag74 ,,land ,,and ,,sea ,,breezes4 ,a breeze which blows from the sea on to the land during the day often reverses its direction during the evening4 ,why is this8 ,the earth grows hot or cold more rapidly than the sea4 ,when the sun shines hotly1 the land warms quickly and heats the air over it1 which becomes light1 and is displaced by the cooler air over the sea4 ,when the sun sets1 the earth and the air over it lose their warmth quickly1 while the sea remains at practically the same temperature as before4 ,so the balance is changed1 the heavier air now lying over the land4 ,it therefore flows seawards1 and drives out the warmer air there4 ,,light ,,air ,,and ,,moisture4 ,light1 warm air absorbs moisture4 ,as it cools1 the moisture in it condenses4 ,breathe on a plate1 and you notice that a watery film forms on it at once4 ,the cold surface condenses the water suspended in the warm breath4 ,if you wish to dry a damp room you heat it4 ,moisture then passes from the walls and objects in the room to the atmosphere4 ,,the ,,barometer4 ,this property of air is responsible for the changes in weather4 ,light1 moisture-laden air meets cold1 dry air1 and the sudden cooling forces it to release its moisture1 which falls as rain1 or floats about as clouds4 ,if only we are able to detect the presence of warm air-strata above us1 we ought to be in a position to foretell the weather4 ,we can judge of the specific gravity of the air in our neighbourhood by means of the barometer1 which means weight-measurer4 ,the normal air-pressure at sea-level on our bodies or any other objects is about #ae lbs4 to the square inch--that is to say1 if you could imprison and weigh a column of air one inch square in section and of the height of the world's atmospheric envelope1 the scale would register #ae lbs4 ,many years ago 7#afdc7 ,torricelli1 a pupil of ,galileo1 first calculated the pressure by a very simple experiment4 ,he took a long glass tube sealed at one end1 filled it with mercury1 and1 closing the open end with the thumb1 inverted the tube and plunged the open end below the surface of a tank of mercury4 ,on removing his thumb he found that the mercury sank in the tube till the surface of the mercury in the tube was about #cj inches in a vertical direction above the surface of the mercury in the tank4 ,now1 as the upper end was sealed1 there must be a vacuum above the mercury4 ,what supported the column8 ,the atmosphere4 ,so it was evident that the downward pressure of the mercury exactly counterbalanced the upward pressure of the air4 ,as a mercury column #cj inches high and #a inch square weighs #ae lbs41 the air-pressure on a square inch obviously is the same4 ,illustration3 ,,fig4 #aeb4--;,a ,fortin barometer4 ,,fortin',s ,,column ,,barometer is a simple ,torricellian tube1 ,t1 with the lower end submerged in a little glass tank of mercury 7,fig4 #aeb74 ,the bottom of this tank is made of washleather4 ,to obtain a reading the screw ,s1 pressing on the washleather1 is adjusted until the mercury in the tank rises to the tip of the little ivory point ,p4 ,the reading is the figure of the scale on the face of the case opposite which the surface of the column stands4 ,illustration3 ,,fig4 #aec4 ,,the ,,wheel ,,barometer also employs the mercury column 7,fig4 #aec74 ,the lower end of the tube is turned up and expanded to form a tank1 ,c4 ,the pointer ,p1 which travels round a graduated dial1 is mounted on a spindle carrying a pulley1 over which passes a string with a weight at each end4 ,the heavier of the weights rests on the top of the mercury4 ,when the atmospheric pressure falls1 the mercury in ,c rises1 lifting this weight1 and the pointer moves4 ,this form of barometer is not so delicate or reliable as ,fortin's1 or as the siphon barometer1 which has a tube of the same shape as the wheel instrument1 but of the same diameter from end to end except for a contraction at the bend4 ,the reading of a siphon is the distance between the two surfaces of the mercury4 ,a ,,very ,,simple ,,barometer is made by knocking off the neck of a small bottle1 filling the body with water1 and hanging it up by a string in the position shown 7,fig4 #aed74 ,when the atmospheric pressure falls1 the water at the orifice bulges outwards2 when it rises1 the water retreats till its surface is slightly concave4 ,illustration3 ,,fig4 #aed4 ,,the ,,aneroid ,,barometer4 ,on account of their size and weight1 and the comparative difficulty of transporting them without derangement of the mercury column1 column barometers are not so generally used as the aneroid variety4 ,aneroid means without moisture1 and in this particular connection signifies that no liquid is used in the construction of the barometer4 ,fig4 #aee shows an aneroid in detail4 ,the most noticeable feature is the vacuum chamber1 ,v ,c1 a circular box which has a top and bottom of corrugated but thin and elastic metal4 ,sections of the box are shown in ,figs4 #aef1 #aeg4 ,it is attached at the bottom to the base board of the instrument by a screw 7,fig4 #aef74 ,from the top rises a pin1 ,p1 with a transverse hole through it to accommodate the pin ,k ,e1 which has a triangular section1 and stands on one edge4 ,illustration3 ,,fig4 #aee4--;,an aneroid barometer4 ,returning to ,fig4 #aee1 we see that ,p projects through ,s1 a powerful spring of sheet-steel4 ,to this is attached a long arm1 ,c1 the free end of which moves a link rotating1 through the pin ,e1 a spindle mounted in a frame1 ,d4 ,the spindle moves arm ,f4 ,this pulls on a very minute chain wound round the pointer spindle ,b1 in opposition to a hairspring1 ,h ,s4 ,b is mounted on arm ,h1 which is quite independent of the rest of the aneroid4 ,illustration3 ,,fig4 #aef4 ,,fig4 #aeg4 ,the vacuum chamber of an aneroid barometer extended and compressed4 ,the vacuum chamber is exhausted during manufacture and sealed4 ,it would naturally assume the shape of ,fig4 #aeg1 but the spring ,s1 acting against the atmospheric pressure1 pulls it out4 ,as the pressure varies1 so does the spring rise or sink2 and the slightest movement is transmitted through the multiplying arms ,c1 ,e1 ,f1 to the pointer4 ,a good aneroid is so delicate that it will register the difference in pressure caused by raising it from the floor to the table1 where it has a couple of feet less of air-column resting upon it4 ,an aneroid is therefore a valuable help to mountaineers for determining their altitude above sea-level4 ,,barometers ,,and ,,weather4 ,we may now return to the consideration of forecasting the weather by movements of the barometer4 ,the first thing to keep in mind is1 that the instrument is essentially a weight recorder4 ,how is weather connected with atmospheric weight8 ,in ,england the warm south-west wind generally brings wet weather1 the north and east winds fine weather2 the reason for this being that the first reaches us after passing over the ,atlantic and picking up a quantity of moisture1 while the second and third have come overland and deposited their moisture before reaching us4 ,a sinking of the barometer heralds the approach of heated air--that is1 moist air--which on meeting colder air sheds its moisture4 ,so when the mercury falls we expect rain4 ,on the other hand1 when the glass rises1 we know that colder air is coming1 and as colder air comes from a dry quarter we anticipate fine weather4 ,it does not follow that the same conditions are found in all parts of the world4 ,in regions which have the ocean to the east or the north1 the winds blowing thence would be the rainy winds1 while south-westerly winds might bring hot and dry weather4 ,,the ,,diving-,,bell4 ,water is nearly #ggc times as heavy as air4 ,if we submerge a barometer a very little way below the surface of a water tank1 we shall at once observe a rise of the mercury column4 ,at a depth of #cd feet the pressure on any submerged object is #ae lbs4 to the square inch1 in addition to the atmospheric pressure of #ae lbs4 per square inch--that is1 there would be a #cj-lb4 absolute pressure4 ,as a rule1 when speaking of hydraulic pressures1 we start with the normal atmospheric pressure as zero1 and we will here observe the practice4 ,illustration3 ,,fig4 #aeh4--;,a diving bell4 ,the diving-bell is used to enable people to work under water without having recourse to the diving-dress4 ,a sketch of an ordinary diving-bell is given in ,fig4 #aeh4 ,it may be described as a square iron box without a bottom4 ,at the top are links by which it is attached to a lowering chain1 and windows1 protected by grids2 also a nozzle for the air-tube4 ,illustration3 ,,fig4 #aei4 ,a simple model bell 7,fig4 #aei7 is easily made out of a glass tumbler which has had a tap fitted in a hole drilled through the bottom4 ,we turn off the tap and plunge the glass into a vessel of water4 ,the water rises a certain way up the interior1 until the air within has been compressed to a pressure equal to that of the water at the level of the surface inside4 ,the further the tumbler is lowered1 the higher does the water rise inside it4 ,evidently men could not work in a diving-bell which is invaded thus by water4 ,it is imperative to keep the water at bay4 ,this we can do by attaching a tube to the tap 7,fig4 #afj7 and blowing into the tumbler till the air-pressure exceeds that of the water1 which is shown by bubbles rising to the surface4 ,the diving-bell therefore has attached to it a hose through which air is forced by pumps from the atmosphere above1 at a pressure sufficient to keep the water out of the bell4 ,this pumping of air also maintains a fresh supply of oxygen for the workers4 ,illustration3 ,,fig4 #afj4 ,inside the bell is tackle for grappling any object that has to be moved1 such as a heavy stone block4 ,the diving-bell is used mostly for laying submarine masonry4 ,the bell1 slung either from a crane on the masonry already built above sea-level1 or from a specially fitted barge1 comes into action4 ,the block is lowered by its own crane on to the bottom4 ,the bell descends upon it1 and the crew seize it with tackle suspended inside the bell4 ,instructions are sent up as to the direction in which the bell should be moved with its burden1 and as soon as the exact spot has been reached the signal for lowering is given1 and the stone settles on to the cement laid ready for it4#cd ,for many purposes it is necessary that the worker should have more freedom of action than is possible when he is cooped up inside an iron box4 ,hence the invention of the ,,diving-,,dress1 which consists of two main parts1 the helmet and the dress proper4 ,the helmet 7,fig4 #afa7 is made of copper4 ,a breastplate1 ,b1 shaped to fit the shoulders1 has at the neck a segmental screw bayonet-joint4 ,the headpiece is fitted with a corresponding screw1 which can be attached or removed by one-eighth of a turn4 ,the neck edge of the dress1 which is made in one piece1 legs1 arms1 body and all1 is attached to the breastplate by means of the plate ,p#a1 screwed down tightly on it by the wing-nuts ,n ,n1 the bolts of which pass through the breastplate4 ,air enters the helmet through a valve situated at the back1 and is led through tubes along the inside to the front4 ,this valve closes automatically if any accident cuts off the air supply1 and encloses sufficient air in the dress to allow the diver to regain the surface4 ,the outlet valve ,o ,v can be adjusted by the diver to maintain any pressure4 ,at the sides of the headpiece are two hooks1 ,h1 over which pass the cords connecting the heavy lead weights of #dj lbs4 each hanging on the diver's breast and back4 ,these weights are also attached to the knobs ,k ,k4 ,a pair of boots1 having #ag lbs4 of lead each in the soles1 complete the dress4 ,three glazed windows are placed in the headpiece1 that in the front1 ,r ,w1 being removable1 so that the diver may gain free access to the air when he is above water without being obliged to take off the helmet4 ,illustration3 ,,fig4 #afa4--;,a diver's helmet4 ,by means of telephone wires built into the life-line 7which passes under the diver's arms and is used for lowering and hoisting7 easy communication is established between the diver and his attendants above4 ,the transmitter of the telephone is placed inside the helmet between the front and a side window1 the receiver and the button of an electric bell in the crown4 ,this last he can press by raising his head4 ,the life-line sometimes also includes the wires for an electric lamp 7,fig4 #afb7 used by the diver at depths to which daylight cannot penetrate4 ,the pressure on a diver's body increases in the ratio of #d-a_/c lbs4 per square inch for every #aj feet that he descends4 ,the ordinary working limit is about #aej feet1 though old hands are able to stand greater pressures4 ,the record is held by one ,james ,hooper1 who1 when removing the cargo of the ,cape ,horn sunk off the ,south ,american coast1 made seven descents of #bja feet1 one of which lasted for forty-two minutes4 ,illustration3 ,,fig4 #afb4--;,diver's electric lamp4 ,a sketch is given 7,fig4 #afc7 of divers working below water with pneumatic tools1 fed from above with high-pressure air4 ,owing to his buoyancy a diver has little depressing or pushing power1 and he cannot bore a hole in a post with an auger unless he is able to rest his back against some firm object1 or is roped to the post4 ,pneumatic chipping tools merely require holding to their work1 their weight offering sufficient resistance to the very rapid blows which they make4 ,illustration3 ,,fig4 #afc4--;,divers at work below water with pneumatic tools4 ,,air-,,pumps4 ,illustration3 ,,fig4 #afd4 ,illustration3 ,,fig4 #afe4 ,mention having been made of the air-pump1 we append diagrams 7,figs4 #afd1 #afe7 of the simplest form of air-pump1 the cycle tyre inflator4 ,the piston is composed of two circular plates of smaller diameter than the barrel1 holding between them a cup leather4 ,during the upstroke the cup collapses inwards and allows air to pass by it4 ,on the downstroke 7,fig4 #afe7 the edges of the cup expand against the barrel1 preventing the passage of air round the piston4 ,a double-action air-pump requires a long1 well-fitting piston with a cup on each side of it1 and the addition of extra valves to the barrel1 as the cups under these circumstances cannot act as valves4 ,,pneumatic ,,tyres4 ,illustration3 ,,fig4 #aff4 ,illustration3 ,,fig4 #afg4 ,the action of the pneumatic tyre in reducing vibration and increasing the speed of a vehicle is explained by ,figs4 #aff1 #afg4 ,when the tyre encounters an obstacle1 such as a large stone1 it laps over it 7,fig4 #aff71 and while supporting the weight on the wheel1 reduces the deflection of the direction of movement4 ,when an iron-tyred wheel meets a similar obstacle it has to rise right over it1 often jumping a considerable distance into the air4 ,the resultant motions of the wheel are indicated in each case by an arrow4 ,every change of direction means a loss of forward velocity1 the loss increasing with the violence and extent of the change4 ,the pneumatic tyre also scores because1 on account of its elasticity1 it gives a kick off against the obstacle1 which compensates for the resistance during compression4 ,illustration3 ,,fig4 #afh4--,section of the mechanism of an air-gun4 ,,the ,,air-,,gun4 ,this may be described as a valveless air-pump4 ,fig4 #afh is a section of a ,gem air-gun1 with the mechanism set ready for firing4 ,in the stock of the gun is the cylinder1 in which an accurately fitting and hollow piston moves4 ,a powerful helical spring1 turned out of a solid bar of steel1 is compressed between the inside end of the piston and the upper end of the butt4 ,to set the gun1 the catch is pressed down so that its hooked end disengages from the stock1 and the barrel is bent downwards on pivot ,p4 ,this slides the lower end of the compressing lever towards the butt1 and a projection on the guide ,b1 working in a groove1 takes the piston with it4 ,when the spring has been fully compressed1 the triangular tip of the rocking cam ,r engages with a groove in the piston's head1 and prevents recoil when the barrel is returned to its original position4 ,on pulling the trigger1 the piston is released and flies up the cylinder with great force1 and the air in the cylinder is compressed and driven through the bore of the barrel1 blocked by the leaden slug1 to which the whole energy of the expanding spring is transmitted through the elastic medium of the air4 ,there are several other good types of air-gun1 all of which employ the principles described above4 ,,the ,,self-,,closing ,,door-,,stop is another interesting pneumatic device4 ,it consists of a cylinder with an air-tight piston1 and a piston rod working through a cover at one end4 ,the other end of the cylinder is pivoted to the door frame4 ,when the door is opened the piston compresses a spring in the cylinder1 and air is admitted past a cup leather on the piston to the upper part of the cylinder4 ,this air is confined by the cup leather when the door is released1 and escapes slowly through a leak1 allowing the spring to regain its shape slowly1 and by the agency of the piston rod to close the door4 ,,the ,,action ,,of ,,wind ,,on ,,oblique ,,surfaces4 ,why does a kite rise8 ,why does a boat sail across the wind8 ,we can supply an answer almost instinctively in both cases1 ,because the wind pushes the kite or sail aside4 ,it will1 however1 be worth while to look for a more scientific answer4 ,the kite cannot travel in the direction of the wind because it is confined by a string4 ,but the face is so attached to the string that it inclines at an angle to the direction of the wind4 ,now1 when a force meets an inclined surface which it cannot carry along with it1 but which is free to travel in another direction1 the force may be regarded as resolving itself into two forces1 coming from each side of the original line4 ,these are called the component forces4 ,illustration3 ,,fig4 #afi4 ,to explain this we give a simple sketch of a kite in the act of flying 7,fig4 #afi74 ,the wind is blowing in the direction of the solid arrow ,a4 ,the oblique surface of the kite resolves its force into the two components indicated by the dotted arrows ,b and ,c4 ,of these ,c only has lifting power to overcome the force of gravity4 ,the kite assumes a position in which force ,c and gravity counterbalance one another4 ,illustration3 ,,fig4 #agj4 ,a boat sailing across the wind is acted on in a similar manner 7,fig4 #agj74 ,the wind strikes the sail obliquely1 and would thrust it to leeward were it not for the opposition of the water4 ,the force ,a is resolved into forces ,b and ,c1 of which ,c propels the boat on the line of its axis4 ,the boat can be made to sail even up the wind1 her head being brought round until a point is reached at which the force ,b on the boat1 masts1 etc41 overcomes the force ,c4 ,the capability of a boat for sailing up wind depends on her lines and the amount of surface she offers to the wind4 ,,the ,,balloon is a pear-shaped bag--usually made of silk--filled with some gas lighter than air4 ,the tendency of a heavier medium to displace a lighter drives the gas upwards1 and with it the bag and the wicker-work car attached to a network encasing the bag4 ,the tapering neck at the lower end is open1 to permit the free escape of gas as the atmospheric pressure outside diminishes with increasing elevation4 ,at the top of the bag is a wooden valve opening inwards1 which can be drawn down by a rope passing up to it through the neck whenever the aeronaut wishes to let gas escape for a descent4 ,he is able to cause a very rapid escape by pulling another cord depending from a ripping piece near the top of the bag4 ,in case of emergency this is torn away bodily1 leaving a large hole4 ,the ballast 7usually sand7 carried enables him to maintain a state of equilibrium between the upward pull of the gas and the downward pull of gravity4 ,to sink he lets out gas1 to rise he throws out ballast2 and this process can be repeated until the ballast is exhausted4 ,the greatest height ever attained by aeronauts is the #g-a_/d miles1 or #cg1jjj feet1 of ,messrs4 ,glaisher and ,coxwell on ,september #e1 #ahfb4 ,the ascent nearly cost them their lives1 for at an elevation of about #cj1jjj feet they were partly paralyzed by the rarefaction of the air1 and had not ,mr4 ,coxwell been able to pull the valve rope with his teeth and cause a descent1 both would have died from want of air4 ,illustration3 ,,fig4 #aga4 ,the flying-machine1 which scientific engineers have so long been trying to produce1 will probably be quite independent of balloons1 and will depend for its ascensive powers on the action of air on oblique surfaces4 ,sir ,hiram ,maxim's experimental air-ship embodied the principles shown by ,fig4 #aga4 ,on a deck was mounted an engine1 ,e1 extremely powerful for its weight4 ,this drove large propellers1 ,s ,s4 ,large aeroplanes1 of canvas stretched over light frameworks1 were set up overhead1 the forward end somewhat higher than the rear4 ,the machine was run on rails so arranged as to prevent it rising4 ,unfortunately an accident happened at the first trial and destroyed the machine4 ,in actual flight it would be necessary to have a vertical rudder for altering the horizontal direction1 and a horizontal tail for steering up or down4 ,the principle of an aeroplane is that of the kite1 with this difference1 that1 instead of moving air striking a captive body1 a moving body is propelled against more or less stationary air4 ,the resolution of forces is shown by the arrows as before4 ,up to the present time no practical flying-machine has appeared4 ,but experimenters are hard at work examining the conditions which must be fulfilled to enable man to claim the dominion of the air4 #cd ,the ,romance of ,modern ,mechanism1 p4 #bdc ,chapter ,,xviii4 ,,hydraulic ,,machinery4 ,the siphon--,the bucket pump--,the force-pump--,the most marvellous pump--,the blood channels--,the course of the blood--,the hydraulic press--,household water-supply fittings--,the ball-cock--,the water-meter--,water-supply systems--,the household filter--,gas traps--,water engines--,the cream separator--,the hydro4 ,in the last chapter we saw that the pressure of the atmosphere is #ae lbs4 to the square inch4 ,suppose that to a very long tube having a sectional area of one square inch we fit an air-tight piston 7,fig4 #agb71 and place the lower end of the tube in a vessel of water4 ,on raising the piston a vacuum would be created in the tube1 did not the pressure of the atmosphere force water up into the tube behind the piston4 ,the water would continue to rise until it reached a point #cd feet perpendicularly above the level of the water in the vessel4 ,the column would then weigh #ae lbs41 and exactly counterbalance the atmospheric pressure2 so that a further raising of the piston would not raise the water any farther4 ,at sea-level1 therefore1 the lifting power of a pump by suction is limited to #cd feet4 ,on the top of a lofty mountain1 where the air-pressure is less1 the height of the column would be diminished--in fact1 be proportional to the pressure4 ,illustration3 ,,fig4 #agb4 ,illustration3 ,,fig4 #agc4 ,,the ,,siphon is an interesting application of the principle of suction4 ,by its own weight water may be made to lift water through a height not exceeding #cd feet4 ,this is explained by ,fig4 #agc4 ,the siphon pipe1 ,a ,b ,c ,d1 is in the first instance filled by suction4 ,the weight of the water between ,a and ,b counter-balances that between ,b and ,c4 ,but the column ,c ,d hangs1 as it were1 to the heels of ,b ,c1 and draws it down4 ,or1 to put it otherwise1 the column ,b ,d1 being heavier than the column ,b ,a1 draws it over the topmost point of the siphon4 ,any parting between the columns1 provided that ,b ,a does not exceed #cd feet1 is impossible1 as the pressure of the atmosphere on the mouth of ,b ,a is sufficient to prevent the formation of a vacuum4 ,,the ,,bucket ,,pump4 ,we may now pass to the commonest form of pump used in houses1 stables1 gardens1 etc4 7,fig4 #agd74 ,the piston has a large hole through it1 over the top of which a valve is hinged4 ,at the bottom of the barrel is a second valve1 also opening upwards1 seated on the top of the supply pipe4 ,in sketch 7a7 the first upstroke is in progress4 ,a vacuum forms under the piston1 or plunger1 and water rises up the barrel to fill it4 ,the next diagram 7b7 shows the first downstroke4 ,the plunger valve now opens and allows water to rise above the piston1 while the lower closes under the pressure of the water above and the pull of that below4 ,during the second upstroke 7c7 the water above the piston is raised until it overflows through the spout1 while a fresh supply is being sucked in below4 ,illustration3 ,,fig4 #agd4 ,,the ,,force-,,pump4 ,illustration3 ,,fig4 #age4 ,force-pump2 suction stroke4 ,illustration3 ,,fig4 #agf4 ,force-pump2 delivery stroke4 ,for driving water to levels above that of the pump a somewhat different arrangement is required4 ,one type of force-pump is shown in ,figs4 #age1 #agf4 ,the piston now is solid1 and the upper valve is situated in the delivery pipe4 ,during an upstroke this closes1 and the other opens2 the reverse happening during a downstroke4 ,an air-chamber is generally fitted to the delivery pipe when water is to be lifted to great heights or under high pressure4 ,at each delivery stroke the air in the chamber is compressed1 absorbing some of the shock given to the water in the pipe by the water coming from the pump2 and its expansion during the next suction stroke forces the water gradually up the pipe4 ,the air-chamber is a very prominent feature of the fire-engine4 ,a double-action force-pump is seen in ,fig4 #agg1 making an upward stroke4 ,both sides of the piston are here utilized1 and the piston rod works through a water-tight stuffing-box4 ,the action of the pump will be easily understood from the diagram4 ,illustration3 ,,fig4 #agg4 ,,the ,,most ,,marvellous ,,pump known is the heart4 ,we give in ,fig4 #agh a diagrammatic sketch of the system of blood circulation in the human body1 showing the heart1 the arteries1 and the veins1 big and little4 ,the body is supposed to be facing the reader1 so that the left lung1 etc41 is to his right4 ,illustration3 ,,fig4 #agh4--;,a diagrammatic representation of the circulatory system of the blood4 ,the heart1 which forces the blood through the body1 is a large muscle 7of about the size of the clenched fist7 with four cavities4 ,these are respectively known as the right and left auricles1 and the right and left ventricles4 ,they are arranged in two pairs1 the auricle uppermost1 separated by a fleshy partition4 ,between each auricle and its ventricle is a valve1 which consists of strong membranous flaps1 with loose edges turned downwards4 ,the left-side valve is the mitral valve1 that between the right auricle and ventricle the tricuspid valve4 ,the edges of the valves fall together when the heart contracts1 and prevent the passage of blood4 ,each ventricle has a second valve through which it ejects the blood4 7,that of the right ventricle has been shown double for the sake of convenience47 ,the action of the heart is this3--,the auricles and ventricles expand2 blood rushes into the auricles from the channels supplying them1 and distends them and the ventricles2 the auricles contract and fill the ventricles below quite full 7there are no valves above the auricles1 but the force of contraction is not sufficient to return the blood to the veins72 the ventricles contract2 the mitral and tricuspid valves close2 the valves leading to the arteries open2 blood is forced out of the ventricles4 ,,the ,,blood ,,channels are of two kinds--7#a7 ,the arteries1 which lead the blood into the circulatory system2 7#b7 the veins1 which lead the blood back to the heart4 ,the arteries divide up into branches1 and these again divide into smaller and smaller arteries4 ,the smallest1 termed capillaries 7,latin1 capillus1 a hair71 are minute tubes having an average diameter of #a_/cjjjth of an inch4 ,these permeate every part of the body4 ,the capillary arteries lead into the smallest veins1 which unite to form larger and larger veins1 until what we may call the main streams are reached4 ,through these the blood flows to the heart4 ,there are three main points of difference between arteries and veins4 ,in the first place1 the larger arteries have thick elastic walls1 and maintain their shape even when empty4 ,this elasticity performs the function of the air-chamber of the force-pump4 ,when the ventricles contract1 driving blood into the arteries1 the walls of the latter expand1 and their contraction pushes the blood steadily forward without shock4 ,the capillaries have very thin walls1 so that fluids pass through them to and from the body1 feeding it and taking out waste matter4 ,the veins are all thin-walled1 and collapse when empty4 ,secondly1 most veins are furnished with valves1 which prevent blood flowing the wrong way4 ,these are similar in principle to those of the heart4 ,arteries have no valves4 ,thirdly1 arteries are generally deeply set1 while many of the veins run near the surface of the body4 ,those on the front of the arm are specially visible4 ,place your thumb on them and run it along towards the wrist1 and you will notice that the veins distend owing to the closing of the valves just mentioned4 ,arterial blood is red1 and comes out from a cut in gulps1 on account of the contraction of the elastic walls4 ,if you cut a vein1 blue blood issues in a steady stream4 ,the change of colour is caused by the loss of oxygen during the passage of the blood through the capillaries1 and the absorption of carbon dioxide from the tissues4 ,the lungs are two of the great purifiers of the blood4 ,as it circulates through them1 it gives up the carbon dioxide which it has absorbed1 and receives pure oxygen in exchange4 ,if the air of a room is foul1 the blood does not get the proper amount of oxygen4 ,for this reason it is advisable for us to keep the windows of our rooms open as much as possible both day and night4 ,fatigue is caused by the accumulation of carbon dioxide and other impurities in the blood4 ,when we run1 the heart pumps blood through the lungs faster than they can purify it1 and eventually our muscles become poisoned to such an extent that we have to stop from sheer exhaustion4 ,,the ,,course ,,of ,,the ,,blood4 ,it takes rather less than a minute for a drop of blood to circulate from the heart through the whole system and back to the heart4 ,we may briefly summarize the course of the circulation of the blood thus3--,it is expelled from the left ventricle into the aorta and the main arteries1 whence it passes into the smaller arteries1 and thence into the capillaries of the brain1 stomach1 kidneys1 etc4 ,it here imparts oxygen to the body1 and takes in impurities4 ,it then enters the veins1 and through them flows back to the right auricle2 is driven into the right ventricle2 is expelled into the pulmonary 7lung7 arteries2 enters the lungs1 and is purified4 ,it returns to the left auricle through the pulmonary veins2 enters the left auricle1 passes to left ventricle1 and so on4 ,a healthy heart beats from #abj times per minute in a one-year-old infant to #fj per minute in a very aged person4 ,the normal rate for a middle-aged adult is from #hj to #gj beats4 ,heart disease signifies the failure of the heart valves to close properly4 ,blood passes back when the heart contracts1 and the circulation is much enfeebled4 ,by listening through a stethoscope the doctor is able to tell whether the valves are in good order4 ,a hissing sound during the beat indicates a leakage past the valves2 a thump1 or clack1 that they shut completely4 ,,the ,,hydraulic ,,press4 ,it is a characteristic of fluids and gases that if pressure be brought to bear on any part of a mass of either class of bodies it is transmitted equally and undiminished in all directions1 and acts with the same force on all equal surfaces1 at right angles to those surfaces4 ,the great natural philosopher ,pascal first formulated this remarkable fact1 of which a simple illustration is given in ,fig4 #agi4 ,two cylinders1 ,a and ,b1 having a bore of one and two inches respectively1 are connected by a pipe4 ,water is poured in1 and pistons fitting the cylinders accurately and of equal weight are inserted4 ,on piston ,b is placed a load of #aj lbs4 ,to prevent ,a rising above the level of ,b1 it must be loaded proportionately4 ,the area of piston ,a is four times that of ,b1 so that if we lay on it a #dj-lb4 weight1 neither piston will move4 ,the walls of the cylinders and connecting pipe are also pressed outwards in the ratio of #aj lbs4 for every part of their interior surface which has an area equal to that of piston ,b4 ,illustration3 ,,fig4 #agi4 ,illustration3 ,,fig4 #ahj4--,the cylinder and ram of a hydraulic press4 ,the hydraulic press is an application of this law4 ,cylinder ,b is represented by a force pump of small bore1 capable of delivering water at very high pressures 7up to #aj tons per square inch74 ,in the place of ,a we have a stout cylinder with a solid plunger1 ,p 7,fig4 #ahj71 carrying the table on which the object to be pressed is placed4 ,bramah1 the inventor of the hydraulic press1 experienced great difficulty in preventing the escape of water between the top of the cylinder and the plunger4 ,if a gland packing of the type found in steam-cylinders were used1 it failed to hold back the water unless it were screwed down so tightly as to jam the plunger4 ,he tried all kinds of expedients without success2 and his invention1 excellent though it was in principle1 seemed doomed to failure1 when his foreman1 ,henry ,maudslay1#ce solved the problem in a simple but most masterly manner4 ,he had a recess turned in the neck of the cylinder at the point formerly occupied by the stuffing-box1 and into this a leather collar of ,u-section 7marked solid black in ,fig4 #ahj7 was placed with its open side downwards4 ,when water reached it1 it forced the edges apart1 one against the plunger1 the other against the walls of the recess1 with a degree of tightness proportionate to the pressure4 ,on water being released from the cylinder the collar collapsed1 allowing the plunger to sink without friction4 ,the principle of the hydraulic press is employed in lifts2 in machines for bending1 drilling1 and riveting steel plates1 or forcing wheels on or off their axles2 for advancing the boring shield of a tunnel2 and for other purposes too numerous to mention4 ,,household ,,water-,,supply ,,fittings4 ,among these1 the most used is the tap1 or cock4 ,when a house is served by the town or district water supply1 the fitting of proper taps on all pipes connected with the supply is stipulated for by the water-works authorities4 ,the old-fashioned plug tap is unsuitable for controlling high-pressure water on account of the suddenness with which it checks the flow4 ,lest the reader should have doubts as to the nature of a plug tap1 we may add that it has a tapering cone of metal working in a tapering socket4 ,on the cone being turned till a hole through it is brought into line with the channel of the tap1 water passes4 ,a quarter turn closes the tap4 ,illustration3 ,,fig4 #aha4--;,a screw-down water cock4 ,its place has been taken by the screw-down cock4 ,a very common and effective pattern is shown in ,fig4 #aha4 ,the valve ,v1 with a facing of rubber1 leather1 or some other sufficiently elastic substance1 is attached to a pin1 ,c1 which projects upwards into the spindle ,a of the tap4 ,this spindle has a screw thread on it engaging with a collar1 ,b4 ,when the spindle is turned it rises or falls1 allowing the valve to leave its seating1 ,v ,s1 or forcing it down on to it4 ,a packing ,p in the neck of ,b prevents the passage of water round the spindle4 ,to open or close the tap completely is a matter of several turns1 which cannot be made fast enough to produce a water-hammer in the pipes by suddenly arresting the flow4 ,the reader will easily understand that if water flowing at the rate of several miles an hour is abruptly checked1 the shock to the pipes carrying it must be very severe4 ,,the ,,ball-,,cock is used to feed a cistern automatically with water1 and prevent the water rising too far in the cistern 7,fig4 #ahb74 ,water enters the cistern through a valve1 which is opened and closed by a plug faced with rubber4 ,the lower extremity of the plug is flattened1 and has a rectangular hole cut in it4 ,through this passes a lever1 ,l1 attached at one end to a hollow copper sphere1 and pivoted at the other on the valve casing4 ,this casing is not quite circular in section1 for two slots are cast in the circumference to allow water to pass round the plug freely when the valve is open4 ,the buoyancy of the copper sphere is sufficient to force the plug's face up towards its seating as the valve rises1 and to cut off the supply entirely when a certain level has been attained4 ,if water is drawn off1 the sphere sinks1 the valve opens1 and the loss is made good4 ,illustration3 ,,fig4 #ahb4--;,an automatic ball-valve4 ,,the ,,water-,,meter4 ,illustration3 ,,fig4 #ahc4 ,some consumers pay a sum quarterly for the privilege of a water supply1 and the water company allows them to use as much as they require4 ,others1 however1 prefer to pay a fixed amount for every thousand gallons used4 ,in such cases1 a water-meter is required to record the consumption4 ,we append a sectional diagram of ,kennedy's patent water-meter 7,fig4 #ahc71 very widely used4 ,at the bottom is the measuring cylinder1 fitted with a piston1 7#f71 which is made to move perfectly water-tight and free from friction by means of a cylindrical ring of india-rubber1 rolling between the body of the piston and the internal surface of the cylinder4 ,the piston rod 7#be71 after passing through a stuffing-box in the cylinder cover1 is attached to a rack1 7#ae71 which gears with a cog1 7#ac71 fixed on a shaft4 ,as the piston moves up and down1 this cog is turned first in one direction1 then in the other4 ,to this shaft is connected the index mechanism 7to the right74 ,the cock-key 7#bd7 is so constructed that it can put either end of the measuring cylinder in communication with the supply or delivery pipes1 if given a quarter turn 7see ,fig4 #ahd74 ,the weighted lever 7#ad7 moves loosely on the pinion shaft through part of a circle4 ,from the pinion project two arms1 one on each side of the lever4 ,when the lever has been lifted by one of these past the vertical position1 it falls by its own weight on to a buffer-box rest1 7#ah74 ,in doing so1 it strikes a projection on the duplex lever 7#ai71 which is joined to the cock-key1 and gives the latter a quarter turn4 ,in order to follow the working of the meter1 we must keep an eye on ,figs4 #ahc and #ahd simultaneously4 ,water is entering from ,a1 the supply pipe4 ,it flows through the cock downwards through channel ,d into the lower half of the cylinder4 ,the piston rises1 driving out the water above it through ,c to the delivery pipe ,b4 ,just as the piston completes its stroke the weight1 raised by the rack and pinion1 topples over1 and strikes the key-arm1 which it sends down till stopped by the buffer-box4 ,the tap is then at right angles to the position shown in ,fig4 #ahd1 and water is directed from ,a down ,c into the top of the cylinder1 forcing the piston down1 while the water admitted below during the last stroke is forced up the passage ,d1 and out by the outlet ,b4 ,before the piston has arrived at the bottom of the cylinder1 the lifter will have lifted the weighted lever from the buffer-box1 and raised it to a vertical position2 from there it will have fallen on the right-hand key-arm1 and have brought the cock-key to its former position1 ready to begin another upward stroke4 ,illustration3 ,,fig4 #ahd4 ,the index mechanism makes allowance for the fact that the bevel-wheel on the pinion shaft has its direction reversed at the beginning of every stroke of the piston4 ,this bevel engages with two others mounted loosely on the little shaft1 on which is turned a screw thread to revolve the index counter wheels4 ,each of these latter bevels actuates the shaft through a ratchet2 but while one turns the shaft when rotating in a clockwise direction only1 the other engages it when making an anti-clockwise revolution4 ,the result is that the shaft is always turned in the same direction4 ,,water-,,supply ,,systems4 ,the water for a town or a district supply is got either from wells or from a river4 ,in the former case it may be assumed to be free from impurities4 ,in the latter1 there is need for removing all the objectionable and dangerous matter which river water always contains in a greater or less degree4 ,this purification is accomplished by first leading the water into large settling tanks1 where the suspended matter sinks to the bottom4 ,the water is then drawn off into filtration beds1 made in the following manner4 ,the bottom is covered with a thick layer of concrete4 ,on this are laid parallel rows of bricks1 the rows a small distance apart4 ,then come a layer of bricks or tiles placed close together2 a layer of coarse gravel2 a layer of finer gravel2 and a thick layer of sand at the top4 ,the sand arrests any solid matter in the water as it percolates to the gravel and drains below4 ,even the microbes1#cf of microscopic size1 are arrested as soon as the film of mud has formed on the top of the sand4 ,until this film is formed the filter is not in its most efficient condition4 ,every now and then the bed is drained1 the surface mud and sand carefully drained off1 and fresh sand put in their place4 ,a good filter bed should not pass more than from two to three gallons per hour for every square foot of surface1 and it must therefore have a large area4 ,it is sometimes necessary to send the water through a succession of beds1 arranged in terraces1 before it is sufficiently pure for drinking purposes4 ,,the ,,household ,,filter4 ,when there is any doubt as to the wholesomeness of the water supply1 a small filter is often used4 ,the microbe-stopper is usually either charcoal1 sand1 asbestos1 or baked clay of some kind4 ,in ,fig4 #ahe we give a section of a ,maignen filter4 ,r is the reservoir for the filtered water2 ,a the filter case proper2 ,d a conical perforated frame2 ,b a jacket of asbestos cloth secured top and bottom by asbestos cords to ,d2 ,c powdered carbon1 between which and the asbestos is a layer of special chemical filtering medium4 ,a perforated cap1 ,e1 covers in the carbon and prevents it being disturbed when water is poured in4 ,the carbon arrests the coarser forms of matter2 the asbestos the finer4 ,the asbestos jacket is easily removed and cleansed by heating over a fire4 ,illustration3 ,,fig4 #ahe4 ,the most useful form of household filter is one which can be attached to a tap connected with the main4 ,such a filter is usually made of porcelain or biscuit china4 ,the ,berkefeld filter has an outer case of iron1 and an interior hollow candle of porcelain from which a tube passes through the lid of the filter to a storage tank for the filtered water4 ,the water from the main enters the outer case1 and percolates through the porcelain walls to the internal cavity and thence flows away through the delivery pipe4 ,whatever be the type of filter used it must be cleansed at proper intervals4 ,a foul filter is very dangerous to those who drink the water from it4 ,it has been proved by tests that1 so far from purifying the water1 an inefficient and contaminated filter passes out water much more highly charged with microbes than it was before it entered4 ,we must not therefore think that1 because water has been filtered1 it is necessarily safe4 ,the reverse is only too often the case4 ,,gas ,,traps4 ,dangerous microbes can be breathed as well as drunk into the human system4 ,every communication between house and drains should be most carefully trapped4 ,the principle of a gas trap between1 say1 a kitchen sink and the drain to carry off the water is given in ,fig4 #ahf4 ,enough water always remains in the bend to rise above the level of the elbow1 effectually keeping back any gas that there may be in the pipe beyond the bend4 ,illustration3 ,,fig4 #ahf4--;,a trap for foul air4 ,,water-,,engines4 ,before the invention of the steam-engine human industries were largely dependent on the motive power of the wind and running water4 ,but when the infant nursed by ,watt and ,stephenson had grown into a giant1 both of these natural agents were deposed from the important position they once held4 ,windmills in a state of decay crown many of our hilltops1 and the water-wheel which formerly brought wealth to the miller now rots in its mountings at the end of the dam4 ,except for pumping and moving boats and ships1 wind-power finds its occupation gone4 ,it is too uncertain in quantity and quality to find a place in modern economics4 ,water-power1 on the other hand1 has received a fresh lease of life through the invention of machinery so scientifically designed as to use much more of the water's energy than was possible with the old-fashioned wheel4 ,illustration3 ,,fig4 #ahg4--;,a ,pelton wheel which develops #e1jjj horse-power4 ,observe the shape of the double buckets4 ,the turbine1 of which we have already spoken in our third chapter1 is now the favourite hydraulic engine4 ,some water-turbines work on much the same principle as the ,parsons steam-turbine2 others resemble the ,de ,laval4 ,among the latter the ,pelton wheel takes the first place4 ,by the courtesy of the manufacturers we are able to give some interesting details and illustrations of this device4 ,illustration3 ,,fig4 #ahh4--,pelton wheel mounted1 with nozzle in position4 ,the wheel1 which may be of any diameter from six inches to ten feet1 has buckets set at regular intervals round the circumference1 sticking outwards4 ,each bucket1 as will be gathered from our illustration of an enormous #e1jjj h4p4 wheel 7,fig4 #ahg71 is composed of two cups4 ,a nozzle is so arranged as to direct water on the buckets just as they reach the lowest point of a revolution 7see ,fig4 #ahh74 ,the water strikes the bucket on the partition between the two cups1 which turns it right and left round the inside of the cups4 ,the change of direction transfers the energy of the water to the wheel4 ,illustration3 ,,fig4 #ahi4--,speed regulator for ,pelton wheel4 ,the speed of the wheel may be automatically regulated by a deflecting nozzle 7,fig4 #ahi71 which has a ball and socket joint to permit of its being raised or lowered by a centrifugal governor1 thus throwing the stream on or off the buckets4 ,the power of the wheel is consequently increased or diminished to meet the change of load1 and a constant speed is maintained4 ,when it is necessary to waste as little water as possible1 a concentric tapered needle may be fitted inside the nozzle4 ,when the nozzle is in its highest position the needle tip is withdrawn2 as the nozzle sinks the needle protrudes1 gradually decreasing the discharge area of the nozzle4 ,pelton wheels are designed to run at all speeds and to use water of any pressure4 ,at ,manitou1 ,colorado1 is an installation of three wheels operated by water which leaves the nozzle at the enormous pressure of #ice lbs4 per square inch4 ,it is interesting to note that jets of very high-pressure water offer astonishing resistance to any attempt to deflect their course4 ,a three-inch jet of #ejj-lb4 water cannot be cut through by a blow from a crowbar4 ,in order to get sufficient pressure for working hydraulic machinery in mines1 factories1 etc41 water is often led for many miles in flumes1 or artificial channels1 along the sides of valleys from the source of supply to the point at which it is to be used4 ,by the time that point is reached the difference between the gradients of the flume and of the valley bottom has produced a difference in height of some hundreds of feet4 ,illustration3 ,,fig4 #aij4--,the ,laxey water-wheel1 ,isle of ,man4 ,in the top right-hand corner is a ,pelton wheel of proportionate size required to do the same amount of work with the same consumption of water at the same pressure4 ,the full-page illustration on p4 #chj affords a striking testimony to the wonderful progress made in engineering practice during the last fifty years4 ,the huge water-wheel which forms the bulk of the picture is that at ,laxey1 in the ,isle of ,man4 ,it is #gb-a_/b feet in diameter1 and is supposed to develop #aej horse-power1 which is transmitted several hundreds of feet by means of wooden rods supported at regular intervals4 ,the power thus transmitted operates a system of pumps in a lead mine1 raising #bej gallons of water per minute1 to an elevation of #a1bjj feet4 ,the driving water is brought some distance to the wheel in an underground conduit1 and is carried up the masonry tower by pressure1 flowing over the top into the buckets on the circumference of the wheel4 ,the little cut in the upper corner represents a ,pelton wheel drawn on the same scale1 which1 given an equal supply of water at the same pressure1 would develop the same power as the ,laxey monster4 ,by the side of the giant the other appears a mere toy4 ,,the ,,cream ,,separator4 ,in #ahfd ,denmark went to war with ,germany1 and emerged from the short struggle shorn of the provinces of ,lauenburg1 ,holstein1 and ,schleswig4 ,the loss of the two last1 the fairest and most fertile districts of the kingdom1 was indeed grievous4 ,the ,danish king now ruled only over a land consisting largely of moor1 marsh1 and dunes1 apparently worthless for any purpose4 ,but the ,danes1 with admirable courage1 entered upon a second struggle1 this time with nature4 ,they made roads and railways1 dug irrigation ditches1 and planted forest trees2 and so gradually turned large tracts of what had been useless country into valuable possessions4 ,agriculture being much depressed1 owing to the low price of corn1 they next gave their attention to the improvement of dairy farming4 ,labour-saving machinery of all kinds was introduced1 none more important than the device for separating the fatty from the watery constituents of milk4 ,it would not be too much to say that the separator is largely responsible for the present prosperity of ,denmark4 ,illustration3 ,,fig4 #aia4--,section of a ,cream ,separator4 ,how does it work8 asks the reader4 ,centrifugal force#cg is the governing principle4 ,to explain its application we append a sectional illustration 7,fig4 #aia7 of ,messrs4 ,burmeister and ,wain's hand-power separator1 which may be taken as generally representative of this class of machines4 ,inside a circular casing is a cylindrical bowl1 ,d1 mounted on a shaft which can be revolved #e1jjj times a minute by means of the cog-wheels and the screw thread chased on it near the bottom extremity4 ,milk flows from the reservoir ,r 7supported on a stout arm7 through tap ,a into a little distributer on the top of the separator1 and from it drops into the central tube ,c of the bowl4 ,falling to the bottom1 it is flung outwards by centrifugal force1 finds an escape upwards through the holes a a1 and climbs up the perforated grid e1 the surface of which is a series of pyramidical excrescences1 and finally reaches the inner surface of the drum proper4 ,the velocity of rotation is so tremendous that the heavier portions of the milk--that is1 the watery--crowd towards the point furthest from the centre1 and keep the lighter fatty elements away from contact with the sides of the drum4 ,in the diagram the water is represented by small circles1 the cream by small crosses4 ,as more milk enters the drum it forces upwards what is already there4 ,the cap of the drum has an inner jacket1 ,f1 which at the bottom all but touches the side of the drum4 ,the distance between them is the merest slit2 but the cream is deflected up outside ,f into space ,e1 and escapes through a hole one-sixteenth of an inch in diameter perforating the plate ,g4 ,the cream is flung into space ,k and trickles out of spout ,b1 while the water flies into space ,h and trickles away through spout ,a4 ,,the ,,hydro41 used in laundries for wringing clothes by centrifugal force1 has a solid outer casing and an inner perforated cylindrical cage1 revolved at high speed by a vertical shaft4 ,the wet clothes are placed in the cage1 and the machine is started4 ,the water escapes through the perforations and runs down the side of the casing to a drain4 ,after a few minutes the clothes are dry enough for ironing4 ,so great is the centrifugal force that they are consolidated against the sides of the cage1 and care is needed in their removal4 #ce ,inventor of the lathe slide-rest4 #cf ,living germs2 some varieties the cause of disease4 #cg ,that is1 centre-fleeing force4 ,water dropped on a spinning top rushes towards the circumference and is shot off at right angles to a line drawn from the point of parting to the centre of the top4 ,chapter ,,xix4 ,,heating ,,and ,,lighting4 ,the hot-water supply--,the tank system--,the cylinder system--,how a lamp works--,gas and gasworks--,automatic stoking--,a gas governor--,the gas meter--,incandescent gas lighting4 ,,hot-,,water ,,supply4 ,a well-equipped house is nowadays expected to contain efficient apparatus for supplying plenty of hot water at all hours of the day4 ,there is little romance about the kitchen boiler and the pipes which the plumber and his satellites have sometimes to inspect and put right1 but the methods of securing a proper circulation of hot water through the house are sufficiently important and interesting to be noticed in these pages4 ,in houses of moderate size the kitchen range does the heating4 ,the two systems of storing and distributing the heated water most commonly used are--7#a7 ,the tank system2 7#b7 the cylinder system4 ,,the ,,tank ,,system is shown diagrammatically in ,fig4 #aib4 ,the boiler is situated at the back of the range1 and when a damper is drawn the fire and hot gases pass under it to a flue leading to the chimney4 ,the almost boiling water rises to the top of the boiler and thence finds its way up the flow pipe into the hot-water tank ,a1 displacing the somewhat colder water there1 which descends through the return pipe to the bottom of the boiler4 ,water is drawn off from the flow pipe4 ,this pipe projects some distance through the bottom of ,a1 so that the hottest portion of the contents may be drawn off first4 ,a tank situated in the roof1 and fed from the main by a ball-cock valve1 communicates with ,a through the siphon pipe ,s4 ,the bend in this pipe prevents the ascent of hot water1 which cannot sink through water colder than itself4 ,from the top of ,a an expansion pipe is led up and turned over the cold-water tank to discharge any steam which may be generated in the boiler4 ,a hot-water radiator for warming the house may be connected to the flow and return pipes as shown4 ,since it opens a short circuit for the circulation1 the water in the tank above will not be so well heated while it is in action4 ,if cocks are fitted to the radiator pipes1 the amount of heat thus deflected can be governed4 ,illustration3 ,,fig4 #aib4--,the tank system of hot-water supply4 ,a disadvantage of the tank system is that the tank1 if placed high enough to supply all flows1 is sometimes so far from the boiler that the water loses much of its heat in the course of circulation4 ,also1 if for any reason the cold water fails1 tank ,a may be entirely emptied1 circulation cease1 and the water in the boiler and pipes boil away rapidly4 ,,the ,,cylinder ,,system 7,fig4 #aic7 is open to neither of these objections4 ,instead of a rectangular tank up aloft1 we now have a large copper cylinder situated in the kitchen near the range4 ,the flow and return pipes are continuous1 and the cold supply enters the bottom of the cylinder through a pipe with a siphon bend in it4 ,as before1 water is drawn off from the flow pipe1 and a radiator may be put in the circuit4 ,since there is no draw-off point below the top of the cylinder1 even if the cold supply fails the cylinder will remain full1 and the failure will be discovered long before there is any danger of the water in it boiling away4 ,illustration3 ,,fig4 #aic4--,the cylinder system of hot-water supply4 ,boiler explosions are due to obstructions in the pipes4 ,if the expansion pipe and the cold-water supply pipe freeze1 there is danger of a slight accumulation of steam2 and if one of the circulation pipes is also blocked1 steam must generate until something has to go1#ch which is naturally the boiler4 ,assuming that the pipes are quite full to the points of obstruction1 the fracture would result from the expansion of the water4 ,steam cannot generate unless there be a space above the water4 ,but the expanding water has stored up the heat which would have raised steam1 and the moment expansion begins after fracture this energy is suddenly let loose4 ,steam forms instantaneously1 augmenting the effects of the explosion4 ,from this it will be gathered that all pipes should be properly protected against frost2 especially near the roof4 ,another cause of disaster is the furring up of the pipes with the lime deposited by hard water when heated4 ,when hard water is used1 the pipes will sooner or later be blocked near the boiler2 and as the deposit is too hard to be scraped away1 periodical renewals are unavoidable4 ,,how ,a ,,lamp ,,works4 ,from heating we turn to lighting1 and first to the ordinary paraffin lamp4 ,the two chief things to notice about this are the wick and the chimney4 ,the wick1 being made of closely-woven cotton1 draws up the oil by what is known as capillary attraction4 ,if you dip the ends of two glass tubes1 one half an inch1 the other one-eighth of an inch in diameter1 into a vessel of water1 you will notice that the water rises higher in the smaller tube4 ,or get two clean glass plates and lay them face to face1 touching at one end1 but kept slightly apart at the other by some small object4 ,if they are partly submerged perpendicularly1 the water will rise between the plates--furthest on the side at which the two plates touch1 and less and less as the other edge is approached4 ,the tendency of liquids to rise through porous bodies is a phenomenon for which we cannot account4 ,mineral oil contains a large proportion of carbon and hydrogen2 it is therefore termed hydro-carbon4 ,when oil reaches the top of a lighted wick1 the liquid is heated until it turns into gas4 ,the carbon and hydrogen unite with the oxygen of the air4 ,some particles of the carbon apparently do not combine at once1 and as they pass through the fiery zone of the flame are heated to such a temperature as to become highly luminous4 ,it is to produce these light-rays that we use a lamp1 and to burn our oil efficiently we must supply the flame with plenty of oxygen1 with more than it could naturally obtain4 ,so we surround it with a transparent chimney of special glass4 ,the air inside the chimney is heated1 and rises2 fresh air rushes in at the bottom1 and is also heated and replaced4 ,as the air passes through1 the flame seizes on the oxygen4 ,if the wick is turned up until the flame becomes smoky and flares1 the point has been passed at which the induced chimney draught can supply sufficient oxygen to combine with the carbon of the vapour1 and the free carbon escapes as smoke4 ,the blower-plate used to draw up a fire 7,fig4 #aid7 performs exactly the same function as the lamp chimney1 but on a larger scale4 ,the plate prevents air passing straight up the chimney over the coals1 and compels it to find a way through the fire itself to replace the heated air rising up the chimney4 ,illustration3 ,,fig4 #aid4--,showing how a blower-plate draws up the fire4 ,,gas ,,and ,,gasworks4 ,a lamp is an apparatus for converting hydro-carbon mineral oil into gas and burning it efficiently4 ,the gas-jet burns gases produced by driving off hydro-carbon vapours from coal in apparatus specially designed for the purpose4 ,gas-making is now1 in spite of the competition of electric lighting1 so important an industry that we shall do well to glance at the processes which it includes4 ,coal gas may be produced on a very small scale as follows3--,fill a tin canister 7the joints of which have been made by folding the metal1 not by soldering7 with coal1 clap on the lid1 and place it1 lid downwards1 in a bright fire1 after punching a hole in the bottom4 ,vapour soon begins to issue from the hole4 ,this is probably at first only steam1 due to the coal being more or less damp4 ,but if a lighted match be presently applied the vapour takes fire1 showing that coal gas proper is coming off4 ,the flame lasts for a long time4 ,when it dies the canister may be removed and the contents examined4 ,most of the carbon remains in the form of coke4 ,it is bulk for bulk much lighter than coal1 for the hydrogen1 oxygen1 and other gases1 and some of the carbon have been driven off by the heat4 ,the coke itself burns if placed in a fire1 but without any smoke1 such as issues from coal4 ,illustration3 ,,fig4 #aie4--,sketch of the apparatus used in the manufacture of coal gas4 ,our home-made gas yields a smoky and unsatisfactory flame1 owing to the presence of certain impurities--ammonia1 tar1 sulphuretted hydrogen1 and carbon bisulphide4 ,a gas factory must be equipped with means of getting rid of these objectionable constituents4 ,turning to ,fig4 #aie1 which displays very diagrammatically the main features of a gas plant1 we observe at the extreme right the retorts1 which correspond to our canister4 ,these are usually long fire-brick tubes of ,d-section1 the flat side at the bottom4 ,under each is a furnace1 the flames of which play on the bottom1 sides1 and inner end of the retort4 ,the outer end projecting beyond the brickwork seating has an iron air-tight door for filling the retort through1 immediately behind which rises an iron exit pipe1 ,a1 for the gases4 ,tar1 which vaporizes at high temperatures1 but liquefies at ordinary atmospheric heat1 must first be got rid of4 ,this is effected by passing the gas through the hydraulic main1 a tubular vessel half full of water running the whole length of the retorts4 ,the end of pipe ,a dips below the surface of the water1 which condenses most of the tar and steam4 ,the partly-purified gas now passes through pipe ,b to the condensers1 a series of inverted ,u-pipes standing on an iron chest with vertical cross divisions between the mouths of each ,u4 ,these divisions dip into water1 so that the gas has to pass up one leg of a ,u1 down the other1 up the first leg of the second pipe1 and so on1 till all traces of the tar and other liquid constituents have condensed on the inside of the pipe1 from which they drop into the tank below4 ,the next stage is the passage of the scrubber1 filled with coke over which water perpetually flows4 ,the ammonia gas is here absorbed4 ,there still remain the sulphuretted hydrogen and the carbon bisulphide1 both of which are extremely offensive to the nostrils4 ,slaked lime1 laid on trays in an air-tight compartment called the lime purifier1 absorbs most of the sulphurous elements of these2 and the coal gas is then fit for use4 ,on leaving the purifiers it flows into the gasometer1 or gasholder1 the huge cake-like form of which is a very familiar object in the environs of towns4 ,the gasometer is a cylindrical box with a domed top1 but no bottom1 built of riveted steel plates4 ,it stands in a circular tank of water1 so that it may rise and fall without any escape of gas4 ,the levity of the gas1 in conjunction with weights attached to the ends of chains working over pulleys on the framework surrounding the holder1 suffices to raise the holder4 ,illustration3 ,,fig4 #aif4--,the largest gasholder in the world3 ,south ,metropolitan ,gas ,co41 ,greenwich ,gas ,works4 ,capacity1 #ab1aeh1fjj cubic feet4 ,some gasometers have an enormous capacity4 ,the record is at present held by that built for the ,south ,metropolitan ,gas ,co41 ,london1 by ,messrs4 ,clayton ,son of ,leeds4 ,this monster 7of which we append an illustration1 ,fig4 #aif7 is #cjj feet in diameter and #ahj feet high4 ,when fully extended it holds #ab1aeh1fjj cubic feet of gas4 ,owing to its immense size1 it is built on the telescopic principle in six lifts1 of #cj feet deep each4 ,the sides of each lift1 or ring1 except the topmost1 have a section shaped somewhat like the letter ,n4 ,two of the members form a deep1 narrow cup to hold water1 in which the dip member of the ring above it rises and falls4 ,illustration3 ,,fig4 #aig4--;,drawing retorts4 7,photo by ,f4 ,marsh47 ,,automatic ,,stoking4 ,the labour of feeding the retorts with coal and removing the coke is exceedingly severe4 ,in the illustration on p4 #djj 7made from a very fine photograph taken by ,mr4 ,f4 ,marsh of ,clifton7 we see a man engaged in drawing the retorts through the iron doors at their outer ends4 ,automatic machinery is now used in large gasworks for both operations4 ,one of the most ingenious stokers is the ,de ,brouwer1 shown at work in ,fig4 #aih4 ,the machine is suspended from an overhead trolley running on rails along the face of the retorts4 ,coal falls into a funnel at the top of the telescopic pipe ,p from hoppers in the story above1 which have openings1 ,h ,h1 controlled by shutters4 ,the coal as it falls is caught by a rubber belt working round part of the circumference of the large wheel ,w and a number of pulleys1 and is shot into the mouth of the retort4 ,the operator is seen pulling the handle which opens the shutter of the hopper above the feed-tube1 and switching on the #d h4p4 electric motor which drives the belt and moves the machine about4 ,one of these feeders will charge a retort #bj feet long in twenty-two seconds4 ,illustration3 ,,fig4 #aih4--;,de ,brouwer automatic retort charger4 ,a ,,gas ,,governor4 ,some readers may have noticed that late at night a gas-jet1 which a few hours before burned with a somewhat feeble flame when the tap was turned fully on1 now becomes more and more vigorous1 and finally may flare up with a hissing sound4 ,this is because many of the burners fed by the main supplying the house have been turned off1 and consequently there is a greater amount of gas available for the jets still burning1 which therefore feel an increased pressure4 ,as a matter of fact1 the pressure of gas in the main is constantly varying1 owing partly to the irregularity of the delivery from the gasometer1 and partly to the fact that the number of burners in action is not the same for many minutes together4 ,it must also be remembered that houses near the gasometer end of the main will receive their gas at a higher pressure than those at the other end4 ,the gas stored in the holders may be wanted for use in the street lamps a few yards away1 or for other lamps several miles distant4 ,it is therefore evident that if there be just enough pressure to give a good supply to the nearest lamp1 there will be too little a short distance beyond it1 and none at all at the extreme point2 so that it is necessary to put on enough pressure to overcome the friction on all these miles of pipe1 and give just enough gas at the extreme end4 ,it follows that at all intermediate points the pressure is excessive4 ,gas of the average quality is burned to the greatest advantage1 as regards its light-giving properties1 when its pressure is equal to that of a column of water half an inch high1 or about #a_/ej lb4 to the square inch4 ,with less it gives a smoky1 flickering light1 and with more the combustion is also imperfect4 ,illustration3 ,,fig4 #aii4 ,every house supply should therefore be fitted with a gas governor1 to keep the pressure constant4 ,a governor frequently used1 the ,stott1 is shown in section in ,fig4 #aii4 ,gas enters from the main on the right1 and passes into a circular elbow1 ,d1 which has top and bottom apertures closed by the valves ,v ,v4 ,attached to the valve shaft is a large inverted cup of metal1 the tip of which is immersed in mercury4 ,the pressure at which the governor is to act is determined by the weights ,w1 with which the valve spindle is loaded at the top4 ,as soon as this pressure is exceeded1 the gas in ,c ,c lifts the metal cup1 and ,v ,v are pressed against their seats1 so cutting off the supply4 ,gas cannot escape from ,c ,c1 as it has not sufficient pressure to force its way through the mercury under the lip of the cup4 ,immediately the pressure in ,c ,c falls1 owing to some of the gas being used up1 the valves open and admit more gas4 ,when the fluctuations of pressure are slight1 the valves never close completely1 but merely throttle the supply until the pressure beyond them falls to its proper level--that is1 they pass just as much gas as the burners in use can consume at the pressure arranged for4 ,governors of much larger size1 but working on much the same principle1 are fitted to the mains at the point where they leave the gasometers4 ,they are not1 however1 sensitive to local fluctuations in the pipes1 hence the necessity for separate governors in the house between the meter and the burners4 ,,the ,,gas-,,meter commonly used in houses acts on the principle shown in ,fig4 #bjj4 ,the air-tight casing is divided by horizontal and vertical divisions into three gas-chambers1 ,b1 ,c1 and ,d4 ,gas enters at ,a1 and passes to the valve chamber ,b4 ,the slide-valves of this allow it to pass into ,c and ,d1 and also into the two circular leather bellows ,e1 ,f1 which are attached to the central division ,g1 but are quite independent of one another4 ,illustration3 ,,fig4 #bjj4--,sketch of the bellows and chambers of a dry gas meter4 ,we will suppose that in the illustration the valves are admitting gas to chamber ,c and bellows ,f4 ,the pressure in ,c presses the circular head of ,e towards the division ,g1 expelling the contents of the bellows through an outlet pipe 7not shown7 to the burners in operation within the house4 ,simultaneously the inflation of ,f forces the gas in chamber ,d also through the outlet4 ,the head-plates of the bellows are attached to rods and levers 7not shown7 working the slide-valves in ,b4 ,as soon as ,e is fully in1 and ,f fully expanded1 the valves begin to open and put the inlet pipe in communication with ,d and ,e1 and allow the contents of ,f and ,c to escape to the outlet4 ,the movements of the valve mechanism operate a train of counting wheels1 visible through a glass window in the side of the case4 ,as the bellows have a definite capacity1 every stroke that they give means that a certain volume of gas has been ejected either from them or from the chambers in which they move3 this is registered by the counter4 ,the apparatus practically has two double-action cylinders 7of which the bellows ends are the pistons7 working on the same principle as the steam-cylinder 7,fig4 #ba74 ,the valves have three ports--the central1 or exhaust1 leading to the outlet1 the outer ones from the inlet4 ,the bellows are fed through channels in the division ,g4 ,,incandescent ,,gas ,,lighting4 ,the introduction of the electric arc lamp and the incandescent glow-lamp seemed at one time to spell the doom of gas as an illuminating agent4 ,but the appearance in #ahhf of the ,welsbach incandescent mantle for gas-burners opened a prosperous era in the history of gas lighting4 ,the luminosity of a gas flame depends on the number of carbon particles liberated within it1 and the temperature to which these particles can be heated as they pass through the intensely hot outside zone of the flame4 ,by enriching the gas in carbon more light is yielded1 up to a certain point1 with a flame of a given temperature4 ,to increase the heat of the flame various devices were tried before the introduction of the incandescent mantle1 but they were found to be too short-lived to have any commercial value4 ,inventors therefore sought for methods by which the emission of light could be obtained from coal gas independently of the incandescence of the carbon particles in the flame itself2 and step by step it was discovered that gas could be better employed merely as a heating agent1 to raise to incandescence substances having a higher emissivity of light than carbon4 ,dr4 ,auer von ,welsbach found that the substances most suitable for incandescent mantles were the oxides of certain rare metals1 thorium1 and cerium4 ,the mantle is made by dipping a cylinder of cotton net into a solution of nitrate of thorium and cerium1 containing #ii per cent4 of the former and #a per cent4 of the latter metal4 ,when the fibres are sufficiently soaked1 the mantle is withdrawn1 squeezed1 and placed on a mould to dry4 ,it is next held over a ,bunsen gas flame and the cotton is burned away1 while the nitrates are converted into oxides4 ,the mantle is now ready for use1 but very brittle4 ,so it has to undergo a further dipping1 in a solution of gun-cotton and alcohol1 to render it tough enough for packing4 ,when it is required for use1 it is suspended over the burner by an asbestos thread woven across the top1 a light is applied to the bottom1 and the collodion burned off1 leaving nothing but the heat-resisting oxides4 ,the burner used with a mantle is constructed on the ,bunsen principle4 ,the gas is mixed1 as it emerges from the jet1 with sufficient air to render its combustion perfect4 ,all the carbon is burned1 and the flame1 though almost invisible1 is intensely hot4 ,the mantle oxides convert the heat energy of the flame into light energy4 ,this is proved not only by the intense whiteness of the mantle1 but by the fact that the heat issuing from the chimney of the burner is not nearly so great when the mantle is in position as when it is absent4 ,the incandescent mantle is more extensively used every year4 ,in ,germany #ij per cent4 of gas lighting is on the incandescent system1 and in ,england about #dj per cent4 ,we may notice1 as an interesting example of the fluctuating fortunes of invention1 that the once doomed gas-burner has1 thanks to ,welsbach's mantle1 in many instances replaced the incandescent electric lamps that were to doom it4 #ch ,if1 of course1 there is no safety-valve in proper working order included in the installation4 ,chapter ,,xx4 ,,various ,,mechanisms4 ,,clocks ,,and ,,watches3--,a short history of timepieces--,the construction of timepieces--,the driving power--,the escapement--,compensating pendulums--,the spring balance--,the cylinder escapement--,the lever escapement--,compensated balance-wheels--,keyless winding mechanism for watches--,the hour hand train4 ,,locks3--,the ,chubb lock--,the ,yale lock4 ,,the ,,cycle3--,the gearing of a cycle--,the free wheel--,the change-speed gear4 ,,agricultural ,,machines3--,the threshing-machine--,mowing-machines4 ,,some ,,natural ,,phenomena3--,why sun-heat varies in intensity--,the tides--,why high tide varies daily4 ,,clocks ,,and ,,watches4 ,a ,,short ,,history ,,of ,,timepieces4 ,the oldest device for measuring time is the sun-dial4 ,that of ,ahaz mentioned in the ,second ,book of ,kings is the earliest dial of which we have record4 ,the obelisks of the ,egyptians and the curious stone pillars of the ,druidic age also probably served as shadow-casters4 ,the clepsydra1 or water-clock1 also of great antiquity1 was the first contrivance for gauging the passage of the hours independently of the motion of the earth4 ,in its simplest form it was a measure into which water fell drop by drop1 hour levels being marked on the inside4 ,subsequently a very simple mechanism was added to drive a pointer--a float carrying a vertical rack1 engaging with a cog on the pointer spindle2 or a string from the float passed over a pulley attached to the pointer and rotated it as the float rose1 after the manner of the wheel barometer 7,fig4 #aec74 ,in #hjg ,a4,d4 ,charlemagne received from the ,king of ,persia a water-clock which struck the hours4 ,it is thus described in ,gifford's ,history of ,france3--,the dial was composed of twelve small doors1 which represented the division of the hours4 ,each door opened at the hour it was intended to represent1 and out of it came a small number of little balls1 which fell one by one1 at equal distances of time1 on a brass drum4 ,it might be told by the eye what hour it was by the number of doors that were open1 and by the ear by the number of balls that fell4 ,when it was twelve o'clock twelve horsemen in miniature issued forth at the same time and shut all the doors4 ,sand-glasses were introduced about #ccj ,a4,d4 ,except for special purposes1 such as timing sermons and boiling eggs1 they have not been of any practical value4 ,the clepsydra naturally suggested to the mechanical mind the idea of driving a mechanism for registering time by the force of gravity acting on some body other than water4 ,the invention of the weight-driven clock is attributed1 like a good many other things1 to ,archimedes1 the famous ,sicilian mathematician of the third century ,b4,c42 but no record exists of any actual clock composed of wheels operated by a weight prior to #aabj ,a4,d4 ,so we may take that year as opening the era of the clock as we know it4 ,about #aejj ,peter ,hele of ,nuremberg invented the mainspring as a substitute for the weight1 and the watch appeared soon afterwards 7#aebe ,a4,d474 ,the pendulum was first adopted for controlling the motion of the wheels by ,christian ,huygens1 a distinguished ,dutch mechanician1 in #afei4 ,to ,thomas ,tompion1 the father of ,english watchmaking1 is ascribed the honour of first fitting a hairspring to the escapement of a watch1 in or about the year #affj4 ,he also introduced the cylinder escapement now so commonly used in cheap watches4 ,though many improvements have been made since his time1 ,tompion manufactured clocks and watches which were excellent timekeepers1 and as a reward for the benefits conferred on his fellows during his lifetime1 he was1 after death1 granted the exceptional honour of a resting-place in ,westminster ,abbey4 ,,the ,,construction ,,of ,,timepieces4 ,a clock or watch contains three main elements3--7#a7 ,the source of power1 which may be a weight or a spring2 7#b7 the train of wheels operated by the driving force2 7#c7 the agent for controlling the movements of the train--this in large clocks is usually a pendulum1 in small clocks and watches a hairspring balance4 ,to these may be added1 in the case of clocks1 the apparatus for striking the hour4 ,,the ,,driving ,,power4 ,weights are used only in large clocks1 such as one finds in halls1 towers1 and observatories4 ,the great advantage of employing weights is that a constant driving power is exerted4 ,springs occupy much less room than weights1 and are indispensable for portable timepieces4 ,the employment of them caused trouble to early experimenters on account of the decrease in power which necessarily accompanies the uncoiling of a wound-up spring4 ,jacob ,zech of ,prague overcame the difficulty in #aebe by the invention of the fusee1 a kind of conical pulley interposed between the barrel1 or circular drum containing the mainspring1 and the train of wheels which the spring has to drive4 ,the principle of the drum and fusee action will be understood from ,fig4 #bja4 ,the mainspring is a long steel ribbon fixed at one end to an arbor 7the watchmaker's name for a spindle or axle71 round which it is tightly wound4 ,the arbor and spring are inserted in the barrel4 ,the arbor is prevented from turning by a ratchet1 ,b1 and click1 and therefore the spring in its effort to uncoil causes the barrel to rotate4 ,illustration3 ,,fig4 #bja4 ,a string of catgut 7or a very fine chain7 is connected at one end to the circumference of the drum1 and wound round it1 the other end being fixed to the larger end of the fusee1 which is attached to the driving-wheel of the watch or clock by the intervention of a ratchet and click 7not shown74 ,to wind the spring the fusee is turned backward by means of a key applied to the square end ,a of the fusee arbor1 and this draws the string from off the drum on to the fusee4 ,the force of the spring causes the fusee to rotate by pulling the string off it1 coil by coil1 and so drives the train of wheels4 ,but while the mainspring1 when fully wound1 turns the fusee by uncoiling the string from the smallest part of the fusee1 it gets the advantage of the larger radius as its energy becomes lessened4 ,the fusee is still used for marine chronometers1 for some clocks that have a mainspring and pendulum1 and occasionally for watches4 ,in the latter it has been rendered unnecessary by the introduction of the going-barrel by ,swiss watchmakers1 who formed teeth on the edge of the mainspring barrel to drive the train of wheels4 ,this kind of drum is called going because it drives the watch during the operation of winding1 which is performed by rotating the drum arbor to which the inner end of the spring is attached4 ,a ratchet prevents the arbor from being turned backwards by the spring4 ,the adoption of the going-barrel has been made satisfactory by the improvements in the various escapement actions4 ,,the ,,escapement4 ,illustration3 ,,fig4 #bjb4 ,the spring or weight transmits its power through a train of cogs to the escapement1 or device for regulating the rate at which the wheels are to revolve4 ,in clocks a pendulum is generally used as the controlling agent4 ,galileo1 when a student at ,pisa1 noticed that certain hanging lamps in the cathedral there swung on their cords at an equal rate2 and on investigation he discovered the principle that the shorter a pendulum is the more quickly will it swing to and fro4 ,as has already been observed1 ,huygens first applied the principle to the governing of clocks4 ,in ,fig4 #bjb we have a simple representation of the dead-beat escapement commonly used in clocks4 ,the escape-wheel is mounted on the shaft of the last cog of the driving train1 the pallet on a spindle from which depends a split arm embracing the rod and the pendulum4 ,we must be careful to note that the pendulum controls motion only2 it does not cause movement4 ,the escape-wheel revolves in a clockwise direction4 ,the two pallets a and b are so designed that only one can rest on the teeth at one time4 ,in the sketch the sloping end of b has just been forced upwards by the pressure of a tooth4 ,this swings the pallet and the pendulum4 ,the momentum of the latter causes a to descend1 and at the instant when b clears its tooth a catches and holds another4 ,the left-hand side of a1 called the locking-face1 is part of a circle1 so that the escape-wheel is held motionless as long as it touches a3 hence the term1 dead beat--that is1 brought to a dead stop4 ,as the pendulum swings back1 to the left1 under the influence of gravity1 a is raised and frees the tooth4 ,the wheel jerks round1 and another tooth is caught by the locking-face of b4 ,again the pendulum swings to the right1 and the sloping end of b is pushed up once more1 giving the pendulum fresh impetus4 ,this process repeats itself as long as the driving power lasts--for weeks1 months1 or years1 as the case may be1 and the mechanism continues to be in good working order4 ,,compensating ,,pendulums4 ,metal expands when heated2 therefore a steel pendulum which is of the exact length to govern a clock correctly at a temperature of #fj would become too long at #hj1 and slow the clock1 and too short at #dj1 and cause it to gain4 ,in common clocks the pendulum rod is often made of wood1 which maintains an almost constant length at all ordinary temperatures4 ,but for very accurate clocks something more efficient is required4 ,graham1 the partner of ,thomas ,tompion1 took advantage of the fact that different kinds of metal have different ratios of expansion to produce a self-compensating pendulum on the principle illustrated by ,fig4 #bjc4 ,he used steel for the rod1 and formed the bob1 or weighted end1 of a glass jar containing mercury held in a stirrup2 the mercury being of such a height that1 as the pendulum rod lengthened with a rise of temperature1 the mercury expanded upwards sufficiently to keep the distance between the point of suspension and the centre of gravity of the bob always the same4 ,with a fall of temperature the rod shortened1 while the mercury sank in the jar4 ,this device has not been improved upon1 and is still used in observatories and other places where timekeepers of extreme precision are required4 ,the milled nut ,s in ,fig4 #bjc is fitted at the end of the pendulum rod to permit the exact adjustment of the pendulum's length4 ,for watches1 chronometers1 and small clocks ,,the ,,spring ,,balance takes the place of the pendulum4 ,we still have an escape-wheel with teeth of a suitable shape to give impulses to the controlling agent4 ,there are two forms of spring escapement1 but as both employ a hairspring and balance-wheel we will glance at these before going further4 ,illustration3 ,,fig4 #bjc4 ,the hairspring is made of very fine steel ribbon1 tempered to extreme elasticity1 and shaped to a spiral4 ,the inner end is attached to the arbor of the balance-wheel1 the outer end to a stud projecting from the plate of the watch4 ,when the balance-wheel1 impelled by the escapement1 rotates1 it winds up the spring4 ,the energy thus stored helps the wheel to revolve the other way during the locking of a tooth of the escape-wheel4 ,the time occupied by the winding and the unwinding depends upon the length of the spring4 ,the strength of the impulse makes no difference4 ,a strong impulse causes the spring to coil itself up more than a weak impulse would2 but inasmuch as more energy is stored the process of unwinding is hastened4 ,to put the matter very simply--a strong impulse moves the balance-wheel further1 but rotates it quickly2 a weak impulse moves it a shorter distance1 but rotates it slowly4 ,in fact1 the principle of the pendulum is also that of the hairspring2 and the duration of a vibration depends on the length of the rod in the one case1 and of the spring in the other4 ,motion is transmitted to the balance by one of two methods4 ,either 7#a7 directly1 by a cylinder escapement2 or 7#b7 indirectly1 through a lever4 ,illustration3 ,,fig4 #bjd4--;,cylinder watch escapement4 ,,the ,,cylinder ,,escapement is seen in ,fig4 #bjd4 ,the escape-wheel has sharp teeth set on stalks4 7,one tooth is removed to show the stalk47 ,the balance-wheel is mounted on a small steel cylinder1 with part of the circumference cut away at the level of the teeth1 so that if seen from above it would appear like a in our illustration4 ,a tooth is just beginning to shove its point under the nearer edge of the opening4 ,as it is forced forwards1 b is revolved in a clockwise direction1 winding up the hairspring4 ,when the tooth has passed the nearer edge it flies forward1 striking the inside of the further wall of the cylinder1 which holds it while the spring uncoils4 ,the tooth now pushes its way past the other edge1 accelerating the unwinding1 and1 as it escapes1 the next tooth jumps forward and is arrested by the outside of the cylinder4 ,the balance now reverses its motion1 is helped by the tooth1 is wound up1 locks the tooth1 and so on4 ,,the ,,lever ,,escapement is somewhat more complicated4 ,the escape-wheel teeth are locked and unlocked by the pallets ,p ,p#a projecting from a lever which moves on a pivot 7,fig4 #bje74 ,the end of the lever is forked1 and has a square notch in it4 ,on the arbor of the balance-wheel is a roller1 or plate1 ,r1 which carries a small pin1 ,i4 ,two pins1 ,b ,b1 projecting from the plate of the watch prevent the lever moving too far4 ,we must further notice the little pin ,c on the lever1 and a notch in the edge of the roller4 ,illustration3 ,,fig4 #bje4--,lever watch escapement4 ,in the illustration a tooth has just passed under the impulse face b of ,p#a4 ,the lever has been moved upwards at the right end2 and its forked end has given an impulse to ,r1 and through it to the balance-wheel4 ,the spring winds up4 ,the pin ,c prevents the lever dropping1 because it no longer has the notch opposite to it1 but presses on the circumference of ,r4 ,as the spring unwinds it strikes the lever at the moment when the notch and ,c are opposite4 ,the lever is knocked downwards1 and the tooth1 which had been arrested by the locking-face a of pallet ,p1 now presses on the impulse face b1 forcing the left end of the lever up4 ,the impulse pin ,i receives a blow1 assisting the unwinding of the spring1 and ,c again locks the lever4 ,the same thing is repeated in alternate directions over and over again4 ,,compensating ,,balance-,,wheels4 ,the watchmaker has had to overcome the same difficulty as the clockmaker with regard to the expansion of the metal in the controlling agent4 ,when a metal wheel is heated its spokes lengthen1 and the rim recedes from the centre4 ,now1 let us suppose that we have two rods of equal weight1 one three feet long1 the other six feet long4 ,to an end of each we fasten a #b-lb4 weight4 ,we shall find it much easier to wave the shorter rod backwards and forwards quickly than the other4 ,why8 ,because the weight of the longer rod has more leverage over the hand than has that of the shorter rod4 ,similarly1 if1 while the mass of the rim of a wheel remains constant1 the length of the spokes varies1 the effort needed to rotate the wheel to and fro at a constant rate must vary also4 ,graham got over the difficulty with a rod by means of the compensating pendulum4 ,thomas ,earnshaw mastered it in wheels by means of the compensating balance1 using the same principle--namely1 the unequal expansion of different metals4 ,any one who owns a compensated watch will see1 on stopping the tiny fly-wheel1 that it has two spokes 7,fig4 #bjf71 each carrying an almost complete semicircle of rim attached to it4 ,a close examination shows that the rim is compounded of an outer strip of brass welded to an inner lining of steel4 ,the brass element expands more with heat and contracts more with cold than steel2 so that when the spokes become elongated by a rise of temperature1 the pieces bend inwards at their free ends 7,fig4 #bjg72 if the temperature falls1 the spokes are shortened1 and the rim pieces bend outwards 7,fig4 #bjh74ci ,this ingenious contrivance keeps the leverage of the rim constant within very fine limits4 ,the screws ,s ,s are inserted in the rim to balance it correctly1 and very fine adjustment is made by means of the four tiny weights ,w ,w4 ,in ships' chronometers1#dj the rim pieces are sub-compensated towards their free ends to counteract slight errors in the primary compensation4 ,so delicate is the compensation that a daily loss or gain of only half a second is often the limit of error4 ,illustration3 ,,fig4 #bjf4 ,,fig4 #bjg4 ,,fig4 #bjh4 ,a compensating watch balance1 at normal1 super-normal1 and sub-normal temperatures4 ,,keyless ,,winding ,,mechanism ,,for ,,watches4 ,the inconvenience attaching to a key-wound watch caused the ,swiss manufacturers to put on the market1 in #ahea1 watches which dispensed with a separate key4 ,those of our readers who carry keyless watches will be interested to learn how the winding and setting of the hands is effected by the little serrated knob enclosed inside the pendant ring4 ,there are two forms of going-barrel keyless mechanism--7#a7 ,the rocking bar2 7#b7 the shifting sleeve4 ,the rocking bar device is shown in ,figs4 #bji1 #baj4 ,the milled head ,m turns a cog1 ,g1 which is always in gear with a cog1 ,f4 ,this cog gears with two others1 ,a and ,b1 mounted at each end of the rocker ,r1 which moves on pivot ,s4 ,a spring1 ,s ,p1 attached to the watch plate presses against a small stud on the rocking bar1 and keeps ,a normally in gear with ,c1 mounted on the arbor of the mainspring4 ,illustration3 ,,fig4 #bji4--,the winding mechanism of a keyless watch4 ,to wind the watch1 ,m is turned so as to give ,f an anti-clockwise motion4 ,the teeth of ,f now press ,a downwards and keep it in gear with ,c while the winding is done4 ,a spring click 7marked solid black7 prevents the spring uncoiling 7,fig4 #bji74 ,if ,f is turned in a clockwise direction it lifts ,a and prevents it biting the teeth of ,c1 and no strain is thrown on ,c4 ,to set the hands1 the little push-piece ,p is pressed inwards by the thumb 7,fig4 #baj7 so as to depress the right-hand end of ,r and bring ,b into gear with ,d1 which in turn moves ,e1 mounted on the end of the minute-hand shaft4 ,the hands can now be moved in either direction by turning ,m4 ,on releasing the push-piece the winding-wheels engage again4 ,the shifting sleeve mechanism has a bevel pinion in the place of ,g 7,fig4 #bji7 gearing with the mainspring cog4 ,the shaft of the knob ,m is round where it passes through the bevel and can turn freely inside it1 but is square below4 ,on the square part is mounted a little sliding clutch with teeth on the top corresponding with the other teeth on the under side of the bevel-wheel1 and teeth similar to those of ,g 7,fig4 #bji7 at the end4 ,the clutch has a groove cut in the circumference1 and in this lies the end of a spring lever which can be depressed by the push-piece4 ,the mechanism much resembles on a small scale the motor car changing gear 7,fig4 #di74 ,normally1 the clutch is pushed up the square part of the knob shaft by the spring so as to engage with the bevel and the winding-wheels4 ,on depressing the clutch by means of the push-piece it gears with the minute-hand pinion1 and lets go of the bevel4 ,illustration3 ,,fig4 #baj4--,the hand-setting mechanism in action4 ,in one form of this mechanism the push-piece is dispensed with1 and the minute-wheel pinion is engaged by pulling the knob upwards4 ,,the ,,hour-,,hand ,,train4 ,illustration3 ,,fig4 #baa4--,the hour-hand train of a clock4 ,the teeth of the mainspring drum gear with a cog on the minute-hand shaft1 which also carries one of the cogs of the escapement train4 ,the shaft is permitted by the escapement to revolve once an hour4 ,fig4 #baa shows diagrammatically how this is managed4 ,the hour-hand shaft ,a 7solid black7 can be moved round inside the cog ,b1 driven by the mainspring drum4 ,it carries a cog1 ,c4 ,this gears with a cog1 ,d1 having three times as many teeth4 ,the cog ,e1 united to ,d1 drives cog ,f1 having four times as many teeth as ,e4 ,to ,f is attached the collar ,g of the hour-hand4 ,f and ,g revolve outside the minute-hand shaft4 ,on turning ,a1 ,c turns ,d and ,e1 ,e turns ,f and the hour-hand1 which revolves #a_/c of #a_/d #a_/ab as fast as ,a4#da 99 99 99 99 99 ,,locks4 ,on these unfortunately necessary mechanisms a great deal of ingenuity has been expended4 ,with the advance of luxury and the increased worship of wealth1 it becomes more and more necessary to guard one's belongings against the less scrupulous members of society4 ,illustration3 ,,fig4 #bab4 ,the simplest form of lock1 such as is found in desks and very cheap articles1 works on the principle shown in ,fig4 #bab4 ,the bolt is split at the rear1 and the upper part bent upwards to form a spring4 ,the under edge has two notches cut in it1 separated by a curved excrescence4 ,the key merely presses the bolt upwards against the spring1 until the notch1 engaging with the frame1 moves it backwards or forwards until the spring drives the tail down into the other notch4 ,this primitive device affords1 of course1 very little security4 ,an advance is seen in the ,,tumbler ,,lock4 ,illustration3 ,,fig4 #bac4 ,the bolt now can move only in a horizontal direction4 ,it has an opening cut in it with two notches 7,figs4 #bac1 #bad74 ,behind the bolt lies the tumbler ,t 7indicated by the dotted line71 pivoted at the angle on a pin4 ,from the face of the tumbler a stud1 ,s1 projects through the hole in the bolt4 ,this stud is forced into one or other of the notches by the spring1 ,s#a1 which presses on the tail of the tumbler4 ,illustration3 ,,fig4 #bad4 ,in ,fig4 #bac the key is about to actuate the locking mechanism4 ,the next diagram 7,fig4 #bad7 shows how the key1 as it enters the notch on the lower side of the bolt to move it along1 also raises the tumbler stud clear of the projection between the two notches4 ,by the time that the bolt has been fully shot1 the key leaves the under notch and allows the tumbler stud to fall into the rear locking-notch4 ,a lock of this type also can be picked very easily1 as the picker has merely to lift the tumbler and move the bolt along4 ,barron's lock1 patented in #aggh1 had two tumblers and two studs2 and the opening in the bolt had notches at the top as well as at the bottom 7,fig4 #bae74 ,this made it necessary for both tumblers to be raised simultaneously to exactly the right height4 ,if either was not lifted sufficiently1 a stud could not clear its bottom notch2 if either rose too far1 it engaged an upper notch4 ,the chances therefore were greatly against a wrong key turning the lock4 ,illustration3 ,,fig4 #bae4--,the bolt of a ,barron lock4 ,,the ,,chubb ,,lock is an amplification of this principle4 ,it usually has several tumblers of the shape shown in ,fig4 #baf4 ,the lock stud in these locks projects from the bolt itself1 and the openings1 or gates1 through which the stud must pass as the lock moves1 are cut in the tumblers4 ,it will be noticed that the forward notch of the tumbler has square serrations in the edges4 ,these engage with similar serrations in the bolt stud and make it impossible to raise the tumbler if the bolt begins to move too soon when a wrong key is inserted4 ,illustration3 ,,fig4 #baf4--,tumbler of ,chubb lock4 ,fig4 #bag is a ,chubb key with eight steps4 ,that nearest the head 7#h7 operates a circular revolving curtain1 which prevents the introduction of picking tools when a key is inserted and partly turned1 as the key slot in the curtain is no longer opposite that in the lock4 ,step #a moves the bolt4 ,illustration3 ,,fig4 #bag4--;,a ,chubb key4 ,in order to shoot the bolt the height of the key steps must be so proportioned to the depth of their tumblers that all the gates in the tumblers are simultaneously raised to the right level for the stud to pass through them1 as in ,fig4 #bah4 ,here you will observe that the tumbler ,d on the extreme right 7lifted by step #b of the key7 has a stud1 ,d ,s1 projecting from it over the other tumblers4 ,this is called the detector tumbler4 ,if a false key or picking tool is inserted it is certain to raise one of the tumblers too far4 ,the detector is then over-lifted by the stud ,d ,s1 and a spring catch falls into a notch at the rear4 ,it is now impossible to pick the lock1 as the detector can be released only by the right key shooting the bolt a little further in the locking direction1 when a projection on the rear of the bolt lifts the catch and allows the tumbler to fall4 ,the detector also shows that the lock has been tampered with1 since even the right key cannot move the bolt until the overlocking has been performed4 ,illustration3 ,,fig4 #bah4--;,a ,chubb key raising all the tumblers to the correct height4 ,each tumbler step of a large ,chubb key can be given one of thirty different heights2 the bolt step one of twenty4 ,by merely transposing the order of the steps in a six-step key it is possible to get #gbj different combinations4 ,by diminishing or increasing the heights the possible combinations may be raised to the enormous total of #g1ggf1jjj6 ,illustration3 ,,fig4 #bai4--,section of a ,yale lock4 ,,the ,,yale ,,lock1 which comes from ,america1 works on a quite different system4 ,its most noticeable feature is that it permits the use of a very small key1 though the number of combinations possible is still enormous 7several millions74 ,in our illustrations 7,figs4 #bai1 #bbj1 #bba7 we show the mechanism controlling the turning of the key4 ,the keyhole is a narrow twisted slot in the face of a cylinder1 ,g 7,fig4 #bai71 which revolves inside a larger fixed cylinder1 ,f4 ,as the key is pushed in1 the notches in its upper edge raise up the pins ,a#a1 ,b#a1 ,c#a1 ,d#a1 ,e#a1 until their tops exactly reach the surface of ,g1 which can now be revolved by the key in ,fig4 #bbj1 and work the bolt through the medium of the arm ,h4 7,the bolt itself is not shown47 ,if a wrong key is inserted1 either some of the lower pins will project upwards into the fixed cylinder ,f 7see ,fig4 #bba71 or some of the pins in ,f will sink into ,g4 ,it is then impossible to turn the key4 ,illustration3 ,,fig4 #bbj4--,yale key turning4 ,there are other well-known locks1 such as those invented by ,bramah and ,hobbs4 ,but as these do not lend themselves readily to illustration no detailed account can be given4 ,we might1 however1 notice the time lock1 which is set to a certain hour1 and can be opened by the right key or a number of keys in combination only when that hour is reached4 ,another very interesting device is the automatic combination lock4 ,this may have twenty or more keys1 any one of which can lock it2 but the same one must be used to unlock it1 as the key automatically sets the mechanism in favour of itself4 ,with such a lock it would be possible to have a different key for every day in the month2 and if any one key got into wrong hands it would be useless unless it happened to be the one which last locked the lock4 ,illustration3 ,,fig4 #bba4--,the wrong key inserted4 ,the pins do not allow the lock to be turned4 99 99 99 99 99 ,,the ,,cycle4 ,there are a few features of this useful and in some ways wonderful contrivance which should be noticed4 ,first1 ,,the ,,gearing ,,of ,a ,,cycle4 ,to a good many people the expression geared to #gj inches1 or #fe1 or #hj1 as the case may be1 conveys nothing except the fact that the higher the gear the faster one ought to be able to travel4 ,let us therefore examine the meaning of such a phrase before going farther4 ,the safety cycle is always geared up--that is1 one turn of the pedals will turn the rear wheel more than once4 ,to get the exact ratio of turning speed we count the teeth on the big chain-wheel1 and the teeth on the small chain-wheel attached to the hub of the rear wheel1 and divide the former by the latter4 ,to take an example3--,the teeth are #ge and #cj in number respectively2 the ratio of speed therefore #ge_/cj #e_/b #b-a_/b4 ,one turn of the pedal turns the rear wheel #b-a_/b times4 ,the gear of the cycle is calculated by multiplying this result by the diameter of the rear wheel in inches4 ,thus a #bh-;inch wheel would in this case give a gear of #b-a_/b #bh #gj inches4 ,one turn of the pedals on a machine of this gear would propel the rider as far as if he were on a high ordinary with the pedals attached directly to a wheel #gj inches in diameter4 ,the gearing is raised or lowered by altering the number ratio of the teeth on the two chain-wheels4 ,if for the #cj-tooth wheel we substituted one of #be teeth the gearing would be-- #ge_/be #bh inches #hd inches4 ,a handy formula to remember is1 gearing ,t_/t ,d1 where ,t teeth on large chain-wheel2 t teeth on small chain-wheel2 and ,d diameter of driving-wheel in inches4 ,two of the most important improvements recently added to the cycle are--7#a7 ,the free wheel2 7#b7 the change-speed gear4 ,,the ,,free ,,wheel is a device for enabling the driving-wheel to overrun the pedals when the rider ceases pedalling2 it renders the driving-wheel free of the driving gear4 ,it is a ratchet specially suited for this kind of work4 ,from among the many patterns now marketed we select the ,micrometer free-wheel hub 7,fig4 #bbb71 which is extremely simple4 ,the ratchet-wheel ,r is attached to the hub of the driving-wheel4 ,the small chain-wheel 7or chain-ring1 as it is often called7 turns outside this1 on a number of balls running in a groove chased in the neck of the ratchet4 ,between these two parts are the pawls1 of half-moon shape4 ,the driving-wheel is assumed to be on the further side of the ratchet4 ,to propel the cycle the chain-ring is turned in a clockwise direction4 ,three out of the six pawls at once engage with notches in the ratchet1 and are held tightly in place by the pressure of the chain-ring on their rear ends4 ,the other three are in a midway position4 ,illustration3 ,,fig4 #bbb4 ,when the rider ceases to pedal1 the chain-ring becomes stationary1 but the ratchet continues to revolve4 ,the pawls offer no resistance to the ratchet teeth1 which push them up into the semicircular recesses in the chain-ring4 ,each one rises as it passes over a tooth4 ,it is obvious that driving power cannot be transmitted again to the road wheel until the chain-wheel is turned fast enough to overtake the ratchet4 ,,the ,,change-,,speed ,,gear4 ,a gain in speed means a loss in power1 and vice vers4 ,by gearing-up a cycle we are able to make the driving-wheel revolve faster than the pedals1 but at the expense of control over the driving-wheel4 ,a high-geared cycle is fast on the level1 but a bad hill-climber4 ,the low-geared machine shows to disadvantage on the flat1 but is a good hill-climber4 ,similarly1 the express engine must have large driving-wheels1 the goods engine small driving-wheels1 to perform their special functions properly4 ,in order to travel fast over level country1 and yet be able to mount hills without undue exertion1 we must be able to do what the motorist does--change gear4 ,two-speed and three-speed gears are now very commonly fitted to cycles4 ,they all work on the same principle1 that of the epicyclic train of cog-wheels1 the mechanisms being so devised that the hub turns more slowly than1 at the same speed as1 or faster than the small chain-wheel1#db according to the wish of the rider4 ,we do not propose to do more here than explain the principle of the epicyclic train1 which means a wheel on 7or running round7 a wheel4 ,lay a footrule on the table and roll a cylinder along it by the aid of a second rule1 parallel to the first1 but resting on the cylinder4 ,it will be found that1 while the cylinder advances six inches1 the upper rule advances twice that distance4 ,in the absence of friction the work done by the agent moving the upper rule is equal to that done in overcoming the force which opposes the forward motion of the cylinder2 and as the distance through which the cylinder advances is only half that through which the upper rule advances1 it follows that the force which must act on the upper rule is only half as great as that overcome in moving the cylinder4 ,the carter makes use of this principle when he puts his hand to the top of a wheel to help his cart over an obstacle4 ,illustration3 ,,fig4 #bbc4 ,illustration3 ,,fig4 #bbd4 ,illustration3 ,,fig4 #bbe4 ,now see how this principle is applied to the change-speed gear4 ,the lower rule is replaced by a cog-wheel1 ,c 7,fig4 #bbc72 the cylinder by a cog1 ,b1 running round it2 and the upper rule by a ring1 ,a1 with internal teeth4 ,we may suppose that ,a is the chain-ring1 ,b a cog mounted on a pin projecting from the hub1 and ,c a cog attached to the fixed axle4 ,it is evident that ,b will not move so fast round ,c as ,a does4 ,the amount by which ,a will get ahead of ,b can be calculated easily4 ,we begin with the wheels in the position shown in ,fig4 #bbc4 ,a point1 ,i1 on ,a is exactly over the topmost point of ,c4 ,for the sake of convenience we will first assume that instead of ,b running round ,c1 ,b is revolved on its axis for one complete revolution in a clockwise direction1 and that ,a and ,c move as in ,fig4 #bbd4 ,if ,b has #aj teeth1 ,c #cj1 and ,a #dj1 ,a will have been moved #aj_/dj #a_/d of a revolution in a clockwise direction1 and ,c #aj_/cj #a_/c of a revolution in an anti-clockwise direction4 ,now1 coming back to what actually does happen1 we shall be able to understand how far ,a rotates round ,c relatively to the motion of ,b1 when ,c is fixed and ,b rolls 7,fig4 #bbe74 ,b advances #a_/c of distance round ,c2 ,a advances #a_/c #a_/d #g_/ab of distance round ,b4 ,the fractions1 if reduced to a common denominator1 are as #d3g1 and this is equivalent to #dj 7number of teeth on ,a73 #dj #cj 7teeth on ,a teeth on ,c47 ,to leave the reader with a very clear idea we will summarize the matter thus3--,if ,t number of teeth on ,a1 t number of teeth on ,c1 then movement of ,a3 movement of ,b33 ,t t3 ,t4 ,here is a two-speed hub4 ,let us count the teeth4 ,the chain-ring 7 ,a7 has #fd internal teeth1 and the central cog 7 ,c7 on the axle has #af teeth4 ,there are four cogs 7 ,b7 equally spaced1 running on pins projecting from the hub-shell between ,a and ,c4 ,how much faster than ,b does ,a run round ,c8 ,apply the formula3--,motion of ,a3 motion of ,b33 #fd #af3 #fd4 ,that is1 while ,a revolves once1 ,b and the hub and the driving-wheel will revolve only #fd_/hj #d_/e of a turn4 ,to use scientific language1 ,b revolves #bj per cent4 slower than ,a4 ,this is the gearing we use for hill-climbing4 ,on the level we want the driving-wheel to turn as fast as1 or faster than1 the chain-ring4 ,to make it turn at the same rate1 both ,a and ,c must revolve together4 ,in one well-known gear this is effected by sliding ,c along the spindle of the wheel till it disengages itself from the spindle1 and one end locks with the plate which carries ,a4 ,since ,b is now being pulled round at the bottom as well as the top1 it cannot rotate on its own axis any longer1 and the whole train revolves solidly--that is1 while ,a turns through a circle ,b does the same4 ,to get an increase of gearing1 matters must be so arranged that the drive is transmitted from the chain-wheel to ,b1 and from ,a to the hub4 ,while ,b describes a circle1 ,a and the driving-wheel turn through a circle and a part of a circle--that is1 the driving-wheel revolves faster than the hub4 ,given the same number of teeth as before1 the proportional rates will be ,a #hj1 ,b #fd1 so that the gear rises #be per cent4 ,by means of proper mechanism the power is transmitted in a three-speed gear either 7#a7 from chain-wheel to ,a1 ,a to ,b1 ,b to wheel low gear2 or 7#b7 from chain-wheel to ,a and ,c simultaneously solid1 normal1 or middle gear2 or 7#c7 from chain-wheel to ,b1 ,b to ,a1 ,a to wheel high gear4 ,in two-speed gears either #a or #c is omitted4 99 99 99 99 99 ,,agricultural ,,machines4 ,,the ,,threshing-,,machine4 ,bread would not be so cheap as it is were the flail still the only means of separating the grain from the straw4 ,what the cream separator has done for the dairy industry 7p4 #chd71 the threshing-machine has done for agriculture4 ,a page or two ought therefore to be spared for this useful invention4 ,illustration3 ,,fig4 #bbf4--,section of a threshing machine4 ,in ,fig4 #bbf a very complete fore-and-aft section of the machine is given4 ,after the bands of the sheaves have been cut1 the latter are fed into the mouth of the drum ,a by the feeder1 who stands in the feeding-box on the top of the machine4 ,the drum revolves at a very high velocity1 and is fitted with fluted beaters which act against a steel concave1 or breastwork1 ,b1 the grain being threshed out of the straw in passing between the two4 ,the breastwork is provided with open wires1 through which most of the threshed grain1 cavings 7short straws71 and chaff passes on to a sloping board4 ,the straw is flung forward on to the shakers ,c1 which gradually move the straw towards the open end and throw it off4 ,any grain1 etc41 that has escaped the drum falls through the shakers on to ,d1 and works backwards to the caving riddles1 or moving sieves1 ,e4 ,the main blower1 by means of a revolving fan1 ,n1 sends air along the channel ,x upwards through these riddles1 blowing the short straws away to the left4 ,the grain1 husks1 and dust fall through ,e on to ,g1 over the end of which they fall on to the chaff riddle1 ,h4 ,a second column of air from the blower drives the chaff away4 ,the heavy grain1 seeds1 dust1 etc41 fall on to ,i1 ,j1 and ,k in turn1 and are shaken until only the grain remains to pass along ,l to the elevator bottom1 ,m4 ,an endless band with cups attached to it scoops up the grain1 carries it aloft1 and shoots it into hopper ,p4 ,it then goes through the shakers ,q1 ,r1 is dusted by the back end blower1 ,s1 and slides down ,t into the open end of the rotary screen-drum ,u1 which is mounted on the slope1 so that as it turns the grain travels gradually along it4 ,the first half of the screen has wires set closely together4 ,all the small grain that falls through this1 called thirds1 passes into a hopper1 and is collected in a sack attached to the hopper mouth4 ,the seconds fall through the second half of the drum1 more widely spaced1 into their sack2 and the firsts fall out of the end and through a third spout4 ,,mowing-,,machines4 ,illustration3 ,,fig4 #bbg4 ,the ordinary lawn--mower employs a revolving reel1 built up of spirally-arranged knives1 the edges of which pass very close to a sharp plate projecting from the frame of the mower4 ,each blade1 as it turns1 works along the plate1 giving a shearing cut to any grass that may be caught between the two cutting edges4 ,the action is that of a pair of scissors 7,fig4 #bbg71 one blade representing the fixed1 the other the moving knife4 ,if you place a cylinder of wood in the scissors it will be driven forward by the closing of the blades1 and be marked by them as it passes along the edges4 ,the same thing happens with grass1 which is so soft that it is cut right through4 ,,hay-,,cutter4 ,the hay-cutter is another adaptation of the same principle4 ,a cutter-bar is pulled rapidly backwards and forwards in a frame which runs a few inches above the ground by a crank driven by the wheels through gearing4 ,to the front edge of the bar are attached by one side a number of triangular knives4 ,the frame carries an equal number of spikes pointing forward horizontally4 ,through slots in these the cutter-bar works1 and its knives give a drawing cut to grass caught between them and the sides of the spikes4 99 99 99 99 99 ,,some ,,natural ,,phenomena4 ,,why ,,sun-,,heat ,,varies ,,in ,,intensity4 ,the more squarely parallel heat-rays strike a surface the greater will be the number that can affect that surface4 ,this is evident from ,figs4 #bbh1 #bbi1 where ,a ,b is an equal distance in both cases4 ,the nearer the sun is to the horizon1 the more obliquely do its rays strike the earth4 ,hence midday is necessarily warmer than the evening1 and the tropics1 where the sun stands overhead1 are hotter than the temperate zones1 where1 even in summer at midday1 the rays fall more or less on the slant4 ,illustration3 ,,fig4 #bbh4 ,illustration3 ,,fig4 #bbi4 ,the atmospheric envelope which encompasses the earth tends to increase the effect of obliquity1 since a slanting ray has to travel further through it and is robbed of more heat than a vertical ray4 ,,the ,,tides4 ,all bodies have an attraction for one another4 ,the earth attracts the moon1 and the moon attracts the earth4 ,now1 though the effect of this attraction is not visible as regards the solid part of the globe1 it is strongly manifested by the water which covers a large portion of the earth's surface4 ,the moon attracts the water most powerfully at two points1 that nearest to it and that furthest away from it2 as shown on an exaggerated scale in ,fig4 #bcj4 ,since the earth and the water revolve as one mass daily on their axis1 every point on the circumference would be daily nearest to and furthest from the moon at regular intervals1 and wherever there is ocean there would be two tides in that period1 were the moon stationary as regards the earth4 7,it should be clearly understood that the tides are not great currents1 but mere thickenings of the watery envelope4 ,the inrush of the tide is due to the temporary rise of level47 ,illustration3 ,,fig4 #bcj4 ,illustration3 ,,fig4 #bca4 ,,why ,,high ,,tide ,,varies ,,daily4 ,the moon travels round the earth once in twenty-eight days4 ,in ,fig4 #bca the point a is nearest the moon at1 say1 twelve noon4 ,at the end of twenty-four hours it will have arrived at the same position by the compass1 but yet not be nearest to the moon1 which has in that period moved on #a_/bhth of a revolution round the earth4#dc ,consequently high tide will not occur till a has reached position b and overtaken the moon1 as it were1 which takes about an hour on the average4 ,this explains why high tide occurs at intervals of more than twelve hours4 ,illustration3 ,,fig4 #bcb4--,relative positions of sun1 moon1 and earth at spring tides4 ,illustration3 ,,fig4 #bcc4--,relative positions of sun1 moon1 and earth at neap tides4 ,,neap ,,tides ,,and ,,spring ,,tides4 ,the sun1 as well as the moon1 attracts the ocean1 but with less power1 owing to its being so much further away4 ,at certain periods of the month1 sun1 earth1 and moon are all in line4 ,sun and moon then pull together1 and we get the highest1 or spring tides 7,fig4 #bcb74 ,when sun and moon pull at right angles to one another--namely1 at the first and third quarters--the excrescence caused by the moon is flattened 7,fig4 #bcc71 and we get the lowest1 or neap tides4 #ci ,in both ,figs4 #bjg and #bjh the degree of expansion is very greatly exaggerated4 #dj ,as the sun passes the meridian 7twelve o'clock1 noon7 the chronometer's reading is taken1 and the longitude1 or distance east or west of ,greenwich1 is reckoned by the difference in time between local noon and that of the chronometer4 #da ,for much of the information given here about clocks and watches the author is indebted to ,the ,history of ,watches1 by ,mr4 ,j4,f4 ,kendal4 #db ,we shall here notice only those gears which are included in the hub of the driving-wheel4 #dc ,the original position of the moon is indicated by the dotted circle4 ,,index4 ,,note4--,figures in italics signify that an illustration of the thing referred to appears on the page4 ,aberration1 spherical1 of lens1 #bdc4 ,acoustics1 #bid4 ,achromatic lens1 #bdc4 ,action carriage of piano1 #bhc4 ,advancing the spark1 #ajb4 ,air-gun1 #cdb4 ,air-pump for cycle tyres1 #cdj2 for ,westinghouse brake1 #aii4 ,alternating currents1 #afd2 dynamo1 #afd4 ,amperage1 #abe4 ,angle of advance1 #eg1 #eh2 incidence1 #bfh2 reflection1 #bfh4 ,aorta1 #cfj4 ,arc lamp1 #ahb4 ,archimedes1 #dab4 ,armature1 #afb4 ,arteries1 #ceh4 ,arterial blood1 #cei4 ,atmospheric pressure1 #cej4 ,auditory nerve1 #bgb4 ,automatic brakes1 #ahh2 signalling1 #bbh2 stoker1 #cii4 ,backfall1 #bih4 ,balance-wheel1 #dai4 ,ball cock1 #cff1 #cfg4 ,balloon1 fire1 #cbc2 gas1 #cdg4 ,barometer1 aneroid1 #cbh1 #cbi2 and weather1 #cca2 ,fortin's1 #cbf2 meaning of1 #cbe2 simple1 #cbh2 wheel1 #cbg4 ,beau de ,rochas1 #hi4 ,bell1 diving1 #ccb2 electric1 #aai1 #abj4 ,bellows of organ1 #cjc4 ,bioscope1 #bff4 ,blades1 turbine1 #ha1 #hc4 ,block system1 #bja1 #bab4 ,blood1 arterial1 #cei2 circulation of1 #cef1 #ceg1 #cfj2 venous1 #cei4 ,blower-plate1 #cic1 #cid4 ,boat1 sails of1 #cdf4 ,boiler1 ,babcock and ,wilcox1 #ba1 #bb2 explosions1 #cd1 #cia2 fire-tube1 #ba2 fittings1 #ca2 ,lancashire1 #be1 #bf2 locomotive1 #bj1 #bc2 multitubular1 #ba2 principle of1 #ae2 stored energy in1 #cb2 vertical1 #be2 water supply to1 #ci2 water-tube1 #ba4 ,brakes1 hydraulic1 #ahh2 motor car1 #aaj2 railway1 #ahg2 vacuum1 #ahi1 #aij1 #aia2 ,westinghouse1 #aid1 #aie1 #aig4 ,bramah1 #cfc1 #dcg4 ,breezes1 land and sea1 #cbd4 ,brushes of dynamo1 #afa1 #agb4 ,bunsen burner1 #dji4 ,burning-glass1 #bcb4 ,camera1 the1 #bcc2 pinhole1 #bcd1 #bce4 ,canals1 semicircular1 #bgc4 ,capillary attraction1 #cib2 veins1 #ceh4 ,carbon dioxide1 #bg1 #cei2 monoxide1 #bg4 ,carburetter1 #ih1 #ii4 ,cardan shaft1 #ic4 ,carmania1 the1 #hc4 ,centrifugal force1 #chb4 ,change-speed gear1 #aje1 #ddb4 ,chassis of motor car1 #ib4 ,circulation of water in a boiler1 #ag1 #ah1 #ai2 of water in a motor car1 #ie1 #ig4 ,clarionet1 #cjh4 ,clock1 first weight-driven1 #dab2 water1 #daj4 ,clutch of motor car1 #aje4 ,coal1 as fuel1 #ae2 gas1 #cid2 gas making1 #cid2 gas plant1 #cif2 gas1 purification of1 #cig4 ,cochlea1 #bgc4 ,coherer1 #adj4 ,coil1 ,ruhmkorff1 #aba4 ,coke1 #cie4 ,combinations in ,chubb lock1 #dcf2 ,yale lock1 #dcf4 ,combustion1 #bf1 #cic2 perfect1 #bh4 ,compensating gear1 #ajg1 #ajh4 ,compound engines1 #ei2 arrangement of1 #fa2 invention of1 #ei4 ,compound locomotives1 #fb4 ,compound microscope1 #bfa4 ,condenser1 marine1 #ga1 #gb2 of ,ruhmkorff coil1 #abc4 ,conduit1 #agf4 ,convex lens1 image cast by1 #bcf4 ,conjugate foci1 #bfb4 ,cornet1 #cjh4 ,corti1 rods of1 #bgd4 ,coxwell1 #cdh4 ,cream separator1 #cha1 #chc4 ,current1 reversal of electric1 #acj1 #aca2 transformation of1 #abd4 ,cushioning of steam1 #ee4 ,cycle1 gearing of1 #dci4 ,cylinder1 hydraulic press1 #cfc2 steam1 #di4 ,danes1 #chb4 ,dead point1 #dg4 ,de ,brouwer stoker1 #dja4 ,detector in ,chubb lock1 #dce4 ,diving-bell1 #ccb2 simple1 #ccc1 #ccd4 ,diving-dress1 #cce4 ,direction of current in dynamo circuit1 #afc4 ,diver's feats1 #cch2 helmet1 #ccf2 lamp1 #cch4 ,donkey-engines1 #fh4 ,doorstop1 self-closing1 #cdd4 ,double-cylinder engines1 #dg4 ,draught1 forced1 #bh1 #bi2 induced1 #bi4 ,drum and fusee1 #dad4 ,durability of motor-car engine1 #if4 ,d-valve1 #fg4 ,dynamo1 alternating1 #afd1 #agd2 brushes1 #agb2 compound1 #agd2 continuous-current1 #afe2 multipolar1 #afi2 series wound1 #agc2 shunt wound1 #agc2 simple1 #afa1 #afb4 ,ear1 the1 #bga1 #bgc2 a good1 #bgd1 #cjg2 sensitiveness of1 #bge4 ,eccentric1 #eb1 #ec2 setting of1 #ec4 ,edison1 ,thomas1 #caj4 ,edison-,bell phonograph1 #caj4 ,electricity1 current1 #aae2 forms of1 #aac2 nature of1 #aab2 static1 #aad4 ,electric bell1 #aai1 #abj2 signalling1 #bbe2 slot1 #bbf4 ,electroplating1 #ahe1 #ahf4 ,electro-magnets1 #aag4 ,endolymph1 #bgb4 ,engines1 compound1 #ei2 donkey1 #fh2 double-cylinder1 #dg2 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