,transcriber's note3 ,for this text version passages in italics are indicated by underscores4 ,small caps have been replaced by ,,all ,,caps and subscripts are denoted as #b4 99 99 99 99 99 ,illustration3 cover ,illustration3 ,by permission of ,messrs4 ,chance ,bros1 and ,co41 ,ltd4 ,a ,,huge ,,lamp ,the marvellous arrangement of lenses and prisms which enables the lighthouse to send out its guiding flashes1 with the mechanism for turning it4 ,made for ,chilang ,lighthouse1 ,china ,frontispiece ,,marvels ,,of ,,scientific ,,invention ,,an ,,interesting ,,account ,,in ,,non-,,technical ,,language ,,of ,,the ,,invention ,,of ,,guns1 ,,torpedoes1 ,,submarines ,,mines1 ,,up-,,to-,,date ,,smelting1 ,,freezing1 ,,colour ,,photography1 ,,and ,,many ,,other ,,recent ,,discoveries ,,of ,,science ,,by ,,thomas ,w4 ,,corbin ,,author ,,of ,,engineering ,,of ,,to-,,day1 ,,mechanical ,,inventions ,,of ,,to-,,day1 ,,the ,,romance ,,of ,,submarine ,,engineering1 c41 c4 ,,with #cb ,,illustrations ,,diagrams ,,philadelphia ,j4 ,b4 ,,lippincott ,,company ,,london3 ,,seeley1 ,,service ,,co4 ,,ltd4 #aiag ,,contents ,,chapter ,,page ,i4 ,,digging ,,with ,,dynamite #i ,,ii4 ,,measuring ,,electricity #bb ,,iii4 ,,the ,,fuel ,,of ,,the ,,future #db ,,iv4 ,,some ,,valuable ,,electrical ,,processes #ee ,v4 ,,machine-,,made ,,cold #fg ,,vi4 ,,scientific ,,inventions ,,at ,,sea #gh ,,vii4 ,,the ,,gyro-,,compass #ij ,,viii4 ,,torpedoes ,,and ,,submarine ,,mines #ih ,,ix4 ,,gold ,,recovery #aji ,x4 ,,intense ,,heat #abc ,,xi4 ,,an ,,artificial ,,coal ,,mine #acg ,,xii4 ,,the ,,most ,,striking ,,invention ,,of ,,recent ,,times #adi ,,xiii4 ,,how ,,pictures ,,can ,,be ,,sent ,,by ,,wire #agf ,,xiv4 ,a ,,wonderful ,,example ,,of ,,science ,,and ,,skill #aia ,,xv4 ,,scientific ,,testing ,,and ,,measuring #aih ,,xvi4 ,,colour ,,photography #bab ,,xvii4 ,,how ,,science ,,aids ,,the ,,stricken ,,collier #bbj ,,xviii4 ,,how ,,science ,,helps ,,to ,,keep ,,us ,,well #bca ,,xix4 ,,modern ,,artillery #bcf ,,appendix #bde ,,index #bdg ,,list ,,of ,,illustrations ,a ,huge ,lamp ,frontispiece ,,facing ,,page ,first ,effect of the ,dynamite #af ,a ,fine ,crop #bd ,apple-tree planted by ,spade #dh ,machine-made ,ice #gb ,a ,cold ,store #hj ,dassen ,island ,lighthouse #hh ,measuring ,heat #abh ,the ,telewriter #ahd ,a ,miners' ,rescue ,team #bjh ,pneumatic ,hammer ,drill #baf ,an ,artificial ,coal ,mine #bbd ,sectional view of a #fj-pounder ,gun #bcb ,rifles of different ,nations #bdj ,,diagrams ,,fig4 ,,page #a4 ,principle of ,galvanometer #cj #b4 ,string ,galvanometer #ca #c4 ,duddell ,thermo-,galvanometer #ci #d4 ,construction of a ,voltmeter #fd #e4 ,the ,working of a ,refrigerating ,machine #gj #f4 ,hertz's ,machine #aee #g4 ,hertz ,detector #aef #h4 #i4 #aj4 ,wireless ,waves #aeh #aa4 ,a ,wireless ,antenna #afd #ab4 ,poulsen's ,machine #aff #ac4 #ad4 ,how ,pictures are sent by ,wire #agg #ae4 ,message received by ,telewriter #ahi ,,marvels ,,of ,,scientific ,,invention ,,chapter ,i ,,digging ,,with ,,dynamite ,most people are afraid of the word explosion and shudder with apprehension at the mention of dynamite4 ,the latter1 particularly1 conjures up visions of anarchists1 bombs1 and all manner of wickedness4 ,yet the time seems to be coming when every farmer will regard explosives1 of the general type known to the public as dynamite1 as among his most trusty implements4 ,it is so already in some places4 ,in the ,united ,states explosives have been used for years1 owing to the exertions of the ,du ,pont ,powder ,company1 while ,messrs ,curtiss' and ,harvey1 and ,messrs ,nobels1 the great explosive manufacturers1 are busy introducing them in ,great ,britain4 ,it will perhaps be interesting first of all to see what this terror-striking compound is4 ,one essential feature is the harmless gas which constitutes the bulk of our atmosphere1 nitrogen4 ,ordinarily one of the most lazy1 inactive1 inert of substances1 this gas will1 under certain circumstances1 enter into combination with others1 and when it does so it becomes in some cases the very reverse of its usual peaceful1 lethargic self4 ,it is as if it entered reluctantly into these compounds and so introduced an element of instability into them4 ,it is like a dissatisfied partner in a business1 ready to break up the whole combination on very slight provocation4 ,and it must be remembered that an explosive is simply some chemical compound which can change suddenly into something else of much larger volume4 ,water1 when boiled1 increases to about #afjj times its own volume of steam1 and if it were possible to bring about the change suddenly water would be a fairly powerful explosive4 ,coal burnt in a fire changes1 with oxygen from the atmosphere1 into carbonic acid gas1 and the volume of that latter which is so produced is much more than that of the combined volumes of the oxygen and coal4 ,when the burning takes place in a grate or furnace we see nothing at all like an explosion1 for the simple reason that the change takes place gradually4 ,that is necessarily so since the coal and oxygen are only in contact at the surface of the former4 ,if1 however1 we grind the coal to a very fine powder and mix it well with air1 then each fine particle is in contact with oxygen and can burn instantly4 ,hence coal-dust in air is an explosive4 ,it used to be thought that colliery accidents were due entirely to the explosion of methane1 a gas which is given off by the coal1 but it has of recent years dawned upon people that it is the coal-dust in the mine which really does the damage4 ,the explosion of methane stirs up the dust1 which then explodes4 ,the former is comparatively harmless1 but it acts as the trigger or detonator which lets loose the force pent up in the innocent-looking coal-dust4 ,hence the greatest efforts in modern collieries are bent towards ridding the workings of dust or else damping it or in some other way preventing it from being stirred up into the dangerous state4 ,so the essential feature of any explosive is oxygen and something which will burn with it4 ,if it be a solid or liquid the oxygen must be a part of the combination or mixture1 for it cannot get air from the surrounding atmosphere quickly enough to explode2 and1 moreover1 it is generally necessary that explosives should work in a confined space away from all contact with air4 ,so oxygen1 of necessity1 must be an integral part of the stuff itself4 ,but when oxygen combines with anything it usually clings rather tenaciously to its place in the compound and is not easily disturbed quickly1 and that is where the nitrogen seems to find its part4 ,it supplies the disturbing element in what would otherwise be a harmonious combination1 so that the oxygen and the burnable substances readily split up and form a new combination1 with the nitrogen left out4 ,of all the harmless things in the world one would think that that sweet1 sticky fluid1 glycerine1 which most of us have used at one time or another to lubricate a sore throat1 was the most harmless4 ,as it stands in its bottle upon the domestic medicine shelf1 who would suspect that it is the basis of such a thing as dynamite8 ,such is the case1 however1 for glycerine on being brought into contact with a mixture of sulphuric and nitric acids gives birth to nitro-glycerine1 an explosive of such sensitivity1 of such a furious1 violent nature1 that it is never allowed to remain long in its primitive condition1 but is as quickly as possible changed into something less excitable4 ,glycerine is one of those organic compounds which is obtained from once-living matter4 ,arising as a by-product in the manufacture of soap1 it consists1 as do so many of the organic substances1 of carbon and hydrogen1 the atoms of which are peculiarly arranged to form the glycerine molecule4 ,to this the nitric acid adds oxygen and nitrogen1 the sulphuric acid simply standing by1 as it were1 and removing the surplus water which arises during the process4 ,so while glycerine is carbon and hydrogen1 nitro-glycerine is carbon1 hydrogen1 nitrogen and oxygen4 ,in this state they form a compact liquid1 which occupies little space4 ,the least thing upsets them1 however4 ,the carbon combines with oxygen into carbon dioxide1 commonly called carbonic acid gas1 the hydrogen and some more oxygen form steam1 while the nitrogen is left out in the cold1 so to speak4 ,and the total volume of the gases so produced is about #fjjj times that of the original liquid4 ,it is easy to see that a substance which is liable suddenly to increase its volume by #fjjj times is an explosive of no mean order4 ,but the fact that it is liable to make this change on a comparatively slight increase in temperature or after a concussion makes it too dangerous for practical use4 ,it needs to be tamed down somewhat4 ,this was first done by the famous ,nobel1 who mixed it with a fine earth known as kieselguhr1 whereby its sensitiveness was much decreased4 ,this mixture is dynamite4 ,it will be seen that the function of the earth is simply to act as an absorbent of the liquid nitro-glycerine1 and several other things can be used for the same purpose4 ,moreover1 there are now many explosives of the dynamite nature but differing from it in having an active instead of a passive absorbent1 so that the decrease in sensitivity is accompanied by an increase in strength4 ,for example1 gelignite1 which is being used for agricultural purposes in ,great ,britain1 consists of nitro-glycerine mixed with nitro-cotton1 wood-meal and saltpetre4 ,the wood-meal acts as the absorbent instead of the kieselguhr1 while the nitro-cotton is another kind of explosive and the saltpetre1 one of the ingredients in the old gunpowder1 provides the necessary oxygen for burning up the wood-meal4 ,nitro-cotton is made in much the same way as nitro-glycerine1 except that cotton takes the place of the glycerine4 ,cotton is almost pure cellulose1 another organic substance1 like glycerine insomuch as it is composed of carbon and hydrogen1 but1 unlike it1 containing also oxygen4 ,treated with nitric acid it also forms a combination of carbon1 hydrogen1 oxygen and nitrogen1 which is called nitro-cotton1 nitro-cellulose1 or gun-cotton4 ,it may be asked1 why1 if these two substances are thus similar1 need they be mixed8 ,the answer is that although alike to a certain degree they are not exactly the same1 and the modern manufacturer of explosives in his strife after perfection finds that for certain purposes one is the best1 and for others another1 while for others again a combination may excel any single one4 ,for some work another kind of explosive altogether is to be preferred4 ,this is based upon chlorate of potash1 a compound very rich in oxygen1 which it is prepared to give up readily to burn any other suitable element which may be at hand4 ,a well-known explosive of this class is that known as cheddite1 since it was first made at a factory at ,chedde1 in ,savoy4 ,for the sake of simplicity1 however1 ,i propose in the following descriptions to refer to all these explosives under the common term dynamite1 since that will probably convey to the general public an idea of their nature better than any other term or terms which ,i could choose4 ,so now we come to the great question1 how can the modern farmer benefit by the use of high explosives such as these8 ,the answer is1 in many ways4 ,let us take the most obvious one first4 ,a farmer has been ploughing his land and growing his crops upon it for years4 ,perchance his forefathers have been doing the same for generations4 ,every year1 for centuries possibly1 a hard steel ploughshare has gone over that ground1 turning over and over the top soil to a depth of six to eight inches4 ,each season the plants1 whatever they may be1 grow mainly in that top layer4 ,they take the goodness or nourishment out of it and it eventually becomes more or less sterile4 ,by properly rotating his crops he mitigates this to a certain extent1 in addition to which he restores to the land some of its old nitrogenous constituents by the addition of manure4 ,yet1 do what he will1 this thin top layer is bound to become exhausted4 ,and all the while a few inches lower down there is almost virgin soil which has scarcely been disturbed since the creation of the world4 ,nay1 more1 that virgin soil1 with all its plant food still in it1 is not only doing little for its owner1 it is positively doing him harm4 ,for every time his plough goes over it it tends to ram it down flat2 every time a man walks over it the result is the same2 every horse that passes1 everything that happens or has happened for centuries in that field1 tends to make that soil just below the reach of the ploughshare a hard1 impervious mass1 through which only the roots of the most strongly growing plants can find a way1 and which tends to make the soil above it wet in wet weather and dry in dry weather4 ,thus roots have to spread sideways instead of downwards2 or1 growing downwards with difficulty1 each plant has to expend vital energy in forcing its roots through the hard ground which it might better employ in producing flowers or fruits4 ,and there is no natural storage of water4 ,a shower drenches the ground4 ,in time it dries1 through evaporation into the air1 and then when the drought comes all is arid as the ,sahara4 ,that hard subsoil is known by the term hard-pan1 and1 as we have seen1 it is produced more or less by all that goes on in the field4 ,even worse is the case--a very frequent one too--wherein there is a natural stratum of clay or equally dense waterproof material lying a few feet down4 ,beyond the reach of any plough1 this hard stratum can be broken up by the use of dynamite4 ,the usual method is to drive holes in the ground about fifteen to twenty feet apart and about three or four feet deep1 right into the heart of the hard layer4 ,at the bottom of each hole is placed a cartridge of dynamite with a fuse and a detonator4 ,this latter is a small tube containing a small quantity of explosive which1 unlike the dynamite1 can be easily fired1 and initiates the detonation of the cartridge4 ,when these miniature earthquakes have taken place all over a field a very different state of things prevails4 ,the hard-pan has been broken4 ,the explosive used for such a purpose has a sudden shattering power1 whereby it pulverises the ground in its vicinity rather than making a great upheaval at the surface4 ,the sudden shock makes cracks and fissures in all directions1 through which roots can easily make their way4 ,moreover1 it permits air to find an entrance1 thereby aerating the soil in such a way as to increase its fertility4 ,the heat1 or else the chemical products of the explosion1 seem to destroy the fungus germs in the ground4 ,finally a natural storage of water is set up4 ,heavy rain1 instead of drenching the upper soil1 simply moistens it nicely1 while the surplus water descends into the newly disturbed layers1 there to remain until the roots pump it up in time of drought4 ,it is stated that an acre of hay pumps up out of the soil #ejj tons of water per annum1 so it is easy to see what an important feature this natural water-storage is4 ,farmers say that their crops have doubled in value after thus dynamiting the subsoil4 ,this operation has been spoken of as a substitute for ploughing1 but that may be put down to journalistic licence1 for while it truly conveys the general idea1 it is hardly correct4 ,the ordinary plough turns over about eight inches1 the special subsoil plough reaches down to about eighteen inches1 but the dynamite method loosens the ground to a depth of six or seven feet4 ,corn roots if given a chance will go downwards from four to eight feet4 ,potatoes go down three feet1 hops eight to eighteen feet and vines twenty feet1 so it is easy to see how restricted the plants are when their natural rooting instincts are restrained by a hard layer at a depth of eighteen inches or so4 ,the holes are made by means of a bar or drill4 ,a great deal depends1 of course1 upon the hardness of the soil4 ,sometimes a steel bar has to be driven in by a sledge-hammer4 ,at others a pointed bar can be pushed down by hand4 ,in some cases it will be found that the best tool to employ is a dirt-auger1 a tool like a carpenter's auger1 which on being turned round and round bores its way into the earth4 ,however it may be done1 one or more cartridges of dynamite are lowered into the finished hole1 one of them being fitted with the necessary detonator and fuse4 ,then a little loose earth or sand is dropped into the hole until it is filled to a depth of six inches or so above the uppermost cartridge4 ,above that it is quite safe to fill the hole with earth1 ramming it in with a wooden rammer4 ,this is called tamping1 and it is necessary in order to prevent the force of the explosion being wasted in simply blowing up the hole4 ,what is wanted is that the explosion shall take place within an enclosed chamber so that its effect may be felt equally in all directions4 ,the holes are generally about an inch and a half or an inch and three-quarters in diameter4 ,there are two ways of firing the charges4 ,one is by means of fuses4 ,the detonator is fastened to one cartridge and a length of fuse is attached to the detonator1 which passing up the hole terminates above the ground4 ,the fuse is a tube of cotton filled with gunpowder1 and it burns at the rate of about two feet a minute4 ,thus if three feet of fuse be used the man who lights it has a minute and a half in which to find a place of safety from falling stones4 ,the other way is by electricity4 ,in this case an electric fuse is attached to the cartridge and two wires are led up the hole4 ,these are connected to an electrical machine1 which causes a current to pass down into the fuse1 where1 by heating a fine platinum wire1 it fires the detonating material with which it is packed4 ,this detonating material in turn fires the dynamite4 ,the advantage of the electrical method is that twenty or thirty holes being simultaneously connected to the same machine can all be fired at once4 ,and now let us think of another kind of farming1 in which fruit trees are concerned4 ,with a large tree the need of plenty of underground space for its roots would seem to be more important even than in the case of annual plants like wheat4 ,yet we know very well that the usual procedure is to dig a small hole just about big enough to accommodate the roots of the sapling when it is planted1 while the ground all round is left undisturbed4 ,the assumption is that the tree will1 in time1 be able to push its roots through anything which is not actually solid rock4 ,so much is this the case that one authority has thought fit to warn tree-growers in this picturesque fashion4 ,when planting a tree1 he says1 forget what it is you are doing1 and think that you are about to bury the biggest horse you know4 ,how many people when planting any tree dig a hole big enough to bury a horse8 ,it is fairly safe to reply1 only those who do it by dynamite4 ,illustration3 ,by permission of ,dupont ,powder ,co41 ,wilmington1 ,delaware ,,first ,,effect ,,of ,,the ,,dynamite ,clearing a field of tree stumps by blowing them up with dynamite4--,see p4 #af ,the method of working is to bore a hole nearly as deep as the hole you want to blast4 ,at the bottom place a powerful charge1 far stronger than you would use for subsoiling1 as just described4 ,that will not only blow a hole big enough for you to put your tree in1 but it will loosen the ground all around the hole for yards4 ,the main debris from the hole will fall back into it1 but that will not matter much1 since1 being all loose1 it is an easy matter to remove as much as is necessary to plant the young tree4 ,the advantages are the same as those enumerated in the previous case--namely1 the loosened ground gives more scope for the roots--apple-tree roots want twenty feet or so--the ground holds moisture better1 and the explosion kills the fungus germs4 ,in addition to these there is the advantage that to blast a hole like this is cheaper than digging it4 ,and that the advantages are not merely theoretical is shown by the fact that trees so planted actually do grow stronger1 bigger and quicker than precisely similar ones under the same conditions1 but set in the ordinary way with a spade4 ,and not only do new trees thus benefit2 old trees can be helped by dynamite4 ,many an existing orchard has been improved by exploding dynamite at intervals between the rows of trees4 ,care has to be taken to see that the disturbance is not so violent or so close as to damage the trees1 but that can be easily arranged1 and then the result is that the soil all around the trees is loosened1 the roots are given more freedom and the water-storing properties of the ground are greatly improved4 ,again1 how often a farmer is troubled with a pond or a patch of marshy ground right in the midst of his fields4 ,it is of no use1 and simply serves to make the field in which it occurs more difficult to plough and to cultivate--besides being so much good land wasted4 ,now the reason for the existence of that pond or marsh is that underneath the surface there is an impervious layer in which1 as in a basin1 the water can collect4 ,make a hole in that and it will no more hold water than a cracked jug will4 ,and to make that hole with dynamite is the easiest thing in the world4 ,if the pond be merely a collection of water which occurs in wet weather1 but which dries up quickly1 there simply needs to be drilled a deep hole and a fairly strong explosion caused at the bottom of it4 ,how deep the hole must be depends upon the formation of the earth at that point1 and how low down is the stratum which1 being waterproof1 causes the water to remain4 ,it is that1 of course1 which must be broken through1 and so the explosion must be caused at a point near the under side of that layer4 ,with a little experience the operator can judge the position by the feel of the tool with which he makes the hole4 ,if the pond is permanent but shallow1 men can wade to about the centre1 there to drill a hole and fire a shot4 ,if it be permanent and deep1 then the work must be done from a raft1 which1 however1 can be easily constructed for the purpose4 ,once broken through1 the water will quickly pass away below the impervious stratum and useless land will become valuable4 ,the same may be done on a larger scale by blasting ditches with dynamite4 ,this is in many cases much cheaper than digging them4 ,a row of holes is put down1 or even two or three rows1 according to the width of the proposed ditch4 ,in depth they are made a little less than the depth of the ditch that is to be4 ,and for a reason which will be apparent they are put very close together1 say three feet or so apart4 ,preparations may thus be made for blasting a ditch hundreds of feet long and then all are fired together4 ,the earth is thrown up by a mighty upheaval1 a ditch being produced of remarkable regularity considering the means by which it is made4 ,the sides1 of course1 take a nice slope1 the debris is thrown away on both sides and spread to a considerable distance1 so that1 given favourable conditions and a well-arranged explosion1 there is constructed a finished ditch which hardly needs touching with spade or other tool4 ,it not being feasible to fire a lot of holes electrically1 the limit being about thirty1 the simultaneous explosion of perhaps hundreds has to be brought about in some other manner1 and usually it is accomplished by the simple device of putting the holes fairly near together and firing one with a fuse4 ,the commotion set up by this one causes the nearest ones to go off1 they in turn detonating those farther on1 with the result that explosion follows explosion all along the line so rapidly as to be almost instantaneous4 ,a farmer who is troubled by a winding stream passing through his land1 cutting it up into awkward shapes1 can straighten it by blasting a ditch across a loop in the manner just described4 ,in the case of low-lying land1 however1 ditches are obviously no use1 since water would not flow away along them4 ,in that case the principle suggested just now for dealing with an inconvenient pond can sometimes be used1 for if the subsoil be blasted through at several points it is very likely that water will find a way downwards by some means or other4 ,and the list of possible uses is by no means exhausted yet4 ,a man opening up virgin land often finds old tree stumps his greatest bother4 ,he can dig round them and then pull them out with a team of horses1 but by far the simpler way is with a few well-placed dynamite cartridges1 for they not only throw up the stump for him1 but they break it up1 shake the earth from it1 and leave it ready for him to cart away or to burn4 ,boulders1 too1 can be blown to pieces far more easily than one would think4 ,the charges may be put underneath them as with the tree stumps1 but in many cases that is not necessary1 all that is needed being some dynamite laid upon the top of the rock and covered with a heap of clay4 ,so sudden is the action of the explosive that its shock will break up the stone underneath it4 ,yet another way1 perhaps the most effective of all1 is to drill a hole into the stone and fire a charge inside it4 ,it behoves the onlooker then to keep away1 for the fragments may be thrown three or four hundred feet1 a fair proof that the stone will be very thoroughly demolished4 ,even in the digging of wells explosives may be useful4 ,in that case the holes are made in a circle1 and they slant downwards and inwards1 so that their lower ends tend to meet4 ,the result of simultaneously exploding the charges in these holes is to cut out a conical hole a little larger in size than the ring and a little deeper than the point at which the explosion took place4 ,the bottom of that hole can be levelled a little and the operation repeated1 and so stage by stage the well will proceed to grow downwards4 ,the thought that naturally occurs to one is this4 ,all the operations described may be very well1 the cost may be low1 and the effect good1 but are they sufficient to compensate for the risks necessarily dependent upon the use of explosives8 ,the doubt implied in that question1 natural though it be1 is based upon prejudice1 with which we are all more or less afflicted4 ,the art of making these explosive substances has been brought to such a pitch that with reasonable care there is no risk whatever4 ,the greatest possible care is used in the factory to see that all explosives sent out are what they are meant to be1 and that they can therefore be relied upon to behave according to programme and not to play any tricks4 ,that is the first step1 and what with competition between makers1 ,government inspection1 and searching inquiry into the slightest accident1 and the desire of each maker to keep up the credit of his name1 it is safe to say that modern explosives may be relied upon to do their duty faithfully4 ,the second step in the process of securing safety is that the powerful explosive1 the one that does the work1 is made very insensitive1 so that it is really quite hard to explode it4 ,with reasonable care1 then1 it will never go off by accident4 ,on the other hand1 the sensitive material1 which is easy to let off1 is in very small quantities1 so small that an accident with it would not1 again with reasonable precautions1 be a serious matter4 ,fuse1 too1 is very reliable nowadays4 ,the man who lights the fuse may be absolutely sure that he will have that time to get to a place of safety which corresponds to the length of fuse which he employs4 ,with electrical firing1 too1 it is quite easy to arrange that the final electrical connection shall not be made until all are at a safe distance1 so that a premature explosion is impossible4 ,in many of the cases described1 the shock takes place almost entirely within the earth and there is very little debris thrown about4 ,indeed the only danger which is to be feared with these operations is about on a par with that which every farm hand runs from the kick of a horse4 ,any careful1 trustworthy man could be quite safely taught to do this work1 and with the assistance of a labourer he could do all that is necessary4 ,given a fair amount of intelligence1 too1 he would take but little teaching4 ,altogether there is no doubt that the use of explosives is going to have a marked effect upon farming operations in the near future4 ,,chapter ,,ii ,,measuring ,,electricity ,there are many people whose acquaintance with electricity consists mainly in paying the electric light bill4 ,to such the instruments whereby electricity is measured will make a specially interesting appeal4 ,current is sold in ,great ,britain at so much per ,board of ,trade ,unit4 ,to state what that is needs a preliminary explanation of the other units employed in connection with electric currents4 ,the public electricity supply in any district is announced to be so many volts1 it may be #ajj1 #bjj or perhaps #bcj1 but whatever it be1 it is always so many volts4 ,then the electrician speaks lightly of numbers of amperes1 he may even talk of the number of watts used by the lamps1 while occasionally the word ohm will leak out4 ,among these terms the general reader is apt to become completely fog-bound4 ,but really they are quite simple if once understood1 and1 as we shall see in a moment1 there are some very remarkable instruments for measuring them1 some of which exhibit a delicacy truly astonishing4 ,it is well at the outset to try and divest ourselves of the idea that there is anything mysterious or occult about electricity4 ,it is quite true that there are many things about it very little understood even by the most learned1 but for ordinary practical purposes it may be regarded as a fluid1 which flows along a wire just as water flows along a pipe4 ,the wire is1 electrically speaking1 a hole through the air or other non-conducting substance with which it is surrounded4 ,a water-pipe being a hole through a bar of iron1 so the copper core of an electrical wire is1 so far as the current is concerned1 but a hole through the centre of a tube of silk1 cotton1 rubber or whatever it be4 ,electricity can flow through certain solids just as water can flow through empty space4 ,water will not flow through a pipe unless a pressure be applied to it somewhere4 ,in a pipe the ends of which are at the same level water will lie inert and motionless4 ,lower one end1 however1 and the pressure produced by gravity--in other words1 the weight of the water--will cause it to move4 ,in like manner pressure produced by the action of a pump will make water flow4 ,on the other hand1 when it moves it encounters resistance1 through the water rubbing against the walls of the pipe4 ,similarly1 an electrical pressure is necessary before a current of electricity will flow4 ,and every conductor offers more or less resistance to the flow of current1 thus opposing the action of the pressure4 ,before current will flow through your domestic glow-lamps and cause them to give light there must be a pressure at work1 and that pressure is described as so many volts4 ,a battery is really a little automatic electrical pump for producing an electrical pressure4 ,and the volt1 which is a legal measure1 just as much as a pound or a yard1 is a certain fraction of the pressure produced by a certain battery known as ,clark's ,cell4 ,it is not necessary here to say exactly what that fraction is1 but it will give a general idea to state that the ordinary ,leclanche or dry cell1 such as is used for electric bells1 produces a pressure of about one and a half volts4 ,thus we see the volt is the electrical counterpart of the term pound per square inch which is used in the case of water pressure4 ,a flow of water is measured in gallons per minute4 ,an electrical current is measured in coulombs per second4 ,thus the coulomb is the electrical counterpart of the gallon4 ,but in this particular we differ slightly in our methods of talking of water and electricity4 ,gallons per minute or per hour is the invariable term in the former case1 but in the latter we do not speak of coulombs per second1 although that is what we mean1 for we have a special name for one coulomb per second1 and that same is ampere4 ,one ampere is one coulomb per second1 two amperes are two coulombs per second1 and so on4 ,there is no recognised term to denote the resistance which a water-pipe offers to the passage of water through it1 but in the similar case with electricity there is a term specially invented for the purpose1 the ohm4 ,legally it is the resistance of a column of mercury of a certain size and weight4 ,a rough idea of it is given by the fact that a copper wire a sixteenth of an inch thick and #djj feet long has a resistance of about one ohm4 ,the three units--the volt1 ampere and ohm--are so related that a pressure of one volt acting upon a circuit with a resistance of one ohm will produce a current of one ampere4 ,a current can do work2 when it lights or heats your room or drives a tramcar it is doing work2 and the rate at which a current does work is found by multiplying together the number of volts and the number of amperes4 ,the result is in still another unit1 the watt4 ,and #ajjj watts is a kilowatt4 ,finally1 to crown the whole story1 a kilowatt for one hour is a ,board of ,trade unit4 ,so for every unit which you pay for in the quarterly bill you have had a current equal to #ajjj watts for an hour4 ,to give a concrete example1 if the pressure of your supply is #bjj volts1 and you take a current of five amperes for an hour1 you will have consumed one ,b4,t4,u4 ,perhaps it will give added clearness to this explanation to tabulate the terms as follow3-- ,volt ,the unit of pressure1 analogous to pounds per square inch in the case of water4 ,coulomb ,the measure of quantity1 analogous to the gallon4 ,ampere ,the measure of the strength of a current1 meaning one coulomb per second4 ,watt ,the unit denoting the power for work of any current4 ,it is the result of multiplying together volts and amperes4 ,kilowatt #ajjj watts4 ,board of ,trade ,unit ,a current of one kilowatt flowing for one hour4 ,illustration3 ,by permission of ,dupont ,powder ,co4 ,a ,,fine ,,crop ,celery grown on soil tilled by dynamite4--,see p4 #bd ,in practice the measurements are generally made by means of the connection between electricity and magnetism4 ,a current of electricity is a magnet4 ,whenever a current is flowing it is surrounded by a region in which magnetism can be felt4 ,this region is called the magnetic field1 and the strength of the field varies with the strength that is the number of amperes in the current4 ,if a wire carrying a current be wound up into a coil it is evident that the magnetic field will be more intense than if the wire be straight1 for it will be concentrated into a smaller area4 ,iron1 with its peculiar magnetic properties1 if placed in a magnetic field seems to draw the magnetic forces towards itself1 and consequently1 if the wire be wound round a core of iron1 the magnetism due to the current will be largely concentrated at the ends of the core4 ,but the main principle remains--in any given magnet the magnetic power exhibited will be in proportion to the current flowing4 ,the switchboard at a generating station is always supplied with instruments called ammeters1 an abbreviation of amperemeters1 for the purpose of measuring the current passing out from the dynamos4 ,each of these consists of a coil of wire through which the current passes4 ,in some there is a piece of iron near by1 which is attracted more or less as the current varies1 the iron being pulled back by a spring and its movement against the tension of the spring being indicated by a pointer on a dial4 ,in others the coil itself is free to swing in the neighbourhood of a powerful steel magnet1 the interaction between the electro-magnet1 or coil1 and the permanent magnet being such that they approach each other or recede from each other as the current varies4 ,a pointer on a dial records the movements as before4 ,in yet another kind the permanent magnet gives way to a second coil1 the current passing through both in succession1 the result being very much the same1 the two coils attracting each other more or less according to the current4 ,another kind of ammeter known as a thermo-ammeter works on quite a different principle4 ,it consists of a piece of fine platinum wire which is arranged as a shunt--that is to say1 a certain small but definite proportion of the current to be measured passes through it4 ,now1 being fine1 the current has considerable difficulty in forcing its way through this wire and the energy so expended becomes turned into heat in the wire4 ,it is indeed a mild form of what we see in the filament of an incandescent lamp1 where the energy expended in forcing the current through makes the filament white-hot4 ,the same principle is at work when we rub out a pencil mark with india-rubber1 whereby the rubber becomes heated1 as most of us have observed4 ,the wire1 then1 is heated by the current passing through it1 and accordingly expands1 the amount of expansion forming an indication of the current passing4 ,the elongation of the wire is made to turn a pointer4 ,a simple modification makes any of these instruments into a voltmeter4 ,this instrument is intended to measure the force or pressure in the current as it leaves the dynamo4 ,a short branch circuit is constructed1 leading from the positive wire near the dynamo to the negative wire1 or to the earth1 where the pressure is zero4 ,in this circuit is placed the instrument1 together with a coil made of a very long length of fine wire so that it has a very great resistance4 ,very little current will flow through the branch circuit because of the high resistance of the coil1 but what there is will be in exact proportion to the pressure4 ,the voltmeter is therefore the same as the ammeter1 except that its dial is marked for volts instead of for amperes1 and it has to be provided with the resistance coil4 ,instruments of the ammeter type can also be used as ohmmeters4 ,in this case what is wanted is to test the resistance of a circuit1 and it is done by applying a battery1 the voltage of which is known1 and seeing how much current flows4 ,all the voltmeters and ohmmeters mentioned owe their method of working to what is known as ,ohm's law4 ,one of the greatest steps in the development of electrical science was taken when ,dr ,ohm put forward the law which he had discovered whereby pressure1 current and resistance are related4 ,the reader will probably have noticed from what has already been said about the units of measurement--the volt1 the ampere and the ohm--that the current varies directly as the pressure and inversely as the resistance4 ,that is the famous and important ,ohm's law and anyone who has once grasped that has gone a long way towards understanding many of the principal phenomena of electric currents4 ,but the instruments so far referred to are of the big1 clumsy type1 suitable for measuring large currents and great pressures4 ,they are like the great railway weigh-bridges1 which weigh a whole truck-load at a time and are good enough if they are true to a quarter of a hundredweight4 ,the instruments about to be described are more comparable with the delicate balance of the chemist1 which can detect the added weight when a pencil mark is made upon a piece of paper4 ,indeed beside them such a balance is quite crude and clumsy4 ,they may be said to be the most delicate measuring instruments in existence4 ,we will commence with the galvanometer4 ,the simplest form of this is a needle like that of a mariner's compass very delicately suspended by a thin fibre in the neighbourhood of a coil of wire4 ,the magnetic field produced by the current flowing in the wire tends to turn the needle1 which movement is resisted by its natural tendency to point north and south4 ,thus the current only turns the needle a certain distance1 which distance will be in proportion to its strength4 ,the deflection of the needle1 therefore1 gives us a measure of the strength of the current4 ,but such an instrument is not delicate enough for the most refined experiments1 and the improved form generally used is due to that prince of inventors1 the late ,lord ,kelvin4 ,he originally devised it1 it is interesting to note1 not for laboratory experiments1 but for practical use as a telegraph instrument in connection with the early ,atlantic cables4 ,before describing it1 it may sharpen the reader's interest to mention a wonderful experiment which was made by ,varley1 the famous electrician1 on the first successful ,atlantic cable4 ,he formed a minute battery of a brass gun-cap1 with a scrap of zinc and a single drop of acidulated water4 ,this he connected up to the cable4 ,probably there is not one reader of this book but would have thought1 if he had been present1 that the man was mad4 ,what conceivable good could come of connecting such a feeble source of electrical pressure to the two thousand miles of wire spanning the great ocean2 the very idea seems fantastic in the extreme4 ,yet that tiny battery was able to make its power felt even over that great distance1 for the ,thomson ,mirror ,galvanometer was there to detect it4 ,two thousand miles away1 the galvanometer felt and was operated by the force generated in a battery about the size of one of the capital letters on this page4 ,this wonderful instrument consisted of a magnet made of a small fragment of watch-spring1 suspended in a horizontal position by means of a thread of fine silk1 close to a coil of fine wire4 ,when current flowed through the coil the magnetic field caused the watch-spring magnet to swing round1 but when the current ceased the untwisting of the silk brought it back to its original position again4 ,so far it seems to differ very little from the ordinary galvanometer previously mentioned1 but the stroke of genius was in the method of reading it4 ,with a small current the movement of the magnet was too small to be observed by the unaided eye1 so it was attached to a minute mirror made of one of those little circles of glass used for covering microscope slides1 silvered on the back4 ,the magnet was cemented to the back of this1 yet both were so small that together their weight was supported by a single thread of cocoon silk4 ,light from a lamp was made to fall upon this mirror1 thereby throwing a spot of light upon a distant screen4 ,thus the slightest movement of the magnet was magnified into a considerable movement of the spot of light4 ,the beam of light from the mirror to the screen became1 in fact1 a long lever or pointer1 without weight and without friction4 ,the task of watching the rocking to and fro of the spot of light was found to be too nerve-racking for the telegraph operators1 and so ,lord ,kelvin improved upon his galvanometer in two ways4 ,he first of all managed to give it greater turning-power1 so that1 actuated by the same current1 the new instrument would work much more strongly than the older one4 ,then he utilised this added power to move a pen whereby the signals were recorded automatically upon a piece of paper4 ,the new instrument is known as the ,siphon ,recorder4 ,the added power was obtained by turning the instrument inside out1 as it were1 making the coil the moving part and the permanent magnet the fixed part4 ,this enabled him to employ a very powerful permanent magnet in place of the minute one made of watch-spring4 ,the interaction of two magnets is the result of their combined strength1 and that of the coil being limited by the strength of the minute current the only way to increase the combined power of the two was to substitute a large powerful magnet for the small magnetised watch-spring4 ,this large magnet would1 of course1 have been too heavy to swing easily and therefore the positions had to be reversed4 ,so now we have two types of galvanometer1 both due originally to the inventions of ,lord ,kelvin4 ,for some purposes the ,thomson type 7his name was ,thomson before he became ,lord ,kelvin7 are still used1 but in a slightly elaborated form4 ,its sensitiveness is such that a current of a thousandth of a micro-ampere will move the spot of light appreciably4 ,and when one comes to consider that a micro-ampere is a millionth part of an ampere this is perfectly astounding4 ,but there is a more wonderful story still to come1 of an instrument which can detect a millionth of a micro-ampere1 or one millionth of a millionth of an ampere4 ,it is not generally known that we are all possessors of an electric generator in the form of the human heart1 but it is so1 and ,professor ,einthoven1 of ,leyden1 wishing to investigate these currents from the heart1 found himself in need of a galvanometer exceeding in sensitiveness anything then known4 ,even the tiny needles or coils with their minute mirrors have some weight and so possess in an appreciable degree the property of inertia1 in virtue of which they are loath to start movement and1 having started1 are reluctant to stop4 ,this inertia1 it is easy to see1 militates against the accurate recording of rapid variations in minute currents1 so the energetic ,professor set about devising a new galvanometer which should answer his purpose4 ,this is known as the ,string ,galvanometer4 ,illustration3 ,,fig4 #a4-,this shows the principle of this wonderful ,galvanometer invented by ,lord ,kelvin in its latest form4 ,current enters at a1 passes round the coils1 as shown by the arrows1 and away at b4 ,a light rod1 c1 is suspended by the fine fibre1 d1 so that the eight little magnets hang in the centres of the coils--four in each4 ,the current deflects these magnets and so turns the mirror1 m1 at the bottom of the rod4 ,at e are two large magnets which give the little ones the necessary tendency to keep at zero4 ,illustration3 ,,fig4 #b4--;,here we see the working parts of the ,string ,galvanometer1 by which the beating of the heart can be registered electrically4 ,the current flows down the fine silvered fibre1 between the poles1 a and b1 of a powerful magnet4 ,as the current varies1 the fibre bends more or less4 ,the main body of the instrument is a large1 powerful electro-magnet1 in shape like a large pair of jaws nearly shut4 ,energised by a strong current1 this magnet produces an exceedingly strong magnetic field in the small space between the teeth as it were4 ,in this space there is stretched a fine thread of quartz which is almost perfectly elastic4 ,it is a non-conductor1 however1 so it is covered with a fine coating of silver4 ,silver wire is sometimes used1 but no way has yet been found of drawing any metallic wire so thin as the quartz fibre1 which is sometimes as thin as two thousandths of a millimetre1 or about a twelve-thousandth of an inch4 ,a hundred pages of this book make up a thickness of about an inch1 so that one leaf is about a fiftieth of an inch4 ,consequently the fibre in question could be multiplied #bdj times before it became as stout as the paper on which these words are printed4 ,the current to be measured1 then1 is passed through the stretched fibre and the interaction of the magnetic field by which the fibre is then surrounded1 with the magnetic field in which it is immersed1 causes it to be deflected to one side4 ,of course the deflection is exceedingly small in amount1 and as it is undesirable to hamper its movements by the weight of a mirror1 no matter how small1 some other means of reading the instrument had to be devised4 ,this is a microscope which is fixed to one of the jaws1 through a fine hole in which the movements of the fibre can be viewed4 ,or what is often better still1 a picture of the wire can be projected through the microscope on to a screen or on to a moving photographic plate or strip of photographic paper4 ,in the latter case a permanent record is made of the changes in the flowing current4 ,an electric picture can thus be made of the working of a man's heart4 ,he holds in his hands two metal handles or is in some other way connected to the two ends of the fibre by wires just as the handles of a shocking coil are connected to the ends of the coil4 ,the faint currents caused by the beating of his heart are thus set down in the form of a wavy line4 ,such a diagram is called a cardiogram1 and it seems that each of us has a particular form of cardiogram peculiar to himself1 so that a man could almost be recognised and distinguished from his fellows by the electrical action of his heart4 ,the galvanometer has a near relative1 the electrometer1 the astounding delicacy of which renders it equally interesting4 ,it is particularly valuable in certain important investigations as to the nature and construction of atoms4 ,the galvanometer1 it will be remembered1 measures minute currents2 the electrometer measures minute pressures1 particularly those of small electrically charged bodies4 ,every conductor 7and all things are conductors1 more or less7 can be given a charge of electricity4 ,any insulated wire1 for example1 if connected to a battery will become charged--current will flow into it and there remain stationary4 ,and that is what we mean by a charge as opposed to a current4 ,air compressed into a closed vessel is a charge4 ,air1 however compressed1 flowing along a pipe would be better described as a current4 ,imagine one of those cylinders used for the conveyance of gas under pressure and suppose that we desire to find the pressure of the gas with which it is charged4 ,we connect a pressure-gauge to it1 and see what the finger of the gauge has to say4 ,what happens is that gas from the cylinder flows into the little vessel which constitutes the gauge and there records its own pressure4 ,and just the same applies with electrometers4 ,precisely as the pressure-gauge measures the pressure of air or gas in some vessel1 so the electrometer measures the electrical pressure in a charged body4 ,further1 some of the charged bodies with which the student of physics is much concerned are far smaller than can be seen with the most powerful microscope4 ,how wonderfully minute and delicate1 therefore1 must be the instrument which can be influenced by the tiny charge which so small a body can carry4 ,it will be interesting here to describe an experiment performed with an electrometer by ,professor ,rutherford1 by which he determined how many molecules there are in a centimetre of gas1 a number very important to know but very difficult to ascertain1 since molecules are too small to be seen4 ,this number1 by the way1 is known to science as ,avogadro's ,constant4 ,everyone has heard of radium1 and knows that it is in a state which can best be described as a long-drawn-out explosion4 ,it is always shooting off tiny particles4 ,night and day1 year in and year out1 it is firing off these exceedingly minute projectiles1 of which there are two kinds1 one of which appears to be atoms of helium4 ,some years ago1 when radium was being much talked about and the names of ,m4 and ,madame ,curie were in everyone's mouth1 little toys were sold1 the invention1 ,i believe1 of ,sir ,william ,crookes1 called spinthariscopes4 ,each of these consisted of a short brass tube with a small lens in one end4 ,looking through the lens in a dark room1 one could see little splashes of light on the walls of the tube4 ,those splashes were caused by a tiny speck of radium in the middle of the tube1 the helium atoms from which1 by bombarding the inner surface of the tube1 produced the sparks4 ,now if we can count those splashes we can tell how many atoms of helium are being given off per minute4 ,and if then we reckon how many minutes it takes to accumulate a cubic centimetre of helium we can easily reckon how many atoms go to the cubic centimetre4 ,but the difficulty is to count them4 ,so the learned ,professor called in the aid of the electrometer4 ,he could not count all the atoms shot off1 so he put the piece of radium at one end of a tube and an electrometer at the other4 ,every now and then an atom shot right through the tube and out at the farther end4 ,and since each of these atoms from radium is charged with electricity1 each as it emerged operated the electrometer4 ,by simply watching the twitching of the instrument1 therefore1 it was possible to count how many atoms shot through the tube--one atom one twitch4 ,and from the size and position of the tube it was possible to reckon what proportion of the whole number shot off would pass that way4 ,the result of the experiment showed that there are in a cubic centimetre of helium a number of atoms represented by #bef followed by seventeen noughts4 ,and as helium is one of the few substances in which the molecule is formed of but one atom1 that is also the number of molecules4 ,and now consider this1 please4 ,a cubic centimetre is about the size of a boy's marble4 ,that contains the vast number of molecules just mentioned4 ,and the electrometer was able to detect the presence of those one at a time4 ,need one add another word as to the inconceivable delicacy of the instrument4 ,in its simplest form the electrometer is called the electroscope4 ,two strips of gold-leaf are suspended by their ends under a glass or metal shade4 ,as they hang normally they are in close proximity4 ,their upper ends are1 in fact1 in contact and are attached to a small vertical conductor4 ,a charge imparted to the small conductor will pass down into the leaves1 and since it will charge them both they will repel each other so that their lower ends will swing apart4 ,such an instrument is very delicate1 but because of the extreme thinness of the leaves it is very difficult to read accurately the amount of their movement and so to determine the charge which has been given to them4 ,in a more recent improvement1 therefore1 only one strip of gold-leaf is used1 the place of the other being taken by a copper strip4 ,the whole of the movement is thus in the single gold-leaf1 as the copper strip is comparatively stiff1 and it is possible to arrange for the movement of this one piece of gold-leaf to be measured by a microscope4 ,the other principal kind of electrometer we owe1 as we do the galvanometers1 to the wonderful ingenuity of ,lord ,kelvin4 ,in this the moving part is a strip of thin aluminium1 which is suspended in a horizontal position by means1 generally1 of a fine quartz fibre4 ,since it is necessary that this fibre should be a conductor1 which quartz is not1 it is electro-plated with silver4 ,thus a charge communicated to the upper end of the fibre1 where it is attached to the case1 passes down to the aluminium needle1 as it is called4 ,now the needle is free to swing to and fro1 with a rotating motion1 between two metal plates carefully insulated4 ,each plate is cut into four quadrants1 the opposite ones being electrically connected1 while all are insulated from their nearest neighbours4 ,one set of quadrants is charged positively1 and one set negatively1 by a battery1 but these charges have no effect upon the needle until it is itself charged4 ,as soon as that occurs1 however1 they pull it round1 and the amount of its movement indicates the amount of the charge upon the needle1 and therefore the pressure existing upon the charged body to which it is connected4 ,the direction of its movement shows1 moreover1 whether the charge be positive or negative4 ,a little mirror is attached to the needle1 so that its slightest motion is revealed by the movement of a spot of light1 as in the case of the mirror galvanometers4 ,instruments such as these are called ,quadrant ,electrometers4 ,my readers will remember1 too1 the ,string ,galvanometer already mentioned4 ,the same idea has been adapted to this purpose4 ,a fine fibre is stretched between two charged conductors while the fibre is itself connected to the body whose charge is being measured4 ,the charge which it derives from the body causes it to be deflected1 which deflection is measured by a microscope4 ,in all cases of transmission of electricity over long distances for lighting or power purposes the currents are alternating4 ,they flow first one way and then the other1 reversing perhaps twenty times a second1 or it may be two hundred1 or even more times in that short period4 ,some electric railways are worked with alternating current1 and it is used for lighting quite as much as direct current and is equally satisfactory4 ,in wireless telegraphy it is essential4 ,in that case1 however1 the reversals may take place millions of times per second4 ,consequently1 to distinguish the comparatively slowly changing currents of a frequency or periodicity of a few hundreds per second from these much more rapid ones1 the latter are more often spoken of as electrical oscillations4 ,and these alternating and oscillating currents need to be measured just as the direct currents do4 ,yet in many cases the same instruments will not answer4 ,there has therefore grown up a class of wonderful measuring instruments specially designed for this purpose1 by which not only does the station engineer know what his alternating current dynamos are doing1 but the wireless operator can tell what is happening in his apparatus1 the investigator can probe the subtleties of the currents which he is working with1 and apparatus for all purposes can be designed and worked with a system and reason which would be impossible but for the possibility of being able to measure the behaviour of the subtle current under all conditions4 ,one trouble in connection with measuring these alternating currents is that they are very reluctant to pass through a coil4 ,one method by which this difficulty can be overcome has been mentioned incidentally already4 ,i refer to the heating of a wire through which current is passing4 ,this is just the same whether the current be alternating or direct4 ,one of the simplest instruments of this class has been appropriated by the ,germans1 who have named it the ,reiss ,electrical ,thermometer1 although it was really invented nearly a century ago by ,sir ,william ,snow ,harris4 ,it consists of a glass bulb on one end of a glass tube4 ,the current is passed through a fine wire inside this bulb1 and as the wire becomes heated it expands the air inside the bulb4 ,this expansion moves a little globule of mercury which lies in the tube1 and which forms the pointer or indicator by which the instrument is read4 ,as the temperature of the wire rises the mercury is forced away from the bulb1 as the temperature falls it returns4 ,and as the temperature is varied by the passage of the current1 so the movement of the mercury is a measure of the current4 ,another way is to employ a ,rectifier4 ,this is a conductor which has the peculiar property of allowing current to pass one way but not the other4 ,it thus eliminates every alternate current and changes the alternating current into a series of intermittent currents all in the same direction4 ,rectified current is thus hardly described by the term continuous1 but still it is continuous current in the sense that the flow is always in the same direction1 and so it can be measured by the ordinary continuous current instruments4 ,the difficulty about it is that there is some doubt as to the relation between the quantity of rectified current which the galvanometer registers and the quantity of alternating current1 which after all is the quantity which is really to be measured4 ,how the rectification is accomplished will be referred to again in the chapter on ,wireless ,telegraphy4 ,but to return to the thermo-galvanometers1 as those are termed which ascertain the strength of a current by the heat which it produces1 the simple little contrivance of ,sir ,william ,snow ,harris has more elaborate successors1 of which perhaps the most interesting are those associated with the name of ,mr ,w4 ,duddell1 who has made the subject largely his own4 ,besides their interest as wonderfully delicate measuring instruments1 these have an added interest1 since they introduce us to another strange phenomenon in electricity4 ,we have just noted the fact that electricity causes heat4 ,now we shall see the exact opposite1 in which heat produces electrical pressure and current4 ,and the feature of ,mr ,duddell's instruments is the way in which these two things are combined4 ,by a roundabout but very effective way he rectifies the current to be measured1 for he first converts some of the alternating current into heat and then converts that heat into continuous current4 ,if two pieces of dissimilar metals be connected together by their ends1 so as to form a circuit1 and one of the joints be heated1 an electrical pressure will be generated which will cause a current to flow round the circuit4 ,the direction in which it will flow will depend upon the metals employed4 ,the amount of the pressure will also depend upon the metals used1 combined with the temperature of the junctions4 ,with any given pair of metals1 however1 the force1 and therefore the volume of current1 will vary as the temperature4 ,really it will be the difference in temperature between the hot junction and the cold junction1 but if we so arrange things that the cold junction shall always remain about the same1 the current which flows will vary as the temperature of the hot one4 ,the volume of that current will therefore be a measure of the temperature4 ,such an arrangement is known as a thermo-couple1 and is becoming of great use in many manufacturing processes as a means of measuring temperatures4 ,in the ,duddell ,thermo-galvanometers1 therefore1 the alternating current is first led to a heater consisting of fine platinised quartz fibre or thin metal wires4 ,just above the heater there hangs a thermo-couple1 consisting of two little bars1 one of bismuth and the other of antimony4 ,these two are connected together at their lower end1 where they nearly touch the heater1 but their upper ends are kept a little apart1 being joined1 however1 by a loop formed of silver strip4 ,this arrangement will be quite clear from the accompanying sketch1 and it will be observed that the loop is so shaped that the whole thing can be easily suspended by a delicate fibre which will permit it to swing easily1 like the coil in a mirror galvanometer4 ,it is indeed a swinging coil of a galvanometer formed with a single turn instead of the many turns usual in the ordinary instruments1 and it will be noticed from the sketch that there is a mirror fixed just above the top of the loop4 ,this coil1 then1 with the thermo-couple at its lower extremity1 is hung between the ends of a powerful magnet much as the fibre of the ,einthoven ,galvanometer is situated4 ,the alternating current to be measured comes along through the heater4 ,the heater rises in temperature4 ,that warms the lower end of the thermo-couple4 ,instantly a steady1 continuous current begins to circulate round the silver strip which forms the coil1 and that1 acting just as the current does in the ordinary galvanometer1 causes the coil to swing round more or less1 which movement is indicated by the spot of light from the mirror4 ,a current as small as twenty micro-amperes 7or twenty millionths of an ampere7 can be measured in this way4 ,mr ,duddell has also perfected a wonderful instrument called an ,oscillograph1 for the strange purpose of making actual pictures of the rise and fall in volume of current in alternating circuits4 ,illustration3 ,,fig4 #c4--,the ,duddell ,thermo-galvanometer4 ,in this remarkable instrument alternating current enters at a1 passes through the fine wire and leaves at b4 ,in doing this it heats the wire1 which in turn heats the lower end of the bismuth and antimony bars4 ,this generates continuous current1 which circulates through the loop of silver wire1 c1 which1 since it hangs between the poles1 d and e1 of a magnet1 is thereby turned more or less4 ,the amount of the turning indicates the strength of the alternating current4 ,to realise the almost miraculous delicacy of these wonderful instruments we need first of all to construct a mental picture of what takes place in a circuit through which alternating current is passing4 ,the current begins to flow3 it gradually increases in volume until it reaches its maximum3 then it begins to die away until it becomes nil3 then it begins to grow in the opposite direction1 increases to its maximum and dies away once more4 ,that cycle of events occurs over and over again at the rate it may be of hundreds of times per second4 ,now for the actual efficient operation of electrical machinery working on alternating current it is very necessary to know exactly how those changes take place--do they occur gradually1 the current growing and increasing in volume regularly and steadily1 or irregularly in a jumpy manner8 ,engineers have a great fancy for setting out such changes in the form of diagrams1 in which case the alternations are represented by a wavy line1 and it is of much importance to obtain an actual diagram showing not what the changes should be according to theory1 but what they really are in practice4 ,it is then possible to see whether the wave-form of the current is what it ought to be4 ,once again we must turn our thoughts back to the string galvanometer4 ,in that case1 it will be remembered1 there is a conducting fibre passing between the ends or poles of a powerful magnet1 the result of which arrangement is that as the current passes through the fibre it is bent by the action of the magnetic forces produced around it4 ,if the current pass one way1 downwards let us say1 the fibre will be bent one way1 while if it pass upwards it will be bent the opposite way4 ,suppose then that we have two fibres instead of one1 and that we send the current up one and down the other4 ,one will be bent inwards and the other outwards4 ,then suppose that we fix a little mirror to the centre of the fibres1 one side of it being attached to one fibre and the other to the other4 ,as one fibre advances and the other recedes the mirror will be turned more or less4 ,consequently1 as the current flowing in the fibres increases or decreases1 or changes in direction1 the mirror will be slewed round more or less in one direction or the other4 ,the spot of light thrown by the mirror will then dance from side to side with every variation1 and if it be made to fall upon a rapidly moving strip of photograph paper a wavy line will be drawn upon the paper which will faithfully represent the changes in the current4 ,in its action1 of course1 it is not unlike an ordinary mirror galvanometer1 but its special feature is in the mechanical arrangement of its parts which enable it to move with sufficient rapidity to follow the rapidly succeeding changes which need to be investigated4 ,it is far less sensitive than1 say1 a ,thomson ,galvanometer1 but the latter could not respond quickly enough for this particular purpose4 ,,chapter ,,iii ,,the ,,fuel ,,of ,,the ,,future ,we now enter for a while the realm of organic chemistry1 a branch of knowledge which is of supreme interest1 since it covers the matters of which our own bodies are constructed1 the foods which we eat and the beverages which we drink1 besides a host of other things of great value to us4 ,although the old division of chemistry into inorganic and organic is still kept up as a matter of convenience1 the old boundaries between the two have become largely obliterated4 ,the distinction arose from the fact that there used to be 7and are still to a very great extent7 a number of highly complex substances the composition of which is known1 for they can be analysed1 or taken to pieces1 but which the wit of man has failed to put together4 ,consequently these substances could only be obtained from organic bodies4 ,the living trees1 or animals1 could in some mysterious way bring these combinations about1 but man could not4 ,the molecules of these substances are much more complicated than those with which the inorganic chemist deals4 ,the important ingredient in them all is carbon1 which with hydrogen1 nitrogen and oxygen almost completes the list of the simple elements of which these marvellous substances are compounded4 ,in some cases there appear to be hundreds of atoms in the molecule4 ,if one takes a glance at a text-book on organic chemistry the pages are seen to be sprinkled all over with ,c's and ,o's1 ,n's and ,h's1 with but an occasional symbol for some other element4 ,another feature of this branch which cannot fail to strike the casual observer is the queer names which many of the substances possess4 ,trimethylaniline1 triphenylmethane and mononitrophenol are a few examples which happen to occur to the memory1 and they are by no means the longest or queerest-sounding4 ,another peculiarity about these organic substances is that a number of them1 each quite different from the others1 can be formed of the same atoms4 ,certain atoms of hydrogen1 sulphur and oxygen form sulphuric acid1 and under whatever conditions they combine they never form anything else4 ,on the other hand1 there are sixty-six different substances all formed of eight of carbon1 twelve of hydrogen and four of oxygen4 ,this can only mean that in such cases as the latter the atoms have different groupings and that when grouped in one way they form one thing1 in another way some other thing1 and so on4 ,this explains the extreme difficulty which the chemist finds in building up some of these organic substances4 ,every now and again we are startled by some eminent man stating that the time will come when we shall be able to make living things1 when the laboratory will turn out living cows and sheep1 birds and insects1 even man with a mind and soul of his own4 ,yet one cannot but feel that such men1 no matter how great their authority1 are simply pulling the public's leg1 to use a colloquial expression4 ,for they hopelessly fail to make many of the commonest things4 ,in many cases where they wish to produce an organic substance they have to call in the aid of some living thing to do it for them1 even if it be but a humble microbe4 ,for the microbes perform wonderful feats in chemistry1 far surpassing those of the most eminent men4 ,hence the latter very sensibly use the microbe1 employ it to work for them1 just set things in order and then stand by while the microbe does the work4 ,thus most things can be analysed--that is to say1 taken to pieces--while many things can now be synthesised--that is to say1 built up from their constituent atoms--but still a great many remain1 and among them the most important1 the synthesis of which completely baffles man4 ,one of the most useful and widespread substances1 for example1 cellulose1 is1 at present at least1 utterly beyond us4 ,we do not even know how many atoms there are in the cellulose molecule4 ,the molecules may1 for all we know1 contain thousands of atoms4 ,indeed many of these organic matters have very large molecules4 ,and even if the chemist were able to make all kinds of organic matter1 he would still be as far off as ever from making living matter4 ,indigo used to be derived entirely from plants of that name4 ,one of the greatest triumphs of the organic chemist was when he produced artificial or synthetic indigo4 ,but he is as far off as ever from making the indigo plant4 ,it is claimed that synthetic rubber is exactly the same as natural rubber1 although some users say it is not quite the same4 ,still1 if it be so1 it is dead rubber1 not the living part of the plant4 ,the time1 then1 is infinitely far distant when the chemist will be able to make anything with the characteristics of life--namely1 to grow by accretion from within and to reproduce its kind4 ,the most wonderful product of the laboratory is dead4 ,at most it simply resembles something which once was alive4 ,but that is somewhat of a digression4 ,this dissertation on organic chemistry was simply intended to lead up to the question of liquid fuels1 all of which are organic4 ,in the life of to-day one of the most important things is petroleum4 ,this is a kind of liquid coal4 ,just how it was formed down in the depths of the earth is not clear4 ,one idea is that it is due to the decomposition of animal and vegetable matter4 ,another is that certain volcanic rocks which are known to contain carbide of iron might1 under the influence of steam1 have in bygone ages given off petroleum1 or paraffin1 to use the other name for the same thing4 ,in many parts of the world these deposits of oil are obtained by sinking wells and pumping up the oil4 ,in others the liquid gushes out without the necessity of pumping at all4 ,this is believed to be due to the fact that water pressure is at work4 ,artesian wells1 from which the water rushes of its own accord1 are quite familiar1 and are due to the fact that some underground reservoir tapped by the well is fed through natural pipes1 really fissures in the rock1 from some point higher than the mouth of the well4 ,now supposing that a reservoir of oil were also in communication with the upper world in the same way1 the descending water would go to the bottom1 underneath the lighter oil1 and would thus lift it up1 so that on being tapped the oil would rush out4 ,another source of mineral oil is shale1 such as is to be found in vast deposits in the south-east of ,scotland4 ,this shale is mined much as coal is3 it is then heated in retorts as coal is heated at the gas-works3 and the vapour which is given off1 on being condensed1 forms a liquid like crude petroleum4 ,in all these cases the original oil is a mixture of a great number of grades differing from each other in various ways4 ,they are all hydro-carbons1 which means compounds of carbon and hydrogen1 and they extend from cymogene 7the molecules of which contain four atoms of carbon and ten of hydrogen7 to paraffin wax1 which has somewhere about thirty-two of carbon to sixty-six of hydrogen4 ,for practical purposes their most important difference is the temperature at which they boil1 or turn quickly into vapour4 ,this forms the means by which they are sorted out4 ,in a huge still1 like a steam-boiler1 the crude or mixed oil is gradually heated1 and the gas given off is led to a cooling vessel where it is chilled back into liquid4 ,the lightest of all1 cymogene1 is given off even at the freezing-point of water4 ,that is led into one chamber and condensed there4 ,then1 as the temperature rises to #ah deg4 ,c41 rhigolene is given off3 that is collected and condensed in another vessel4 ,between #gj deg4 and #abj deg4 petroleum ether and petroleum naphtha are produced1 and they together constitute what is commonly called petrol4 ,between #abj deg4 and #aej deg4 petroleum benzine arises4 ,all the foregoing taken together constitute about #h to #aj per cent4 of the whole crude oil4 ,then between #aej deg4 and #cjj deg4 there comes off the great bulk of the oil1 nearly #hj per cent41 the kerosene or paraffin which we burn in lamps4 ,above #cjj deg4 there is obtained another oil1 which is used for lubrication1 also the invaluable vaseline1 and finally1 when the still is allowed to cool1 there remains a solid residuum known as paraffin wax4 ,this process is known as fractional distillation1 and it will be noticed that it consists essentially in collecting and liquefying separately those vapours which are given off at different ranges of temperature4 ,for our purpose in this chapter we are mainly concerned with the petrol and the kerosene4 ,many efforts have been made in times gone by to use kerosene for firing the boilers of steam-engines4 ,in naval vessels a great deal is so used at the present time4 ,but the chief method of employing oil for generating power is to use it in an internal combustion-engine4 ,these machines have been dealt with at length in ,engineering of ,to-day and ,mechanical ,inventions of ,to-day and so must be simply mentioned here4 ,they consist of two types4 ,in one1 which is exemplified by the ordinary car or bicycle motor1 the oil is gasified in a vessel called a carburetter or vaporiser and then led into the cylinder of the engine1 together with the necessary air to enable it to burn4 ,at the right moment a spark ignites the mixture1 which burns suddenly1 causing a sudden expansion1 in other words1 an explosion4 ,thus the power of the engine is derived from a succession of explosions4 ,if the fuel be petrol it vaporises at the ordinary temperature of the engine and needs no added heat4 ,with kerosene1 however1 heat has to be employed in the vaporiser to make it turn readily into a gas4 ,the other method is employed in engines of the new ,diesel type1 in which the cylinder of the engine1 being already filled with hot air1 has a jet of oil sprayed into it4 ,the heat of the air causes it to burst into flame1 causing an expansion which drives the engine4 ,an important feature in the latter type of engine is that the oil is very completely burnt1 so that very heavy oils can be used1 oils which1 if employed in an engine of the other kind1 would choke up the cylinder with soot4 ,in other words1 the range of oils which can be used in this new kind of engine is much wider than is possible in the others4 ,the latter may be likened to a fastidious man who is very particular about his food1 while the former resembles the man of hearty appetite who can eat anything4 ,and just as a man of the latter sort is more easily provided for by the domestic authorities1 so the ,diesel engine makes the problem of the provision of liquid fuel much simpler4 ,for it must never be forgotten that the provision of liquid fuel for the world is by no means a simple matter1 since the supply is by no means adequate4 ,the output runs into thousands of millions of gallons1 and the whole world is being searched for new fields of oil1 and yet it is all swallowed up as fast as it can be produced1 while the coal mines do not feel the competition4 ,a year or so ago the ,united ,states and ,russia between them 7and they are the greatest producers7 obtained #e1jjj1jjj1jjj gallons of oil1 seemingly an enormous quantity4 ,but1 on the other hand1 ,great ,britain alone produces over #bej1jjj1jjj tons of coal per annum4 ,if1 therefore1 liquid fuel is to displace coal1 as some people lightly think it is going to do1 the supply will have to be multiplied many times4 ,in the amount of heat which it is capable of giving the coal of ,great ,britain alone beats the oil produced by the whole world4 ,and another thing to be borne in mind is that as the coal miner goes down to the seam and sees for himself what is there1 while the oil producer simply stays at the surface and draws it up with a pump1 the coal man knows far more as to how much there is still left than the oil man does4 ,we know that the coal deposits will last for many years to come1 even if the production go on increasing1 whereas the oil supply may fall off in the near future instead of increasing4 ,and in both cases we are using up capital4 ,coal is not being made on the earth now1 at any rate in any appreciable quantity4 ,the stage of the earth's history favourable to the formation of coal measures has long gone by4 ,and the same probably applies to oil4 ,it is interesting in this connection to note that coal itself is to a certain extent1 or can be at all events1 a source of oil4 ,when coal is heated in order to make it give up its gas1 or to turn it into coke1 vapours are given off which on cooling become coal-tar4 ,at one time regarded only as a crude sort of paint1 this is now the source from which many chemical substances are obtained1 varying from photographic chemicals to saccharine1 a substitute for sugar4 ,so valuable are these products that there is a brisk demand for the tar1 in other directions than the manufacture of oils1 but oils of various kinds are also obtained from it4 ,the first step in the operations is fractional distillation1 after the manner just described for petroleum4 ,the first fraction is coal-tar naphtha4 ,then follows carbolic oil1 after that heavy or creosote oil1 anthracene oil1 and finally there remains in the still on cooling a solid residue known as coal-pitch4 ,the naphtha1 on being distilled again1 gives1 among other things1 benzine1 from which the famous aniline dyes are made1 and which is useful in many industries4 ,creosote is largely employed as a preservative for wood1 being forced into the timber under high pressure1 so that it penetrates right into it and tends to prevent rotting1 no matter how wet it may be4 ,railway sleepers are thus treated1 small truck-loads of them being run into a cast-iron tunnel which is then sealed at both ends1 while the creosote is forced in by powerful pumps4 ,after such treatment they can lie nearly buried in the damp ballast for a long time without any deterioration4 ,these coal-tar substances are all very similar to petroleum and its products1 hydro-carbons1 compounds of hydrogen and carbon in various proportions4 ,many of them could be used for fuel4 ,illustration3 ,by permission of ,dupont ,powder ,co4 ,,apple ,,tree ,,planted ,,with ,a ,,spade ,this apple tree was planted in the ordinary way with a spade4 ,compare its size with that in following illustration at p4 #dh4 ,but since they are based upon the supply of coal1 which is itself limited1 they cannot1 however they may be used1 do more than stave off the evil day when the supply will be exhausted4 ,quite different is it with alcohol1 which it seems likely may be the fuel of the future4 ,some people will be inclined to exclaim ,what a pity to burn it6 since to many the word conveys ideas of another sort altogether4 ,there are many nowadays1 however1 who1 like the writer1 have none but a scientific interest in it4 ,to such whisky1 for example1 is but impure alcohol1 and it is without the impurities that it may become of vast use to the world1 thereby possibly repaying man for some of the harm which in the past it has inflicted upon him4 ,alcohol1 again1 is a hydrocarbon4 ,it is really more correct to speak of it in the plural1 as alcohols1 since there is a large group of substances all of the same name4 ,two of these are of the greatest importance1 methyl alcohol and ethyl alcohol4 ,the former is obtained from wood1 hence it is sometimes called wood spirit4 ,wood is strongly heated in an iron still1 and the methyl alcohol is given off in the form of vapour1 which on being collected and cooled condenses into liquid4 ,it is exceedingly unpleasant to the taste3 if it were the only kind there would be no consumption of alcohol as a drink4 ,the second kind mentioned is obtained by the agency of germs or microbes1 and the story of its production is so interesting as to demand a little space4 ,we will commence with the maltster4 ,he performs the first part of the operation4 ,starting with ordinary barley1 by the action of heat1 aided by natural growth1 he produces the raw material on which the brewer may work4 ,now barley1 like all grain1 is largely made up of starch1 and although starch will not make alcohol1 it can be turned into sugar1 which will4 ,so the task of the maltster is to commence the change of the starch in the grain into sugar4 ,first of all it is soaked in water and spread upon floors and heated until it begins to sprout4 ,there is a little part in each grain called the endosperm1 which is the embryonic plant1 and the starch is really the food provided by nature to nourish the growing endosperm until such time as it shall be strong enough to draw its nourishment from the soil4 ,in order that it may not be washed away prematurely1 the starch is locked up by nature in closely fastened cells1 and1 moreover1 it is insoluble1 so that water cannot carry it away4 ,the endosperm1 however1 has at its disposal certain substances known as enzymes 7and it increases its store of these as it grows71 one of which is able to dissolve away the walls of the cells1 to unlock the treasures1 as it were1 while the other turns the insoluble starch into soluble matter1 in which state the growing organism is able to make use of it as food4 ,so as the grain sprouts upon the maltster's floor this process is going on--the cells are being opened and their contents converted from starch into soluble matters4 ,then1 when the growth has gone far enough1 the grain is transferred to a kiln1 where it is subjected to heat1 by which the growth is stopped4 ,the living part of the grain is1 in fact1 killed4 ,that is mainly to stop the young plant from eating up the altered starch1 which it would do if allowed time1 but which the brewer wants to be kept for his own use4 ,the maltster's task is now finished1 and we come to the brewer's4 ,the first thing he does with the malt is to crush it between rolls1 thereby liberating thoroughly those substances which have been formed from the starch and which he intends to turn into sugar4 ,having crushed it1 he places it in the mash tun1 a large tank of wood or iron1 in which it is mixed with water and subjected to heat4 ,while in this vessel the enzymes become active again and turn the soluble starch1 or a part of it1 into a kind of sugar4 ,the liquid drawn off from the mash tun1 containing1 of course1 the sugar1 is subsequently boiled1 numerous flavouring matters 7including hops7 are added1 and then it is cooled again1 ready for the final process--fermentation4 ,this takes place in a large vat or tun and is brought about by the agency of yeast which is added to the liquid4 ,now yeast is a multitude of microscopic plants round in shape and about one three-thousandth of an inch in diameter4 ,though so small1 this little organism is really quite complicated in its structure1 and within its little body there are carried on complicated chemical changes which baffle entirely the most learned chemist to imitate4 ,further1 he has yet to find out how the little yeast plant does it4 ,he not only cannot imitate the process1 he does not know what the process is4 ,these little organisms multiply mainly by the process of budding4 ,a new one grows out of the side of each old one1 rapidly reaches maturity1 breaks away and commences an independent existence4 ,no sooner is it free than it in turn gives birth to another4 ,indeed so great is its hurry to propagate itself that sometimes the new cell begins to throw out a bud before it has itself separated from its parent4 ,it is therefore easy to see that yeast increases in quantity by what some call leaps and bounds1 but which the mathematically minded know as geometrical progression4 ,the particular form of sugar with which we are concerned here is known as dextro-glucose4 ,this the yeast splits up into alcohol and carbonic acid gas4 ,the latter bubbles up to the surface1 and escapes into the air1 while the alcohol becomes dissolved in the watery liquid4 ,it is believed that the yeast performs this operation not directly1 but by the production of certain enzymes1 which in their turn act upon the sugar4 ,the liquid so formed is beer4 ,but since it is alcohol with which we are concerned1 and not beer1 many details connected with its manufacture have been omitted4 ,enough has been said1 however1 to show that by comparatively simple processes grain of all sorts1 in fact1 anything which contains starch1 and such things are to be found in worldwide profusion1 can be turned into alcohol4 ,all the really intricate chemical functions are performed readily and cheaply by living organisms4 ,all man has to do is to set up the conditions under which the organisms can work4 ,in the process just described only a portion of the starch in the grain is converted into sugar1 hence the percentage of alcohol in beer is comparatively small4 ,if all the starch be converted a liquid much stronger in alcohol is produced1 and if that be distilled1 so as to separate the spirit from the water with which it is mixed1 there results whisky4 ,brandy1 likewise1 is the spirit distilled from wine1 rum from molasses1 and so on4 ,in all these familiar beverages the essential feature is this same alcohol1 of the variety known as ethyl alcohol4 ,it will be noticed that in the making of beer the alcohol is actually formed in water4 ,there is a sugary water which under the action of the yeast becomes an alcoholic water4 ,and this indicates a very useful feature about the liquid when used for industrial purposes4 ,a tank full of petrol is extremely dangerous1 so much so that the storage of petrol is hedged about by all manner of precautions4 ,the danger is that it gives off an inflammable vapour and that if it once begin to burn there is practically no possibility of putting it out4 ,being lighter than water1 it simply clothes with a layer of fire any water which may be thrown on to it4 ,the water in such circumstances simply serves to spread the naming petrol about and so to make matters worse4 ,now alcohol1 with its partiality for the companionship of water1 behaves quite differently4 ,true1 it also may give off an inflammable vapour1 but if a quantity of it catch fire it can be extinguished in the usual way by a fire-engine4 ,the water and alcohol immediately combine--the alcohol becomes dissolved in the water just as sugar may do1 and as soon as the percentage of water in the mixture becomes considerable the burning stops4 ,it may be that some readers will have discovered this fact for themselves without knowing precisely what it was4 ,it is a common dodge with amateur photographers if they want to dry a negative quickly to immerse it in methylated spirit4 ,the spirit seems to take the water out of the film and1 itself drying quickly1 leaves the negative in a perfectly dry condition in a few minutes4 ,now after using spirit in that way it is useless to put it in a spirit stove or lamp4 ,it will not burn4 ,methylated spirit is alcohol1 and the reason why it has such a quick drying action is that it and the water in the wet film quickly mix4 ,after immersion the film is wet1 not with water merely1 but with a mixture of a lot of spirit and a little water4 ,hence the speed with which it evaporates4 ,and the non-inflammability of the mixture is due to the presence of the water4 ,methylated spirit only differs from the alcohol in alcoholic beverages in that something is added to make it undrinkable4 ,owing to the craving for it1 which is so widespread1 and the doubtful effect which it has on certain citizens1 most states regard it as pre-eminently a subject for taxation1 thereby on the one hand bringing in a good revenue1 and on the other discouraging its too free use4 ,but those considerations apply only to drinkable alcohol4 ,that which is to be used for industrial purposes is not in any way a legitimate object for taxation4 ,hence the problem arises of making a form of alcohol which shall answer all the needs of the industries which use it1 and at the same time be so repulsive to the senses that no one can possibly drink it4 ,this result is achieved by adding some of the methyl alcohol derived from the vapour given off by wood when heated4 ,commonly known as wood spirit1 this is so unpleasant that it renders the mixture of no use for drinking1 and so it can safely be freed from taxation4 ,unfortunately this spirit has less heating value than petrol4 ,that means that a given quantity of each liquid will produce more heat in the case of petrol than in the case of alcohol4 ,indeed the difference is about two to one4 ,hence an engine to give out a certain horse-power would need to have its cylinders twice as big if it were to use alcohol instead of the other fuel4 ,there is a certain compensation1 however1 in the fact that alcohol is very easily compressible4 ,in modern internal combustion-engines much of the efficiency is due to the explosive charge which is drawn into the cylinder being compressed into a small space before it is fired4 ,it was the discovery of the value of compressing the gas which made the gas-engine so formidable a rival to the steam-engine1 and the wonderful performances of the ,diesel engines are due very largely to the fact that the air is compressed in the cylinder to a very high pressure4 ,the jet of oil burns in highly compressed air4 ,and because of the facility with which alcohol can be compressed it is said to be more effective as a source of motive power than would be expected from its comparatively feeble heat4 ,thus we may sum up the possibilities of the future4 ,coal1 petroleum and their derivatives exist in limited quantities in the world1 and so far as we can see the vast drafts which we are taking from them are not being replaced1 indeed at this stage of the earth's development cannot be replaced1 by any more4 ,sooner or later we must come to an end of them4 ,is it not comforting1 therefore1 to know that there is another source of fuel at hand1 inexhaustible1 since it can be produced as needed4 ,we have only to set the sun and the ground to work to produce grain1 rice1 potatoes1 or any of the myriad substances which contain starch1 and from that1 by simple and well-known processes1 we can obtain a cheap1 safe and reliable fuel4 ,indeed there seems nothing but the ultimate loss of sunlight1 countless millions of years hence1 which can ever check the supply of this valuable commodity4 ,what has doubtless1 in many cases1 been a curse in the past may turn out to be the great boon of the future4 ,,chapter ,,iv ,,some ,,valuable ,,electrical ,,processes ,students of that branch of science known as physics are coming to the conclusion that electricity plays a much more important part in the universe than was supposed4 ,they are led to believe that electrical attraction is the cement which binds together those exceedingly minute particles out of which everything is built up4 ,whether electricity binds them together or not1 it is certain that electrical action can in some cases separate those particles1 and this process of separation provides a means of carrying on some very remarkable and useful industrial processes4 ,let us imagine a vessel filled with water to which has been added a little sulphuric acid1 while suspended in it are two strips of platinum4 ,there is a space between the strips1 so that when their upper ends are suitably connected to a source of electric current that current flows from one strip to the other through the liquid4 ,that is an example of the apparatus for carrying out this electrical separation in its simplest form1 and it will facilitate the further description if the names of various parts are enumerated4 ,the process itself is electrolysis2 the liquid is the electrolyte1 while the strips are the electrodes4 ,the individual electrodes1 again1 have special names1 that by which the current enters being the anode and that by which it leaves the cathode4 ,it is not difficult to remember which is which if we bear in mind that the current traverses them in alphabetical order4 ,since1 however1 it may not be easy for the general reader to carry all these terms in his mind1 we will1 when it is necessary to differentiate between the two electrodes1 call one the in-electrode and the other the out-electrode4 ,returning now to our imaginary apparatus1 let us turn on the current4 ,at first nothing seems to be happening1 although suitable instruments would show that current was flowing4 ,soon1 however1 little bubbles appear upon the electrodes1 and these grow larger and larger1 until they detach themselves from the platinum to which they have been adhering1 float up to the surface and burst4 ,the question which naturally arises is1 ,what do those bubbles consist of8 ,are they air8 ,if we take means to collect the gases which formed them we get an unmistakable answer4 ,the bubbles which arise from the in-electrode are oxygen1 those from the other hydrogen4 ,if we allow our apparatus to work for some time1 and collect all the gas which arises1 we shall find that there is twice as much hydrogen as oxygen4 ,we shall also find1 as the process goes on1 that the quantity of water diminishes4 ,perhaps ,i may be allowed at this point to remind my readers that water is a collection of minute ultra-microscopic particles called molecules1 each of which is formed of three smaller particles still called atoms4 ,of the three atoms two are hydrogen and one oxygen4 ,water therefore consists of hydrogen and oxygen1 there being twice as much of the former as there is of the latter4 ,we see1 therefore1 that electrolysis gives us hydrogen and oxygen in exactly those proportions in which they occur in water1 and since we also see that as these gases appear the water itself disappears1 we are led to conclude that the current is splitting up the water into the gases of which it is formed4 ,but the strange thing is that this will not work with pure water4 ,we have to add something to it4 ,in the case of our imaginary experiment it was sulphuric acid4 ,what part does that play8 ,this is not fully understood1 but we may be able to form a mental picture of what is believed to happen as follows4 ,the in-electrode is surrounded by a vast assemblage of these tiny molecules1 most of them those of water1 but a few those of the acid4 ,the latter are more complex in their structure than the former1 but they too contain hydrogen4 ,current flows into the electrode and instantly hydrogen atoms from the acid molecules crowd round it1 like boatmen at the seaside anxious to secure a passenger4 ,each takes on board a quantity of electricity and with it darts across the intervening space to the other electrode4 ,arrived there1 it gives up its load and1 its work done1 remains lying upon the electrode until enough others like unto itself have gathered there to form a bubble and so escape4 ,these hydrogen atoms are thought to be the craft which carry the current through the liquid and enable it to pose1 as it were1 as a conductor of electricity1 which in reality it is not4 ,but where does the oxygen come from8 ,to find the answer to that we must add a second chapter to our story4 ,when the hydrogen boats took on board their load of electricity they left their former associates1 and these forthwith set upon neighbouring water molecules and with the audacity of highwaymen stole from them enough hydrogen atoms to take the place of those they had lost4 ,thus the acid molecules became complete once more1 while the scene of the conflict near the in-electrode was strewn with the remains of the water molecules from which the hydrogen atoms had been stolen4 ,these remains1 of course1 would be oxygen1 and they1 collecting together on the electrode1 would eventually be in numbers sufficient to form bubbles and so escape4 ,thus it may be the acid which really does the work1 yet because of its subsequent raid upon the water it is the latter which disappears1 and it is the materials of the latter which are bought to the surface in the bubbles4 ,and there we see the mechanism whereby1 so it is believed1 electric current can pass through otherwise non-conducting liquids4 ,and the important point1 as far as practical utility is concerned1 is that the passage of the current is accompanied by a splitting up of something or other1 either the water or something in it1 the materials of which are deposited1 one on one electrode and the other on the other4 ,and now we can proceed to those useful applications of electrolysis1 the commonest of which1 perhaps1 is electro-plating4 ,we have seen how electrolysis causes hydrogen1 probably out of the acid1 to be deposited upon one electrode4 ,suppose that1 instead of an acid1 we put in the water one of those substances known to chemists as a salt1 the commonest example of which is ordinary table salt4 ,this well-known condiment is caused by the interaction of hydrochloric acid and the metal sodium and will serve to illustrate what all salts are4 ,all acids are compounds of hydrogen and something else1 and their biting action is due to the readiness with which the something else evicts the hydrogen and takes in a metal in its place4 ,thus hydrochloric acid1 given the opportunity1 gets rid of its hydrogen and takes in sodium1 thereby forming chloride of soda or common salt4 ,another example is the gold chloride familiar to photographers4 ,this is the result of the action of certain acids upon gold1 wherein the acids throw out their hydrogen and take in gold instead4 ,to sum up1 then1 a salt is just the same sort of thing as an acid1 like the sulphuric acid which we used in our experiment1 except that some metal has taken the place of the hydrogen4 ,it is not surprising1 then1 to find that if we put a salt in the electrolyte instead of an acid we get a similar result4 ,in the one case hydrogen is deposited upon the out-electrode1 in the other the metal4 ,in the former case1 since hydrogen is a gas1 it forms bubbles and floats away1 but in the latter the solid metal remains a thin1 even coating upon the electrode4 ,that is the principle of electro-plating4 ,the electrolyte consists of a suitable solution containing a salt of the metal to be deposited1 and it is placed in an insulating vessel or vat4 ,the articles to be plated form the out-electrode1 so that they have to be suspended in some convenient way from a metal conductor by conducting wires4 ,of course they are entirely immersed in the liquid4 ,the in-electrode is sometimes a plate of platinum 7the reason that expensive metal is used being that it is unaffected by the chemicals7 or else a plate of the metal being deposited4 ,in the former case1 the solution becomes weaker as the work proceeds1 and more salt has to be added4 ,in the latter1 however1 the strength of the solution remains unchanged1 for by an interesting interchange the in-electrode adds to it just what it loses by deposition upon the other one4 ,the effect is therefore just as if the current tore off particles from the one and placed them upon the other4 ,this is believed to be due to the agency of the oxygen which in the case of the electrolysis of water becomes free1 but which in this case forms with the metal electrode a layer of oxide upon its surface1 this oxide being then dissolved away by the liquid4 ,thus as fast as the metal is deposited upon the out-electrode its place is taken by more metal from the in-electrode4 ,in some processes it is desired to deposit metal upon a non-conducting surface1 and it is evident that such cannot be used as an electrode4 ,nor is it any use to attempt to deposit upon anything except an electrode4 ,the only thing to do1 then1 is to make the object a conductor by some means4 ,models in clay1 wax and plaster1 once-living objects like small animals1 fruit1 flowers or insects1 can1 however1 have a perfect replica made of them by electrical deposition1 by the simple method of coating the surface to be plated with a thin layer of plumbago4 ,this skin1 although extremely thin1 is a sufficiently good conductor to make the process possible4 ,process blocks for printing are copied in this way1 so that a particularly delicate example of the blockmaker's art need not be worn down by much pressing1 copies or electros being made off it for actual use in the press4 ,the original block is a plate of copper on which the picture is represented by minute depressions and prominences4 ,on this a layer of soft wax is pressed1 so as to obtain a perfect but reversed copy4 ,having been coated with plumbago1 this is then put into a vat containing a solution of copper salts and is used as the out-electrode1 the other being a plate of copper4 ,when the current is turned on the copper is thus deposited on the wax until a thin sheet of copper is formed which is an exact but reversed copy of the wax1 a direct copy1 that is1 of the original block4 ,the back of this thin sheet is then covered with molten lead or type metal to fill up any depressions and to give it sufficient strength4 ,anyone who has seen one of these half-tone blocks covered with minute depressions so slight that they can scarcely be seen1 yet so perfect that a beautiful print can be obtained from them1 will realise the wonderful power of this electrolytic process1 the marvellous accuracy with which the original is copied1 and the unerring way in which the electric current carries the particles of copper into every one of the myriad recesses in the wax4 ,another specimen of the marvellous work of this system is the wax cylinder of the phonograph4 ,the sound is produced by a needle trailing along a groove of varying depth cut in the surface of the cylinder4 ,this groove forms a spiral1 passing round and round like the thread of a screw1 and it encircles the cylinder one hundred times in every inch of its length4 ,consequently1 at any point one may take1 there is but one one-hundredth of an inch from the centre of one turn to the centre of the turn on either side of it4 ,and at its deepest the groove is less than one-thousandth of an inch deep4 ,the phonograph itself cuts the first master record1 as it is termed1 and the problem is to take a number of casts off this model of such delicacy and accuracy that every variation in that exceedingly fine groove shall be faithfully reproduced4 ,such a task might well be given up as hopeless1 but with the help of electrolysis it is accomplished easily and cheaply4 ,to attempt to press anything upon the surface of the master would but smooth out the soft wax and obliterate the groove altogether4 ,to apply anything softened by heating would be to melt it4 ,but electrolysis1 without tending in any way to distort or damage the delicately cut surface1 deposits upon it a surface of metal from which thousands of casts can be made4 ,the gentle fingers of the electricity overlay the soft wax with the hard1 strong metal with a minute perfection almost beyond belief4 ,to commence with1 the master record is placed upon a sort of turntable in a vacuum and turned round in the neighbourhood of two strips of gold-leaf strongly electrified4 ,by this means the gold is vaporised and a perfect coating of gold is laid upon the wax4 ,this is far too thin to be of any use1 except to render the cylinder a conductor1 for the coating is so fragile that it is no stronger than the wax itself4 ,it enables the cylinder1 however1 to be electro-plated with copper until it is surrounded by a strong metallic shell a sixteenth of an inch thick4 ,it takes about four days to deposit this thickness4 ,the copper shell is then turned smooth in a lathe and fitted tightly into a brass jacket4 ,a little cooling causes the wax record to shrink sufficiently to free it from the copper shell and allow it to be lifted out4 ,a copper mould is thus formed in which any number of additional records can be cast4 ,the molten wax is simply introduced into the inside1 and allowed to set2 the inside is bored out in a lathe1 and then with a little cooling it shrinks and can be withdrawn1 a completely finished record1 every tiny depression or swelling in the original master being reproduced with an accuracy almost incredible4 ,another valuable use to which this process is put is the purification of metals4 ,the electro-chemical action works with unerring precision3 it never mistakes an atom of iron for an atom of copper1 for example4 ,passing through a solution of copper salt1 the current deposits only copper4 ,for modern electrical machinery and apparatus copper is required of the utmost possible purity1 for every impurity adds to its electrical resistance1 in other words1 diminishes its value as a conductor4 ,consequently thousands of tons of electrolytic copper1 as it is termed1 are produced every year4 ,the electrodes used are plates of ordinary copper4 ,a coating of pure metal is deposited by electrolysis upon the out-electrode from the other one4 ,when the deposit is thick enough the out-electrode is taken out and the deposit torn off it1 the union between the two being sufficiently imperfect for this to be done without difficulty4 ,the metal of which the in-electrode is made has already been purified by other processes1 until it contains but one per cent4 of foreign matter1 and by this means even that small percentage is entirely got rid of4 ,the impurities fall to the bottom of the vessel in the form of slime1 which is periodically removed4 ,and not only is electrolysis thus unerring in picking out certain atoms from among a mixture1 but there is an exact relation between the work done and the quantity of current used4 ,consequently it forms a very exact method of measuring currents4 ,the method of measuring current by the strength of the magnetic field which it produces has been mentioned already1 and such measurements can be checked by electrolysis4 ,thus the practical definition of the ampere is that current which when passed through a solution of silver nitrate in water will deposit silver at the rate of 4#jjaaah gramme per second4 ,the electric accumulator or secondary battery1 one of the most useful appliances1 is the result of electrolysis reversed4 ,many large electric-lighting plants have in addition to their generating machinery a large battery of secondary cells1 which1 being kept charged1 are able to help the machinery in times of heavy demand1 or even to supply the whole current needed for1 say1 half-an-hour1 so that the whole of the machinery could1 in the event of an accident1 be shut down for that time and the supply maintained from the batteries4 ,this would be sufficient in many cases for fresh machinery to be brought into action or emergency arrangements to be made4 ,it may be that this book is being read by someone seated serenely in his arm-chair while engineers and workmen at the generating station are working in frantic haste to set right some sudden breakdown before the batteries are run down4 ,the batteries may have saved the town half-an-hour's darkness4 ,large telegraph offices are fitted with secondary batteries4 ,many motorists owe the ignition which keeps their engines at work to secondary batteries4 ,it is secondary batteries which keep the wireless apparatus at work on a wrecked vessel after the engines have stopped4 ,indeed secondary batteries are one of the most beneficent inventions4 ,and if only they could be made in a lighter form than is possible at present their value would be infinitely increased4 ,we have seen how the passage of current through acidulated water produces hydrogen and oxygen4 ,if those gases be collected in closed vessels over the water1 so that they remain in contact with the water1 as soon as the current is stopped a reverse action sets in4 ,the gases tend to recombine with the electrolyte and in so doing to give back a current equal to that which formed them4 ,fig4 #d shows the construction of what is called a voltameter1 in which the gases arising from the electrodes are collected in little glass vessels placed just above them4 ,such an apparatus enables us to see easily how the accumulator works4 ,the picture shows the battery which is effecting the separation of the oxygen and hydrogen4 ,if that be disconnected1 and the wires joined1 as shown by the dotted line1 a current will flow back until the oxygen and hydrogen have returned into the solution again4 ,the apparatus will1 in fact1 work like an ordinary battery1 except that instead of a plate or rod of zinc a mass of hydrogen will form the essential part4 ,an appliance such as a voltameter is not of much use for the practical purpose of storing large quantities of electrical energy1 because the surfaces of the electrodes are so small and the surfaces where liquid and gases are in contact are small too4 ,it is clear that the larger the electrodes are the wider will be the passage for the current1 just as a wide road can accommodate more traffic than a narrow path4 ,we may regard the electrodes as like gateways through which the current passes4 ,by making them large1 therefore1 we enable a large current to pass1 and consequently permit electrolysis to take place with great comparative rapidity4 ,illustration3 ,,fig4 #d4 ,the plates1 as the electrodes in a secondary battery are termed1 are generally large metal plates4 ,experiment has shown that lead is the best for this purpose4 ,it has also been found that it can be improved by making it porous1 since the inner surfaces of the pores are so much added surface through which current can pass into the electrolyte4 ,there are various ways of producing this porosity1 which need not trouble us here1 however4 ,it will suffice for our purpose to understand that an ordinary secondary cell consists of two lead plates1 with the largest possible surface1 immersed in a liquid1 generally a dilute solution of sulphuric acid in water4 ,to charge the battery1 current is sent to one plate1 through the liquid to the other plate1 and so away4 ,a thin film of hydrogen is thus formed upon the outgoing plate1 while oxygen is formed at the incoming one4 ,since the hydrogen is spread over such a large area1 it does not accumulate sufficiently for much of it to rise to the surface4 ,most of it remains adhering to the plate4 ,the oxygen combines with the lead of its plate and so is safely stored up there in the form of oxide of lead4 ,this storage of hydrogen upon the one plate and oxygen on the other cannot go on indefinitely1 and so as soon as the limit is reached the cell is fully charged4 ,passage of further current is then simply waste4 ,the dynamo or primary batteries which are used for charging having been disconnected1 the two plates can be connected together through lamps1 motors1 or in any other desired way1 and the current will then flow out again1 the opposite way to that in which it entered1 just as a stone thrown up in the air returns the opposite way4 ,the current which comes out is1 in fact1 a sort of reflex action arising from that which went in1 the mechanism by which it is produced being the reabsorption of the oxygen and hydrogen into the electrolyte4 ,whether a cell is fully charged or not is ascertained by weighing the electrolyte1 an operation which at first sight seems to have nothing whatever to do with the matter4 ,it arises from the difference in weight between water and sulphuric acid1 the latter being the heavier4 ,we have seen that while a little acid must be added to water before it can be electrolysed1 it is the water which is ultimately resolved into its constituent gases4 ,hence the result of electrolysis is to increase not the amount1 but the proportion of acid4 ,therefore it increases the weight of the electrolyte4 ,this weight is ascertained by means of a hydrometer1 a glass tube1 stopped1 and loaded with some small shot at its lower end4 ,on the upper part is engraved a graduated scale1 so that the exact depth to which it sinks can be easily read4 ,this depth will1 of course1 vary with the specific gravity of the liquid1 and so the depth recorded by the scale will be an indication of the proportion of acid1 and that in turn will show how far the process of charging has progressed4 ,accumulators are1 or have been hitherto at any rate1 very troublesome things4 ,they are apt to lose their power4 ,if not properly charged they are easily damaged4 ,too rapid charging or too rapid discharging1 standing for a while only partly charged--all these things have a bad effect1 in extreme cases even destroying them altogether4 ,because of the use of lead they are terribly heavy too1 so much so that for traction purposes they are of very little use1 for a large amount of the energy stored in the accumulators is then used up in hauling them about4 ,yet what a field there is for the successful accumulator6 ,take the one instance of the electrification of a railway4 ,if good light and efficient accumulators were to be had1 no alteration at all would be necessary to the permanent way4 ,the engines or motor carriages would need to go periodically to a depot to be re-charged1 but that could easily be arranged4 ,such things as conductor rails1 overhead conductors and so on would be needless4 ,the world has therefore been interested for years in the rumour that ,t4 ,a4 ,edison was engaged upon this problem1 and at last he has produced his accumulator1 by which he has removed many of the difficulties1 if not all4 ,instead of a case of glass or celluloid1 as is usual with the older cells1 his cells are enclosed in strong boxes of nickel steel4 ,the positive plate consists of nickel tubes filled with alternate layers of nickel hydroxide1 while the negative plate is formed of prepared oxide of iron in a nickel framework4 ,the electrolyte is a solution of potassium hydroxide4 ,the chemical action and the electrical reaction is1 of course1 on the same principle precisely as in the older cells1 but it is claimed that the ,edison cells are fool-proof--that is to say1 they cannot be damaged by careless handling1 and they appear to be a little lighter4 ,thus the problem is partly solved1 and with that as a fresh starting-point someone may sooner or later give us a secondary battery which is light as well as strong4 ,if any would-be scientific inventor reads these words there is a suggestion for a promising line of investigation4 ,,chapter ,v ,,machine-,,made ,,cold ,one of the most remarkable adaptations of scientific knowledge is the manufacture of cold4 ,at first that phrase seems strange1 but it is really quite legitimate4 ,there are machines at work at this moment which are turning out cold as if it were any other manufactured article4 ,it is not that they manufacture cold water or cold air1 it is the cold itself which they produce4 ,of course1 cold has no real existence1 since it is simply a negative quantity1 an absence of heat1 yet its effects are so real that we are in the habit of talking of it as if it were a reality1 and in that sense we can regard it as a product of manufacture4 ,moreover1 we see in this a conspicuous instance of the interdependence of invention and science1 for scientific principles were first adapted to produce cold1 and then artificial cold was employed in scientific investigations1 whereby the rare gases of the atmosphere have been discovered1 as we shall see presently4 ,in ,mechanical ,inventions of ,to-day ,i have dealt with the uses which can be made of heat as a motive power4 ,here we have in some sense a reversal of the process4 ,in the heat-engine the expenditure of heat produces motion4 ,in the refrigerating machine motion produces heat1 on the face of it a strange way of producing cold4 ,yet it is by the production of heat in the first instance that we are ultimately able to obtain the cold4 ,one way to make a thing cold is to place it in contact with ice4 ,but that process suffers from severe limitations4 ,in the first place1 we may not be able to procure ice when we want it4 ,and in the second place1 we may want to produce a temperature much lower than that of ice4 ,now a machine can produce any degree of coldness1 almost down to the absolute zero1 the point at which a body is absolutely devoid of any heat whatever1 the condition in which its molecules are absolutely still4 ,that point is #bgd deg4 ,c4 below freezing-point4 ,freezing-point on that scale is zero1 and so this absolute zero is minus #bgd deg4 ,or1 to put it another way1 freezing-point is #bgd deg4 absolute temperature4 ,the absolute zero has never been reached1 and there is reason to believe that it never can be quite reached1 but by methods about to be described a temperature within a few degrees of it has been attained4 ,and all of this can be done without any cooling agent colder than water at an ordinary temperature4 ,there are several systems1 but the one which illustrates the principle most simply is that in which carbonic acid gas is the working fluid4 ,this is a very compressible gas1 and so is well fitted for the purpose4 ,first of all a pump or compressor compresses it4 ,that has the effect of heating it4 ,such we might expect from the fact that heat is molecular activity3 when by compressing the gas we force the molecules closer together1 they naturally hit each other and the sides of the containing vessel harder than they did before1 and the increased activity is manifested as increased heat4 ,so the first effect1 as was remarked just now1 is to produce1 apparently1 increased heat4 ,but then the hot compressed gas1 by being passed through a coil of pipe surrounded by cold water1 can be robbed of that heat4 ,according to the speed at which it traverses the coil it will be more or less cooled3 by causing it to travel slowly it can be brought down almost to the temperature of the water4 ,so we start with the gas at atmospheric pressure and at somewhere about atmospheric temperature too4 ,this we convert into compressed gas at a high temperature4 ,after cooling it we have compressed gas at a moderate temperature4 ,then1 to complete the process1 we let the gas expand again4 ,now just as compressing a gas heats it1 letting it expand cools it4 ,if we compressed it and then expanded it again we should be just as we were to commence with4 ,but since1 in between the two operations we extract a quantity of heat by means of the cooling water1 we get at the end a very much lower temperature than that with which we started4 ,we cannot cool the gas without compressing it1 because heat will only flow from one body into another when the second is cooler than the first4 ,but by making the gas hot temporarily by compression we enable the water to draw some heat from it1 and then1 allowing it to sink back to its original state1 we get practically the old temperature1 less what the water has extracted4 ,the principle is really absurdly simple when one once gets to understand it4 ,the application is not so simple as far as the designer of the machine is concerned1 for he has to adjust the various parts to exactly the right shape and dimensions1 so that they may work well with one another and produce the desired result with the minimum expenditure of power4 ,to the observer1 however1 and to the user too1 the finished machine is wonderful in its simplicity4 ,the principle is illustrated diagrammatically in ,fig4 #e4 ,in the centre is the compressor4 ,its action forces the gas along the pipe to the right and down into the condenser4 ,as it flows downwards through the coil there cold water enters at the bottom of the tank1 flows upward past the coil and escapes again at the top4 ,thus the coil is kept in contact with cold water4 ,passing then through the bottom of the tank the gas travels from right to left through the regulating valve and into an arrangement almost exactly similar to the condenser but called the evaporator4 ,here the gas expands and suffers a great fall in temperature4 ,this cold is communicated to liquid circulating in the tank which forms a part of the evaporator1 and this liquid can be circulated through pipes into any rooms to be cooled or around vessels of water which it is desired to freeze4 ,this liquid1 which acts as the carrier of the cold1 is called brine1 and is water to which is added calcium chloride to keep it from freezing4 ,illustration3 ,,fig4 #e4--,this diagram shows the working of the ,refrigerating ,machine4 ,the pump compresses the gas and drives it through the coil in the condenser1 where it is cooled by water4 ,it passes thence through the coil in the evaporator1 where it expands and cools the surrounding brine4 ,now the observant reader may have noticed that there is no apparent reason for the name of the left-hand vessel4 ,it will be quite clear1 however1 when ,i explain that although ,i have spoken of the working fluid all along as gas1 ,i have only done so to avoid bringing in too many explanations at once4 ,it is actually liquid for a good part of its journey4 ,carbonic acid gas liquefies at a very moderate temperature and pressure1 and so while it leaves the compressor as a gas it becomes liquid in the condenser and remains so until it has passed the regulating valve4 ,then it begins to expand into gas once more1 and in that state it passes back to the compressor4 ,there is a pressure-gauge on the pipe leaving the compressor and another on the one entering it4 ,a comparison of the readings on these two tells how the apparatus is working4 ,the difference between them indicates how much compression is being given to the gas4 ,assuming that the compressor is working at a constant speed1 this compression can be regulated to a nicety by the valve3 close it a little and the compression will increase3 open it a little and the compression will decrease4 ,by this means the degree of cold produced can be varied at will4 ,this is the way in which many ships are enabled to carry cargoes of frozen meat4 ,the chambers in which the meat is stowed are insulated--that is to say1 their walls are made as impervious as possible to heat4 ,then the brine is carried into the chambers in pipes1 cooling them much as the hot-water pipes heat an ordinary public building4 ,or another method is to carry the pipe which constitutes the evaporator into the chamber to be cooled4 ,a third way is to dispense with brine and to blow air through the coils of the evaporator1 whereby the air is made to carry away the cold to wherever it is needed4 ,ice can be made easily in moulds of metal or wood around which brine circulates4 ,if made of ordinary water the ice is likely to be cloudy and opaque1 which is quite good enough for many purposes4 ,in cases where it is desired that it should be clear1 the water is agitated during freezing1 or else distilled water is used4 ,to enable the blocks to be got out of the moulds it is sometimes arranged to circulate warm brine for a few moments4 ,ice skating rinks are formed by making1 first1 an insulating layer of sawdust1 slag-wool or something of that sort 7those by the way1 being the materials generally used for insulating cold chambers7 underneath the floor4 ,the floor1 too1 is made waterproof and then upon it is laid as closely as possible a series of iron pipes4 ,water is flooded on to the floor until the pipes are covered to a depth of several inches1 and then brine is pumped through the pipes4 ,in time the water freezes1 and so long as the brine circulates it remains so4 ,but although the ,,co#b process described above is the simplest illustration of the principle1 there are other systems4 ,in one very popular form ammonia gas is the working fluid4 ,this is liquefied by pressure and cooling with water1 being subsequently expanded just as described above4 ,another much-used system is the ammonia-absorption process1 in which the ammonia is not liquefied1 but when under pressure is absorbed by water1 returning to gas again when the pressure is released4 ,but the degree of cold attained in these commercial machines is as nothing to the extremely intense cold generated on the same principles in the liquid-air machine1 which is found in every well-equipped physical laboratory4 ,briefly1 this consists of a coil of many turns of small tube enclosed in a small double vessel1 the space between the inner and outer skins of which is packed with insulating material4 ,a compressor pumps air in at the top of the coil at a pressure of from #aej to #bjj atmospheres4 ,an atmosphere1 it may be remarked1 is a unit often used in scientific matters1 meaning the normal pressure of the atmosphere1 which is1 roughly speaking1 #ae lb4 per square inch4 ,hence #bjj atmospheres is about #cjjj lb4 per square inch4 ,of course air so highly compressed as that is hot1 but after it has passed down the coil and has escaped from the valve which liberates it at the bottom it is much cooler4 ,but that is only the beginning of the operation4 ,the expanded1 and therefore cooled1 air finds its way upward through the turns of the coil down which the following air is coming4 ,that1 expanding in its turn1 is colder still1 because of the cooling action of the first air1 and so the process goes on4 ,illustration3 ,by permission of ,messrs4 ,j4 and ,e4 ,hall1 ,ltd41 ,london and ,dartford ,,machine-,,made ,,ice ,here we see a huge block of ice being lifted 7it may be on a hot summer day7 from the mould in which it has been made ,this is perhaps easier to understand if we imagine that the air comes through the coil in gusts and we notice what happens to each succeeding gust4 ,the first comes down1 expands1 cools and ascends1 thereby cooling the second gust as it comes down4 ,the second then1 after expansion1 will be cooler than the first was4 ,that in its turn will cool the third1 and so the third after expansion will be cooler than the second4 ,and that will go on1 each succeeding gust being cooler than the one before4 ,and although the flow of air is continuous1 and not in gusts1 the result is just the same3 it goes on getting cooler and cooler until at last the air comes out in its liquid form4 ,this liquid collects in a little chamber formed at the bottom of the vessel which contains the coil and can be drawn off when desired4 ,air in its liquid state looks very much like water4 ,in fact it is difficult to get chance observers to believe that it is not water4 ,it boils at a temperature far below the freezing-point of water1 so that liquid air if placed in a cup made of ice will boil furiously4 ,ice is so much the hotter that it behaves towards liquid air as a very hot fire does to water4 ,the feature of the above machine1 it will be noticed1 is that no cooling water is required1 as in the refrigerating machine1 although the principle of the two is the same4 ,the coil is the condenser and the vessel in which it is enclosed is the evaporator1 and so the cold air produced by the process in the evaporator cools the coil of the condenser4 ,thus it is self-intensive1 as the makers call it4 ,hydrogen can be liquefied in a similar machine1 except that it needs a little preliminary cooling with liquid air4 ,liquid hydrogen is the coolest thing known approaching the region of absolute zero4 ,and now we can turn to the wonderful discoveries which have followed upon the manufacture of liquid air4 ,to make the story complete we need to go back to the time of ,priestly and ,cavendish1 early in last century4 ,they investigated the atmosphere and showed that it consisted of oxygen and nitrogen in certain invariable proportions1 with under certain conditions a small proportion of carbonic acid4 ,these facts were so well authenticated1 and they seemed to explain everything so satisfactorily1 that it was quite thought almost up to the end of the nineteenth century that there was nothing more to learn about the atmosphere4 ,nevertheless there was an idea in the minds of some scientists that there must be another group of elements somewhere1 the existence of which was then undiscovered1 but it was never dreamed that these were in the air4 ,soon after the weights of the atoms had been found a medical student named ,prout in an anonymous essay called attention to the fact that there were curious numerical relationships between them4 ,speculation on the subject went on for many years1 until in #ahfe the great ,russian chemist ,mendeleeff published his conclusions4 ,he had arranged the elements in the form of a table in the order of their atomic weights4 ,the table consisted of twelve rows of names forming eight vertical columns1 and the remarkable thing was that all those elements which fell into any particular column1 although their atomic weights were very widely different1 had similar properties4 ,this enabled him to predict the discovery of certain new elements1 for the table contained a number of blank spaces4 ,three elements have been found since1 and their atomic weights and properties are just such as to fill three of the blank spaces4 ,one blank space1 it is thought1 may be filled some day by the gas coronium1 which like helium has been discovered in the sun1 but unlike it has not yet been detected here4 ,when it is1 there is the place in the table which it may fill4 ,the table then commenced with what is still called ,group #a1 but for reasons too complicated to explain here it appeared as if there must be a group before that1 a group the chief characteristic of which would be the inactivity of the elements included in it4 ,these were expected to be of various atomic weights1 but these weights1 it was anticipated1 would so occur in the intervals between the others that they would all fall into a new column to the left of ,group #a4 ,in the year #ahib ,lord ,rayleigh was investigating the question of the density of a number of different gases1 including1 so it happened1 nitrogen4 ,now there are several ways of procuring nitrogen4 ,one is to get it from the atmosphere by ridding it of the oxygen with which it is normally mixed4 ,another way is to split up some compound1 such as ammonia1 of which it forms a part1 in such a way as to catch the nitrogen and leave the other elements with which it was combined elsewhere4 ,lord ,rayleigh tried both ways1 and he found that the nitrogen from the atmosphere was denser than that derived from ammonia4 ,sir ,william ,ramsey then carried the matter a step further4 ,he heated atmospheric nitrogen in the presence of magnesium1 under which conditions some of the nitrogen combines with the latter element to form nitride of magnesium4 ,that1 it was found1 made the remaining nitrogen denser still4 ,the explanation then seemed obvious4 ,suppose we imagine a mixture of sawdust and iron filings3 it will be heavier than an equal quantity of pure sawdust4 ,and if we contrive to take away some of the sawdust from the mixture we shall find that what is left is heavier still1 when compared with an equal bulk of pure sawdust4 ,for it is clear that as we take away sawdust we thereby increase the proportion of the heavier iron filings and so we make the mixture heavier4 ,applying a similar process of reasoning to these discoveries1 the conviction grew that the nitrogen of the air was not pure1 but that it had mixed with it a small proportion of some other gas of greater density4 ,they soon succeeded in isolating this denser gas1 to which they gave the name of argon4 ,its atomic weight was found1 and1 wonderful to relate1 it was such that argon fell into a new column to the left of ,group #a1 as had been anticipated4 ,the discovery of argon was announced in #ahid4 ,the next year ,sir ,william ,ramsey1 investigating a gas which had been discovered locked up in the interstices of a mineral called clevite1 was able to state that it was helium1 the element which had been previously noticed by the spectroscope in the sun4 ,like argon1 it was found to be extremely inactive1 and its atomic weight turned out to be such that it too fell into the ,zero ,group4 ,in #ahih ,professors ,ramsey and ,travers found two more gases in the air1 krypton and neon1 and a little later still1 there was found mixed with the krypton a further new gas1 xenon4 ,all of these had their atomic weights found1 and fell into that new column in the periodic table4 ,but what has all this got to do with liquid air8 ,the two subjects are closely related1 for it is by liquid-air machines that these rare gases are now obtained1 and it was from liquid air that the last three were first discovered4 ,for air1 as we well know1 is a mixture of gases1 and when extreme cold and pressure are applied these gases liquefy1 each behaving according to its own nature4 ,they do not all liquefy at the same time1 nor on being relieved from the pressure and heated do all evaporate again at the same temperature4 ,although they emerge from the liquid-air machine in the form of a single liquid1 it is really a mixture of liquids1 each with its own boiling-point4 ,in an earlier chapter we saw how petroleum can be separated into its various constituents1 such as petrol1 by fractional distillation1 advantage being taken of the difference in the boiling-point of the various fractions4 ,the boiling-point of a liquid is1 of course1 the temperature at which it turns freely into vapour1 and just as petroleum when heated gives off first cymogene1 next rhigolene1 then petrol1 benzine1 kerosene and so on1 in the order named1 so liquid air1 when it is evaporated1 gives off its different constituents in order4 ,nitrogen1 oxygen1 argon1 helium1 krypton1 neon and xenon can all be separated each from the others in this way1 by fractional distillation4 ,the heat from the surrounding objects is allowed to get at the liquid1 and the gases are then given off in the order of their boiling-points4 ,and thus we see how the mechanical production of cold has assisted in the pursuit of pure science4 ,the newly-found gases are not of any great use at present4 ,they are so inactive that possibly they never will be1 with one exception1 and that is neon4 ,if an electric discharge be made to pass through a tube filled with this gas1 a beautiful glow is the result1 and it is just possible that neon tubes may become the electric light of the future4 ,that is only a prediction1 however1 and a hesitating one at that4 ,the inactive elements may become of value in explosives4 ,we have seen how important nitrogen is in these dangerous substances1 the chief feature of which is their instability--their readiness1 that is1 to change into something else--which instability is due to the reluctance with which nitrogen enters into them4 ,now nitrogen1 though inactive1 is much less so than these others1 and if a way should ever be found of inducing them to enter into a compound1 that compound will probably be an extremely powerful explosive4 ,,chapter ,,vi ,,scientific ,,inventions ,,at ,,sea ,the safety of our fellow-creatures has always been a strong stimulus to our inventive faculties4 ,the occurrence of a bad railway accident1 and1 roughly1 its nature1 can be inferred from the files of the ,patent ,office1 for such an event brings men's thoughts to devising ways and means of preventing a recurrence1 and an avalanche of such inventions descends upon the patent department in consequence4 ,in like manner a particularly distressing accident to a lifeboat some years ago brought out many inventions for the improvement of those romantic craft4 ,many of the inventions which arise under these conditions are1 of course1 utterly worthless1 but some of them come to stay4 ,it is not surprising1 therefore1 when we think of the almost innumerable wrecks which happen1 even with modern shipping1 that human ingenuity has been extremely busy in devising ways for bringing more of safety and less of risk into the lives of those who go down to the sea in ships4 ,of these perhaps none is more fascinating than the modern lighthouse1 with its tall tower1 its brightly flashing light1 standing undisturbed in the wildest storm1 quietly and persistently sending forth its guiding rays1 no matter how the elements may be buffeting it4 ,there is something specially attractive in this perfect embodiment of quiet strength and devotion to duty4 ,of course1 its origin is very ancient4 ,one of the earliest inventions1 no doubt1 was the bright thought of a very primitive man who lit a fire on a hill to serve as a guide to some belated friends out in their fishing canoes4 ,from some such beginning the modern lighthouse1 a magnificent product of the science of civil engineering and the science of optics1 has arisen4 ,of the difficulties encountered in the construction of lighthouse towers on outlying rocks much has been written4 ,the historic ,eddystone1 for example1 has quite a voluminous literature of its own4 ,of the light itself1 however1 much less is known4 ,it will be interesting first to note the different purposes for which a light may be required1 and then see how the apparatus of the lighthouse is made to serve these purposes4 ,there is the making light1 perched1 if possible1 upon some high eminence1 deriving its name from the fact that the sailor sights it as he is making the land4 ,vessels approaching ,england from the south-west by night first see the light at the ,lizard4 ,the transatlantic vessels know they are approaching land by catching sight of the ,fastnet ,rock light off the coast of ,ireland4 ,cape ,race light serves in the same way for those about to enter the ,st ,lawrence and ,navesink for the entrance to ,new ,york harbour4 ,all such as these have to be of the greatest power practicable1 so that they may be visible not only at the longest possible distance1 but also under unfavourable conditions1 such as haze and slight fog4 ,no light1 of course1 can penetrate thick fog1 but in light fog and haze a powerful light can be seen at considerable distances4 ,for the same reason these lights must be high up1 or the curvature of the ocean's surface will limit their range4 ,a light elevated #ajj feet above the sea-level will be visible nearly #af miles away1 but if only #ej feet up it will be invisible at #ac miles4 ,to be seen #dj miles away it must be as high as #ajjj feet4 ,but then again height is in some cases a disadvantage1 for sometimes fog hovers a little distance above the sea1 while below it the air is clear1 and the higher a light may be the more likely is it to have its lantern immersed in a floating cloud of fog4 ,many readers familiar with the south coast of ,britain will remember that the light which used to show on the summit of ,beachy ,head is there no more1 but has been replaced by a tower at the foot of the cliffs1 the reason being that it may be below the clouds of fog which are prevalent at that point4 ,but the mention of ,beachy ,head introduces us to another class of lights1 known as coasting lights1 since they are intended to lead the mariner on from point to point along a coast4 ,it will be seen at once that in many cases they do not need to be visible at such great distances as the making lights4 ,when the mariner has sighted the ,lizard1 for example1 he knows where he is4 ,in order that he may learn that important fact as soon as possible it is desirable that that light should have the greatest possible range1 but having thus located himself1 when he begins to feel his way along the ,english ,channel he is guided by the coasting lights1 and so long as they are of such range that he will never be out of sight of one or two of them that will be sufficient4 ,thus the ,beachy ,head light1 in its present low position1 has a sufficient range for its purpose1 with the added advantage of more freedom from obscuration by fog4 ,thus we see how the local conditions and the purpose of each particular light have to be taken into consideration in determining its position and power4 ,the ,eddystone1 again1 is an example of a further class4 ,it simply serves to denote the position of a group of dangerous rocks4 ,its function is not so much guidance1 although no doubt it often serves for that1 but for warning4 ,the ,lizard light beckons the on-coming ship to the safety of the ,english ,channel2 the ,eddystone warns it away from danger4 ,the latter1 therefore1 and similar lights are warning lights4 ,illustration3 ,by permission of ,messrs4 ,j4 and ,e4 ,hall1 ,ltd4 ,a ,,cold ,,store ,interior of a cold store1 in which meat and poultry are kept good and fresh by the use of machine-made cold4--,see p4 #fg ,right at the entrance to the ,english ,channel1 that greatest of all highways for shipping1 there lie the ,scilly ,isles4 ,this group comprises some few islands of fair size from which we draw those plentiful supplies of beautiful spring flowers1 but it also includes a large number of rocky islets which have sent many a strong ship to its doom4 ,on one of the islets1 therefore1 the ,bishop's ,rock1 there now stands a very powerful light which exemplifies many whose purpose is the double one of welcoming the mariner as he approaches our shores and at the same time warning him of a local danger4 ,such are both making and warning lights4 ,of no less importance1 though less impressive1 are the guiding lights1 which guide the ships into and out of harbours and through narrow channels4 ,these are generally arranged in pairs1 one of the pair being a little way behind and above the other4 ,thus when the sailor sees them both1 one exactly over the other1 he knows he is on the right course4 ,sometimes lighthouses have subsidiary lights as well as the main light1 to mark a passage between two dangers1 or to give warning of some danger4 ,the subsidiary lights are often coloured1 and they are generally sectors showing not all round a complete circle1 or even a considerable portion of one1 but just in one certain direction4 ,they are generally shown from a window in the tower lower down below the main light4 ,finally1 it is important to remember that every light must be distinguishable from its neighbours4 ,hence every one in any given locality has a different character from all the others4 ,this character is given to it by means of flashes4 ,instead of showing1 as the primitive lights did1 a steady light1 the modern lighthouse exhibits a series of flashes1 the duration of which1 together with the intervals between1 give it its distinctive character4 ,this flashing arrangement has a further advantage over the steady light4 ,each flash can be made more powerful than a steady light could be4 ,but of that more later4 ,the actual source of light varies with circumstances4 ,the electric arc is1 as we all know1 a very powerful light1 in fact it can be made the most powerful of all1 but its light is decidedly bluish4 ,now the time when a light is most of all needed is when the weather is thick4 ,fogs varying from a slight haze to a thick pall of darkness are of very common occurrence1 and the lighthouse light must be able as far as possible to penetrate them4 ,as a matter of fact clean fog1 such as one gets at sea1 is not by any means opaque4 ,the black fogs of the great cities are another matter1 but they are not the sort which afflict the mariner4 ,on a foggy day in the open country or by the sea it is often particularly light2 indeed the light is of a peculiarly diffuse nature which gives a nice even illumination to everything4 ,thus we see that fog is really transparent1 but it diffuses the light4 ,it does not stop the light rays1 but simply bends them about and scatters them in all directions4 ,thus we can see nothing through the fog1 yet a flood of light reaches us through it4 ,in its effect it is like that crinkled glass which is often used for partitions between rooms1 which lets light through1 but which cannot be seen through4 ,we see1 then1 that the effect which a fog produces is mainly to refract the light rays4 ,each little drop of water 7for it must be remembered that fog is myriads of tiny drops of liquid2 it is not vapour7 acts like a minute lens1 and bends the rays which pass through it4 ,and the more blue a ray is the more it is bent4 ,on the contrary1 the more red it is the less is it bent4 ,when a beam of light is analysed in the spectroscope the red rays are bent least and the blue rays most1 so that the red rays fall at one end of the spectrum and the blue at the other4 ,now we only see a thing when light rays proceeding from every part of it fall straight 7or nearly so7 upon our eyes4 ,consequently1 since red rays are bent and scattered by the fog less than blue rays are1 a red light will be more easily seen through a fog than a blue one4 ,it might seem from this that a red glass put in front of a light would make it better for this purpose1 but that is not the case1 for the simple reason that filtering the light through red glass does not really make it any redder than it was before3 it simply makes it look redder by extracting from the original light all except the red4 ,but a source of light which is naturally reddish is so because it is more plentifully endowed with red rays1 while a bluish light like the electric arc is naturally deficient in red rays4 ,consequently we should be inclined to expect from theory that the electric arc would not be a good light for a lighthouse1 since it would lack penetrating power in foggy weather4 ,some readers may have noticed themselves1 in towns where electric lights and gas lamps are in use near each other1 that the latter1 though relatively feebler under normal conditions1 seem to give more light in fog4 ,and experiments show that this is really the case4 ,so although there are some lighthouses with electric arc lights1 that which is now believed to be the best is an oil lamp of special design1 using a mantle of the ,welsbach type4 ,the oil is stored in strong steel reservoirs into which air is pumped by means of a pump not unlike those used to inflate bicycle tyres4 ,by this means a pressure is maintained upon the oil of about #fe lb4 per square inch4 ,this forces the oil up a pipe and drives it in a jet into a vaporiser1 a tube heated from the outside so that in it the oil is turned into gas4 ,this gas then rises to the burner and heats the mantle1 just as the gas does in the ordinary incandescent gas light4 ,indeed in the case of lights on the mainland near a town the gas from the town main is often utilised4 ,but this simple arrangement for using vaporised oil1 as will readily be seen1 can be employed anywhere4 ,a little of the gas produced is led through a branch pipe and burnt to heat the vaporiser4 ,to start the apparatus the vaporiser is heated with a little methylated spirit4 ,thus everything is quite self-contained and so simple that there is little to get out of order4 ,the largest size of lamp will give #bdjj candle-power1 with an expenditure of #b-a_/d pints of oil per hour1 just common oil1 too1 of the kind used with ordinary wick lamps4 ,having got a source of powerful light1 the next thing is to collect that light and throw it in the direction required4 ,for the light proceeds from the lamp in all directions 7practically71 and much of it would be entirely wasted could it not be collected and guided in the required direction4 ,the earliest attempt at this was to use a reflector of bright polished metal4 ,in the most improved form these were made to that peculiar curve known as a parabola4 ,this is a curve obtained by cutting a cone in a certain way1 wherefore it is one of the conic sections1 and its particular appropriateness for this work resides in the fact that if a light be placed at a certain point known as the focus all the diverging rays which fall upon the reflector will be reflected in the same direction1 parallel to each other4 ,an ordinary spherical mirror would reflect them either back to the lamp or in diverging directions4 ,at any distance the beam from the parabolic reflector will be more intense than that from the spherical one1 since the rays will be closer together4 ,but even with the parabolic one there is some diffusion1 for the simple reason that whereas the focus is a mathematical point 7position without magnitude7 the most concentrated form of light known has a considerable magnitude4 ,hence the rays proceeding from the centre of the mantle are reflected as per the theory1 but those from the outlying parts of it are somewhat diffused4 ,this difficulty cannot possibly be overcome1 and hence even in the finest examples of lighthouse architecture the flashes are not quite sharp and clear-cut4 ,there is a central moment1 so to speak wherein the flash is almost blinding in its intensity1 but it is preceded by a period of growing brightness and succeeded by one of decreasing light4 ,in the modern apparatus1 however1 metallic mirrors are entirely dispensed with1 their place being taken by reflecting prisms of glass4 ,the metallic ones had to be continually rubbed to keep them clean1 and this soon dulled their brightness1 while the glass prisms need only to be wiped carefully1 which operation has little effect upon their surface4 ,it may come as a surprise to some that reflecting prisms are possible4 ,the idea of refraction through a prism is quite familiar4 ,such forms the essential principle of the spectroscope4 ,refraction is explained to every school child in order to account for the rainbow4 ,but reflection by a piece of the clearest glass seems a contradiction in terms almost4 ,yet it is only a question of shape4 ,in some prisms the light is simply bent as it passes through4 ,in others it is bent twice1 so that it leaves the prism just as if it had been reflected off a mirror4 ,both devices are used in the lighthouse4 ,let us see how they are combined so as to perform the work to be done4 ,take first of all the case of a light upon an isolated rock where the warning is needed equally all round4 ,all that is necessary here is to pick up those rays which1 if left to themselves1 would fall upon the water near the foot of the tower1 and those which would waste themselves skywards1 and then to gather all the rays into several bundles or beams4 ,we will suppose a simple case in which the light is supposed to give flashes at regular intervals4 ,we are in the topmost room of the lighthouse1 the lantern1 as it is called4 ,in the centre there stands the murette or pedestal4 ,in this several columns support a circular platform on the top of which there moves what we might call a turntable1 which in turn bears a frame of gun-metal into which are fitted a maze of glass bars triangular in section and curved to form concentric circles4 ,the whole structure1 possibly1 is of great size4 ,from the floor to the platform is as high as an ordinary man4 ,indeed around the turntable there is a gallery which forms a roof over our heads1 so that it is only after mounting some iron steps on to this gallery that we are able to examine the glass part4 ,as we ascend we notice that the walls of the chamber as far up as the gallery are formed of iron plates1 while above that there is a metal framework filled in with glass panes1 and above all a dome-shaped roof4 ,having reached the platform we proceed to examine the glass1 and we find that the metal framework forms a cage with four sides1 each approximately flat1 but really slightly spherical4 ,each of these sides is called a panel4 ,in the centre of each is a lens4 ,peeping through the interstices between the prisms1 we perceive that the lamp is inside this structure1 exactly in the centre1 so that its light shines directly through the central lens or bull's eye4 ,around this bull's-eye are many circles of glass bar1 forming refracting prisms4 ,around this again are more bars in the form of segments1 which together form circles1 some being refracting prisms and others reflecting prisms4 ,all the light rays from the lamp which fall on any one prism are deflected1 so that they proceed approximately in the same direction4 ,those prisms in the upper part lay hold of the rays which would otherwise go up into the sky4 ,those at the bottom collect those which would fall near the foot of the tower4 ,so scarcely any are lost4 ,but for the fact that the lamp itself is comparatively large and not a theoretical point1 as already explained1 the beam from this panel would be perfectly straight1 parallel1 and of uniform density everywhere4 ,as it is1 it widens slightly as it proceeds1 but1 practically speaking1 we might call it a solid beam of light4 ,each of the panels sends forth such a beam1 so that they strike out in four directions from the central lamp much as four spokes from the hub of a wheel4 ,then descending once more to the floor from which we started1 we see that among the columns there is a large clockwork arrangement1 the purpose of which is to drive round the turntable and all that it carries--in the language of the lighthouse engineer the optical apparatus or1 more briefly1 the apparatus4 ,and as this turns the radiating beams of light sweep round the horizon and in succession strike into the eyes of any mariner who may be within range4 ,each time a beam strikes him he sees a flash4 ,if the apparatus revolve once a minute he will see four flashes every minute1 one from each panel4 ,let us consider1 then1 the advantages of this wonderful mechanism1 with its cunning arrangement of prisms4 ,it is these latter1 of course1 which are the important thing4 ,the rest1 the mechanical portion1 is simply for the purpose of holding them and turning them at the proper speed4 ,in the first place1 the contrivance gives us flashes instead of a steady light2 it gives the lighthouse its character4 ,then again it enhances the brightness of the light4 ,instead of shining all round1 the light is concentrated in four special directions1 and the light which would be wasted upwards or downwards is saved and brought into use4 ,but suppose that the lighthouse we are considering be near the shore1 so that there is no need for it to throw any light in one--the landward--direction4 ,then we should see inside the revolving framework with its prisms a fixed frame with reflecting prisms which would catch any rays going from the lamp in the direction of the land and simply hurl them1 as it were1 back into the flame4 ,thus the intensity of the flame becomes increased by those rays thrown back which would else have been wasted4 ,or suppose that the character of the light is such that the flashes have to be at irregular intervals4 ,then the framework1 instead of being symmetrically four-sided1 would be of an irregular shape4 ,and that brings us to a beautiful feature of the mechanism of the apparatus4 ,we have been discussing a four-panel arrangement4 ,suppose that we were to reduce it to three4 ,then1 since all the light would be concentrated into three beams instead of four1 each beam would be more intense4 ,we should thereby have increased the range of our apparatus without any increase in the cost of oil--for nothing1 as it were4 ,but to get the same number of flashes per minute we should have to drive it round so much the faster4 ,but increased speed means increased burden on the keepers who have to wind up the heavy weights which operate the clockwork4 ,so there is a limit to the speed which can be attained4 ,but if friction can be almost eliminated the apparatus can revolve at a high speed without throwing undue burden upon the men4 ,but how can friction thus be got rid of8 ,messrs ,chance ,bros41 the great lighthouse constructors1 of ,birmingham1 have done it1 almost entirely1 by floating the apparatus on mercury4 ,the turntable has on its under side a large ring which nearly fits a cast-iron trough on the top of the pedestal4 ,in this trough there is mercury1 so that upon the liquid metal the apparatus floats as if upon a circular raft4 ,the table with its lenses1 prisms and other fittings may weigh six or seven tons1 yet it can be pushed round by one finger4 ,the various sizes of optical apparatus are known as orders4 ,one of the first order has a focal distance of #ibj millimetres4 ,this means that there is that distance between the centre of the lamp and the bull's-eye4 ,they descend by successive stages down to the sixth order1 with a focal distance of #aej millimetres1 while the most important lights are of an order superior even to the so-called first1 termed the hyper-radial1 the focal distance of which is #accj millimetres4 ,a recent example of a hyper-radial light is at the well-known ,cape ,race in ,newfoundland4 ,it revolves once every #cj seconds1 giving a flash of #c seconds every #g-a_/b seconds4 ,the optical apparatus weighs seven tons4 ,illustration3 ,by permission of ,messrs4 ,chance ,bros4 and ,co41 ,ltd41 ,birmingham ,,dassen ,,island ,,lighthouse1 ,,cape ,,of ,,good ,,hope ,this lighthouse1 #hj feet high1 is built of cast-iron plates1 bolted together ,most lighthouses are fitted with fog signals of some kind which have a distinctive character the same as the lights4 ,some are horns blown at intervals by compressed air often obtained from a special air-pump driven by an oil-engine4 ,another thing is to let off detonators at stated intervals4 ,but perhaps the most interesting of all is the submarine telephone4 ,the trouble with audible signals is that they are apt to vary as the conditions of the atmosphere change4 ,for1 strange though it may appear1 the air which is the natural medium by which sounds are carried to our ears is really a very bad substance for the purpose4 ,water is much superior4 ,a swimmer who cares to try the experiment of lying upon the water with his ears immersed while a friend beats a gong under the water some distance off will be astounded at the result4 ,so many modern ships are fitted with under-water ears1 waterproof telephone receivers1 really4 ,one is fixed each side of the vessel1 the wires from them being led to telephone receivers near the bridge4 ,many lighthouses and lightships in like manner are fitted with under-water bells which can be rung at intervals4 ,the sounds so conveyed through the water are always the same4 ,atmospheric or similar changes have no effect upon them4 ,and1 moreover1 the officer can tell which side of his ship the bell is4 ,if it be on his port-side it sounds louder in his port telephone1 and vice versa4 ,by turning his ship until he hears them equally he knows that he is pointing directly to or from the bell4 ,thus if the bell belong to a warning light he can steer confidently right away from the danger even in the thickest fog4 ,but science has not only provided the mariner with lights of marvellous power and of strange distinctive characters1 and reliable sound-signals for foggy weather1 it has also found him a reliable compass1 but that is worthy of a chapter to itself4 ,,chapter ,,vii ,,the ,,gyro-,,compass ,the magnetic compass has been for ages the mariner's guide over the trackless waters4 ,in cloudy weather it has been his only means of knowing the direction in which his craft was heading4 ,indeed1 it is not too much to say that the maritime commerce of the world was based upon the behaviour of that little piece of magnetised steel4 ,it has always1 however1 been subject to certain faults4 ,to commence with1 it points1 not to the geographical north1 but to the magnetic pole1 a point some distance from the geographical pole1 and one1 moreover1 which is not quite permanent4 ,the fact that the magnetic pole varies its position is impressively shown by the fact that a special department at ,greenwich ,observatory is continually employed1 by the aid of delicate self-recording instruments1 watching and setting down its fluctuations4 ,and the premier observatory of the world1 it should be remembered1 exists primarily1 not in the interests of pure science1 but as a department of the ,british ,admiralty in order to study matters of interest to navigation4 ,thus we have testimony to the importance of these little vagaries on the part of the magnetic compass4 ,but in addition to these inherent faults there is a new source of error in the magnetic compass which man has introduced himself by making his ships of iron instead of wood4 ,every ship of the present day is a huge magnet4 ,a piece of iron left in the same position for a length of time becomes polarised1 which is to say that it acquires the properties of a magnet2 and two magnets always exert an influence upon each other4 ,consequently the ship1 after lying for perhaps a year in one position1 during the period of building1 becomes itself magnetic and interferes with its own compass4 ,then1 again1 our methods of ship construction aggravate this trouble4 ,it is believed that every molecule of iron is itself a minute magnet with a north and south pole of its own4 ,these lying in confusion in the mass of unmagnetised iron neutralise each other1 so that the mass1 taken as a whole1 does not exhibit any magnetic power4 ,but if by some means the whole of the millions of millions of molecules can be set the same way--with all their north poles in one direction1 and their south poles in the opposite direction--then they will all act together4 ,instead of neutralising each other they will then help each other1 and under those conditions the mass of iron will possess that peculiar power which is distinctive of a magnet4 ,so long as a piece of iron is left in the same position the magnetism of the earth is thus acting upon the molecules4 ,just as it tends to place the compass needle north and south1 so it does with every molecule in the iron mass4 ,and if1 while lying still1 the iron be hammered1 the shaking of the molecules due to the hammering loosens them as it were and assists the earth's power in pulling them into position4 ,one has only1 then1 to watch the riveting up of a ship1 and to see the vigorous way in which the riveters wield their hammers1 to realise that when the thousands or even millions of rivets have all been finished the material of that ship will have had the very best possible chance of becoming magnetic4 ,to make matters worse still1 ships are often loaded with great weights of iron among their cargo4 ,that1 too1 may affect the compass4 ,on warships there are the heavy guns1 each weighing1 with its turret1 hundreds of tons1 and they move1 so that their effect upon the compass is not always the same1 but may vary from time to time4 ,and finally one may mention the electrical machinery in a modern ship consisting largely of powerful magnets4 ,altogether1 then1 it is not surprising that the old magnetic compass is somewhat unreliable4 ,it has to be coaxed into doing its duty4 ,pieces of iron and magnets have to be disposed about it to counteract these disturbing influences with which it is surrounded4 ,before a voyage experts have to come on board to adjust the compasses1 and even then there is reason to believe that the instrument sometimes plays the ship false4 ,it is not to be wondered at1 then1 that the naval authorities in particular throughout the world have welcomed the advent of a new compass which appears to possess none of these drawbacks4 ,it points to the geographical north1 to the actual pivot1 if one may so speak1 upon which the earth turns4 ,it is non-magnetic1 so that the presence of iron or magnets even in its immediate neighbourhood has little or no effect upon it4 ,on the other hand1 it has to be driven by a current of electricity1 and it seems just possible that in some great crisis it might fail1 although every provision is made for alternative sources of supply in case of one failing1 and there is always the possibility of falling back upon the old magnetic compass should the new one go wrong4 ,in principle the improved compass is1 like its older brother1 simplicity itself4 ,the latter is but a small piece of iron magnetised2 the former is nothing more than a spinning-top4 ,it is rather strange that although the spinning object has been a familiar toy for years1 and that1 moreover1 its behaviour has been the subject of investigation by some very eminent scientific men1 it is only of recent years that its principles have been put to practical use4 ,everyone is familiar with the fact that a round block of wood will support itself upon a comparatively tall peg so long as it is rapidly rotating4 ,and that is but one of the curious things which a rotating body will do4 ,for example1 imagine a wheel mounted upon an axle the ends of which are supported inside a ring1 while the ring again is supported on pivots between the two prongs of a fork1 the fork being free to swivel round in a socket4 ,the wheel is then free to move in any direction4 ,technically1 it is said to have three degrees of freedom4 ,it can spin round1 its axle can turn over and over with the pivoted ring inside which it is fixed1 while it can also swing round and round as the fork turns in its socket4 ,assuming that the joints are all perfectly free1 that the pivots move in their sockets with perfect freedom--which1 of course1 they do not--then a wheel so mounted could move in any direction under the influence of any force that might act upon it4 ,now a wheel so mounted if left alone remains in precisely the same position so long as it goes on rotating4 ,if it be turning sufficiently quickly its tendency to remain will be strong enough to overcome the friction of any ordinarily well-made instrument4 ,consequently a wheel of that description has been used to demonstrate the rotation of the earth1 it remaining still 7except1 of course1 for its rotating movement7 while the earth has moved under it4 ,could we entirely eliminate the effects of friction that might be used as a compass1 for it could be set1 say with its axle pointing north and south1 at the commencement of the voyage1 and it would remain so despite all the evolutions through which the ship might go4 ,but there is a better scheme even than that1 based upon the peculiar behaviour of a revolving wheel when it has only two degrees of freedom4 ,suppose that we dispense with the ring employed in the previous arrangement1 pivoting the ends of the axle between the prongs of the fork4 ,the wheel is then free to rotate1 and its axle can slew round through a complete circle by the turning of the fork in its socket1 but there can be no tilting of the axle4 ,being thus deprived of one of its movements the gyroscope with three degrees becomes a gyroscope with two degrees of freedom1 and in that form it supplies the need for an efficient and reliable compass4 ,the secret of the whole thing is the curious fact that a gyroscope with two degrees of freedom exhibits a keen desire to place its axis parallel with the axis of the earth4 ,owing to the shape of the earth1 a device such as has been described1 with its fork standing up vertically1 cannot possibly have its axis really parallel with that of the earth1 except on the ,equator4 ,still it gets as nearly parallel as possible4 ,to be scientifically accurate1 we ought to say that it places it own axis in the same plane as that of the earth4 ,to understand this we need to realise that all movement is relative4 ,in ordinary language1 when we say a thing is still we mean that it is still in relation to the surface of the earth1 but since the earth is moving the stillest thing1 apparently1 is really travelling at enormous speed4 ,saint ,paul's ,cathedral in ,london1 or a tall sky-scraper in ,new ,york1 would usually be regarded as supreme instances of immobility4 ,it would be hard to find better examples of stationariness1 as we ordinarily look at things4 ,each stands1 firm and strong1 upon a horizontal base4 ,yet each is really turning a somersault every twenty-four hours4 ,the plateau upon which ,st ,paul's stands1 though it seems still and motionless beneath our feet1 is continually tilting2 its eastern edge is continually going downwards and its western edge upwards1 as the earth performs its daily spin4 ,it is only a north and south line which does not share in some degree this continual tilting action4 ,every plane1 large or small1 so long as it remains horizontal1 is being tilted thus1 down at the eastern edge and up at the western4 ,and the plane in which the axle of a gyroscope with two degrees is free to move is a horizontal plane4 ,owing to its being held between the prongs of the fork1 while it can swing round to point north1 south1 east or west1 or towards any point between them1 it cannot deviate from the horizontal plane4 ,therefore such axle is always being tilted by the motion of the earth1 except when it happens to be lying exactly north and south4 ,now for a reason which is too complex to go into here a gyroscope strongly objects to having its axle tilted in this manner4 ,if it be compelled by superior force to submit to tilting1 it tries to wrench itself round sideways4 ,anyone who has a gyroscope top and cares to try the experiment will feel this action quite easily4 ,hold the spinning-top in your hand and turn it over so as to tilt the axle1 when it will1 if you are not careful1 twist itself out of your grasp4 ,so a gyroscope of the kind we are considering1 when the motion of the earth tilts its axis1 turns itself round in its socket until at last it reaches the north and south position1 when the tilting1 and therefore the twisting1 ceases4 ,hence the axle of the gyroscope if left to itself 7the rotation of the wheel being maintained the while7 will place itself in a north and south direction4 ,and1 moreover1 it will keep in that direction4 ,it will take some force to slew it round into any other4 ,and if moved into any other by some extraneous means it will restore itself to the old position again4 ,hence a wheel thus arranged has all the attributes which we need for a mariner's compass4 ,but unfortunately there are mechanical difficulties in the way of using such a simple contrivance for that purpose4 ,chief of all these is the fact that it is not what engineers call dead-beat4 ,that means that it will not go to the proper position and then remain there quite still4 ,instead1 it will first slightly overshoot the mark1 which being followed by the reverse action1 it will come back and overshoot it just as far in the opposite direction4 ,instead1 therefore1 of a steady pointing1 always in the same direction precisely1 it will oscillate more or less1 the exact north and south line being the mean or average position1 the centre of the oscillations4 ,it would of course be possible to damp this1 to apply a break as it were1 if the apparatus were to remain stationary4 ,for example1 if the whole concern were immersed in water the resistance of the liquid would restrain any quick movement of the axle1 yet it would not prevent it from slowly finding its true position4 ,thus the oscillations would be reduced to such a small range as to be for practical purposes negligible4 ,but the drawback to a device of that kind1 applied to a gyroscope on board ship1 would be that the axle would be carried round to some extent every time the ship turned4 ,as she changed direction it would more or less carry round the water with it2 that in turn would carry the gyroscope1 and so the direction of the latter would be for a time untrue4 ,it would in course of time regain its accuracy1 but in the meantime it would be leading the ship astray4 ,consequently the application of this1 in itself wonderfully simple1 idea1 to this extremely important purpose was accompanied with a difficulty which was for a long time insuperable4 ,but all was overcome at last by the genius of ,dr ,anschutz1 of ,hamburg1 whose firm were the first to turn out the practicable article4 ,taking advantage of another movement of the gyroscope when arranged as has been described1 and using the revolving wheel itself as a centrifugal fan1 he was able to make the wheel blow air against itself1 as it were1 when in any position other than north and south4 ,thus1 if it deviates towards the east1 this jet of air tends to blow it back2 if it turns westwards the jet again comes into operation1 tending to bring the erring gyro back to its proper place2 and so the tendency to oscillate is checked4 ,the finished instrument as it is installed on the latest warships is1 of course1 quite different in detail from the simple contrivance which we have been considering so far1 although it is the same precisely in principle4 ,the essential part is a heavy metal wheel combined with which is an electric motor which keeps it rotating at a speed of #bj1jjj or so times per minute4 ,the bearings of the wheel are supported upon a metal ring which floats upon the surface of a trough of mercury4 ,thus friction is brought down almost to the irreducible minimum4 ,the only place where the wheel and its supports touch anything solid is at one delicately made pivot which serves to keep the floating mechanism in the centre of the mercury basin1 and to prevent it from rubbing against the side of it4 ,the current which drives the motor reaches it through this pivot and leaves through the mercury4 ,thus arranged1 although the floating part is of considerable weight1 a very slight force indeed is enough to move it2 while1 looking at it the other way1 we can see that the ship might turn rapidly to right or to left1 carrying round the mercury bowl with it1 without turning the floating part at all4 ,thus the gyroscopic action is very free indeed to exercise its function of keeping the contrivance pointing always in the one way4 ,the float has mounted upon it a compass card much like that of the ordinary magnetic instrument1 and the sailor reads it in precisely the same way4 ,to outward appearance there is little essential difference2 in one case there is a magnet under the card to keep it still1 in the other there is the float with the revolving wheel mounted upon it4 ,it is customary to have one master compass of this kind on a ship1 with an electrical repeater in each of the steering positions4 ,as the master turns in its casing it sends a rapid series of currents to all the others1 causing them to turn in unison with it4 ,the master is fitted in some safe part of the ship where it is least likely to be the victim of any accidental damage4 ,,chapter ,,viii ,,torpedoes ,,and ,,submarine ,,mines ,it is sad to think how much scientific skill and learning has1 during the ,great ,war1 been devoted to killing people4 ,it used to be thought that one day a great scientific invention would arise1 of such deadly power that for ever afterwards war would be unthinkable2 its horrors would be such that all nations would shrink from it4 ,that prophecy1 however1 has not been fulfilled1 nor are there any signs of it4 ,on the contrary1 each scientific achievement in the realm of warfare is quickly countered by another3 so much so that with all our science in the manufacture of weapons1 and our skill in using them1 warfare in the twentieth century is if anything less deadly in proportion to the numbers engaged than it used to be4 ,there are1 however1 two weapons which in this war have reached a deadly efficiency which they did not seem to possess before1 and to which satisfactory antidotes have not yet appeared4 ,these two are the submarine mine and the torpedo4 ,the latter1 particularly1 had been a dismal failure previously1 but as the weapon of the submarine it has now established itself4 ,it is1 however1 only in connection with the submarine that it has achieved any measure of success1 and1 as there are strong indications that very soon the submarine itself will be robbed of its terrors1 it is quite likely that the reign of the torpedo will be brief4 ,although it has only just made itself felt seriously in warfare1 the torpedo is a fairly old idea4 ,in fact we can trace the general idea of it back to very ancient times4 ,the modern weapon1 however1 dates from the year #ahfd1 when an ,austrian inventor approached an ,english engineer named ,whitehead with a request to take up his idea4 ,mr ,whitehead had at that time a works at ,fiume1 on the ,adriatic1 and it was really his genius that developed the crude idea into a practicable invention4 ,thus there came into existence the ,whitehead ,torpedo1 now used in a great many navies1 and also the ,schwartzkopff1 which may be regarded as the ,german variety of the same thing4 ,speaking generally1 it may be described as a small automatic submarine boat4 ,externally1 it naturally follows somewhat the lines of a fish4 ,deriving its name from that curious fish which is able to give electric shocks from its snout1 it likewise carries on its nose that appliance whereby it gives a shock1 not electric it is true1 but equally deadly1 to anything which it may touch4 ,since no man-made mechanism can approach the marvellous action of the fish's fins and tail1 the propulsion is achieved by a propeller like that of a steamboat1 but of course on a very small scale4 ,a single propeller1 however1 would tend to turn the torpedo over and over in the water1 and so it has two1 one behind the other1 driven in opposite ways1 so that the turning tendency of one is neutralised by that of the other4 ,the blades of the propellers are1 however1 set in opposite ways1 so that although rotating in different directions they both push the torpedo along4 ,behind the propellers1 again1 there are rudders for steering4 ,one steers to right or left1 as does that of an ordinary ship1 while two others are so placed that they can steer upwards and downwards4 ,so there we have the general picture of the outside3 a smooth1 fish-like body with a sting in its nose1 propellers at the rear to drive it along1 and rudders to guide it4 ,inside are various chambers4 ,one contains the explosive which blows up when the nose strikes something4 ,this head1 as it is termed1 is detachable1 so that it can be left off until it is really required for war4 ,the peace-head1 which is of the same size1 shape and weight as the war-head1 is what the torpedo carries during its earlier career4 ,with this it can be tried and tested in safety1 the war-head being substituted when the real business of the torpedo begins4 ,another chamber contains the compressed air which furnishes the motive power4 ,this also serves to give buoyancy4 ,another chamber1 again1 contains the engines1 beautiful little things of the finest workmanship almost exactly like the finest steam-engine1 but of course very small in comparison4 ,in the early stages the range of the torpedo was limited by the amount of compressed air which it could carry4 ,at first sight there seems no reason why any limit should be placed upon this1 but in practice there are often limitations in engineering matters which are not apparent on the surface4 ,for example1 to increase the air chamber would mean enlarging the whole torpedo1 calling for more propulsive power and larger engines1 and these larger engines would call for more air1 thus defeating the object in view4 ,forcing more air in by using a higher pressure1 in a similar way would necessitate a thicker chamber1 to resist the higher pressure4 ,this would add weight1 calling for more buoyancy4 ,thus there seemed to be a practical limit beyond which it was impossible to go4 ,the difficulty was overcome1 however1 in a very cunning way4 ,when the engines have used some of the air1 and the store is somewhat exhausted1 chemicals come into action which generate heat1 which is imparted to the air which is left4 ,this heat expands the air1 producing in effect a larger supply of it1 and enabling the torpedo to make a longer journey4 ,steering in a horizontal direction--that is to say1 to left or right--is done by a gyroscope4 ,the action of a rotating wheel is discussed in the last chapter1 and it is not necessary here to say more than this3 a rotating wheel always tries to keep its axle pointed in the same direction4 ,just at the moment of starting such a wheel is set going inside the torpedo1 and its arrangement is such that1 should the torpedo swerve to the left1 the gyroscope operates the rudder and steers it back4 ,in the same way1 if it tends to turn to the right1 the ever-watchful gyroscope brings it to its true course once more4 ,the effect of the gyroscope1 therefore1 acting upon the rudder1 is to keep the torpedo faithfully to the direction upon which it is started4 ,the up and down rudders are likewise controlled quite automatically1 but in a different way4 ,their function1 clearly1 is to keep the thing at a certain uniform level4 ,without such control a torpedo would be equally likely to jump out of the water altogether1 or to go downwards vertically and bury its nose in the mud4 ,the depth at which it is to move is determined beforehand1 certain necessary adjustments are made1 and the torpedo then pursues its even way1 neither coming to the surface nor driving beneath its target4 ,for this purpose there is first of all a hydrostatic valve4 ,this little appliance1 which is open to the action of the water1 responds to changes in pressure4 ,the pressure at any point under water is exactly proportional to the depth4 ,at ten feet1 for example1 it is precisely ten times what it is at one foot4 ,so the hydrostatic valve is adjusted to set the rudders straight when the water-pressure upon it is a certain amount4 ,if1 then1 it dives downwards the pressure increases and the valve operates the rudders so as to bring it upwards1 while if it rise too high the decrease of pressure causes it to be guided downwards4 ,this action1 however1 is too sudden and violent1 so that with it alone the torpedo would proceed by leaps and bounds4 ,after being low it would come up too suddenly1 overshoot the mark1 only to be steered downwards again equally suddenly4 ,the valve1 therefore1 is combined with a pendulum1 whose action tends to restrain these too sudden changes1 with the result that under the influence of the two things combined the torpedo keeps fairly well to an even course1 only varying upwards or downwards to an extent which is negligible4 ,finally1 there is an interesting little feature about the firing mechanism which merits a description4 ,the actual firing is caused by the driving in of a little pin which projects at the nose of the torpedo4 ,suppose that1 in the process of pointing the torpedo and launching it upon its course1 that pin were to be knocked accidentally1 an awful disaster would result4 ,it must be provided against1 therefore1 and the method adopted is beautiful in its certainty and simplicity4 ,normally1 the firing-pin is fixed by a screw so securely that no accidental firing is possible4 ,there is1 however1 a little propeller-like object associated with it1 which is driven round by the water as the torpedo is pushed through it1 and this unscrews1 and thereby releases the pin4 ,the little fan has to rotate a certain number of times before the pin is released1 and it is quite impossible for this number to be accomplished before the torpedo has proceeded to a safe distance from the ship which fires it4 ,on board the ship1 therefore1 and so long as it is near the ship1 it is quite safe1 but by the time it reaches its target it is ready to explode4 ,as far as is known1 the foregoing description gives a true general description of the torpedoes now in use4 ,those of different powers may vary in detail1 but1 broadly1 they are as just described4 ,there are others1 however4 ,the ,brennan1 for instance1 was once adopted and largely used by the ,british for harbour defence4 ,this was controlled from the shore by wires4 ,it was driven1 so to speak1 with wire reins1 and thus guided it could fairly hunt down its prey1 turning to right and to left as required4 ,of greater scientific interest1 perhaps1 still1 is the ,armor#a wireless controlled torpedo4 ,this is the invention of two gentlemen1 ,messrs ,armstrong and ,orling1 whose first syllables combine to form the title of the torpedo4 ,of this1 two very interesting features may be mentioned4 ,firstly1 the wireless control4 ,in the chapter on ,wireless ,telegraphy there is described the coherer1 a simple little apparatus which we might describe as a door which is opened by the waves which travel through the ether from the sending apparatus4 ,whenever the key of the sending apparatus is depressed these waves travel forth1 and when they fall upon the coherer it opens4 ,normally1 the coherer is shut1 but when acted upon by the incoming waves it opens and lets through current from a battery1 which current can be caused to perform any duty which we may wish4 ,thus1 ignoring the intermediate steps1 we get this3 whenever the sending key is depressed current flows through the coherer and performs whatever duty is set before it4 ,and now picture to yourself a tooth wheel with four teeth4 ,a catch normally holds one of the teeth1 but when the catch is lifted for a moment it lets that tooth slip and the next one is caught4 ,at every lifting of the catch the wheel turns a quarter of a turn4 ,then imagine that that catch is operated by an electro-magnet energised by the current which passes through the coherer4 ,we see1 then1 that every time the sending key is depressed the wheel turns a quarter turn4 ,attached to the wheel is a little crank which turns with it1 and the pin of this crank fits in a slot in the end of a bar like the tiller of a boat4 ,suppose that1 to commence with1 the tiller is straight1 so as to steer the boat straight4 ,depress the key1 the wheel turns a quarter turn and the tiller is set so as to steer to one side1 say the left4 ,another pressure upon the key and a second quarter turn brings the tiller straight again4 ,yet another pressure1 another quarter turn1 and the tiller is steering to the right4 ,thus by simply pressing the key the correct number of times the torpedo can be made to travel in any desired direction4 ,the second ingenious feature of this weapon is the means by which it is made visible to the man who is controlling it from the shore or ship4 ,probably the reason why these torpedoes are not used more is that the man who guides them is of necessity himself visible4 ,he has to be posted somewhere where he can follow its course1 or he has no idea how to steer it4 ,consequently1 he would be an object for attack by the enemy4 ,such a torpedo would be useless in a submarine1 for the submarine would need to come to the surface in order that the observer might get a sufficiently good view to be able to steer the torpedo1 and we all know that when upon the surface a submarine is a very vulnerable craft4 ,but that is by the way4 ,the point is how to make the torpedo very clearly visible while it is still under water4 ,a short mast might be used1 but that would be liable to be shot away4 ,the inventor had a happy inspiration when he made it blow up a jet of water1 like a whale does4 ,this jet is quite easy to see1 yet no shot can destroy it4 ,compressed air blows up this tell-tale jet which the observer can see1 and by its means he can guide the torpedo at will4 ,a submarine mine may be regarded as a stationary torpedo4 ,it consists of a metal case filled with a powerful charge of explosive which floats harmlessly in the water until some unfortunate vessel strikes against it1 when it blows up with sufficient force to make a hole in the stoutest ship4 ,there are two classes of mine3 one which is laid in peace time1 to protect harbours and channels2 and the other1 which is laid during actual warfare4 ,the former are anchored in a more or less permanent way4 ,the services of divers are used to place them in position4 ,in some cases they float well down in the water1 out of the way of passing ships1 but come up nearer the surface when needed4 ,this result is achieved by having an anchor chain of such a length that when fully extended the mine floats a little way under the surface1 just high enough to be struck by a passing ship1 together with what is called an explosive link4 ,the link is used to loop together two parts of the chain1 and so1 in effect1 to reduce its length4 ,wires pass from the link to the shore1 and when an electric current is sent along these wires the link bursts asunder1 liberates the chain1 and the mine floats up to the full length of its chain4 ,another plan is to let the mines float high up always1 but to fire them1 not by the touch of the ship but by electricity from the shore4 ,in this way a safe channel is kept for friendly vessels1 while an enemy can be destroyed4 ,necessarily1 those mines which are hurriedly laid in war time are very different from these4 ,to be of much use1 a mine must be concealed below the surface4 ,if it floats upon the water it will be visible1 and can be avoided1 or1 at all events1 easily picked up4 ,it is practically impossible to set a floating object at a certain depth in the water1 except by anchoring it to another1 heavier1 object1 which will lie at the bottom4 ,therefore mines have to be anchored in some way4 ,but the sea varies in depth1 so that the length of the anchor chain must be varied1 or else some of the mines will be on the surface1 thereby advertising the presence of the mine-field1 while others will be below the depth of even the biggest ship4 ,in warfare1 however1 mines need to be laid quickly4 ,there is no time to sound for the depth and then to adjust the length of cable accordingly4 ,hence the mine must be so made as to set itself correctly at a pre-determined depth4 ,possibly some readers may think that such things might be made to float1 of themselves1 at the right depth4 ,it is a fact1 however1 that a thing either floats upon the surface of water or falls to the bottom4 ,water is practically incompressible1 so that the water at the bottom of the sea is no heavier than that near the surface4 ,the conditions which prevail in air and allow a balloon to float at any desired height do not apply4 ,the only thing1 in this case1 is to have an anchor chain or rope of the right length4 ,so let us picture a mine-laying ship steaming along1 probably in the dead of night1 surreptitiously laying mines in the hope that the enemy will run into them on the morrow4 ,along the deck of the ship are small railway lines1 and on these lines stand what appear to be trains of small trucks1 each truck having small wheels to run on1 and each bearing a large round metal ball4 ,as the ship travels along1 the crew1 handling these deadly things quite freely1 as if they were innocent of any danger1 propel them along to the stern1 and at regular intervals push one overboard4 ,that is all4 ,the freedom with which the men handle them is not folly1 for they are then quite harmless4 ,nor need they trouble about the length of rope1 for that adjusts itself4 ,just tumble the things overboard1 and in due time they anchor themselves at the right depth and set themselves in the right condition for blowing up any ship which may get amongst them4 ,the truck-like object upon wheels is not the mine itself3 it is the sinker which lies at the bottom of the sea4 ,the round ball which it bears is the mine1 and the two are connected together by a wire rope4 ,to commence with1 this rope is coiled upon a drum in the sinker1 which drum is either held tightly or is free to revolve according to the position of a catch4 ,that catch is held open1 so that the drum is free1 by a weight at the end of a short rope4 ,let us assume that that rope is ten feet long4 ,then1 when the whole thing is tumbled into the water1 the weight sinks first ten feet below the sinker1 which1 being more bulky in proportion to its weight1 follows downwards more slowly4 ,while sinking1 the weight is pulling upon its rope and holding open that catch1 so that the drum pays out its rope and the mine lies serenely upon the surface4 ,as soon as the weight touches bottom1 however1 the pull on the short rope ceases1 the catch grips the drum1 no more rope is paid out1 and the sinker1 in settling down its last ten feet1 has to drag the mine down too4 ,thus1 quite automatically1 by what is really a beautifully simple arrangement1 the mine becomes automatically anchored at a depth below the surface equal to the length of the short rope4 ,by making that rope the desired length1 the depth of the mine under the water can be fixed4 ,there are various methods of firing these mines1 all of which work perforce by the concussion of the ship itself4 ,in some cases the sudden tilting over causes an electric contact to be made1 and permits a battery in the mine to cause the explosion4 ,another way is to furnish the mine with projecting horns of soft metal1 inside which are glass vessels containing chemicals4 ,the ship1 striking a horn1 bends it1 breaks the glass1 and liberates the chemicals which cause the explosion4 ,in the type of mine largely used by the ,british ,navy there is a projecting arm pivoted on the top of the mine and projecting from it horizontally4 ,the mine itself rolls along the side of the passing ship1 but the arm simply trails or scrapes along4 ,thus the mine turns in relation to the arm1 and a trigger is thereby released1 which fires the mine4 ,in this1 be it noted1 the ship only pulls the trigger1 so to speak1 and releases a hammer which does the work1 just as the trigger of a gun releases the hammer4 ,the motive force which makes the hammer do its work when the trigger is pulled is the pull on the anchor rope4 ,that arrangement has a virtue which is not apparent at first sight4 ,since it is the pull on the anchor rope which actually fires the mine1 it follows that if such a mine break away from its moorings it instantly becomes harmless4 ,safety for the men who lay the mines is secured in several ways4 ,one is by the use of a hydrostatic valve4 ,the firing mechanism is locked until the pressure of water releases it1 and that pressure does not exist until the mine is several feet under water4 ,another way is to seal up the firing mechanism with a soluble seal made of some substance such as sal-ammoniac4 ,the mine cannot then explode until it has been under water long enough for the seal to be melted4 ,it now remains to relate how these mines are swept up and removed1 yet there is very little really to tell1 for the process is so exceedingly simple4 ,so far as is generally known1 no method has been found that is superior to the primitive plan of dragging a rope along between two ships so as to catch the anchor ropes4 ,the vessels employed are usually of very light draft1 so that they stand a good chance of passing over the mines themselves1 and the rope used is as long as possible1 so that a mine1 if exploded by being caught in the loop of the rope1 explodes so far away as to do no harm4 ,when dragged to the surface the mines are exploded from a distance by shots from a small gun1 or even from a rifle4 ,in the case of those mines which have horns1 a blow from a bullet is enough to break the glass and cause explosion1 and in all cases mines seem sooner or later to succumb to a sharp blow4 ,thus they are destroyed1 by their own action1 at a safe distance from the sweepers4 ,accidents happen1 however1 and mine-sweeping is no job for anyone but the bravest4 ,it has been somewhat difficult to crowd a description of torpedoes and mines into the small space of one chapter1 and so many details have had to be omitted1 but the above descriptions give the broad1 general principles underlying practically all forms of these terrible weapons4 ,,chapter ,,ix ,,gold ,,recovery ,there has always been something very fascinating about gold4 ,even in ancient times it was prized above all other things1 and apparently it was comparatively plentiful4 ,it is estimated1 for example1 that ,king ,solomon possessed over ,l#d1jjj1jjj worth of it1 while the little gift which the ,queen of ,sheba brought him was of the handsome value of ,l#fjj1jjj1 so that she too must have been plentifully supplied with it4 ,probably it was more easily come by in those days1 owing to the richness of the primitive deposits1 the best of which1 perchance1 have been worked out4 ,in one respect gold differs from all other metals 7with the single exception of platinum1 which is scarcer still7 in that it appears naturally as gold1 not as ore4 ,the little pieces of gold lie in the mine ready to be picked out1 and so if the deposit in which it occurs be near the surface1 and the particles be of any considerable size1 they are sure to be found4 ,a savage may be1 and often is1 very anxious to secure weapons and tools of iron1 little knowing that the very ground upon which he stands is possibly of iron ore4 ,he covets the single article of iron1 and in some cases is willing to give much gold for it1 or ivory1 or some such treasure1 while thousands or millions of tons of iron lie at his feet1 only he does not recognise it1 nor would he know how to utilise it if he did4 ,for iron1 like all other metals except the two just referred to1 is found naturally in combination with something else1 generally oxygen1 and the combination bears no resemblance at all to the metal4 ,the red rust so familiar to us on iron is a combination of iron and oxygen1 and it is fairly typical of the kind of state in which iron is found in the earth4 ,nor would anyone recognise copper ore1 lead ore1 tin ore1 or any of the ores1 any better than iron ore4 ,all are difficult to recognise4 ,it is said that the highest compliment that a ,cornish miner--the finest metalliferous miners in the world come from ,cornwall1 or are the product of ,cornish influence--the highest compliment that such a man can pay to another is to say that he knows tin1 meaning that he can tell tin ore when he sees it4 ,contrasted with these other metals1 gold is easy to find4 ,it does1 it is true1 under certain conditions1 form chemical compounds with other things1 as1 for instance1 in gold chloride1 which is present in sea-water1 but it does not oxidise as the others do1 and so when it is in the earth it is in the bright yellow grains such as 7if they be large enough7 can easily be recognised at sight4 ,and it is often found in beds of loose gravel1 alluvial deposits1 as they are termed4 ,in such cases the gold is to be had simply for the picking up4 ,sometimes a lucky find occurs in the form of a big nugget1 but more often the metal lies in tiny grains at long distances apart1 so that a ton of gravel has to be sorted over to find a paltry ounce or so of gold4 ,yet so desired is it that gold will always fetch its price1 and an ounce to the ton 7even less7 is sometimes worth getting4 ,but in the early history of the world there were possibly particularly generous deposits with plenty of gold in good-sized pieces1 and such would be quickly discovered and worked by primitive man4 ,no doubt the chieftains of those days took much1 if not all1 of the gold that their people found1 and more powerful chiefs and kings would1 in turn1 either by force or in trade1 take it from the weaker1 so that it is not surprising to learn that some of the mighty kings and potentates of long ago were well supplied with gold4 ,yet there are few things more useless4 ,its value in the first instance was probably entirely due to its beautiful colour1 and the fact that it does not easily tarnish4 ,for this reason1 coupled with the fact that it was by no means plentiful1 men liked to deck themselves with it1 not only adding to their beauty by so doing1 but advertising to their fellows the fact that they were men of wealth1 men who possessed what few others had1 or at all events possessed it more abundantly4 ,these three basic facts about gold1 its beauty1 its freedom from deterioration and its comparative scarcity1 give it its peculiar status among the commodities of commerce1 in that for it1 and for it alone1 there is a continuous and universal demand4 ,no gold-mining company ever shut down its properties because of the falling off in the demand for gold4 ,no one ever had to hawk gold about to find a purchaser2 it is always saleable4 ,and hence its value to humanity as the great medium of exchange4 ,when a tailor wants bread1 as has been pointed out by a great political economist1 he does not go searching for a baker who happens to need a coat4 ,if he did1 he might starve before he found one4 ,instead1 he gives his coat to anyone who needs one1 no matter what his trade may be1 taking gold in exchange4 ,then he goes with confidence to the baker1 knowing full well that he1 in turn1 will be perfectly ready to give bread in exchange for gold4 ,that is the principle upon which gold1 and in a few cases silver1 has become the foundation of trade4 ,we use it for toning photographs and a few other things1 but1 practically speaking1 it is useless stuff1 yet certain special circumstances have given it a special function in civilised society1 and so governments now make it up into little flat discs1 putting their own special stamp upon them as a guarantee of size and quality1 and it is by handing those little discs about that we carry on our trade4 ,or even where we use no actual disc1 we pretend that we do1 and use a piece of paper the value of which we say is so many discs1 but that value depends entirely upon the fact that someone has guaranteed1 on demand1 to give so many discs for it4 ,and the strange thing about it is that although this usefulness of gold depends upon its rarity1 we lose no opportunity of looking for new sources of supply1 and so diminishing that rarity4 ,as has been said1 gold is present in sea-water1 although no one knows how to get it out1 except at a cost which makes it not worth while4 ,but suppose that some genius found a way1 and gold thus became twice as plentiful as it is now1 the world would be no better off4 ,everything would cost twice as much as it does now2 that is all4 ,a pound is merely so much gold4 ,if gold be twice as plentiful people will want twice as much of it in exchange for what they have to sell4 ,yet1 all the same1 the man who could solve that problem of getting gold from sea-water1 or from anywhere else1 in fact1 would be hailed as a benefactor1 and for a time at least he would reap a generous harvest4 ,even as it is1 science has done much for the production of gold4 ,not1 as in other metals1 in finding ways for extracting it from its ores1 for1 strictly speaking1 it has none1 but in finding ways of catching the tiny particles of metal from the gangue1 as it is called1 the rock or earth in which they are embedded4 ,the trouble is that they are so small1 so infinitesimally small1 almost4 ,there are two great types of place where gold is found4 ,in the alluvial deposits1 the beds of old rivers1 the gold is quite loose4 ,the convulsions of ages ago have1 in many cases1 elevated these beds1 until now they are on the sides of mountains4 ,in such cases the loose1 gravelly stuff of which they are composed is washed down by a powerful stream of water from a huge hose-pipe terminating in a nozzle called a monitor4 ,this process1 called hydraulicing1 brings down everything into a pond formed at the foot of the hill1 and in some cases a boat or raft is floated upon the pond with machinery on board for dredging up the material4 ,often a powerful centrifugal pump sucks up the water through a pipe reaching to the bottom of the pond1 bringing gravel and gold with it4 ,arrived in this way upon the raft1 it all goes on to separating tables1 by which the gold1 being heavier1 is divided from the gravel1 which is lighter4 ,these tables will be referred to again later4 ,in non-alluvial workings the gold is embedded in rock of some kind1 such as that called quartz4 ,this is hard1 somewhat of the nature of granite1 and before the gold can be liberated it has to be crushed to the likeness of fine sand1 so that the tiny grains of gold can be captured4 ,the quartz is found in veins or lodes1 fissures1 evidently1 in the original crust of the earth1 produced probably as the earth cooled4 ,these have been gradually filled up by hot volcanic streams of water1 which carried not only the gold in solution but also the materials of which the quartz is formed4 ,it used to be thought that the veins were the result of hot liquids forced up from below by volcanic action1 the rock and metal being themselves in the liquid state through intense heat4 ,it is now more generally held that water was the vehicle by which the materials were brought in1 and the vein formed4 ,the gold in the alluvial deposits1 too1 is now thought to have come there in solution in water1 and not by the erosion and washing down of rocks higher up the original river4 ,however that may be1 and it is the subject of discussion among geologists and metallurgists1 there the gold is to-day1 firmly fixed in the hard rock1 and the problem which confronts the metallurgist is to get it out with the least expense4 ,the old historic way of breaking up the quartz rock is with what are called stamps1 pestles and mortars on a huge scale4 ,there are a number of vertical beams of wood1 each shod with iron1 fixed in a wooden frame1 so that they are free to slide up and down4 ,running along behind these stamps is a horizontal shaft with projections upon it called cams4 ,there is one cam for each stamp1 and as the shaft turns slowly round this projection catches under a projection on the stamp1 and after lifting it up a short distance drops it suddenly4 ,thus1 as the machine works1 the stamps are lifted and dropped in rapid succession4 ,the rock is fed into a box into which the feet of the stamps fall1 and thus it is pounded until it is quite small4 ,meanwhile a stream of water flows through the box and carries away the finely broken particles through a kind of sieve which forms the front of the box1 and which allows the fine1 small pieces to escape1 while holding back the larger ones and keeping them until they too have been crushed4 ,an average stamp will weigh #fjj to #gjj lb41 and the repeated blows of such a hammer are enough to pulverise the hardest rock4 ,machines such as these have been employed since the sixteenth century1 at all events1 and the improvements of modern times are only as regards details4 ,it may well be wondered1 then1 why such an old device is still in use and how it comes about that it has not been displaced by something newer and better4 ,the answer1 which is an instructive one1 well worth bearing in mind by many inexperienced inventors1 is that it is so simple4 ,it can be shipped in comparatively small parts1 and so taken cheaply to any outlandish place4 ,a good deal of it can be made roughly of wood1 so that if native timber is available it can be made partly at the mine1 and carriage costs saved4 ,finally1 it is so easy to work and to understand that the most inexperienced workman can handle it1 and there is so little that can go wrong that the most careless attendant cannot damage it4 ,in the bottom of the boxes there is placed some mercury1 for which gold has a curious affinity4 ,if a particle of gold once gets into contact with the surface of the mercury it will not get away again easily4 ,thus the mercury catches and holds many of the gold particles which are liberated when the rock is broken up4 ,as it reaches the required fineness1 then1 the crushed rock escapes from the stamp machine and flows away in the stream of water1 and although much gold is caught by the mercury1 it is by no means all4 ,the stream is therefore directed over tables formed of copper sheets coated with mercury1 so that additional opportunities are given to mercury to catch the grains of gold4 ,moreover1 the table1 which1 by the way1 is placed at a slight incline1 is broken at intervals by little troughs of mercury called riffles1 which assist in the depositing and catching of the metal particles4 ,but even then all the gold is not captured4 ,the crushed rock is now like sand1 and some of the grains still contain gold1 which has not been detached by the crushing4 ,the gold1 however1 makes such grains slightly heavier than the others1 and because of that they can be separated4 ,the old way is to use a blanket table1 a table1 that is1 covered with coarse flannel or baize1 the hairs of which catch these heavier particles as the water stream carries them along1 the lighter particles escaping4 ,the grains so caught form what are known as concentrates1 since in them the gold is concentrated4 ,the concentrates are subsequently treated as we shall see later4 ,now we can see how modern scientific methods have supplemented the old ways4 ,take first the case of the stamp mill or stamp battery4 ,in spite of that prime virtue of simplicity which has kept it at work almost unchanged for centuries1 it has its weaknesses1 and no doubt for some purposes crushing mills are better4 ,of these there are a great variety1 several of which depend for their action upon centrifugal force1 or1 as it is more correctly termed1 centrifugal tendency4 ,in these crushing mills there is a ring1 generally of steel1 inside which are suspended one or more heavy iron rollers4 ,the shafts which carry these rollers are attached by their upper ends to the driving mechanism on the top of the mill1 and when that is set in motion the rolls are carried round and round inside the ring4 ,because of the centrifugal tendency1 they swing outwards1 pressing heavily against the inner surface of the ring4 ,the rock is fed in in such a way that the rollers1 as they roll round the inside of the ring1 repeatedly travel over it and crush it4 ,in another type of mill1 called the ball mill1 the principle is different4 ,there you have a cylinder of steel which turns upon a horizontal axis4 ,this cylinder is partly filled with steel balls of various sizes1 and as the mill turns1 the rock1 being mixed with these balls1 is pounded and broken up4 ,as the mill turns over and over the balls fall upon the pieces of rock1 thus producing a fine powder4 ,other mills1 again1 are but refined editions of the common mortar mill so often seen where building operations are going on1 in which heavy iron rollers travel over the material to be crushed as it lies in a round pan4 ,the blanket table1 too1 gives place at the modern mine to the vanner1 of which there are several varieties4 ,essentially they are much the same1 and a description of two will serve to give an idea of them all4 ,let us take the ,record vanner4 ,imagine a large table formed of wood1 the upper surface covered with linoleum4 ,it is fixed on slides so that it can move to and fro endwise4 ,it is given a slight slope in the direction at right angles to its length--that is to say1 one edge is a little lower than the other4 ,the material is fed on at one end1 at the higher edge1 and naturally tends to run down and off at the lower edge4 ,it is restrained somewhat from doing this by the presence of rows of riffles or ridges running lengthwise4 ,nevertheless it does in a short time find its way off the table at the lower end4 ,but all the time that it is at work the table is being slidden backwards and forwards on the slides4 ,by a simple but curious mechanism it is arranged so that it moves quickly in one direction and slowly in the other1 with the result that the heavier particles of sand--those which contain gold--are carried to the farther end of the table4 ,thus1 as has been said1 all the stuff is fed on to the higher edge and carried down by the water1 until it falls off at the lower edge1 but during the journey from edge to edge the peculiar motion of the table causes the different kinds of sand to separate themselves1 so that the concentrates fall off near one end1 and the rest near the other end4 ,another interesting example of ingenuity is the well-known ,frue vanner4 ,in this the table is a broad1 endless band of india-rubber1 extended upon two rollers1 one of which is slightly higher than the other4 ,the stream of water and crushed ore flows on at the upper end1 and runs down to the lower1 the lighter particles being carried down and dropped off at the lower end1 while the heavier rest upon the band4 ,meanwhile the turning of the rollers carries the band slowly along1 so that the heavier particles gradually ascend and are carried over at the upper end4 ,to assist in the separation1 the whole concern is given a side-to-side shaking motion while it is at work4 ,we have seen so far how the ore is crushed1 and the coarser grains of gold got out of it by the aid of mercury4 ,the mixture of mercury and gold is termed amalgam1 and the process of extracting gold by mercury is called amalgamation4 ,the gold is actually dissolved in the mercury1 and so when the amalgam has been 7as it is periodically7 collected from the plant1 it has to be filtered and then evaporated in a retort4 ,the mercury vapour is caught and condensed back into a liquid1 while the gold is left in the retort4 ,in fact the amalgam is distilled in order to separate the gold and mercury4 ,but when all that is done we still have the concentrates from the vanners1 or whatever be used1 to deal with4 ,mercury is useless with them1 for the gold is covered probably with a coating of the other substances1 whatever they may be1 with which it has been associated1 or else there is mixed with the gold some substances which make amalgamation impossible1 or at least difficult4 ,often roasting is necessary before anything more can be done4 ,if arsenic or sulphur be present1 for example1 they interfere with the recovery of the gold1 and roasting will disperse them4 ,so the concentrates are passed through great furnaces1 in which they are heated in contact with air until these objectionable matters have been oxidised or burnt4 ,then finally we come to some process by which the remaining gold is dissolved out from its admixtures in some solvent liquid from which it can be subsequently precipitated4 ,this is rather interesting1 because it means that man has adopted1 to recover this gold from the ore1 the very method which it is believed nature employed to put it there4 ,as already said1 the latest idea is that the gold was carried into and deposited in the lodes where it is now to be found by water--that the gold was actually dissolved in water at the time4 ,but1 of course1 gold in its metallic state will not dissolve in water4 ,salts of gold1 however 7the meaning of the term salt1 as applied to a metal1 has been explained earlier71 will dissolve in water1 as every photographer who makes up his own toning solution knows from experience4 ,gold will not dissolve in water1 but chloride of gold will4 ,and so the gold must have been carried to its resting-place as a salt1 and converted into the metallic form after arrival4 ,in the same way1 to recover these finest particles of all1 it has to be converted back into a salt2 then that salt must be dissolved and drained away from the other stuff2 and1 finally1 the gold must be thrown out of solution again in some way4 ,the great example of this operation is the familiar cyanide process4 ,the word familiar is appropriate to this matter in only one way1 however4 ,holders of shares in mining companies1 for example1 may hear about it repeatedly at shareholders' meetings and in prospectuses1 but very few have any clear idea as to what it is4 ,so ,i cannot be accused of telling an oft-told tale if ,i devote a short space to its consideration4 ,the combination of one atom of carbon and one atom of nitrogen is called cyanogen4 ,if cyanogen be given the chance it will take unto itself an atom of hydrogen1 producing the deadly hydrocyanic or prussic acid4 ,alternatively1 if potassium be brought into combination with it1 there results potassium cyanide1 which1 with the assistance of water and oxygen1 can dissolve gold4 ,in applying this scientific fact to the purpose of recovering gold from the concentrates1 the latter are placed in vats with a weak solution of the cyanide in water4 ,the time during which they are allowed to remain depends upon the size of the gold particles4 ,if they be comparatively large1 it stands to reason that it must be longer than if they be small1 for they will take longer to dissolve4 ,after the proper time1 which is found by experiment1 the liquid is drawn off1 and in some cases the concentrates are given a second dose to ensure that the gold shall be thoroughly removed and none left undissolved4 ,if the material being operated upon be very fine1 as it often is1 forming what the mining people call slimes1 then mechanical stirrers have to be used in the vats to keep the stuff moving1 as otherwise the cyanide would not get to all the particles and some would not be acted upon4 ,the liquid1 having been the appropriate time in the vat1 is drawn off1 placed in wooden tanks or boxes1 and fine shreds of zinc are added to it4 ,discs of sheet zinc are put into a lathe and a fine shaving taken off them1 and it is these fine shavings which are used4 ,now zinc1 as we know from the fact that it is the essential part in electric batteries1 has very pronounced electrical properties1 and it is believed that these come into play here4 ,at all events the gold becomes deposited upon the zinc1 while the zinc itself is to a certain extent eaten away by the solution4 ,the result is 7a7 a solution weaker than it was before1 7b7 the remains of the shavings1 and 7c71 at the bottom of the box in which this process takes place1 a dark mud4 ,that black mud1 on being heated1 produces the bright metallic gold1 and the object of the whole operation is achieved4 ,the solution is then led to another tank1 brought up to its proper strength again and is ready to be used once more1 while the remains of the shavings are used for the next batch of material to be treated4 ,in some cases the crushed ore straight from the crushing mill is cyanided1 in others it is simply the remains left over from the previous amalgamating process which is thus treated4 ,all depends upon the nature of the material in question4 ,there are other chemical methods besides the cyaniding1 but it is the chief4 ,it has been found specially useful with the ,johannesburg ores1 and to it the ,south ,african goldfields owe a great deal of their success4 ,there is a more modern form of it1 although the whole process is quite novel1 having been introduced only in the nineties of last century4 ,this development1 it is almost wearying to repeat1 is electrical4 ,instead of the zinc shavings being used to precipitate the gold out of the solution1 the process is electrolytic4 ,a lead anode is used while the process is carried on in a box the bottom of which is covered with mercury1 which forms the cathode4 ,the precipitated gold is thus amalgamated1 the amalgam being removed at intervals1 retorted1 and the gold recovered4 ,the idea of recovering gold from the waters of the sea is certainly a most attractive one4 ,to some1 it is true1 the suggestion may bring thoughts the reverse of pleasant1 for there have been several partially successful attempts to delude the public with specious promises of vast dividends to be gathered in the form of pure gold from the inexhaustible sea4 ,still1 there is something in it1 and some day the dreams may be realised4 ,the quantity of gold dissolved in sea-water is so small that in #bjj cubic centimetres it is impossible to detect it1 even by the most delicate tests known4 ,the quantity needs to be multiplied threefold before the quantity of gold becomes even detectable1 to say nothing of being recoverable4 ,a writer in ,cassier's ,magazine1 a few years ago1 related how he had actually obtained gold from the water of ,long ,island ,sound4 ,but whereas he got two dollars' worth1 it cost him over #djjj dollars to do it4 ,no company will ever be floated on results such as that4 ,from the mud of a creek near ,new ,york1 however1 he did a little better1 for there ten dollars' worth of gold only cost #cgi dollars4 ,a company promoter would still look askance at even that comparatively successful undertaking4 ,as usual1 authorities differ1 but there is a consensus of opinion that in every ton of sea-water there is from one-half to one grain of gold1 besides silver and iodine4 ,it seems as if the water were able to dissolve that amount and no more4 ,if1 as has been suggested earlier in this chapter1 all the gold which is now found in mines and in gravel beds was carried there in water1 it is probable that the sea obtains its gold from the same original sources1 and that1 just as the hot ocean of ages ago carried its burden of gold in solution1 so the colder water of to-day has its share1 the cold water naturally carrying less than the hot did4 ,it is quite likely1 then1 that1 could we find out how to rob the sea of its precious metal1 it could replenish its store from some secret hoard of its own4 ,but even if it could not1 it would make little difference to us1 since what it holds is far more than we could ever use4 ,put it at half-a-grain per ton3 there are #dbje million tons in every cubic mile of ocean1 and #cjj million cubic miles of water in the ocean4 ,if all the gold that man has ever handled were to be dissolved in the sea1 no chemist would be able to discover the fact4 ,on the other hand1 if that half-grain per ton which we believe to be in the ocean now were to be recovered we should have about #dj1jjj million tons of gold1 a prospect which is enough to make the political economist turn pale with apprehension4 ,what is required is some substance which1 on being added to sea-water1 will combine with the gold1 and then be precipitated--that is to say1 fall to the bottom4 ,the precipitate--that which falls to the bottom--would need to be heavy1 so that it would fall quickly and not necessitate the water being left standing for long periods4 ,it would need to be cheap1 too1 or easily recoverable1 so that it could be used over and over again4 ,and1 finally1 it would need to be such that the gold1 having been captured by it1 could be easily obtained from it4 ,given such a precipitant1 the process of recovering the gold would be simple and cheap4 ,tanks would be formed in sheltered bays and inlets4 ,at every tide these would be filled1 and when full the precipitant would be added4 ,the tide falling1 the water would run out again and leave the precipitate on the floor of the tanks1 whence it could be removed by scraping4 ,simple treatment would release the gold from its partner1 which would then be returned to the tanks to act as the precipitant once more4 ,thus by simple means1 the tide itself assisting1 the gold could be obtained from the sea4 ,and there is nothing inherently impossible about this suggestion4 ,the necessary precipitant may exist1 awaiting discovery4 ,a large works operating in this manner would produce1 it is estimated1 about thirteen tons of gold per annum4 ,it looks as if it would be a bad day for the ,rand when that discovery is made4 ,and there is yet another possibility1 though less alluring than what has just been described4 ,the ,american writer mentioned a little while back got a better return from the mud of a creek than from the water itself4 ,in all probability this is due to the action of organic matter carried down by streams1 or in some other way introduced into the waters of the creek whence the mud was obtained4 ,this organic matter would possibly have an effect as a precipitant upon the dissolved gold1 causing it to be thrown out of solution and deposited in the mud4 ,thus the mud around our shores1 and particularly in the creeks and estuaries1 may be potential gold mines whence in time to come we may draw supplies of the precious metal4 ,the cyanide or some similar process may be needed in order that we may extract the metal from its enclosing mud1 but the time may not be so very far distant when dredging for gold may be a regular occupation at1 for example1 the mouths of the ,thames and the ,hudson4 ,,chapter ,x ,,intense ,,heat ,many of the useful and interesting manufacturing processes of to-day are based upon the intense heat which science has taught the manufacturer how to produce4 ,tasks which our forefathers dreamed of1 but were unable to accomplish1 are easy to-day because of the facility with which great heat can be generated4 ,the burning fiery furnace seven times heated is as nothing to some of the temperatures which are now obtained in the ordinary course of things4 ,the greatest heat of all is that of the electric arc4 ,two conductors1 generally rods of carbon1 are placed with their ends touching1 and the current is turned on so that it passes from one to the other4 ,then they are gradually drawn apart4 ,as the gap widens the current experiences more and more difficulty in passing over this non-conducting gap1 and great electrical energy has to be employed to keep it going4 ,now that wonderful law of the ,conservation of ,energy decrees that no energy can ever be lost4 ,it can only be changed from one form into another4 ,therefore the energy expended upon the arc is not lost1 but is converted into heat4 ,it is that heat1 acting upon the small particles of carbon which are torn off the ends of the rods1 which gives us the arc light4 ,as a matter of fact nearly all artificial light 7and natural light too for that matter#a7 is due to heat4 ,the heat sets the molecules in violent agitation1 which1 acting upon the corpuscles in the atoms1 sets them in violent motion too1 so that light is often the companion of heat4 ,some substances give light more readily than others1 under the influence of heat1 and we may reasonably believe that they are those whose corpuscular arrangements are such that they can be readily accelerated by the molecular action4 #a ,the glow-worm is an example of the few exceptions4 ,to take a familiar instance1 coal-gas is mainly methane1 one of the many combinations of carbon and hydrogen1 and when it is burnt in air the hydrogen and oxygen combine1 liberating heat1 which causes the carbon liberated at the same time to glow4 ,as each methane molecule breaks up the carbon atoms are thrown out1 forming solid particles of carbon1 and it is they really which give the light4 ,it is therefore the combustible gas heating the solid particles of carbon which forms the luminous part of the gas flame4 ,the non-luminous part of the flame1 near the burner 7,i am now speaking of the old-fashioned burner71 is the burning gas before the carbon particles have had time to heat up4 ,and the old gas flame1 as we know1 is now being rapidly displaced by the incandescent mantle1 the reason being simply that ,von ,welsbach discovered how certain rare minerals gave a more brilliant light when heated than particles of carbon do4 ,in other words1 it is easier to accelerate the motion of the corpuscles in ceria1 thoria and the other ingredients of the mantle1 than it is those of carbon4 ,consequently1 they sooner reach that degree of agitation which will send forth electro-magnetic waves of the high frequency necessary to produce the sensation of light4 ,for this reason the mantle heated by gas gives as bright a light as the carbon particles in the electric arc1 although the latter are subjected to a much more intense heat4 ,but the arc can be1 and often is1 used as a source of heat1 apart altogether from the light which it gives4 ,in ,sweden1 for example1 where coal is rare1 but water-power plentiful1 the power of the waterfalls is made to smelt iron4 ,hence the waterfalls are sometimes termed the white coal of that country4 ,needless to say1 it is the ubiquitous electricity which performs the change from the force of falling water into heat4 ,the furnaces are in shape much like those in which iron is smelted with coal--namely1 tall chimney-like structures at the bottom of which is the fire4 ,in the arc furnaces there are1 passing in through the side1 near the bottom1 a number of electrodes1 and between these a series of arcs are formed4 ,coke and ironstone are thrown in from the top into this region of intense heat1 and there the iron is liberated from the oxygen with which it is combined in the ore4 ,liberated1 it flows out through a spout at one side of the furnace4 ,but the question will arise in the reader's mind3 ,why is coke needed in an electric furnace8 ,it is for metallurgical reasons4 ,the heat of the arc loosens the bonds between the iron and oxygen1 but it needs the presence of some carbon to tempt the oxygen atoms away4 ,therefore coke1 as the most convenient form of carbon1 has to be there4 ,it is there1 however1 in much smaller quantity than it would be in an ordinary furnace4 ,it is not there as fuel1 but simply as the counter-attraction to draw the oxygen atoms away from their old love4 ,the arc is also used for welding pieces of iron together1 for which purpose it is eminently suitable1 since what is wanted is intense heat at a particular point4 ,but perhaps the reader will be wondering by this time what the heat of the arc is4 ,it has been repeatedly referred to as intense1 but something more definite may be demanded4 ,in theory it is unlimited4 ,apply more pressure--more volts1 that is--thereby driving more current across1 and the temperature will rise4 ,it is only a question of making dynamos large enough1 and driving them fast enough1 and any temperature is possible4 ,but there are practical difficulties which limit the degree of heat4 ,one is the melting-point of the furnace itself4 ,fire-clay melts at about #agjj deg4 to #ahjj deg4 ,c4 ,so in a furnace which has to be lined with fire-clay that is about the limit4 ,in welding two pieces of iron together1 the iron1 of course1 defines what the limit shall be4 ,it needs to be heated to welding heat and no more--that is1 a little short of melting--so that the parts to be joined are soft1 and1 with a little hammering1 will join thoroughly together4 ,if too much heat were to be applied the parts would melt away4 ,but the heat of the arc can be controlled by simply varying the current1 and so the right heat can be applied at the right place1 than which little more is wanted4 ,one very simple way of doing this is for the workman to hold one of the electrodes--a rod of carbon suitably insulated--in his hand4 ,the current is led to it through a flexible wire4 ,the iron itself is made the other electrode by being gripped in a vice which is itself insulated but connected to the source of current4 ,thus on bringing the point of his rod near to the part to be heated the man causes an arc to be created there4 ,by moving the rod he can move the arc about1 heating one part more than another1 distributing his heat if he wants to do so over a larger area1 or keeping it to a small one1 just as he wills4 ,on reaching the right heat the rod is withdrawn1 the arc destroyed1 and the iron can be hammered just as if it had been heated in a fire4 ,yet another way still is known as resistance welding4 ,in it an enormous current at an extremely low voltage is used4 ,the fundamental principle is the same1 since the heat is formed by forcing current past a point over which it is reluctant to pass4 ,that point of poor conductivity is the ends of the two bars to be joined4 ,they are placed just touching1 but since an imperfect contact like that always offers considerable resistance to the flow of a current1 the passing current needs only to be made large enough for great heat to be generated4 ,this is exceedingly pretty to watch4 ,we will suppose that the article to be operated upon is the tyre of a wheel4 ,the bar of iron has already been bent by rollers into the correct curve and the two ends are touching4 ,brought to the machine1 it is gripped1 each side of the junction1 in the jaws of an insulated vice and the current is turned on4 ,in a few seconds the place where the two ends are just touching begins to glow4 ,rapidly it increases in brightness until in about half-a-minute it is at welding heat4 ,then one vice1 which is movable1 is forced along a little by a screw1 so that the ends are pressed firmly together1 a little judicious hammering meanwhile helping to complete the job4 ,then the current is switched off and the complete tyre taken out of the machine4 ,the current used has a force comparable with that which operates domestic electric bells1 but in volume it is thousands of amperes4 ,alternating current is used1 and it is obtained from a transformer or induction coil4 ,in such a case the primary part of the coil is made of many turns of fine wire1 so that little current passes through it1 while the secondary part is but one or two turns of thick bar4 ,thus the voltage generated in the secondary is very little1 but since the secondary has an almost negligible resistance the current caused by that small voltage is enormous4 ,such an arrangement is in industrial realms generally called a transformer1 the term induction coil being employed more for those things of a similar nature intended for the laboratory4 ,the one just described is1 moreover1 a step-down transformer1 since it lowers the voltage1 to distinguish it from step-up transformers1 which raise the voltage4 ,and the resistance principle is also applied in another way to large furnaces1 such as those for refining iron4 ,in these the resistance of the iron itself is utilised to generate the heat4 ,of course1 it should be well understood1 heat is always generated in everything through which current flows4 ,there is no perfect conductor1 and so every conductor is more or less heated by the passage of current through it4 ,some energy needs to be expended to drive current1 even along large copper wires1 and that energy must be turned into heat in the wires4 ,if the same volume of current be forced along iron wires of the same size1 the heat will be greater1 since iron is but a poor conductor compared with copper1 the relation being about as one to six4 ,and if the iron be hot the resistance will be still more1 for it stands to reason that when heated the molecules1 being farther apart1 will be the less easily able to exchange corpuscles4 ,we have the best reasons for believing1 as has been suggested already1 that a current of electricity is but a flow of corpuscles1 and so we are not surprised to hear that1 as a general rule1 the hotter a thing is the less does it conduct electricity4 ,illustration3 ,by permission of ,cambridge ,scientific ,inst4 ,co41 ,ltd41 ,cambridge1 ,eng4 ,,measuring ,,heat ,,at ,a ,,distance ,this wonderful instrument1 the ,fery ,radiation ,pyrometer1 although itself some distance away from the furnace1 is telling the temperature of its hottest part4 ,so imagine a circular trough of fire-clay or other heat-resisting material filled with fragments of iron1 or1 it may be1 with iron barely above melting-point1 which has come from another furnace1 where it underwent the previous process4 ,circling inside or outside this trough is an enormous coil of wire through which currents of electricity are alternating4 ,that is the primary of a transformer1 and the secondary is--the iron itself1 in the trough4 ,if it be1 as it often is1 in the form of scrap1 or broken pieces1 the heat will begin to show itself where the pieces touch each other4 ,the currents generated in the trough1 by the coil outside1 will1 of course1 pass from piece to piece and the points of contact1 since they offer the greatest resistance1 will show signs of heat4 ,this will increase until the pieces begin to melt4 ,as the separate fragments merge into the molten mass the resistance will in one way decrease1 for the imperfect contacts between the pieces will give place to the perfect contact throughout the mass of liquid metal4 ,but for another reason--namely1 the increase in heat--the resistance will increase4 ,and all the while the alternations in the primary coil will be pumping currents1 as it were1 round and round the ring of molten iron4 ,whether the resistance increase or decrease1 the current will do the opposite1 so that heat will be generated whatever happens4 ,for as resistance decreases current increases1 and vice versa4 ,and the slightest variation in the strength of the primary current will have its effect upon the secondary1 and therefore on the heat generated4 ,so1 by simply regulating the primary current1 the temperature of the metal can be controlled to a nicety4 ,and such furnaces have the immense advantage that there is no possibility of deleterious substances in the fuel getting into and spoiling the metal1 a thing which may very easily happen during the manufacture of high-class steels1 alloys of iron in which the exact quantities1 purity and proportions of the ingredients are of the utmost importance4 ,hence these induction furnaces1 as they are called1 are frequently used quite apart from any question of utilising water-power4 ,and they will probably be used still more as time goes on4 ,for one thing1 they may become valuable adjuncts to the older form of iron and steel furnaces1 from which they will obtain their power free1 gratis and for nothing4 ,in districts such as ,middlesbrough they could generate more electricity than they have any use for4 ,the ordinary iron furnaces belch forth flames which are really good useful gas 7carbon monoxide7 burning to waste4 ,many of the furnaces are covered in at the top1 and this gas is led away to heat boilers for the steam-engines or to drive large gas-engines1 but in a large works there is more of this waste gas than they know what to do with4 ,now that could1 and probably will ere long1 be turned into electricity by means of gas-engines and the current used for making steel in induction furnaces4 ,it will probably surprise many to know that these enormous currents which can thus heat great masses of metal until they melt are no danger at all to the men who work with them4 ,a man might dip an iron rod into the trough of metal and he would scarcely feel the shock4 ,and the same is true of the welding machine1 which can be touched in any part without fear4 ,the reason1 of course1 is that1 broadly speaking1 it is volume of current which does harm1 and the resistance of the human body is so great that with the small voltages used1 the volume which can pass is negligible4 ,it should be mentioned1 however1 that the volume of current in lightning is also small1 but we know that it is capable of inflicting terrible injury4 ,lightning1 however1 is in a class by itself4 ,our terrestrial voltages are baffled by an air-gap of a few inches1 but lightning springs across a gap miles wide4 ,its voltage must1 therefore1 amount to millions1 and the ordinary rules relating to earthly currents do not apply4 ,but other sources of heat besides electricity are at the disposal of our manufacturers nowadays4 ,pre-eminently there is the flame of some gas burning with pure oxygen4 ,the oxyhydrogen jet has been known for many years as the best means of producing the light for a magic lantern4 ,such a jet impinging upon a pencil of lime causes the latter to glow with a dazzling white light4 ,but the oxyhydrogen jet is now employed in many factories for the welding of metals4 ,this is known as fusion welding1 since the two parts are actually reduced to liquid4 ,the usual way to go about this work is to bevel off the ends or edges to be joined4 ,suppose1 for instance1 that we wanted to weld two pieces of brass pipe together4 ,we should first file or otherwise trim the edges to be joined until when put together they form a groove practically as deep as the metal is thick4 ,then with a stick of brass wire in the left hand1 and an oxyhydrogen blowpipe in the right1 we should direct the flame from the pipe on to the metal until1 at one point1 the sides of the groove were beginning to melt4 ,then1 inserting the point of the wire into the groove1 we should melt a little off it4 ,thus we should work all round the joint1 melting the sides of the groove and filling in with melted metal from the wire1 until the whole groove had been filled up and the metal added had been thoroughly amalgamated with that on either side4 ,as a matter of fact1 if it were brass which we were working on we should probably use the cheaper though less pure form of hydrogen--coal-gas--so that it would really be oxycoal-gas that we should use and not oxyhydrogen4 ,the latter is used1 however1 notably for the fusion-welding of lead1 or lead-burning1 as it is termed4 ,the blowpipe is a brass tube about a foot or eighteen inches long1 with two passages in it1 one for the oxygen and the other for the other gas4 ,the gases are brought to one end of it through rubber pipes1 while at the other end there is a nozzle in which the gases mingle and from which they emerge in a fine jet4 ,the oxyhydrogen flame has a temperature of about #bjjj deg4 ,c41 hot enough to melt fire-clay4 ,that does not matter in the case of welding1 however1 since the molten metal is very small in quantity at any given moment1 and is allowed to cool before it can run away4 ,it would be an awkward temperature to deal with1 nevertheless1 in a furnace4 ,it seems strange that it does not burn the nozzle of the blowpipe1 but the fact that it does not is1 it is believed1 explained by the fact that the expansion of the gas1 as soon as it emerges from the hole out of which it shoots1 causes a comparatively cool space just there1 shielding it from the intense heat farther on4 ,an exceedingly interesting use of the oxyhydrogen flame is in the manufacture of artificial rubies4 ,these stones are made in ,paris by a very simple means4 ,the necessary chemicals are prepared and ground to an exceedingly fine powder4 ,this is then allowed to fall through an oxyhydrogen flame4 ,thus there is no need for a crucible capable of withstanding this high temperature1 since the melting takes place as the particles are in the act of falling4 ,when they reach the support prepared to catch them they have cooled somewhat4 ,stones so called are real rubies--artificial1 but not shams4 ,they possess every property of the ruby from the mine4 ,another product of the oxyhydrogen flame is the quartz fibres which are used for suspending the needles in the finest galvanometers4 ,the quartz is melted1 in this case a crucible being employed4 ,an arrow is then dipped in the liquid quartz and immediately fired into the air4 ,the thick treacly liquid is thus drawn out into a thread of such fineness that a microscope is necessary to find it with4 ,hotter even than oxyhydrogen is the oxyacetylene flame1 which at its hottest point reaches nearly #cejj deg4 ,c4 ,the gas1 which is another of the combinations of carbon and hydrogen 7its molecules containing two atoms of each71 is easily made by allowing water to come into contact with calcium carbide4 ,the latter1 which is ,ca,c#b1 is made by heating coke and lime together in the intense heat of an electric furnace4 ,this accounts largely for the great heating power of acetylene1 for since great heat is necessary to cause the elements to combine great heat is given out by them when they ultimately separate4 ,here again is the conservation of energy4 ,the heat energy of the electric furnace is largely expended in forcing these two elements into partnership4 ,they are1 as it were1 given a large amount of capital in the form of heat4 ,it ceases to be sensible heat1 becoming latent in the compound1 but still it is there4 ,so a lump of calcium carbide1 with which many readers are familiar1 has vast stores of heat locked up within it4 ,when water comes into contact with the carbide the partnership is broken1 but the heat is not liberated then1 since another partnership is formed1 which still retains the old heat-capital4 ,the calcium in the carbide is displaced by the hydrogen from the water1 and so ,c#b;,h#b comes into being1 while the rejected calcium consoles itself by entering into combination with the equally forsaken oxygen from the water1 forming ,ca,o1 which is but another name for lime4 ,then the acetylene 7,c#b;,h#b7 is mixed with oxygen in the blowpipe and burnt1 under which conditions the pent-up heat1 borrowed originally from the electric furnace1 is brought into play4 ,with this flame the harder metals can be fused and welded4 ,wrought iron1 cast-iron1 steel in all its forms1 all can be melted by the oxyacetylene flame1 almost as easily as snow by a hot iron4 ,the fusion welding of these metals is then carried on just as already described for brass4 ,by means of a special blowpipe1 wherein an excess of oxygen is introduced at the hot point1 hard steel plates can be cut to pieces almost as easily as a grocer cuts cheese4 ,even thick1 hard armour-plate can thus be cut1 almost the only way1 indeed1 in which it can be cut4 ,and for purposes such as welding and cutting this flame has an interesting and peculiar advantage over all other kinds of heat4 ,when a metal is heated in the air there is usually trouble from oxidation4 ,the domestic poker1 for example1 after it has been left to get red-hot in the fire is seen to be coated1 in the part which has been heated1 with scales which will flake off if the thing be struck4 ,those scales are oxide of iron1 caused by the union of iron and oxygen when the poker was hot4 ,but if the heat be applied by the oxyacetylene flame that will not happen4 ,the oxygen and the carbon from the acetylene will burn1 and if the supply of the former be properly regulated it will be entirely used up in the process4 ,the hydrogen from the acetylene is1 strange to say1 unable to unite with oxygen at such a high temperature as that of the oxygen and carbon1 so that it passes on beyond the oxygen-carbon flame and ultimately burns on its own account with the oxygen from the atmosphere in a second flame surrounding the first4 ,thus there is a double flame3 inside1 a little pointed cone of white flame1 that is the oxygen and carbon2 and outside that a bluish flame1 the hydrogen and the atmospheric oxygen4 ,the latter flame forms a kind of jacket entirely enveloping the former4 ,and so when one melts metal by means of the white cone the hydrogen jacket shields the molten metal from oxygen and prevents the oxidation4 ,only one who knows the bother caused by oxidation whenever metals are heated can realise the wonderful advantage of this4 ,and now we can turn to even another source1 also quite modern1 of high temperature4 ,if the oft-quoted man in the street were asked the two commonest things on earth he might possibly name oxygen as one1 and so far he would be right1 but the chances are much against his naming aluminium as the second4 ,if he did not1 however1 he would be wrong4 ,aluminium and oxygen form alumina1 of which are constituted the sapphire1 the ruby and other precious stones1 but alumina is most commonly found in combination with silica1 or silicon and oxygen4 ,this compound is called silicate of aluminium1 and of it are formed clay and many rocks4 ,the reason why the metal aluminium was until recently rare and expensive was because of the great difficulty of disentangling the metal from this rather complex combination4 ,and these two commonest elements have1 under certain conditions1 a rare affinity for each other4 ,they join forces with such energy that great heat is given out in the process4 ,this1 again1 we may regard as an example of the conservation of energy4 ,heat had to be used up1 apparently1 in separating the aluminium and oxygen as they were found together in the natural state4 ,and that heat reappears when they combine together again4 ,this is a most useful principle1 for if heat has disappeared anywhere in the course of some operation1 we know that in all probability1 if we go about it the right way1 we can get that heat back again1 perhaps in a more convenient form4 ,that is so in this case at all events4 ,now aluminium will not readily combine with atmospheric oxygen1 but it will readily do so with oxygen from the oxide of a metal4 ,so if we put into a vessel some oxide of iron and some finely powdered aluminium1 and give it some heat at one point1 just to set the process going1 the whole mass will burn with intense heat4 ,and when the burning is finished the crucible will be found to contain 7#a7 some molten iron1 the oxide of iron with the oxygen gone1 and 7#b7 some oxide of aluminium or alumina1 in the form which we call corundum1 a very hard substance which in a powdered form is used for grinding hard metals4 ,we start1 you will notice1 with a pure metal and an oxide4 ,we finish with a pure metal and an oxide1 only the oxygen has changed its quarters1 having passed from the iron to the aluminium4 ,and in the course of the change a vast amount of pent-up heat has been liberated4 ,aluminium is thus a fuel1 strange though it may seem to say so1 just as coal is4 ,coal1 however1 is willing to pair off with oxygen from the air1 while aluminium1 more fastidious1 will only accept it as partner when it can steal it from another combination4 ,but the practical result is eminently satisfactory1 for the action of the aluminium and iron oxide is to leave us with a crucible full of molten iron at a very high temperature4 ,and this can be used in various ways4 ,tramway rails1 for example1 can be joined together by it4 ,a mould is formed around the ends of two rails1 where they butt together1 and into this mould a quantity of the melted iron can be poured4 ,so hot is it that it partially melts the ends of the rails1 and then1 amalgamating with them1 it forms a perfectly homogeneous connection between them4 ,the same method can be applied to the repair of iron structures of all kinds4 ,the propeller shaft of a ship1 for example1 sometimes breaks on a voyage4 ,such a catastrophe is fraught with the most serious consequences1 unless it can be quickly repaired4 ,thermit1 as this process is called1 is perhaps the only means whereby1 under certain conditions1 this can be accomplished4 ,the extraordinary heat of the metal produced in this way is demonstrated by the fact that if it be poured on to an iron plate an inch thick it goes clean through it4 ,it melts its way through instantly4 ,but although such high temperatures are at the command of the modern manufacturer1 there are some things--indeed many things--which still baffle him1 the diamond1 for example4 ,it is true that diamonds of small size have been made1 but larger ones have so far defied all efforts4 ,one very interesting fact about this may be mentioned in concluding this chapter4 ,sir ,andrew ,noble1 a member of the great firm of ,armstrong1 ,whitworth ,co41 of ,elswick1 tried the experiment of exploding some cordite1 a high explosive1 inside a steel vessel of enormous strength4 ,he thus produced what is believed to be the highest temperature ever produced on earth4 ,it is reckoned to have been #ebjj deg4 ,c41 and the pressure at the same time was1 it is calculated1 #ej tons per square inch4 ,his intention was not to make diamonds1 but ,sir ,william ,crookes predicted that diamonds would be the result4 ,for the cordite consisted mainly of carbon1 which1 as is well known1 is the material of which the diamond is formed1 and the combination of high temperature and high pressure is just what is needed1 so it is believed1 to bring the carbon into this particular form4 ,and true enough1 on the iron being examined after the explosion1 there were seen tiny diamonds4 ,for larger ones even higher temperatures and greater pressures are1 no doubt1 necessary1 and as the diamond1 like gold1 has a peculiar fascination for mankind1 so the efforts to manufacture it will continue4 ,in years to come the means may be found of creating these extreme conditions of temperature and pressure1 and so another of the problems of the ages will be solved4 ,illustration3 ,by permission of the ,british ,aluminium ,co ,a ,,striking ,,feature ,,of ,,modern ,,aluminium ,,works ,for the production of aluminium water power is required4 ,water is stored at a high level and is then brought down to the factory in pipes4 ,the illustration shows the pipe track recently laid down for this purpose at ,kinlochleven in ,argyleshire4 ,the six pipes1 each of which is thirty-nine inches in diameter1 run down the hillsides for one mile and a quarter ,,chapter ,,xi ,,an ,,artificial ,,coal ,,mine ,those countries which are blessed with a plentiful supply of coal are periodically shocked and saddened by a terrible calamity--an explosion in one of the mines1 in which often scores of poor fellows lose their lives1 and hundreds of widows and orphans find themselves without a breadwinner4 ,one has only to recall that heart-rending calamity of the ,courrieres mines in ,france1 where over a thousand lives were lost1 to realise how important is the question of the cause and the cure of the colliery explosion4 ,it used to be thought a settled matter that these were due to the accidental ignition of a gas called1 scientifically1 methane1 but by the miners fire-damp4 ,this undoubtedly does collect in many mines1 and since it is much the same as the domestic coal-gas 7indeed methane forms the bulk of coal-gas7 it is not surprising that the explosions were attributed to it4 ,at times shots were fired1 to blast down the coal1 and although the greatest precautions are taken to prevent any accident resulting1 it seems certain that explosions have occasionally followed the firing of shots4 ,but still more dangerous is the adventurous miner who1 for some reason1 opens his safety lamp4 ,it is lit for him before he enters the workings1 and locked up1 so that1 theoretically1 he cannot tamper with it2 but it has to be a cleverly devised lock that cannot be picked in some way1 and with the carelessness born of long immunity from accident these are got open sometimes1 with1 it may be1 disastrous results4 ,even a spark struck from a miner's pick may ignite the gas2 or a spark from some electrical machine used in the mine4 ,that is one of the reasons why electrical apparatus is suspect in colliery matters and machines worked by the less convenient and more costly means of compressed air are preferred4 ,in some such manner the fire-damp is ignited1 and then there follows the fiery blast1 which1 sweeping through the narrow galleries and passages which constitute the workings1 simply licks up the life of the men whom it encounters4 ,others1 in byways and sheltered corners1 escaping the burning cloud of flame1 are poisoned by the deadly fumes of carbon monoxide which it leaves when its force is spent4 ,while others1 perchance the most unfortunate of all1 are saved for a time1 but1 being imprisoned by falls from the roof and walls1 die a lingering death of hunger and slow suffocation4 ,a colliery explosion is one of the ghastliest events imaginable1 the only relief from which is the noble heroism with which the survivors1 from the mine managers to the humblest workmen1 crowd round the pit-mouth1 eager to risk their own lives for the faint chance of saving some below4 ,not infrequently these brave volunteers only share the fate of the men they would rescue4 ,now all that1 as ,i have said1 used to be put down to the effect of the fire-damp4 ,but it dawned upon men's minds some years ago that the damage seemed to be out of proportion to the power of the gas4 ,modern mines are well ventilated by large fans1 which impel great volumes of air through all the workings4 ,the air currents are cunningly guided by partitions or brattices1 so that every nook and corner shall be scoured out by the plentiful draught of pure fresh air4 ,consequently the amount of explosive gas which can collect in any one place is but small4 ,how1 then1 can so small a volume of gas do so large an amount of damage8 ,coupled with this was the fact that explosions take place in flour mills1 where there is no gas1 and experimenters had found in their laboratories that almost any burnable substance1 if ground up finely enough and blown into a cloud1 would explode4 ,coal-dust would naturally do this4 ,indeed anyone throwing the dust from the bottom of the coal-shovel upon a fire will see for himself how1 quickly such dust will burn1 and1 as has been pointed out in an earlier chapter1 an explosion is but rapid burning4 ,so the blame was largely transferred from the shoulders of the fire-damp to those of the clouds of coal-dust which collect throughout the workings of a mine4 ,but then a difficulty arose from the fact that there is dust in all mines1 yet some districts are quite free from explosions4 ,and such districts are those where there is little or no fire-damp4 ,these two facts seem to be explainable in one way1 and in one way only4 ,it must be that the gas first of all explodes feebly1 and so1 stirring up the dust lying along the roads and passages1 prepares the way for the powerful1 deadly explosion of coal-dust which follows4 ,but that was only a guess1 and the matter was of such importance that it needed something more certain than mere assumption4 ,so the ,mining ,association of ,great ,britain decided to have a series of experiments which should settle once and for all what part the coal-dust played in these catastrophes1 and how best they could be prevented4 ,it was at first thought that an old mine might be utilised for the experiments1 but there was the difficulty that such always become wet after work has ceased in them1 and so the dust would not behave normally4 ,moreover1 the work would be extremely dangerous and the results difficult to observe4 ,then a culvert was suggested built of concrete1 partly buried in the ground1 but that too was dismissed4 ,finally it was decided to make an imitation mine of steel1 using old boiler shells with the ends taken out4 ,the sum of ,l#aj1jjj was subscribed for the purpose by the coal-owners of ,great ,britain1 and the great work was carried out at ,altofts1 in ,yorkshire1 close to a colliery where a terrible disaster occurred in #ahhf4 ,here the great tube or gallery was built4 ,roughly the shape of a letter ,l1 one leg is over #ajjj feet long1 while the other is #bie feet4 ,the longer leg is #g-a_/b feet in diameter and the shorter #f feet4 ,at the end of the shorter part a large fan is installed which can force #ej1jjj to #hj1jjj cubic feet of air per minute through the structure1 so producing the conditions of a well-ventilated mine4 ,the shorter length has several sharp turns in it for the purpose of breaking the force of the explosion along that part1 and so shielding the fan from damage1 while a tall chimney is provided there1 so that1 the door being shut to cut off the fan1 the gases from the explosion can find a harmless way out4 ,inside the tube1 shelves are fixed along the sides so as to reproduce the effect of the timbering in a real mine1 upon the beams of which the dust finds lodgment4 ,props were put up too1 just as they would be in the real mine4 ,everything1 in fact1 was done to make the place as perfect a replica as possible of actual underground workings4 ,and then1 added to this huge and costly structure1 was an outfit of scientific instruments worthy of the important investigations which were to be carried on4 ,to grasp the purpose and working of these we need to remind ourselves of the aims and intentions of the experiments4 ,first of all it was desired to find out how various quantities and qualities of coal-dust behaved4 ,the dust was laid along the floor of the tube and along the shelves4 ,a small gun fired at some point in the tube raised a cloud of this dust just as the gas explosion in the real mine would do4 ,then another gun was fired to explode the dust-cloud4 ,so far all is quite simple and easy4 ,but to do that would be of no value without the means of finding out exactly what resulted from the explosion4 ,and that is the function of the instruments4 ,to commence with1 there is the great wave or tide of force or pressure which surges along the gallery immediately the cloud bursts into flame4 ,how fast does that wave travel8 ,how long is it after the explosion before the shattering effects of it are felt a hundred yards away8 ,to solve that problem electrical contact-breakers are fixed at intervals of fifty yards along the gallery4 ,each of these consists of a cylinder with a piston inside it something like1 shall we say1 a cycle pump4 ,the piston1 held down normally by a spring1 is blown upwards by the force of the explosion4 ,the spring is adjustable1 and so it can be arranged that the feeble force of the gun cannot lift the piston1 but the more powerful coal-dust explosion which follows can4 ,thus when the explosion takes place these contact-breakers are operated in succession4 ,the one nearest the seat of the disturbance is operated first2 next the one fifty yards farther away2 then the one a hundred yards away1 and so on4 ,the moments when they work will tell the speed at which the blast travels along the gallery4 ,but it travels with great speed1 and so to measure and record the exact moment when each contact-breaker is moved is a matter of no little difficulty4 ,electricity1 however1 makes this1 like so many other things1 comparatively easy4 ,there is an apparatus used in astronomical observatories called a chronograph1 which registers1 within a small fraction of a second1 the moment when a star seems to pass across a wire in the transit circle1 the telescope by which the positions of stars are determined and the exact time kept4 ,the observer sits with his eye to the telescope1 watching the apparent movement of the star4 ,in his hand he holds a small push1 pressure on which by his fingers operates a minute pricker1 which acts upon a moving strip of paper4 ,the paper travels along with the utmost steadiness and regularity1 while a clock drives a sharply pointed pricker on to it every two seconds4 ,thus the clock marks out the paper into lengths1 each of which represents two seconds4 ,but the other pricker1 worked electrically by the observer's hand1 also makes its mark upon the paper1 and so1 while the regular marks indicate intervals of two seconds1 each irregular one marks the time of a transit or passing of a star across the wire4 ,an examination of the strip subsequently enables the times of a transit to be seen with great accuracy1 from the position of the corresponding mark between two of the regular marks4 ,and the same principle was applied to the circuit-breakers of this artificial mine4 ,normally1 current flows through the circuit-breaker1 but the lifting of the piston breaks the circuit 7whence the name of the contrivance71 and that breaking of the circuit and consequent cessation of the current operates the chronograph4 ,by a cleverly constructed device1 the details of which are too complicated to set out here1 each circuit-breaker in turn makes its mark on the same strip1 so that the distances apart of these marks show the time taken by the force of the explosion to travel fifty yards4 ,meanwhile the clock goes on making its regular marks 7in this case every half-second71 so that they form a scale by which the other intervals can be measured very exactly4 ,the chronograph used here is more accurate than that in use at ,greenwich ,observatory1 the reason being that in this case the recording currents are sent mechanically by the contact-breakers operated by the explosion itself1 while in the case of the astronomer the human element comes in4 ,to watch a moving speck of light and to tell exactly when it crosses a fine line is by no means easy1 and so to tell the time within a tenth of a second1 is about the limit of possible accuracy4 ,the instrument we have been referring to1 however1 can register the time which a gaseous wave moving #cjjj feet per second takes to travel fifty feet4 ,in other words1 the circuit-breakers can be operated so fast that when only a sixtieth of a second intervenes between the action of one and that of the next the chronograph can duly record the fact4 ,the records of the chronograph can be made in two ways3 one by a pen on a piece of paper tape1 and the other by a scratch on a piece of smoked paper4 ,so by that means the progress of the force of the explosion can be measured4 ,it is necessary also to time the movement of the heat of the explosion1 for the two may not travel together1 and the difference between them may let in some light as to the nature and behaviour of the explosion4 ,so for this second purpose a second set of circuit-breakers are used4 ,each of these consists of a strip of thin tinfoil stretched across the gallery4 ,being placed edgeways to the moving current of gas1 the force of the explosion has no effect upon it1 but the heat instantly melts it4 ,normally1 current flows through the strip1 and so the melting is signalised by the cessation of the current1 which event is recorded by the chronograph4 ,thus the speeds at which the force and the heat of the explosion travel are ascertained4 ,another important fact which needs to be found is the amount of the force1 or the pressure1 at different points4 ,for this purpose pressure-gauges can be connected to the gallery at the desired spots by means of flexible tubes4 ,this flexible tube is necessary in order that the vibration of the steel shell1 due to the explosion1 shall not be communicated to the instrument4 ,the pressure1 finding its way along the flexible pipe1 raises a piston against the force of a spring1 and the distance to which it is raised forms1 of course1 a measure of the pressure inside the gallery at the point to which the tube is connected4 ,the pressure is recorded by the action of the piston in moving a style which just touches against the surface of a moving paper4 ,there are three styles in all marking this paper4 ,the first is the one just mentioned4 ,the second is held down on to the paper by an electro-magnet energised by current flowing through a fine wire stretched across the gallery just where the explosion originates4 ,this fine wire is broken at the moment of the explosion1 whereby the current is cut off and the style raised4 ,it therefore makes its mark until the moment the explosion occurs1 and then leaves off4 ,the end of that line1 therefore1 shows the time of the explosion4 ,meanwhile the first style is drawing a straight line1 but as soon as the pressure begins to be felt by the pressure recorder this style moves and the line slopes upward4 ,upward it goes as the pressure increases1 until it has reached its height1 after which it descends1 until the style is drawing a straight line once more4 ,thus the rise and fall of the line represents the rise and fall of the force of the explosion4 ,then comes the matter of time4 ,how soon after the explosion occurred did the pressure begin to be felt8 ,how long did it take to reach its maximum and how long to die out again8 ,these questions need answers which the apparatus so far described does not give4 ,true1 the speed of the paper may be known approximately1 but all that ,i have described will occur within the space of a fraction of a second1 and it is difficult to tell the speed of the paper with sufficient accuracy4 ,therein we see the purpose of the third style4 ,it is attached electrically to the tenth-of-a-second time-marker4 ,this consists of a weight suspended at a height4 ,the force of the explosion lets it drop4 ,the moment it starts to fall it causes the style to make a mark on the paper4 ,when it has fallen a certain distance the style makes another mark4 ,and the distance that the weight falls between the making of the two marks is so adjusted that the space between them on the chart represents exactly a tenth of a second4 ,thus a scale is formed upon the chart by which the other times can be measured4 ,there is the line terminating at the moment of explosion2 the straight line changing into an up-and-down curve1 representing the time and the variation of the pressure2 finally there are the two marks representing a tenth of a second by which the other marks recorded upon the chart can be interpreted4 ,but the mere pressure and velocity of the explosion form but a part of the knowledge desired4 ,how the explosion is formed1 whether or not the coal-dust is burnt up entirely1 whether1 indeed1 it be the dust itself which burns or coal-gas given off by the dust under the heat of the preliminary explosion1 what the gas is which is left by the explosion at various stages--these are important things to be known1 and they can only be ascertained by taking samples of the gases in the gallery at different moments during and after the explosion4 ,to obtain these samples bottles are used1 but the question is how to get them filled at just the right time4 ,into the shell of the gallery holes are drilled1 and to these the metal bottles or flasks are screwed1 a pipe leading from the mouth of each bottle well in towards the centre of the gallery4 ,the end of this tube is closed by a cap of glass above which there stands poised a little hammer4 ,controlling the hammer is an electrical device called a contact-maker1 so arranged that just at the desired moment the hammer falls1 breaking the glass1 and admitting a sample of the gas in the gallery1 the bottle and its tube having previously had the air exhausted from them1 so that on the glass being broken the gas is sucked in4 ,at the same moment a weight falls1 attached to the end of a cord1 and this1 on reaching the end of its tether1 closes the end of the tube1 and the sample is imprisoned until such time as the bottle can be disconnected and taken away to the laboratory for its contents to be analysed4 ,the contact-makers are of two kinds4 ,in one the pressure of the explosion raises a piston which completes a circuit allowing current to flow through the very fine wire which prevents the fall of the hammer4 ,this fine wire being fused by the current1 the hammer falls and does its work4 ,the other kind1 which are used when the force of the explosion is not enough to raise a piston1 is operated by one of the tinfoil circuit-breakers4 ,a magnet1 being energised by current passing through the foil1 holds up a curved bar over two cups of mercury4 ,broken by the heat of the explosion1 the foil cuts off this current1 de-energises the magnet1 and allows the bar to fall with its ends in the mercury4 ,this completes another circuit1 permitting current to pass to the fine wire1 whereby the hammer is released4 ,by connecting a bottle to a contact-maker at a distance the sample can be obtained at any desired period of the explosion4 ,if1 for instance1 the sample is to represent the immediate products of combustion1 it is placed near to the contact-maker4 ,then the sample is drawn in practically at the moment of explosion4 ,if1 on the other hand1 it is the after-damp that is to be sampled1 then the bottle would be connected to a contact-maker a long way from the seat of the explosion1 with the result that its glass cap would not be broken until some considerable time had elapsed after the explosion has passed the bottle4 ,the time also during which the bottle is drawing in its sample can be adjusted by varying the length of the cord to which the weight is attached4 ,and last of all must be mentioned the employment of a kinematograph1 capable of taking twenty-two photographs per second1 for observing the effects at the ends of the gallery 7see illustrations74 ,thus records are obtained of the force and heat of the explosion1 its mechanical and thermal effects upon the walls of the gallery1 or1 if it were in a real pit1 the effects which it would have in shaking and in heating the workings1 and the men labouring in them4 ,this and the analysis of the gases producing and produced by the explosion1 derived from the contents of the bottles1 give sound data upon which can be built up reliable theories as to the nature of colliery explosions and the way to prevent them1 results which could be obtained in no other way4 ,no one can help being struck with the thoroughness and ingenuity of the means adopted to these ends1 and it is no exaggeration to say that it is a splendid example of thoroughly scientific methods applied to an important industrial investigation4 ,it will be interesting to conclude this account with a brief mention of some of the results to which these painstaking efforts have led4 ,first in importance the fact is placed beyond doubt that coal-dust1 which in bulk will only burn slowly1 will1 when well mixed with air1 explode4 ,and no combustible gas need be present to aid in the explosion4 ,the dust-raising gun1 by blowing some dust into a cloud which was ignited by the second gun1 caused an explosion powerful enough to do all the damage experienced in the most disastrous natural explosions4 ,so it is practically certain that the function of the gas is but that of the first gun1 to raise the cloud of dust4 ,a typical experimental explosion may be briefly described4 ,on the cloud-raising gun being fired a small cloud of dust was driven out of the ends of the gallery1 even that end at which the fan was blowing air in4 ,in other words1 the current of air was checked1 even reversed1 by the preliminary shock4 ,this cloud was1 of course1 shown by the kinematograph4 ,then when the second gun was fired1 and the real coal-dust explosion occurred1 there was first a cloud of dust shot out larger than the other one1 to be followed by a cloud of flame #ahj feet long4 ,these also were recorded by the kinematograph4 ,the sound was heard seven miles away4 ,pressures as high as #ib lb4 per square inch were recorded1 and the force of the blast was found to travel well over #bjjj feet per second4 ,in many cases1 strange to say1 the effects were very slight at the seat of the disturbance1 the force seeming to increase as the wave travelled along the gallery4 ,probably the dust had not time to burn completely but only partially at the first onset4 ,where props or timbers checked the flow of the flaming gases there the damage was most1 for no doubt the eddies caused the air and coal to be particularly well mixed at such points4 ,an encrustation of coke was found on the sides and the timbers after all was over1 probably because there was not sufficient air to burn all the dust1 and some was only heated into coke to be deposited on the nearest surface1 where the tarry matters would make it stick4 ,finally1 the most important1 perhaps1 of all1 it was demonstrated that an admixture of stone-dust with the coal-dust made it non-inflammable4 ,if a small zone were treated in this way1 stone-dust being mingled with the other1 the explosion became stifled at that point4 ,true1 the poisonous after-damp swept on beyond1 so that men there might have been poisoned by it1 but the stone zone would certainly save them from the direct effects of the blast4 ,if1 however1 stone-dust be mingled with coal-dust all along the gallery1 then no explosion at all would occur1 again proving that it is the coal-dust which does the damage4 ,in the colliery adjoining the experimental gallery this plan had been in use for years4 ,soft shale is ground to fine powder1 and is sprinkled wherever coal-dust has collected4 ,it is just strewn by hand1 giving the workings the appearance of having been roughly whitewashed4 ,and since that has been done there has been no explosion in that pit4 ,the experiments showed beyond doubt that that was no chance occurrence4 ,they showed that in some way not thoroughly understood this addition of stone-dust renders the coal-dust harmless4 ,it may be that it merely dilutes it4 ,it may be that in some way it takes some of the heat and so prevents the coal particles becoming hot enough4 ,it may be that1 being a little heavier1 it checks the formation of the dust-cloud4 ,however that may be1 there is no doubt now that stone-dust is the salvation of the miner so far as explosions are concerned4 ,water sprinkled upon the coal-dust1 by laying it and keeping it from forming a cloud1 has the same effect1 but it is less convenient1 for the simple reason that water evaporates1 while stone-dust stays where it is put4 ,,chapter ,,xii ,,the ,,most ,,striking ,,invention ,,of ,,recent ,,times ,probably no invention has made such a sensation during recent years as wireless telegraphy4 ,and since it is the direct outcome of the most abstruse1 purely scientific investigations1 there could be no more appropriate subject for a place in this book4 ,for many years there has been a belief in the existence of a mysterious something to which has been given the name of ,the ,ether4 ,totally different1 it should be noted1 from the chemical of the same name1 it is entirely a creature of the intellect4 ,none of our senses give us the slightest direct indication of its existence4 ,no one has either seen1 felt1 heard1 smelt or tasted it4 ,yet we feel that it must exist1 for the simple reason that some things which our senses do tell us of are utterly inexplicable without it4 ,it was originally thought of in connection with light4 ,standing at night upon the top of a hill1 we see the lights of a town a mile away4 ,how is it that those distant gas or electric lamps affect our eyes8 ,they are a mile away2 and the idea that one object can affect another at a distance is one which the human mind refuses to accept4 ,we feel compelled to believe that there is something in contact with the source of light which is affected first1 and through which the disturbance1 whatever it may be1 is conveyed to our eyes1 with which it must also be in contact4 ,we feel that there must be a something stretching from our eyes to the distant objects1 by which the light is carried4 ,of course the air fills the space referred to1 but that cannot be the carrier of light1 for if we look through a glass vessel from which the air has been exhausted we see distant objects undimmed4 ,we also have good reason to believe that the air belongs specially to our globe1 and does not extend upwards for more than a few miles4 ,consequently it cannot be air which brings sunlight and starlight4 ,we are forced to fall back1 therefore1 upon the belief in something1 of which we have no other knowledge1 which must fill all the vacant spaces in the whole universe1 passing1 even1 between the particles of which ordinary matter is composed1 reaching as far as the remotest star1 able to penetrate everything1 and consequently not excludable from the most perfect vacuum4 ,it is something so different from anything of which we have any direct knowledge that one is tempted sometimes to doubt whether there must not be some other explanation of light4 ,in order to transmit light at the speed at which we find that it does in fact travel1 the ether must be more rigid than the hardest substance we know of4 ,many1 many thousand times more rigid1 indeed4 ,yet it seems to offer no resistance to the passage of the planets through it4 ,still1 there is no other alternative1 so far as men can conceive1 and we are compelled1 therefore1 to believe in the existence of the ether4 ,the first things discovered by the telescope were the larger satellites of ,jupiter4 ,with that precision for which astronomers are noted1 they soon drew up time-tables1 showing not only the past movements of these bodies1 but also their future ones4 ,they were soon puzzled1 however1 by the obvious fact that the moons of ,jupiter were not working according to schedule1 to use a railway expression4 ,they got later and later for a time1 and then gradually quickened up until they got too fast4 ,then they slowed down again4 ,this repeated itself1 and is going on still1 with this difference1 however1 that the cause has been discovered and the schedules amended accordingly4 ,the solution of the puzzle was that when the earth and the great planet are on the same side of the sun they are some #ahf millions of miles nearer together than when they are on opposite sides of the sun4 ,the evolutions of the satellites are quite regular1 according to the astronomers' calculations1 but they seemed to the earthly astronomers to vary1 because of the time which light took to traverse that #ahf millions of miles4 ,when the two bodies were nearest together the occurrences seemed to happen about #ajjj seconds 7#af minutes7 earlier than when they were farthest apart4 ,consequently it became evident that light took #ajjj seconds to travel #ahf million miles1 or that1 in other words1 it moved at the prodigious speed of #ahf thousand miles per second4 ,that discovery was1 of course1 many years ago1 but experiments since have proved the figure mentioned to be about right4 ,it put beyond question the fact that the action of a distant light upon the eye was not an action at a distance1 for such action1 were it possible1 would take effect at once4 ,seeing that light passed from the distant satellites at a definite velocity1 and took a certain time to reach us1 it was evident that it was1 during that time1 passing through a medium of some sort1 and that medium must be the ether1 for no alternative explanation will suffice4 ,so it became recognised that light really consists of waves or undulations of some sort in the ether2 that a distant1 luminous body set these waves going2 that they travelled with a definite velocity1 and then1 striking our eyes1 produced the sensation known as light4 ,many things were found out about light in the years which followed the discovery of its velocity4 ,the lengths of the waves were ascertained--that is to say1 the distance from the crest of one to the crest of the next4 ,the different lengths were sorted out and found to give rise to different colours1 while longer waves1 which produced no sensation of light1 were found to carry heat1 thereby explaining how the heat reaches us from a distant fire1 or from the sun4 ,of the actual nature of the waves1 however1 little was known1 although there was a vague idea that they were connected in some way with electricity1 at which point in the story there comes in the famous name of ,james ,clerk ,maxwell1 a professor of ,cambridge ,university1 who in #ahfd produced before the ,royal ,society the explanation of the nature of the waves and their connection with electricity and magnetism4 ,that in itself was a wonderful achievement1 but far more wonderful still is the fact that he truly predicted the existence of longer waves than any then known1 which no one knew how to cause1 or how to detect if caused4 ,that prediction has since been fulfilled4 ,the long waves have been found2 we know how to make them and how to perceive their presence4 ,they are the messengers which carry our wireless messages4 ,the discovery of these1 at that time unknown waves1 on paper1 by simply calculating and reasoning about them1 is more marvellous even than the feat of ,adams and ,le ,verrier in discovering a planet on paper before anyone had seen it4 ,it established ,maxwell among the heroes of science for all time4 ,a magnet acts upon a piece of iron some distance away4 ,the pull must be transmitted through some kind of ether4 ,a current of electricity behaves in the same way1 acting precisely as a magnet1 with power to affect things at a distance4 ,again an ether is necessary4 ,a dynamo works by moving a magnet past a wire which it does not touch1 thereby generating current in it4 ,there again an ether is necessary to transmit the effect from the one to the other4 ,taking1 then1 the known magnetic effects of an electric current and the electrifying effects of magnets1 he was able to show that the same ether accounted for all1 and for the transmission of light as well1 that1 in fact1 there was but one ether which performed all these various duties4 ,he proved from the known facts about electricity and magnetism that waves such as he imagined would1 in fact1 move with the speed of light4 ,and once knowing the nature of the waves1 he asserted that in all probability there were others of which men had then no practical knowledge4 ,maxwell's theory soon set experimenters searching for the means of producing the long waves which he had predicted would be found4 ,several authorities had before then stated their belief that the current derived from a ,leyden jar was not simply a flow in one direction4 ,they suggested1 and gave grounds for the belief1 that the current surged to and fro for some time before it settled down2 that it swung to and fro1 indeed1 like a pendulum4 ,there may be some of my readers who are unacquainted with this interesting piece of electrical apparatus the ,leyden jar4 ,it is a convenient form of what is called an electrostatic condenser4 ,this is two conductors1 generally in the form of two plates with an insulator between them4 ,in the ,leyden jar the insulator is a glass jar1 while the plates are coatings of tinfoil1 one inside and the other outside4 ,on connecting one coating to one pole of a battery1 and the other to the other pole1 they become charged1 one positively and the other negatively4 ,one1 that is1 acquires an excess of electricity1 while the other becomes deficient to an exactly similar extent4 ,when the two are afterwards connected by a wire the surplus on one flashes through it to make good the deficiency on the other4 ,rushing first of all from positive coating to negative1 electrical inertia causes it to overshoot the mark and to recharge the jar with the charges reversed4 ,then current begins to flow back again1 doing the same several times over1 until at last equilibrium is established4 ,the power to absorb and hold a charge of electricity1 which is the characteristic of a condenser1 is called capacity4 ,what1 then1 is electrical inertia8 ,i have already referred to the effect which the creation of a magnetic field around a current has upon neighbouring conductors4 ,it also has an effect upon itself4 ,as soon as the current begins to flow it builds up the magnetic field1 and in the process some of its energy is exhausted4 ,on the original current ceasing1 however1 the magnetic field collapses back on to the conductor once more and in so doing restores that energy4 ,this occurs whenever current flows1 but it is specially noticeable in long conductors1 like submarine cables4 ,in them the battery has to act for a considerable time before any current reaches the farther end4 ,it is in the meantime employed in building up the magnetic field around the wire4 ,then when the battery has ceased to act the current still comes flowing out at the farther end--the magnetic field is giving back the energy expended upon it4 ,thus a current is reluctant to start flowing through a conductor1 and1 having started1 is disinclined to stop4 ,this is called inductance1 and it has exactly the same effect upon the current that inertia has upon a body4 ,what inertia is to a material body inductance is to an electric current4 ,and lastly1 the resistance which the conductor offers to the passage of the current is precisely analagous to the friction of the water in a pipe4 ,so1 we see1 the capacity of the two coatings of the jar and the inductance which occurs in the connecting wire cause the current to oscillate to and fro for a while when the jar is discharged1 which surging or oscillation is ultimately stopped by the resistance of the wire4 ,the two coatings and the wire form what is called an oscillatory circuit4 ,we can now resume our story4 ,after much experimenting ,hertz1 of ,carlsruhe1 discovered the fact that when a discharge was taking place in an oscillatory circuit tiny sparks passed between the ends of a curved wire held some distance away4 ,his apparatus is illustrated in ,figs4 #f and #g4 ,the former1 which is termed nowadays a ,hertz ,oscillator1 is simply two metal discs almost connected by a thick wire4 ,the wire is broken1 however1 at the centre1 and the two halves terminate in two metal balls4 ,each ball is connected to one terminal of an induction coil4 ,now the current comes from an induction coil in a series of spurts4 ,it is not an alternating current exactly 7since every alternate current is so feeble as to be negligible71 but is practically an intermittent current always in the same direction4 ,thus we may call one the positive end of the coil and the other the negative4 ,a short current comes along with every backward movement of the little vibrating arm which forms a part of the apparatus4 ,this breaking of the primary circuit may take place perhaps fifty times per second1 so that the intermittent secondary currents will succeed each other at intervals of a fiftieth of a second1 or even less4 ,the brain reels at the attempt to think of a fiftieth of a second1 but it is really quite a long interval as these things go1 and during that interval quite a lot happens4 ,for the current first of all charges the two plates as a condenser4 ,illustration3 ,,fig4 #f4--,the apparatus by which ,hertz made his discoveries1 hence called the ,hertz ,oscillator4 a a are metal plates2 d is the spark-gap between the two metal balls2 b is the battery1 and c the induction coil4 ,when they are as full as they will hold the current overflows1 as it were1 across the gap between the two balls4 ,now an air-gap--a gap that is filled with air1 between two conductors--is a very strong insulator4 ,but when current has once broken through it it becomes a fairly good conductor4 ,hence as soon as the first spark has passed between the two knobs the plates become connected almost as if a wire were passed from one to the other4 ,and there we have quite a good oscillatory circuit4 ,there is capacity at each end and a fairly long length of wire to provide the inductance4 ,consequently that breakdown of the insulation of the air in the spark-gap is followed by electrical oscillations which take place with inconceivable rapidity4 ,yet because of the resistance of the spark-gap1 which is considerable even after it has been broken through1 the oscillations do not continue for long4 ,they have died away long before the lapse of a fiftieth of a second1 when the next impulse comes along from the coil4 ,in the meantime the air-gap regains its insulating properties1 and so1 on the arrival of the next impulse1 the whole thing occurs once more4 ,thus a little train of oscillations is produced for every impulse from the coil4 ,every train causes a corresponding disturbance in the ether1 and sends off a train of electro-magnetic waves1 and these1 falling upon the distant wire1 generate in it a train similar to that which brought them into being4 ,these trains1 in ,hertz' simple apparatus1 manifested themselves in the form of minute sparks leaping across the small gap between the ends of the curved wire 7,fig4 #g74 ,illustration3 ,,fig4 #g4--;,hertz ,detector4 ,it was with this simple apparatus that ,hertz discovered how to detect the wireless waves4 ,it was in #ahhh that ,hertz made this discovery of a way to detect long electric waves4 ,he subjected the matter to many more experiments and found that the waves have many points in common with light rays4 ,he found that they were reflected from certain surfaces1 just as light is reflected from the surface of a mirror4 ,he made prisms which were able to bend them as light waves are bent by a prism of glass4 ,some things appeared to be transparent to them1 as clear glass is to light1 while others are opaque4 ,it does not follow that the same things which reflect light waves reflect electric waves1 and so on4 ,the latter can pass through a brick wall1 for example4 ,but the same divergence is to be observed between light and radiant heat1 of which the action of glass is a familiar example4 ,clear glass will let light through almost undimmed1 yet we use it for fire-screens to shield us from too much radiant heat4 ,the important fact is that all three--light1 radiant heat and ,hertzian waves--in addition to travelling at the same speed1 are reflected1 absorbed or refracted1 according to precisely the same principles4 ,this is almost perfect testimony to their essential identity4 ,the difference between them1 as has been said already1 is the distance from crest to crest of the waves--the wave-length1 that is4 ,and the reader will wonder by what manner of means this mysterious dimension can be ascertained4 ,in spite of its seeming mystery the method is very simple4 ,it is based upon the fact that two sets of similar waves travelling at the same speed in opposite directions interfere with one another in a peculiar way4 ,suppose that one set of waves travel along to a reflector and strike it vertically2 then another set will travel back from the reflector exactly similar to the first1 except that their direction will be opposite4 ,and the result will be that at certain intervals they will exactly neutralise each other1 so that at those points there will be no wave-action appreciable at all4 ,those points where no action is to be perceived are called nodes1 and they are exactly half a wave-length apart4 ,this will be quite easily understood from the accompanying diagrams4 ,in each of these diagrams the set of waves marked a are supposed to be moving from left to right1 while those denoted by b are reflected back and are moving from right to left4 ,it will be noticed that each wavy line has a straight line drawn through it1 dividing it into alternate crests and hollows1 which line is known as the axis of the waves4 ,now notice that in ,fig4 #h there are points marked x1 where the a waves are just as much above the axis as the b waves are below it1 and vice versa4 ,hence at those points the two sets of waves will neutralise each other4 ,now turn to the next figure1 which1 be it remembered1 shows the same waves a moment later1 when they have moved a little farther on in their respective journeys1 and it will be seen that there1 too1 are places marked x where the two sets of waves neutralise each other4 ,and the same with the third diagram4 ,and finally observe that the places marked x are always the same in all the diagrams--that is to say1 they are always the same distance from the line on the right-hand side1 which denotes the reflector4 ,it will be clear1 too1 that each node is half a wave-length from the next4 ,thus it can be shown that at every moment1 and not merely at the three indicated in the diagrams1 the two sets neutralise each other at the nodes1 that the nodes are always in the same places and half a wave-length apart4 ,illustration3 ,,figs4 #h1 #i and #aj4--,these diagrams help us to see how the wireless waves are measured4 ,the a waves are supposed to be moving from left to right and the b waves from right to left4 ,at the points marked x they neutralise each other4 ,it is then easy to discover those points and the distance apart of any two adjacent ones is half the wave-length4 ,n4,b4--,in ,fig4 #aj the b waves fall exactly on top of the a waves4 ,everywhere else1 except at the nodes1 there is action more or less energetic1 but there is perpetual calm4 ,but how can we tell where the nodes are8 ,when we recollect that they are points at which no wave-motion at all takes place it is easy to see that we shall at those points get no spark in our detector4 ,so what ,hertz did was to set his oscillator going so that it threw waves upon a reflecting surface and then move his detector to and fro in the neighbourhood until he found the nodes4 ,between the nodes1 as will be seen by an inspection of the curves once more1 there are other points at which the wave-action will be twice as great as with the single wave1 and so at those points the response of the detector would be especially energetic4 ,this mutual action between an incident wave and a reflected wave is termed interference1 and by it the wave-lengths of all the ethereal waves have been measured4 ,the plan used in the case of light waves1 although the same in principle1 is somewhat different because of the extreme shortness of the waves4 ,so the experiments of ,hertz not only showed that long electric waves existed1 but that they were in all essentials similar to light1 and their wave-lengths were ascertained4 ,on that basis has been built up modern wireless telegraphy4 ,it may be interesting to mention at this point a very curious1 and in a sense pathetic1 incident4 ,professor ,hughes1 whose name is associated with certain well-known instruments for ordinary telegraphy1 nine years before ,hertz' discovery noticed that a microphone was affected by the action of an induction coil some distance away4 ,he himself attached some importance to the matter1 but he allowed himself to be dissuaded from following up the discovery by other scientists1 more eminent than himself at the time1 who thought that it was not a promising field for investigation4 ,but for the influence of these friends he would possibly be the hero of this story in place of ,hertz4 ,professor ,silvanus ,thomson has said that he too noticed the sparks produced at a distance when a ,leyden jar was discharged1 but he makes no claim to precedence over ,hertz1 since1 seeing the phenomenon1 he did not perceive its real meaning1 while ,hertz1 though a little later in time1 realised the profound significance of it4 ,hertz himself in his account of his experiments is generous enough to assert that1 had he not discovered the waves when he did1 he is quite certain that ,sir ,oliver ,lodge would have done so4 ,before proceeding to describe the principal apparatus used in the wireless station ,i should like to devote a little space to the explanation of a term which will come up again and again1 and which represents that which is responsible1 in the main1 for the marvellous advances which the art of sending wireless messages has achieved in the last few years4 ,i refer to resonance4 ,it will be a great help if the reader will try for himself a simple1 inexpensive little experiment4 ,stretch a string horizontally across a room and on to it tie two other strings so that they hang down vertically a little distance apart4 ,to the ends of the two strings tie some small objects--a cotton reel on each will answer admirably4 ,they will thus form two pendulums1 and1 to commence with1 they should be just the same length4 ,having rigged all this up1 give one pendulum a good swing4 ,it will impart motion of a to-and-fro variety to the supporting string1 which in its turn will pass that motion on to the other pendulum4 ,in a very short time1 then1 the second pendulum will be vibrating like the first4 ,indeed the whole motion of the first will shortly become transferred to the second1 so that the second will be swinging and the first still4 ,then the second will re-transfer its energy back to the first1 and so they will go on until the original energy given to the first pendulum is exhausted4 ,the point to be observed is the quickness with which one pendulum responds to the impulses given it by the other1 and the ease with which the energy of the one passes to the other4 ,now reduce the length of one pendulum4 ,on setting the first in motion a certain irregular spasmodic action is to be observed in the second1 but it is very different from the whole-hearted response in the previous instance4 ,in the former case the second one responded naturally and readily to the first4 ,now its response is reluctant in the extreme4 ,it moves somewhat because it is forced to1 but it is apparently unwilling4 ,energy has to be impressed upon it4 ,there is no readiness1 because there is no sympathy between them4 ,that sympathy between the two equal pendulums is resonance4 ,the same occurs between two violin or piano strings when they are in tune4 ,the explanation is that a pendulum has a certain natural frequency which depends upon its length4 ,another pendulum of the same length1 arranged as just described1 therefore imparts impulses to it at just the frequency which is natural to it4 ,consequently the effect is a cumulative one1 and it responds quickly4 ,impulses at any other frequency tend more or less to neutralise each other4 ,in the same way a string1 of a certain length and a certain tension1 has a frequency peculiarly its own1 and it will respond to another similar string because the other gives its impulses at its own natural frequency4 ,it is on record that an engine in a factory happened to run at precisely the same speed as the natural frequency of the building1 with the result that after a little time the structure shook so much that it collapsed4 ,now electrical circuits in which currents oscillate have a natural frequency of their own4 ,that frequency depends upon the two electrical properties of the circuit3 capacity and inductance4 ,and if you want to set up an electrical oscillation in any circuit you can best do it by giving it impulses at intervals which agree with its natural frequency4 ,sir ,oliver ,lodge seems to have been the first to appreciate fully the effects of resonance in wireless telegraphy4 ,it is strange that in ,england the work of this eminent man in wireless matters is not more fully recognised4 ,when wireless telegraphy reached the point at which the public became interested1 ,marconi was just coming to the front and so1 for ever1 will his name be foremost in the public estimation4 ,indeed more than foremost1 for in the minds of many he monopolises the credit for this invention4 ,many people are under the impression that he is the one and only1 or at any rate the original1 inventor of wireless telegraphy4 ,now ,marconi has done exceedingly valuable work in this field4 ,moreover1 he has been the means of placing the affair on a good commercial footing4 ,but all the same he is by no means the original or only inventor4 ,while admitting that he is a remarkable man1 who has done wonders1 it is only common justice to refer to the others whose contributions to the solution of the problem are possibly of equal value4 ,and1 of these1 few can compare with ,sir ,oliver ,lodge4 ,but to return to the question of resonance4 ,at first the distances over which messages could be sent were but small4 ,now a marconigram can be flung across a hemisphere4 ,at first little could be done by day1 work had to be done mainly at night4 ,now communication passes by day and night alike4 ,yet in principle1 and in many details1 the instruments are unaltered from what they were several years ago4 ,the main source of all this improvement is the use of resonance4 ,to enumerate broadly the apparatus used for the dispatch and receipt of messages the following list will be useful3-- ,transmitting ,end 7#a7 ,an ,antenna1 consisting of a number of wires raised to a considerable height above the ground4 7#b7 ,a ,spark-gap1 consisting of a series of metal balls with gaps between them1 the outer ones being connected to the antenna and to the induction coil4 7#c7 ,a powerful ,induction ,coil with batteries or other source of current to work it4 7#d7 ,a ,telegraph ,key1 by which the induction coil can be started and stopped at will4 ,receiving ,end 7#a7 ,an ,antenna precisely similar to the other4 7#b7 ,a ,coherer or other oscillation detector4 7#c7 ,a ,receiving ,instrument which may be a writing telegraph instrument1 a telephone1 any of a number of ordinary telegraph instruments1 or a galvanometer4 ,transmitting and sending instruments are1 of course1 installed at both ends and either of them can be connected to the antenna at will by the simple movement of a switch4 ,the antenna plays the part of one of the metal plates in the ,hertz oscillator4 ,early experiments were made with ,hertz apparatus1 but the range of such a contrivance is very limited4 ,for one thing1 it neglects to take advantage of the earth4 ,it is little realised what an important part the earth plays in the carrying of wireless messages4 ,a very great step was taken when ,marconi dispensed with one of the plates of ,hertz1 and used the earth instead2 while the other plate gave place to the elevated wires1 the most familiar part of the apparatus to most people4 ,the condenser is thus formed by the earth as one plate1 the elevated wires as the other1 and the intervening air as the insulator4 ,the capacity must be exceedingly small in such an apparatus1 but it is sufficient2 while the long lines of electrical force stretching from the high antenna to the earth produce waves of great carrying power4 ,lastly1 when the earth forms a part of the condenser the waves cling to it1 so that instead of being largely dissipated into space1 they move along the surface of the earth4 ,the advantage of this is obvious4 ,at first it was customary to place the spark-gap in the wire leading from the antenna to the earth1 as in the accompanying sketch4 ,later1 however1 it was found better to place the coil and spark-gap in a local circuit in which the oscillations are first produced4 ,these oscillations pass through a coil which is interwound with another one connected to the antenna and to earth1 and thus the local oscillations1 as we might call them1 induce similar oscillations in the antenna1 just as the fluctuations in one part of an induction coil induce fluctuations in the other4 ,indeed the coil in the local circuit and the one in the antenna circuit actually constitute an induction coil4 ,the advantage of this is that by introducing condensers the capacity of which can be varied1 and coils the inductance of which can be varied1 into the oscillation circuit it becomes possible to tune the circuits effectively4 ,thus resonance comes into play and the power expended can be made to produce the maximum effect4 ,some attempts have been made to displace the induction coil in wireless telegraphy altogether by a specially made dynamo4 ,these machines can produce either alternating or continuous currents1 in fact the alternating current dynamo is really simpler than the more familiar continuous-current machine4 ,the difficulty is1 however1 to run it sufficiently fast to produce sufficiently rapid alternations4 ,nicola ,tesla made an alternator 7to give the alternating current dynamo its short title7 which could produce #aejj alternations per second1 while ,mr ,w4 ,duddell made one which produced #abj1jjj1 but neither was satisfactory for the work in question4 ,could such a machine be made1 it would be invaluable1 for it will be apparent that a continuous succession of waves would be formed by it and not a succession of short trains of waves such as is produced by the induction coil and spark-gap4 ,the difficulties are not electrical1 but mechanical4 ,it seems doubtful if a machine will ever be made to run with sufficient rapidity which would not knock itself to pieces in a very short time4 ,illustration3 ,,fig4 #aa4--,the simplest form of wireless antenna4 ,small alternators are used sometimes1 however1 to supply alternating current to the primary of an induction coil1 or transformer1 as it is more often called in its larger sizes4 ,the interrupter is only needed when the primary current is continuous--from batteries1 for example4 ,alternating current needs no interrupter1 and so that bother is removed4 ,the alternations of a hundred or so per second1 which are quite the common thing with alternators1 are just what is needed to excite an induction coil4 ,consequently small machines of this kind are to be found in many stations4 ,a ,danish inventor1 ,valdemar ,poulsen1 has adopted an altogether different method of producing electrical oscillations1 which method is the distinctive feature of his mode of telegraphy4 ,he takes advantage of a curious effect of passing current between two rods1 one of which is carbon1 so as to form an arc such as we see in arc lamps4 ,my readers are already familiar with the term shunt in connection with electrical matters1 and so will perceive at once what is meant when a second circuit is said to be arranged as a shunt to the arc4 ,the accompanying diagram will in any case make the matter clear4 ,the current comes along from the battery or continuous-current dynamo to a hollow rod of copper which1 to prevent it being melted1 has cold water continually circulating inside it4 ,thence the current jumps across to a carbon rod1 forming an arc between the two rods1 and returns whence it came4 ,in its journey it traverses the coils of an electro-magnet1 the poles of which are one each side of the arc4 ,this tends to blow the arc out1 as a puff of wind blows out a candle1 an effect which a magnet always has upon an electric arc4 ,the shunt consists of a wire leading from the copper to the carbon rod with a condenser and an inductance coil inserted in it4 ,the latter coil also forms one part of that coil by which the oscillations in the local circuit are transferred to the antenna4 ,the electrical explanation of what happens when the current is turned on to an arrangement like this is rather too complex to set out here4 ,it depends upon a curious behaviour of the arc4 ,it is really a conductor1 yet it does not behave as ordinary conductors do1 and the result is that the continuous current flowing through the arc is accompanied by an oscillating current in the shunt circuit4 ,and the important feature of the arrangement is that these oscillations are continuous1 in one long train1 not in a succession of trains4 ,the advantage of this has already been referred to4 ,one other feature of the apparatus just described should be mentioned1 since it will seem curious to the general reader4 ,for it to work properly it is necessary that the arc should be enclosed in a chamber filled with hydrogen or a hydro-carbon gas4 ,coal-gas is generally used4 ,hertz' original discovery was that small sparks could be seen to pass between the ends of a curved wire when the electric waves fell upon it4 ,such spark detectors1 as they are called1 are useful in the laboratory1 but not for practical telegraphy4 ,illustration3 ,,fig4 #ab4--;,diagram 7simplified7 showing how ,poulsen generates oscillations4 ,current from a dynamo flows through the arc1 whereupon currents oscillate through the condenser and coil 7as described in the text74 ,several people seem to have noticed in years gone by that a mass of loose metal filings1 normally a very bad conductor of electricity1 became a much better conductor when an electrical discharge of some sort occurred near by4 ,the demand for a wireless receiver had not then arisen1 however1 and so the discoveries were not followed up4 ,consequently it remained to be rediscovered by ,branly1 of ,paris1 in #ahij4 ,he placed some metal filings in a glass tube1 the ends of which he closed with metal plugs4 ,lying loosely together the filings would not conduct the current of a small battery from one plug to the other1 but when a spark occurred not far away they suddenly became conductive and allowed it to pass4 ,several years after this ,sir ,oliver ,lodge took up the idea as a receiver for wireless messages1 and believing that its action was due to the waves causing the filings to cling together1 he christened it ,coherer4 ,marconi succeeded in making a very delicate form of this1 although working on strictly the same lines4 ,the trouble with a coherer is that when once it becomes conductive it remains so unless the filings be shaken apart4 ,lodge therefore arranged for the tube to be continually struck by clockwork or by a mechanism like that of an electric bell4 ,marconi effected a further improvement by making the current passing through the coherer control the striking mechanism1 so that the latter is normally quiet but administers one or two taps at just the right moment4 ,sir ,oliver ,lodge and ,dr ,muirhead devised another detector which1 though quite different in form1 is really much the same in principle4 ,a steel disc with a sharp knife-like edge is made to rotate above a vessel of mercury4 ,the edge just touches the mercury but no more4 ,on the top of the mercury there floats a thin layer of oil1 a bad conductor4 ,now as the disc revolves it picks up on its edge a film of oil1 which it carries down into the mercury4 ,the film adheres so tightly that it prevents the moving disc from actually touching the liquid metal4 ,thus1 under normal conditions1 the two are electrically insulated from each other by the film of oil and no current can pass from mercury to disc4 ,oscillations1 however1 caused by incoming electric waves1 are able to break through the oil film and so bring disc and mercury into contact1 whereupon the current flows4 ,the constant movement of the disc restores the oil-film as soon as the oscillations cease4 ,the reason why these detectors act as they do is not quite understood4 ,one suggested explanation is that the oscillating currents heat the particles and so partially weld them together4 ,another is that adjacent particles become charged as the plates of a minute condenser1 and so are drawn tightly together as the plates in an electrostatic voltmeter are drawn towards each other4 ,supposing that the original non-conductivity of the loose filings be due to the film of air which may surround them1 either of these things would account for the film being broken or squeezed out1 resulting in better contact and improved conducting power4 ,but both suggestions seem to be contradicted by the fact that if the pieces in contact be of certain substances the coherer works the opposite way4 ,under those conditions the conductivity is normally good1 but the influence of the incoming waves causes it to become bad4 ,in #ahif ,professor ,rutherford1 now of ,manchester1 described some discoveries which he had made as to the magnetic effects of oscillations4 ,a simple little contrivance which he had constructed was operated by the discharge of a coil half-a-mile away1 at that time a great performance4 ,this detector was simply an electro-magnet with a steel core instead of the usual soft iron core4 ,the reason the latter is used in the ordinary magnet is that it loses its magnetism the moment the current ceases to pass through the coil with which it is surrounded1 while a steel core retains its magnetism4 ,for most purposes a steel core would render an electro-magnet useless1 but in this case it was desired that the core should be permanently magnetised4 ,so a current was first passed through the coil to magnetise the core1 and then the coil was connected to a simple form of antenna while a swinging magnet was brought near so that the magnetic power of the core would be indicated and any change made apparent4 ,the effect of the discharge half-a-mile away was to demagnetise the core slightly4 ,this was shown by the movement of the swinging magnet1 and so the first magnetic detector was found4 ,but here1 perhaps1 ,i ought to explain the use of the antenna at the receiving station--its function at the sending end has already been made clear4 ,the electro-magnetic waves1 coming from the distant transmitter1 strike the receiving antenna and in so doing set up in it oscillations such as those which set them in motion4 ,for every oscillation in the sending antenna there will be another1 similar in every respect except that it will be feebler1 in the receiving antenna4 ,and the oscillations are here led to the detector1 of whatever form it may be1 and in it they make their presence felt4 ,in some few cases a ,duddell thermo-galvanometer has been employed as the detector1 in which the oscillating currents report themselves directly4 ,in coherers the detector works by causing the oscillating currents to control a continuous current from a battery and it is the latter which actually gives the signal1 but there are a number of extremely interesting means which have been invented to detect the oscillating currents by their heating effect4 ,r4 ,a4 ,fessenden1 for instance1 has perfected one which is a marvel of delicate workmanship4 ,he depends upon the heating of a wire by the currents passing through it4 ,such heating is the result of the electrical force acting against resistance1 and the difficulty is that if the resistance be great it will almost entirely kill the faint oscillating forces in the receiving antenna1 while if1 on the other hand1 it be small1 the rise in temperature will be inappreciable4 ,so he encloses a fine thread of platinum in a glass bulb from which the air is exhausted4 ,the platinum wire is first of all embedded in a wire of silver3 the silver wire is given a core of platinum1 in fact4 ,then the compound wire is drawn down until it is so thin that the platinum core is only one and a half thousandths of an inch in diameter4 ,a short length of this compound wire is then bent into a ,u-shaped loop and its ends connected to thicker wires4 ,finally the bottom of the loop is immersed in nitric acid1 which eats away the silver at that point and leaves the bare platinum4 ,thus is produced a very short length 7a few millimetres7 of exceedingly thin platinum wire supported at its ends by comparatively thick wires4 ,being so short1 this wire does not offer much resistance1 and consequently does not materially check the oscillations4 ,at the same time1 since it is so fine1 it does offer some resistance1 and finally1 since what heat is generated will be in an exceedingly small space1 it will be appreciable there4 ,a telephone is arranged so that its current also passes through the fine wire1 and every slight variation in the temperature of the platinum wire1 by varying its resistance1 varies the current through the telephone4 ,and exceedingly slight variations can be detected by sound in the telephone4 ,thus the oscillations generated in the antenna affect the heat in the wire2 that affects its resistance2 and that again affects the telephone1 which1 finally1 affects the ear of anyone who is listening to it4 ,it must be understood1 however1 that this is not a wireless telephone1 for the sounds heard are not articulate but merely long and short sounds1 representing the dots and dashes of the ,morse ,code4 ,electrolysis provides us with another form of detector4 ,an exceedingly small platinum wire forms one electrode and a large lead plate the other1 and both are immersed in dilute acid4 ,the passage of current from a local battery sets up electrolysis1 and so stops itself by forming a film of oxygen on the small electrode4 ,this film1 however1 is broken by the oscillating currents from the antenna1 so that as long as they are coming the battery current can flow1 but as soon as they cease the battery current stops itself again4 ,thus the flowing and stopping of the oscillating currents is exactly copied by the current from the battery1 which current is led through a telephone or a sensitive galvanometer4 ,it may occur to readers to inquire why the oscillating currents are not passed direct to a galvanometer4 ,the answer is that because they are oscillating a very sensitive galvanometer is not possible4 ,true1 the ,duddell thermo-galvanometer has been mentioned in this connection1 but although it is a beautiful instrument it cannot compare for delicacy with the direct-current galvanometers4 ,the latter are easily a hundred thousand times more sensitive4 ,but the trouble can be overcome by rectifying the oscillating currents1 by passing them through a unidirectional conductor--one1 that is1 which passes current one way only4 ,these remind one of a turnstile as installed at certain public places1 which let you out but will not let you in unless you pay4 ,in fact they will not let you in at all4 ,in like manner rectifiers will only allow those currents to pass which are flowing in one direction1 and so they cut out every alternate oscillation1 thus producing something very like continuous current1 which can be detected by the very delicate galvanometers which are usable where continuous currents are concerned1 or more often by a telephone receiver4 ,the rectifying conductors are in many cases crystals1 hence these detectors are called ,crystal ,detectors4 ,carborundum is a favourite for this purpose4 ,and that brings us to the important question of the secrecy of wireless communication1 and the measures taken to prevent confusion from the number of independent messages flying through the air at the same time4 ,this can be largely achieved by the aid of resonance4 ,trains of waves flung out by one antenna may strike several other antennae1 but unless the latter are in tune with the sending apparatus they will probably not be affected appreciably4 ,let one of them1 however1 be in tune1 and it will pick up easily the message which is not noticed by the others4 ,it is as if three people watching a distant lamp were affected by a form of colour-blindness which rendered them practically blind to all colours except one4 ,suppose one could see red only1 the other blue and the third yellow4 ,a light sent through a blue glass being robbed of all rays except the blue ones would be visible only to the man who could see blue4 ,the man who could see blue would1 in like manner1 be quite blind to light sent through red or yellow glass4 ,each of them1 in fact1 could be signalled to quite independently of the others by simply sending him rays of the colour to which his eyes were sensitive4 ,in precisely the same way each wireless receiver is or can be made most sensitive to waves of a particular length and practically blind to all others4 ,the operator can adjust his apparatus for certain prearranged wave-lengths1 and so he can communicate with secrecy to stations whose wave-length he knows4 ,the change1 of course1 is made by altering the capacity1 or inductance1 or both4 ,the instruments can be so calibrated that it is quite easy to make the alteration4 ,then1 antennae can be so constructed that messages can be received with most readiness from one particular direction4 ,in others1 they can be received from any direction1 but the direction can be discovered4 ,this1 it will be easy to see1 is of great value to ships in a fog4 ,antennae made with a short vertical part and a long horizontal part radiate best in the direction away from which their horizontal part points4 ,this is of great advantage in stations which are built specially to communicate with other particular stations4 ,in such cases the antenna is carefully built1 so as to point in the required direction4 ,such antennae also receive more readily those signals which come from the direction away from which they are pointing4 ,reference has been made already to the interesting fact that wireless communication is easier at night than in the daytime4 ,that is probably because of the ionisation of the atmosphere by the action of sunlight4 ,along with the visible sunlight there comes to us from the sun a quantity of light known as ultra-violet1 since it makes its effect known in the spectrum of sunlight beyond the violet1 which is the limit of visibility at one end of the spectrum4 ,we cannot see it but it affects photographic plates powerfully4 ,it has energetic chemical powers1 and it has the ability to make the air more conductive than it is ordinarily4 ,comparatively little of it penetrates our atmosphere1 but it must exercise a good deal of influence a little higher up4 ,now readers will remember that the process by which electro-magnetic waves are propagated is checked when the waves strike a conductor4 ,the energy in the waves is then employed in causing currents in the conductor instead of forming more waves4 ,and so partially conductive air forms a partial barrier to the waves4 ,the effect is not appreciable in the case of the tiny waves of light and heat1 but it is in the case of the long wireless waves4 ,everyone has seen the waves of an advancing tide coming up a sandy beach1 and has noticed how the dry sand 7a good conductor of water7 sucks up and destroys the foremost ripples4 ,in like manner are the wireless waves sucked up by the partially conductive atmosphere4 ,but the effect of the ultra-violet light does not last long1 and so1 at night-time1 it disappears4 ,therefore messages can be sent better at night than by day4 ,for wireless telephony what is wanted is a continuous uninterrupted train of waves1 such as those from the ,poulsen arc1 and a receiver of the magnetic type4 ,the coherer is no good for this purpose1 since it either stops the current entirely or lets it flow copiously4 ,the magnetic detectors1 however1 respond to the variations in the strength of the incoming waves4 ,as the latter increase or decrease in strength so does the magnetic detector give out stronger or weaker signals4 ,so a telephone transmitter of the ordinary type is made to vary the strength of the oscillations at the sending end1 while an ordinary telephone receiver is placed in series with the detector at the receiving end4 ,thus every slight variation corresponding to sound waves spoken into the transmitter is reproduced in the receiver4 ,it is strange that wireless telephony has not made greater progress1 for it may be said1 on the word of one of the greatest authorities1 that wireless telephony is simpler and easier than telephony through a submarine cable4 ,in the latter there are almost insuperable obstacles caused by the capacity and inductance of the circuit1 while in the wireless method there is very little difficulty4 ,there are1 of course1 several so-called systems of wireless telegraphy in use4 ,there is the ,marconi in ,great ,britain2 the secret ,admiralty system in the ,british ,navy2 the ,de ,forest in the ,united ,states2 the ,telefunken in ,germany1 not to mention the promising ,poulsen system4 ,and there are still others4 ,but it would be futile to attempt to explain how they differ from one another in a work like this4 ,in principle they are alike4 ,the precise forms of instrument used may vary1 but even there there is much in common between them4 ,as time goes on there will inevitably be a tendency to more and more uniformity4 ,that is always the case1 for some things are inherently better than others1 and rival systems1 although each is working along its own lines1 always come to very much the same result in the end4 ,without making any comparisons1 it is safe to say that if the ,telefunken system1 for example1 has any points of superiority over the ,marconi1 the latter will sooner or later find out the fact1 and will modify their apparatus accordingly4 ,in all probability this will operate both ways1 and some things which the ,german system is now using will give place to those which the ,british have in operation4 ,in another very modern industry this is very apparent4 ,having attended and carefully studied several annual exhibitions of flying machines1 ,i have noticed with great interest how the varying types of a few years ago are merging into the more or less uniform types of to-day4 ,and it has been the same with wireless telegraphy1 and will be still more so in the future4 ,the best means of generating the waves and the best means of detecting them at a distance--that is the whole problem1 and all the workers in it will sooner or later come to much the same conclusions as to which are the best ways4 ,patents may do a little to delay this1 but not much4 ,for one thing1 patents only last a few years4 ,for another1 a patent only covers a particular way of doing a particular thing4 ,a machine that is termed patent is often the subject of a hundred patents1 each covering a particular little point4 ,it is well-nigh impossible to patent a whole machine4 ,a general principle cannot be patented1 only a particular application of that principle1 and so there are in a great many cases little variations of a patented method which are quite as good as the patented one1 and which can be used freely4 ,so even patents will not have much effect1 in all probability1 upon this unification process4 ,but1 however that may be1 there is no doubt that the whole world owes a deep debt of gratitude to the men who have worked out this most beneficent of inventions4 ,it is difficult to think of a single one which has ever brought such a load of benefits to poor1 struggling humanity as this has4 ,the ship in distress1 the lighthouse man on his lonely islet1 the explorer in the ,polar regions1 the pioneer settler in the new lands--in fact1 just those who most need some connecting link with their fellows--are the people to whom the wireless telegraph brings aid and comfort4 ,all honour to the men who have done it4 ,,chapter ,,xiii ,,how ,,pictures ,,can ,,be ,,sent ,,by ,,wire ,the sending of a message by telegraph is easily understandable4 ,various combinations of two simple signs1 such as short sounds and long sounds1 can readily be made to indicate letters by which the words can be spelt out4 ,nor does the sending of sound over a wire make a very great demand upon the credulity4 ,we all know that sound consists of innumerable little waves in the air1 and by the simplest of devices these can be converted into variations in an electric current1 which variations1 by means equally simple1 can be made to re-convert themselves back into sound waves at the other end4 ,but to transmit a picture is another matter altogether4 ,it seems barely possible in the case of a drawing such as a pen-and-ink sketch1 which consists of a comparatively small number of definite lines2 but with a shaded sketch or a photograph1 with its gradations of light and shadow--to transmit such would seem to be beyond the bounds of possibility1 did we not know that it has been done4 ,the description of the methods will therefore constitute a not uninteresting subject for a chapter4 ,it is worthy of remark that an attempt along these lines was made many years ago by a man named ,caselli1 and a description of this pioneer apparatus will form a good introduction to the later developments4 ,in ,fig4 #ac we see a square which represents a sheet of tinfoil1 upon which is drawn1 in non-conductive ink1 a simple geometrical figure4 ,the ink may be grease1 or shellac varnish1 indeed there are many substances which are available for use as an insulating ink4 ,across the square there are a number of parallel dotted lines1 but these1 it must be understood1 are not actually drawn upon the foil--their purpose will be apparent in a moment4 ,suppose that we connect the foil to one pole of a battery1 and the other pole by a flexible wire to a metal pen or stylus4 ,if we place the point of the pen in contact with the foil1 we make a complete circuit1 through which1 of course1 current will flow4 ,but if1 with it1 we touch one of the non-conductive lines1 there will be no current4 ,illustration3 ,,fig4 #ac ,illustration3 ,,fig4 #ad ,taking a ruler1 then1 let us draw the point of the stylus across the foil in a series of parallel straight lines4 ,it is these excursions of the stylus which the dotted lines are intended to represent4 ,for nearly the whole of the time current will be flowing2 but whenever the stylus is crossing one of the lines of non-conductive ink there will be a momentary cessation4 ,thus1 the reader will begin to perceive1 we obtain what we may call an electrical representation of the figure drawn upon the foil4 ,and now let us turn to ,fig4 #ad4 ,there1 too1 is a square1 but in this case it is not foil1 but paper which has been soaked in prussiate of potash4 ,the reason for introducing this chemical is that it is susceptible to electrical action4 ,wherever current passes through it1 it becomes changed into ,prussian blue1 so that if we place the point of a pen upon the paper1 and cause current to flow out of that point through the paper1 there we get a blue dot4 ,if1 while the current is flowing1 we draw the pen along1 we get a blue line4 ,fig4 #ac therefore represents in principle the sending apparatus of ,caselli's writing telegraph1 while ,fig4 #ad represents the receiving instrument4 ,the two pens are connected together by the main wire1 in such a manner that1 when the point of the one is in contact with the bare foil current flows out of the other and into the paper2 but as the former crosses an ink line all current ceases4 ,if1 then1 while the sending pen is drawn line by line across the foil1 the other is drawn at the same speed1 line by line1 across the chemically prepared paper1 we shall get on the latter a series of lines as shown in ,fig4 #ad almost continuous1 but broken here and there4 ,each breakage represents a passage of the sending pen across a line1 and taken together1 as will be seen1 they constitute a reproduction of the geometrical figure drawn upon the foil4 ,as shown1 the lines are rather far apart1 and so the reproduction is not a very good one4 ,they are only drawn so1 however1 in order that the principle may be shown the more clearly4 ,they may be drawn so that they overlap1 and then the effect is very much better1 the received picture being almost an exact reproduction of the other4 ,it will be noticed that an essential to the success of this method is that the two pens should move in perfect unison1 and that was the great difficulty4 ,caselli used an arrangement of pendulums1 the best thing available at the time4 ,the reproduction is1 in photographic language1 a negative1 a somewhat unsatisfactory feature of the method4 ,a simple modification1 however1 of the electrical connections will reverse that1 so that the reproduction shall be a positive4 ,there are two ways of cutting off a current from any particular circuit4 ,one is to interpose a resistance1 through which current cannot pass in an appreciable quantity1 and the other is to provide a second path for the current so much easier than the first that practically all the current will pass that way1 leaving the first circuit1 to all intents and purposes1 free4 ,it is as if a farmer wished to stop people passing across a certain field4 ,two methods would be open to him3 one to put up a high gate over which no one would dare to climb1 and the other to provide a short cut so much more pleasant and convenient than the old path that no one having the choice of the two ways would think of going the old way4 ,what the farmer would call a short cut the electrician calls a short circuit1 and a short circuit is often a more convenient way of cutting off a current than a switch which interposes resistance4 ,at all events1 in a case like this1 a short circuit enables that to be accomplished which would be very difficult by any other means4 ,in the apparatus as already described the battery had to drive the current along a long wire1 terminating at the distant receiving instrument1 whence the current returned via the earth4 ,the foil and pen1 acting as a kind of electrical tap1 controlled this4 ,when foil and pen touched1 the tap was open and current flowed4 ,when the line of non-conductive ink interposed itself1 the tap was off and the flow ceased4 ,but connect the battery directly to the wire1 and place the foil and pen in a short branch circuit1 and the whole thing is reversed4 ,then the opening of the tap sent current to the other end2 now the opening of the tap causes it to flow round the short branch and leave the main wire4 ,then the closing of the tap stopped the current reaching the farther end2 now it causes it to do so4 ,in fact1 the entire action of the apparatus is completely reversed1 and the bare parts of the foil become represented by blank paper1 while the insulating lines produce the marks4 ,in short1 a positive results instead of a negative4 ,such was the scheme of ,caselli years ago4 ,it is mentioned here at some length1 since the principle of it is largely re-used in an improved form in the most successful of modern apparatus for a like purpose4 ,it undoubtedly was a very excellent scheme1 simple and effective1 which ought to have succeeded2 but it did not do so1 for the sufficient reason that at that time knowledge of electricity and skill in constructing delicate mechanism were not so highly developed as they are to-day4 ,for success1 as has already been said1 one thing was essential1 and that thing very difficult to obtain--a perfect synchronism between one stylus and the other4 ,if the one were but the slightest degree out of step with the other1 failure followed inevitably4 ,so the electrical transmission of sketches dropped for the time being4 ,more recently a perfectly successful solution of the problem has come in another way altogether4 ,this apparatus1 at first called the telautograph1 but now known as the telewriter1 it will be more convenient to refer to later4 ,of modern systems for the transmission of pictures the most successful1 probably1 are the ,korn telautograph and the ,thorn-,baker telectrograph4 ,both of these are able to transmit very fair reproductions of photographs besides line drawings4 ,the difficulty with photographs is1 of course1 that many parts of them are not of equal blackness or whiteness1 but shade off gradually from one into the other4 ,take the case of a simple portrait4 ,part of the subject's face will be pure white1 while the side in shadow will be comparatively dark4 ,there is no hard and fast line between the two1 but by a gradation through an infinite number of shades the one tones into the other4 ,how can it be possible to convey that1 more or less mechanically1 over a wire8 ,the solution is due to the fact that the eye will blend together a number of distinctly different shades1 if properly arranged1 into a gradual change4 ,really the change is step by step1 but the effect is apparently quite continuous4 ,this can be seen in the half-tone illustrations in this book4 ,close examination will show that such a picture is cut up into small squares4 ,in the pure white part the squares are invisible1 while in the perfectly black parts1 if there be any1 they are so merged into one another as to be inseparable4 ,but everywhere else in the picture it will be seen that there are squares each with a dot in the middle4 ,in the darker parts the dots are large2 in the lighter ones they are small4 ,we get the effect almost of colour1 although the picture is done entirely in black ink4 ,the eye does not see the individual dots when we are just looking at the picture2 we have to examine it very closely to find them4 ,yet they are there all the time1 and it is simply the peculiar action of the eye which sees beautiful half-tones1 shading imperceptibly one into another1 whereas in real fact there are only a vast number of equidistant dots1 all equally black4 ,we see1 therefore1 that it is possible to split up a picture of any kind into a number of small squares and to treat each square as being of equal darkness throughout4 ,then1 if we can communicate by wire that particular degree of darkness to a distant station1 where the small parts can be put together in their proper order and given their correct shade1 the picture as constructed at the receiving end will be something like that at the sending end4 ,and we have only to make the size of each separate square small enough to obtain a copy which will resemble the original very closely indeed4 ,in the early days it was actually proposed to telegraph pictures by ordinary telegraphy1 using this principle4 ,the suggestion was to agree upon a code of twenty-six shades1 each called by a letter of the alphabet4 ,one shade was to be a1 the next b1 and so on4 ,then the picture was to be divided up into squares1 and the particular shade of each square telegraphed by means of the corresponding letter4 ,the shades thus communicated were to be put together at the receiving end1 on a prearranged system1 and so the picture was to be built up4 ,given plenty of time1 that scheme might be moderately successful1 but to get a really good reproduction the subdivision needs to be so minute1 and the number of squares1 therefore1 so immense1 that it would be quicker to send the picture by train than to telegraph it by such laborious means4 ,in a fairly coarse half-tone block the squares are1 say1 #bejj to the square inch4 ,that number of letters would therefore have to be telegraphed for every square inch of picture transmitted1 to say nothing of the difficulty of building up a picture of such a great number of parts and giving to each the desired shade4 ,that idea1 abortive though it is in its crude form1 illustrates very clearly the fundamental principle on which this work is done4 ,the problem is really to devise a machine which will do that same thing rapidly and automatically divide up the original into a large number of squares1 and then send an electric current to represent each square1 such current by its strength to indicate the shade of the square3 and finally a similar instrument is needed to act as receiver1 and to reproduce those squares in the proper order1 giving to each its correct shade4 ,in practically all of them the mechanism is rotatory1 the original being placed upon a drum which turns round under a stylus1 or its equivalent1 while the stylus gradually travels along from end to end after the manner of the needle of a phonograph1 or else the same result being achieved by the drum itself having an endwise movement as well as a rotative one4 ,the receiving instrument is of similar form1 and both must start together1 move at the same speed and indeed preserve a perfect correspondence with each other4 ,if the distance be great between the two there may be difficulties due to the retardation of the currents passing between them4 ,electricity does not pass through long wires1 particularly if they be under the sea1 with anything like the quickness which we are apt to think4 ,over a short line and under favourable circumstances the receipt of a telegraph signal at the farther end is practically instantaneous1 but on long lines1 and under certain conditions1 that is far from being the case4 ,then something has to be done to quicken the action of the current1 or else the receiving drum must be made to lag behind the sending drum by the requisite amount4 ,in some cases1 too1 the transmitting apparatus loses a little time in sending off the currents1 and that1 too1 has to be allowed for1 so that1 all things considered1 the reader will see that the successful solution of this problem is hedged about with many subtle difficulties which are probably only appreciated by those who are well acquainted by sad experience with the little vagaries of both electricity and mechanical devices4 ,neither of them does quite what we want it to do2 each suffers from little faults1 which in the case of a delicate problem like this1 where a difference of a hundredth of a second would be fatal to success1 introduce difficulties almost insuperable4 ,to transmit line drawings1 ,professor ,korn uses a sending instrument very like that of ,caselli4 ,the picture is placed1 either by hand or photographically1 upon a sheet of copper foil1 which is fixed round the rotating cylinder1 the lines being formed of non-conducting material4 ,the foil being electrified and the stylus connected to the line or main wire1 currents pass to the farther end just as in the old apparatus4 ,at the receiving end the drum is covered with photographic paper and enclosed in a light-tight box4 ,through a hole in this box a fine pencil of light passes from a lamp1 suitable lenses being used to ensure that the pencil shall have1 as it were1 a very fine point1 producing a very small spot of light upon the paper4 ,if the light remains quite steady1 the drum meanwhile rotating1 a line will be drawn by it upon the paper which will be visible when the latter is developed4 ,since the drum not only turns upon its axis1 but also moves endwise one hundredth of an inch at every revolution1 this line will be a spiral1 the turns of which will be one hundredth of an inch apart4 ,thus the paper will be blacked1 practically uniformly1 all over4 ,should the intensity of the light vary1 however1 the line will at times be lighter than at others1 while1 should it be cut off altogether for a moment1 then there will be a corresponding gap in the line1 and it is easy to see that if these lighter parts or gaps occur in the correct places they will form a picture4 ,in other words1 by controlling that light we can build up a picture upon the paper4 ,the question is how to control it4 ,professor ,korn uses a form of the ,einthoven galvanometer already described4 ,instead of the silvered fibre generally employed in this instrument1 a silver wire is fitted1 the movement of which partly or entirely cuts off the pencil of light4 ,the ,korn transmitter for photographs is quite different1 although the receiver is practically the same as what has just been described4 ,the basis of it is a peculiar power possessed by the metal selenium when in a certain state4 ,this1 like all metals1 is a conductor of electricity1 but of course offers resistance in some degree4 ,now the special feature of selenium is that its resistance is reduced if light shine upon it4 ,suppose1 then1 that current be flowing through a mass of selenium and that the latter be suddenly illuminated brightly1 the resistance will at once fall and the current increase4 ,on the other hand1 should the light falling upon the selenium diminish1 its resistance will increase and the current flowing through it will decrease4 ,in short1 given a suitable arrangement1 the current flowing in a circuit of which a selenium cell forms a part will increase or decrease with the increase or decrease in the light falling upon the cell4 ,a while ago the papers were telling striking stories of a way by which blind people1 so it was said1 were to be recompensed for the loss of their sight--a new sense1 as it were1 was to be given them by which they could hear light1 even if they could not see it4 ,all this had reference to this curious property of selenium1 it being1 of course1 an undoubted fact that it will vary an electric current in accordance with the variations in the light1 and if that current be led through a telephone receiver a man1 by holding that to his ear1 could1 in a sense1 hear the variations in the light4 ,illustration3 ,,the ,,telewriter ,this remarkable instrument transmits actual writing and drawings1 the receiving pen copying precisely the movements of the sending pen ,in the ,korn transmitter for photographs selenium is employed as follows3--,a transparent photograph is made1 on a celluloid or gelatine film1 and this is fixed upon a glass cylinder mounted as already described4 ,a pencil of light falls upon this in much the same way as in the case of the receiver just described1 and1 as the cylinder revolves1 describes a fine spiral line all round and round it4 ,moreover1 the light passes right through the photograph and falls upon a mirror inside1 off which it is reflected on to a selenium cell4 ,at every moment1 then1 the light is falling upon some small part of the photograph1 and the amount of it which gets through and ultimately reaches the selenium depends upon the density of that part4 ,current1 meanwhile1 is flowing from a battery through the selenium1 and thence over the main wire to the distant station4 ,as the light pencil traces its spiral path over the rolled up photograph every variation in the density of the picture is reproduced as a variation in the current through the selenium4 ,this1 at the remote end1 operates the ,einthoven galvanometer1 the movements of which vary the shade of the spiral line being drawn upon the photographic paper4 ,this process takes place with remarkable celerity1 so that in a few minutes the innumerable variations constituting a complete photograph can be transmitted and faithfully recorded at the distant end of the wire4 ,but perhaps the most successful of these methods is that known as the telectrograph4 ,it is surprisingly like the scheme of ,caselli in principle1 and forms another example of the fact that good ideas often fail through lack of the proper means to carry them out4 ,mr ,thorne-,baker1 the inventor of the telectrograph1 has had at his disposal accumulated stores of knowledge and skill which did not exist in ,caselli's time4 ,consequently the former has made a brilliant success where his predecessor produced only an interesting but somewhat ineffective attempt4 ,reference has been made already to the half-tone blocks wherein a host of small dots of varying sizes make up a picture4 ,now instead of parallel rows of dots parallel lines of varying thickness will give very much the same result4 ,the former are made by photographing the picture through a sheet of glass ruled with two sets of lines at right angles to each other4 ,the latter can be made by using a screen with lines one way only instead of two ways4 ,it is therefore quite easy for a blockmaker to produce a process block wherein lines are used instead of dots4 ,for this particular purpose1 however1 it is not an ordinary block that is needed1 although it is in essentials very similar4 ,the picture to be transmitted is photographed through a screen as if a half-tone block were to be made4 ,the negative so obtained is then printed by the gum process on to a sheet of soft lead and1 after washing1 the picture remains upon the lead in the form of lines of insoluble gum on a background of bare lead4 ,a squeeze in a press drives the gum into the lead1 and so gives the whole sheet a smooth surface over which a stylus will ride easily1 but which is1 nevertheless1 made up of conductive parts and non-conductive parts1 the latter forming the picture4 ,the lead sheet is then put upon a revolving cylinder and turned under a moving stylus in the manner with which we are now familiar4 ,the sheet is placed with the lines lengthwise of the cylinder so that current passes to the stylus except as it passes over the breadth of the lines1 and so similar lines are built up at the distant end4 ,the receiving mechanism is of the electro-chemical type which ,caselli used4 ,the current passes from the receiving stylus to the paper1 and there makes its mark in a way that will be understood from the description of the earlier apparatus4 ,the supreme advantage of this method of working1 over that of ,professor ,korn1 is that the operator can see what he is doing4 ,to obtain good results1 a number of electrical adjustments have to be made1 and whether he has got them right or wrong can be seen as soon as the picture begins to grow upon the receiving paper4 ,if a little readjustment be needed the operator sees it and can set things right before the really important part of the picture begins to appear1 whereas with the ,korn apparatus he does not know what is happening at all1 since he can see nothing until the picture is finished and the photographic paper has been developed4 ,it will be apparent1 too1 to anyone who has carefully considered the wireless telegraphy chapters1 that it ought to be possible to make the sending stylus or its equivalent control a wireless transmitter and a wireless receiver to operate the receiving stylus1 so as to be able to send pictures by wireless4 ,experiments to this end have been made with some measure of success1 and sooner or later we are almost sure to hear that the difficulties1 which are by no means small1 have been overcome4 ,but we cannot conclude this chapter without a fuller reference to that marvellous invention1 the telewriter4 ,in this a man makes a sketch with a pen on a piece of paper1 or maybe he writes a message1 and simultaneously a pen1 hundreds of miles away if need be1 does precisely the same thing4 ,the receiving instrument draws the sketch line by line1 or it transcribes the message in the actual handwriting of the sender4 ,a little touch1 almost weird in its naturalness1 is that every now and then the receiving pen leaves the paper and dips itself into a bottle of ink1 after which it resumes its work at the very spot where it left off4 ,now how the complicated lines and curves1 the strokes and dots which make up a written language1 even the little shakes and defects which give each man's writing a personality of its own1 how all these can be sent over a wire is at first sight very difficult to understand4 ,the inventor of this apparatus has discovered an extremely simple way of doing it4 ,but even he does not attempt to do it with one wire1 it should be said1 for he uses two4 ,this is no drawback when1 as is often the case1 it is used in conjunction with a telephone1 for the latter1 to be effective1 also requires two wires4 ,years ago single wires were employed for telephones as for telegraphs1 the circuit being completed through the earth4 ,but the difficulty arose that every wire through which currents flow is apt to induce currents in neighbouring wires--the induction coil is based upon that fact--and so messages in one wire were overheard on others1 or1 what was perhaps more annoying still1 the dots and dashes passing in a telegraph wire would produce loud noises in a telephone wire that happened to be near4 ,the use of two wires1 however1 entirely removes that trouble1 for the neighbouring current then induces two currents instead of one1 one in each1 and it so happens that these are opposed to each other1 so that they neutralise each other4 ,so every telephone wire now is double and therefore is ready1 as it were1 to have the telewriter fitted to it4 ,but even with two wires the difficulty seems insuperable until we remember that the most complex of curves can be resolved into two simple movements4 ,the sending pen1 with which the original writing or drawing is done1 is attached to the junction of two light rods4 ,the farther end of each rod is attached to the end of a light crank fixed so that it can rotate or oscillate1 after the manner of cranks1 in the plane of the desk upon which the paper lies4 ,all the joints mentioned are of the hinge nature1 so that as the pen is moved about the rods turn1 more or less1 one way or the other1 the two cranks4 ,this simple mechanism1 it will be observed1 carries out very effectively the principle just mentioned1 for it resolves the motion of the pen1 no matter how complicated it may be1 into a simple rotating motion of the two cranks4 ,so the cranks turn this way or that as the draughtsman makes his picture1 and it is very easy to arrange that their movement shall vary the strength of two electric currents1 whereby we obtain electric currents varying in accordance with the movement of the cranks4 ,this is done by making each crank operate a variable resistance or rheostat4 ,when in its extreme position on one side the crank permits current to flow freely1 but as it moves over to the other extreme position the resistance in the path of the current is increased4 ,such an arrangement is a common feature in electrical apparatus4 ,so current from a battery flows to the two wires leading to the distant station1 each passing through the rheostat connected to one of the cranks4 ,we may think of the rheostats as taps which can be turned on or off by the action of the cranks4 ,let us imagine that crank a is in the position when the current flows freely--when the electrical tap is fully open2 then a strong current will flow along wire a1 returning to the sending battery via the earth4 ,as that crank is moved the current will gradually be reduced1 until1 if it be moved right over to the other extreme1 the current will be at its feeblest4 ,illustration3 ,,fig4 #ae4--;,a ,message received by ,telewriter4 ,arrived at the other end1 this current passes to a device which we may describe simply as a magnet so arranged that its action pulls round a crank against the restraining action of a spring4 ,now the stronger the current the more does that magnet pull and the farther does the receiving crank turn4 ,the sending crank varies the resistance1 the resistance varies the current1 the current varies the strength of the receiving magnet1 and the magnet varies the position of the receiving crank4 ,properly adjusted1 then1 the motion of the crank at the one end is communicated through that long chain of causes and effects1 until at last it is repeated exactly by the movement of the crank at the other end4 ,the same thing occurs simultaneously over each of the two wires1 crank a at the sending end communicating over wire a to crank a at the other end1 while crank b communicates its motion over wire b to the other crank b4 ,each sending crank is closely imitated in its every action by the corresponding one at the distant station4 ,the two receiving cranks are connected by light rods to the receiving pen in precisely the same way that the sending pen is connected4 ,consequently1 not only are the separate movements of the two cranks repeated at the remote station but the complex movements of the sending pen1 which gave rise to the actions of the cranks1 are also conveyed to1 and repeated by1 the recording pen4 ,the movements of the first pen are resolved into rotating motions by the two cranks1 these are transferred to the other cranks1 and their movements are in turn converted back into the written curves4 ,thus as the pen in the artist's hand draws his sketch1 so does the automatic hand at the other place1 it may be at a great distance1 repeat faithfully his work1 and the sketch grows line by line simultaneously at both ends4 ,there is not space here to detail how1 by another current superposed upon those referred to already1 the receiving-pen is made to dip itself periodically into the inkwell at the will of the sender4 ,by a cunning use of alternating current this is done without in any way interfering with the action of the cranks as described above4 ,but of course there is a severe limitation to the usefulness of this machine1 inasmuch as the drawing has to be made at the time of transmission1 and it can only be put on the wire by the hand of the artist himself4 ,,chapter ,,xiv ,a ,,wonderful ,,example ,,of ,,science ,,and ,,skill ,in the preceding chapter reference was made to the fact that for the successful sending of pictures by wire one thing was necessary above all others4 ,that one thing consists in making two machines1 perhaps hundreds of miles apart1 start working together1 stop together and1 when working1 turn at exactly the same speed4 ,let the reader just picture the problem to himself1 and ask himself how such an arrangement can be possible4 ,let him think of a town two hundred miles away and then meditate on the possibility of making a machine working in his own room and another in that distant town maintain perfect unanimity in their movements4 ,the result of such reflection will probably be the assertion that such a thing is beyond the bounds of possibility4 ,then he will find the following description of how it is done extremely interesting4 ,in the first place it must be understood that each machine is driven by an electric motor4 ,the motors are designed to run at #cjjj revolutions per minute1 and they drive the cylinders of the machines through gearing so arranged that the latter turn at #ej revolutions per minute4 ,now of all machines perhaps the most docile and easily managed is the direct-current electric motor4 ,each such machine is made with a view to its working at a certain speed1 but that can be varied within certain limits1 by simply varying the force of the current which drives it4 ,and that force can be very easily varied by the use of an instrument called a rheostat or variable resistance4 ,we are all familiar with the way in which the engine-driver regulates the speed of a locomotive1 by means of a valve in the steam-pipe4 ,the opening and closing1 more or less1 of the valve enables the speed to be changed at will and adjusted to a nicety4 ,the rheostat is to the electric current what the valve is to the steam2 it can be opened and closed1 more or less1 as necessary4 ,by it the current driving the motor can be made stronger or weaker1 and as that change is made so does the speed of the motor change accordingly4 ,thus we see that there is at hand the means of setting a motor to work at any desired speed4 ,the difficulty1 however1 is to tell when the desired speed has been attained4 ,one can count the revolutions of a machine at two or three revolutions per minute with a certain amount of accuracy1 but fifty revolutions per minute are more than one could count correctly4 ,still less could we count the #cjjj revolutions every minute of the motors4 ,thus1 even if we had the two motors side by side1 we should have extreme difficulty in making them work at the same speed exactly4 ,one might be doing #cjjj while the other did #biij or #cjaj and we should be none the wiser4 ,and when we separate the two by a distance of many miles1 the task of synchronising them is even worse4 ,but fortunately there is a simple contrivance by which we can tell very accurately the speed of a motor4 ,the reader has already been familiarised1 in previous chapters1 with the difference between direct or continuous electric currents and alternating ones4 ,it is the continuous sort which is used to drive these motors1 but a slight addition to the machine will make it so that while direct current is put in1 to drive it1 alternating current can be drawn out of it4 ,two little insulated metal rings are fitted on to the spindle of the machine1 and these are connected in certain ways to the wires of the motor2 then against these rings1 as they turn1 there rub two little metal arms1 called1 because of their sweeping action1 brushes2 and from these brushes we can draw the alternating current4 ,for our present purpose the importance of this lies in the fact that the rate at which that current will alternate depends upon the speed of the motor4 ,as the motor increases or decreases in speed1 so will the rate of alternation increase or decrease4 ,so that if we can measure the rate at which the current drawn from the motor is alternating1 we shall know from that the rate at which the machine is working4 ,this we can do by the aid of a frequency meter4 ,the working of this is based upon the acting of a tuning-fork4 ,everyone knows that a given tuning-fork always gives out the same note4 ,the note depends upon the rate at which the fork vibrates1 and the reason that one fork always gives the same note is because it always vibrates at the same rate4 ,that rate1 in turn1 depends upon its length4 ,if one were to file a little off the end of a tuning-fork1 its note would be raised1 because its rate of vibration would become faster4 ,similarly1 lengthening the fork would result in a lower note being given4 ,thus1 a tuning-fork1 or any bar of steel held by one end1 and free to vibrate at the other1 gives us a standard of speed which is very reliable4 ,and it so happens that we can easily use a set of such forks to test the rate of alternation of an alternating current4 ,generally speaking1 alternating current is no use for energising a magnet4 ,the chief reason for that is that the current tends to get choked up1 as it were1 in the coil4 ,alternating current traverses a coil very reluctantly indeed4 ,it is1 however1 possible to make an electric magnet of special design which will work sufficiently well with alternating current to answer our present purpose4 ,and it will be clear that just as the alternating current itself consists of a series of short currents1 so the force of the magnet will be intermittent2 it will give not a steady pull1 as is usually the case with magnets1 but a succession of little tugs4 ,there will1 in fact1 be one tug for every alternation of the current4 ,a simple form of motor fitted up as just described1 and rotating at #cjjj revolutions per minute1 would give out #ajj alternations per second4 ,if1 then1 such current were employed to energise a magnet1 that magnet would give #ajj tugs per second4 ,so a small steel bar of the right length to give #ajj vibrations per second can be fixed with its free end nearly touching such a magnet1 and when the current is turned on it will very soon be vibrating vigorously4 ,for the tugs of the magnet will agree with the natural rate of vibration of the bar4 ,and just as the two pendulums described in ,chapter ,,xii4 responded readily to each other1 so the bar responds readily to the pulls of the magnet4 ,but increase or decrease the rate of alternation ever so slightly1 and that sympathy between magnet and bar is destroyed4 ,the bar will not then respond4 ,it will only answer when the pulls of the magnet and the natural rate of vibration of the bar exactly correspond4 ,so it is usual to place five or six such bars with their ends near the one magnet4 ,the lengths of the bars vary slightly1 so that the rates of vibration are1 say1 #ih1 #ii1 #ajj1 #aja1 #ajb respectively4 ,let us1 in imagination1 adjust the speed of a supposititious motor until we get that which corresponds to #ajj alternations4 ,we switch on the current and at first1 possibly1 we get no response from any of the vibrating bars4 ,just a touch to the handle of the rheostat and we notice that bar #ajb shows signs of life4 ,we see then that our first speed was much too fast1 and that reducing it has brought it down to #ajb1 which is still a little too fast4 ,just a little more movement of the handle1 and #ajb begins to relapse into quiet1 while #aja shows animation4 ,a little more movement and #aja gives place to #ajj1 and then we know that our motor is working at the desired speed4 ,if our motor had been too slow to commence with1 it would have been #ih which first got into action1 but the method of adjustment would have been precisely the same4 ,and thus we see the whole scheme4 ,we regulate the speed by the rheostat1 and meanwhile that tell-tale stream of alternating current comes flowing out of the motor to indicate to us what the speed is1 while the frequency meter1 with its various vibrating bars1 interprets to us the message which the alternating current brings to us4 ,so by watching the meter we know when we have got the speed that we desire4 ,but even that is only half the battle4 ,we have seen how to make a machine turn at any desired speed1 and so we can adjust any two1 so that they revolve at the same speed1 but we have not seen how to start and stop the two machines at the same time4 ,first of all1 it must be understood that in the case of the receiving machine there is a friction clutch1 as it is termed1 between the motor and the cylinder which it is driving4 ,that means that while1 under ordinary circumstances1 the motor drives the cylinder round1 we can1 if we like1 hold the latter still without stopping the motor4 ,when we do so1 the connection between the two simply slips4 ,so if we fit a catch on the cylinder which is capable of holding it from rotating1 we can still start the motor1 and the latter will work4 ,then1 the moment the catch is released the cylinder will begin to turn too4 ,the commonest form of friction drive is the flat leather belt upon two pulleys1 which everyone has seen at some time or other in a factory4 ,and it will be quite easy to conceive how1 if one of the driven machines were to stick1 the belt might simply slip upon one of the pulleys1 yet1 as soon as the machine became free again1 it would rotate just as it did before4 ,it is just the same with what we are considering4 ,the motor works continuously at its proper speed1 but the cylinder can be stopped when desired by the catch4 ,combined with the catch is an electro-magnet1 and through its coils there flows the current of electricity which is engaged in printing the picture on the cylinder4 ,if a magnet be arranged to attract another magnet1 it will do so only when the energising current flows one way4 ,when it flows the other way1 it does not attract4 ,therefore it is easy to arrange matters so that the printing current1 though passing through the coil of the magnet1 shall not pull open the catch4 ,but if that current be reversed in direction for a moment the magnet gives a pull1 open flies the catch1 and away goes the cylinder upon its revolution4 ,thus1 we see1 all that is necessary to start the receiving cylinder is to reverse the current for a moment4 ,and now let us turn our attention to the sending machine4 ,upon its cylinder there is an arrangement which automatically reverses the current flowing to the main wire once in every revolution4 ,normally the current flows to the wire as described in the last chapter1 carrying by means of its variations the details of the picture for reproduction by the receiving machine at the other end4 ,but for an instant once in every revolution that current is interrupted and a current sent in the opposite direction instead4 ,this the sending machine does of itself1 quite automatically4 ,and now the reader knows of all the apparatus2 it remains only to see how the different parts work in combination4 ,standing by the sending machine we first of all turn on the current1 which goes coursing along the wire to the distant station4 ,then we set the motor to work and the cylinder begins to rotate4 ,before it has completed a single revolution the reverser is operated1 and just for a moment the reverse current goes to the wire4 ,on arrival at the other end that lifts the catch and the receiving cylinder starts4 ,that first partial revolution of the sending cylinder counts for nothing4 ,real business begins when the reverser first acts1 and that is the moment when the receiving cylinder also begins to move4 ,similarly1 when the sending cylinder stops it sends no more reversed currents1 and so the receiving cylinder is caught by the catch and not released4 ,so starting and stopping are quite automatic4 ,the same arrangement enables a continual readjustment of the relative speed of the two cylinders to take place4 ,with all the best devices1 the tuning-forks and the rest1 it is still impossible to attain perfect unanimity1 but the variation in a single revolution cannot be enough to matter2 it is only when the error in one revolution goes on multiplying itself that serious difference might arise1 and that is prevented in the following beautifully simple way4 ,the motor which drives the receiving drum is so regulated that it travels slightly faster than does the other4 ,thus the receiving cylinder completes every revolution slightly in advance of the other1 and consequently it is stopped and held by the catch every time4 ,the catch retains it1 of course1 until the reverse current arrives and releases it4 ,thus not only does the sending cylinder start the other when the operations first commence1 but it does so every revolution4 ,every revolution1 therefore1 the two cylinders start together4 ,so the two cylinders are set1 according to the frequency meter1 at as nearly as possible exactly the correct speeds1 and the action of the reverser1 the reverse current and the catch1 ensures quite automatically that at the commencement of every revolution there shall be perfect agreement between the two4 ,no accumulation of errors can possibly occur1 and the problem1 though apparently so difficult1 if not insuperable1 at first sight1 is surmounted4 ,,chapter ,,xv ,,scientific ,,testing ,,and ,,measuring ,science1 whether it be of the pure variety1 that which is pursued for its own sake--for the mere greed for knowledge--or applied science1 the purpose of which is to assist manufacture1 is based entirely upon accurate testing and measuring4 ,it is only by discovering and investigating small differences in size1 weight or strength that some of the most important facts can be brought to light4 ,there are some problems1 too1 that defy theory1 since they are too complicated2 they involve too many theories all at once1 and such can only be solved by accurate tests4 ,and all these necessitate the use of very ingenious and often costly devices4 ,electrical measuring instruments were of sufficient importance and interest to warrant a chapter of their own1 but there are many others of great value1 and not without interest to the general reader4 ,for example1 some years ago there was a collision in the ,solent1 just off ,cowes1 between the cruiser ,hawke and the giant liner ,olympic4 ,the cause of this was a subject of dispute and of litigation2 the theorists theorised2 some reached the conclusion that the ,hawke was to blame1 and others the ,olympic2 and where doctors disagree who shall decide8 ,it was wisely decreed that tests should be made to settle the question4 ,the main point was this4 ,the officers of the ,hawke1 by far the smaller vessel1 averred that they were drawn out of their course by suction caused by the movement of so large a ship as the ,olympic in the comparatively narrow and shallow waters of the ,solent2 in other words1 that the ,olympic in moving through the water caused a swirling1 eddying motion in the water1 tending to draw a lighter vessel towards itself4 ,and that is just one of those problems with which theory is unable to deal4 ,so it was transferred to the ,national ,physical ,laboratory at ,teddington1 near ,london1 for investigation by experiment4 ,at this institution1 which is a semi-national one1 there is a tank constructed for purposes such as this4 ,the word tank leads us to underestimate its size somewhat1 for it is #did feet long and #cj feet wide4 ,it is solidly constructed of concrete1 with a miniature set of docks at one end1 and a sloping beach at the other4 ,on either side are rails upon which run trollys which support the ends of a bridge which spans the whole4 ,this bridge can be propelled along1 by means of electric motors operating the wheels of the trollys1 from one end of the tank to the other1 at any desired speed1 within1 of course1 reasonable limits1 and from it may be towed any model which it is desired to test4 ,the models used are usually made of wax1 by means of a machine specially designed for the purpose4 ,it should be explained that the plans of a ship consist of a series of curves1 each of which represents the contour of the vessel at one particular height4 ,for example1 if you can imagine a ship cut horizontally into slices of uniform thickness1 then each slice could be shown on the drawing 7the shear plan1 as it is termed7 by a curved line4 ,near the keel the lines would1 of course1 be almost straight1 but they would bulge more and more as they occur higher up4 ,and what this machine is required to do is to make1 quickly and economically1 a wax model which shall be an exact reproduction1 on a small scale1 of the vessel under discussion4 ,it may be--it most often is--a ship as yet unbuilt1 the behaviour of which it is desired to test4 ,or it may be an existing vessel1 as it was in the case mentioned just now4 ,however that may be1 the model is made from the drawings4 ,a block of wax rests upon a table1 while the drawing is spread upon a board near by4 ,a pointer is moved by hand along one of the lines1 and its movement is repeated by a rapidly revolving cutter which cuts away the wax to a similar curve4 ,by suitable adjustments the cutter can be made to magnify or reduce the size1 so as to produce any desired scale4 ,thus every line is gone over and a similar curve cut in the wax at the correct height4 ,of course this only produces a lump of wax shaped in steps1 as it were1 but it is then quite easy to trim it down by hand1 so as to produce a smooth model of the ship1 perfectly accurate in its shape1 and a copy on a small scale of the vessel portrayed on the drawing4 ,it can also be hollowed out1 ballasted with weights inside1 and so made to sink to any desired level1 thereby representing the vessel when fully loaded1 half loaded and so on4 ,all sorts of unequal loading can be produced if needed1 indeed every condition of the real ship can be imitated in the model4 ,it can then be towed to and fro in the tank by the travelling carriage described above4 ,the speed of towing can be varied by changing the speed of the motors which drive it4 ,the force needed to pull the model through the water is measured by means of a dynamometer which registers the pull on the towing apparatus4 ,a matter very often needing investigation is the shape and size of the wave thrown up by the bow of the vessel1 and of that left behind her1 known as the bow wave and the stern wave respectively4 ,these waves represent wasted energy1 for they are no use and are produced actually by the power of the engines of the ship as they drive her along4 ,the ideal ship would cause no waves1 but since that is a degree of perfection impossible even to hope for1 the shipbuilder has to content himself by so designing his ships that these waves shall be as small as possible4 ,the waves are recorded photographically1 in some cases by the kinematograph4 ,some of the large shipbuilders have their own tanks1 and so have the naval authorities of the great naval ,powers4 ,the one at ,teddington was established through the munificence of a famous ,british shipbuilder1 ,mr ,yarrow1 who not only defrayed the cost of construction1 but gave an endowment to assist in its upkeep4 ,it is intended to serve the needs of the smaller builders who have not tanks of their own1 and also for the investigation of matters of general interest to shipbuilders1 and for such tests as that relating to the ,hawke and ,olympic4 ,in this last-named case1 of course1 two models were made1 one to represent each ship1 and they were towed along in such a way as to imitate very closely the movements of the ships at the time when they collided4 ,it was as the result of these tests that the ,olympic was ordered to pay damages to the ,admiralty1 it being held that she was the cause of the accident4 ,a very interesting investigation of this kind was recently carried out in the tank at the ,united ,states ,navy ,yard4 ,the port of ,new ,york consists very largely of jetties projecting out from the banks of the river4 ,with the growth of the ,atlantic liner the old jetties had become too short1 and questions arose as to the elongation of them4 ,if it were done1 how would it effect the current in the river1 and the handling of shipping generally8 ,if1 on the other hand1 it were not done1 what would be the effect of the ships lying with their ends projecting out into the stream unprotected by a jetty4 ,to determine these points the experimental tank was converted into a model of the ,new ,york ,harbour1 or at all events of that part in connection with which these questions arose4 ,a false floor was put in1 so as to make the depth exactly right in proportion to the width4 ,little model jetties were arranged to represent exactly the real ones1 while against them were moored model vessels1 so that the effect upon them could be observed as the model of the large vessel was towed past4 ,in addition to this1 special appliances were arranged for finding out what the disturbance might be which the movement of a giant liner produces under the surface as well as above it4 ,for this purpose buoyant balls were employed1 moored at various distances below the surface1 from which thin rods projected upwards1 the movement of which rendered visible the movements of the submerged balls and therefore the effects of the under-water currents4 ,all these things had to be observed at one and the same time--the moving model itself1 the models alongside the jetties1 the commotion on the surface1 the swayings to and fro of the rods attached to the submerged floats--all1 or most of which1 at all events1 it was impossible to make self-recording4 ,yet1 seeing that it was of the utmost importance that the relations between all these things should be observed1 and recorded from time to time as the model was towed along1 it is evident that something must be done1 and a cunning use of the kinematograph solved the problem quite easily4 ,at various points commanding a good view of the model harbour and its shipping these machines were placed1 and so several series of photographs were obtained1 by the study of which all the different movements could be seen and compared4 ,a large dial too was rigged up upon the travelling carriage by which the model was towed1 a finger on which denoted the distance which the carriage had travelled at any moment4 ,this large dial came into each photograph1 of course1 and so each picture bore upon itself a clear record of that particular moment in the voyage of the model to which it referred4 ,thus we see an instance of how the very latest and most up-to-date methods of amusement are sometimes applied to serve very practical purposes4 ,akin to the experiments upon ships are aerial experiments to determine matters connected with the navigation of the air4 ,at ,barrow-in-,furness the great firm of ,vickers1 shipbuilders and armament manufacturers1 and latterly builders of aerial craft for the ,british ,admiralty1 have erected a machine for testing the efficiency of aerial propellers and other things of a kindred nature4 ,upon the top of a tall tower there is pivoted a long arm of light iron framework4 ,to the end of this a propeller can be fixed1 so that as the arm revolves there is produced almost exactly the same conditions as those which prevail when a propeller drives an aeroplane or steerable balloon4 ,by means of suitable mechanism the propeller can be turned at any desired speed1 with the result that it drives the arm round and round upon its pivot on the top of the tower4 ,the force which the propeller thus exerts can easily be measured1 and so can be determined such questions as the most efficient speed for each type of propeller1 the power which any particular one can develop1 the best form for each particular need1 and so on4 ,materials1 too1 require the most careful testing1 in order that they may be put to the best possible use in modern machinery and structures4 ,for example1 anyone can measure the strength of a spring1 but what do we know as to its lasting power8 ,springs often have to form part of a machine in which they are stretched and compressed millions of times1 and the question arises as to what is the best shape and material for the purpose4 ,it may be that the spring which works best a few times will be the first to become weary1 for with repeated strain such things as steel get tired1 just as the human frame does4 ,now that is a matter which will yield to no calculation1 the only way to determine it is actual test4 ,so a mechanism has to be employed which will extend and compress the spring over and over again1 just as it will be in actual use1 with a counter of the nature of a cyclometer to count how many times it has been subjected to this distortion4 ,then the apparatus is set going and left to itself for hours1 or even for days1 during which time it may work the spring millions of times4 ,this may go on until it breaks1 or else it may be done a prearranged number of times1 and then the spring taken out and tested by other means to see how its strength has been affected4 ,metal bars are often subjected to sudden blows1 light in themselves but oft repeated4 ,the point to be determined then is how many times the blow may fall before permanent injury is done to the bar4 ,to investigate such matters we have the repeated-impact machine4 ,the bar is held in a suitable holder1 under a hammer which gives it a blow1 the force of which can be easily regulated1 at regular intervals1 the number of blows being counted by a suitable recording mechanism4 ,ultimately the bar breaks1 under a blow the like of which it can endure singly without any apparent strain at all4 ,the machine1 by the way1 can be caused to turn the bar round to some degree after each blow1 so that it is struck from all directions in succession4 ,the microscope1 too1 has established its place in the testing laboratory4 ,it is a very valuable adjunct to chemical and mechanical tests4 ,suppose1 for example1 that a bar of steel is being investigated2 it can be put into a machine and pulled until it breaks in two4 ,the machine registers the amount of the pull which was applied4 ,or a small piece can be put under a press and compressed to any desired degree4 ,it can also be tested by impact or even pulled apart by a sudden blow1 as described in ,mechanical ,inventions of ,to-day4 ,the bar can be supported by its ends and loaded or pulled down in the centre1 so that its power of resisting bending can be determined4 ,it can be judged1 too1 from its chemical composition4 ,steel1 in particular1 depends for its properties very largely upon its chemical composition4 ,the difference between cast-iron1 wrought-iron and steel1 also the differences between the innumerable varieties of steel1 are due almost entirely to the admixture of a certain percentage of carbon with the metal4 ,this can be ascertained by chemical analysis4 ,this form of inquiry has the advantage over the more purely mechanical methods in that the latter1 for the most part1 have to be applied to the bar as a whole1 whereas the quality may vary in different parts1 the surface in particular being liable to differ from the interior4 ,in such cases1 one analysis can be made of a piece cut from the surface and another of a piece from the centre4 ,and it is here1 too1 that microscopical analysis comes in4 ,for this purpose a piece is sawn off the bar1 and the end ground perfectly smooth4 ,this is then washed in a suitable chemical1 such as a mild acid1 which acts differently upon the different materials of which the metal is built up1 thereby rendering them visible one from another4 ,a photograph taken through a microscope then shows the structure of the metal2 how the different constituents are built together4 ,this is known as metallographic testing1 and its advantage as compared with chemical analysis is that the latter shows1 as we might say1 what are the bricks of which the thing is built1 while the former shows how the bricks are arranged4 ,indeed it is hardly correct to speak of the advantage or superiority of one over the other1 since each is the complement of the other1 supplying the information which the other fails to give4 ,and there are other mechanical tests which have not yet been mentioned4 ,there are machines which twist a bar so as to discover its power to resist torsion1 there are others which apply a downward pressure on one part of the bar and an upward one on an adjacent part1 so as to show its capabilities in withstanding shearing strain4 ,moreover1 many of these tests are nowadays1 in a well-equipped testing-house1 carried out in conjunction with the use of heat4 ,it stands to reason that a part of a machine which will have to work under considerable heat may have to be of different material from a part which works under a normal temperature4 ,in some cases the bar is surrounded by a spiral wire through which electric current is passing1 and by the regulation of this current any desired temperature can be set up in the bar4 ,or it may be placed in a bath of hot oil in such a way that the bar shall be raised to any temperature required1 without interfering with the machinery which exerts the tension or pressure1 or whatever it be4 ,years ago such elaborate tests as these were never thought of4 ,there are certain well-known figures1 to be found in all engineering text-books1 which give what stresses different materials ought to be able to stand1 and these were1 and are still1 to a large extent1 relied upon1 it being taken for granted that the material used will be up to the average standard4 ,in large and important works1 however1 the testing has been developed upon scientific lines1 so that it is known from actual experiment what each particular thing is capable of4 ,this not only means security but economy1 for it is sometimes found that a substance is stronger than it is thought to be1 and so things made of it can be designed to give the requisite strength lighter and cheaper than they would have been otherwise4 ,some of the machines employed are of enormous strength1 capable of exerting a pull or a compression of1 it may be1 #ajj tons or more4 ,they are often made1 too1 with self-recording appliances1 whereby the course of the test is set down automatically upon a chart4 ,for example1 when a bar is being tested for tension1 it is desirable to know not only the actual pull under which it came in two1 but the behaviour of the test piece during the period before that4 ,it begins to stretch as soon as the tension is applied1 theoretically at all events1 and if the metal were perfectly ductile it would stretch continuously as the load increases1 until at last the breaking stress is reached4 ,but in actual practice it probably stretches somewhat by fits and starts1 and a record of that fact will be of great value in estimating the strength of the material in actual work4 ,for such1 an automatically made record1 which can be studied at leisure1 is of the utmost importance4 ,but perhaps the finest instance of scientific methods in manufacture is to be found in the methods by which standard parts of machines are measured1 so as to ensure that they shall be interchangeable4 ,it may surprise the casual reader to be told that an absolutely exact measurement is an impossibility4 ,it is safe to say that out of a million similar articles--articles made with the intention that they shall be exactly alike--there are no two which are1 in fact1 absolutely similar4 ,they may be made with the same machines and the same tools1 handled by the same man1 but machines and tools wear or get out of adjustment1 while man's liability to err is proverbial4 ,astronomers are the greatest experts in the art of measurement1 and they recognise the possibility1 nay1 the probability1 of error so frankly as to make every measurement several times over2 if it be an important one they make it1 if possible1 a great many times over1 and then take the average of the results4 ,by this means they eliminate1 to a certain extent at any rate1 the error which cannot be avoided4 ,that process is to allow for errors on the part of their instruments1 for the most part4 ,to deal with personal errors another method is used as well1 for it is known that some observers have a natural tendency to err on one side more or less1 while others tend to make mistakes in some degree on the other side4 ,this tendency to err is known as the personal equation of the observer1 and there are machines and tests by which the personal equation of each man can be determined1 or perhaps it would be more correct to say estimated1 so that in all observations made by him the proper allowance can be added or deducted4 ,but of course it would be extremely difficult to apply such methods in a workshop4 ,it would never do to have to measure everything several times over1 hoping that the average would come out in such manner as to indicate that the thing being measured was the size required4 ,instead1 therefore1 of wasting time seeking an accuracy which is known to be unattainable1 the manufacturing engineer adopts a scientific system of measurement wherein a certain amount of inaccuracy is determined upon as permissible1 and then simple appliances are used to see that it does1 in fact1 fall within those limits4 ,for instance1 a round bar is to be made1 say1 an inch in diameter4 ,now we know from what has just been said that1 when made1 we have no means of telling whether the bar is really and truly an inch in diameter or not4 ,we consider1 then1 what it is for1 and decide1 say1 that it will be near enough so long as we are sure that it is not larger than one inch plus one thousandth1 nor less than one inch minus one thousandth4 ,so long as it does not exceed or fall short of its reputed size by more than one thousandth of an inch1 then we know that it will answer its purpose4 ,now1 having come to that decision1 we can build up a system upon which any intelligent workman can proceed1 with the result that all the inch bars which he makes will be the same size within the limits of #a_/ajjj over or under1 so that the greatest possible difference between any two will be #a_/ejj4 ,this system involves the use of two gauges for every size4 ,the man employed upon making one-inch bars has a plate with a hole in it #a-a_/ajjj inches in diameter and another hole #iii_/ajjj of an inch in diameter4 ,one of these is the go in gauge2 the other is the not go in4 ,so that all he has to do1 in order to be quite sure that his work is right1 is to see that it can be poked through one of these holes1 but not through the other4 ,no trouble at all1 it will be observed1 adjusting fine measuring appliances1 simply a plate with two holes in it1 and the workman can be sure that he is turning out articles every one of which is practically correct1 with no variation beyond a slight inequality too small to matter4 ,and probably at some other part of the factory there is a man making articles each of which has a hole in it1 into which this bar must fit4 ,how does he manage8 ,he is provided with a gauge somewhat the shape of a dumb-bell1 one end of which is slightly larger than the other4 ,one is the go in end1 the other the not go in end4 ,if the hole which he makes will permit the former to enter1 but will refuse admittance to the latter1 then he knows that that hole is sufficiently near its reputed size to answer its purpose4 ,illustration3 ,by permission of ,the ,mining ,engineering ,co41 ,sheffield ,a ,,miners' ,,rescue ,,team ,these men are equipped with breathing apparatus which enables them to pass safely through the deadly fumes after an explosion1 to rescue their unfortunate comrades ,in the instances mentioned1 a thousandth of an inch either way has been mentioned as the limit of inaccuracy1 or the tolerance1 as it is sometimes termed1 but often the limits are much narrower than that4 ,the gauges themselves are a case in point1 for they must be true within1 say1 a ten-thousandth1 or even less4 ,and they too are checked by master gauges of a finer degree of accuracy still1 being made by the most laborious methods1 and checked over and over again1 so as to reach the utmost limits in the way of correctness4 ,so this methodical scientific system of limit gauges is based upon the principle of having one gauge limiting the error one way and another defining it in the other4 ,anything simpler or more effective it would be impossible to conceive4 ,it is due very largely to this system that many manufactured articles are now so much cheaper than they used to be4 ,for it enables each individual part to be made wholesale on a large scale1 by machines specially adapted to the work1 operated by men specially trained to work them1 with the practical certainty that these parts when assembled together will fit each other4 ,in conclusion1 there is another very interesting instrument which was first made for a purely utilitarian use--namely1 the investigation of the methods of making coloured glass--but which has since been applied to some interesting problems in pure science4 ,it is called the ultra-microscope4 ,it must first be pointed out that there is a limit to the power of the ordinary microscope1 beyond which the skill of the optician cannot go4 ,he is baffled at that point not because of any lack of ability on his own part1 but because of the nature of light itself4 ,an opaque object1 unless it be self-luminous1 which few things are1 can only be seen by reflected light4 ,generally speaking1 we see things because they reflect in some degree the light which falls upon them4 ,but light consists of waves1 and when we reach an object so minute that its diameter is about half the wave-length of light1 then we cannot see it because it is unable to reflect the light on account of its smallness4 ,we can see this any day by the seaside1 or by a river or large pond4 ,there it is evident that the waves and ripples are reflected by such things as large stones1 wood posts or anything of any size which come in their way2 but when a wave encounters an object much smaller than itself it simply swallows it up1 as it were1 flows all over it or around it1 without being in any way reflected by it4 ,and it is just the same with the waves of light2 they are unaffected by obstacles below a certain size1 and so are not reflected by them4 ,for this reason things smaller than about a seven-thousandth of a millimetre cannot possibly be seen by a microscope in the ordinary way4 ,but if an object can be made self-luminous1 then it can be seen1 whatever its size1 if the magnifying power of the microscope be great enough4 ,so this ultra-microscope1 as it is called1 is really an ordinary microscope of the highest power possible1 with an added apparatus for making the tiny particles which are being sought for self-luminous4 ,this is done by directing upon them a pencil of light of exceeding intensity4 ,generated by powerful arc lamps1 the light is concentrated by a system of lenses until it is of an almost incredible brightness1 after which it falls upon the object4 ,now at first sight this seems to be no different from the usual procedure with a microscope1 and there appears to be no reason why it should be more successful1 but the explanation is this3 light is a form of energy1 and the waves of this very intense beam1 falling upon the object1 throw it into a state of violent agitation1 by virtue of which it shines1 not with reflected light1 but with light of its own4 ,it is not that the waves are reflected1 but that they so shake up the particle that it gives off light waves itself4 ,and thus it comes within the range of human vision4 ,in this way1 not only have the very small particles of colouring matter in glass been seen individually1 but it is thought that the actual molecules of matter have been seen1 or if not the molecules individually1 little groups of molecules1 dancing and capering about1 just as scientific people for years have believed them to be doing1 although they could not see them4 ,so here we have an instance in which manufacture has aided science--an inversion of the usual order of things4 ,,chapter ,,xvi ,,colour ,,photography ,photography has introduced many of the general public to a branch of practical science which otherwise they would never have cared much about4 ,the action of light upon certain chemicals1 the subsequent action upon the same of other chemicals1 such as developers1 toning solutions and so on1 form a very well-known region of the domain of science4 ,and this is1 too1 a branch of chemistry in which the practical inventor has been very busy4 ,the efforts1 therefore1 which have been made to invent ways of producing photographic pictures which shall give to the objects their natural colours1 will probably be of special interest in a book like this4 ,of these there are two very well-known systems1 and to them we will mainly confine our attention4 ,it should first be pointed out1 however1 that what we are discussing is quite different from the simple orthochromatic plates which are used by many photographers4 ,these latter are coated somewhat differently from other plates1 with a view to their giving a more realistic picture1 but the result is still in one colour4 ,they are1 in fact1 a little more sensitive to differences in colour than ordinary plates1 so that colours which appear1 when the latter are used1 very much the same1 appear1 when orthochromatic plates are employed1 a little different4 ,but the difference in colour in the object photographed is only1 even then1 represented by a difference in shade in the picture4 ,the object is1 it may be1 in many colours1 in all the colours1 very likely1 but the picture is only in one4 ,and the step from that to a coloured picture is a very long one4 ,true1 the solution of the problem is very simple in principle1 yet the practical difficulties are so great that even now they have not been entirely overcome4 ,let us first of all examine the principle4 ,sunlight1 by which photographs are usually taken1 appears to the eye white and colourless4 ,it is not really so1 however1 as can be proved by analysing it with the spectroscope4 ,in this instrument a flat beam of light1 having passed through a narrow slit1 falls upon a prism of glass1 from which it emerges as a broad band1 known as the spectrum4 ,this band can be seen upon a screen1 or can be examined through a telescope4 ,so far from being white and colourless1 it consists of the most lovely colours4 ,at one end of the spectrum is a beautiful red1 which1 as the eye travels along1 imperceptibly merges into orange1 which in turn merges into yellow1 after which we find green1 blue1 indigo and violet1 in the order named4 ,these seven are known as the primary colours1 but it is quite a mistake to suppose that there are seven clearly defined and distinct colours4 ,the colours so change1 one into another1 that their number is really infinite4 ,the seven names indicate seven points in the spectrum1 whereat the colours are sufficiently distinct from others to warrant a separate name being given to them4 ,we call the starting colour red1 for example1 and as we pass our eyes along we perceive a constant change1 and when that change has become sufficiently pronounced to justify our doing so1 we call the new colour orange4 ,continuing1 we find the orange changing into something else1 and when it has gone far enough1 we bring in a third name1 yellow1 and so on to the violet4 ,thus we see the division into seven colours is arbitrary1 and only for our own convenience1 since the whole number of colours is innumerable4 ,passing through a prism is not1 however1 the only means by which white light can be split up4 ,when the sun shines upon a blue flower1 for instance1 the blue petals perform a partial separation2 they reflect the blue part of the sunlight1 and absorb all the rest4 ,a red flower likewise reflects the red part of the sunlight and absorbs the rest4 ,it is because things can thus discriminate1 reflecting some kinds of light and absorbing the remainder1 that we perceive things in different colours4 ,it follows1 therefore1 that when we look upon a landscape1 or a field of flowers1 we receive into our eyes an enormous variety of coloured lights4 ,the white sunlight furnishes each thing we see with a flood of white light1 and each thing according to its nature1 reflects more or less4 ,a white flower reflects the whole1 a pure black object reflects none1 but the great majority of things reflect some part or other of that infinite variety of which white light really consists4 ,so a view at all varied sends to our eyes a variety of colours1 almost as manifold as the colours of the spectrum1 which1 as has been said1 are infinite4 ,and the task of reproducing them1 or even of producing a similar general effect1 upon a piece of paper seems at first sight beyond the bounds of possibility4 ,but fortunately there is a way by which we can produce1 approximately at all events1 the intermediate colours by mixtures of the others4 ,the second colour of the spectrum1 for example1 orange1 can be obtained by mixing its neighbours on either hand--namely1 red and yellow4 ,we can1 indeed1 imitate very closely the imperceptible change from red to yellow through orange1 by skilful mixture of red and yellow pigments4 ,first there is the pure red1 then just a suggestion of yellow is added2 more and more yellow brings us to orange2 after which by gradually diminishing the amount of red we reach the pure yellow4 ,next1 by introducing blue pigment1 we can gradually change the yellow into green1 and further manipulation of the same two colours will lead us on to pure blue4 ,indeed by mixtures of red1 yellow and blue we can obtain almost all the perceptible varieties of colour4 ,and it must be remembered that when1 by mixing blue and yellow pigments1 we get the effect of green1 that is only the result of an optical illusion4 ,the particles of which the yellow pigment is made remain yellow1 and the particles of blue remain blue4 ,the one sort reflect yellow light to our eyes1 the other sort reflect blue light1 and owing to what in one sense may be called a defect in our vision1 these two mingling together look as if the whole were green4 ,in the spectrum we see real green light2 from green paint made by mixing yellow and blue1 we only see an imitation or artificial green4 ,if the particles were large enough1 we should see the yellow and the blue ones quite separate1 but since they are too small for us to see at all1 except in the mass1 our eyes blend the whole together into the intermediate colour4 ,thus we see that1 although the variety of colours is infinite1 we can for practical purposes reproduce as much difference as our eyes can perceive by the judicious blending of three--namely1 red1 yellow and blue4 ,and there is a further fortunate fact--we can filter light4 ,the red glass with which the photographer covers his dark-room lamp looks red1 and throws a red light into the room1 because it is acting as a filter to the light proceeding from the lamp behind it4 ,the lamp is sending out light of many colours1 but the glass is only transparent to the red4 ,it holds up all the others but lets the red pass freely4 ,so if we were to take a photograph through a red screen1 we should get on the plate only those parts which were more or less red in colour4 ,for example1 if we thus photographed a group of three flowers1 one red1 one orange and one yellow1 the red one would come out prominently1 the orange one would come out faintly1 and the yellow one not at all4 ,then suppose we took the same picture again through a yellow screen4 ,in that case the yellow flower would be prominent1 the orange would again be faint1 but the red would be absent4 ,having got1 in imagination1 two such negatives1 let us make two carbon prints1 one off each4 ,and let the print off the first negative be red1 while that off the second is yellow4 ,let each be1 in fact1 of the same colour as the screen through which the picture was taken4 ,finally1 let the two films be placed in contact one upon the other4 ,on holding the two up to the light1 what should we see8 ,we should see a red flower1 for there would be a red flower clearly defined upon one film coinciding with a blank transparent space upon the other film4 ,we should see1 too1 a yellow flower1 for a clearly defined yellow flower on the second film would coincide with a clear space upon the first4 ,we should see also an orange-coloured flower1 for there would be a faint red image of it1 and a faint yellow image of it1 one on each film1 lying one over the other1 producing the same effect as a mixture of yellow and red pigments4 ,thus by taking two negatives through two coloured screens1 and then colouring the prints to correspond1 we can obtain three colours in the finished picture4 ,by taking a third negative1 through a blue screen1 we could add immensely to the range of colours obtainable4 ,indeed1 with three films1 red1 yellow and blue respectively1 made through three screens of the same colour1 a variety of colours practically infinite can be obtained4 ,so the principle is quite simple2 the difficulty is in carrying it out4 ,for the three kinds of light have not the same photographic power1 and so to avoid upsetting the balance of the colours different exposures would be required for each4 ,then there is the difficulty of so manipulating the films as to get them one over another exactly4 ,anyone who has tried the handling of carbon prints will readily realise how difficult this would be4 ,it is possible and has been done1 but the process is too uncertain and too laborious to be of general use4 ,but the same result can be attained more or less automatically1 as the following descriptions will show4 ,let us turn to the ,lumiere autochrome process1 by which the results desired can be in a large measure attained by methods of manipulation comparatively simple4 ,illustration3 ,by permission of ,the ,mining ,engineering ,co41 ,ltd41 ,sheffield ,,pneumatic ,,hammer ,,drill ,this tool is used by miners for making holes in hard rock1 preliminary to blasting4 ,note the spray of water1 which prevents the stone dust rising and getting into the miner's lungs4--,see p4 #bbj ,the plates used for this are of a very special nature4 ,in the first place1 there is the basis of glass1 but upon that there is laid what we might term the selective screen4 ,this is a layer of starch grains1 of exceeding smallness4 ,the size of them is as little as a half a thousandth of an inch and there are about four millions of them on every square inch of plate4 ,next1 upon the screen of starch grains is a layer of waterproof varnish1 while over that is the ordinary sensitive emulsion such as forms the essential part of the usual non-colour plate4 ,now the starch grains which form the screen are1 before they are laid on1 stained in three colours4 ,some are blue1 some red1 and some a yellowish-green1 which experience shows is preferable to pure yellow4 ,the differently coloured grains are well mixed1 and when the screen is held to the light and looked through the effect is almost that of clear glass4 ,that is because red rays from the red grains1 and green and blue rays from the grains of those colours1 all proceed to the eye mingled together4 ,this plate is placed in the camera differently from the usual way1 since the glass side is turned towards the lens4 ,the light1 therefore1 after entering the camera1 passes through the glass1 then through the screen1 and finally falls upon the sensitive film4 ,suppose1 then1 that the camera were pointed to a red wall2 red light would fall upon the plate and1 passing through the red grains1 would act upon the sensitive film behind them4 ,the blue and green grains1 on the other hand1 would stop those rays which fell upon them1 and so those parts of the sensitive film which they cover would remain unaffected by light4 ,then1 if that plate were to be developed1 a dark1 opaque spot would be produced upon the film under each red grain1 the film under the other grains remaining transparent4 ,hence1 when held up to the light and looked through1 the plate would appear a greenish-blue1 for all the red grains would be covered up4 ,in like manner1 if the wall were blue instead of red1 a greenish-red plate would result1 while if it were green1 the plate would be a purple1 the result of the combination of red and blue4 ,but this1 it will be seen1 is a topsy-turvy effect1 the exact opposite of what we want1 so that it is fortunate that by a simple chemical method we can set it right4 ,after a first development in the ordinary way the plate is placed in another bath and exposed to strong daylight1 with the result that those parts which were darkened by the first development become clear and the parts which were clear become opaque4 ,thus1 after this twofold development of the photograph of the red wall1 we find ourselves in possession of a red plate1 in which only the red grains are visible1 since all the others are covered up by opaque parts of the sensitive film4 ,the photograph of the blue wall will also1 after it has been subjected to the double development1 show blue only1 and the same with the green4 ,but suppose that instead of a red wall or a blue wall we focus our camera upon one which is half red and half blue4 ,then it is easy to perceive that we shall get a plate which is half one colour and half the other4 ,moreover1 it follows that a wall covered with a mosaic of red1 blue and green would give us a plate duly coloured in the same way4 ,but when we go a step further and photograph1 say1 a landscape1 which may contain a vast range of colours1 we find a difficulty in believing that they can all be rendered by the simple process of covering or leaving uncovered grains either blue1 red or green4 ,it can be done1 however1 since the other colours may be made up of two or more of these three in varying proportions4 ,for example1 should there be something in the landscape of a darker1 more blue1 shade of green than the green grains1 then the light proceeding from that object1 while passing freely through the green grains upon which it falls1 will slightly penetrate the neighbouring blue ones as well1 and so at that point on the plate there will be not only green grains visible1 but some of the blue grains partly visible also4 ,the light from the blue grains will enter the eye along with that from the green grains1 and by so doing will add just that amount of blue to the green as to give it the right shade4 ,after this manner is the whole picture built up4 ,it is1 of course1 really a mosaic1 consisting entirely of little coloured patches1 but since they are so small none can be seen individually1 all merging together in the eye so as to form a picture in which colours change imperceptibly from one into another4 ,to sum up1 then1 what happens is this4 ,we start with a layer of coloured grains2 the action of taking and developing the photograph covers up some of these grains and leaves others exposed1 and the action of the light is such that those which are left visible produce a picture closely resembling the original1 not only in form but in colour4 ,but there is one other interesting point about this process which deserves mention4 ,the differently coloured lights are not of the same power photographically4 ,red light1 as we know well1 is very weak in this respect1 wherefore1 we use it in the dark-room4 ,a faint red light will have no perceptible effect upon a plate unless it be exposed to it for some time4 ,blue light1 on the other hand1 is very active1 and were the blue and red lights to be allowed to act equally on the autochrome plate1 the result would be much too blue4 ,it is therefore necessary to handicap the blue light1 as it were1 by placing a reddish-yellowish screen either just in front of1 or just behind1 the lens to cut off a proportion of the blue rays4 ,the other very successful process is known as the ,dufay dioptichrome process4 ,it differs very little from the ,lumiere except in detail1 the selective screen being formed of small coloured squares instead of by a mass of little grains4 ,in both1 it will be noticed1 the result is a single positive4 ,it is not1 as in ordinary photography1 a negative off which any desired number of positive prints can be made4 ,and1 moreover1 it is a transparency3 it cannot be viewed except by light shining through it4 ,the results are1 however1 extremely beautiful1 when well done1 and anyone who cares to try either of these methods of working will be well repaid for the trouble involved4 ,,chapter ,,xvii ,,how ,,science ,,aids ,,the ,,stricken ,,collier ,nothing is more characteristic of the present age than the care which is1 quite rightly1 expended upon the comfort and safety of those who do the manual labour of the community4 ,the stores of scientific knowledge and skill are drawn upon freely for this end1 and some very interesting examples can be given of the truly scientific methods which have been evolved1 not only for preventing injuries of any kind1 but for succouring those who may1 despite those precautions1 fall victims to disease or accident4 ,an example has already been given of the scientific investigation into the nature of colliery explosions and the best means of preventing them4 ,we have seen there how expense has been poured out lavishly in fitting up the experimental gallery or artificial pit1 and how the most cunning mechanical and electrical devices have been pressed into the service in order to find out just what happens when an explosion occurs4 ,it has been related how these investigations have revealed with certainty the true cause of the explosions and thereby led the way to their prevention4 ,but with it all there is still an occasional disaster1 occurring1 sometimes1 at the best and most carefully managed collieries4 ,and therefore it is still necessary to provide for rescuing the unfortunate men who are affected4 ,it is worth remark1 here1 that colliery explosions are1 all things considered1 a very rare occurrence4 ,because of their dramatic suddenness1 and the number of lives which are commonly lost in a single disaster1 we are apt to magnify their severity in our minds and to picture the life of the miner as a very hazardous one4 ,in point of fact1 the expectation of life1 as the insurance people call it1 is quite as great among the coal-miners as among any class of manual labour4 ,and of those who do meet an untimely end there are more lost through isolated accidents1 involving one or two men1 than in the great disasters4 ,to meet these isolated cases science is almost powerless4 ,for the most part1 they are due to falls of material from the roof of the mine1 or some simple accident of that kind1 caused by an error of judgment or lack of care on the part of fellow-workmen1 and the only safeguard against such is the most careful and systematic supervision1 which1 in ,great ,britain at all events1 is rigidly applied4 ,the underground staff are very carefully organised with this end in view1 and the whole is supervised by ,government inspectors4 ,no amount of scientific investigation or invention will help much in these matters4 ,with the explosion or fire1 however1 it is different1 for there subtle forces and strange chemical influences come into play with which science is specially well fitted to deal4 ,to a great many people the first news of organised1 trained and scientifically equipped rescue parties came at the time of the terrible ,courrieres disaster in ,france1 when over #ajjj men lost their lives4 ,for then a party with apparatus hurried from ,germany and played a prominent part in the rescue operations4 ,but unfortunately the glamour of their performance was somewhat dimmed by the fact that after they had done all they could1 and had gone home again1 more men were rescued4 ,many1 reading of that fact1 were inclined to scoff at the new-fangled ideas1 thinking that after all the old way of working with a party of brave but untrained and often ignorant volunteers was better than the new way of working with equipped and trained men4 ,it certainly did seem as if the former had succeeded where the latter had failed4 ,but that was quite a mistake1 as subsequent events have shown1 and in all probability it was due to the fact that the uninstructed party were local men1 thoroughly familiar with the mine in which they were working1 its geography and its special local conditions1 whereas the trained men came from far away4 ,at all events the pioneer work of the ,germans in the matter of rescue teams has been amply justified by the fact that other people have copied them1 and none more thoroughly than the mining authorities of ,great ,britain4 ,indeed we see here another instance of the remarkable way in which the ,british people1 though a little slow to take up a new idea1 do take it up when it has once been established1 and in such a way that they are soon among the foremost in its use4 ,the ,germans1 all honour to them1 started the rescue teams1 but at this moment there are rescue teams and stations for their training in ,britain second to none in the world4 ,of these there is a splendid example in the ,rhondda ,valley1 in ,south ,wales1 supported and worked by the owners of the pits in that district1 besides others at ,aberdare1 in the same neighbourhood1 at ,mansfield1 to serve the collieries in ,derbyshire and ,nottinghamshire2 indeed rescue stations are now dotted throughout the mining districts4 ,the general idea of these stations is as follows4 ,the building is centrally situated in the district which it is intended to serve1 and in it are kept an ample supply of the necessary appliances1 in the shape of breathing apparatus1 which enables men to walk unhurt through poisonous gas1 reviving apparatus1 by which partially suffocated men can be brought round again by the administration of oxygen1 together with quantities of that valuable gas in suitable portable cylinders4 ,everything which forethought can suggest as even possibly useful in an emergency is kept in a constant state of readiness4 ,and all the while a swift motor car stands ready to carry them to the scene of operations4 ,but the appliances are of little use without men to work them1 who know them and can trust them4 ,the case of ,david1 who felt able to do better work with his sling and stone than in all the panoply of ,saul's armour1 because he had not proved it1 is typical of a universal human instinct4 ,a man feels safer unarmed1 or simply armed1 than he does with the most elaborate weapons in which he has not learned to have confidence4 ,and therefore the men who may be called upon to work this apparatus are first taught to have confidence in it4 ,each station has its instructor1 who is usually also the general superintendent of the station1 and galleries in which the instruction can be carried out4 ,volunteers are called for in each colliery and a number of the most suitable men are chosen to undergo training1 preference being given1 very naturally1 to those who are already trained1 as fortunately so many workmen are nowadays1 in ambulance work4 ,these chosen men then repair at intervals to the station to undergo a proper course of instruction4 ,the instructor1 often an ex-non-commissioned officer in the ,royal ,engineers1 accustomed1 therefore1 to engineering matters1 and also to systematic discipline1 there puts them through a course of drill the object of which is to teach them to work together as a squad under the orders of a properly constituted chief4 ,thus when called upon in some emergency there will be no confusion1 but each man will know what to do1 and a few short words of command from the chief will serve better than the long explanations which would be necessary with an undisciplined body4 ,it welds the individual men1 as it were1 into a smoothly working machine1 thereby increasing the efficiency of the whole4 ,and arrangements are made whereby1 should the leader fail1 another man steps into his place of authority at once and without question4 ,then1 having thus brought them under a suitable discipline1 the instructor takes his men into the experimental gallery4 ,this may be described as a long1 low1 narrow shed1 in which are timber props and beams1 rails on the floor1 heaps of coal1 all things1 in fact1 which may tend to make it closely resemble the actual workings of a coal-mine after they have been shaken and shattered by the force of an explosion4 ,the great difficulty1 in a real disaster1 arises from what are known as falls4 ,the roof of the mine is normally supported by timbers1 and these the explosion moves1 so that in places many tons of the earth of which the roof of the mine consists will fall and block completely the roads or tunnels which communicate from the shaft to the places where the men are at work4 ,these1 of course1 have to be removed or burrowed through before the men imprisoned in the distant workings can be reached4 ,the rescue party do not1 of course1 wait to clear away the whole of this debris1 only just enough to enable them to crawl through or over it1 but even then it often represents the waste of precious hours1 and the expenditure of great exertions1 to get past a fall4 ,so at intervals falls are made in the gallery1 in order that men may be practised in dealing with them4 ,illustration3 ,by permission of ,w4 ,e4 ,garforth1 ,esq41 ,pontefract ,,an ,,artificial ,,coal ,,mine ,these two photographs show the clouds of flame and smoke issuing from the mouth of the ,artificial ,coal ,mine during the experiments described in the text ,it may be interesting to give a brief statement of the training undergone by the men at the ,mansfield ,rescue ,station4 ,in that case1 it should be stated1 the gallery is made double1 so that men can go one way and return the other back to their starting-point4 ,having donned their breathing apparatus1 they enter the gallery1 which1 by the way1 is filled with smoke and foul gas4 ,passing along it1 they encounter two falls1 which they must get over or through2 then they have to set twelve timber props as might be necessary to maintain the safety of a damaged road in the mine2 all that they do three times over4 ,then they are required to bring up and lay #bej bricks1 a thing which might also be necessary in an actual emergency1 after which they have to fix up brattice cloth in a part of the gallery4 ,one of the first duties1 of course1 for a rescue party is to restore the circulation of air in the mine1 and brattice cloth is a rough kind of cloth which is put to guide the air currents4 ,that done1 they have to take a dummy representing a man of #ad stone1 put it on a stretcher1 and carry it round the gallery and over the falls4 ,finally1 they restore the timber1 bricks and cloth1 and their turn of work is done4 ,the total time required for this is two hours1 and during the whole of that period they are1 of course1 breathing not the natural air1 but the artificial atmosphere provided for them by the apparatus with which each man is provided4 ,the chief point of this part of the training1 as has been remarked already1 is to accustom the men to the wearing of the apparatus and to doing work in it4 ,by this means they gain confidence in it1 and get to know that it will not fail them in the time of trial4 ,the course of instruction consists of ten drills such as has been described1 after which the men are called up twice a year1 just to refresh their memories4 ,one side of the gallery is glazed1 so that the instructor can watch his men at work without of necessity being inside himself1 and there are emergency doors as well1 which can be opened to let a man out should the ordeal be too much for him4 ,the necessary fumes are generated in a stove and driven into the gallery by a fan4 ,the stations are beautifully fitted up1 with baths for the men to wash after their somewhat dirty experience in the gallery1 and everything is done for their convenience and welfare4 ,the advantage of this systematic training of a great number of men is that there are men at each colliery who can be called upon when needed4 ,the team of strangers1 as has been remarked1 partially failed at ,courrieres1 largely because they were strangers1 but when every colliery has a team ready1 composed of its own men1 then clearly there is the greatest chance of success4 ,the central station of the district is the training-ground where the men go from all the collieries to get the experience and instruction1 and where a reserve store of appliances is kept4 ,in many cases1 of course1 the collieries have their own appliances1 so that work can be begun at once1 without having to wait for that from the rescue station1 but the latter forms a reserve in case of need1 and1 being kept under the care of an expert1 it is naturally always in the best possible working order4 ,to give an idea of the cost of these stations1 it may be stated that the one at ,porth1 in the ,rhondda ,valley1 cost1 including equipment1 ,l#gjjj1 while the one at ,mansfield cost ,l#cjjj4 ,this first cost and the expense of maintenance is borne by the collieries of the district in proportion to the quantity of coal which they raise4 ,and now we can turn to the apparatus itself1 without which the organisation already described would be of little value4 ,there are several makes of these1 but a description of the particular apparatus used at the two stations mentioned will serve as an illustration4 ,the purpose1 of course1 is to give the wearer an atmosphere of his own1 which he can carry about with him1 and which will render him quite independent of the ordinary atmosphere and quite indifferent to the poisonous nature of the gases around him4 ,to this end his mouth and nostrils must be cut off from the outer world altogether4 ,there are two ways of doing this4 ,in the one there is used a helmet1 or perhaps mask would be the better term4 ,this fits right over the man's face1 an air-tight joint being made between the helmet and his head by means of a rubber washer which can be inflated with air4 ,the inflation is accomplished by squeezing a rubber ball on the right-hand side of the helmet4 ,in the centre is a glass window through which he can see easily1 and since this is apt to become clouded by the dampness of his breath there is a wiper inside1 which can be turned by a knob on the outside1 so that by simply turning his knob with his hand he can clean the window at any time that may be necessary4 ,two soft pads inside the helmet bear one on the man's forehead and the other on his chin1 and these1 working in conjunction with a strap which passes right round the back of his head1 keep the thing firmly in position4 ,in addition there is combined with the helmet a leather skull-cap which1 being continued down behind1 gives good protection to the head and neck4 ,the other form of apparatus consists of a mouth-piece and nose-clip4 ,the mouth-piece1 as its name implies1 fits in the man's mouth1 being supported and kept in position by a strap passing behind the back of his head4 ,combined with it is a little screw clip which closes his nostrils4 ,the man also wears a leather skull-cap1 from which straps depend to bear the weight of the mouth-piece and its attached tubes1 so that the weight does not fall upon his mouth4 ,either of these arrangements1 it is clear1 cuts him off from communication with the outer air1 but that is only half the problem1 for he must be given a substitute or he will be suffocated4 ,this part of the appliance he carries1 knapsack fashion1 upon his back4 ,first there is a rectangular case1 called the regenerator1 with1 below it1 two small cylinders of compressed oxygen4 ,a suitable arrangement of pipes connects these together1 and to the helmet or mouth-piece as the case may be4 ,when the man exhales1 as we all know1 the air which he then discharges from his lungs is deficient in oxygen and instead contains carbonic acid gas4 ,the latter must be got rid of and replaced by pure oxygen4 ,the exhaled air is therefore led down a pipe to the regenerator1 where it comes into contact with several trays of caustic soda1 a chemical which has a great affinity for carbonic acid4 ,the result is that the latter gas is extracted from the impure air1 finding a more congenial home in the caustic soda4 ,it is then necessary to restore the normal quantity of oxygen1 and so1 as the air passes on1 it meets1 in a little apparatus known as an injector1 a spray of pure oxygen from the cylinders4 ,thus1 after being purified and re-oxygenated1 the air passes on through more pipes to the helmet or mouth-piece1 to be breathed once more4 ,the apparatus contains sufficient oxygen and caustic soda for this to go on for a space of two hours4 ,but during times of extra exertion a man needs more air than at others1 for which provision has to be made1 and so on his chest the rescuer carries a flexible bag divided into two compartments4 ,through one of these the exhaled air passes on its way to the regenerator1 while through the other the oxygenated air flows on its way to the man's mouth4 ,when he is breathing hard1 then1 during a moment of extra exertion1 and when1 therefore1 he is turning out bad air faster than it can be purified1 and drawing in pure air faster than it can be produced1 this bag comes to his aid4 ,from the store of oxygenated air in one side of it he draws the extra which he requires1 while the other side stores up temporarily the excess of vitiated air1 until the regenerator is able to overtake its work4 ,thus at all times1 whether breathing ordinarily or heavily1 the apparatus can respond to his demands4 ,the spray of oxygen as it escapes from the cylinders into the injector has the effect of driving the air along1 so that the circulation through the tubes and the regenerator is automatic1 and the foul air flows away from the man's mouth and the new air comes back to him quite without effort on his part4 ,as time goes on1 of course1 and the stored oxygen becomes used up1 the pressure in the cylinders falls1 which fall1 shown upon a little pressure-gauge1 tells the man how much longer time he has before he must return for fresh supplies of oxygen and soda4 ,fresh cylinders of oxygen can be connected up very quickly in place of the empty ones1 while a fresh regenerator can be put in1 or new caustic soda supplied1 in a very short time4 ,the superintendent of the ,mansfield station has invented what is termed a self-rescue apparatus1 to be used in conjunction with that which has been described above4 ,it is simpler and lighter than the rescue apparatus1 and will not keep a man supplied with air for more than an hour or an hour and a quarter4 ,moreover1 it is not automatic1 since the flow of oxygen has to be controlled by the man himself4 ,since1 however1 it consists only of a mouth-piece1 a breathing-bag and a cylinder of oxygen1 it is very portable1 and may well be carried by a rescue party for the use of any men who may be discovered alive beyond the danger zone4 ,it may well happen1 indeed it often has happened1 that a remote part of a mine1 although cut off from the shaft by passages full of after-damp1 as the foul gases caused by the explosion are termed1 may itself contain fairly pure air in which men can live for a long time4 ,if such men be reached1 the difficulty is to get them through the passages containing the bad air4 ,consequently a rescue party which carried one or two of these light forms of apparatus could equip such men with them and then they could pass out with safety4 ,another use1 the one1 in fact1 from which the appliance draws its name1 is the facility with which1 by its aid1 a man could set right a chance defect in his ordinary rescue apparatus4 ,suppose1 for example1 that a fully equipped man found something wrong1 whereby he was prevented from getting his proper supply of purified air4 ,then1 if the party had one of the self-rescue sets with them1 he could slip off his helmet or mouth-piece1 quickly replacing it1 for a time1 with the self-rescue mouth-piece4 ,this might enable him to reach safety1 or even to put the other apparatus right and then don it once more4 ,the whole thing can be packed up into a small tin case which can be slung over one shoulder1 and with the oxygen cylinder slung over the other one the complete outfit can be carried quite easily by a man in addition to what he is wearing himself4 ,still another form of breathing appliance may well be taken on these rescue expeditions1 and that is the reviving apparatus1 for use upon those who have apparently ceased to breathe4 ,in this case a mask is put over the sufferer's mouth and nose1 and then the turning of a lever into a certain position causes oxygen to escape from a cylinder in such a way as to cause a suction which empties the man's lungs of the bad gases which have laid him low4 ,that done1 another movement of the lever and a deep breath of oxygen flows into his lungs in their place4 ,thus by alternating the positions of the lever an artificial respiration is set up far more effective than can possibly be attained by the ordinary method of moving the man's arms and pressing his chest4 ,indeed there are cases1 such as when his arms or ribs are injured1 when the ordinary method is impossible1 but it is hard to imagine an instance when this beneficent apparatus could not be used1 and so long as there be any spark of life left in the poor fellow there seems to be every reason to expect a complete revival as the result of its use4 ,of course there are many other places where poisonous gases are likely to be met with1 such as gas-works1 chemical-works1 limeworks1 and so on1 where this apparatus may be kept with advantage1 in case of accident4 ,indeed all that has been described above has its use apart from colliery explosions1 although they are the outstanding opportunities for its employment4 ,old workings1 tunnels which have been empty for a time1 sewers--all these have1 on occasion1 to be entered1 not to mention houses full of smoke1 or factories full of chemical fumes1 all of which form cases in which the rescue apparatus would find useful employment4 ,,chapter ,,xix ,,how ,,science ,,helps ,,to ,,keep ,,us ,,well ,one branch of science--medical science--concerns itself almost entirely with health1 but it would be out of place to refer to such matters here1 even if the present writer were capable of doing justice to the subject4 ,a new medicine or a new method of operating upon a suffering patient would be quite correctly described as a scientific marvel1 but it is not of such that this chapter deals1 but rather with those great works by which the engineer1 often taught by the medical man1 promotes the health of a whole community4 ,most important of these1 perhaps1 is the provision of pure water4 ,some places are more fortunately situated than others in this respect1 being near streams flowing down from mountains clear and unpolluted1 which can be drunk after the minimum of purification4 ,others have to make use of the waters of a moderately clean river1 as ,london does those of the ,thames and ,lea1 in which cases the greatest care has to be exercised in the filtration of the liquid before it can be sent out through the mains for domestic consumption4 ,in this particular domain invention has been comparatively slow4 ,there are novel pumps1 it is true1 for handling the water1 such as the ,humphrey ,gas ,pump1 which the ,metropolitan ,water ,board 7,london7 have installed for filling their great reservoirs at ,chingford4 ,in these an explosion of gas is the motive force4 ,water flows by gravitation into a huge iron pipe closed at the top but open at the bottom4 ,it is so arranged that a quantity of gas shall be entrapped in the upper end1 which1 being exploded by an electric spark1 drives the mass of water out4 ,some of it1 together with a quantity of fresh water1 presently comes surging back1 entrapping a fresh supply of gas and causing a new explosion2 and so it goes on over and over again4 ,the particular pumps at the waterworks referred to discharge about fourteen tons of water at each explosion1 of which there are nine every minute4 ,the special effect of these machines1 however1 is not to improve the public health so much as to relieve the public pocket1 for their chief feature is that they work more economically than any other kind of pump4 ,the filters1 by which the water is purified1 are simply layers of sand1 much the same as have been in use for many years1 although in some cases chemistry is brought in and the work of the filters aided by the action of precipitants4 ,these are substances which combine in some way with the impurities in the water1 and carry them to the bottom of the tank or reservoir1 while the pure water remains to be drawn off from the top4 ,this is also the most usual method by which water is softened4 ,hardness in water is due to the presence of certain salts which are dissolved out of the ground as the water percolates through it1 and which are absent from rain-water4 ,to get rid of these the hard water has chemicals mixed with it in a tank1 from which it flows slowly through another tank4 ,the effect of the added chemicals is to convert the soluble salts in the water into insoluble particles1 which then tend to fall down to the bottom of the containing vessel4 ,the slow passage through the second tank is intended to give the particles time to settle4 ,illustration3 ,,sectional ,,view ,,of ,,hydraulic ,,buffer ,,and ,,running-,,out ,,presses ,,of ,a #fj-,,pounder ,,gun ,finally1 to make sure that these have been all got rid of1 the water traverses a filter1 and then it is for all practical purposes as soft as rain-water4 ,some people are frightened of this artificially softened water1 on the ground that chemicals have been added to it1 supposing1 apparently1 that when they use such water they are really employing a chemical solution4 ,that is quite wrong1 however1 for the added chemicals1 combining with the hardness1 form substances which are quite easily extracted from the water altogether4 ,if we liken the hardness to a number of pickpockets in a crowd1 and the added chemicals to a number of policemen who come in to arrest the said pickpockets1 finally leaving the crowd free from both pickpockets and policemen1 we get a simple illustration of what takes place4 ,but almost equally important as the provision of pure water is the effective dealing with the drainage of a large town4 ,much offensive matter flows under the streets of our towns and cities1 and if it is not to become a nuisance it must be scientifically dealt with4 ,years ago the drains of ,london simply emptied themselves into the ,thames1 until1 in #ahfd1 two large drains were constructed1 one on each side of1 and approximately parallel with1 the river1 to intercept the old drains and to carry their contents to points many miles down towards the sea4 ,even that1 however1 by no means abated the evil1 for it simply transferred it to a new place4 ,the river was as foul as ever4 ,william ,morris1 in ,news from ,nowhere1 pictures the catching of salmon in the ,thames off ,chelsea1 while one of ,london's prominent citizens1 referring to what was being done in the direction of purifying the river1 jocosely promised the members of ,parliament a little fly-fishing at ,westminster4 ,equally remote1 it is to be feared1 from actual accomplishment1 these two prophecies do certainly indicate the tendency of events1 for science has enabled the authorities to relieve the long-suffering river of much of the pollution which they used to thrust into it4 ,the first great step was the introduction1 in #ahhg1 of a treatment in principle very like that just described for softening water4 ,the liquid from the drains is gathered into large reservoirs1 where chemicals are added to it1 causing the heavier matter to be precipitated in the form known as sludge4 ,the liquid portion1 or effluent1 as it is called1 which is left is discharged into the river just as the tide is ebbing1 so that it is carried right away1 and1 being comparatively inoffensive1 it pollutes the river very little indeed4 ,the sludge1 on the other hand1 is pumped into special steamers1 which carry it down to a certain spot off the ,thames ,estuary1 where they drop it into the sea4 ,the currents at the particular spot chosen are such that none of it returns to the river4 ,for a similar purpose electrolysis has been employed4 ,in this process the sewage is made to flow between two iron plates which are connected up to a source of electric current so that they form electrodes1 while the sewage is the electrolyte4 ,the current decomposes the liquid sewage1 causing chlorine and oxygen to be deposited upon that plate which forms the anode4 ,this deodorises and purifies the sewage1 in addition to which iron salts are formed on the iron plates1 the effect of which is to precipitate the solid particles4 ,thus the same result is achieved as when chemicals are used1 the main difference being that instead of chemicals being added1 they are produced by the passage of the current4 ,but1 from the scientific point of view1 the most interesting process of all is that in which bacteria or microbes are brought into the service4 ,the fact is familiar to most people that there are certain minute organisms which cause terrible diseases4 ,it is not so well known that there are still more of them whose action is extremely beneficent4 ,the writer has seen these minute living things described in a popular book as insects1 but they really belong to a low order of plant life1 and1 as has been said in an earlier chapter1 in spite of the lowliness of their status in the order of creation1 they are able to accomplish certain chemical processes which baffle the cleverest men4 ,they are particularly good1 or some of them are at any rate1 at forming compounds in which nitrogen forms a part4 ,further1 they can be divided into two classes1 the aerobic and the anaerobic4 ,the former work best in air1 while the latter need an absence of air while they perform their functions4 ,after which preliminary explanation we can proceed to describe how they are induced to carry on this valuable work for mankind4 ,the sewage flows first of all into a tank from which light and air are excluded as far as possible4 ,there the anaerobic microbes flourish and multiply1 and in the course of their life work they convert the sewage into an inoffensive liquid4 ,after an appropriate interval the liquid passes to filter-beds1 where it trickles over and through beds of coke1 the effect of which is to aerate it very thoroughly1 whereby the aerobic microbes come into action1 completing the good work1 so that nothing is left except a clean1 colourless and odourless liquid4 ,indeed it is more than that1 for the microbes have turned the offensive matter into nitrogenous compounds which1 as we have seen in a previous chapter1 are the best fertilisers4 ,hence this effluent1 if placed upon the soil1 is of great value4 ,the advantage of this to towns which are not blessed1 like ,london1 with a broad river and the sea near at hand needs no explanation4 ,the bacteria necessary to carry on the process are always present in sewage1 and after any particular plant has been in operation for a little while there results an accumulation of them1 so that the process becomes more and more active as time goes on4 ,mechanical ingenuity has so arranged matters that a sewage disposal plant on this system can be made quite automatic1 requiring little or no attention for months together1 the raw sewage flowing in at one end1 while the odourless1 harmless effluent pours out at the other4 ,and1 moreover1 so powerful is the action of these beneficent bacteria that should disease germs come down in the sewage they soon destroy them4 ,no chemicals are needed1 for the bacteria replenish themselves4 ,no sludge is left1 everything being turned into the harmless effluent4 ,and1 it may be said once more1 disease germs are destroyed4 ,of all the valuable inventions of modern times this is surely not one of the least4 ,,chapter ,,xix ,,modern ,,artillery ,even as late as the time of the ,crimean ,war guns1 even the largest1 were made of that extremely common material1 cast-iron4 ,in fact1 so far as material went1 there was no difference between a gun and a water-pipe4 ,it was the need for some material possessing strength comparable with that of steel combined with the ease of production of cast-iron which led ,sir ,henry ,bessemer to experiment in the manufacture of steel4 ,out of those experiments came ,bessemer steel and its near relative1 ,siemens steel1 two materials of universal application at the present time1 so that to the needs of the artilleryman we owe two inventions which have proved of infinite value in peace as well as in war4 ,if any particular piece of ordnance can be said to be the prime favourite with the ,english-speaking peoples1 it is the big naval gun4 ,with both ,british and ,americans the navy takes pride of place2 both nations are given to contemplating with pleasure the number of dreadnoughts which they possess1 and the distinguishing feature of a dreadnought is the large number of big guns which it carries4 ,of the latest of these gigantic weapons one may not speak1 but much is already public property concerning the #ab-;inch gun which the original ,dreadnought carried1 and which is probably followed in its general features by the still greater guns of the most recent ships4 ,a gun is spoken of by its calibre1 which means the inside diameter1 or1 to use another expression1 the size of the bore4 ,so the #ab-;inch naval gun is #ab inches in the bore4 ,its length is in some cases #de calibres and in others #ej calibres4 ,in other words1 some are #de feet long and others #ej feet4 ,why the difference8 someone may ask4 ,the answer is that the longer ones are an improved type4 ,the extra length gives longer range and harder hits1 as is quite apparent after a little thought4 ,the explosive goes off and forthwith commences to drive the shell towards the muzzle4 ,so long as it is in the gun the shell is being pushed faster and faster1 but so soon as it leaves the muzzle the pushing ceases and the shell is left to pursue its course with its own momentum4 ,therefore1 generally speaking1 one may say that the longer the gun the faster will be the speed of the shell as it leaves the muzzle1 the farther will it go and the harder will be the blow at a given range4 ,incidentally this explanation reveals the need for different kinds of explosive4 ,the propellant whose function it is to drive the shell out of the gun is different from that with which the shell is itself filled4 ,the former needs to act comparatively slowly1 so that it may continue its pushing action during the whole time that the shell is travelling along the gun4 ,it might be ever so powerful1 but were its action too sudden it would simply tend to burst the gun1 without imparting very much speed to the shell4 ,on arrival at its destination1 however1 the shell needs to burst suddenly and violently4 ,another interesting question arises at this point4 ,seeing how fast is even the slowest speed at which a projectile travels1 how can it be possible to measure the rate at which a shell issues from one of these monster guns4 ,needless to say1 it is electricity which makes a thing apparently so difficult really quite easy4 ,near the gun is set up a frame with a wire zigzagging to and fro across it1 in such a manner that when the gun is fired the shell is bound to cut the wire4 ,electric current is made to pass through this wire on its way to a suitable house in which are recording instruments1 where it energises a magnet and so holds something up4 ,now it is easy to see that as soon as the shell cuts the wire the current will stop1 the magnet will let go and the something will drop4 ,at a certain distance farther on there is a second frame with wires upon it1 through which passes a second current1 which is also led to the instrument house1 where it again operates a second magnet4 ,when the first magnet releases its hold it drops something1 to wit1 a long lead weight4 ,when the second magnet lets go it permits a second weight to fall against the first and make a dent or scratch upon it4 ,the longer the interval between the action of the two magnets the higher up upon the lead weight will the scratch be4 ,the apparatus1 in short1 will register the distance fallen through by the lead weight between the breaking of the wire in the first frame and the breaking of the wire in the second frame4 ,now a falling object1 if only it has such weight that the resistance of the air is negligible1 falls according to a well-understood law1 which law it obeys with the utmost accuracy4 ,therefore the distance fallen by the weight between the passage of the shell through two points gives a very accurate record of the time taken to travel from one to the other4 ,of course several such frames can be used if desired in the same way4 ,but to return to the gun itself4 ,it is not merely one piece of metal but several tubes beautifully fitted one inside another4 ,moreover1 in the ,british gun at all events1 between two of the tubes there is a space filled with wire4 ,this wire is perhaps better described as steel tape1 and is of the finest material for the purpose1 flexible and tremendously strong4 ,it is wound round and round one of the tubes until there are many miles of it on a single gun4 ,it is wound tightly1 too1 by means of special machinery4 ,the purpose of the wire is to resist cracking4 ,the solid steel tubes may crack1 and1 as is the way with all cracks1 these will tend to grow longer and longer4 ,the many turns of wire1 however1 will not crack4 ,even if a few turns should break1 the damage will not spread1 and the gun can probably go on as if nothing had happened4 ,the material of which these guns are made is nickel chrome gun steel4 ,steel is ordinarily an alloy of iron and carbon1 but this metal also contains traces of nickel and chromium1 which make it specially suitable for its special purpose4 ,each of the tubes of which the gun is formed start as an ingot1 a mere lump of metal1 but roughly shaped4 ,the requisite mixture is obtained in a furnace and the molten metal is run out into a mould4 ,the ingot is heated again and pressed under enormous hydraulic presses until it is approximately the shape required4 ,this pressing not only produces the desired shape1 it also improves the quality of the metal4 ,the rough forging is then bored out1 to make it into a tube4 ,one is inclined to wonder why the ingot is not cast hollow to commence with1 and so save the labour of boring it all out later4 ,the explanation of this is that certain impurities are always present in the metal and these always gather together in the part which sets last4 ,now in a solid block or ingot it is clear that the centre is the part which will set last1 and hence that is the part where the impurities will congregate4 ,then1 when the centre part is all bored out the impurities are entirely removed4 ,the tube is shaped externally by being turned in a lathe4 ,the innermost tube is not simply smooth4 ,there is a spiral groove1 called the rifling1 running round and round1 screw fashion1 inside it4 ,the purpose of this is to give the shell a spinning action which causes it to keep point foremost throughout its flight4 ,but for this the shell would tend to turn over and over1 resulting in uncertain and inaccurate flight4 ,the shell is a little smaller than the bore of the gun1 but near its base it has an encircling band of soft copper1 which band is a tight fit in the gun4 ,the soft copper crushes into the rifling1 whereby the shell obtains its spinning action4 ,the large guns are mounted in pairs1 each pair on a turntable1 by the movement of which to right or left they are trained upon the distant target4 ,the turntable is surrounded by a wall of thick armour and is covered by an iron hood or roof4 ,in addition to being turnable to right or left1 there is1 of course1 provision for raising or depressing the direction in which each gun is pointing4 ,they need always to point more or less upwards1 and the particular angle depends upon the range or distance of the object aimed at4 ,this is ascertained by range-finding instruments and communicated to the officers in the turrets1 as the covered turntables are called4 ,the guns are then elevated or depressed to suit the range4 ,each gun rests upon a cradle which is itself fitted upon a slide4 ,when it is fired it kicks backwards1 against the force of a buffer of springs1 or a hydraulic or pneumatic cylinder4 ,thus after each shot the gun moves backwards upon the slide1 but the hydraulic apparatus brings it back again into position for firing almost instantaneously4 ,in naval guns all the movements1 including that of the turntable1 are by power1 either hydraulic or electric1 or a combination of the two4 ,the loading is also by power4 ,the shells and ammunition are kept well down towards the bottom of the ship1 under each turret4 ,lifts bring them up from there to a chamber just beneath the turntable1 known as the working chamber4 ,here a small quantity only is kept1 and that for as short a time as possible before it is sent up by other hoists straight to the guns themselves4 ,the hoists are so arranged that1 no matter how they may be elevated or depressed1 the ammunition is delivered exactly opposite the breech1 as the rear end of a gun is termed4 ,then a mechanical rammer pushes it straight in4 ,illustration3 ,,rifles ,,of ,,different ,,nations 7,see ,appendix7 ,the breech of the gun is closed by a beautiful piece of mechanism called the breech-block4 ,it is really a huge plug which securely closes the end of the gun1 a partial turn after it is in place fixing it firmly enough to resist all the force of the explosion4 ,yet it can be freed and swung back upon hinges in a few seconds4 ,at the same moment that it is opened a jet of air blows into the gun1 clearing out all effects of the recent explosion4 ,the process of firing one of these guns may thus be summarised4 ,the turntable is swivelled to right or left until the gunners1 looking through the sights1 which are really telescopes1 see the object straight in front of them4 ,meanwhile the sights have been set according to the range--that is to say1 they have been so set in relation to the gun itself that when they point directly at the target the gun will be pointed upwards at exactly the right angle for that range4 ,the whole thing1 therefore1 gun and sights combined1 is tilted upwards or downwards as may be necessary until the sights point directly at the object aimed at4 ,then at a signal the gun is fired by electricity4 ,the shock causes the gun to slide backwards upon its supporting slide1 but the buffers1 having taken the shock automatically1 return it to its position again2 the aim is thus undisturbed and it is ready for the next shot4 ,these enormous guns can be fired at the rate of one shot every fifteen seconds4 ,field guns are in principle very similar to these1 only1 of course1 they are much smaller and are mounted upon carriages1 so that they can be quickly moved from place to place4 ,it must be borne in mind1 however1 that there are in the case of land guns two distinct types4 ,field guns1 like naval guns1 fire straight at their target2 howitzers or mortars fire upwards with a view to letting the shell fall on the target from above4 ,the latter are1 generally speaking1 short1 fat1 stumpy guns1 as compared with the long1 slender field guns4 ,in the field all guns have to be loaded by hand4 ,the elaborate system of hoists which enables the great naval guns to be loaded with such rapidity is obviously impossible4 ,that has to be compensated for by the skill and quickness of the gunners themselves1 and it is indeed astonishing to see with what deftness they can handle the heavy and dangerous projectiles4 ,with all guns1 of whatever kind1 range-finding is of the utmost importance4 ,no projectile1 however fast it may travel1 really moves in a straight line4 ,it must be fired more or less upwards in order to compensate for the downward pull of gravity4 ,if the elevation be insufficient the shell will fall short2 if it be too much it may go beyond the mark1 or it may fall short1 according to circumstances4 ,just the right elevation is absolutely essential for good shooting4 ,and for that to be achieved the range must be known with the utmost possible accuracy4 ,there are various systems and instruments used for this purpose1 but all depend upon the same principle4 ,it is the principle underlying all surveying and all astronomy2 indeed it is the only possible principle for measuring a distance when you cannot actually go and lay a measure upon it or by it4 ,it is based upon a peculiar property of a triangle4 ,in the case of every triangle which has straight sides1 if we know the size of two of the angles and the length of one of the sides we can easily calculate all that there is to be known about that triangle4 ,we unconsciously use the principle when we judge a distance with our eyes4 ,we focus each eye separately upon the object which we are looking at4 ,in other words1 each of our eyes looks along a straight line terminating in the object4 ,those two lines1 together with a line joining our two eyes1 form a triangle4 ,the line between our eyes is the base1 the line of which we know the length1 while the directions in which we point our eyes give us the angles at each end of the base4 ,from this we are able to judge the distance of the object4 ,of course there is probably not one of us who knows the length of that natural base in inches1 but that does not matter in this case1 since it is always the same whatever we may look at1 and so the mere inclination of the eyes gives us a means of comparing distances4 ,when we judge by the eye alone1 what we really do is to draw upon our experience and consciously or unconsciously compare the distance which we are estimating with some others which we already know4 ,in surveying1 a telescope is set up at one end of a base-line and pointed first at the other end of the base-line and then at the distant object4 ,a scale with which the instrument is provided gives us the size of the angle between the two4 ,then the same thing is done at the other end of the base and the similar angle there is obtained4 ,the length of the base being known1 the distance of the remote object can then be calculated4 ,in the same way two observations can be made1 one at each end of a ship1 the length of the ship forming the base-line4 ,or an instrument can be made by which two observations can be made simultaneously by the same man4 ,this is done by means of mirrors which are turned so that the same object is seen in both of them1 apparently in a straight line4 ,the extent to which one of them has to be turned gives the angle1 and the instrument forms the base4 ,anyone with the slightest geometrical experience will perceive at once that the best results are obtained when the base-line is of considerable length1 and hence small portable range-finding instruments such as can be easily carried and used by one man are necessarily less accurate than an arrangement such as has been suggested above1 where two observers work simultaneously from the two ends of a ship4 ,in many cases1 however1 the self-contained instrument is the only one which it is possible to use1 and when the instrument is well made and in experienced hands the results are surprisingly good4 ,as used in surveying1 for example1 where the base-line may be anything1 according to circumstances1 and the angles may likewise vary at both ends1 elaborate trigonometrical calculations have to be performed to arrive at the desired result4 ,if1 however1 the base-line be always the same1 and one of the angles be always a right angle1 the distance of the distant object will vary with the remaining angle4 ,indeed the scale by which that angle is measured can be made to give not degrees1 but the distance of the object4 ,portable range-finders1 therefore1 in many cases have one reflector set for a right angle and only one of the reflectors movable4 ,the instrument then shows the distance of the object at a glance4 ,this is impossible in the case of two separate observations on a ship4 ,in that case the base is always the same1 but since the ship cannot be set at right angles to the object whenever a range has to be found1 both angles have to be measured4 ,there is1 however1 a beautifully simple little mechanism in which two pointers are set one to each of the two angles1 and the distance is then shown instantly4 ,,appendix ,a ,,description ,,of ,,the ,,rifles ,,shown ,,at ,,page #bdj ,,the ,,german ,,mauser can fire forty rounds a minute--more than any other rifle used in the war4 ,the rifle is of the #ahih pattern1 weighs #i lb4 #ad oz4 with bayonet fixed1 and is sighted from #bai to #bahg yards4 ,the magazine holds five cartridges1 packed in chargers4 ,as the rifle is not provided with a cut-off1 it cannot be used as a single-loader4 ,with its long barrel and long bayonet it gives a stabbing length of #e ft4 #i in4--#h in4 longer than the ,british4 ,,the ,,austrian ,,rifle is the ,mannlicher4 ,this rifle is very fast in action as a snap back and forth of the wrist is sufficient to operate it4 ,it is1 however1 more trying for prolonged work1 owing to the throwing of the strain only on the wrist4 ,without the bayonet the rifle weighs only #h lb4 #e oz41 the lightest of all1 yet the bullet--#bdd grains--is the heaviest used by any of the belligerents4 ,the rifle is sighted from #daj to #bacb yards1 and the barrel has a four-groove rifling4 ,,the ,,british ,,lee-,,enfield--,,mark ,,iii--is the outcome of the ,south ,african ,war4 ,it is not too long for horseback and is yet quite efficient for infantry4 ,the barrel is #be in4 long and has five grooves in the rifling4 ,it is sighted from #bjj to #bhjj yards4 ,the rifle is fitted with a magazine which holds ten cartridges packed in chargers1 each of which contains five rounds1 so that the magazine is filled with ten rounds in two motions4 ,the rifle is also fitted with a cut-off1 which enables it to be used as a single-loader4 ,it is altogether a most efficient weapon4 ,,the ,,french ,,lebel is of the #ahhf-ahic pattern1 and with bayonet fixed is longer than any other rifle4 ,it weighs1 without bayonet1 #i lb4 #c-a_/b oz4 ,the tube magazine under the barrel contains eight cartridges2 it takes1 of course1 longer to charge than a magazine loaded with a charger4 ,it does not fire as many shots a minute as some of the other rifles in the field4 ,the position of the magazine is indicated by the crosses4 ,the rifle is sighted from #bgc to #bahg yards1 and the bullet weighs #aih grains4 ,,the ,,belgian ,,army uses the #ahhi pattern ,mauser1 which weighs just over #h lb4 and is sighted from #edg to #bahg yards4 ,the magazine holds five cartridges carried in clips2 not having a cut-off1 the rifle cannot be used as a single-loader4 ,it has four grooves in its rifling and measures #d ft4 #b-a_/d in41 or1 with the bayonet1 #d ft4 #aa-c_/d in4 ,the bayonet is short and flat4 ,,the #c ,,line ,,nagant of ,russia is #a_/d lb4 heavier than the ,british rifle and is over #g in4 longer4 ,the triangular bayonet is always fixed and never removed from the rifle4 ,the magazine of the rifle is of the box type and holds five cartridges4 ,the rifle is capable of discharging twenty-four bullets to the minute4 ,a useful feature is the interrupter1 which prevents jamming of two cartridges4 ,,the ,,italian ,,mannlicher-,,carcano is of the #ahia pattern4 ,it weighs1 without bayonet1 just over #h lb4 #f oz4 and measures #ej-c_/d in4 ,the barrel1 #cj-c_/d in4 long1 has a four-groove rifling4 ,the box magazine1 fixed under receiver without cut-off1 holds six cartridges4 ,the magazine holds six rounds1 and the rifle is capable of discharging fifteen rounds a minute4 ,,index ,a ,accumulators or secondary batteries1 #fe ,aerial craft experiments1 #bjb ,aerobic and ,anaerobic bacteria1 #bcd ,afterdamp1 #bbh ,alcohol as a fuel1 #di ,alternating current1 #ce1 #aic ,altofts1 artificial coal mine at1 #aci ,aluminium1 #acc ,amalgam1 #aag ,ammeters1 #be ,ammonia in making ice1 #gb ,ammunition for big guns1 #bdj ,amperes1 #bb1 #bd ,analysis and synthesis1 #dc ,anode1 #ee ,anschutz1 ,dr1 #if ,antennae1 #afb1 #aga ,anthracene oil1 #dh ,arc1 the1 in wireless1 #afe ,argon1 the gas1 #ge ,artesian wells1 #de ,atmosphere1 a unit of measure1 #gb ,atoms1 #ef ,avogadro's ,constant1 #cc ,b ,bacteria1 beneficent1 #bcd ,ball mill1 the1 #aae ,battery1 electrical1 #bc ,benzine1 #de1 #dh ,bessemer1 ,sir ,h41 #bcf ,blowpipe1 oxyhydrogen1 #abj ,board of ,trade ,unit1 the1 #bb ,boiling water1 #aj1 #gf ,bore of a gun1 #bcf ,boulders1 blasting1 #bj ,branly1 #aff ,brattice cloth1 #bbd ,breech of a big gun1 #bdj ,brennan torpedo1 the1 #ajb ,brewing1 #ej ,brine in machine-made cold1 #gj ,budding of yeast1 the1 #ea ,c ,calibre of a gun1 #bcf ,capacity1 #aec ,capacity and inductance1 electrical properties1 #afa ,carbolic oil1 #dh ,carbon1 #aa ,carbonic acid gas1 #aj ,carburetter1 the1 #df ,cardiograms1 #cb ,caselli1 #agf ,cathode1 #ee ,cavendish1 investigations of1 #gc ,cellulose1 #ab1 #dd ,centrifugal tendency1 #aae ,character of a lighthouse1 #hf ,charge and current1 #cb ,cheddite1 #ac ,chemicals in waterworks1 #bcb ,chemistry1 organic and inorganic1 #db ,chlorate of potash1 #ab ,chloride of soda1 #eh ,chronograph1 the1 #ada ,clark's ,cell1 #bc ,coal and oil1 #dg ,coal1 burnt1 #aj ,coal-dust an explosive1 #aj ,coal-dust1 explosions from1 #aci ,coal-pitch1 #dh ,coal-tar1 #dh ,coasting lights1 #hj ,coherer1 the1 #ajc1 #afb1 #afg ,coke in smelting1 #abe ,colliery explosions1 #acg ,colliery explosions1 rescue apparatus1 #bbf ,colours of the spectrum1 #bac ,colours of flowers1 #bac ,compass1 a ship's1 #ia ,compressed air in torpedoes1 #ajj ,concentrates1 #aae ,condensers in wireless1 #afc ,conservation of energy1 #acb ,contact makers1 #ade ,coronium1 the gas1 #gd ,corundum1 #acd ,coulombs1 #bc ,courrieres colliery disaster1 #bba ,creosote1 #dh ,creosote oil1 #dh ,crooks1 ,sir ,w41 #cc ,crushing mills1 #aae ,crystal detectors1 #aga ,curie1 ,m4 and ,mme41 #cc ,curtis and ,harvey1 #i ,cyanide process1 the1 #aah ,cyanogen1 #aah ,cymogene1 #de ,d ,detectors1 #afg ,detonator1 the1 #ad ,dextro-glucose1 #ea ,diamonds1 #ace ,diesel engines1 #df ,direct-current electric motor1 #aia ,dirt-auger1 the1 #ae ,ditches1 blasting1 #ah ,drainage1 #bcc ,du ,pont ,powder ,company1 #i ,duddell1 ,w4 ,h41 #cg ,dufay dioptichrome process1 #bai ,dynamite1 what it is1 #i1 #ab2 in agriculture1 #ac2 firing a charge1 #af2 fruit trees1 #af2 marshy ponds1 #ag2 ditches1 #ah2 tree stumps1 #ai2 boulders1 #ai2 wells1 #bj ,dynamo1 the1 #fe ,e ,eddystone ,lighthouse1 #hj ,edison's accumulator1 #ff ,einthoven1 ,prof41 #cj ,electric arc1 the1 #abc ,electric furnace1 #abe ,electric fuse1 the1 #af ,electrical ,inertia1 #aec ,electrical battery1 #bc2 pressure1 #bc2 cells1 #bc2 measure1 #bd2 magnetism1 #be ,electricity1 #bb2 the current1 #ef2 electro-plating1 #eh2 purification of metals1 #fa2 secondary batteries1 #fb ,electrode1 #ee ,electrolysis1 #ee1 #agj2 in drainage1 #bcd ,electrolyte1 #ee ,electrometer1 the1 #cb1 #cd ,electro-plating1 #eh ,electros1 #fj ,electroscope1 the1 #cd ,endosperm1 the1 #ej ,engines driven by oil fuel1 #df ,enzymes1 #ej ,ether1 #de1 #adi ,ethyl alcohol1 #di ,explosions1 #i2 in mines1 #acg ,explosive link1 the1 #ajd ,explosives for guns1 #bcg ,f ,falls in a coal mine1 #bbc ,fermentation1 #ej ,fessenden1 ,r4 ,a41 #afi ,field guns1 #bda ,filters in waterworks1 #bcb ,fire-damp1 #acg ,firing-pin of torpedo1 #ajb ,flashing lights1 #ha ,fog1 effects of1 #hb ,fog signals1 #hh ,fractional distillation1 #gf ,frequency1 #cf ,frequency meter1 #aic ,friction clutch1 #aie ,frue vanner1 the1 #aaf ,fruit trees and dynamite1 #af ,fuses1 firing1 #bj ,g ,galvanometer1 the1 #bg1 #agj ,gangue1 the1 #aab ,gauges1 #bjh ,gelignite1 #ab ,glycerine in explosives1 #aa ,gold1 #aaj ,guiding lights1 #ha ,gyroscope1 the1 #ic1 #ajj ,h ,half-tone illustrations1 #aha ,hard-pan1 #ad ,harris1 ,sir ,w4 ,s41 #cf ,hawke and ,olympic1 collision between1 #aih ,head of the torpedo1 #ii ,heat and electricity1 #cg ,heat of the electric arc1 #abc ,heat1 testing by1 #bje ,helium1 #cc1 #ge ,hertz1 #aed ,howitzers1 #bda ,hughes1 ,prof41 #aei ,humphrey ,gas ,pump1 #bca ,hydraulicing1 #aab ,hydro-carbons1 #de ,hydrogen1 liquid1 #gc ,hydrometer1 the1 #fe ,hydrostatic valve of torpedo1 #aja ,hyper-radial apparatus1 #hh ,i ,ice1 machine-made1 #ga ,indigo1 synthetic1 #dd ,inductance1 #aed ,induction coil for wireless1 #afb ,induction furnaces1 #abi ,insulating ink1 #agg ,interference of light waves1 #aei ,ionisation of the atmosphere1 #agb ,iron1 #aji ,j ,jupiter's moons1 #aej ,k ,kelvin1 ,lord1 #bh ,kerosene1 #df ,kieselguhr1 #ab ,kilowatt1 the1 #be ,kinematograph in coal mine experiments1 #adf ,korn1 ,prof41 #ahc ,krypton1 the gas1 #ge ,l ,leclanche cell1 the1 #bc ,leyden jar1 the1 #aec ,light1 speed of1 #aea ,light waves1 #aea ,lighthouse1 the1 #gh ,lighthouse lamp1 the1 #hc ,limit gauges1 #bji ,liquid air1 #gc ,lodge1 ,sir ,o41 #aei1 #afa ,lumiere autochrome process1 #baf ,m ,magnetic detector1 the first1 #afh ,magnetic pole1 the1 #ij ,magnetism1 #be ,magnets1 #be ,making light1 the1 #gi ,maltster1 the1 #ej ,mansfield ,rescue ,station1 the1 #bbd ,marconi1 #afa ,marshy ponds1 to remove by dynamite1 #ag ,mash tun1 the1 #ej ,master compass1 the1 #ig ,master records1 #fj ,maxwell1 ,j4 ,c41 #aeb ,measuring by electrolysis1 #fb ,mendeluff's table1 #gd ,mercury1 #aad ,metallographic testing1 #bje ,metals1 testing1 #bjd ,methane gas1 #aj1 #abd ,methyl alcohol1 #di1 #ec ,microbes1 their use1 #dc ,mine-laying1 #aje ,mine-sweeping1 #ajg ,molecules1 #ef ,morris1 ,william1 #bcc ,mud1 gold from1 #abb ,muirhead1 ,dr1 #afg ,murette or pedestal of lighthouse lamp1 #he ,n ,naphtha1 #de ,national ,physical ,laboratory1 #aii ,natural frequency1 #afa ,neon1 the gas1 #ge ,nickel chrome gun steel1 #bci ,nitric acid1 #aa ,nitro-cotton1 #ab ,nitro-glycerine1 #aa ,nitrogen gas1 #i ,nobel1 inventor of dynamite1 #ab1 #ace ,nodes1 #aeg ,o ,ohm1 the1 #bb1 #bd ,ohmmeter1 the1 #bg ,ohm's law1 #bg ,oil1 mineral1 #dd ,oil-producing countries1 #dg ,optical apparatus of lighthouse1 #hf ,orders of lighthouse apparatus1 #hh ,ores1 #aaj ,orthochromatic plates1 #bab ,oscillations1 electrical1 #cf ,oscillatory circuit1 #aed ,oscillograph1 ,duddell's1 #ci ,oxide of iron1 #acc ,oxyacetylene flame1 the1 #aca ,oxygen gas1 #aj ,oxyhydrogen jet1 #acj ,p ,paraffin wax1 #de ,patents1 #agd ,periodicity1 #cf ,personal equation1 the1 #bjg ,petrol1 #de1 #eb ,petroleum1 #dd ,phonograph1 the1 #fj ,plans of a ship1 #aii ,plates of the secondary battery1 #fd ,platinum1 #ahd ,plumbago in plating1 #ei ,poulsen arc1 the1 #agc ,poulsen1 ,valdemar1 #afe ,pressure gauges1 #adc ,priestly1 investigations of1 #gc ,primary colours1 #bac ,prisms1 reflection of1 #he ,process blocks1 #ahf ,projectiles1 velocity of1 #bcg ,propellers of the torpedo1 #ii ,propellers1 testing aerial1 #bjc ,prout's anonymous essay1 #gd ,prussiate of potash1 #agg ,purification of metals1 #fb ,q ,quadrant electrometer1 the1 #ce ,quartz1 #aac2 fibre1 #ca1 #aca ,r ,radium1 #cc ,ramsey1 ,sir ,w41 #ge ,range-finding1 #bdj1 #bdb ,rayleigh1 ,lord1 #gd ,receiving instruments for wireless1 #afb ,record vanner1 the1 #aaf ,rectifier1 the1 #cg1 #aga ,red rays of light1 #hb ,reflection by prisms1 #hd ,reflectors1 lighthouse1 #hd ,reiss electrical thermometer1 #cf ,repeated-impact testing machine1 #bjd ,rescue teams for colliery accidents1 #bba1 #bbb ,resistance welding1 #abf ,resonance1 an experiment1 #afj ,reviving apparatus for coal mines1 #bbi ,rheostat1 the1 #ahh1 #aia ,rhigolene1 #de ,rifling of a gun1 #bci ,rubber1 synthetic1 #dd ,rubies1 artificial1 #aca ,rudders of a torpedo1 #ajj ,rutherford1 ,prof41 #cc1 #afh ,s ,saccharine1 #dh ,saltpetre1 #ab ,schwartzkopff torpedo1 the1 #ii ,scilly ,island lighthouse1 #hj ,sea1 gold in the1 #abj ,secondary battery1 the1 #fb ,sectors1 #ha ,selenium1 #ahd ,self-rescue apparatus1 a1 #bbh ,shale1 oil from1 #de ,shells for guns1 #bci ,ships1 testing by models1 #bjj ,short circuit1 #agi ,shunt1 the1 #afe ,sighting a big gun1 #bda ,silica1 #acc ,skating rinks1 ice in1 #ga ,sludge and effluent of drainage1 #bcc ,spark detectors1 #aff ,spark-gap1 #afb ,spectrum1 the1 #bac ,spinthariscopes1 #cc ,spirits1 #eb ,springs1 testing1 #bjc ,stamps for crushing quartz1 #aac ,starch grains in colour photography1 #bag ,step-down and step-up transformers1 #abg ,string galvanometer1 the1 #cj ,submarine mines1 #ajd ,submarine telephone1 #hh ,sulphuric acid1 #aa1 #dc ,sunlight1 composition of1 #bac ,synchronism1 difficulties of1 #ahb1 #aia 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