206 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[July, 



of the air is maintained at an uniform degree. Now as the piston ad- 

 vances it would speedily condense the air in the tube unless the pump- 

 ing were carried on at a rate corresponding to tlie velocity of the pis- 

 ton's motion. Suppose that in advancing a foot forward it disp aced 

 a cubic foot of air, that quantity of air must have passed through the 

 pumps: had a less quantity been pumped out the rarefaction would 

 be diminished, had a greater quantity been pumped out the r.irefaction 

 would have increased. In other words, in order to the maintenance 

 of an uniform degree of rarefaction after the piston has been set in 

 motion, the quantity of rarefied air pumped out must be equal to the 

 quantity of that air which the piston displaced during that period. 



Now it need scarcely be stated that during tliis state of tlie mechamc 

 operation of the air there is neither gain nor loss of power. That is, 

 while the piston is in motion and the rarefaction uniform, the work 

 done by tlie prime mover is exactly measured by the quantity of mo- 

 tion transferred to the train of carriages; for every cubic foot of air 

 pumped out at the station the travelling piston describes a foot ot 

 space. . , . 



This equivalence then being established, namely the equivalence ot 

 the work of the station engines to the eft'ect produced on the train, U 

 follows necessarilvtlv.it that effect is in no way attributable to the 

 agency of the preliminary exhaustion, and that therefore— and this is 

 the point we had to prove— the preliminary exhaustion before motion 

 contributed nothing to this subsequent motion of the train. 



In order to clearly establish this position, we must show that no use 

 is made subsequently of the elasticity or restituiin force ol the air. For 

 It is by no means necessary that the use of an elastic agent should be 

 aUavs attended with a loss of power; in a steel bow for instance the 

 force exerted in bending the bow is almost exactly measured by the 

 velocity generated in the arrow by the bow while unbending. Here, 

 and indeed in all phenomena of elasticity, there are two distinct ope- 

 rations, first the exUrnal force exerted to make the elastic body change 

 its form, secondly the intirnal force exerted by the body itself in re- 

 covering its form, and this second force is aptly called the Jorce of 

 restitution. In the case of atmospheric power, the first external force 

 is that of the station engines in rarifying or stretching the air. Now 

 ifafter this, the air were allowed to recover its original form by its 

 natural elasticity, " the force of restitution" would be exerted. But 

 in the atmospheric railway the force of restitution never is in fact 

 made use of. After the air is stretched to its utmost it is not allowed 

 to recover its original form, but is pumped out and removed altoge- 

 ther, so that its restitutive powers could never be used, even if it 

 were desired to employ them. As a matter of fact the pumping is 

 cor.tiiiued till the carriages have nearly finished their journey, and 

 then their momentum is destroyed by the external application of 

 breaks. 



So then, neither at the end of the journey nor during the progress 

 of it is the labour of preliminary exhaustion in any way rendered use- 

 ful—it is wholly lost and wasted, and we therefore arrive demonstra- 

 /ire/y at this conclusion: — 



To prccure a working power of ten pounds to the square inch in an 

 atmospheric railway two-thirds of the strokes of the station engines are 

 nailed from a cause independent of leakage and friction. 



12 



For a working power of 12 ;6. the waste is — or FouR-niTHS. 



' 15 ' 



5 



74 



: or ONE-HALF. 



15 



or ONE-THIRD. 



The reader may easily verify these results for himself. 



In conclusion, it must be carefully borne in mind that the loss here 

 demonstrated is one wholly different from the losses by friction or 

 leakage : those are losses ai ising from defects of mechanism, but this 

 13 essentially the immediate effect of the fundamental laws of matter ; 

 and even could an atmospheric railway be constructed wholly free 

 from leakage or friction, no art could abate, avoid, or even increase, 

 this defnct. It is wholly beyond ihe poroer, sagacity, or perseverance of 

 man. Unless we can make the air cease to be extensible — that is, 

 cease to be air ; unless we can subvert the Laws of Nature, we can- 

 not remove the evil. We mav have the most extended notion of the 

 powers of human ingenuity and the future triumphs of science, — but 

 there is one clear distinction respecting fitting olijects for men's inge- 

 nuity, one obvious limit to the triumphs of his scieic^. The distinc- 

 tion is between efforts suggested by the known laws of matter, and 

 efforts to resist those laws : the limit confines man's successful ener- 

 gies to the former of thfse tasks. And no one has thoroughly com- 

 prehended the investigatii n here made unless he be certain that all 

 the mechanical genius which the woild has produced could not, if 



combined, remove from atmospheric railways the defect we have de- 

 monstrated ; no committee report could remove it, no omnipotent act 

 of parliament could remove it, no " monied interest," however vast, 

 could remove it, no fortunate accident could remove it, and lastly, by 

 no stretch of the imagination, by no freak of the fancy however whim- 

 sical and fantastic, can we picture the bare possibility of its removal. 



H. C. 



ON THE MECHANICAL THEORY OF STEAM. 

 Proposed by the " Artizan Club." 

 A Treatise on the Steam Engine has recently been published bv a 

 person or persons callins; himself or themselves the "Artizan Club;" 

 the viork appears in shiding monthly parts, and has now reached its 

 twelfth number. The prosiiectus, after setting forth the defects of all 

 similar works which have hitherto been written, states the object of 

 the present publication in the following terms : — " Engineers are, up 

 to the present time, unprovided with a key to the difficulties of their 

 calling, such as may be found in almost every other department of in- 

 dustry. It is to extinguish this want, and to enable every man of or- 

 dinary intelligence and assiduity, and however humble his means, to 

 become thoroughly acquainted with the Steam Engine, in all its phases, 

 that the present work has been projected ; and it may be satisfactory 

 if a summary be here given of its intended contents." 



In the eleventh number of the Treatise is a chapter on the "Me- 

 chanical Power of Steam," aud as the remarks are invested with a 

 certain authority by the presence ot a quantity of mathematical sym- 

 bols, it seems highly important that the conclusions arrived at should 

 be correct, for with them only can a large proportion of the readers 

 who have not learned the unknnwn language of analysis acquaint 

 themselves. The mechanical effect of the condenser of a steam en- 

 gine is thus detailed — 



Suppose that vviien the steam had raised the piston to 142 feet above the 

 base of the cylinder it were suddenly condensed, it is obvious that the piston 

 would he impelled with a force equal to the pressure of the atmosphere on 

 the piston, and through a heigl.t equal to that the piston had been raised by 

 the generation of the steam. In doing this it is obvious that the piston in 

 its descent would raise a weight attached to it equal to tlie atmospheric 

 pressure, and through a height exactly equal to tlie maximum height of the 

 piston above the base of the cylinder. If a weight be placed upon the piston 

 in addition to the atmospheric pressure, then, as we said formerly, the piston 

 will not rise to such a height, and consequently upon condensation the weight 

 will not De raised so high either ; but to counteract this the weight raised 

 will be greater, and that very nearly in the same proportion. Hence the 

 mechanical power of the steam of a given quantity of water as developed by 

 condensation continues very nearly constant whatever be the force at which 

 it is generated, the ditference being in favour of the greatest pressure. We 

 see from this that the mechanical power of the steam of a given quantity of 

 water is the same, whether it be developed by generation or by momentary 

 condensation, and that it remains very nearly constant whatever be the tem- 

 perature of the steam. Hence we may now state this general fact, which it 

 may be useful to treasure up in the memory. 



" A cubic inch of water converted into steam will supply a mechanical 

 force very nearly equal to a ton weight raised a foot high, whether this force 

 be developed by generation or by rapid condensation ; and this force wdl not 

 be suhject to considerable variation, whatever be the temperature or pressure 

 at which the water may be evaporated j the small difference being in favour 

 of the greatest pressure." 



Now this passage is not a little remarkable, and the conclusion is 

 somewhat startling. We are told that a cubic inch of water will pro- 

 duce exactly the same mechanical tffect by generation as by condensa- 

 tion, and this remark is not confined to sieam of any particular pres- 

 sure, but applies as well to a pressure of sixty pounds to the square 

 inch as to a pressure of three or four. The obvious conclusion would 

 be, as the effect is the same lor both f^eneration and condensation, 

 that if both those actions were applied to an engine the power pro- 

 duced by the same quantity of water wculd be in all cases doubled, — 

 that in a high pressure engine working at a pressure of (JO lb. the ap- 

 plication of a condenser would produce an additional pressure of 601b. 

 to the inch, and that in a low pressure engine working at the common 

 pressure of 3 or 4 pounds the gain would be other 3 or 4 pounds. 

 This conclusion, we repeat, is somewhat startling, and we think never 

 before occurred to anyone who ever saw a steam engine, or even read 

 about one; for it is generally imagined that the variation of atmo- 

 spheric pressure is confined to mucii narrower limits than those here 

 suggisted. The pressure of the air in well constructed condensing 

 engines (such as those of the Government steamers for example), is 

 usually ttkeii at about 12 lb. to the square inch, but it appears from 



