528 



SCIENCE. 



MECHANICAL SCIENCE* 



By Sir W. Armstrong, C.B., D.C.L., LL. D., F.R.S. 



The astonishing progress which has been made in the 

 construction and application of machinery during the 

 half century which has elapsed since the nativity of the 

 British Association for the Advancement of Science, is a 

 theme which I might with much complacency adopt in 

 this address, but instead of reviewing the past and ex- 

 ulting in our successes, it will be more profitable to look 

 to the future and to dwell on our failures. It is but jus- 

 tice to say that by growing experience, by increasing fa- 

 cilities of manufacture, and by the exercise of much skill 

 and ingenuity, we have succeeded in multiplying and ex- 

 panding the applications of our chief motor, the steam- 

 engine, to an extent that would have appeared incredible 

 fifty years ago ; but the gratulation inspired by this suc- 

 cess is clouded by the reflection that the steam-engine, 

 even in its best form, remains to this day a most waste- 

 ful apparatus for converting the energy of heat into mo- 

 tive power. Our predecessors of that period had not the 

 advantage of the knowledge which we possess of the 

 true nature of hea f , and the conditions and limits affect- 

 ing its utilization. In their time heat was almost uni- 

 versally regarded as a fluid which, under the name of 

 caloric, was supposed to lie dormant in the interstices of 

 matter until forced out by chemical or mechanical means. 

 Although Bacon, Newton, Cavendish, and Boyle all 

 maintained that heat was only internal motion, and al- 

 though Davy and Rurnford not only held that view, but 

 proved its accuracy by experiment, yet the old notion of 

 caloric continued to hold its ground, until in more recent 

 times Joule, Meyer, Codling, and others, put an end to all 

 doubt on the subject, and established the all-important 

 fact that heat is a mode of motion having, like any other 

 kind of motion, its exact equivalent in terms of work. 

 By their reasonings and experiments it has been definitely 

 proved that the quantity of heat which raises the tem- 

 perature of a pound of water i° Fahrenheit, has a me- 

 chanical value equal to lifting 772 lbs. one foot high, and 

 that conversely the descent of that weight from that 

 height is capable of exactly reproducing the heat ex- 

 pended. 



The mechanical theory of heat is now universally ac- 

 cepted, although a remnant of the old doctrine is dis- 

 played in the continued use of the misleading term 

 "latent heat." According to the new theory, heat is an 

 internal motion of molecules capable of being communi- 

 cated from the molecules of one body to those of another, 

 the result of the imparted motion being either an increase 

 of temperature, or the performance of work. The work 

 may be either external, as where heat, in expanding a 

 gas, pushes away a resisting body, or it may be internal, 

 as where heat pulls asunder the cohering particles of ice 

 in the process of liquefaction, or it may be partly internal 

 and partly external, as it is in the steam engine, where the 

 first effect of the heat is to separate the particles of water 

 into vapor, and the second to give motion to the piston. 

 Internal as well as external work may be reconverted into 

 heat, but until the reconversion takes place, the heat 

 which did the work does not exist as heat, and it is delu- 

 sive to call it " latent heat." All heat problems are com- 

 prised under the three leading ideas of internal work, ex- 

 ternal work, and temperature, and no phraseology should 

 be used that conflicts with those ideas. 



The modern theory of heat has thrown new light upon 

 the theory of the steam-engine. W e now know what is the 

 mechanical value in foot pounds of the heat evolved in the 

 combustion of one pound of coal. In practice we can 

 determine how much of that heat is transmitted to the 

 water in the boiler, and we are taught how to calculate 

 the quantity which in the process of vaporisation takes 

 the form of internal work. We can determine how much 

 disappears in the engine in the shape of external work, 



*Britisk Association, 1881. 



including friction, and the remainder.with the exception of 

 the trifling quantity saved in the feed-water, we know to 

 be lost. Taking a good condensing engine as an example, 

 we may roughly say that, dividing the whole heat energy 

 into ten equal parts, two escape by the chimney, one is 

 lost by radiation and friction, six remain unused when the 

 steam is discharged, and only one is realized in useful 

 work. It may be fully admitted that the greater part of 

 the aggregate loss is inevitable ; but are we to suppose 

 that the resources of science, ingenuity, and skill have 

 been exhausted in the attainment of so miserable a re- 

 sult ? Nothing but radical changes can be expected to 

 produce any great mitigation of the present monstrous 

 waste, and without presuming to say what measures are 

 practicable and what are not, I will briefly point out the 

 directions in which amelioration is theoretically possible, 

 and shall afterwards advert to the question whether we 

 may hope to evade the difficulties of the steam-engine 

 by resorting to electrical methods of obtaining power. 



To begin with the loss which takes place in the appli- 

 cation of heat to the boiler ; why is it that we have to 

 throw away, at the very outset of our operations, twice as 

 much heat as we succeed in utilizing in the engine ? The 

 answer is, that in order to force a transmission of heat 

 from the fire to the water in the boiler, a certain excess of 

 temperature over that of the water must exist in the fur- 

 nace and flues, and the whole of the heat below the re- 

 quired excess must pass away unused, except the trifling 

 portion of it which disappears in the production of 

 draught. Further, that since we cannot avoid admitting 

 the nitrogen of the air along with the oxygen, we 

 have to heat a large volume of neutral gas which 

 has no other effect than to rob the fire. Con- 

 sidering what efforts have been made to facilitate the 

 transmission of the heat by augmenting the evaporative 

 surface, and using thin tubes as flues, it is vain to expect 

 any great result from further perseverance in that direc- 

 tion, and unless a method can be devised of burning the 

 fuel inside instead of outside the apparatus, so as to use 

 the heated gases conjointly with the steam as a working 

 medium in the engine, a remedy appears to be hopeless. 

 We already practice internal combustion in the gas-en- 

 gine, and it is clear that with gaseous fuel, at all events, 

 we could associate such a mode of combustion with the 

 vaporization of water. We may even regard a gun as 

 an engine with internally-burnt fuel, and here I may re- 

 mark that the action of heat in a gun is strictly analo- 

 gous to that of heat in a steam-engine. In both cases 

 the heat is evolved from chemical combination, and the 

 resulting pressures differ only in degree. The gun is the 

 equivalent of the cylinder, and the shot of the piston, and 

 the diagrams representing the pressure exerted in the 

 two cases bears a close resemblance to each other. 

 While the powderis burning in the gun we have a nearly 

 uniform pressure, just as we have in the cylinder while 

 the steam is entering, and in both cases the uniform 

 pressure is followed by a diminishing pressure, repre- 

 sented by the usual curve of expansion. If in the steam- 

 engine we allowed the piston to be blown out it would 

 act as a projectile, and if in the gun we oppose mechan- 

 ical resistance to the shot, we might utilize the effect in a 

 quieter form of motive power. But it is a remarkable 

 fact that such is the richness of coal as a store of me- 

 chanical energy that a pound of coal, even as used in the 

 steam-engine, produces a dynamic effect about five times 

 greater than a pound of gunpowder burnt in a gun. I 

 cannot, however, on this account encourage the idea that 

 steam may be advantageously substituted for gunpowder 

 in the practice of gunnery. 



And now to turn from the fire which is the birthplace 

 of the motive energy, let us follow it in the steam, to the 

 condenser, where most of it finds a premature tomb. 

 From the point at which expansion commences in the 

 cylinder the temperature and pressure of the steam begin 

 to run down, and if we could continue to expand indefi- 



