ON THE T1IEOKT Of THE STEAM-ENGINE. 571 



formance of work by expenditure of heat through the action of elastic 

 bodies. 



The development of the science of thermodynamics into available and 

 satisfactory form was effected mainly by Professors Rankine and Clausius, 

 working independently but contemporaneously from 1849. Clausius de- 

 veloped the general theory -with beautiful clearness and conciseness of 

 mathematical method and work, and succeeded in constructing a com- 

 plete system, almost equal in extent and exactness to the geometrical 

 system of Euclid. Rankine, producing the same results, in part, by his 

 wonderfully condensed method of treatment, turned his attention more 

 closely to the application of the theory to the case of the steam and 

 other heat-engines, giving finally, in his • Prime Movers ' (1859), a 

 concise yet full exposition of the correct theory of those motors, so far as 

 it is possible to do so by purely thermodynamic treatment. He was 

 unaware, apparently, as were all the scientific men of his time, of the 

 extent to which the conclusions reached by such treatment of the case 

 are modified, in real engines, by the interference of other physical prin- 

 ciples than those taken cognisance of by his science. Sir William 

 Thomson, partly independently, and partly working with Joule, has 

 added much valuable work to that done by Clausius and Rankine. In 

 the hands of these great men the science took form, and has now 

 assumed its place among the most important of all branches of physical 

 science. 



The theory of the steam-engine, like every other scientific system, rests 

 upon a foundation of facts ascertained by experiment, and of principles 

 determined by the careful study of the laws relating to those facts, and con- 

 trolling phenomena, properly classed together by that science. Like every 

 other element entering into the composition of a scientific system, this theory 

 has been developed subsequently to the establishment of its fundamental 

 facts, and the history of progress in the art to which it relates shows that 

 the art has led the science from the first. The theory of the steam-engine 

 includes all the phenomena and all the principles involved in the produc- 

 tion of power, by means of the steam-engine, from the heat-energy derived 

 from the chemical combination of a combustible with the oxygen of the 

 air acting as a supporter of combustion. The complete theory therefore 

 includes the theory of combustion ; the consideration of the methods of 

 development and transfer, and of losses of heat in the steam boiler ; the 

 examination of the methods of transfer of heat-energy from boiler to 

 engine, and of waste of heat in this transfer ; and, finally, the development 

 of mechanical energy in the engine, and its application, beyond the engine, 

 to the machinery of transmission, with an investigation of the nature and 

 method of waste in this last transformation. It is, however, only the last 

 of these divisions of the subject that it is here proposed to consider. The 

 remaining portion of this paper will be devoted to the tracing of the 

 growth of the theory of the steam-engine, simply as a mechanical instru- 

 ment for transformation of the one form of energy into the other — of the 

 molecular energy of heat motion, as stored in the vapour of water, into 

 mass energy, mechanical energy, as applied to the driving of mechanism. 

 The theory thus limited includes a study of the thermodynamic phenomena, 

 as the principal and essential operations involved in the performance of work 

 by the engine ; it further includes the consideration of the other physi- 

 cal processes which attend this main function of the engine, and which, 



