ON THE THEORV OF THE STEAM-ENGINE. .369 



On the Theory of the Steam-Engine. 

 By Professor Kobert H. Thurston. 



[A communication ordered by the General Committee to be printed in extenso among 



the Reports.] 



The following paper is intended to present, in the briefest possible form, 

 an outline of the growth of the theory of the steam-engine, from its 

 first and most primitive form to its most recent and most thoroughly- 

 practical development in application. It is nob proposed to make this 

 sketch in any sense complete, and it is hardly expected that it can be 

 critically accurate. It may, however, prove interesting, and may be of 

 real service, it is hoped, as presenting a listinct outline of what will, 

 when more completely worked up, prove to be an exceedingly interesting 

 and important detail of the history of applied science. 



A complete history of the development of the theory of the steam- 

 engine would include, first, the history of the mecbanical theory of 

 heat ; secondly, the history of the science of thermodynamics, which 

 has been the outgrowth of that theory ; thirdly, the history of the appli- 

 cation of the science of heat transformation to the case of the steam- 

 engine ; and fourthly, an account of the completion of the theory of the 

 steam and other heat-engines by the introduction of the theory of losses 

 by the more or less avoidable forms of waste, as distinguished from those 

 necessary and unavoidable wastes indicated by the pure theory of ther- 

 modynamics. The first and second of these divisions are treated of in 

 works on thermodynamics, and in treatises on physics. The third division 

 is briefly considered, and usually very incompletely, in treatises on the 

 steam-engine ; while the last is of too recent development to be the sub- 

 ject of complete treatment, as yet, in any existing works. The principal 

 object of the present paper is simply to collect into a condensed form, 

 and in proper relations, these several branches of the subject, leaving for 

 another time and place that more full and complete account which might, 

 did opportunity offer, be pi-epared to-day. 



The ' Mechanical Theory of Heat,' as is now well understood, existed, 

 as a speculation, from the days of the earliest philosophies. The contest 

 which raged with such intensity, and sometimes acrimony, among specu- 

 lative men of science during the last century was merely a repetition of 

 struggles of which we find evidences at intervals throughout the whole 

 period of recorded history. The closing period of this, which proved to 

 be an important, revolution in science marked the beginning of the 

 nineteenth century. It was inaugurated by the introduction of ex- 

 perimental investigation directed toward the crucial point of the question 

 at issue. It tei'minated, about the middle of the century, with the 

 acceptance of the general results of such experiment by every scientific 

 man of acknowledged standing on either side the Atlantic. The 

 doctrine that heat was materia), and its transfer a real movement of 

 substance from the source to the receiver of heat, was thus finally com- 

 pletely superseded by the theory, now become an ascertained truth, that 

 heat is a form of energy, and its transformation a change in the location 

 and method of molecular vibration. The dynamical theory of heat was 



