Manchester Memoirs, Vol. Ixv. (1921), No. 3 13 



engine indicator and determined the value of the horse-power 

 as 33,000 ft. lbs. per minute. Certain methods of improving 

 economy were evident, and engineers were busily engaged in 

 testing these modes by the slow but sure inductive methods 

 of invention, design, construction and operation in practical 

 work; but in this progress up to the time of Joule they had 

 but little aid from pure science. 



The science of thermodynamics did not yet exist. To 

 Joule belongs the credit of bringing it into full existence in 

 a form capable of use by the engineer. Thus, in a paper 

 published in 1843, he states : — 



" In the case of the steam engine, by ascertaining the 

 quantity of heat produced by the combustion of coal, we 

 find out how much of it is converted into mechanical power, 

 and thus come to the conclusion as to how far the steam 

 engine is susceptible of further improvement. Calculations 

 made upon this principle have shown that at least ten times 

 as much power might be produced as is now obtained by 

 the combustion of coal." 



At this date, 1843, Joule had fully proved that mechanical 

 work produced heat in a fixed equivalent quantity, and he 

 here assumed that the reverse process may be also followed, 

 by which heat in the same proportion can be transformed into 

 mechanical work. Here he did not appreciate the limitations 

 imposed by nature on the reversed process, and later he 

 recognised that although any heat which disappears in per- 

 forming mechanical work does so in the Joule equivalent 

 proportion ; yet under conditions of limitation of temperature 

 range only a fraction of the total heat can be so converted, 

 the remaining part being passed by conduction to the bodv 

 at lower temperature. 



The ideas of Joule, Macquorn Rankine, Clausius, and 

 Thomson now form so much of the basis of all reasoning 

 upon motive power engines that there is some little danger 

 to the present generation of forgetting what they owe to this 

 great group of distinguished men, but principally to Joule. 

 To appreciate the great advance made it is desirable to con- 

 sider the position of motive power produced by heat at the 

 middle of the last century. At that time many attempts had 

 been made to displace the steam engine by various forms of 

 air engines. The knowledge of engineers at that time mav 

 be found by the consideration of papers read at the Institution 

 of Civil Engineers in the years 1845 an d 1853 and the discus- 



