Manchester Memoirs, Vol. Ixv. (1921), No. 3 15 



transmission of heat. Clearly on this view it was logical to 

 expect an indefinitely great economy if heat loss could be 

 suppressed. C. W. Siemens was the only reader and speaker 

 at the 1853 meeting who fully grasped Joule's idea, and he 

 gave a table showing the theoretical performance of different 

 steam and hot air engines referred to the mechanical equiva- 

 lent of 770 foot pounds. He states, however, that a perfect 

 heat engine would give a performance of 770 foot pounds per 

 heat unit, which clearly shows that he had not grasped the 

 Carnot cycle in conjunction with Joule's law. 



This short discussion of the position of the hot air engine 

 is most useful in proving how much practical men were in 

 need of the work of Joule and Thomson. 



It is not surprising that, of all the engineers present, 

 Siemens appeared to be alone in thoroughly grasping the new 

 ideas. Thomson's own conversion from the material theory 

 of heat to the dynamical theory was not complete until 1851, 

 and although he had then succeeded in reconciling the ideas 

 of Joule and Carnot, it is not to be wondered at that engineers 

 two years later had not quite succeeded in grasping the 

 combination of the two laws. This combination, however, 

 supplied engineers with a new and accurate standard of 

 measurement for studying and improving upon their heat 

 engines, and they were by no means slow in grasping the help 

 thus offered them by the abstract scientific man. The broad 

 laws of thermodynamics have placed the theory of the heat 

 engine in a position of certainty, which was much needed. 

 It would be a mistake to assume, however, that even the 

 determination of the mechanical equivalent of heat and the 

 second law of thermodynamics expressed in terms of an 

 absolute thermometric scale had solved all the difficulties of 

 the engineer desiring to determine the efficiency of his heat 

 engines. Joule, Thomson, Rankine, and their great Con- 

 tinental colleagues, it is true, settled once and for all the broad 

 laws of thermodynamics, but the Carnot cycle is a cycle which 

 is, as has been repeatedly shown, an impossible one in prac- 

 tice. Accordingly actual engines have to operate upon 

 imperfect cycles. The theory of these imperfect cycles has 

 been worked out mostly during the last twenty-five years, 

 although Rankine made a beginning in dealing with the theory 

 of the Joule air engine. For the first time he showed the 

 existence of what may be termed a cycle of constant efficiency 

 in the case of the Joule air engine. Assuming constant 

 specific heat for the working fluid, he calculated the efficiency 

 of what we now call a constant-pressure air engine between 



