JOULE'S AIR ENGINE. 139 



Thermo-dynamics, obtain greater economy. A description 

 of this invention, with a discussion of its possible economic 

 advantages as shown by the new theory, was read before 

 the Royal Society, June, 185 1, and published in the Phil. 

 Trans., 185 1. 



Joule did not construct an engine or undertake anyexperi- 

 mental investigation connected with it ; but merely described 

 in detail his engine, and gave the mathematical theory of its 

 action, in which he acknowledges the assistance of Sir 

 William Thomson. Under the name of "Joule's Air Engine," 

 the theory of this engine is included in almost all text 

 books on Thermo-dynamics. Joule's air engines have been 

 tried over again, and although, like all hot air engines, have 

 (from incidental causes not to be realized without experience) 

 failed to rival the steam engine in general economy or use- 

 fulness, a number of Joule's engines have been usefully 

 employed for special purposes. The paper is, however, 

 chiefly interesting as showing that in the early stages of 

 the mathematical development of the theory of Thermo- 

 dynamics, Joule followed until he perfectly realized the 

 general significance of the theory as depending on Carnot's 

 theory as well as his own. Beyond this Joule does not 

 appear to have gone. 



From his attempts to start incidental lines of experi- 

 mental research, Joule was diverted in 1852 by the require- 

 ments that became apparent for further experiments to 

 determine the data to complete the application of Thermo- 

 dynamics to gases, and to verify the conclusions arrived at 

 by those who were developing the results of this theory. 

 This led him to undertake a joint investigation with Sir 

 William Thomson, in 1852, which was continued, with 

 interruptions, till 1859. 



