566 POPULAR LECTURES AND ADDRESSES. 



always remembered as one of the most illustrious 

 members of that great family. Carnot's theory 

 gave an important fundamental principle regarding 

 the development of motive power from heat. 

 Joule's work, on the other hand, so far as the 

 mechanical equivalent was concerned, was the 

 generation of heat by mechanical work. It was 

 quite the middle of the century before Carnot's 

 theory began to attract attention ; but Joule was 

 early made acquainted with it, and after fighting 

 a little against it, as differing from his own theory, 

 he of all others took it up in the most hearty 

 manner. I can never forget the British Association 

 at Oxford in the year 1847, when in one of the 

 sections I heard a paper read by a very unassuming 

 young man who betrayed no consciousness in his 

 manner that he had a great idea to unfold. I was 

 tremendously struck with the paper. I at first 

 thought it could not be true because it was different 

 from Carnot's theory, and immediately after the 

 reading of the paper I had a few words of conversa- 

 tion with the author James Joule, which was the 

 beginning of our forty years' acquaintance and 



