564 POPULAR LECTURES AND ADDRESSES. 



investigation on the surface-condensation of steam 

 with the assistance of a grant from the Royal 

 Society for the purpose. The results of this very 

 elaborate investigation, communicated to the 

 Royal Society on December 13, 1860, and 

 published in the PJiilosophical Transactions, have 

 proved to be of enormous practical importance. 

 They led directly and speedily to the present 

 practical method of surface-condensation which is 

 one of the most valuable improvements of the 

 steam engine, especially for marine use, since the 

 time of Watt. 



But I have not yet touched upon Joule's great 

 fundamental discovery, the discovery which is first 

 in every one's mouth when asked what was Joule's 

 work ? The Mechanical Equivalent of Heat, You 

 must understand that it was not merely by a 

 chance piece of experiment or of guessing that he 

 stumbled on a result which was afterwards found 

 to be of great value. It was measurement, 

 rigorous experiment and observation, and philo- 

 sophic thought all round the field of physical 

 science that made the discovery possible to him. 



