THE COPLEY MEDALIST OF 1870 449 



ments. The mean of them was 802 foot-pounds. From 

 experiments with water agitated by a paddle-wheel, he 

 deduced, in 1845, an equivalent of 890 foot-pounds. In 

 1847 he again operated upon water and sperm -oil, agitated 

 them by a paddle-wheel, determined their elevation of 

 temperature, and the mechanical power which produced 

 it. From the one he derived an equivalent of 781-5 foot- 

 pounds ; from the other an equivalent of 782 -1 foot-pounds. 

 The mean of these two very close determinations is 781*8 

 foot-pounds. 



By this time the labors of the previous ten years had 

 made Mr. Joule completely master of the conditions essen- 

 tial to accuracy and success. Bringing his ripened experi- 

 ence to bear upon the subject, he executed, in 1849, a 

 series of 40 experiments on the friction of water, 50 ex- 

 periments on the friction of mercury, and 20 experimentg 

 on the friction of plates of cast iron. He deduced from 

 these experiments our present mechanical equivalent of 

 heat, justly recognized all over the world as "Joule's 

 equivalent. ' ' 



There are labors so great and so pregnant in conse- 

 quences that they are most highly praised when they are 

 most simply stated. Such are the labors of Mr. Joule. 

 They constitute the experimental foundation of a princi- 

 ple of incalculable moment, not only to the practice, but 

 still more to the philosophy of Science. Since the days 

 of Newton, nothing more important than the theory, of 

 which Mr. Joule is the experimental demonstrator, has 

 been enunciated. 



I have omitted all reference to the numerous minor 

 papers with which Mr. Joule has enriched scientific liter- 

 ature. Nor have I alluded to the important investigations 



