THE COPLEY MEDALIST OF 1870. 427 



attained in his first experiments. 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 labours of the previous ten years 

 had made Mr. Joule completely master of the conditions 

 essential to accuracy and success. Bringing his ripened 

 experience to bear upon the subject, he executed in 

 1849 a series of 40 experiments on the friction of water, 

 50 experiments on the friction of mercury, and 20 

 experiments on the friction of plates of cast-iron. He 

 deduced from these experiments our present mechanical 

 equivalent of heat, justly recognised all over the world 

 as 'Joule's equivalent.' 



There are labours so great and so pregnant in conse- 

 quences, that they are most highly praised when they 

 are most simply stated. Such are the labours of Mr. 

 Joule. They constitute the experimental foundation of 

 a principle 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. 



1 have omitted all reference to the numerous minor 

 papers with which Mr. Joule has enriched scientific 

 literature. Nor have I alluded to the important inves- 

 tigations which he has conducted jointly with Sir 



