Dynamical Theory of Heat. 383 



"As the experience of the experimenter increased, we find that 

 the coincidence of his results becomes closer. In 1845 Mr. 

 Joule deduced from experiments with water agitated by a paddle- 

 wheel an equivalent of 



890 foot-pounds. 



" Summing up his results in 1845, and taking the mean, he 

 found the equivalent to be 



817 foot-pounds. 



"In 1847 he found the mean of two experiments to give as 

 equivalent 



781*8 foot-pounds. 



"Finally, in 1849, applying all the precautions suggested by 

 seven years' experience, he obtained the following numbers for 

 the mechanical equivalent of heat : — 



772*692 from friction of water, mean of 40 experiments. 

 774-083 „ mercury, „ 50 „ 



774*987 „ cast iron, „ 20 



" For reasons assigned in his paper, Mr. Joule fixes the exact 

 equivalent of heat at 



772 foot-pounds. 

 " According to the method pursued by Mayer in 1842, the me- 

 chanical equivalent of heat is 



77T4 foot-pounds. 



" Such a coincidence relieves the mind of every shade of uncer- 

 tainty regarding the correctness of our present mechanical equi- 

 valent of heat." 



It was in connexion with this resume that I wrote the follow- 

 ing paragraph, which has been already quoted in the Philoso- 

 phical Magazine. 



H Do I refer to these things in order to exalt Mayer at the 

 expense of Joule ? It is far from my intention to do so. The 

 man who through long years, without encouragement, and in the 

 face of difficulties which might well be deemed insurmountable, 

 could work with such unswerving steadfastness of purpose to so 

 triumphant an issue, is safe from depreciation. And it is not 

 the experiments alone, but the spirit which they incorporate, and 

 the applications which their author made of them, that entitle 

 Mr. Joule to a place in the foremost rank of physical philoso- 

 phers. Mayer's labours have in some measure the stamp of a 

 profound intuition, which rose, however, to the energy of un- 

 doubting conviction in the author's mind. Joule's labours, on 

 the contrary, are an experimental demonstration. True to the 

 speculative instinct of his country, Mayer drew large and weighty 



