1 62 Proceedings. 



Hirn considers the great merit of the founders of the 

 mechanical theory of heat to have consisted not so much in 

 the production of a more correct hypothesis, as in placing 

 themselves above hypotheses, and considering the effects not 

 only as equivalent among themselves, but as the measure of 

 the causes whatever they might be. Just as he considers 

 that the great merit of Newton lay in stating that the 

 celestial motions ensue as if the bodies attract each other 

 across space, and in avoiding any explanation, more or less 

 plausible, of the mechanism of this attraction ; adding, that 

 if Newton had presented his equations as the results of a 

 hypothesis of this nature, the mechanical sciences might 

 have been still kept in their cradle. 



"M. Hirn's own experiments on the numerical measure 

 of the mechanical equivalent of heat began in 1844, a t a 

 time when he was ignorant of Joule's work, and the first 

 publication of his results appeared in the " Bulletin de la 

 Societe industrielle de Mulhouse" in 1855. Other results 

 were published in 1857 and in 1858, and a final series of 

 experiments was conducted in i860 and 1861, to the 

 correctness of which he makes a strong claim. Reviewing 

 his own experiments in 1875, Hirn selects four series 

 as those in which he feels most confidence ; these depend 

 upon hammering lead, the friction of water, the flow 

 of water under great pressure, and drilling iron, and the 

 values of the mechanical equivalent in kilogram metres 

 deduced are 424*5, 432, 433, and 425. Among these he 

 inclines to 432 as giving the value most approximately. 

 Also, from a method dependent upon the expansion of air 

 and the ratio of the specific heats, he concludes that 440 is 

 greater than the possible maximum value. It will be re- 

 membered that Joule's determination by different methods 

 varied from 419*5 to 488*3, and that it was only after a very 

 careful consideration that he decided upon 424 as the most 

 satisfactory value. It is evidently with regret that Hirn 



