126 ACCEPTS JOULE'S VIEWS. 



papers. This Rankine acknowledges in his first paper, 

 and, at the same time, declares his conviction of the 

 unexceptional character of the evidence adduced by Joule 

 of the convertibility of heat and mechanical effect ; but, at 

 the same time, declares himself dissatisfied with the value 

 of the equivalent as determined by Joule, on account of the 

 smallness of the differences of temperature measured in 

 Joule's experiments, and the numerous possible sources of 

 error to which such experiments are necessarily exposed, 

 which sources he points out in a somewhat patronizing 

 manner. 



Rankine then proceeds : — 



" The best means of determining the mechanical equiva- 

 lent of heat are furnished by those experiments in which 

 no machinery is employed. Of this kind are experiments 

 on the velocity of sound in air and other gases, which, 

 according to the received and well-known theory of Laplace, 

 is accelerated by the heat developed by the compression of 

 the medium." 



There is a speciousness in this argument arising from 

 the fact that these experiments on sound only become 

 available for the purpose when the true value of at least 

 one of the specific heats of air is known, and that the deter- 

 mination of this involves " machinery," and is a matter of 

 extreme difficulty. Rankine, in the natural anxiety to render 

 his theory, which he had considered in 1842, independent of 

 Joule's subsequent work, for the moment overlooked this 

 speciousness, and in his first paper deduces, from the data 

 already mentioned, the mechanical equivalent of heat to be 

 695*6, whereupon he remarks, " I have already pointed out 

 the causes which tend to make the apparent value of the 

 mechanical equivalent of heat, in Mr. Joule's experiments, 



