2 Mr. W. J. ]\I. Rankine on the Mechanical Action of Heat. 



pressure as compared witli that of water, according to the expe- 

 riments of Dc la Roche and Berard. The result thus obtained 

 was 695 feet of fall per degi-ee of Fahrenheit, being about one- 

 tenth part less than Joule's equivalent. But shortly afterwards 

 Mr, Joide's latest and most accui'ate series of experiments were 

 published, the mutual agreement of which made it evident that 

 his determination of the value of one degree of Fahrenheit's 

 scale in liquid water, viz. 772 feet of fall, is coiTect to about 

 ^~dth of its amount. All the numerical results in the fol- 

 lowing paper, therefore, have been corrected to correspond with 

 this value ; and amongst others, the specific heat of air, the 

 theoretical computation of which has recently been exactly con- 

 firmed by the experiments of M. Regnault. 



With respect to the consequences of the law of the mutual 

 convertibility of heat and expansive power, the results of the 

 following investigations will be found to coincide in many points 

 with those arrived at by Messrs. Mayer, Helmholtz, Holtzmann, 

 Clausius, and Thomson. 



Considered as the development of the consequences of the hy- 

 pothesis of molecular vortices, these researches constitute the 

 second part of a paper on the Centrifugal Theory of Elasticity, 

 the first part of which, treating of the Statical Relations of Heat 

 and Elasticity, was read to the Royal Society of Edinburgh along 

 with the present investigation, and published in the Philosophical 

 Magazine for December 1851. 



The introduction of an hypothesis into the theory of a phy- 

 sical science has for its sole legitimate object, to deduce the laws 

 of one class of phsenomena from those of another class whose 

 laws are better known, and thus to diminish the number and 

 the complexity of first principles. 



Motion and force being the only phsenomena of which we 

 thoroughly and exactly know the laws, and mechanics the only 

 complete physical science, it has been the constant endeavour of 

 natural philosophers, by conceiving the other phsenomena of 

 nature as modifications of motion and force, to reduce the other 

 physical sciences to branches of mechanics. 



In the preface to the Principia, we find Newton expressing a 

 wish for the extension of this kind of investigation in these 

 words : — " Utinam ctetera naturae phsenomena ex princi- 

 PIIS mechanicts derivare liceret." 



The theory of radiant heat and light having been reduced to 

 a branch of mechanics by means of the hypothesis of undulations, 

 it is the object of the hypothesis of molecular vortices to reduce 

 the theory of thermometric heat and elasticity also to a branch 

 of mechanics, by so conceiving the molecular structure of matter, 

 that the laws of these phsenomena shall be consequences of those 

 of motion and force. 



