HISTORY OF DYNAMICAL THEORY OF HEAT. 337 



to a principle which he subsequently enunciated in a work on rail- 

 ways, 1 in treating of the motive-power of heat, namely : 



" La force mecanique qu'apparait pendent l'abaissement de temperature d'un 

 gas, comme de tout autre corps qui se dilate, est la mesure et la representation 

 de cette diminution de chaleur." 



If in the single term "chaleur" Seguin intended to include both 

 sensible and latent heat, his principle was undoubtedly correct ; but 

 it is to be inferred from an indicated method of determining the 

 relative dynamical value of heat and mechanical units, 3 that he had 

 quite neglected to take into account any change of molecular energy 

 other than that of sensible heat. 



Nearly identical with these, though much more celebrated, w T ere 

 the subsequent speculations of Dr. J. R. Mayer upon this subject. 

 In a memoir published in Liebig's Annalen, for May, 1842, entitled 

 "Bemerkungen iiberdie Krafte der neubelebten Natur," he undertook 

 to answer the questions : " What are we to understand by force? and 

 how are different forces related to each other?" Toward the latter 

 part of the disquisition he entered upon the subject of the mutual 

 convertibility of heat and mechanical energy, considering the genera- 

 tion of heat by the shock or gradual stopping of a falling body, by 

 friction, and by compression ; and illustrating by the heat excited in 

 the bearings and rubbing surfaces of water-mills and railway-trains; 

 and by the diminution of the earth's bulk in the falling of a body to 

 the ground. 



In this he first expressly used the term equivalent, in speaking of 

 the relation of heat, to mechanical effect ; and by the same method as 

 that employed in the deduction of Seguin's value, though with more 

 accurate data, found the distance through which any mass of water 

 would have to fall, in order that its temperature, by the shock of sud- 

 den stoppage, might be raised from to 1 Cent., to be 365 metres. 



The physical reasoning upon which he founded this determination 

 was manifestly incomplete, if not erroneous ; and, on this account, his 

 claims as an original promoter of correct theory have been made of 

 late the subject of considerable dispute. In view of the historical im- 

 portance attaching to this point, and because an allowable explana- 

 tion of the phenomenon referred to will illustrate very fully the re- 

 ceived distinction between sensible and latent heat, we here make a 

 slight digression to consider more particularly the thermal effect 

 attending the compression of elastic fluids. 



The term specific heat is ordinarily employed to designate that 

 quantity which it is necessary to impart to unity of w T eight of any 



1 Entitled " Etudes sur l'lnfluence des Chemins de Fer," p. 378, et seq. Paris, 1838. 



2 The method indicated, with the data then at his command, for steam, gave 650 kilo- 

 grammetres as the mechanical value of an increase of temperature of 1 Cent, in one 

 kilogramme of water. 



TOL. XII. 22 



