19G Mr. J. Croll on supposed Objections to 



steam-engine, according to the dynamical theory of heat, ranges 

 from ., l 4 th to A-tfa or thereabouts, and that by the aid of ex- 

 pansion that efficiency can be increased in ordinary cases to -j^th 

 or Jth, and in some special cases to Ith. 



There is one passage in Mr. Gill's letter which I trust that 

 his own sense of justice will, on further consideration, induce 

 him to withdraw. It is that in which he brings against M. Him 

 the serious charge of having allowed himself to be "borne down 

 by scientific authority " at the time of the change in his opinions 

 as to the theory of heat — a change which M. Hirn has distinctly 

 stated to have been the result of his own deliberate and indepen- 

 dent judgment. Such a charge ought not to have been made 

 in the absence of all proof, and on the sole ground of a difference 

 of opinion*. 



I am, Gentlemen, 



Your most obedient Servant, 



W. J. Macquorn Ranktne. 

 Glasgow, Februarv 15, 1863. 



XXXV. On supposed Objections to the Dynamical Theory of Heat. 

 By Mr. James Croll. 



To John Tyndall, Esq., F.R.S. %c. 

 Sir, 



IN a paper by Mr. Gill, addressed to you, which appeared in 

 the Philosophical Magazine of this month, he adduces cer- 

 tain facts which he considers cannot well be explained upon the 

 principles of the modern dynamical theory of heat. Mr. GilPs 

 conclusions appear, however, to have been drawn rather hastily. 



Perhaps the following considerations will tend to remove his 

 difficulties, in so far as his first two objections are concerned. 



Let us suppose with this writer that 100 represents the quan- 

 tity of heat required to double the absolute temperature of the 

 cubic foot of air in his cylinder under constant volume. When 



* The following are the words in which M. Hirn sums up the results of 

 his experiments on the steam-engine : — "Le resultat positif auquel je suis 

 arrive dans des reeherches tout-a-fait recentes, e'est que quelle que soit la 

 maniere dont travaille une machine a vapeur; que ce soit avec detente 

 ou non, avec vapeur saturee ou avec vapeur surchauffee, que ce soit avec 

 de la vapeur a une pression peu differente de celle de la chaudiere, ou avec 

 une vapeur qui eprouve en passant de la chaudiere au cylindre une chute de 

 pression considerable, on trouve toujours qu'il y a une proportionnalite 

 exacte entre le travail rendu par le moteur et la difference qui existe entre 

 la quantite de chaleur fournie avant l'entree dans le cylindre moteur et 

 la quantite de chaleur qui reste a la vapeur a sa sortie du cylindre." (Ex- 

 position analytique et experimentale de la Theorie Mecanique dela Chaleur, 

 p. 84. Colmar, 18G2.) 



