Dynamical Theory of Heat. 151 



consequence de cette maniere cPenvisager les faits, que si Pon 

 fait passer directeinent de la vapeur d'eau, de la chaudiere qui la 

 produit a travers une masse d'eau dans laquelle elle se condense, 

 cette vapeur elevera plus la temperature de Peau que si on la 

 faisait servir prealablement a mettre en jeu une machine a vapeur, 

 dans laquelle elle perdrait une partie de son ressort." (p. 882.) 



t( I/abaisseinent de temperature qui accompagne P expansion 

 de tout fluide aeriform e dans un espace plus grand que celui qui 

 repond au degre de tension ou il etait d'abord, et le phenomene 

 oppose, ou la production de chaleur qui est toujours la suite de 

 sa compression, me paraisseut deux circonstances qui viennent 

 k Pappui de cette assertion." (p. 383.) 



" Je vais done raisonner dans Phypothese que Pabaissement 

 de temperature de la vapeur, lorsqu'elle se dilate, represente 

 exactement la quantite de puissance qui apparait alors." (p. 384.) 



"Enfin Pabaissement subit de temperature de la vapeur 

 lorsqu'elle s'echappe dans Pair, circonstance mise a profit de nos 

 jours pour utiliser son ressort et sa puissance, montre que, dans 

 ce cas, Peffort qu'elle exerce en recul contre les appareils qui la 

 laissent echapper, ou la vitesse qu'elle communique a Pair ara- 

 biant, forment un equivalent de la perte de chaleur qu'elle 

 eprouve." (p. 394.) 



Seguin gives data from which the mechanical equivalent of heat 

 may be readily deduced on his hypothesis, the result being too 

 great in consequence of the thermal effect of the compression of 

 vapour being understated. Neither in SeguhPswritingof 1839, nor 

 in Mayer's paper of 1842, were there such proofs of the hypothesis 

 advanced as were sufficient to cause it to be admitted into science 

 without further inquiry. I believe that the experiment attributed 

 to Gay-Lussac was not referred to by Mayer previously to the year 

 1845. Mayer appears to have hastened to publish his views for 

 the express purpose of securing priority. He did not wait until 

 he had the opportunity of supporting them by facts. My course, 

 on the contrary, was to publish only such theories as I had esta- 

 blished by experiments calculated to commend them to the scien- 

 tific public, being well convinced of the truth of Sir J. HerschePs 

 remark, that " hasty generalization is the bane of science." 



I applied the dynamical theory to steam-engines, to electro- 

 magnetic engines, to vital processes, and to chemistry in 1843. 

 In the postscript alluded to in § 50 of Mr. TyndalPs article, I 

 intended the word " apprehend" to express the meaning applied 

 to it in Johnson's Dictionary, viz. "to conceive by the mind," 

 not "to conjecture*". My popular lecture will show that the 

 outlines of cosmical and other applications of the theory were so 



* [With respect to this passage, we had. before receiving Dr. Joule's 

 communication, been requested by Prof. Tyndall, in a letter dated Pontre- 

 sina, July 18, to substitute the word " statement" for "conjecture".— W. F.] 



