118 



SCIENTIFIC THOUGHT. 



he found in the ideas expounded by Poncelet, Sadi 

 Carnot, and Clapeyron, the means of accomplishing the 

 object. We now see how there lay, in the fundamental 

 problem of thermo-dynamics, the unifying idea of sciences 

 hitherto far apart and working on independent lines and 

 with independent standards of measurement, speaking, as 

 it were, separate languages. And what was the new idea 

 which lay concealed in Sadi Carnot's forgotten pamphlet ? l 

 In Carnot's original memoir it appears as an axiom at 

 the beginning of his reflections. " The production of 

 motion," he says, " in steam-engines is always accompanied 

 by a circumstance on which we must fix our attention. 

 This circumstance is the re-establishment of equilibrium, 

 or level, in the caloric that is to say, its passage from 

 one body where the temperature is more or less elevated, 



1 The story of Sadi Carnot's 

 memoir is not less curious than that 

 of Mohr's first paper. It was first 

 given by Lord Kelvin in his earliest 

 article, "On an Absolute Thermo- 

 metric Scale" (1848), reprinted in 

 ' Math, and Phys. Papers,' vol. i. 

 p. 100), and "An Account of Car- 

 not's Theory" (1849, ibid., p. 113). 

 He had in 1845 searched in vain for 

 the ' Puissance motrice du Feu ' in 

 all the bookshops of Paris. In 1848 

 he obtained a copy from Lewis 

 Gordon in Glasgow. It was known 

 to him before through Clapeyron's 

 memoir in the 14th vol. of the 

 ' Journal de 1'Ecole polytechnique ' 

 (1834). Sadi Carnot published his 

 memoir as a pamphlet in 1824. It 

 has since been republished by his 

 brother, Hippolyte Carnot (' Re"flex- 

 ions sur la Puissance motrice du 

 Feu et sur les Machines propres a 

 de"velopper cette Puissance,' Paris, 

 Gauthier-Villars, 1878), with im- 

 portant posthumous papers, from 

 which, inter alia, it is evident that 



Carnot, before he died, had aban- 

 doned the material theory of heat, 

 and actually, by an unknown pro- 

 cess, calculated the mechanical 

 equivalent of heat as 360 kilogram- 

 metres. As in several other cases, 

 so also in that of Sadi Carnot, the 

 line of reasoning initiated by La- 

 place, and brilliantly developed by 

 his school, militated against the 

 acceptance of the dynamical as 

 opposed to the material conception 

 of the phenomena of heat ; and 

 M. Bertin, in his "Rapport sur le 

 Progres de la Thermodynamique en 

 France" ('Recueil de Rapports,' 

 &c., p. 5) could write in 1867 : "II 

 faut bien 1'avouer, parceque c'est la 

 veYite" : nous sommes reste"s long- 

 temps, je ne dis pas rebelles, mais 

 Strangers aux nouvelles id^es : elles 

 nous sont rest^es trop longtemps 

 inconnues, et encore aujourd'hui, 

 on peut regretter qu'elles n'occupent 

 pas une place plus considerable dans 

 notre enseignement scientifique." 



