122 SCIENTIFIC THOUGHT. 



not find it necessary to enter upon any theory of the 

 nature of heat, the analogy with the flow of water from 

 higher to lower levels would naturally present itself. 

 For his purpose this analogy had no importance. For 

 the purposes of Sadi Carnot, who noticed that upon 

 the difference of temperature depended not only the 

 flow of heat, but also the work it might eventually 

 His influ ^' ^ e same analogy seemed all-important. " We may," 

 camot! ne savs > "J us tly compare the motive power of heat with 

 that of a fall of water: both have a maximum which 

 cannot be exceeded. The motive power of a fall of 

 water depends upon its height and the quantity of the 

 liquid ; the motive power of heat likewise depends on 

 the quantity of caloric employed and on what we will 

 take the liberty of calling the height of its drop that 

 is, the difference of temperature of the bodies between 

 which the exchange of caloric has taken place." 1 In 

 this analogy two further assumptions seem to be im- 

 plied : First, that the work capable of being done is in 

 direct proportion to the difference of levels of height 

 or of temperature; secondly, that the quantities with 

 which we operate, of water or of caloric, remain the 

 same, before and after the fall. Neither of these 

 inferences is necessary ; neither is permissible. Carnot 

 does not adopt the first inference, 2 but he does adopt 

 the second, 3 though he significantly remarks that the 



1 ' Puissance inotrice du feu,' 

 ed. 1878, p. 15. 



2 " Daus la chute d'eau, la puis- 

 sance motrice est rigoureusement 

 proportionelle a la difference de 

 niveau entre le reservoir supeYieur 

 et le reservoir inferieur. Dans la 

 chute du calorique, la puissance 



niotrice auginente sans doute avec 

 la difference de temperature entre 

 le corps chaud et le corps froid ; 

 inais nous ignorons si elle est 

 proportionelle a cette difference" 

 (ibid., p. 15 ; compare also pp. 



38, 39). 



3 '- 



La production de la puissance 



