136 THOMSON ON CARNOT'S 



the boiler;* provided the total mass of water and 

 steam be invariable, and be restored to its primitive 

 physical condition (which will be the case rigorously, 

 if the condenser be kept cool by the external appli- 

 cation of cold water instead of by injection, as is 

 more usual in practice), and if the condensed 

 water be restored to the boiler at the end of each 

 complete revolution. Thus we perceive that a cer- 

 tain quantity of heat is let down from a hot body, 

 the metal of the boiler, to another body at a lower 

 temperature, the metal of the condenser; and that 

 there results from this transference of heat a certain 

 development of mechanical effect. 



11. If we examine any other case in which 

 mechanical effect is obtained from a thermal origin, 

 by means of the alternate expansions and contrac- 

 tions of any substance whatever, instead of the 

 water of a steam-engine, we find that a similar 

 transference of heat is effected, and we may there- 

 fore answer the first question proposed, in the fol- 

 lowing manner : 



The thermal agency ~by which mechanical effect 

 may be obtained is the transference of heat from 

 one body to another at a lower temperature. 



* So generally is Carnot's principle tacitly admitted as an 

 axiom, that its application in this case has never, so far as 

 I am aware, been questioned by practical engineers. (1849). 



