MOTIVE POWER OF HEAT. 139 



of the thermal agency is wasted. Hence it is of 

 primary importance to discover the criterion of a 

 perfect engine. This has been done by Carnot, who 

 proves the following proposition : 



13. A perfect thermodynamic engine is such 

 that, whatever amount of mechanical effect it can 

 derive from a certain thermal agency, if an equal 

 amount be spent in working it baclcivards, an equal 

 reverse thermal effect will be produced* 



14. This proposition will be made clearer by the 

 applications of it which are given later ( 29), in 

 the cases of the air-engine and the steam-engine, 

 than it could be by any general explanation ; and it 

 will also appear, from the nature of the opera- 

 tions described in those cases, and the principles of 

 Carnot's reasoning, that a perfect engine may be 

 constructed with any substance of an indestructible 

 texture as the alternately expanding and contract- 

 ing medium. Thus we might conceive thermo- 

 dynamic engines founded upon the expansions and 

 contractions of a perfectly elastic solid, or of a 

 liquid; or upon the alterations of volume experi 

 enced by substances in passing from the liquid to 

 the solid state, f each of which being perfect, would 



* For a demonstration, see 29. 



f A case minutely examined in another paper, to be laid 

 before the Society at the present meeting. ' ' Theoretical 



