MOTIVE POWER OF HEAT. 135 



repetition of doubts, I shall refer to Carnot's funda- 

 mental principle, in all that follows, as if its truth 

 were thoroughly established. 



9. We are now led to the conclusion that the 

 origin of motive power, developed by the alternate 

 expansions and contractions of a body, must be 

 found in the agency of heat entering the body and 

 leaving it ; since there cannot, at the end of a com- 

 plete cycle, when the body is restored to its primi- 

 tive physical condition, have been any absolute ab- 

 sorption of heat, and consequently no conversion 

 of heat, or caloric, into mechanical effect; and it 

 remains for us to trace the precise nature of the 

 circumstances under which heat must enter the 

 body, and afterwards leave it, so that mechanical 

 effect may be produced. As an example, we may 

 consider that machine for obtaining motive power 

 from heat with which we are most familiar the 

 steam-engine. 



10. Here, we observe, that heat enters the ma- 

 chine from the furnace, through the sides of the 

 boiler, and that heat is continually abstracted by 

 the water employed for keeping the condenser cool. 

 According to Carnot's fundamental principle, the 

 quantity of heat thus discharged, during a complete 

 revolution (or double stroke) of the engine, must be 

 precisely equal to that which enters the water of 



