44 MOTIVE POWER OF HEAT. 



ever the working substance and whatever the 

 method by which it is operated. 



Machines which do not receive their motion from 

 heat, those which have for a motor the force of 

 men or of animals, a waterfall, an air-current, etc., 

 can be studied even to their smallest details by 

 the mechanical theory. All cases are foreseen, all 

 imaginable movements are referred to these general 

 principles, firmly established, and applicable under 

 all circumstances. This is the character of a com- 

 plete theory. A similar theory is evidently needed 

 for heat-engines. We shall have it only when the 

 laws of Physics shall be extended enough, general- 

 ized enough, to make known beforehand all the 

 effects of heat acting in a determined manner on 

 any body. 



We will suppose in what follows at least a 

 superficial knowledge of the different parts which 

 compose an ordinary steam-engine; and we con- 

 sider it unnecessary to explain what are the 

 furnace, boiler, steam-cylinder, piston, condenser, 

 etc. 



The production of motion in steam-engines is 

 always accompanied by a circumstance on which 

 we should fix our attention. This circumstance 

 is the re-establishing of equilibrium in the caloric; 

 that is, its passage from a body in which the 



