MOTIVE POWER OF HEAT. 43 



have long sought, and are seeking to-day, to ascer- 

 tain whether there are in existence agents preferable 

 to the vapor of water for developing the motive 

 power of heat; whether atmospheric air, for ex- 

 ample, would not 'present in this respect great ad- 

 vantages. We propose now to submit these ques- 

 tions to a deliberate examination. 



The phenomenon of the production of motion 

 by heat has not been considered from a sufficiently 

 general point of view. We have considered it only 

 in machines the nature and mode of action of 

 which have not allowed us to take in the whole 

 extent of application of which it is susceptible. 

 In such machines the phenomenon is, in a way, 

 incomplete. It becomes difficult to recognize its 

 principles and study its laws. 



In order to consider in the most general way 

 the principle of the production of motion by heat, 

 it must be considered independently of any mecha- 

 nism or any particular agent. It is necessary to 

 establish principles applicable not only to steam- 

 engines* but to all imaginable heat-engines, what- 



* We distinguish here the steam-engine from the heat- 

 engine in general. The latter may make use of any agent 

 whatever, of the vapor of water or of any other, to develop 

 the motive power of heat, 



