72 CORRELATION OF PHYSICAL FORCES. 



no part of the power can be abstracted so as to be employed 

 for any practical purpose : this question then follows, what 

 takes place with regard to the initial heat, if, after the ascent 

 of the piston, the weight be removed so as not to help the pis- 

 ton in its descent, but to fall upon a lever or produce some 

 extraneous mechanical effect? 



To answer this question, let us suppose a weight to rest 

 on a piston which confines air at a definite temperature, say 

 for example 50, in a cylinder, the whole being assumed 

 to be absolutely non-conducting for heat. A part of the heat 

 of this confined air will be due to the pressure, since, as 

 we have seen, compression of an elastic fluid produces 

 heat. 



Suppose, now, the confined air to be heated to 70, the 

 piston with its superincumbent weight will ascend, and tlie 

 temperature, in consequence of the dilatation of the air, will 

 be somewhat lowered, say to G9 (we will assume, for the 

 sake of simplicity, that the heat engendered by the friction of 

 the piston compensates the force lost by frietion). 



The piston having reached its maximum of elevation, let 

 a cold body or condenser take away 20 from the temperature 

 of the confined air ; the piston will now descend, and by the 

 compression which the weight on it produces, will restore the 

 1 lost by dilatation, and when the piston reaches its original 

 position the temperature of the air will be restored to 50. 

 Suppose this experiment repeated up to the rise of the piston ; 

 but when the piston is at its full elevation, and the cold body 

 applied, let the weight be removed, so as drop upon a wheel, 

 or to be used for other mechanical purposes. The descend- 

 ing piston will not now reach its original point without more 

 heat being abstracted ; in consequence of the removal of the 

 weight, there will not be the same force to restore the 1, and 

 the temperature will be 49, or some fraction short of the 

 original 50. If this were otherwise, then, as the weight in 

 falling may be made to produce heat by friction, we should 



