. HEAT. 57 



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

 piston compensates the force lost by friction). 



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 

 i 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 maximum rise of 

 the piston ; but when the piston is at its full elevation, and 

 the cold body applied, let the weight be tilted off, so as to 

 drop upon a wheel, or be used for other mechanical pur- 

 poses. The descending 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 i, 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 fric- 

 tion, we should have more heat than at first, or a creation of 

 heat out of nothing in other words, perpetual motion. 



Let us now assume that this 20 supplied in the first 

 instance was yielded by a body at 90, of such size and mate- 

 rial that its total capacity for heat is equal to that of the mass 

 of confined air : this body would be reduced in temperature 

 to 70, in other words, our furnace would have lost 20 of 

 heat. Let the cold body of the same size and material, used 

 as a condenser, be at 30. In the first experiment, the body 

 at 30 would bring back the piston to its original point ; but 

 in the second experiment, or that where the weight has been 

 removed, the body at 30 would not suffice to restore the 

 piston : to effect this, the cold body or condenser must be at 

 a lower temperature. 



The question in Carnot's theory, which is not experi- 

 mentally resolved, and which presents extreme experimental 

 difficulty, is the following : Granted that a piston with a super- 

 imposed weight be raised by the thermic expansion of confined 

 gas or vapour below it ; if the elastic medium be restored to its 

 original temperature by cooling, the weight -in depressing the 



