164 Prof. Barnard on the comparative Expenditure of Heat — 
lowing illustration is from Carnot, by whom the first attempt was 
i , to present a dynamic theory of heat.* 
Let A be a body constantly atthetem- 1 , 
perature S°, and capable of preserving 
another body in contact with it at the 



serving another body in contact with it 
at the same temperature. Let CDEF be 
a cylinder in which moves an air-tight 
piston P; both cylinder and piston being 
impervious to heat, with the exception eae ea 
that the bottom, C D, of the cylinder is a perfect conductor, and 
without capacity for heat. K represents a stand, impervious to 
heat, designed, when necessary, to neutralize the conducting 
power of CD. 
If this cylinder contain beneath the piston P, in its present po 
sition, a gas, or liquid capable of conversion into vapor, and be 
placed upon the body A, the piston is to be supposed to rise, 
while the confined gas, or liquid and vapor, is kept at the temper 
ature S°, by the body A. If now we force down the piston t 
its original position, the body A being supposed to withdraw heat 
as it before imparted it, and always to maintain the temperature 
of the gas at S°, we shall have to expend precisely as much force 
as was developed during the expansion. But if, when the piston 
has reached the top of the cylinder, we remove the whole to the 
body B, and suppose the temperature to be instantly reduced by 
this body to T°, and afterward maintained at that point, then Wé 
may force down the piston with less labor than before. Thus 
NS 
* See Thompson’s “ Account of Carnot’s Theory,” in the Trans, of the Roy. Sot 
of Edinburgh, vol. xvi, part 5, 1849. 
