CARNOT’S THEORY OF THE MOTIVE POWER OF HEAT. 547 
pressure of the vapour above the water will tend to push up the piston, and 
must be resisted by a force applied to the piston,* till the commencement of the 
operations, which are conducted in the following manner. 
(1.) The cylinder being placed on the body A, so that the water and vapour 
may be retained at the temperature S, let the piston rise any convenient height 
EE,, to a position E, F,, performing work by the pressure of the vapour below it dur- 
ing its ascent. 
[During this operation a certain quantity, H, of heat, the amount of latent heat in the fresh 
vapour which is formed, is abstracted from the body A.] 
(2.) The cylinder being removed, and placed on the impermeable stand K, 
let the piston rise gradually, till, when tt reaches a position KE, F,, the temperature of 
the water and vapour is T, the same as that of the body B. 
[During this operation the fresh vapour continually formed requires heat to become latent ; 
and, therefore, as the contents of the cylinder are protected from any accession of heat, their tem- 
perature sinks. | 
(3.) The cylinder being removed from K, and placed on B, let the piston be 
pushed down, till, when it reaches the position E; ¥;, the quantity of heat evolved and 
abstracted by B amounts to that which, during the first operation, was taken from A. 
[During this operation the temperature of the contents of the cylinder is retained constantly at 
T°, and all the latent heat of the vapour which is condensed into water at the same temperature, is 
given out to B.] 
(4.) The cylinder being removed from B, and placed on the impermeable 
stand, let the piston be pushed down from E; F, to its original position EF. 
[During this operation, the impermeable stand preventing any loss of heat, the temperature of 
the water and air must rise continually, till (since the quantity of heat evolved during the third ope- 
ration was precisely equal to that which was previously absorbed), at the conclusion it reaches its 
primitive value, S, in virtue of Carnot’s fundamental axiom.] 
16. At the conclusion of this cycle of operations} the total thermal agency 
has been the letting down of H units of heat from the body A, at the temperature 
S, to B, at the lower temperature T; and the aggregate of the mechanical effect 
has been a certain amount of work produced, since during the ascent of the piston 
in the first and second operations, the temperature of the water and vapour, and 
therefore the pressure of the vapour on the piston, was on the whole higher than 
during the descent, in the third and fourth operations. It remains for us actually 
to evaluate this aggregate amount of work performed; and for this purpose the 
* Tn all that follows, the pressure of the atmosphere on the upper side of the piston will be in- 
cluded in the applied forces, which, in the successive operations described, are sometimes overcome by 
the upward motion, and sometimes yielded to in the motion downwards. It will be unnecessary, in 
reckoning at the end of a cycle of operations, to take into account the work thus spent upon the atmo- 
sphere, and the restitution which has been made, since these precisely compensate for one another. 
{ In Carnor’s work some perplexity is introduced with reference to the temperature of the 
water, which, in the operations he describes, is not brought back exactly to what it was at the com- 
mencement ; but the difficulty which arises is explained by the author. No such difficulty occurs 
with reference to the cycle of operations described in the text, for which I am indebted to Mons. 
CLAPEYRON. 
