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 

 E El, to a position Ei Fi, performing work by the pressure of the vapour helow 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 it reaches a position E^ Fo, the temper-atiire 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 Es Fa, 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 E3F3 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 amoimt 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 



* In 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 xmnecessary, in 

 reckoning at the errd of a cycle of operations, to take into account the work thus spent upon the atmo- 

 sphere, and the restitution wliich has been made, since these precisely compensate for one another. 



\ In Caknot'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 Mens. 

 Clapetkon. 



