546 PROFESSOR WILLIAM THOMSON’S ACCOUNT OF 
and contractions of a perfectly elastic solid, or of a liquid; or upon thealtera- 
tions of volume experienced by substances, in passing from the liquid to the solid 
state,* each of which being perfect, would produce the same amount of mechanical 
effect from a given thermal agency; but there are two cases which Carnot has 
selected as most worthy of minute attention, because of their peculiar appropriate- 
ness for illustrating the general principles of his theory, no less than on account 
of their very great practical importance ; the steam-engine, in which the substance 
employed as the transferring medium is water, alternately in the liquid state, and 
in the state of vapour ; and the air-engine, in which the transference is effected 
by means of the alternate expansions and contractions of a medium, always in 
the gaseous state. The details of an actually practicable engine of either kind 
are not contemplated by Carnot, in his general theoretical reasonings, but he con- 
fines himself to the ideal construction, in the simplest possible way in each case, 
of an engine in which the economy is perfect. He thus determines the degree of 
perfectibility which cannot be surpassed; and, by describing a conceivable method 
of attaining to this perfection by an air-engine or a steam-engine, he points out 
the proper objects to be kept in view in the practical construction and working of 
such machines. I now proceed to give an outline of these investigations. 
Carnot’s Theory of the Sieam-Engine. 
15. Let CDF, E, be a cylinder, of which the curved surface is perfectly imper- 
meable to heat, with a piston also im- 
permeable to heat, fitted in it; while 
the fixed bottom C D, itself with no ca- 
pacity for heat, is possessed of perfect 
conducting power. Let K be an im- 
permeable stand, such that when the 
cylinder is placed upon it, the con- 
tents below the piston can neither gain 



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nor lose heat. Let A and B be two 
bodies permanently retained at con- 


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stant temperatures, S° and T°, respec- 7 Or 
tively, of which the former is higher 
than the latter. Let the cylinder, i 
placed on the impermeable stand, K, x 
be partially filled with water, at the ] 

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temperature S, of the body A, and 
(there being no air below it) let the 7/ // 
piston be placed in a position E F, 7/ 
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near the surface of the water. The da 

ai A case minutely examined in another paper, to be laid before the Society at the present 
meeting. 

