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 amoimt 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 eflFected 

 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 Steam-Engine. 



15. Let CDF2 E2 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 

 nor lose heat. Let A and B be two 

 bodies permanently retained at con- 

 stant temperatures, S" and T", respec- 

 tively, of which the former is higher ^_ 



than the latter. Let the cylinder, ..., 

 placed on the impermeable stand, K, ^ 



be partially filled with water, at the 

 temperature S, of the body A, and 

 (there being no air below it) let the 

 piston be placed in a position E F, 

 near the surface of the water. The 



D 



* A case minutely examined in another p.^pe^, to be laid Lefore the Society at the present 

 meeting. 



