140 THOMSON ON CARNOT'S 



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 appropriateness 

 for illustrating the general principles of his theory, 

 no less than on account of their very great practi- 

 cal 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 vapor ; 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 con- 

 templated by Carnot in his general theoretical rea- 

 sonings, but he confines himself to the ideal con- 

 struction, 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 conceiv- 

 able 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. 



Considerations on the Effect of Pressure in Lowering the 

 Freezing-point of Water," by Prof. James Thomson. 



