232 GREAT MEN OF SCIENCE 



transference of heat that takes place without change of vol- 

 ume, for example by simple conduction, means a loss in work. 



Carnot then further remarked, that the changes in volume 

 associated with the performance of work are reversible, 

 whereby his ideas rest upon facts already determined by 

 Dalton and Gay-Lussac, and likewise upon Laplace's calcu- 

 lation of the velocity of sound, which was found to agree with 

 experiment. He says: 'If a gas is compressed quickly, its 

 temperature is raised: conversely, it falls, when the gas is 

 quickly expanded. This is one of the best ascertained facts 

 of experience; we shall take it as a foundation of our proofs.'^ 

 He thus arrives at the conclusion that complete reversibility 

 of the process used must be regarded as a condition of 

 maximum production of work by heat. The process in a 

 steam engine is actually reversible. For if the flywheel of 

 the engine were turned backwards, it would act as a pump, 

 removing the water vapour from the condenser into the 

 boiler, whereby the first would be cooled,^ the latter heated, 

 so that along with the flywheel and the steam, the heat also 

 would move in a reverse direction, from the condenser to the 

 boiler, from the colder to the hotter body. 



The efficiency of the engine depends upon the perfection 

 of this reversibility; everything that is not reversible, such 

 as loss of heat by conduction, frictional processes, and also 

 the passage of steam from the cylinder into the condenser, 

 before it - after being cut off from the boiler - has assumed 

 of itself the condenser temperature;^ all such processes 

 diminish the efficiency of the engine. In the case of a 



^ In this connection, Carnot was also the first to suspect the true 

 explanation of the temperature distribution in the earth's atmosphere, in- 

 asmuch as he remarks: 'Must not the cooling of the air by expansion 

 account for the cold in the upper regions of the atmosphere? The rea- 

 sons hitherto given as explanations of this cold are completely insufficient.' 



2 The steam engine running in a reverse direction is actually, in 

 principle, a refrigerator. 



3 Watt already introduced the practice of cutting the steam off from the 

 cylinder long before the end of the piston stroke, in order to make his 

 engines operate more economically. 



