SADI CARNOT 



1796-1832 



This man of science was the first since Watt's time to bring 

 an essentially new idea into the science of the steam engine, 

 and this was an idea which he himself already showed to be 

 of much wider importance, namely, as holding for all appara- 

 tus which produces work by means of heat.^ His discovery 

 proved of continually increasing and more general impor- 

 tance, until finally, twenty-five years later, Clausius made it 

 into the 'second law of thermo-dynamics,' which allows us to 

 calculate so many phenomena of heat in all their details in 

 the most astonishing manner. In this connection, Carnot 

 also arrived in his short lifetime very near to the nature of 

 heat itself, almost to the point of Robert Mayer's later dis- 

 coveries, as we now know from his posthumous papers, 

 which only came to light long after. 



His starting point was the question of the greatest possible 

 efficiency of heat engines, that is to say, the conditions 

 which finally determine how many horse-power hours of work 

 such machines, and in particular a Watt's steam engine, can 

 deliver, in the best case, by the consumption of a given 

 amount of heat in the boiler. He first realised that, in all 

 such apparatus, heat necessarily passes from a hot body to 

 a colder body, in the case of the steam engine from the 

 boiler to the condenser. Only where such passage of heat is 

 possible, that is to say, only when temperature differences 

 are present, can heat be turned into work. If as much work 

 as possible is to be done, only such transference of heat must 

 take place as is associated with changes in volume; for the 

 work is done by these changes in volume, as in the cylinder 

 of the steam engine, where the steam expands. Every 



1 Nicolas Leonard (called Sadi) Carnot, Reflexions sur la puissance 

 motrice du feu et sur les machines propres a develloper cette puissance, Paris, 

 1824; facsimile, Paris, 1912. Eng. trans, by R. H. Thurston, London, 

 1890, 



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