16 THE WORK OF SADI CARNOT. 



possible, in order to secure a commensurate range 

 of temperature ; (2) The cooling must be carried 

 to the lowest point on the scale that may be found 

 practicable ; (3) The passage of the fluid from the 

 upper to the lower limit of temperature must be 

 produced by expansion;" i.e., "it is necessary 

 that the cooling of the gas shall occur sponta- 

 neously by its rarefaction ;" which is simply his 

 method of stating the now universally understood 

 principle that, for highest efficiency, the expansion 

 must be adiabatic, from a maximum to a mini- 

 mum temperature. He goes on to explain these 

 principles, and then says that the advantage of 

 high-pressure engines lies " essentiellement dans la 

 faculte de rendre utile vne plus grande chute de ca- 

 loriqne." This principle, as a practical system of 

 operation, had already, as he tells us, been enunci- 

 ated by M. Clement, and had been practised, as 

 we well know, since the days of its originator, 

 Watt ; but Carnot saw clearly the thermodynamic 

 principle which underlies it, and as clearly states 

 it, for the first time. 



He sees clearly, too, the reasons for the attempts 

 of Hornblower and of Woolf, premature as they 

 proved and as he also sees, in the introduction of 

 the compound engine, and even suggests that this 

 idea might be still further developed by the use 



