SADI CARNOT 233 



perfectly reversible steam engine, the efficiency could only 

 be increased by an increase in the temperature difference 

 between boiler and condenser. 



There now only remained open the important question 

 whether machines using another vapour than water, or 

 generally, another body capable of doing work by expansion, 

 such as some gas or other, might not be more advantageous, 

 inasmuch as with the same range of temperature, and com- 

 plete reversibility, differences in the power of doing work 

 might exist according to the substance used or according to 

 peculiarities in the construction of the machine. Carnot is 

 able to finally answer this question in the negative by means 

 of an imaginary experiment. He bases this - as was done 

 more than two hundred years previously by Stevin - on our 

 experience of the impossibility of perpetual motion. If one 

 of the two engines compared would work more efficiently 

 than the other, we could allow one to use part of the work 

 produced to drive the other backwards, whereby the tem- 

 perature range, used in the driving machine, would be main- 

 tained without addition of heat, on account of the action of 

 the other machine; while in addition, the excess of work de- 

 livered by the better machine would be at our disposal. This 

 would amount to perpetual motion. Since such an arrange- 

 ment is known to be impossible, the efficiencies of the two 

 different machines cannot differ from one another.^ 



Carnot thus concludes that all completely reversible 

 machines acting over the same range of temperature must be 

 equally efficient, and that their delivery of work can only de- 

 pend, apart from the available quantity of heat, upon the 

 utilisable range of temperature, increasing with both of 

 these quantities. This discovery has been of fundamental 



1 In this connection Carnot also draws attention to the experience 

 gained with Volta's batteries, and points out that these arrangements also 

 can in no way be regarded as inexhaustible sources of, say, magnetic 

 force, but always show clear signs of exhaustion, and hence cannot pro- 

 duce perpetual motion. 



