The Contrast of Practice and Theory. 25 



version of heat into work in a practical way might therefore 

 naturally be expected as the result of the advance that theoretical 

 thermodynamics had made, corresponding to the advance in 

 our power of building bridges and other structures, resulting 

 from advances in the theory of strains. 



Among the widest and most far-reaching of the results esta- 

 blished by Carnot's and Joule's researches two stood out 

 pre-eminent, and seemed specially well adapted for practical 

 application. One was the familiar law of Joule telling us how 

 to measure heat by an equivalent quantity of work. The other 

 was Carnot's, which declared that not all the heat supplied to 

 an engine could be converted into work, but only a certain 

 proportion, and that this proportion was fixed by a very simple 

 relation between the highest and lowest temperatures of the 

 steam or other vapour used in the engine. 



The temperatures of boiler and condenser are things easily 

 ascertained, and the whole heat supply can be found without 

 any great difficulty ; consequently one would naturally jump 

 eagerly at a pair of great overriding rules, such as those given 

 by the laws of Joule and Carnot, which appeared to save all 

 trouble about calculating details, and to arrive directly at the 

 desired result of finding how much steam of a given pressure 

 would be required to give, in an engine, a certain return in the 

 form of work. 



Carnot's law, for instance, states in effect that if you want 

 twenty pounds worth of work out of steam in a locomotive with 

 180 lbs. pressure per square inch, you must supply no less than 

 about a hundred pounds worth of work in the form of heat to 

 the boUf;r. The rule for making the calculations is wel^ 

 known, and so simple that anyone can apply it. 



Joule's and Carnot's laws were therefore applied over and 

 over again both by theorists and practical men to the case of 

 the steam engine, and always brought out grossly erroneous 

 results. Nevertheless these laws are certainly true, though it 

 is also certainly true that no law of nature ever fails to work 

 out its own arithmetic correctly. Much doubt and difficulty 



