THE DIFFICULTIES. 79 



sion and contraction of elastic fluids are unquestionably the 

 most important of any. This Joule recognises. In the 

 introduction to his paper he says, " An inquiry of great 

 interest in a practical as well as theoretical point of view, 

 owing to its bearing upon the theory of the steam 

 engine." The difficulties of the enquiry were, however, so 

 great as to have been impossible to any one but Joule. 

 And besides presenting those difficulties which Joule 

 actually overcame, perhaps the greatest difficulty was to 

 conceive any possible determination that would not depend 

 on a knowledge of the properties of gases and the relations 

 amongst these properties which were not measured or 

 formulated till some years afterwards, and then by the aid 

 of Joule's discoveries ; so that not the least interesting part 

 of this research is the way in which Joule, having carefully 

 considered the subject, and perceived the difficulties of the 

 reasoning, and the inadequacy of the existing knowledge, 

 was able, by keeping in view the simple fact that he wanted 

 to determine, namely, the relation between "work" done in 

 compressing air and heat developed, to devise experiments 

 which would yield this relation without raising any question 

 as to the other properties of the air. Having devised the 

 experiments, the courage he displayed in undertaking what 

 obviously required a degree of experimental accuracy far 

 beyond anything previously attained, and which has never 

 been repeated, is no less interesting. From the light they 

 throw on this part of his research, and from other reasons, 

 the first few paragraphs of this paper are of interest. He 

 says : — 



" Dr. Cullen and Dr. Darwin appear to have been the first 

 who observed that the temperature of air is decreased by 

 rarefaction and increased by condensation. Other philo- 



