THE CENTURY'S PROGRESS IN METEOROLOGY 



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searches of Black, Hutton, and their confreres of that 

 Edinburgh school, Wells made observations on evapora- 

 tion and precipitation as early as 1784, but other things 

 claimed his attention ; and though he asserts that the 

 subject was often in his mind, he did not take it up 

 again in earnest until about 1812. 



Meantime the observations on heat of Rumford and 

 Davy and Leslie had cleared the way for a proper in- 

 terpretation of the facts about the facts themselves 

 there had long been practical unanimity of opinion. Dr. 

 Black, with his latent-heat observations, had really given 

 the clew to all subsequent discussions of the subject of 

 precipitation of vapor; and from his time on it had been 

 known that heat is taken up when water evaporates, 

 and given out again when it condenses. Dr. Darwin 

 had shown in 1788, in a paper before the Royal Society, 

 that air gives off heat on contracting, and takes it up on 

 expanding; and Dalton in his essay of 1793 had ex- 

 plained this phenomenon as due to the condensation and 

 vaporization of the water contained in the air. 



But some curious and puzzling observations which 

 Professor Patrick "Wilson, Professor of Astronomy in 

 the University of Glasgow, had communicated to the 

 Royal Society of Edinburgh in 1784, and some similar 

 ones made by Mr. Six of Canterbury a few years later, 

 had remained unexplained. Both these gentlemen ob- 

 served that the air is cooler where dew is forming than 

 the air a few feet higher, and they inferred that the dew 

 in forming had taken up heat, in apparent violation of 

 established physical principles. 



It remained for Wells, in his memorable paper of 

 1816, to show that these observers had simply gotten 

 the cart before the horse. He made it clear that the 



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