RELATION OF VAPOUR AND AIR. .~>77 



how incomplete their knowledge was, by the very 

 gradual manner in which the consequences of this 

 principle were traced out. We have already noticed, 

 as one of the books which most drew attention 

 to the true doctrine, in this country at least, Dr. 

 Wells's Essay on Dew, published in 1814. In this 

 work the author gives an account of the progress 

 of his opinions 19 : " I was led," he says, " in the 

 autumn of 1784, by the event of a rude experi- 

 ment, to think it probable that the formation of 

 dew is attended with the production of cold." This 

 was confirmed by the experiments of others. But 

 some years after, " upon considering the subject 

 more closely, I began to suspect that Mr. Wilson, 

 Mr. Six, and myself, had all committed an error 

 in regarding the cold which accompanies the dew, 

 as an effect of the formation of the dew." He 



mr 



now considered it rather as the cause: and soon 

 found that he was able to account for the circum- 

 stances of this formation, many of them curious 

 and paradoxical, by supposing the bodies on which 

 dew is deposited, to be cooled down, by radiation 

 into the clear night-sky, to the proper temperature. 

 The same principle will obviously explain the for- 

 mation of mists over streams and lakes when the 

 air is cooler than the water; which was put forward 

 by Davy, even in 1819, as a new doctrine, or at 

 least as not familiar. 



Hygrometers. According as air has more or 



' Essay on Den\ p. I. 

 V OL. II. 1* p 



