186 ESSAY 



atmosphere, by the cold of the body on which it 

 appears, the same degree of cold, in the pre- 

 cipitating body, may be attended with much, 

 with little, or with no dew, according to the 

 existing state of the air in regard to moisture ; 

 all of which circumstances are found actually 

 to take place. 



III. The formation of dew, indeed, not only 

 does not produce cold, but like every other pre- 

 cipitation of water from the atmosphere, pro- 

 duces heat. I infer this, partly because very 

 little dew appeared upon the two nights of the 

 greatest cold I have ever observed on the sur- 

 face of the earth, relatively to the temperature 

 of the air, both of them having occurred after a 

 long tract of dry weather ; and partly from the 

 most dewy night, which I have ever seen, having 

 been attended, during the greater part of it, 

 with no considerable degree of cold. On this 

 night, the difference between the temperatures 

 of grass and of air was at first 7^, the dew 

 being then not very abundant. But, after the 

 dew had become very abundant, the difference 

 of those temperatures never exceeded 4, and 

 was frequently only 3. 



With the view of obtaining, though indirectly, 

 some knowledge of the quantity of cold, which 

 had been prevented, by the formation of dew, 

 from appearing on the surface of the earth, in 



