ON DEW, &c. 277 



the evening in the open air to the sky, lost a 

 little weight, in the course of a ..clear night. 

 This I imputed to evaporation taking place, 

 before the water had been cooled enough to 

 condense the vapour of the atmosphere, and to 

 the weight gained afterwards being insufficient 

 to compensate the previous loss. I exposed, 

 therefore, on this night, water to the influence 

 of the sky, until it was cooled to 34. Of this 

 I put 2 ounces into each of two china saucers? 

 which had also been exposed to the air, and 

 then placed the saucers upon the straw bed. 

 In the morning, a thin cake of ice was found in 

 both saucers, one of which had gained 2^, and 

 the other 3 grains, in weight. Dew was also 

 copious on this night. At one time, grass was 

 9j, and the exposed part of the straw bed 12, 

 colder than the air*. 



It must be evident to every person, that the 

 formation of ice, in the three preceding experi- 

 ments, was the effect of a natural operation, 

 similar to that by which the same substance is 

 produced in Bengal. These two facts must, 

 therefore, have a common cause, and this has 



* The greater cold, observed in this and the preceding 

 experiment, upon straw than upon grass, is to be referred 

 to the shortness of the latter, by reason of which heat was 

 readily communicated to its upper parts by the earth. 



