ON DEW, &c. 237 



were altogether derived from watery vapour pre- 

 viously diffused through the atmosphere ; this 

 appearing to me to be by far its most considera- 

 ble source, and none of my conclusions of any 

 importance being liable to be affected, even by 

 the establishment of a contrary opinion. Other 

 writers, however, have regarded dew as being 

 entirely the product of vapour emitted, during 

 the night, by the earth and plants upon it. 

 According to this theory, dew is said to rise. 



The first trace, which I have found of the 

 opinion, that dew rises from the earth at night, 

 occurs in the History of the Academy of 

 Sciences for 1687. It is mentioned there briefly 

 and obscurely, and was, probably, shortly for- 

 gotten ; for Gersten, who advanced it anew in 

 1733, held himself to be its author. Muss- 

 chenbroek and Dufay embraced it immediately 

 after Gersten ; but the former soon admitted, 

 that dew sometimes falls. As far as I have 

 learned, no writer upon dew has since ascribed 

 its total production to vapour, emitted by the 

 earth at night, except Mr. Webster of New 

 England*. But this opinion is frequently ad- 

 vanced in conversation by persons, not much 

 accustomed to philosophical pursuits, chiefly, I 

 think, because it contradicts a popular belief. 



* Mem. of American Acad. vol. III. 



