356 THEORY OF DEW. 



only two or three degrees. In the United States 

 of America, the difference is sometimes 00 F. 

 the consequence of which is copious depositions 

 of dew, resembling the effects of a shower of 

 rain. They are also copious in England, during 

 the prevalence of warm winds from the ocean. 



All other things being equal, a greater amount 

 of dew is condensed between midnight and sun- 

 rise, than between sun-set and midnight, accord- 

 ing to the observations and experiments of Dr. 

 Wells obviously because the surface of the 

 earth is then colder, from the loss of caloric by 

 radiation. Dew, then, is not condensed and 

 precipitated like rain by the coldness of the 

 atmosphere ; but by the coldness of the earth's 

 surface, which is proved by the well known fact 

 that dew and frost are formed on the under sur- 

 face of planks, &c. as well as the upper. 



It has been long known, that little or no dew 

 is formed under a cloudy canopy ; but that it 

 is abundant during clear nights. Dr. Wells 

 referred this circumstance to the obstruction of 

 caloric radiated from the earth's surface by 

 clouds. It is extremely probable, however, from 

 his own statement of facts, that he mistook a 

 mere concomitant for the real cause of the phe- 

 nomenon. He informs us, that when clouds 

 begin to form during the night, the surface of 

 the earth rises several degrees, and that dew 

 formed during clear intervals, often evaporated 

 again when the sky becomes thickly overcast ; 



