r>BW. 1 



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occur, the ground becomes 12 or 15 colder than the air a few- 

 feet above it. Dew is deposited very unequally upon various 

 substances; plants and vegetables, which need this sustenance, 

 receiving the greatest abundance ; but little is deposited upon the 

 dry land, still less upon polished metalic bodies, and .none at nil 

 upon the ocean. The deposition in these cases is proportional lo 

 the temperature, some bodies growing much colder than others 

 when exposed to the same cooling influence ; the surface of the 

 ocean, as wo have before remarked, remains at nearly the same 

 temperature as the air incumbent upon it. 



The surface of the earth is cooled by radiation of the hent it 

 has received during the day, and thus prepared for the deposition 

 of the dew. Hence dews are most abundant in a clear night 

 when the heat radiated from the earth is not intercepted and 

 thrown back from overhanging clouds. It is from this circum- 

 stance that the vulgar notion arises that the rays of the moon 

 have a chilling influence. When the ground becomes cooled by 

 the radiation of heat from its surface, below 32, the dew is fro- 

 zen, and then takes the name of white, or hoar frost. 



It will be apparent from what we have said, that dew will occur 

 most frequently when there is a considerable difference between 

 the heat of the day and the night. It is on this account that we 

 seldom have dews either in mid-winter, or mid-summer, i. e. at 

 the solstices, but generally just after the vernal, and before (he 

 autumnal equinox, viz : in May, and August. Dew is most co- 

 pious in those places which are sheltered from the wind, and high 

 winds therefore are a sure preventive to its formation, and of hoar 

 frost. Dew and frost occur most frequently in clear weather, 

 when the radiations of heat from the ground are thrown up inlo 

 the sky without being again reflected, hence the thinnest screen, 

 or the shade of a tree is a protection, and a cloth thrown over 

 delicate plants will preserve, them from frost. The quantity of 

 moisture precipitated from the atmosphere, depends upon a va- 

 riety of circumstances ; on the previous dampness of the com- 

 mingled portions of the fluid, their difference of heat, the eleva- 

 tion of their mean temperature, and the extent of the combina- 

 tion which takes place. When- the deposition is slow the very 



