1814.] Dr. IVells' Essay on Dew. 305 



radiated by the walls and furniture of the room, which struck 

 against the uncovered panes ; hence the former were colder than 

 the latter. 



2. When we go out of a house into the open air during the night, 

 we are frequently sensible of a considerable chill. This, according 

 to Dr. Wells, is greatest in a clear night, and is more sensible in 

 the country than in town. He conceives it to be owing to the 

 heat radiated from our bodies. To this cause the noxious effects ot 

 the night air are probably owing. 



3. Gardeners are in the habit of covering plants with mats in 

 order to prevent the bad effect of the cold. The use of this cover- 

 ing is to prevent the heat from escaping from the plants by ra- 

 diation. Dr. Wells found grass, over which a thin cambric hand- 

 kerchief was placed horizontally a few inches above it, several de- 

 grees hotter than grass fully exposed to the aspect of the sky. The 

 effect is not so great when the mat touches the body as when it is a 

 few inches distant from it. 



4. The covering of snow upon the surface of the earth during 

 winter in northern regions is chiefly useful by preventing the loss of 

 heat from radiation ; which would be sufficient in a few hours to 

 destroy vegetables altogether. 



5. It is believed in the West Indies, and seems to have been ad- 

 mitted by the ancients, that the exposure of animal substances to 

 moonlight promotes their putrefaction. Moonlight nights are al- 

 ways clear, and usually calm : hence dew will be deposited most 

 copiously during sucli nights. Covering animal substances with 

 dew, in addition to their own moisture, must in hot climates pro- 

 mote their putrefaction : hence probably the origin of the notion. 



6'. Ice is formed in Bengal by exposing water in shallow un- 

 glazed pans to the air, placed upon dry straw or sugar-canes, and 

 sometimes sunk artificially below the surface of the earth. Ice 

 forms only in clear and serene nights. The formation of ice in 

 these cases has been usually ascribed to the evaporation of water 

 from the outer surface of the pans. But Dr. Wells has shown, 

 both by the clearest arguments and by experiments, that this ex- 

 planation is not the real cause; but that the formation of ice is 

 owing to the heat radiated from the surface of the water. The straw 

 serves only to prevent heat from being conducted into the water 

 from the earth. Hence the reason why the process requires a clear 

 and serene night, and why wind and clouds prevent it from suc- 



ing. 

 •Such is a short analysis of this very interesting performance. 

 Few bookl have made their appearance of late years containing a 



t( r number of new and importunt facts; and few scientific 

 theories have ever been presented to the world either in a more sa- 

 tisfactory or luminous manner. 1< may be proper to mention that 

 there occurs a word in Dr. WeUs's essay which 1 do nut recollect 

 to have ever seen before. 1 mean the word conduction, The Eng- 

 u»h languaj destitute of a single word bearing the meaning 



Vol. 1\ V IV. U 



