DEW. 31 



the air cools, the moisture in it is condensed into clouds, 

 fogs, or mists, and finally into rain. 



97. There are many other atmospheric appearances or 

 phenomena which it is important for the husbandman to 

 be acquainted with, such as dew and hoar frosts, which 

 take place during the night, when the sky is clear ; snow, 

 which seems to be frozen mist ; hail, and hurricanes, 

 which are by some persons attributed to the action of 

 electricity. 



98. The Formation of Dew depends upon a property 

 which all solid substances have, in a greater or less 

 degree, according to their nature and outer surface. 



When I hold my hand towards the fire, I feel the heat 

 darting out from the fire to my hand. I feel it darting 

 out, in the same manner, from a hot stove or from a hot 

 flat-iron, on whatever side of the stove or iron I hold my 

 hand. The heat which darts out thus in every direction 

 from any hot thing is said to radiate from it, because it 

 comes out straight from it, just as the spokes, (radii, in 

 Latin,) come out on every side from the hub of a wheel. 

 If I observe carefully, I find that the heat comes out 

 more abundantly from a stove the surface of which is 

 very rough, than from one which is very smooth ; and I 

 discover that the reason is, that every little projecting 

 point radiates a stream of heat. 



Now, what I find to be true of the surface of a hot 

 stove is true of every surface. Every solid body is con 

 tinually sending out heat in straight lines, radiating 

 heat, from its surface. If several bodies are heated to 

 the same degree, the one which is roughest will radiate 

 and consequently cool most rapidly. 



When the sun sets, all things which have been exposed 

 to his heat send it forth by radiation, and grow cool. 



