270 EVAPORATION. 



of these and that of the atmosphere is the greatest. 

 Hence we may see the reason why dew is more abun- 

 dant in some places than in others. The soil being com- 

 posed of different materials, gives off heat with different 

 degrees of rapidity. It may perhaps seem surprising, 

 that so much dew should be deposited from so small a 

 quantity of air, as that which comes in contact with the 

 earth ; but the quantity of water deposited on a tumbler 

 of cold water in hot weather, shows that a small portion 

 of the atmosphere may hold in solution an amount of 

 moisture by no means inconsiderable. It might perhaps 

 seem that more dew would be deposited in very windy 

 nights, than at other times, but generally this is not the 

 case. Probably the particles of air are not in contact 

 with the earth a sufficient length of time to lose much of 

 their heat, and consequently continue to hold in solution 

 most of the water combined with them. 



It is often remarked that on cloudy nights there is 

 little or no dew. This may perhaps be explained a& 

 follows: There is a tendency in heat to diffuse itself 

 uniformly among bodies by a constant radiation from 

 one to another, which is rapid in proportion to the differ- 

 ences of temperature. Bodies therefore differing widely 

 in temperature are soon reduced to nearly the same 

 degree. When there are clouds in the atmosphere at 

 night they receive the heat darted upwards from bodies 

 on the earth's surface, and throw it back again to the 

 earth ; thus keeping the atmosphere near the earth warm- 

 er than it would otherwise be. But in clear weather, the 

 heat thus sent upwards rinding no obstacle to intercept 

 its progress, darts away into boundless space, and is lost 

 altogether to the objects which emitted it. Probably the 

 sultry oppressive heat which is sometimes felt on cloudy 

 days may be attributed to the same cause. 



EFFECTS ON TEMPERATURE, CLIMATE, ETC. 



The effect of evaporation on temperature is strikingly 

 exhibited in the refreshing coolness, which generally fol- 

 lows a summer shower. As heat is the principal, if not the 

 sole agent in the process of evaporation, the rapid evapora- 



