WATER IN THE ATMOSPHERE 219 



cool the surface layers of air below the saturation, or dew-point as it is 

 now termed, the excess of water vapour is deposited as dew, and he 

 made out the chief conditions for a plentiful deposit. When the sky ia 

 clear the radiation is greater, and when the surface layers are at rest 

 they are more rapidly cooled below the dew-point. Hence the conditions 

 favourable are, air with considerable relative humidity, at rest and under 

 a clear sky to allow free radiation. The nature of the ground has, of 

 course, some effect. If it is a good radiator and a bad conductor, its 

 surface will become the more rapidly chilled. 



Aitken (Proc. R.S. Edin., 1886) has supplemented Wells' theory of 

 dew by showing that there are really two kinds, the dew which comes 

 from the air and the dew which is the extruded water or the condensed 

 vapour exhaling from the surface. The surfaces of leaves are always 

 tending to give off vapour, and if the air is not saturated it takes this 

 vapour up. But if the air is saturated, then the exhalation forms drops 

 of dew on the surface. 



