THE SEASON WHY. 103 



" Thou, O God, didst send a plentiful rain, whereby thou didst confirm thine 

 inheritance, when it was weary." PSALM 



Because, being situated in tropical or torrid latitudes, and at a 

 distance from the ocean, the atmosphere above them is always in a 

 dry state. 



430. When is air said to ~be saturated with vapour ? 

 When it cannot take up a larger quantity than that which it 



already holds. 



When common salt is dissolved in water, until the water can take np no 

 more, the water is then said to be saturated with salt. 



431. What proportion of water is air capable of sustaining 

 in the form of vapour ? 



The amount of water held in suspension by the air averages the 

 following proportion : one thousand cubic feet of cftr contain as 

 much vapour as, were it condensed to water, would yield about 

 two fifths of a pint. 



But one thousand cubic feet of air are capable of holding half -a- 

 pint of water; and this may be regarded as the point of 

 saturation. 



Thus, in a room ten feet square and ten feet high, the air, at the point of 

 saturation, would hold in the form of vapour, half-a-pint of water. It must 

 not be forgotten, however, that the point of saturation necessarily varies with 

 the temperature of the air. 



432. Why are cloudy days and nights not always wet ? 

 Because the air has not reached the state of saturation. 



433. Wliy does rain purify the air ? 



Because it produces motion in the particles of the air, by which 

 they are intermixed. And it precipitates noxious vapours, and 

 cleanses the face of the earth from unhealthy accumulations. 



434. Why are mountainous localities more rainy than fiat 

 ones ? 



Because the mountains attract the clouds; and because the 

 clouds that are flying low are borne against the sides of the 

 mountains and directed upwards, where they meet with cold 

 currents of air, 



435. Why does more rain fall ly night than ly day ? 



