RISE OF WARM AIR. 211 



I have already shown you that the air takes its 

 heat, in the first place, chiefly from the ground ; 

 that the warmed and so expanded air rises, and that 

 it is only in this way that the heat of the soil is 

 spread over the higher regions of the atmosphere. 

 Now the soil is not everywhere equally heated. 

 The degree, in which it is favoured in this respect, 

 depends, as you know, greatly upon the situation 

 and latitude of the place. Besides this, certain 

 substances have the property of absorbing more of 

 the sun's rays than can others, supposing equal 

 quantities of heat to fall upon both. Again, other 

 substances allow the sun's heat to penetrate deeper ; 

 and thus from one cause or the other the soil at 

 different places takes a different temperature. Thus 

 during the day-time the temperature in the shade 

 of trees, of houses, and of clouds, in moist meadows 

 and forests/ and on surfaces of water, is usually 

 lower than on dry soils, on rocks, on roofs, and on 

 level plains. The air which rises most quickly 

 over the warmest spots of the soil, is replaced 

 by air coining in from cooler places ; and thus 

 are set up those movements of the air, which we 

 generally find on the borders of forests, in the 

 shade of trees, at the openings of shady mountain 

 glens, and in such valleys themselves, on the banks 

 of rivers and lakes, and on the sea-shore. 



It is impossible that the air can flow from one 

 place to another, without being replaced by a 



P 2 



