258 THE MOISTURE OF THE AIR; ITS FALL, 



the air, besides, is loaded with moisture by the 

 constant evaporation, it will soon be overcharged 

 by the falling clouds, and rain will follow. We 

 are therefore right in commonly saying that the 

 woods fetch down the rain, just as we are in saying 

 that it is drawn to mountains and broad rivers. 

 True it is, too, that the clouds cling to the forests 

 as they do to the mountains, since the cool and 

 moist air which floats over the woods, favours their 

 formation ; when, however, the resulting clouds, 

 driven by the wind beyond the forest, reach the 

 warm air rising up from a dry soil, they are again 

 dissolved into invisible vapour. 



On the other hand, we find that over single 

 woodless islands in the sea, the sky remains cloud- 

 less, while all around thick clouds may be gathered, 

 because the current, mounting up from the ground 

 heated by the sun's rays, raises for a time the capa- 

 city for vapour in the higher layers of the air. The 

 Canary Isles, when they were discovered, were 

 clothed with thick forests, and overgrown with the 

 richest verdure. Great part of these woods were 

 destroyed and burnt by the first settlers ; the result 

 has been the lessening of their rains, and the 

 dwindling away of their springs and brooks. 



Thus the rooting up or even the mere thinning 

 of forests always exerts a striking influence on the 

 moisture of a country. Many districts, which, in 

 former times were known for their rich fruitfulness, 



