94 The Great World's Farm 



which the plants in their turn may derive suppHes when 

 rain fails or is insufficient. 



Then again, vegetation preserves the soil from the 

 assaults of wind and rain, a matter of no small impor- 

 tance, especially in mountain regions, for as we have 

 already seen, the earth on the slopes may be clean washed 

 or blown away, and the fertility of centuries may be thus 

 destroyed. 



But even this is not all. The soil gone, what remains? 



Bare rock or subsoil, which is dried and heated by the 

 sun, growing drier and therefore hotter, till it is quite 

 parched. But a dry, hot surface heats and dries the air 

 above it, for hot air, being lighter than cold, rises. 



From a wide expanse of dry, hot sand, such as that of 

 the Sahara, therefore, there must be a constant upward 

 current of hot air, and this, again, must act like a furnace 

 upon any moist current with which it comes in contact. 

 The moisture has no chance of condensing into a cloud, 

 or rain, as it might if it met with cool air, but is dis- 

 persed — drunk up and evaporated by the hot, thirsty air 

 from below. No wonder, therefore, that the Sahara is a 

 rainless region. 



The Island of St. Helena, again, is a notable instance 

 of what man can do in the way of reducing a luxuriant 

 garden to a barren waste, simply by his ignorant or reck- 

 less destruction of its natural vegetation. When first dis- 

 covered, the island, though very mountainous, and bounded 

 by tremendous precipices rising some two or three thou- 

 sand feet above the sea, was very fertile, and possessed a 

 luxuriant growth of forest. For it is astonishing what 

 a thin film of soil is enough for seeds to sprout in, if only 

 it be moist; and it is astonishing, too, how little soil wiir 



