ii8 Deserts 



has raised the temperature. A similar, but in this case 

 disastrous, result has been produced on the southern 

 slope of the Pyrenees, where what once were wide 

 fertile tracts, covered with vegetation, have been turned 

 into wastes by the destruction of the forests too reck- 

 lessly carried out. 



Wooded countries certainly seem on the whole to 

 receive most rain ; and the clearing away of any kind 

 of vegetation, be it herbage, brushwood, or forest-trees, 

 may be, and often has been, attended by evil conse- 

 quences. For vegetation protects the soil from 

 evaporation, enabling it at least to keep what water 

 it receives, and, as this accumulates, springs, or 

 reservoirs, are formed, from which the plants in their 

 turn may derive supplies when rain fails or is 

 insufficient. 



Then again, vegetation preserves the soil from the 

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

 portance, 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 cen- 

 -turies 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, 

 jrises. 



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 upop any moist current >yith which it 



