LETTER XV. 



THE MOISTURE OE THE AIR, AND THE FALL 

 OF WET. 



Moisture is, as you know, next to heat, a 

 condition the most necessary to the thriving, and 

 even to the life, of plants, and therefore to the 

 fitness of a country for the dwelling of man. 

 Plants are dried up, and their growth stopped, if 

 moisture fails in summer. The smiling fields of 

 Dauphiny, clothed with the loveliest green in 

 spring, and the plains of Southern Italy and of 

 Greece, after being without rain for months in 

 summer, are changed for the most part into dry 

 sandy flats. In many hot regions where water is 

 either quite or nearly wanting, the soil is unfruit- 

 ful, just as in the high north, and almost devoid 

 of herbage. Such is the state of the vast tracts 

 known as the Desert of Sahara, and of great 

 part of Arabia and Persia. 



Springs and rivers can only supply the water 

 required for the nourishment of plants at particular 

 spots, and that constantly, as a general rule, only 

 near high mountain-chains. Thus Lombardy 

 owes its uncommon fruit-fulness chiefly to the 

 industrious care, which the inhabitants have been 



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