242 MOISTURE OF THE AIR. 



taking for centuries, to spread the water from the 

 Alps over the whole face of the land. The fer- 

 tility of the desert-bounded valley of the Nile 

 depends only on the overflowing of the river. And 

 the flourishing appearance of those single spots — 

 the Oases — in the midst of the deserts of Africa 

 and Arabia, is due to the springs which gush forth 

 within them. There are instances too, some of 

 which I have already mentioned to you, of indus- 

 trious people having won for their land a produc- 

 tive power, which nature seems to have denied to 

 it, by artificially opening springs and pouring them 

 upon their soil. 



All these, however, are, as I have said, but 

 single and partial results. The most, universal 

 and active source of the water necessary for the 

 growth of plants is the moisture of the air, the 

 water, which the air carries with it in greater or 

 less quantity as vapour. 



You know the property which water has of 

 evaporation. Water left to itself in open vessels, 

 and cut off from all supplies from without, lessens 

 gradually in bulk, and at last vanishes altogether ; 

 wetted bodies placed in the open air become dry. 

 The moisture that has come in winter and spring 

 always disappears when we have had fine weather 

 for some time in the warm season. The stones 

 wetted by a summer shower, the plants covered 

 with the morning dew, generally become dry soon 



