AS AFFECTED BY FORESTS. 257 



by the plentiful undergrowth of herbage which 

 thrives beneath their shelter, they hinder the rain 

 water, even on sloping ground, from quickly run- 

 ning off. They everywhere check the currents of 

 air, and thus retard the carrying off of the moisture 

 by drying winds. Under their leafy shade, the 

 heat of the summer is assuaged, and the few 

 sun-rays that find their way to the soil, can lick 

 up but little of the moisture for the warm currents 

 to carry to the upper air. Again, because the water 

 sucked up from the earth by the roots is breathed 

 out again by the twigs and leaves as vapour,, 

 the air in the forests always remains moister 

 and nearer to the dew-point than it does where 

 the land is not wooded ; while, therefore, the warm 

 air is mounting up from the dry soil of the open 

 country, the chilled and moister air must fall again 

 from above upon the cooler surface of the wood- 

 land. Thus all the conditions necessary for the 

 fall of moisture from the air meet in much greater 

 force about the forests than over the open country. 

 When the south wind charged with vapour 

 blows over dry ground heated by the sun, no 

 fall of wet can easily take place; but rather 

 the clouds that are ready formed, sinking down 

 into the warmer layers of air below, are dissolved 

 again and vanish. The parched warm soil drives 

 the rain away from itself. But where the tem- 

 perature is softened down by woods, and when 



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