96 OUTLINES OF FORESTRY. 



The vapor of water exerts another and still 

 more powerful influence on the climate of the 

 earth. Water vapor possesses in a marked de- 

 gree the power of absorbing heat rays of the sun. 

 About twenty-eight per cent, of the heat of the 

 vertical rays is absorbed before such rays reach 

 the surface, provided there is a sufficient quantity 

 of vapor in the air. "When the heated earth throws 

 off or radiates its heat into the atmosphere, the 

 same water vapor absorbs a greater part of such 

 rays, and rapid cooling by radiation is thus pre- 

 vented. The presence of the water vapor, there- 

 fore, prevents either the rapid heating of the 

 earth's surface by the direct action of the sun's 

 rays, or the rapid cooling of such surface by 

 radiation. 



If the earth's surface were deprived of this 

 screen of vapor, the air would become so rapidly 

 heated on the rising of the sun, and so rapidly 

 cooled on its setting, that the earth would be 

 unable to sustain its present plant and animal 

 life. 



Tyndall, speaking of the influence that the 

 earth's water vapor exerts on the climate of Eng- 

 land, says, " The removal for a single summer 

 night of the clouds of vapor which cover Eng- 



