<)(> A PRIMER OF FOKKSTRY. 



tin* headwaters of streams, especially in the Rocky 

 Mountain reo-ions of the I'nited States. Kvaporation 

 i> there so active that u'reat banks of snow lyin<r in the 

 full trlare of tlie sun often dis;ip])ear without melting 

 even enough to moisten the ground on tin 1 hillsides 

 below them. Vast quantities of water evaporate in 

 this way without ever reaching the streams. Measure- 

 ments made by the Bureau of Forestry show that 

 evaporation from snow may be four or live times as 

 Lrreat as from water under like circumstances. 



RAINFALL. 



The causes of rain are for the most part wholly 

 beyond the reach of influence from the forest. Such 

 are the o-reat currents of warm and cold water in the 

 ocean, the direction of the prevailing winds, and the 

 presence or absence of mountain ranges. But there 

 are two reasons which lead us to believe that forests 

 do atfect the rainfall. These are their colder and 

 moister air. and the resistance which they oiler to the 

 motion of the winds. A ^reat number of observations 

 has been made in different parts of the world to dis- 

 cover how much the rainfall really is atlected by the 

 forest, but for several reasons no generally accepted 

 result has yet been reached. In the lirst place, accu- 

 rate observations on rainfall are not easy to make. 

 The height above the ground at which a rain ^auire 

 i- placed a fleets it very seriously. A variation of l' 

 fc.-t in height \\ill often make more difference in the 

 amount of rain caught than most observers claim for 

 the whole action of the forest. The rainfall of two 

 -tat ion- at unequal heights above >ea level is some- 



