PBACTICAL FORESTRY. ^ S5 



the force of the winds because the elastic swaying of the twigs and 

 branches is a very effective hindrance to the movement of the air. 

 Strong winds, although they are often dangerous in themselves, do 

 most harm by drying up the moisture in the soil and in the plants 

 which grow from it. Thousands of miles of wind-breaks have been 

 planted by farmers in the western parts of this country to protect 

 their crops and homes against the wind. These wind-breaks serve 

 a most useful purpose, but they are naturally far less effective in 

 preventing evaporation than the forest itself. So great is the power 

 of the latter that direct observations made in Bavaria and Prussia 

 showed that evaporation from a free surface of water in the forest 

 was only 40 per cent of that in the open. 



The presence or absence of leaf mold has a powerful effect on the 

 amount of evaporation from forest soil. The experiments of Doctor 

 Ebermayer, a famous German forest meteorologist, showed that evapo- 

 ration from forest soil without a layer of mold was 47 per cent of that 

 from soil in the open, while with a layer of mold it was less than 

 half as much, or 22 per cent. The greater the altitude above the 

 sea the greater is the effect of the forest in preventing evaporation. 

 This is a powerful reason for preserving mountain forests at the 

 headwaters of streams, especially in the Rocky Mountain regions 

 of the United States. Evaporation is there so active that great 

 banks of snow lying in the full glare of the sun often disappear 

 without melting even enough to moisten the ground on the hillsides 

 below them. Vast quantities of water evaporate in this way with- 

 out ever reaching the streams. Measurements made by the Geo- 

 logical Survey show that evaporation from snow may be four or five 

 times as great 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 great 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 affect the rainfall. 

 These are their colder and moister air, and the resistance which they 

 offer to the motion of the winds. A great number of observations 

 has been made in different parts of the world to discover how much 

 the rainfall really is affected by the forest, but for several reasons no 

 generally accepted result has yet been reached, In the first place, 

 accurate observations on rainfall are not easy to make. The height 

 above the ground at which a rain gauge is placed affects it very seri- 

 ously. A variation of 10 feet in height wQl often make more differ- 



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