30 PRINCIPLES OF AMERICAN FORESTRY. 



evident, then, that the soil covering has an important 

 function in preventing evaporation. 



Wind-breaking Power of Forests. If the loss by evap- 

 oration from an open field be compared with that of a 

 forest-covered ground, as a matter of course it will be 

 found to be less in the latter case, for the shade not only 

 reduces the influence of the sun upon the soil, but also 

 keeps the air under its cover relatively moister, therefore 

 less capable of absorbing moisture from the soil by evapo- 

 ration. In addition, the circulation of the air is impeded 

 between the tiainks, and this influence upon available 

 water supply, the wind-breaking power of the forest, 

 must be considered as among the most important factors 

 of water preservation. Especially is this the case on the 

 Western plains and on those Western mountain ranges 

 bearing only a scattered tree-growth, and where, there- 

 fore, the influence of shade is but nominal. 



The evaporation under the influence of the wind is 

 dependent, not only on the temperature and dryness of the 

 same, but also on its velocity, which being impeded, the 

 rate of evaporation is reduced. 



Interesting experiments for the purpose of ascertaining 

 the changes in the rate of evaporation effected by the 

 velocity of the wind were made by Prof. T. Russell, Jr., 

 of the Signal Service, in 1887. The result of these experi- 

 ments (made with Piche's hygrometers whirled around on 

 an arm 28 feet in length, the results of which were com- 

 pared with those from a tin dish containing 40 cubic centi- 

 meters of water exposed under shelter) show that, with 

 the temperature of the air at 84 degrees and a relative hu- 

 midity of 50 per cent., evaporation at 5 miles an hour was 

 2.2 times greater than in a calm; at 10 miles, 3.8; at 15 

 miles, 4.9; at 20 miles, 5.7; at 25 miles, 6.1, and at 30 miles 

 the wind would evaporate 6.3 times as much water as a 

 calm atmosphere of the same temperature and humidity. 



