Forest Influences 313 



many during the months of July and August, 1883, to deter- 

 mine the amount of evaporation from different soils, it was 

 found that from 1000 square centimeters of bare ground 

 5730 grams of water were evaporated, and that from 

 the same area of similar soil covered with two inches of 

 straw 575 grams were evaporated. This shows that the 

 naked soil evaporated more than ten times as much as 

 the covered soil. It is evident, then, that the soil covering 

 has an important function in preventing evaporation. 



The forest cover acts in the same way. If the loss by 

 evaporation from an open field be compared with that 

 of a forest-covered ground, as a matter of course it will 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 moist, therefore less capable 

 of absorbing moisture from the soil by evaporation. 

 Moreover, the forest cover above, which intercepts the 

 direct rays of the sun and shades the ground, also assures 

 us that the ground will be covered with a layer of dead 

 branches, leaves, twigs and the like. 



The next point to be considered is the effect of wind 

 velocity on evaporation. The evaporation under the 

 influence of the wind is dependent not only on the tem- 

 perature and dryness of the wind, but also on its velocity, 

 which being impeded, the rate of evaporation is reduced. 

 T. Russell, Jr., experimented on this subject in 1887. 

 His experiments were made with Riche's hygrometers 

 whirled around on an arm 28 feet in length, the results of 

 which were compared with those from a tin dish containing 

 40 cubic centimeters of water exposed under shelter. 

 The results show that, with the temperature of the air at 



