FOREST IKFLUENCES. 29 



fulfils an important office in checking a much greater loss 

 due to evaporation and thus becomes an element of con- 

 servation." 



Evaporation. The loss by evaporation after the water 

 has reached the ground depends in the first place upon the 

 amount of direct insolation of the soil, and hence its tem- 

 perature, which again influences the temperature of the air. 

 The nature of the soil cover, the relative amount of moisture 

 in the atmosphere, and the circulation of the air are also 

 factors determining the rate of evaporation. The impor- 

 tance of evaporation as an element of dissipation may be 

 learned from the experiments of Prof. T. Russell, Jr., of the 

 U. S. Signal Service, made in 1888. We learn from these 

 that the evaporation on the Western plains and plateaus 

 may, during the year; amount to from 50 to 80 inches, nay, 

 in spots, 100 inches, while the rainfall (diminishing in re- 

 verse ratio) over this area is from 30 to 12 inches and less. 



''Thus, in Denver, where the maximum annual precipi- 

 tation may reach 20 inches, the evaporation during one 

 year was 69 inches. This deficiency of 49 inches naturally 

 must be supplied by waters coming from the mountains^ 

 where the precipitation is large and the evaporation low. 

 (On Pike's Peak alone there may be 45.6 minus 26.8 or 18.8 

 inches to spare.)'' 



Evaporation from the soil is dependent upon its cover- 

 ing, and this is important, as the soil in forests is always 

 covered with dead branches, leaves, etc. In some experi- 

 ments which were carried on in Germany during the 

 months of July and August, 1883, to determine the amount 

 of evaporation from different soils, it was found that from 

 1,000 square centimeters of bare ground, 5,730 grams of 

 water were evaporated, and that from the same area of simi- 

 lar 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 



