WIND-BREAKING POWER OF FORESTS. 35 
and plateaus may, during the year, amount to from 50 to 80 
inches, nay, in spots, 100 inches, while the rainfall (diminishing 
in reverse ratio) over this area is from 30 to 12 inches and less. 
“Thus, in Denver, where the maximum annual precipitation 
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 covering, 
and this is important, as the soil in forests is always covered 
with dead branches, leaves, etc. In some experiments 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 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. 
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 evaporation. In addition, the circulation of 
the air is impeded between the trunks, and this influence upon 
available water supply, the wind-breaking power of the forest, 
must be considered as among the most important factors o/ 
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, therefore, 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. 
