SHELTER-BELTS. 57 
ter drawn from the soil by growing trees and given off 
in the form of vapor from the leaves is simply im- 
mense. Thus it is stated that the eucalyptus of Austra- 
lia will absorb ten times its weight in a single day (Rept. 
No. 159, H. R. U.S., on Timber Culture, page 94). A 
small pear-tree has been found to absorb and give off 
more than its own weight of water in forty hours. The 
effect of this transpiration is seen in the prevailing moist- 
ure of the forest. We have only to surround a house 
with a dense growth of timber, and we learn the imme- 
diate result in the dampness and mildew which pervade 
the dwelling. Hence the amount of moisture pumped 
up by the growing trees, often from great depths, can 
hardly be measured. This process will be constantly 
varying in its activity with the conditions of vegetable 
life. 
“Extended observations in Europe have proved that 
there is a marked excess in the rainfall of an extensive 
forest over that of the open country. This should be 
expected, since the falling rain, as it reaches the prevail- 
ing moisture of the forest, must condense and carry much 
of its vapor to the ground. 
“Tf the positions above taken be correct, we should 
expect that wooded lands should be cooler than the open 
fields in the daytime, and warmer in the night ; and such 
a conclusion has been clearly established by extended 
observations, made under the direction of the Bavarian 
government during the last six years. 
“ The facts adduced prove that all vegetable life will 
cover itself with a glassy mantle, in density proportioned 
to the luxuriance of growth, and nearly opaque to the 
heat radiated from the earth. 
“ How can this glassy mantle be retained as a nightly 
and constant protection to vegetable life, or must it be 
swept away by the prevailing winds? To answer this 
question intelligently, we must consider briefly some of 
the simple laws which govern atmospheric motion. 
