58 TREES AND TREE-PLANTING. 
“MOTION OF THE ATMOSPHERE. 
“There is a marked contrast in the motion of a liquid 
like water, and an elastic, gaseous fluid like air. If we 
place an impediment in a creek the water immediately 
flows around the impediment, and will not flow over it 
as long as a clear way can be found to either the right 
or the left. But the air not only moves around on either 
side, but piles up in front of whatever checks its course 
and rolls over the top of the impediment as readily as it 
passes around. Thus a grove of timber or a thin shelter- 
belt effectually checks the motion of the wind. 
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The wind rises over the trees, as indicated by the ar- 
rows in the figure, and, instead of falling like water to 
the ground, it flows on, as shown, and does not reach the 
original level until it has gone a distance of eleven times 
the height of the wind-breaks. There will be a quiet 
atmosphere immediately about the trees, but to eleven 
times the height of the shelter-belt, and even in the 
teeth of the wind at D, there will be a quiet atmosphere. 
It is well known that while the wind may sweep with 
fearful velocity over a forest and powerfully agitate the 
tops of trees, the motion is comparatively slight within 
the forest ; the same is true of a succession of shelter-belts. 
The wind will sweep with great force over the trees at 
C, while all below remains quiet. The extent of these 
quiet spaces, A and B, will of course depend upon the 
height of the shelter-belts. Any one who will take the 
