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WIND-BREAKING POWER OF FORESTS. 3 
the humidity of the wind was increased by passing over bodies 
of water or clover fields, the injury was greatly lessened. 
What the possibilities of evaporation from hot and dry winds 
may be can be learned from statements regarding the ‘“Fcehn,” 
which is the hot wind of Switzerland, corresponding to the 
“chinook” of our western country. 
The change in temperature from the normal, experienced under 
the influence of the Fcehn, has been noted as from 28 degrecs 
to 31 degrees Fahr., and a reduction of relative humidity of 58 
per cent. A Foehn of twelve hours’ duration has been known 
to “eat up” entirely a snow cover 2% feet deep. 
In Denver a chinook has been known to induce a rise in 
temperature of 57 degrees Fahr. in twenty-four hours (of which 
360 degrees in five minutes), while the relative humidity sank 
from 100 to 21 per cent. 
The degree of forest influence upon rate of evaporation by 
breaking the force of winds is dependent upon the extent and 
density of the forest, and especially on the height of the trees; 
for according to an elementary law of mechanics the influence 
which breaks the force of the wind is felt at a considerable ele- 
vation above the trees. This can be practically demonstrated 
by passing along a timber plantation on the wind-swept plains. 
Even a thin stand of young trees not higher than five feet will 
absolutely calm the air within a considerable distance and height 
beyond the shelter. 
Professor King found that an oak grove 12 to 15 feet high 
exerted an appreciable effect in a gentle breeze at a distance of 
300 feet. In a strong wind the effect of such a grove would be 
felt at a much greater distance to the leeward. 
At the Dominion Experiment Station in’ Assiniboia Dr. 
Saunders found on one occasion that windbreaks exerted an 
appreciable influence at from 50 to 8o feet to leeward for every 
foot in height, but this was during a very severe wind. It may 
probably be laid down as a general rule that windbreaks will 
exert an appreciable influence for at least one rod for every foot 
in height. ; 
It may not be necessary to state that the damage done to 
crops by the cold, dry winter winds is mainly due to rapid 
evaporation, and that plants are liable to suffer as much by win- 
ter drouth as by summer drouth. 
