32 PRINCIPLES OF AMERICAN FORESTRY. 



the "Foehn," which is the hot wind of Switzerland, corre- 

 sponding 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 degrees to 31 degrees Fahr., and a reduction cf relative 

 humidity of 58 per cent. A Foshn of twelve hours' dura- 

 tion 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 36 degrees in five minutes), while the relative humid- 

 ity 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 elevation above the trees. This can be practi- 

 cally 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 80 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 



