FOEEST INFLUEXCES. 31 



Now, if it is considered that the average velocity of 

 the winds which constantly sweep the Western subarid 

 and arid plains is from 10 to 15 miles, not rarely at- 

 taining a maximum of 50 and more miles, the cause of 

 the aridity is not far to seek, and the function of the 

 timber belt or even simple windbreak can be readily 

 appreciated. 



Professbr King has found in experiments made in "Wis- 

 consin that the influence of even a thin stand of woodland 

 on the rate of evaporation was considerable. In one ex- 

 periment made in the month of May, the instruments were 

 so placed as to measure the evaporation to the leeward of a 

 scant hedgerow six to eight feet high, having in it a few 

 trees twelve feet high and many open gaps. It was found 

 that at 300 feet from the hedge the evaporation was 30.1 

 per cent, greater than at 20 feet, and at 150 feet it was 7.2 

 per cent, less than at 300 feet. The experiment was made 

 during a moist north wind. It is sufficiently evident, there- 

 fore, that even a thin hedgerow exerts an influence that 

 can readily be measured. In fact the presence or absence 

 of protecting belts of trees under the conditions often 

 existing on our prairies may make a difference between a 

 good and a poor crop. All who are acquainted with our 

 prairie sections know that great damage is often done to 

 wheat, corn, and other crops by the hot southwest winds 

 which we are likely to have during the growing months. 

 In Kansas and Nebraska, during the summer of 1894, 

 immense tracts of corn, fully tasselled out, were killed by 

 such winds. At the same time it was noticed that where 

 corn was protected by trees or slopes of land, or where 

 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 



