36 ELEMENTARY FORESTRY. 



Interesting experiments for the purpose of ascertaining the 

 changes in the rate of evaporation effected by the velocity of the 

 wind were made by Prof. T. Russell, Jr., of the Signal Service, 

 in 1887. The result of these experiments (made with Piche's 

 hygrometers whirled around on an arm 28 feet in length, the 

 results of which were compared with those from a tin dish con- 

 taining 40 cubic centimeters of water exposed under shelter) 

 show that, with the temperature of the air at 84 degrees and a 

 relative humidity of 50 per cent, evaporation at 5 miles an hour 

 was 2.2 times greater than in a calm; at 10 miles, 3.8; at 15 

 miles, 4.9; at 20 miles, 5.7; at 25 miles, 6.1, and at 30 miles the 

 wind would evaporate 6.3 times as much water as a calm atmos- 

 phere of the same temperature and humidity. 



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 attaining 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. 



Professor King has found in experiments made in Wiscon- 

 sin that the influence of even a thin stand of woodland on the 

 rate of evaporation was considerable. In one experiment made 

 in the month of May the instruments were so placed as to meas- 

 ure 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, 

 therefore, 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 sum- 

 mer 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 



