ELEMENTS OF DISSIPATION. 29 



"The evaporation under the influence of the wind is de- 

 pendent not only on the temperature and dryness of the same, 

 but also on its velocity, which being impeded, the rate of 

 evaporation is reduced. 



"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 containing 40 cubic centimeters of water exposed un- 

 der 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 atmosphere of the same temperature and 

 humidity. 



"Now, if it is considered that the average velocity of the 

 winds which constantly sweep the Western subarid arid 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 wind- 

 break can be readily appreciated." 



Professor 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 experiment 

 made in the month of May the instruments were so placed as 

 to measure the evaporation to the leeward of a scant hedge 

 row 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 hedge row ex- 

 erts an influence that can readily be measured. In fact the 

 presence or absence of protecting belts of trees under thecondi- 

 tions often existing on our prairies may make a difference be- 

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



