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 4o 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 suthciently 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 
