SOIL MOISTURE AND TO EVAPORATION. 



33 



ceased with the rain and the evaporation rate continued to fall for 

 some time after, not only because of the fall in temperature, but also 

 because of the increase in the moisture content of the air. 



Besides the evidence of the curve just presented, the importance of 

 air currents in determining the rate of evaporation may be illustrated 

 by the following comparative measurements. The velocity of the air 

 current produced by an electric fan, when in motion at each of three 

 different speeds, was taken by means of an air meter at a position 

 30 cm. in front of the fan. Then the air meter was removed and a 

 portable evaporimeter was so placed that its porcelain cylinder occupied 

 the same position. Readings for 5-minute periods were then taken on 

 the evaporimeter, with the air at rest and with three different veloci- 

 ties of air current. Each test consisted of several 5-minute periods. 

 After every test with an air current a test was made in still air, by 

 merely stopping the fan, so as to make absolutely sure that the rate in 

 quiet air had not changed appreciably as time passed. The whole 

 experiment lasted less than two hours and during this time the air 

 temperature remained at 29 C. (84.2 F.), and the relative humidity 

 remained at 63 per cent, as determined by the sling psychrometer. The 

 results are given in Table VI. Air velocities are given in meters and 

 feet per minute and in kilometers and miles per hour; evaporation 

 rates are given in cubic centimeters as observed for 5-minute periods 

 and as calculated for hour periods. 



TABLE VI. Effect of Wind on Evaporation Rate. 



This particular evaporimeter was not calibrated by weighing a vessel 

 of water, but since it was similar in every respect to the fixed one which 

 was calibrated, its readings may be taken as approximately equal to 

 the loss from a free water surface of 100 sq. cm. in the same position 

 and during the same period. It is to be noted from the above data that 

 a breeze with a velocity of only 16.37 kilometers per hour produces an 

 acceleration in water loss by evaporation of 250 per cent, and that with 

 an air current moving at the rate of 29. 1 kilometers per hour, an accelera- 



