Surron—Study of Evuporation from Water-Surfaces. 171 
TaBLE 22.—Comparative Quantities of Evaporation. 
Piche Atmometer | Screened Tub 
inches inches 
January, . b : Sey 7°92 
February, . : : “oul 5°77 
March, : ; : a71 4°54 
April, : : : 4-71 3°45 
May, : : : 5°21 3°48 
June, 4:08 2°68 
July, 4°75 3°03 
August, . é : 6.54 4°41 
September, . 7:88 5°56 
October, 9-21 7°21 
November, . ; j 10°55 9-26 
December, . : 3 | 9°56 8°63 
Wear: s 5 . 84°48 65°94 
According to these results, the annual average evaporation 
from the tub is only 78 per cent. of that from the Piche atmometer, 
the winter average being only 65 per cent. Probably a smaller 
tub would evaporate more than a larger one; but the further 
amount cannot be great, since a copper pan 8 inches in 
diameter and 5 inches deep, exposed to the open sky at 
Kimberley, only loses about 90 inches ina year. Such a gauge 
becomes very hot during the day, and its high temperature and 
free exposure naturally contribute largely to increasing the loss 
by evaporation. Exposed in a louvered screen, it would, of course, 
lose much less than in the open air.’ 
It is to be remembered that the evaporating surface of the 
Piche atmometer is kept completely saturated by the superimposed 
column of water, and therefore the loss is maintained at its 
maximum rate under the given conditions of exposure. But the 
loss will evidently be curtailed if the porous surface become drier. 
What is called a “drying day after rain” only really attracts so 
1 <‘Gauges’’ of this pattern are usually supplied with a wire mesh, a sort of 
cheval de frise, to keep birds and animals from the water. This, perhaps because it 
breaks up the wind-currents, hinders the ordinary process of evaporation. 
