252 
Thomas and Ferguson . — On the 
or, a square centimetre of atmometer surface evaporates 1*43 times as fast 
as a square centimetre of free water surface. 
Evaporation results are often given by ecologists and meteorologists 
in linear measure. The method of reduction is illustrated by Yapp in his 
paper, 1 and the argument may be stated as follows : 
Let the rate of evaporation from the atmometer be ^ c.c. per unit 
time. Then taking the figures given by Yapp’s 1908 I atmometer, the 
circular water surface from which this would evaporate in the same time 
must have an area of 62-07x3*25 (or 141-2x1-43) sq. cm. If d be the 
depth evaporated, then since the volume of a cylinder = A xd 
141-2 x 1-43 xd = x, 
7 X X 
or d — — > 
141-2x1-43 202 
which is the figure given by Yapp. 
But this restrtt again is based on the assumption that the ‘ area 5 law of 
evaporation is the true one. Let us attempt to find the law of reduction from 
cubic to linear measure, making no assumptions as to the law of evaporation. 
If E be the mass of water evaporated per unit time from a circular basin of 
radius r, then 
E = kr n . 
But if d be the depth evaporated 
E = 7t r 2 dp, 
where p is the density of water. 
d = 
Hence 
k_ 
7 Tp 
where K is a constant. If n = 2 — the usual assumption — then d — K, and 
under these circumstances, and only under these circumstances , will the 
evaporation in linear measure be independent of the radius of the basin 
employed. 
The calibrating dish used by Yapp was filled to about 3 mm. below 
its upper edge. Under these conditions the appropriate value of n is 
approximately 1*5. Whence we have 
d = 
that is, the evaporation in linear measure from a circular water surface is 
inversely as the square root of its radius. As an illustration, if we had two 
such dishes, one four times the radius of the other, the depth evaporated in 
unit time from the larger basin would only be one-half of that evaporated 
in the same time from the smaller basin. The possibility of the existence of 
1 Loc. cit , p. 315. 
