Reduction of Transpiration Observations . 
249 
5. Improvements in ‘ Open-pan 5 Methods for calibrating 
Atmometers. 
It appears to us that if a water surface be used as standard, two 
methods of procedure may be followed : 
(1) In calibrating the atmometer, three or more basins of varying radii 
may be used, filled to about 5 millimetres from the rim. The actual value 
of n may then be determined by plotting log E against log a , as previously 
explained. Then if E be the evaporation per unit time from the basin of 
radius a v E 2 the evaporation per unit time from the evaporimeter, and a 2 
the radius of its equivalent surface 
E_i _ (*h\ n 
e 2 ~ V ’ 
or 
l 0O . ^ = n\og ci^ log E t + log E 2 
53 2 11 
giving a 2 . The area (A) of the equivalent water surface is then given by 
A = 7 T a 2 • 
(2) In the second method, although a single basin is sufficient, two or 
three basins may preferably be employed to minimize incidental errors. 
Each basin should be filled to a depth of about 3 centimetres from the rim, 
when it can safely be assumed that n — 2 . Then, using the same notation 
as before, we have 
E\ _ TTa-f 
or 
E, 
A = 
A 
7 ra. 
be 
calculated 
from each of the basins 
The equivalent area A should 
employed, and the mean taken. 
This appears to be the method to be preferred. Such a water surface 
seems to possess the qualities of constancy and of easy reproduction which 
are absolutely necessary in every practical standard, and its general use 
would, we think, remove the great uncertainty which is an undesirable 
feature of the present methods of calibration. Moreover, as will shortly be 
shown, a consideration of the relative errors in the different quantities 
involved will enable us to remove practically every source of uncertainty 
and to make the calibration as accurate as the circumstances may demand. 
6. Example of Method. 
Meantime, a practical example will perhaps help to make matters 
clear : 
The transpiration from a small Pelargonium was determined in the 
usual manner, taking all ordinary precautions. In order to determine the 
