May 28, 1917 
Transpiration Rate of Medicago sativa 
289 
THE ATMOMETER AS A MEANS OF MEASURING SOLAR-RADIATION 
INTENSITY 
The simultaneous measurement in the above determinations of the 
intensity of solar radiation and the evaporation rate from brown and 
white atmometers affords an opportunity of testing the atmometer as a 
means of measuring radiation intensity. The radio-atmometer, in which 
the evaporating surface is darkened so as to absorb as much of the 
incident radiation as possible, was developed by Livingston (1915, p. 147) 
primarily for— 
measuring the effectiveness of solar radiation as an accelerator of evaporation. It 
has not been shown to be available for measuring solar radiation as a whole, though 
its readings no doubt approximate such measurements to a greater or smaller degree. 
However, if the increase in evaporation of the radio-atmometer over the 
white atmometer affords a correct measure of the effect of the intensity 
of “solar radiation as an accelerator of evaporation” independently of 
& . /**/. e /& & /£>AZ G /& S /£>At e /& 
Fig. 4.—A comparison of the hourly radiation with the hourly differences in the evaporation rate from 
a brown cylindrical radio-atmometer and a white atmometer of the same form. 
any peculiarities of the instrument itself, then, conversely, the excess 
evaporation of the radio-atmometer over the corresponding white type 
should afford a measure of the intensity of the incident radiation. It is 
from this latter standpoint that the data are presented. The radio- 
atmometers employed were of the brown cylindrical type 1 mounted 
with the cylinders vertical, and in this position do not present a uniform 
surface area to the march of the sun during the day, as Livingston has 
already pointed out. Furthermore, the radio-atmometers did not absorb 
all the incident radiation, the surfaces being brown in color instead of 
dead black; but, since the coefficient of absorption of a surface does not 
vary with the amount of energy received, this factor is not of importance 
in this connection. * 
The hourly excess of the evaporation from the radio-atmometer over 
that of the corresponding white cylinders is plotted in figure 4 for the 
1 The new spherical radio-atmometers recently developed by Livingston were not available at the time 
these measurements were made. 
