290 
Journal of Agricultural Research 
Vol. IX, No. 9 
three days during which detailed measurements were made. This hourly 
excess represents the difference in the hourly values for the brown and 
white cylinders, as summarized in Table I, which in each instance are 
based upon the mean of four radio-atmometer observations and the mean 
of four white-cylinder observations. For comparison, there is also 
plotted in figure 4 the radiation received on a surface normal to the sun’s 
rays, as measured by a differential telethermograph calibrated by means 
of an Abbot pyrheliometer. The integrated radiation—that is, the area 
inclosed by the graph—for each of the three days investigated, expressed 
as a percentage of the mean of the three, is as follows: 114, 94, 93. The 
corresponding excess evaporation of the cylindrical radio-atmometers 
over the white cylinders for the corresponding three days, expressed 
also as a percentage of the mean, is 113, 101, 85. 
It is of interest to consider in this connection the energy represented 
by the radiation falling on the cylinder compared with the energy 
dissipated in the water evaporated. If we consider the mean of all the 
11 a. m. to 1 p. m. values for the three days, the mean incident radiation 
on the radio-atmometer computed from a formula developed by the writers 
(Briggs andShantz, 1916a, p. 186) is approximately 1,100 gram-calories 
per hour, while the average hourly differential evaporation at 11 a. m. and 
1 p. m is 1.5 gms., corresponding to the dissipation of 800 gram-calories 
per hour. The energy dissipated in the water evaporated thus repre¬ 
sents about three-fourths of the total incident energy. The brown 
cylinder is not a perfect absorbing surface, so that the incident energy 
would be expected to exceed that dissipated in evaporation, as was 
found to be the case. It may be possible with the improved spherical 
atmometers to develop a definite relationship between the incident 
radiation and the excess evaporation from the black surface, in which 
event black and white spherical atmometers, when employed together, 
may be used to measure the intensity of radiation. Black and white 
Bellani plates may perhaps be used also to measure the vertical com¬ 
ponent of radiation. 
SUMMARY 
This paper deals with a comparison of the hourly transpiration rate 
of alfalfa with the hourly evaporation rates from various types of porous- 
cup atmometers, a filter-paper evaporimeter, a blackened, shallow tank, 
and a deep tank. ♦ 
The comparison between the transpiration rate and the evaporation 
rate was made by superimposing the hourly transpiration graph on each 
of the hourly evaporation graphs, choosing the scale of ordinates of the 
transpiration graph so that the total area under the transpiration graph 
was equal to the total area under the evaporation graph. The average 
hourly departure of each of the evaporation graphs from the superimposed 
transpiration graph expressed in percentage of the mean transpiration 
