164 
Journal of Agricultural Research 
Vol. vn. No. 4 
The evaporation from the shallow tank as given in Table I is expressed 
in kilograms per square meter of water surface per day, calculated from 
the observed evaporation from a shallow blackened tank 6,540 sq. cm. in 
area and 2.5 cm. in depth, the depth of water being maintained auto¬ 
matically at about 1 cm. The evaporation from the deep tank (8 feet, 
or 243 cm., in diameter; depth of water, 50 cm.) is expressed in terms of 
the thickness of the layer of water evaporated each day, given both in 
inches and in millimeters. Since the loss of 1 kgm. of water from an area 
of 1 square meter represents a sheet of water 1 mm. in thickness, the 
daily evaporation from the two tanks is easily compared. 
The maximum, minimum, and mean temperature of each day is given 
in Table I in both Fahrenheit and centigrade units. The mean daily 
temperature was determined by integrating the area bounded by the 
thermograph record with the aid of a planimeter, which gives a better 
representation of the mean temperature than the mean of the maximum 
and minimum values. 
The daily radiation represents the total number of small calories 
received during the day on a surface 1 sq. cm. in area kept normal to the 
sun's rays. The radiation values given in Table I were computed from 
the records of a differential thermograph calibrated by means of a stand¬ 
ardized Abbot silver-disk pyrheliometer. 
The wet-bulb depression is expressed in hour degrees on both tempera¬ 
ture scales and represents the summation of the depression for each hour 
of the day beginning at 5 a. m. 
The mean wind velocity for the day as measured by a Robinson 
anemometer 3 feet above the ground is given in miles per hour and 
meters per second. 
The daily values of the weather factors and the daily transpiration 
during the growing season of 1914 as given in Table I are plotted in 
figure 2. The graphs of the two evaporation tanks are seen to be similar 
though not identical. This similarity is of special interest when the 
difference in the hourly distribution of the evaporation from the two 
tanks is considered. The loss from the small shallow tank is confined 
almost wholly to the daylight hours, while the large deep tank shows a 
marked evaporation at night, due to the heat stored during the day in 
the large volume of water. The writers will show later that the daily 
transpiration is more closely correlated with the daily evaporation from 
the shallow tank than with that from the deep tank. 
The daily transpiration is, of course, dependent not only upon the 
environment but upon the relative water requirement of the various 
species, the size of the plants, and the stage of growth. The daily trans¬ 
piration is therefore supplemented in Table II with a statement of the 
period of growth, yield of dry matter, and water requirement of each 
variety. To this table has also been added the variety of plant used, 
the botanical name, and the pot numbers. 
