Transpiration as a Factor in Crop Production. 
125 
was not obstructed at the upper elevation but was greatly re- 
tarded below by the surrounding corn. The wind gauge above 
the corn recorded a total wind velocity of 8,055 miles for the 50- 
day period, while only 521 miles were registered within the corn 
3 feet above the ground. The upper wind velocity exceeded the 
lower fifteen times. 
METHOD STUDY WITH FREE-WATER-SURFACE EVAPORATION 
JARS. 
To secure data regarding the degree of reliability of the 
evaporation rates obtained from the jars used in the preceding 
experiments, an hourly evaporation test was made for 30 days 
in 1914, comparing three styles of jars. They were all 36 square 
inches in surface area and made of gray glazed earthenware 
one-half inch in thickness. They were filled uniformly to within 
\ inch of the top each day, and the surfaces were freely exposed 
5 feet above the ground in the cornfield. Jars Nos. 1 and 2 were 
of the same depth, with 10 inches of water; while No. 3 was 
shallow, with only 1J inches of water. Jar No. 1 was insulated 
with excelsior to eliminate the heating effect of the direct rays of 
the sun on the outside of the jar. Nos. 2 and 3 were not insu- 
lated and stood fully exposed to the sun. Hourly temperature 
records were taken of the water at the surface in order to account 
for differences in evaporation rates. The results are contained 
in Table 51. 
The summary indicates that the shallow jar is most sensitive 
and responds quickly to climatic changes. It warms more quickly 
in the morning and cools more quickly toward evening in response 
to atmospheric changes. As might be expected, the water in the 
deeper jars cools more slowly at night and warms more slowly in 
the daytime. The insulated jar remains 6° F. cooler in the heat 
of the day. The average day temperature of the water in the 
shallow jar is 1 degree cooler than the air, which compares 
favorably with the average transpiring leaf temperature shown in 
Tables 41, 42, and 43, which was 2.1° F. cooler than the air. It 
would appear from these figures that the shallow evaporation 
jar most nearly records the evaporating power of the atmosphere 
at any particular time, and is fairly comparable with the tran- 
spiring leaf except for some difference in exposure to the sun and 
wind. In comparison with the shallow jar, the deep exposed jar 
evaporated 15 per cent more water during the 24 hours of day 
and night, probably because the water remained warmer at night 
and cooled more slowly in the afternoon. 
The results indicate that the evaporation data accompanying 
the daily transpiration tests reported in Tables 27 and 28 were 
