6 i8 
Journal of Agricultural Research 
Vol. V, No. 14 
Considering now the composite graphs based upon the records obtained 
during 26 clear days, it will be seen that the radiation graph is similar in 
form to those already discussed, save that the radiation tends to change 
less rapidly during the early-morning and late-aftemoon hours, owing to 
the fact that the length of the day was not uniform throughout this long 
period. The slight variation in radiation intensity during the midday 
hours and the marked changes between 5 and 7 a. m. and 4 and 6 p. m. 
are in conformity with what has already been noted of the other radiation 
curves. 
The composite temperature graph shows a daily range of 33 degrees, the 
minimum (47 0 F.) occurring between 4 and 5 a. m., and the maximum 
(8o° F.) between 2 and 3 p. m. The graph showing the wet-bulb depres¬ 
sion is very similar in form to the air-temperature graph, and the maxima 
and minima correspond. This is to be expected, since with an unvarying 
amount of water vapor in the air, the wet-bulb depression would be deter¬ 
mined by temperature fluctuations. Furthermore, since the observa¬ 
tions are confined to clear days, sudden changes in absolute humidity are 
not encountered. 
The evaporation graph representing the alfalfa period is nearly symmet¬ 
rical with respect to noon, and the slope of the graph changes but slightly 
during either the morning or afternoon hours. The greater portion of 
the daily evaporation, however, takes place during the afternoon, owing 
probably to the higher temperature prevailing during this part of the day. 
The transpiration graph shows a very low rate of transpiration during 
the night. The rate gradually increases from about one hour after sun¬ 
rise to the maximum at 1.30 p. m. After 2.30 p. m. the curve falls rap¬ 
idly until sundown and remains practically constant throughout the 
night. By far the greater part of the daily transpiration occurs during 
the afternoon. This asymmetry with respect to midday is much more 
apparent in the transpiration graph than in the evaporation graph. 
At the bottom of figure 7 the mean velocity of the wind is shown for 
each hour in the day. During daylight hours the rate is approximately 
7 miles per hour and during the night about 4 miles per hour. It is 
apparent from Table XXIV that the air is never still for an hour at a 
time. 
AMARANTHUS 
The transpiration data so far presented have been confined to crop 
plants. It is also desirable in this connection to study the transpira¬ 
tion of weeds or native plants which have shown themselves adapted to 
regions of limited rainfall. To this end, Amaranthus retroflexus was 
selected as a plant widely distributed throughout the cultivated areas of 
the United States. Amaranthus is also one of the most efficient plants 
known as regards the use of water, its water requirement at Akron being 
below 300, thus comparing favorably with the best of the prosos, millets, 
and sorghums, the most efficient crop plants known. 
