Jan, 3, 1916 
Hourly Transpiration Rate on Clear Days 
5&5 
Wet-bulb depression. —The measurement of the depression of the 
wet bulb was automatically carried on by means of a second differential 
telethermograph. One bulb was surrounded by muslin which was kept 
continuously saturated with water by means of a slowly-dripping Mariotte 
apparatus. In these measurements both bulbs were inside the instru¬ 
ment shelter and protected from solar radiation. The apparatus thus 
measured the depression of the wet bulb corresponding to the ventilation 
afforded by the wind through the shelter, which was similar to that to 
which the plants were subjected. 
Evaporation. —In measuring the evaporation a shallow copper tank 
91.3 cm. (3 feet) in diameter and 2.5 cm. deep was used, being mounted 
on the platform of an automatic scale of the type used in the transpiration 
measurements. The tank was clamped to a heavy, flat, wooden base, 
rf.Af. 
r . .+ S 6 7 0 i 
9 / 
//OO/V 
O // 4? 
/ 2 J 
I** 
!/* 
-JL 
t 
s* 
w 
* >4554 
0 
?or$/os& 
?GS D/m 
P -D/S# /°> 
6/PDA/r/^L 
'*#£'UOMi 
y 
f 
-La 
Fig. I.— Curve showing the comparison of the readings of the differential telethermograph with those of 
Abbot’s silver-disk pyrheliometer. 
which was supported on leveling legs about 3 feet above the scale plat¬ 
form (PI. LV, fig. 3). The inside of the tank was blackened with a 
mixture of lampblack in “ bronzing liquid.” The depth of the water in 
the tank was maintained at approximately 1 cm. by means of a Mariotte 
apparatus supported from the scale platform and located on the north 
side of the tank, so that its shadow did not fall on the tank. 
Air temperature. —-The air temperature was measured by a thermo¬ 
graph calibrated with mercurial thermometers and exposed in a standard 
shelter of the Weather Bureau pattern. 
Wind velocity.— The wind velocity was measured automatically by 
an anemometer of the Weather Bureau pattern, located 3 feet above the 
ground. In the 1914 measurements these measurements were supple¬ 
mented by a special instrument recording each one-twentieth of a mile. 
TRANSPIRATION RATE ON CLEAR DAYS IN RELATION TO PHYSICAL 
FACTORS 
The transpiration graph for a single pot of plants for a single day 
usually shows slight irregularities. In order, therefore, to determine 
whether such departures are normal or accidental, it is necessary to combine 
