120 
Journal of Agricultural Research 
Vol. V, No. 3 
funnel and collected as waste water. When the plant through transpira¬ 
tion causes the balance beam to tip sufficiently to close an electric circuit, 
the funnel 1 ? is withdrawn by the solenoid A, and the water drops fall 
directly into a receiving tube leading to the soil in the pot. Water is thus 
added directly to the 
pot until the balance 
tips sufficiently in the 
opposite direction to 
close a circuit through 
a second solenoid B, 
which restores the 
funnel to its inter¬ 
cepting position. 
The time at which 
the circuit is closed is 
electrically recor d e d 
on a clock drum. 
The position of the 
contacts is adjusta¬ 
ble, so that the 
quantity of water 
added each time—i. e., the size of the steps—may be modified to suit the 
transpiration rate. This method is unique and advantageous in main¬ 
taining the moisture content of the soil constant throughout the experi¬ 
ment. Under freely 
exposed conditions, 
however, the quantity 
of water added each 
time would be variable 
and indeterminate, due 
to the oscillation of the 
balance by the wind. 
TRANSPIRATION BAL¬ 
ANCES OF THE CON- 
TINUOUS-RECORD 
TYPE 
The first continu¬ 
ously recording trans¬ 
piration apparatus ap¬ 
pears to have been de¬ 
vised by Krutizky 
(1878). It is of interest to note that the first step-by-step recording ap¬ 
paratus was described by Vesque in the same year. Krutizky’s appa¬ 
ratus is shown in figure 7. The water lost through transpiration from a 
potometer is continuously replaced through a siphon from a supply con- 
Fig. 4.—Woods’ adaptation of Marvin’s weighing rain gage as a trans¬ 
piration balance. In this apparatus the loss through transpiration 
is counterbalanced by a weight controlled by a screw. 
