122 
Journal of Agricultural Research 
Vol. V, No. 3 
its buoyancy. Temperature effects can, however, be practically elimi¬ 
nated by surrounding the hydrometer tank with a water-jacket, through 
which water is constantly circulating. The sensibility of the apparatus 
is determined by the cross section of the stem of the hydrometer. . 
A NEW AUTOMATIC TRANSPIRATION SCALE OF LARGE CAPACITY 
The requirements of the transpiration studies at Akron necessitated an 
automatic weighing apparatus having a carrying capacity of 150 kgm., 
capable of operating 
positively in the wind, 
and so designed that the 
plants could be freely 
and continuously ex¬ 
posed to the weather 
(PI. IX). A platform 
scale with agate bear¬ 
ings having a carrying 
capacity of 200 kgm. 
and a sensibility of 5 gm. 
was chosen for equip¬ 
ment as an automatic 
balance of the step-by- 
step type (PI. X). The 
scale was fitted with a 
short column so as to bring all the mechanism below the level of the top 
of the pot and was provided with the following auxiliary equipment: 
Fig. 8 .— The transpiration balance of Richard Frfcres with its record¬ 
ing apparatus. 
a. Ball-dropping device. 
b. Ball receiver on beam. 
c. Beam contact and mercury cups. 
d. Oil dashpot on beam. 
e. Spring motor for raising beam. 
f. Adjustable counterpoise for raising the center of gravity of balanced system. 
g. Recorder for registering time at which each ball is dropped. 
h. Batteries and relay. 
i. Case for protecting mechanism from the weather. 
The beam of the scale with a part of the auxiliary equipment is shown 
in fig. 11. The operation of the mechanism is briefly as follows: As the 
plant decreases in weight, the beam falls until an electric contact is 
made at U. This closes a relay circuit, with the following results: 
1. The ball-dropping device A deposits a ball in the receiver L. The weight 
of this ball tends to raise the beam. 
2. The spring motor, by means of a cam K, raises the beam promptly and 
positively to its. upper position. 
3. The time of the event is indicated on the drum of the recorder. 
