124 
Journal of Agricultural Research 
Vol. V, No. 3 
position should extend into the tube approximately one-fourth of the 
tube diameter. It is essential that the valve be accurately made to 
conform to the particular size of ball used as a weight. The inside of the 
valve tube should be kept smooth and clean by the occasional use of 
benzine, and the balls should also be kept polished. 
The balls used for weights were three-sixteenths of an inch in diameter 
and of first-quality hardened steel. They were found to be so nearly 
uniform in weight that no appreciable error is introduced by assuming 
them equal. The individual weights in milligrams of io balls selected at 
random were as follows: 437.0, 438.5, 437-2, 437-7> 436.8, 437.6, 437.3, 
438.0, 437.5, 437.0. 
Mean, 437.4. Prob¬ 
able error for a single 
ball, 0.4 mgm., or 1 
part in 1,000. 
BAnn receiver.— 
The conical receiver 
If for the balls is sus¬ 
pended from an ex¬ 
tension of the beam 
(fig. 11) on the same 
side as the load, since 
the added weight of 
the ball compensates 
for the loss by tran¬ 
spiration. The re¬ 
ceiver is suspended 
from a knife-edge 
which lies in the plane 
determined by the two 
Fig. io.—C orbett's apparatus for measuring transpiration in which the other k nif e-edges on 
plant is carried on the pan of a large Nicholson hydrometer. 
the beam. The dis¬ 
tance from the central knife-edge is so chosen that the weight of a ball 
corresponds to a change of 20 gm. in the weight on the scale platform. 
The measuring tray shown in Plate XI affords a rapid means of count¬ 
ing the balls dropped during any period without touching them. Each 
complete row includes 10 balls, and the rows are graduated accordingly 
on the margin. It is essential that the lower end of the tray be cut 
obliquely so as to form an angle of 6o° with the graduated side. 
Dashpot. —The oil dashpot (fig. 13) is an essential part of the apparatus 
when the balance is exposed to the wind. The resistance can be adjusted 
to some extent by turning the perforated plate on the top of the inner 
cylinder I. The outer cylinder O is mounted directly below the weight 
support on the beam, to which the inner cylinder is attached by the 
rod N. (See fig. 11.) 
