124 



Journal of AgrictUtural 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 lo 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 i 

 part in 1,000. 



Ball receiver. — 

 The conical receiver 

 L 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 

 other knife-edges on 

 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 60° wdth 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.) 



10. — Corbett's apparatus for measuring transpiration in which the 

 plant is carried on the pan of a large Nicholson hydrometer. 



