MEASUREMENT OF TENSIONS 1 87 



least i 5 cm. long. The free end of the open arm should be bent 

 in the same plane at right angles for convenience of attachment. 

 The most convenient form is furnished with a stopcock in the 

 closed arm, as in Fig. 90, but in the use of this instrument great 

 care must be taken to have the fitting perfectly air-tight under 

 possible pressures of several atmospheres. Fill the manometer 

 to half the length of both arms with clean mercury, with air at 

 atmospheric tension in the closed arm. Fill the open arm 

 with distilled water by the aid of a minute metal or glass tube. 

 Branches and stems ranging in diameter from that of the 

 manometer tubing to several cm. in thickness may be tested by 

 the use of adapters. In testing the tensions in stems of the ap- 

 proximate diameter of the manometer arm, cut off the stem 

 cleanly with a sharp knife and bind to the stump a section of 

 heavy rubber tubing 6 cm. in length. Quickly fill with water 

 and place a short section of fine wire in the tubing. Now lift the 

 manometer and drive the open end down into the rubber tubing 

 taking care to admit as little air as possible, although a few bub- 

 bles will not noticeably vitiate the results. The wire, which now 

 lies between the rubber tubing and the manometer tubing, and 

 which served to allow the escape of superfluous water, may be 

 withdrawn, and the rubber tubing bound firmly to the manometer 

 by means of wire clamped and twisted by means of a pair of 

 pliers. Note the height of the mercury column in both arms of 

 the manometer, and measure the exact distance from the mercury 

 to the end of the closed arm. Measure this distance three or four 

 times daily for a week. If exudation pressure, ordinarily known 

 as "root-pressure," is present the air in the closed end of the 

 manometer arm will be compressed. In accordance with Boyle's 

 law the volume of air in this arm varies inversely with the pres- 

 sure. Thus if the column of air in the closed arm measured 8 cm. 

 in length at the beginning of the experiment, and at the next ob- 

 servation it was found to be 6 cm., the pressure indicated is 8/6 

 atmospheres. Or if the column of air measures 10 cm. on the 

 second observation, the pressure will be 8/10 atmosphere, and a 



