Water -Gonducticity of the Wood in Trees and Shrubs. 221 



the water that enters from A, and a screw pinch-cock C serves as a very delicate fine 

 adjustment. The desired pressure within the apparatus, registered by the manometer 

 shown in the figure as graduated in centimetres (but actually each centimetre is sub- 

 divided into millimetres), is thus easily obtained, while the sort of difi'erential pressure 

 in the T-piece secures steadiness, since any sudden increase in A is largely met by an 

 increased outflow in A, and thus is prevented from effecting sudden change in the body 

 of the apparatus. But, although the manometer level may remain constant for a 

 considerable period, any unusual demand on the main supply naturally becomes 



Fig. 1. 



reflected in a change in pressure, but this can instantly be remedied by means of 

 the pinch-cock C. It need hardly be added that the height of the water from the 

 bulb Gj to the twigs must be taken into account in determining the height of the 

 mercury required to indicate the particular pressure desired. 



When the apparatus is used to measure the amount of water sucked up through a 

 twig from a glass, under given conditions of negative pressure, the procedure is 

 somewhat different. For this purpose all the twigs are first cut out of the water circuit 

 by the three-way taps. The water supply is then cut off at the tap F, which at the same 

 time places the manometer in communication solely with the part of the tube below it. 



