556 
Note . 
The sensitiveness of the apparatus can be varied within wide limits by varying 
the diameters of the jar and rod — the magnification increasing as the square of the 
radius of the jar and inversely as the square of the radius of the rod. 
Care should be taken that the root tip is kept away from the sides of the jar and 
from the near neighbourhood of the glass rod, as the water surface will be considerably 
curved there. It is also desirable to have a cylinder of cardboard or black paper to 
slip over the jar to shield the root from the light. 
The inaccuracy due to water adhering to the glass rod when it is raised was found 
to be negligible. 
The following elaborations are suggested where greater accuracy is required or 
when prolonged observations are contemplated. 
I 
(i) The difficulty of accurately measuring the cross-section of the jar at the level 
of the water surface makes it impossible to obtain really precise absolute readings by 
the method of calculation suggested above. The following method of calibration is 
suggested : 
An additional small hole is bored through the cork, through which is inserted 
a long pin (see Fig. II). A small disc of cork, f, too large to pass through the hole, 
slides on the pin, so that the distance of the pin-point from the water surface can be 
adjusted. The position of the glass rod which just causes contact of the water 
surface with the pin-point is determined. A thin washer, g, of known 'thickness is 
then inserted under the supporting cork disc on the pin, and a fresh determination 
made. From these data the actual rise in water level caused by immersion of a given 
length of the rod can be calculated. 
