364 A Self-Recording Porometer and Potometer. 
If the apparatus is required to be used over long periods 
continuously, care must be taken to keep the reservoir I\ from 
becoming depleted of water. The usual precautions in using a 
potometer—use of boiled water, etc.—must of course be adopted. 
The form of tracing obtained on the drum will be evident from 
the following considerations. 
Since contact at t 't 1 is broken almost as soon as it is made 
owing to the opening of the valve at C', the contact of the pen is 
momentary and this record has the form of a dot. The mercury 
then falls rapidly downwards in the tube BB' and on reaching tt 
another dot will be recorded which will be close to the previous dot 
on the drum. 
The valve at C' then closes but not before the mercury has time 
to reach B. After a short pause the index begins to move slowly 
up the tube, so that the contact on passing tt in this direction is a 
long one. This record on the drum has therefore the form of a line, 
the length of which depends on the rate at which the index moves, 
i.e., on the rate of transpiration. The complete record on the drum 
will therefore have the form of a line (as the index passes tt on its 
way upwards) followed after an interval by a dot (as t't' is reached) 
and then immediately afterwards another dot (as the index repasses 
tt on its downward journey to B). Thus - .. - 
•• - •• and so on. The rates of water loss required 
will be measured by the distances between the left-hand ends of the 
lines and the first of the dots following. 
Finally it should be remembered that the voltage of the current 
used across the terminals tt and t't' must be less than that required 
to decompose water, viz., F47 volts. 
Botanical Laboratory, 
Bedford College, 
London. 
R. Madley, Steam Printer, 151, Whitfield Street, Fitzroy Square, London, W. 
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