48 Laidlaw and Knight.— A Description of a Recording 
which pass three glass tubes. Tube B is connected with the leaf-chamber 
attached to the plant, and passes almost to the bottom of the bottle. The 
air drawn from the leaf enters the bottle through B. C is the water-exit 
tube through which water is siphoned from the bottle through tube E, 
whence it flows .drop by drop on to the recorder. The siphon tube C is pro- 
vided with a three-way stopcock, F, by means of which the aspirator bottle 
can be refilled from the funnel when necessary, air escaping through the tube 
and stopcock D during the operation. The head of water used to draw air 
through the leaf is regulated by the difference in level between the lower 
ends of tubes B and E. E is connected to the siphon tube, C, by a piece of 
thick-walled rubber tubing, so that any required pressure difference may 
easily be obtained by raising or lowering E. 
When the rate of dropping is slow, each drop remains at the end 
of tube E for some time before falling, and there is a danger of its being pre- 
maturely removed by air currents. To prevent this, E is surrounded by 
a wide tube, R, secured by a rubber stopper. The inside of R is kept damp 
by lining it with wet blotting-paper, in order to reduce evaporation from the 
drop, which might conceivably be considerable under some conditions. 
The speed at which the water flows from E is measured by the 
frequency of the drops as determined by the recorder. This consists 
