Blackman and Paine.- — A Recording Tran spirometer, i x i 
when the current passes through the other the tube is brought forward 
again. The tube is so placed that in its forward position the funnel stands 
immediately above the glass tube which is fixed in the soil of the pot, and 
the water-dropper is placed so as to deliver its series of regular drops into 
the funnel, whence they run to waste through the tube. 
A balance of almost any type can be used provided that it is sufficiently 
sensitive and that it allows sufficient room above the scale-pan for the 
foliage of the plant. The balance employed in the apparatus photo- 
graphed is of a simple, familiar type, with open pans supported below. 
This is sufficiently sensitive for ordinary work, but one with a bigger 
movement of the scale-pans, such as a large so-called physical balance, 
or Ganong’s special transpiration balance, is more suitable for very accurate 
work. In any case, the balance must be provided with a ‘damper’ and an 
appliance for making contact as the pans rise. The damper is used to 
prevent the unduly large oscillations which would otherwise result from the 
fall of water into the pot, and from air-currents acting on the pans or plant. 
In the form of balance shown in the photograph, this purpose is accom- 
plished in a very simple manner by attaching beneath one of the pans an 
aluminium disc which moves on a vessel containing some viscous fluid such 
as treacle or glycerine. In addition to this ‘ damper ’ there is attached 
to each pan a small vulcanite cup holding mercury. Above each cup 
stand a pair of platinum points, supported on the framework of the 
balance and in electrical connexion with the battery, the solenoids, and the 
recording pen. 
When the apparatus is in use, the pot is placed on the left-hand scale- 
pan and is accurately counterbalanced, and the funnel of the solenoid tube 
and the dropper are placed immediately above the glass collecting-tube in 
the pot. As the plant transpires it loses weight, and the pan rises so that 
the left-hand pair of platinum points dip into the mercury in the cup 
attached to this pan ; the electrical circuit is thus closed, with the result 
that the current passes through one of the solenoids and the metal tube 
with its funnel is drawn back. The water-drops are now free to fall into the 
glass tube fixed in the pot and so reach the soil. The addition of water to 
the pot continues until the increased weight in this pan causes the other 
pan with its attached mercury cup to rise and close another circuit. * As 
a result, a current is sent through the other solenoid, which by its attraction 
brings back the tube to its forward position, and the water now runs again 
to waste. Every time the left-hand pan rises and closes the circuit, a portion 
of the current passes through the magnet of the recording pen, and so 
makes a vertical line on the revolving drum. 
The platinum points can be raised or lowered by means of a simple 
screw arrangement, and so the apparatus can be set to respond to a small 
or a large number of drops. The loss of water from slowly transpiring 
