no Blackman and Paine . — A Recording T ranspirometer. 
often too long in the case of slowly transpiring plants. Of course, balls of 
lighter weight can be used, 1 but they must be of one particular size to suit 
the valve-mechanism. 
The apparatus here described is an attempt to provide an instrument 
of considerable accuracy at a comparatively low cost, since part of the 
apparatus should be available in every well-equipped physiological 
laboratory. It has the further advantage that the water-content of the soil 
is kept constant automatically, a point of considerable importance, since 
soil moisture affects the rate of transpiration. 
The principle on which the apparatus is constructed is, like that of 
Anderson and of Ganong, the addition, at intervals, of a weight equivalent 
to that lost in transpiration ; but the novelty lies in the use of water-drops 
instead of steel balls, the water being added directly to the soil. By this 
means the combined weight of plant and pot and soil is constantly brought 
back to its original amount, and the soil to its original degree of moisture. 
The apparatus consists of four parts : (i) a water-reservoir with a tube 
supplying drops of water of constant size ; (2) an electrical device consisting 
of two solenoids (hollow magnets), which draw a tube backwards or 
forwards when energized by currents of a suitable direction ; (3) a balance 
(provided with a ‘ damper ’ to prevent undue oscillations), the pans of which 
bear cups of mercury into which platinum points dip ; (4) a revolving drum 
with recording pen actuated electrically. The arrangement of the various 
parts is shown in the photograph (Fig. 1), where the apparatus is in duplicate, 
one recording the loss of water from a transpiring plant, the other the loss 
of water from a porous-cup evaporimeter. 
The pot containing the plant is contained in one of Ganong’s aluminium 
‘ shells ’, roofed over by a sheet of indiarubber which surrounds the stem of 
the plant and is perforated for a glass tube, which projects free above and 
is embedded below in the soil. 2 This tube receives water from the dropper 
when the loss of weight by transpiration has risen to a certain prearranged 
value. The ‘ dropper } is a bent tube, provided with a stopcock to regulate 
the rate of flow of the drops, and connected with a reservoir in which 
the pressure of water is kept constant by an air tube which dips below 
the level of the water. The reservoir and tube are supported on a stand 
adjustable in height. 
The tube which serves to catch the water passes, as described above, 
through two solenoids, and is provided with a funnel at one end, the 
other end projecting over a reservoir which catches the waste water. When 
an electric current passes through one solenoid the tube is drawn back ; 
1 Transeau (Bot. Gaz., lii, 1911, p. 54) has made some modifications in this direction and has 
invented a ball-supplying mechanism to be used separately from the recording apparatus. 
2 This tube can be connected below with a small porous pot, so that the water supplied may be 
distributed more evenly to the soil. 
