A Self-Recording Porometer and Potomctcr. 357 
the index has reached the lower end ( b ) of the tube, the position of 
tube is again reversed and another record obtained; in this case, 
however, air is sucked through the stomata whereas in the former 
case it was forced through. In this way, by reversing the position 
of the tube, any number of successive readings can be obtained. 
The small bulbs at the ends of the tube ( b , b') are for the 
following purpose. It was found that if the inside of the tube was not 
fairly uniform in diameter where the platinum terminals were sealed 
through the glass, the mercury index sometimes broke in passing. 
The terminal bulbs allow the drops of mercury to unite should this 
have happened. It is important that the cotton wool plugging the 
ends of the tube should project into these bulbs so that the mercury 
can collect as shown in Fig. 3 b, otherwise the kind of thing illus¬ 
trated in Fig. 3 c occurs— i.e., a small blob of mercury breaks away 
from the index and forms a plug blocking the end of the tube. No 
further movement of the remainder of the mercury into the bulb 
takes place in consequence and the two pieces remain permanently 
separated. If, however, the cotton wool is pushed up one side of 
the bulb so as to keep the mercury to the side as it enters, this 
difficulty does not arise. 
One other modification has been found advantageous, though not 
actually necessary, especially when the apparatus is required to run 
continuously over long periods. In order to ensure the pen writing 
instantaneously on coming in contact with the drum, it should 
strike the drum fairly forcibly. A current of voltage sufficient to 
ensure this, gives rise to considerable sparking at the terminals tt 
and t't' on breaking contact. This leads in time to fouling of the 
mercury, which then ceases to run smoothly in the tube. To avoid 
this a relay is introduced into the circuit. A weak current is used 
through the tube bb' and the closing of the circuit by the mercury 
index, instead of actuating the pen, operates the relay: this in turns 
starts a strong current through the electro-magnet controlling the 
pen. 1 
3. The Driving Mechanism (Figs. 4 and 5). As is evident from 
the above description, whenever a reading is to be taken, the tube 
bb' must be turned upside down. In order to avoid twisting of the 
rubber tubing and wire connections this movement must be 
alternately reversed—first a turn of 180° to the right, then a turn 
of 180° to the left, and so on. For experiments of comparatively 
1 The writer uses, with suitable resistance, the electric light current for 
this latter. The tremulous movement imparted to the pen by the alternating 
current is very satisfactory from the point of view of causing immediate 
writing. 
