1911.] 



The Stomatograph. 



35 



of mercury, through a circular area of leaf tissue of 5 mm. radius, was most 

 convenient to employ ; with these arbitrary constants, the time occupied in 

 one stroke of the pump varied between extremes of 5 seconds and 

 50 minutes, according to the condition of the leaf. The pump had to show a 

 maximum variation of not more than 5 per cent, in its capacity per stroke, and 

 in the pressure exerted during the stroke. It had further to be constructed 

 in such a manner as to be thoroughly protected, together with its recording 

 drum, from wind, dust, and insects, when left in the midst of a field of cotton 

 plants. Lastly, its dimensions had to be such that it should be easily 

 portable, and should not damage nor interfere with the surrounding plants. 

 All these requirements have been met by the appliance here figured and 

 described, which is, moreover, very easy to use. 



The Stomatograph, as set to fulfil the special purpose for which it was 

 designed, consists of two parts ; firstly, a small portable case containing the 

 pump, its operating battery, and a relay ; secondly, an electro-magnetic 

 marker, writing on a chronograph, operated over a telegraph-wire by the 

 relay, from a separate battery. 



(a) The Pump (fig. 1a). — Since the low pressures required made the use 

 of metal faces impracticable, the air was compressed by means of a gas- 

 holder (67) floating in a liquid (Z). This gasholder was suspended by 

 a flexible cord, or metal strip, from the channelled quadrant arm of a short- 

 beam balance (B), and counterpoised on the opposite side from a similar 

 quadrant by a soft iron rod (C). The pressure was then regulated by adding 

 weights to the gas-holder. 



The exit-tube (Ex) from the top of the gas-holder was made of fine 

 rubber of 1 mm. bore, fixed at a distance of a few centimetres to the wall of 

 the enclosing box. Thus arranged, it interfered but slightly with the 

 regularity of the gas-holder's movements, though a central vertical tube 

 with three-point guides would be preferable. 



The inlet valve which admitted air on the up-stroke of the gas-holder and 

 at the same time denned its maximum content of air, consisted simply of two 

 or three holes ( V, V) in its wall, near the foot. When these holes were 

 brought above the surface of the liquid in the beaker, the air rushed in 

 suddenly, and the consequent sudden diminution in weight was utilised to 

 knock off the electric contact mentioned below.* 



To prevent the gas-holder from being jerked upwards too suddenly by the 

 motive power, when taking in a fresh charge of air, a baffle-plate (Bp) was 



* The insertion of a reversible electromagnetic valve on the exit would allow either 

 positive or negative pressures to be employed, would define the pump-capacity with 

 precision, and would maintain a constant pressure. 



