762 Bose. — On Diurnal Variation of 
be the case when the alarum spring is long and fully wound. The succes- 
sions of short release, twenty-four times during the day, produce relatively 
little unwinding of the spring. On account of this and the presence of the 
fan-governor, the period of a single revolution of the wheel remains constant. 
By means of an eccentric the circular movement is converted into an up and 
down movement. The plunging rod R thus dips into a cup of mercury, 
M, for a definite short interval (Fig. 1) and is then lifted off. The duration of 
closure can be regulated by raising or lowering the cup of mercury. In 
practice the duration of tetanizing shock is about 0-2 second. 
The same clock performs three distinct functions. The axis which 
revolves once in twelve hours has attached to it a wheel, and round this is 
wound a thread which allows the recording glass plate to fall through six 
inches in the course of twenty-four hours (Fig. 2). A spoke attached to the 
minute hand releases the alarum at regular and predetermined intervals of 
time, say once in an hour. The plunging rod R, actuated by the eccentric, 
causes a tetanizing shock of uniform intensity and duration to be given to 
the plant at specified times. 
. Constancy of resistance in the secondary circuit . In order that the 
testing electric stimulus shall remain uniform, another condition has to be 
fulfilled, namely, the maintenance of constancy of resistance in the secondary 
circuit, including the plant. Electric connexions have to be made with the 
latter by means of cloth moistened with dilute salt solution ; drying of the 
salt solution, however, gives rise to a variation of resistance in the elec- 
trolytic contact. This difficulty is overcome by making the electrolytic 
resistance negligible compared to the resistance offered by the plant. Thin 
and flexible spirals of silver tinsel attached to the electrodes E, e' are tied 
round the petiole and the stem respectively. In order to secure better 
electric contact, a small strip of cloth moistened with dilute salt and 
glycerine is woun<I round the tinsel. As the resistance of contact is rela- 
tively small, and as drying is to a great extent retarded by glycerine, the 
total resistance of the secondary circuit undergoes practically no variation 
in the course of twenty-four hours. This will be seen from the following 
data: An experiment was commenced one day at 1 p.m., when the resistance 
offered by 8 cm. length of stem and 2 cm. length of petiole was found to be 
1-5 million ohms. After twenty-four hours’ record, the resistance was 
measured the next day and was found unchanged. The fact that the 
stimulus remains perfectly uniform will be quite apparent when the records 
given in the course of this paper are examined in detail. 
I. ( b ) The Response Recorder. 
The amplitude of response affords, as we have seen, a measure of the 
excitability of the plant. In actual record friction of the writer against the 
glass surface becomes a source of error. This difficulty I have been able to 
