730 Hi ley. — On the Value of Different Degrees of 
Working Methods. 
We will take an experiment in which the figures are simple. Suppose 
we try to find the effect of a stimulus of 9 mg. On the rod RR the rider 
carrying the seedling-box should be placed at about 18 cm. from the centre. 
The bolt on bar M (Text-fig. 1) should be screwed to the point which allows 
the bar D to swing out so far that the back of the box lies in a plane 
pointing 6° below the horizontal (tan 6 ° = about §), as it will then lie in the 
line of the resultant of gravity and the centrifugal force. The bands must 
be arranged round the pulleys so that the wheel may rotate at about 210 
revolutions per minute. 
In portion II (Text-fig. 3) we must allow just so much mercury to lie 
in the bath M that in the revolution of the arms, A x touches the mercury for 
1 min., say, and is free of it for 9 min. The points can be removed from 
the other arms so that they do not touch the mercury. Thus the ‘ wheel ’ 
will alternately rotate for 1 min. and be still for 9 min. 
When rotating, the middle of the box will be subject to a centrifugal 
force of 477 2 R 2 r dynes or 4 i: 2 R 2 r ^fi-mg. where r is the radius of rotation 
in centimetres and R the number of revolutions per sec. By our hypothesis 
R — -yg = 3.5 and r — 18. 47 r 2 R 2 r = 8900 dynes = about 9 mg. 
This is at the middle point of the box ; but the radius passed through 
by the outer side of the box is greater than 18 cm., and hence the centri- 
fugal force is greater ; and towards the inner side the radius of rotation is 
less than 18 cm., so that the centrifugal force is less than 9 mg. In fact, 
at the upper (outer) side the centrifugal force will be 11 mg., and at the 
lower (inner) side it will be 7 mg. 
Now, suppose a force of 9 mg. acting for 1 min. exactly counteracts 
the force of gravity (1 mg.) acting for 9 min. ; then, at the centre of the 
box the radicles will not bend in either direction, but will grow straight. 
But in this case a force of 11 mg. acting for 1 min. will more than neutralize 
a force of 1 mg. acting for 9 min., so that the radicles growing above the 
middle will bend outwards. Similarly, below the middle of the box they 
will bend inwards or downwards. This actually happens in all successful 
experiments, and Fig. 6, PI. LVIII, shows such a result. There is thus 
a line of equilibrium across the box, outside which the radicles will bend 
outwards and inside which they will bend inwards, whilst in this line of 
equilibrium the radicles remain approximately straight. This will afterwards 
be referred to as the ‘line’ or ‘point of equilibrium’. 
In actual working, owing to variability in the strength of the town- 
main current, it is impossible exactly to foretell the rate at which the 
wheel will rotate, and consequently the exact force acting at the centre 
of the box will not be known till the end of the experiment, when the data 
supplied by the rotation recorder and the control clock give the required 
