NEWTONIAN CONSTANT OF GRAVITATION. 47 
lutely. I always take two observations with the telescope of the position of rest of 
the gold balls when the vernier readings differ by about 2° at the neutral position, 
and thus, knowing how many scale-divisions deflection are produced at this position 
by a movement of the lead balls of 1°, I am able at once to find the vernier reading of 
the true neutral position (N). Then, when the vernier reading is made N + 65° or 
N — 65°. the deflections in the + and — directions are found to be the same within 
i per cent., from which it is evident that the + and — positions have been set 
truly, with a superabundant degree of accuracy. This preliminary determination I 
generally make the night before the deflections and periods are determined, which in 
Oxford is best done on Sunday night between midnight and 6 a.m. The daytime, of 
course, is out of the question, owing to the rattling traffic on the stones in St. Giles’, 
about a quarter of a mile away; and all nights except Sunday night the railway 
people are engaged making up trains and shunting, which is more continuous and 
disturbing to the steadiness of the ground than a passing train. Hven these come 
through at intervals on a Sunday night, and this limits the accuracy with which the 
periods can be observed. All having been prepared for the Sunday night during the 
previous week, the room is shut up all day, and at midnight or a little later the 
actual observations of deflection and period are begun. 
For a long night’s work without accidents, I am able to take two or three sets of 
observations alternately at each of the + and — positions, with one at the neutral 
position ; one period at each of the + and — positions, lasting about 45 minutes, and 
occasionally one at the neutral position, lasting about 15 minutes, and finally two or 
three sets alternately at each of the + and — positions, followed rarely by one, or 
even two, periods of 30 to 45 minutes. For each set of observations at the + and 
— position, I observe six consecutive elongations, and sometimes eight if I am 
disturbed by the trains. I do not begin them until the apparatus has quieted down 
from any very small tremor which the rotation of the lid may have set up. 
In observing the period, the point of rest is known from previous observation, 
A conspicuous pointer tapering to a point, which can be inserted into any division, 
or if between can be read to a tenth of a division, is placed at the point of rest. 
Air is gently drawn from the mouthpiece for a quarter period and then stopped. 
According to the speed at which the pointer flashes past I determine whether or not 
to draw air again. If I do, I begin about a quarter period after the transit, and 
continue until the next transit, or a quarter period longer still, according to the 
velocity of the transit. It is most important not to begin or leave off drawing the 
air suddenly, lest a quick period movement, of which there are five independent of 
one another, should be set up in the mirror. I begin very gently, gradually increase 
and gradually leave off, the result of which is a beautifully steady motion of the 
mobile system, extending far beyond the limits of the scale. I then start the 
drum and make a dozen marks with the key in rapid succession, to indicate the 
beginning of an observation. At each transit, the key is pressed suddenly and then 
