64 PROFESSOR C. V. BOYS ON THE 
difference of level of the upper lead and lower gold ball, while 6:0139 is the corre- 
sponding difference between the other pair. Similarly ‘0059 is the difference of level 
of the upper lead and upper goid balls, and ‘0216 the difference of level of the lower 
pair of balls. 
In order to eliminate as far as possible any systematic errors that might arise for 
instance from imperfections of the copper central tube, which of course is very near 
the gold balls, or from want of perfection in the lead balls, though nothing comparable 
with the error of experiment need be feared on this account, I made some change in 
the circumstances after every experiment. Thus, after Experiments Nos. 1 and 2 
had been carried out, which were merely preliminary so that I might learn something 
of the behaviour of the apparatus, I arranged the balls in Experiment 3 as follows :— 
Lead ball, No. 1, Wall side, High level, 
san 2 Arche. cows mae 
Golde =. 8, Walla sz anohmes 
wate. ,, 4, Arche ds elrowane 
Neutral reading of lid 267°. 
The arch side is the right as seen from the telescope, or the left as seen from the 
back when the optical compass is in use. This expression depending upon the 
structure of the cellar avoids ambiguity. 
The corresponding arrangement in the whole series of experiments with the dates 
at which they were carried out is given in Table IT. 
As would be expected, I had not at first all the details so complete as at the end 
of the series of experiments. Thus, in Experiment 8, the air drawing arrangement 
for steadying the mirror or starting it into motion, had not been thought of. At 
that time, for want of a better arrangement, I had to enter the corner and 
withdraw the back window, fig. 12, so as to allow the accidental movement of 
the air to start a swing of great amplitude. As I have already indicated, the 
decrement is, in spite of all I can do to prevent it, inconveniently high, so that 
periods extending over forty-five minutes cannot be taken unless the mirror has 
an oscillation of large amplitude at starting, far larger than two or three 
reversals of the lead balls in time with the oscillation would set up. The plan 
was essentially bad owing to temperature, disturbance, and tremor, nevertheless 
the observations made at the time were fairly consistent, thus two periods in 
the + position on October 16th, gave 241:93 and 241:90. The next day I put up 
the felt. screens and two periods were 241°9(4) in the + position, and 241°88 in the 
negative. I then introduced a different method of producing a swing of great 
amplitude, viz., in the air tube then screwed into the back window, fig. 12, I fitted a 
glass stop-cock which could be turned on or off without touching the metal windows 
or screens. I did this hoping that a clockwork fan belonging to a lamp would 
produce a gentle draught upon the end of the mirror when set in action opposite a 
