476 CAPTAIN SABINE ON THE LENGTH OF THE SECONDS PENDULUM 
Stability of the support of the Pendulum in the preceding experiments. 
Being desirous of assuring myself of the stability of the support of the pen- 
dulum in the vacuum apparatus, I undertook a distinct series of experiments 
to obtain the rate of the pendulum on the iron frame, which is permanently 
affixed to the south wall of the pendulum room in the Royal Observatory, 
and designed for the use of observers with invariable pendulums, who 
wish to obtain a basis for their experiments on the variation of gravity at 
other stations. With this intention, I transferred the agate planes, which 
had been employed in the experiments in the vacuum apparatus, to the iron 
frame ; and placed the pendulum on them, with the weight below, and the 
slider at 1 .6 inch, making the observations H H to R R as detailed at the 
close of this paper. The experiments were necessarily made in the free air of 
the apartment, and are reduced to a vacuum by the reduction already found 
for the pendulum with the weight below. The rate of the clock, by Graham, 
which is attached to the wall of the room beneath the iron frame, was fur- 
nished me by Mr. Thomas Glanville Taylor, in a memorandum which is 
subjoined to the detail of the observations. By a mean of the ten experiments 
HH to R R on the iron frame, the pendulum was found to make 86070.98 
vibrations at 53° reduced to a vacuum. The equivalent at 57° is 86069.20, 
which differs by only 0.10 of a vibration per diem from 86069.10, the rate 
ascertained in the vacuum apparatus. The rate on the iron frame was ob- 
tained by thirty-three hours’ vibration of the pendulum ; that in the vacuum 
apparatus by 173 hours. So near an approximation, obtained in less than one 
fifth of the time that the experiments in the vacuum apparatus were continued, 
satisfied me that no permanent cause of difference existed, and that it only 
required that the experiments on the iron frame should be persevered in for the 
same length of time as those were in the vacuum apparatus, to produce the 
closest accordance. We may regard therefore the result of the experiments 
H II to R R, as establishing the equal stability of both supports ; and as 
affording a fair inference that both supports are perfectly stable. 
