332 
CAPTAIN SABINE ON THE REDUCTION TO A VACUUM OF 
to present to the Society the result which shall be obtained, as the result of 
Captain Rater’s pendulum. 
From the principles developed in the recent investigation into the action of 
the air on the vibrations of a pendulum, it was to be inferred that a convertible 
pendulum, such as the one Captain Rater employed, would in two respects 
be affected by the medium in a different manner from that which he had sup- 
posed : namely, first, in respect to its presumed convertibility ; for, since the 
amount of the retardation occasioned by the air is dependent in part on the 
external figure of the body vibrating, and as the two ends of the pendulum are 
not symmetrical, the one being furnished with a large weight, and the other 
with a much smaller weight and of a different form, the reduction to a vacuum 
will not be of the same amount when the pendulum is suspended with the 
great weight uppermost, as when suspended with the great weight below ; and 
consequently the pendulum is erroneously supposed to be convertible when the 
vibrations in air are identical. And second, in respect to the amount of the 
retardation produced by the air, which would be considerably greater than the 
quantity computed on the simple consideration of buoyancy. 
The experiments that have already been made with this pendulum in the 
vacuum apparatus, both in the state in which Captain Rater constructed and 
employed the pendulum, and with certain alterations which I have found it 
expedient to make in its tail pieces, have fully confirmed these inferences ; and 
in the opinion of those whose judgement I have reason to respect, possess an 
interest in the elucidation, and further experimental illustration of the mode in 
which a medium acts on the pendulum in retarding its vibration, which makes 
it desirable that I should communicate the present brief account of them to 
the Society before the recess. 
The pendulum having been conveyed to Greenwich, was examined and found 
in excellent order ; the knife edges were as clean and apparently as perfect as 
when first used ; the smaller weight was well secured by its screws, and the 
slider was at 18.6 divisions towards the greater weight : the rate of vibration 
on each of the knife edges was then tried by a few coincidences, and found so 
nearly identical, as to make it probable that little or no alteration had been 
made in the positions of the weight and slider since Captain Rater’9 experi- 
ments : and as the reduction to a vacuum for each position of the pendulum, 
