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 Kater 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 Kater 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 l>y 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 Kater's experi- 

 ments : and as the reduction to a vacuum for each position of the pendulum. 



