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XXVII. On the reduction to a vacuum of Captain Kater’s convertible pendulum. 
By Captain Edward Sabine, of- the Royal Artillery, Secretary to the Royal 
Society. 
Read June 18, 1829. 
The convertible pendulum with which Captain Kater made his celebrated 
and very admirable experiments, on the length of the pendulum vibrating 
seconds in vacuo in Portland Place, was deposited, after the completion of 
those experiments, in the cabinet of the Royal Society. 
The experiments of Captain Kater were made, as is well known, in the free 
air of the ordinary atmosphere ; and the influence of the air in retarding the 
vibrations, and thereby interfering with the simple effect of the earth’s attrac- 
tion on the pendulum, was computed and allowed for on a principle universally 
received by mathematicians and experimentalists at that period. 
It has been shown by recent investigations, theoretical and experimental, 
that the principle on which the reduction to a vacuum was then computed is 
erroneous : and it is a consequence of those investigations, that further experi- 
ments are necessary with the convertible pendulum employed by Captain 
Kater, in order that the true vibration in a vacuum, corresponding to the 
distance between its knife edges, may be known ; and that the more correct 
length of the seconds pendulum, such as Captain Kater would himself have 
determined it had he been aware in 1817 of what has subsequently been dis- 
covered, may be substituted for the result published by him in the Philoso- 
phical Transactions for 1818. 
The apparatus, of which an account has been presented to the Society in the 
present session, in which pendulums can be vibrated both in air of ordinary 
density and in a highly rarefied medium approaching to a vacuum, affords the 
means of making these further experiments. At the wish of Captain Kater, 
and in compliance with a request of the council of the Royal Society, I have 
undertaken to make them ; and hope, at the commencement of the next session, 
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