414 
MR. BAILY ON THE CORRECTION OF 
Sabine on two similar pendulums, with the vacuum apparatus at Greenwich ; 
as described by him in the Philosophical Transactions for 1829, page 235. 
The specific gravity of this pendulum I have assumed equal to 8’4. Captain 
Kater states that the specific gravity of the first pendulum which he made of 
this kind was 8‘61 (See Philosophical Transactions for 1819, page 354): but 
this is greater than that of any brass that I have yet found, and greater I 
believe than what is usually met with ; it is even considerably more than the 
specific gravity of his convertible pendulum, mentioned in the following arti¬ 
cle, which was formed of nearly similar materials, and which was only 8'248. 
Captain Sabine, relying on this single experiment of Captain Kater’s, has 
assumed 8*6 as the proper specific gravity for a pendulum of this kind: and 
as the results therefore which I have deduced from his experiments will not 
exactly agree with those that he has given, it was necessary here to state the 
principal cause of the discordancy. I estimate the weight of this pendulum 
at 90500 grains, from the mean of the weights of two similar pendulums in 
my possession. 
All the pendulums above described can be swung only in one position. I 
now come to those which are furnished with two or more knife edges; and 
which are of the kind called convertible pendulums. The knife edges of these 
pendulums (at least, all those hitherto constructed,) are placed at unequal 
distances from the centre of gravity; and consequently the same pendulum, 
when swung with that knife edge placed uppermost which is furthest from the 
centre of gravity, will set in motion a different quantity of air, and, as far as 
the subject of the present inquiry is concerned, produce a different result, 
from that which would be produced when the pendulum is swung from the 
other knife edge. I shall therefore consider these knife edges, which I shall 
designate respectively A and B, as two separate and independent pendulums: 
the term A being applied to that knife edge which is the most distant from 
the centre of gravity, and the term B to the knife edge at the other end of the 
pendulum. 
No. 23, 24 are the two knife edges A and B, of Kater’s convertible pen¬ 
dulum, described by him in the Philosophical Transactions for 1818, page 37 : 
the first of these letters designates the pendulum when the great weight is 
below ; and the other, when the great weight is above. This pendulum, having 
