A PENDULUM FOR THE REDUCTION TO A VACUUM. 
415 
been successively altered by Captain Sabine, furnishes us with four separate 
and independent results, according to its form when it was swung : 1°. with 
the wooden tail pieces 17 inches long with which it was originally furnished: 
2°. with those wooden tail pieces reduced to the length of 6*4 inches : 3°. with 
brass tail pieces 7 inches long, instead of the wooden ones : and 4°. without 
any tail pieces whatever, and moreover deprived of the small sliding weight. 
In this last case, it was reduced to nearly the same figure and dimensions as 
the invariable pendulum (No. 22) just described, but without its tail piece. 
As my vacuum apparatus was not sufficiently large (as already mentioned,) for 
a pendulum of this kind, I have deduced the results from the experiments 
made by Captain Sabine with the same pendulum, in the several states above 
alluded to, as detailed by him in the Philosophical Transactions for 1829, 
page 331, &c., and for 1831, page 459, &c. With respect to the specific gra- 
vities, I must take that of the first case, which was the original construction 
of the pendulum, as equal to 7'373 ; which is the value stated by Captain 
Kater in the Philosophical Transactions for 1819, page 415. But this is the 
specific gravity of the body when at rest , deduced in the usual manner, and 
not the vibrating specific gravity of the mass deduced from formula (2) above 
given : and as the weights and distances of the several parts from the axis of 
vibration are not stated, and are now completely destroyed by the alterations 
in the pendulum, I have no means of ascertaining how far the results might 
be affected by this view of the subject. As to the second case, where the 
wooden tail pieces were reduced to 6*4 inches, I have computed the specific 
gravity (on the assumption that 7'373 in the former case was correct,) as equal 
to 7’909. With respect to the remaining two cases, as the pendulum here con- 
sists wholly of brass, I have computed the specific gravity from the data given 
by Captain Kater in the Philosophical Transactions for 1818, page 63, and 
make it equal to 8*248. Captain Kater’s result is 8*469 ; but I apprehend 
there must be some error in his computation. The weight of the pendulum is 
somewhere about 66900 grains : but there appears to be some confusion in 
the weighings. In the Philosophical Transactions for 1818, page 63, the brass 
parts alone are stated to weigh 9*57 pounds ; which, on the presumption that 
these are avoirdupois pounds, will be equal to 66990 grains troy. But, in 
the Philosophical Transactions for 1819, page 415, the weight of the whole 
3 h 2 
