336 
CAPTAIN SABINE ON THE REDUCTION TO A VACUUM OF 
dulum by their loss of moisture as the weather became more dry. As we now 
know that the general effect of the air in retarding the vibration is more than 
twice as great as was then imagined, and that the wooden tail pieces, in con- 
sequence of their position, have a far greater proportionate influence on the 
retardation than would be simply due to the diminution which they occasion in 
the general specific gravity of the pendulum, we perceive increased reason to 
agree with Captain Kater, and to apprehend so much danger of derangement 
from this cause, as to make it desirable to avoid altogether the employment 
of a material susceptible of changes from moisture. 
For this reason I determined to substitute tail pieces of brass, with such 
an alteration in the position or size of the smaller weight as should re-esta- 
blish the equality of vibration. Previously, however, to this being done, and 
for the purpose of illustrating more strongly the effect of the wooden tail 
pieces, I had them reduced to less than half their original length, 10.6 inches 
being taken off from each, leaving them 6.4 inches from the extremity of the 
brass bar. By substituting a still smaller weight (of 1925 grains) of the same 
metal and form, for the smallest weight used by Captain Kater (of 3325 
grains), and securing it very nearly in the same position, the pendulum was 
again rendered nearly convertible, and experiments were made with it in the 
same succession as before, of which the following are the results. 
With the great weight below, there was found a difference of 1 1 .9 vibra- 
tions per diem, corresponding to a difference of atmospheric pressure of 28.741 
inches of mercury at 32°, the temperature of the air of full pressure being 53.2, 
and of the rarefied air 53.8. 
And with the great weight uppermost a difference of 14 vibrations per 
diem, corresponding to a difference of 28.15 inches of mercury at 32°, the 
temperature of the air of full pressure being 51.4, and of the rarefied air 53.8. 
Whence we obtain 12.4 vibrations per diem as the reduction to a vacuum, 
when the wooden tail pieces were shortened from 17 inches to 6.4 inches, and 
the great weight was below, in air of 53°.5, under a pressure of 30 inches of 
mercury at 32° ; and 14.9 vibrations per diem with the great weight uppermost, 
in air of the same temperature and density. 
The wooden tail pieces were then altogether removed, and slips of brass 
substituted, extending 7 inches from each extremity of the bar. The pendulum 
