OF THE VIBRATIONS OF AN INVARIABLE PENDULUM. 
227 
Whence there appears as the result of this experiment, a difference of 5.705 
vibrations per diem, corresponding to a difference of atmospheric pressure of 
16.08 inches of mercury at 32°; the temperature of the air of full pressure 
being 35°.67, and that of the rarefied medium 36°.63. 
Exp. VII. — February 9th and 10th. Clock gaining 4 m 14 s .38. 
This experiment was undertaken for a distinct purpose ; that of ascertaining 
the comparative retardation of a pendulum vibrating in an atmosphere of hy- 
drogen gas, and in an atmosphere of common air. It had been suggested to 
me by Mr. William Hasledyne Pepys, that a nearer practical approach to the 
vibration in a vacuum, than the pump had hitherto effected, might be accom- 
plished, by filling the apparatus with hydrogen gas, and pumping out to the 
extent that the pump could carry the process of exhaustion ; when the small 
portion of the gas remaining in the apparatus, being 13 times lighter than a 
remainder of air which would effect the gauge to an equal amount, might be 
expected to have an influence on the vibration diminished in the ratio of the 
respective specific gravities of air and hydrogen. To ascertain, therefore, 
whether the retardations of air and hydrogen gas were in that ratio was the 
object of this experiment, which was accomplished by vibrating the pendulum, 
1st, in the ordinary atmosphere ; and 2nd, in an atmosphere of hydrogen gas, 
of equal pressure, or as nearly so as circumstances would permit. 
First, in air. 
Observer. 
0-5 
Thermometers. 
Stand- 
Times of 
Arc registered 
Mean 
Correc- 
Reduc- 
Corrected 
o.S 
£ o 
i 
4 
ard 
Barom. 
Disapp. 
Re-app. 
Coincidence. 
and true Arc. 
Interval. 
tion for 
Arc. 
tion to 
36°. 
Vibrations 
at 36°. 
r 
1 
o 
O 
inch. 
m s 
14 45 
m s 
14 58 
Div. o 
0.85 = 1.02 
Captain 
SabineU 
2 
3 
37.7 
37.8 
30.200 
22 58 
31 13 
23 12 
31 25 
>21 23 05.17 
1 . 
>494.745 
+ 0.61 
+ 0.88 
86305.57 
22 
7 43 
15 58 
24 14 
8 16 
23 
24 
38.5 
38.5 
30.156 
16 32 
24 46 
j> 0 16 14.83 
J 
0.235 = 0.28 
J 
38.1 
30.193 
; Capill. + 0.019; Reduction to 32° — 0.019; =30.193. 
86305.57 
The air was then withdrawn until the gauge indicated 0.7 inch ; and 
hydrogen gas was introduced, passed through a cylinder containing muriate 
of lime, till the mercury in the barometer tube rose to 30 inches. The gas was 
2 g 2 
