OF THE VIBRATIONS OF AN INVARIABLE PENDULUM. 
221 
for the present purpose, a similar pendulum. No. 13, destined eventually for 
the Brussels Observatory. 
The arrangement of the observations in each of the succeeding experiments 
was the same as in those already related ; the pendulum was first vibrated in 
air of full pressure ; then in a rarefied medium ; and lastly, again in the air. 
A mean was then taken between the two series in air ; with which mean, the 
intermediate vibration in the rarefied medium was compared. The result of 
the comparison was thus wholly independent of the daily rate of the clock ; and 
in some measure also, of its deviations from an uniform rate in intervals less 
than 24 hours. The clock was compared daily with the transit clock of the 
observatory ; but as the weather in the last half of January and first half of 
February was very rarely clear, and as the transit clock about that period was 
more than usually irregular in its going, it has been deemed preferable to take 
a mean rate for the coincidence clock for the months of January and Febru- 
ary ; its deviations from this mean rate on the days of experiment are trans- 
ferred in appearance to the going of the pendulum on different days : this ap- 
parent irregularity is however wholly inconsequential in respect to the purpose 
of the present experiments, for which it is only necessary that the vibrations 
of the pendulum should be relative in the three series of coincidences forming 
each distinct experiment. 
Exp. III. — Greenwich, January 14th and 15th. Clock gaining 4 m 14 s .38. 
Observer. 
No. of 
Coincid. 
Thermometers. 
Stand- 
ard 
Barom. 
Times of 
Arc registered 
and true Arc. 
Mean 
Interval. 
Correc- 
tion for 
Arc. 
Reduct, 
to 36=. 
Corrected 
Vibrations 
at 36°. 
1 
2 
3 
4 
Disapp. 
Re-app. 
Coincidence. 
Captain 
Sabine.'' 
1 
2 
3 
12 
20 
21 
22 
23 
24 
o 
O 
inch. 
m s 
00 52 
09 05 
17 21 
m s 
01 07 
09 19 
17 33 
] h m s 
>11 09 12.83 
1 54 07.9 
Div. o 
0.69 = 0.83 
0.45 = 0.53 
M94.76 
+ 0.74 
-0.20 
86304.60 
35.4 
35.7 
29.93 
35.6 
35.6 
37 22 
45 39 
53 52 
02 11 
10 24 
37 51 
46 07 
54 22 
02 37 
10 54 
35.6 
35.7 
29.90 
35.53 
29.915 ; Capill. -f 0.019 ; Reduction to 32° — 0.017 ; = 29.917. 
86304.60 
The air was then withdrawn, the pendulum again set in motion, and suffered 
to vibrate an hour before the registry was commenced. 
