OF THE VIBRATIONS OF AN INVARIABLE PENDULUM. 
211 
the true arc of vibration. The telescope was then adjusted for the observa- 
tion of coincidences, the glasses of the apparatus being on, and the joints 
pomatumed. 
A preliminary trial was then made of the facility with which the air could 
be withdrawn. A double pump kept in steady action for fifteen minutes re- 
duced the pressure to 7 inches. More was not then attempted ; but on stop- 
ping the action of the pump, it was soon observed that a leak must exist, as 
the gauge rose at the rate of about an inch in seven or eight minutes. On in- 
tercepting the communication between the pump and the apparatus, the leak 
was shown to be in the latter. The air was then re-admitted; the joints 
examined, as well as the stuffing-box through which the wire passed which 
was employed to set the pendulum in motion. Mr. Newman expressed himself 
satisfied that the leak could be only in the metal of the iron cylinder, notwith- 
standing the thickness of the metal was two inches. The further examination 
of the leak was postponed ; and the pendulum prepared for the next day, 
when it was proposed to try its comparative vibration in air, and in a me- 
dium as rarefied as could be maintained in the then imperfect state of the 
apparatus. 
Exp. I. — June 29. Molyneux losing 0 s . 17 per diem. — The glasses being on 
and prepared for exhausting, but a free communication existing with 
the external air through the exhausting pipe, the following coincidences 
were observed. 
<4-. "O 
o-G 
Barom. 
Therm. 
Times of 
Arc registered 
Mean 
Correc- 
tion for 
Arc. 
Reduc- 
tion to 
72°. 
Corrected 
Vibrations 
at 72°. 
4-5 
Disapp. 
Re-app. 
Coincidence. 
and True Arc. 
Interval. 
1 
inch. 
0 
m s 
23 33 
m s 
23 38 
o / o / 
2 06 = 1 47 
2 
3 
29-91 
70.6 
29 42 
35 52 
29 48 
35 58 
>9 29 45.17 
s 
371.8 
s 
+ 3.06 
+ 0.87 
s 
-0.48 
85937.62 
15 
71-1 
71-3 
50 12 
50 23 
10 50 17-5 
12 29 50.0 
1 10 = 0 59-6 
0 35 = 0 29.7 
31 
29-90 
29 40 
30 00 
373.28 
— 0.34 
85937.45 
29-905; Index + 0.066; Red" to 32° — 0.111 ; = 29-861. Vibrations at 72 ° = 85937.54 
The barometer used in these and the subsequent experiments in London 
belonged to Mr. Browne. By several comparisons with the standard baro- 
2 e 2 
