A PENDULUM FOR THE REDUCTION TO A VACUUM. 
421 
The results of all these pendulums agree very well together, except the 
brass one : and seem to show that in pendulums of equal length and of simi- 
lar construction, the factor for this additional correction depends on the form 
and magnitude of the moving body ; and is not affected by its weight or spe- 
cific gravity. The mean of the wdiole makes n = T864*. I am unable to 
account for the discordancy of the brass sphere from the others ; unless it be 
in the determination of the specific gravity, which is certainly less than that 
of any brass I have yet examined : it being only 7*660 from a mean of three 
different weighings on three different days, and agreeing very well with each 
other. If the specific gravity be assumed equal to 7*8 or 7*9 (which is still 
small,) the result of this pendulum would agree with the others : but I could 
never make it exceed 7'67f- 
Second set . — Results with the 2-inch Spheres. 
On the Knife edge. 
On the Cylinder. 
5) Lead. 
6) Brass. 
7) Ivory. 
8) Lead. 
9) Ivory. 
(Exp. 
n 
Exp. 
n 
Exp. 
n 
Exp. 
n 
Exp. 
n 
49—50 
1-764 
33—34 
1-736 
41—42 
1-752 
61 — 62 
1-811 
57—58 
1-760 
51—52 
1-732 
35—36 
1-732 
43—44 
1-759 
63—64 
1-682 
59—60 
1-722 
53 — 54 
1-717 
37—38 
1-770 
45—46 
1-762 
55 — 56 
1-739 
39—40 
1-767 
47—48 
1-748 
Mean = 
1-738 
Mean = 
1-751 
Mean = 
1-755 
Mean = 
1-746 
Mean = 
1-741 
* There are some singular coincidences and discordancies in these results which though slight are 
worthy of notice. For instance, in the experiments with the lead sphere, No. 17 — 20 are almost 
identical; and so likewise are No. 21 — 24, yet differing from the former. Also in the experiments 
with the brass sphere, No. 9 — ■ 1 2 are almost identical; and so likewise are No. 13 — 16, yet differing 
from the former. These and other cases of a like kind are trifling anomalies for which I cannot 
give any satisfactory explanation. 
t Some persons have supposed that if the hall be greased, the results might he affected : and if so, 
the present discordancy may have arisen from some accidental circumstance of this kind. It is pro- 
bable also that the results may vary according to the state of moisture or dryness of the atmosphere ; 
or from some other unknown cause. On these points, there is certainly a wide field of inquiry open, 
but on which, at present, I have not leisure to enter. The true cause however, of the present dis- 
cordancy, I suspect to arise from some internal cavities in the sphere (indicated by the smallness of its 
specific gravity,) which are connected with the screw-hole, and thus suffer the escape of the included 
air when submitted to the action of the air pump. This contingency cannot be allowed for, in the 
computation ; although it may be appreciable in the result. 
3 i 
MDCCCXXXII. 
