A PENDULUM FOR THE REDUCTION TO A VACUUM. 
423 
The difference between the results of the pendulums 10 and 13 will show 
the effect produced by the substitution of the brass rod for the iron wire*. 
The results of the pendulums 11 and 13 are, as might have been anticipated, 
nearly equal. The comparison of the results of the pendulums 11 and 12 will 
show the difference produced, according to the manner in which the cylinder 
is swung. The whole appear very consistent with the assumption that in pen- 
dulums of equal length and of similar construction, the factor for the addi- 
tional correction depends on the form and magnitude of the moving body. 
Fourth set. — Results with the 4-inch Cylinder. 
Solid. 
Hollow. 
14) Filled with 
Lead. 
15) Both ends 
open. 
16) Top open, 
bottom closed. 
17) Top closed, 
bottom open. 
18) Both ends 
closed. 
19) Hermetically 
sealed. 
Exp. 
n 
Exp. 
n 
Exp. 
n 
Exp. 
n 
Exp. 
n 
Exp. 
n 
97— 98 
99—100 
2-011 
2-052 
85—86 
87 — 88 
1-921 
1-929 
89—90 
91—92 
1-937 
1-943 
93-94 
95—96 
1-983 
1-968- 
81—82 
83—84 
1- 995 
2- 006 
101 — 102 
103—104 
2-055 
2-085 
Mean = 
2-032 
Mean= 
1-925 
Mean= 
1-940 
Mean= 
l-975f 
Mean= 
2-OOOf 
Mean= 
2-070f 
It appears from these last experiments that the effect of the circumambient 
air on the moving pendulum is the same whether a portion of the pendulum 
be solid or hollow ; provided we take into account (in the case of hollow 
bodies,) the diminution of the specific gravity of the pendulum, by reason of 
* It might reasonably he inferred, from this insulated comparison, that the thicker the suspending 
rod, or wire, the greater would he the value of n. But, it will be seen from some additional experi- 
ments, made since this paper was read and which will be given in the sequel, that this is not always 
the case : and that the present results can he satisfactorily accounted for, on a totally different as- 
sumption. 
t The experiments with the top closed and bottom open, and with both ends closed (similar to 
those of pendulums No. 17 and 18), were repeated after the inner sliding tube had been taken away, 
and anew top to the outer cylinder had been soldered on, as mentioned in page 412 : and the results 
were as follow : 
Top closed, and bottom open. Both ends closed. 
1-977 2-111 
1-963 2-094 
Mean = 1-970 Mean = 2- 102 
which agree very well with the preceding results. 
I will also take occasion here to observe that, having reason to suspect the escape of the air from the 
3 I 2 
