A PENDULUM FOR THE REDUCTION TO A VACUUM. 437 
adhering to the pendulum (supposed to be applied to the centre of gyration) 
follow a very different march from the values of the factor n ; and lead to a 
more satisfactory explanation of the effect of the air on the motion of the 
pendulum. For, it evidently appears that the weight of air, dragged by a pen- 
dulum in motion, depends principally on the magnitude of the moving body ; 
the influence of which however seems to be affected by other circumstances at 
present unknown : so that the exact law of the variation of this influence is 
not sufficiently apparent from the examples adduced : and further experiments 
are requisite to clear up this difficult but important point *. 
Difference in the two ends of a convertible pendulum. 
If we examine the results of the several convertible pendulums given in the 
above Table, we shall find that the factor n is not the same for the two knife 
edges. This has been already noticed by M. Bessel, in his work so frequently 
alluded to, and also by M. Poisson in his recent paper in the Con. des Terns 
for 1834 ; both of whom seem to consider that the factor ought to be greater 
when the heaviest end is above the axis of suspension, than in the reversed 
position of the pendulum. This, however, does not appear to be universally 
the case, as will be seen by the following Table ; where I have given the factors 
for the two knife edges of the several convertible pendulums used in the pre- 
ceding experiments : together with the ratio between those factors, assuming 
as unity the factor for the knife edge A, or that position of the pendulum when 
the greatest weight is below the axis of suspension. 
Factors for the correction of a convertible pendulum, for the reduction to a 
vacuum : with the ratio between the corrections for the two knife edges. 
No. 
Pendulums. 
Knife 
edges. 
Ratio. 
A. 
B. 
23 
Kater’s, with wooden tail pieces 
2-144 
2-204 
1-028 
25 
Brass bar, ■§ inch thick 
1-847 
1-968 
1-066 
27 
Copper bar, \ inch thick 
1-891 
1-991 
1-053 
29 
Iron bar, \ inch thick 
1-945 
2-064 
1-061 
31 
Brass bar, inch thick „ 
2-079 
2-085 
1-003 
35 
Brass tube, \ inch diameter 
2-318 
2-262 
0-976 
* The Additional Experiments which I have made on this subject, subsequent to the reading of this 
paper before the Society, •will be given in page 438, &c. 
3 L 
MDCCCXXXII. 
