A PENDULUM FOR THE REDUCTION TO A VACUUM. 
453 
Nineteenth set . — Results with the 2-inch thin brass disc, placed at different 
distances from the knife edge on the long brass rod. 
Distance 
from knife 
edge. 
No. 
n 
Weight of 
adhesive 
air. 
Weight due 
to disc alone. 
inches. 
o-o* 
5-1 
30-0 
38-0 
57*3 
43 
78 
79 
75 
80 
4-083 
4-155 
6-115 
7*439 
12-124 
1-484 
1- 523 
2- 457 
3- 111 
5-368 
1-330 
1-405 
1-426 
The differences in the weight of adhesive air appear, at first sight, very ano- 
malous : especially when we consider that the vibrating specific gravity of the 
mass, and the weight of the pendulum, are exactly alike in each case. But, it 
should be remembered that these weights of adhesive air are supposed, by the 
formula in page 431, to be applied to the centre of gyration', and, if we wish 
to determine the effect due to the disc alone, we must have recourse to the for- 
mula in page 440. It is in this manner that I have obtained the results given 
in the last column of the preceding Table, under the head of “ Weight due to 
the disc alone.” The mean of the last three values gives the weight of air due 
to the disc alone, equal to T387 grain. I have not included the case where 
the disc was only 5T inches from the knife edge ; since no dependence can be 
placed on the result, on account of the magnitude of the multiplier. In fact, 
if the weight of adhesive air at that point of the rod, were only 1*466 instead 
of 1*523, the weight due to the disc alone would correspond with the mean of 
the rest. 
As I was desirous of varying these experiments as much as possible, I next 
tried the effect of two of the thinnest of the cylinders above mentioned (having 
the same diameter as the disc used in the preceding set of experiments), whose 
thickness was respectively 0*18, and 0*50 inch: in order to see whether they 
would exhibit the same law. The cylinders were screwed to the end of the 
long brass rod ; and swung, as in the preceding set, with their flat sides 
opposed to the line of motion. The following are the results : 
MDCCCXXXII. 
* See the first note in page 443. 
3 N 
