A PENDULUM FOR THE REDUCTION TO A VACUUM. 
449 
we shall find that the weight due to the spheres alone, when attached to the 
brass and steel rods, will be as stated in the preceding Table. The values 
annexed to the spheres, when suspended by the iron wire, are deduced from 
the assumption that the weight of the air dragged by the wire is equal to 
0T0 grain, as already stated. These values, like most of those deduced from 
the 14th set of experiments, agree very well with those which result from the 
spheres when annexed at the centre of oscillation: and the whole show that the 
effect of the air on a pendulum consisting of a sphere suspended by a fine rod 
or wire, although principally due to the sphere, is partly owing to the wire also : 
but that this influence of the wire diminishes with its diameter ; and, when 
extremely fine, probably becomes a small constant quantity, of nearly equal 
value in the most usual cases that occur*. 
In order to place the subject of this inquiry in a clearer point of view with 
respect to other bodies, I caused three additional brass cylinders to be made ; 
which, with the cylinder No. 10, described in the preceding part of this paper, 
were proposed to form the subject of a new set of experiments. The diameters 
of all these cylinders were ordered to be made exactly alike ; viz. 2'06 inches: 
and their respective heights, or thicknesses, were 2‘06 inches, TOO inch, 0-50 
inch, and 018 inch. This latter thickness was chosen on account of its being 
precisely the diameter of the brass rod. The 1 inch cylinder weighed 6611 
grains, and its specific gravity I found to be 7'805 : the \ inch cylinder weighed 
3352 grains, and its specific gravity I found to be 8‘116: and the *18 inch 
cylinder weighed 1266^ grains, and its specific gravity I found to be 8T45. 
The other cylinder has been already described. All these cylinders were 
tapped in the circumference, with two screw holes, opposite to each other, for 
the purpose of affixing thereto the two unequal portions of the rods above 
mentioned : whereby the cylinders became placed nearly in the centre of oscil- 
lation of the whole length of the rod. The cylinders, thus placed, were swung 
with their flat sides vertical, and opposed to the line of motion ; similar to the 
* This appears from the slight difference in the quantity of adhesive air dragged by the steel rod 
and iron wire, in this set of experiments ; which is very small. And moreover, in the case of the 
ivory sphere (No. 4), which was suspended hy a very fine silver wire, the result is precisely the mean 
of the other spheres, winch were suspended hy the much coarser iron wire. 
