A PENDULUM FOR THE REDUCTION TO A VACUUM. 
411 
brass rod screwed into its flat side (as in No. 10) : an experiment made for the 
purpose of determining the difference in the results, when suspended by the 
brass rod, and by the iron wire. See the preceding figure, which exhibits this 
pendulum. 
No. 14 is a cylinder of lead, 2‘06 inches in diameter, and 4 inches long; 
tapped with the screw-hole on its flat side, and supported by the same iron 
wire and knife edge as above mentioned. It should here be remarked, how- 
ever, that this cylinder was not wholly of lead ; since it was formed of a thin 
brass tube filled with lead ; and this tube was made to slide into an outer 
cylinder of brass, having the dimensions above described, as will be more 
fully explained in the next article. The specific gravity of the whole I found 
to be 10'237 ; and it weighed 34500 grains. 
No. 15, 16, 17, 18 are cylindrical tubes of brass, 2'06 inches in diameter 
on the outside, 4 inches long, and 043 inch thick. These, however, are not 
different tubes, but consist of one and the same cylindrical outer piece ; and 
is in fact the tube into which the leaden cylinder is made to slide, as men- 
tioned in the preceding article. This cylindrical outer piece is capable of 
being varied in the four following ways, by means of an inner sliding tube. 
No. 15 is when both the ends are open, with the exception of a narrow cross 
piece at the top, to which the screw is attached. No. 16 is when the top is 
still left open, but the bottom closed. No. 17 is when the top is closed, and 
the bottom left open. And No. 18 is when both ends are closed. In all the 
cases, the tube was suspended by the same kind of iron wire as that already 
described ; and from the same knife edge. The specific gravity of the metal 
I found to be 8 - 453 : but here it may be proper to remark (what I shall again 
advert to, in the sequel,) that when a hollow body is swung as a pendulum, we 
must take into account the quantity of air contained within the moving body 
(which, in the present case, is computed to be 3050 grains,) and diminish the 
specific gravity of the metal accordingly*. Proceeding on this principle, I 
* Cases of this kind appear to admit of two distinctions : one, where the hollow body is herme- 
tically sealed ; the other, w T here the included air communicates freely with the surrounding atmo- 
sphere, and consequently escapes under the action of the air-pump. But, in the case of a cylindrical 
tube (like that in question) there will be no difference in the result : as, from the similarity of distri- 
bution of the masses of metal and of air (at least, in the case of the tube, open at both ends ; and ap- 
