A PENDULUM FOR THE REDUCTION TO A VACUUM. 
451 
Rods. 
Thickness of the 2-inch cylinders. 
0*18 
0-50 
1-00 
2-06 
Brass. 
1-284 
1-523 
1-681 
1-902 
Steel. 
1-350 
1-490 
1-661 
1-946 
Mean = 
1-317 
1-506 
1-671 
1-924 
The differences between these mean values would indicate the quantity 
of air dragged by the sides of a cylinder of this diameter, according to its 
thickness: but which does not appear to be very regular in its march ; since 
the thin cylinders drag more in proportion than the thicker ones. Till this 
fact is more fully ascertained, we cannot deduce a correct general formula for 
determining the quantity of air dragged by cylinders of different diameters 
and thicknesses, swung in the manner above mentioned. 
The next set of experiments were made with thin circular discs of brass, 
having about the same thickness as common thick post paper. Twenty 
pieces, screwed together in a vice, measured *08 inch; consequently the thick¬ 
ness of each of the brass discs may be assumed equal to *004 inch. One of 
these discs was intended to be 206 in diameter, in order to correspond with 
the cylinders above mentioned ; but it is in fact somewhat larger, being 2’07 ; 
and weighs 28 grains: the second was 3‘01 inches in diameter, and weighed 
57'5 grains: and the third was 4 inches in diameter, and weighed 106-5 
grains. Their specific gravity I found to be 8-450. The long brass rod above 
mentioned* was then tapped with a screw hole at 38 inches from the knife 
edge, and the three discs, in succession, were respectively fastened thereto; 
and swung with their flat sides opposed to the line of motion. The long steel 
rod could not be used on this occasion, not only because the discs could not 
be conveniently attached thereto, but also on account of its coming to rest 
so soon. 
* This was the second brass rod, 5 6'9 inches long, mentioned in the second note in page 441. 
