A PENDULUM FOR THE REDUCTION TO A VACUUM. 457 
As the quantity of air dragged by spheres is proportionate to the cubes of 
their diameters, I was induced to examine whether the quantity dragged by a 
sphere, and by a cylinder of the same diameter and height, would be propor¬ 
tionate to their solid contents ; or, in the ratio of 1 to 1^. But, from a com¬ 
parison of pendulums No. 6 and 10 (see page 433) it appears that the cylinder 
drags more than that proportion, by about |-th part of the whole. 
If we compare the results of pendulums No. 10 and 13 (see page 433), the 
difference in the quantity of air dragged would appear to be that which is due 
to the difference in the effect produced by the wire and the rod. But we must 
bear in mind what has been stated in page 440, relative to bodies suspended 
at the end of a rod or wire ; and reduce them, by the formula there given, to 
the same point: in which case, the weight of adhesive air, due to the cylinder 
alone , would be very nearly alike in both experiments. 
From a review of the whole, it appears that even when a pendulum is formed 
of materials having the same specific gravity, yet, if it be not of an uniform 
shape throughout, each distinct portion must be made the subject of a separate 
computation, in order to determine the correct vibrating specific gravity of the 
body ; since each part will be variously affected by the circumambient air. 
As an example, take the case of the pendulum No. 3, where the iron wire and 
the brass sphere have almost exactly the same specific gravity, viz. 7‘66. If 
we suppose the sphere drags (MO grain of air, and the wire 0‘10 grain, (or 
about \ of that dragged by the sphere), we shall have the specific gravity of 
the sphere, with its coating of air, reduced to about 4*43, and that of the wire 
with its coating of air, to about O'14. Whence the vibrating specific gravity 
of the whole pendulum, deduced agreeably to the formula (2) in page 405, 
will be about 4 - 21 ; which would give the reduction to a vacuum equal to 
13 - 380 seconds: differing very little from the true correction given in the 
Table in page 433. If the effect of the air on the wire had been neglected, 
this value would have been diminished about one second: which shows that 
in making experiments on pendulums of this kind in water, the whole of the 
wire should be immersed in the fluid, in order to deduce correct results. 
In concluding these experiments I cannot flatter myself that no error has 
escaped me; especially when I consider the vast number of computations 
which have been employed in these investigations. The major part of them. 
