OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
69 
in the whole mass of the surrounding air. I do not however mean to infer 
that such sources of fallacy have not been fully calculated on, or guarded 
against in the many valuable papers already alluded to : I rather mention 
them as a reason for adopting a method of experimenting, differing from the 
preceding, and which might insure an unmixed result. To this end the expe- 
riments were continued in an exhausted receiver, effectually screened from 
vortices, and liberated from the influence of a resisting medium ; and in case 
of employing a rotating body, with a rotation continued so smoothly and evenly, 
as not to cause any vibration capable of inducing motion. 
4. The following is a description of the different mechanical arrangements 
resorted to. A circular plate of close fine-grained slate, a b, fig. 1, 2, 3, of 
about half an inch thick and a foot in diameter, being carefully ground true, 
in order to fit it for the purposes of an air-pump plate, was nicely finished by 
rubbing it over with a mixture of oil and wax.* This plate is supported on a 
firm stand and frame, as represented in fig. 1, furnished with levelling screws 
c, d, e. A short brass cylinder e, f, fig. 2 & 3, is fixed under the plate by 
means of a hole in the centre, and two brass collars ground true to its 
surface, and between which the plate is compressed : so that with a little 
grease, and a strong nut at e, fig. 2, 3, the joint is rendered air-tight. The 
cylinder e f connects laterally with a short pipe and stopcock g h leading to a 
good air-pump ; a long barometer gauge m, fig. 1, 2 & 3, being attached to the 
pipe to indicate the state of the exhaustion. A straight rod of brass n e, having 
a milled head at n, passes through a compressed collar of fine cork at f, and 
is sufficiently free above in the cylinder to allow of the action of the pump ; 
this rod is occasionally employed when furnished with a transverse bar at e, 
having two stops s, s, fig. 2 & 10, to liberate a magnetic bar in an exhausted 
medium, when retained at any angle of deviation from its meridian, or to effect 
any other required operation, as in fig. 3. 
5. The revolving body A, fig. 4 & 5, consists of a circular disc of about five 
inches diameter, and 0.125 of an inch thick ; united to a deep ring of the same 
* The application of this substance to the purposes of an air pump-plate will be found extremely 
convenient, more especially when it is desirable to avoid the presence of a large mass of metal. It 
is a very economical and efficient substitute for glass or brass, and retains its form without liability 
to warp ; and is an application as far as I can learn quite new. 
