OF WHICH VARIOUS SUBSTANCES ARE SUSCEPTIBLE. 
71 
is placed a graduated circle of fine milled board, sustained in a light circular 
ring of wood. The screen and ring are moveable with friction in a hollow 
cylinder of glass 555 ' s", fig. 3, so as to be adjusted at any required altitude. 
7 . The receivers employed to inclose these various bodies are for the most 
part such as are represented in fig. 1 , 3 & 6 ; they have an open neck at B 
ground to a flat surface which admits of the altitude being increased by the 
addition of a second narrow receiver B D : this last is also ground flat at the 
point of union ; thus, by the aid of a little grease, an air-tight joint is easily 
obtained in the usual way. The upper receiver B D is either closed above, 
or open, with a ground flat rim in order to apply a brass plate D carrying a 
brass rod t in an air-tight collar. The rod t is for the convenience of raising 
or depressing light discs or needles suspended in the receiver below : these 
receivers vary from eight to forty inches in altitude, and from one to three 
inches in diameter ; so that the filament of silk for suspension may, when re- 
quired, be upwards of four feet in length. 
8 . The substances exposed to the influence of the revolving bodies are 
formed into light needles or bars, or otherwise into flat circular rings, fig. 9, 
and are either suspended or nicely balanced on points. For the purpose of 
retaining them at rest until the exhaustion is complete, and when the stop e, 
fig. 2, already explained (4) cannot be used, there is an angular lever /, fig. 3, 
which passes in an air-tight collar through the side of the receiver, carrying at 
its extremity a common reed s, which being tubular is easily fixed on it ; hence 
it can be extended at pleasure. 
9. The following are the results of some experiments with the apparatus 
above described. 
(a) A circular disc of copper being put into rotation at the rate of 500 revo- 
lutions in a minute, was left unscreened under a receiver, and a fine reed sus- 
pended over it by a filament of silk ; the reed was retained at rest, until the 
exhaustion was completed to within 0.4 of an inch of a good barometer, when it 
was set free. The reed rotated freely at any distance at which it could be 
placed from the revolving disc, and which at the greatest amounted to eight 
inches. The motion of the reed diminished with the rapidity of the rotation 
of the disc and with the torsion of the silk ; but its motion could be again 
restored by admitting a small quantity of air into the receiver. 
