OF THE VIBRATIONS OF AN INVARIABLE PENDULUM. 
219 
vibration in the air of full pressure and in rarefied air was not sensibly affected 
by a small motion of the support. 
It was now considered, therefore, as established by the experiments, that the 
true reduction to a vacuum is considerably greater than it had been customary 
to suppose ; for the invariable pendulum, for example, nearly as 5 to 3. It was 
also obvious, that all pendulums whatsoever, employed in air, and designed to 
give results which should be independent of the variable retardation occasion- 
ed by their vibration in air, would require to have the influence of the air on 
their respective vibrations, ascertained by experiment, since it is not attainable 
by calculation. Now as the apparatus was suited by its construction, to furnish 
this element with facility and accuracy, for any of the forms in which pen- 
dulums have hitherto been made, either for determining absolute or relative 
lengths, it was probable that it might eventually become more extensively 
useful, than in its present office of furnishing the reduction for an invariable 
pendulum. 
It was thought proper, therefore, that the apparatus should be removed to the 
Royal Observatory at Greenwich and established there, in order that it might 
be hereafter at the command of persons to whom it might be useful, upon their 
application to the Board of Longitude at whose expense it had been con- 
structed. The iron suspension plate with the iron bars supporting it were now 
removed, and the bell metal plate with the circular exterior ring, represented 
in Plate VI, substituted, with the iron frame-work and screws, as represented 
elsewhere in the plate, enabling the support of the pendulum to be fixed im- 
moveably at pleasure in the manner already described. A clock by Dent, with 
a mercurial pendulum carrying a disk, was placed in the angle behind the 
apparatus ; and' the telescope for observing coincidences in front, about 16 feet 
distant from the detached pendulum when suspended. Arrangements were 
made for observing the coincidences by artificial light, without interfering with 
the temperature of the room, by directing the light of an Argand lamp, sta- 
tioned in an adjoining apartment, on the disk of the clock pendulum, through 
a tin tube, which prevented the diffusion of the light in the room. The dia- 
phragm was placed between the glasses and the clock, and the arc within the 
glasses close to the pendulum. The arc was graduated in inches and tenths, 
and was read off to hundredths ; crossing the pendulum at 47-7 inches from 
2 f 2 
