OF THE VIBRATIONS OF AN INVARIABLE PENDULUM. 
209 
ring - thereby the contact of the surfaces of glass and metal. The suspension 
piece being surmounted by the bell glass, the air is withdrawn, and the 
weight of the atmosphere on the exterior presses the several joints into the 
closest contact. Before the air is re-admitted, four screws, destined to connect 
the iron frame in which they work with the suspension piece, are turned until 
their pressure in different directions, against the outside of the ring surrounding 
the suspension plate, attaches it firmly to the iron frame. The frame is itself 
very firmly screwed to stone piers deeply imbedded in the wall on either side, 
and is further strengthened by brackets, fixed in the direction which is most 
immediately opposed to any motion of vibration, which might be communicated 
by the pendulum. The air is then re-admitted, the bell glass taken off, the 
agate planes screwed on and levelled, the pendulum suspended, with such 
thermometers, barometer, and gauge as may be required, and the bell glass re- 
placed. All beneath the bell glass remains from thenceforward a fixture, the 
air being withdrawn and admitted at pleasure through the metallic pipe go- 
verned by the stop-cock. As the three middle glasses are pressed tightly be- 
tween the suspension piece and the pedestal, neither of which can give way to 
their expansion, it might not be prudent perhaps to risk their fracture, by leav- 
ing them so screwed, for such a length of time as should involve a great change 
of temperature. To avoid this, it is only necessary to loosen the screws which 
connect the iron frame with the suspension piece, to tighten them again at a 
new temperature, and to re-level the planes. 
This description applies to the apparatus as it is now established in the south- 
west angle of the quadrant-room at the Royal Observatory at Greenwich : an 
angle being chosen, because the stone piers, to which the iron frame is screwed, 
have in such case but a small distance to project from the walls, in order to 
form an appui on both sides. The apparatus was employed in the two first 
experiments at Mr. Browne’s house in London, where similar means could 
not be resorted to for-rendering the point of suspension of the pendulum im- 
moveable. In these experiments the agate planes were screwed to an iron 
plate, which was supported by four iron bars springing from the interior of 
the cylinder of the pedestal ; and the bell glass rested on the upper glass 
cylinder, without the intervention of the suspension piece. It will be seen by 
the result of those two experiments, compared with the result of others in which 
2 E 
MDCCCXXIX. 
