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 withdmwn, 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 

 thei-mometers, barometer, and gauge as may be requii'ed, and the bell glass re- 

 placed. All beneath the bell glass remains from thencefoi-ward 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 air being withdrawn and admitted at pleasure through the metallic pipe go- 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 pradent 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 Obsei"vatory 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. Tlae 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 



MDCCCXXIX. . 2 E 



