1865.] The Proposed Pendulum Operations for Tndia. 269 



attached to the bar, their bulbs being sunk in the metal at points 

 equidistant from each other and the ends of the bar. The stem of the 

 upper thermometer is inverted, and placed side by side with that of 

 the lower thermometer, in order that, they may both be viewed 

 through a moderate sized glass plate let into the cylinder. In the 

 Lower part of the cylinder there are four other windows, two on the 

 line of the pendulums, to enable their coincidences to be observed ; 

 the other two at right angles to these, to give additional light and 

 enable the observer to ascertain whether the detached pendulum is 

 vibrating truly without wabble. There are two scales fixed at right 

 angles to each other, inside the cylinder, on a level with these windows, 

 one of which is used for measuring the arc of vibration of the pen- 

 dulum, and the other to measure the distance of the pendulum; from 

 the former scale, which is necessary to furnish the correction for 

 parallax in the readings of the arc of vibration : it is useful also in 

 placing the pendulum at a constant distance from the clock, which is; 

 found convenient in practice. 



The upper 4" of the cylinder is made of greater thickness than the 

 rest, and at top is a strong projecting flange which is intended to rest 

 en a strong cast iron frame made in two pieces, so as to grip the 

 cylinder round the thicker part just below the flange ;. the halves of 

 the frame are then firmly bolted together with nuts and screws. The 

 upper surface of the flange is ground perfectly true to receive a bell 

 glass, the cap already mentioned, which is like the receiver of an, 

 ordinary air pump. The eccentric for raising and lowering the pen- 

 dulum on to the agate planes passes through a stuffing box in the 

 upper part of the cylinder. Motion is imparted to the pendulums by 

 means of a fork and crutch turned by a spindle which passes through, 

 another stuffing box. 



The clock with which the vibrations are compared is firmly secured 

 to a wall, and the vacuum apparatus is erected in front, at a distance 

 of about 2 feet from it. The diaphragm for limiting the view of the 

 disc is fitted inside the clock case. 



The telescope for observing the coincidences is placed on a small 

 masonry pier at a distance of about 8 feet from the vacuum apparatus 

 and is mounted so as to slide laterally on a graduated horizontal bar ; 

 it has also a slight vertical motion. The thermometers and barometers 



