
1865.] The Proposed Pendulum Operations for India. 269 
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 are 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 the top is a strong projecting flange which is intended to 
rest on 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 
are read from alongside of this pillar by means of a cathetometer, viz. 

