270 The Proposed Pendulum Operations for India. [No. 4, 
a telescope sliding up and down on a vertical rod. The object of this 
is to obviate the ill effects of any defect in the isolation of the appara- 
tus, as well as the influence of the observer’s person on the thermo- 
meters. 
As the disc on the bob of the clock and the tail-piece of the de- 
tached pendulum are too far apart to be viewed simultaneously by the 
telescope, a lens is placed between them, so as to throw the image of 
the white disc upon ‘the tail-piece of the pendulum. The vacuum 
cylinder and all its adjuncts, air-pump, &c. were made by Adie, and 




























are the only new portions of the apparatus. 
The method of operation is as follows. After setting up the clock, 
the vacuum apparatus is inserted in the iron frame and suspended 
either on wooden trestles or masonry piers; the frame is roughly 
levelled ; the t>mperature bar is fixed in position; the agate planes 
are screwed on firmly to their bed plate, and are very carefully levelled 
by means of delicate spirit levels provided for the purpose. A pen- 
dulum is now inserted and let down upen its planes, but the clock 
must not yet be set in motion. The telescope is next set up on the 
prolongation of the line which passes through the two pendulums, 
when both are at rest. For this purpose it is moved laterally on its 
graduated support, until a very small portion of the paper disc, on the 
bob of the clock pendulum, is visible on one side of the tail-piece of 
the detached pendulum. The reading is noted, and the telescope is then 
moved in the opposite direction, until an equal portion of the dise is 
visible on the other side of the tail-piece; the reading is again noted 
and the telescope is set to the mean position. The pendulum is then 
removed, and the diaphragm in the clock case adjusted, until its cheeks 
are tangents to the disc. The pendulum may now be replaced, and 
nothing remains to be done but to exhaust the air out of the apparatus 
and to set the pendulum in motion. 
The observations are made in exactly the same way as already de- 
scribed in the account of Captain Kater’s apparatus; the times of the 
disappearance and reappearance are both noted, and the mean taken as 
the true time of coincidence. The arc of vibration is then determined 
by noting the reading of the arc, when it is cut by the same edge of 
the tail-piece on each side of the vertical line. The thermometers and 
barometer are read by means of the cathetometer. It is usual to 
