540 GENERAL WALKER ON PENLULUM OPERATIONS FOR DETERAIINING 
no such distinguishing number, but is marked 1821, presumably the year in which it 
was constructed ; Colonel Herschel believes that it is probably No. 6 of the series, 
and has so called it. No, 4 was employed by Sabine in his operations between the 
parallels of 13° South and 80° North Latitude, in 1822-23 ; and No. 6 (1821) was used 
by the late Astronomer Royal, Sir George Airy, in experiments in the Harton 
Colliery Pit, in 1854, to determine the earth's mean density; these two are the 
pendulums of the Royal Society which were employed throughout the operations in 
India. No. 11 was used by Bailey, in London, in 1832, and by Maclear, at the 
Cape of Good Llope, in 1839 ; it was afterwards lent for a while to the Admiralty, 
and eventually deposited in the Kew Observatory. 
Each pendulum is furnished with a pair of agate planes, on which it is intended to he 
swung. The planes are set on either side of a half-inch opening in a solid brass 
frame, which is mounted on a plate at the head of the receiver, and is provided with 
three levelling screws ; outside the frame there is a pair of moveable arms carrying 
Y’s, in which the pendulum rests while not vibratiug, and on lowering wRich the knife 
edge comes in contact with the agate planes for vibration. The pendulum is placed 
midway between the supporting planes by hand and eye estimate, but it is always 
brought by the Y’s down on to the same line across the planes, in all positions of the 
pendulum, whether the marked face is pointing towards the observer or towards the 
clock. 
The length of the pendulum is invariable, excepting from change of temperature for 
which the correction to the vibration-number is known. The shape is that of a 
flexible bar of plate brass, G2 inches long, 1’7 inch broad, and 0T3 inch thick from 
the knife edge downwards for a distance of about 40 inches, where a flat circular bob, 
6 inches in diameter and I’3 inch thick, with a bevelled edge, is soldered on to 
the bar ; the tail piece, below the bob, is reduced to a breadth of 0'7 inch, and is about 
16 inches long. The bar is necessarily very flexible, its thickness being less than a 
tenth of its breadth, and this flexibility is greatly in contrast with the rigidity of the 
German and French pendulums. Kater is believed to have adopted a flexible form 
of bar in preference to a rigid bar designedly, under the impression that it was less 
likely to become permanently bent by accident, and more likely to acquire exact 
verticality wdien its knife-edge is resting on the agate planes during the course of the 
vibrations. 
The Processes of Manipulation. 
When the pendulums were sent out to India, it was intended that they should 
always be swung as nearly as possible in a vacuum. For this purpose a receiver of 
sheet copper, mounted on a substantial and well braced wooden stand, was furnished 
for the pendulums to be swung in ; the receiver was closed above by a hemispherical 
glass cap, which could be removed at pleasure for the insertion or withdrawal of a 
pendulum. Two thermometers were fixed in a dummy pendulum, of the same size as 
