268 The Proposed Pendulum Operations for India. [No. 4, 
than an ith of an inch thick; a strong cross piece of brass is rivetted 
and soldered to the top to hold the knife edge, which consists of a 
prism of very hard steel, passing through the bar and adjusted at right 
angles to its surface, The prism is equilateral in section, but the edge 
on which it vibrates is ground to an angle of about 120°; the length 
of the bar from knife edge to the extremity is about 5 feet 14 inches. 
At 3’ 22” from the knife edge, a flat circular bob, also of brass 
nicely turned and pierced in the direction of its diameter, is firmly 
soldered on; the part of the bar beneath the weight, called the tail- 
piece, which is about 17” in length, is reduced to a breadth of 0.7 of — 
an inch and is varnished black, in order to contrast better with the 
white disc on the clock pendulum, in the observation of the coinci- 

























dences. 
The knife edges rest on agate planes set in a solid brass frame, 
which is provided with three levelling screws. On the outer side of 
each plane are Y¥’s, which are moveable in a vertical direction by 
means of an eccentric ; the knife edges rest in them when the pendu- 
lum is not in use, and by their means the observer is enabled to lower 
the pendulum down gently so as to bear always on the same parts of 
the agate planes. Hach pendulum has its own set of planes, and will 
give different results, if swung on any others. 
It has been decided to swing the Indian pendulums in vacuo, in 
order to secure the following advantages. When the pendulum has 
been set in motion, it will vibrate for a whole day ; its temperature 
will be more equable; it will not be disturbed by currents of air ; 
and errors in the formula for the correction for buoyancy are unimpor- 
tant. The vacuum apparatus consists of a cylinder of sheet copper 
about 1 foot in diameter and rather more than 5 feet long, with 
hemispherical caps, the upper one of glass and moveable, the lower 
one of sheet copper and soldered to the cylinder. The upper end of 
the cylinder carries a strong brass. plate, to which are attached the 
frames containing the agate planes and a bar of the same metal and 
shape as the pendulums ; placed side by side with a pendulum inside the 
apparatus, the bar and pendulum will be of the same temperature, and 
it is evident that thermometers attached to the former will give the 
required temperature of the latter. Two delicate thermometers are 
attached to the bar, their bulbs being sunk in the metal at points 
