268 The Proposed Pendulum Operations for India. [No. 4, 



Each is composed of a bar of plate brass 1.6 inches wide and rather less 

 than an \\X\ 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 \\ inches. 

 At 3' 2J" 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. Each 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 



