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



are read from alongside of this pillar by means of a cathetometer, viz. 

 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 temperature 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 upon 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 disc 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 pendulums 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 



