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



and nicety, and all conceivable sources of error were considered and 

 their effects computed and allowed for. 



The coincidences were observed in a slightly different way from any 

 preceding method. Tire pendulum was enclosed in a wooden case, 

 faced with glass to keep out currents of air, as well as to preserve as 

 constant a temperature as possible ; the clock was placed about 8§- 

 feet in front of the pendulum, and between the two, the object glass 

 of a telescope was adjusted to form an image of the detached pendulum 

 in the plane of the clock pendulum, to enable them both to be seen 

 simultaneously through the observing telescope, which was set up at a 

 distance of about 15 feet. On the wire of the detached pendulum 

 was fixed a small brass cylinder, painted black and called the coinci- 

 dence cylinder ; it weighed something under 4 grains, and could be 

 brought exactly opposite the scale for measuring the arc of vibration. 



Captain Kater's pendulum consisted of a bar of plate brass 1.6 

 inches broad and ^th of an inch thick : two knife edges of the hardest 

 steel, attached to solid pieces of brass, were fixed to the bar at a dis- 

 tance of rather more than 39 inches from each other ; when the pen- 

 dulum was in use, these knife edges rested on horizontal planes of agate. 

 At one end of the bar, immediately below the knife edge, was a large 

 flat brass bob firmly soldered to it ; and on the bar, between the knife 

 edges, were two sliding weights. The plan of operations was to 

 observe the number of vibrations per diem, made by the pendulum 

 when suspended, first, by one knife edge, and then, by the other ; 

 and if these numbers were not identical, to make them so, by means 

 of the sliding weights. The distance between the knife edges, that is, 

 the length of the corresponding simple pendulum, was then measured 

 by a micrometric arrangement. The method of observing the number 

 of vibrations was as follows ; to each extremity of the pendulum, a 

 light deal tail-piece, well blackened, was attached ; and on the bob of 

 the clock pendulum a white paper disc, equal in diameter to the 

 breadth of the tail-piece, was fastened ; the detached pendulum was 

 now placed in front of the clock, and both pendulums being at rest, 

 a telescope was alined, so that the blackened tail-piece exactly covered 

 the paper disc. The telescope was also fitted with a diaphragm, con- 

 sisting of two perpendicular cheeks, which could be adjusted so as to 

 become tangents to the disc. Now, if both pendulums be set in motion, 



