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off on them a scale, transferred from Colonel Lambton’s scale ; the 
length was then measured by a pair of beam compasses. The length 
of the seconds pendulum was found to be 39.0263 inches of this 
scale in air. 
In 1818, Captain Kater published his determination of the length 
of the seconds pendulum in London at Mr. Browne’s house, Portland 
Place, taken for the purpose of fixing the standard of English 
measures. His method was founded on the dynamical theorem due to 
Huyghens, that the centre of oscillation, and axis of guspension, are 
reciprocal in the same body ; that is, if the body be suspended at its 
centre of oscillation, the former axis of suspension will pass through 
the Rew centre of oscillation, and the body will vibrate in the same 
time as before. The distance from the axis of suspension to the point 
called centre of oscillation, is equal to the length of the simple 
pendulum. 
Captain Kater’s pendulum consisted of a bar of plate brass 1.6 
inches broad and 4th 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 t 
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