1865.] The Proposed Pendulum Operations for India. 263 
A toise was set upright on a narrow horizontal plane firmly fixed to 
a perpendicular iron bar, and the contrivance by which the pendulums 
were suspended could be placed either on the horizontal plane, or on 
the top of the toise itself, so that the effective lengths of the wires 
differed in the two cases by an amount exactly equal to the length of 
the toise. The wires, which were of steel, were attached to a thin 
strip of brass which unwound itself over a small cylinder. The pen- 
dulum, thus suspended, described the curve called the evolute of the 
circle. Atthe lower end of the iron bar there was a micrometer 
screw, for measuring small differences in the height of the ball. 
The system ‘of observation was as follows. At the commencement 
of a series of coincidences with the longer pendulum, the thermometers 
attached to the toise were recorded, and the reading of the lower 
surface of the ball was taken with the micrometer screw; the pendulum 
was then set in motion, and after a sufficient number of coincidences 
had been observed, the readings of the ball and thermometers were 
again taken. Exactly the same process was then gone through with 
the shorter pendulum: then, from the times of vibration of the two 
pendulums, whose absolute lengths were unknown, but whose difference 
in length was accurately known, the length of the seconds’ pendulum 
was easily computed.* There were a great many minute details to b® 
attended to, all of which were carried out with the greatest ingenuity 
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. The 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 83 
* Let t, & 1, be times of vibration and Jength of longer pendulum, 

ty ly ” ” ” shorter % 
l, — 1, = difference in length = a 
L =length of seconds’ pendulum. 
U 
Then ¢, ? = 7? a to? 7? 25 l= 7? — 
g gy 
ees Bak Uns 1, —l, l, a 
ne bsg temitys Ly ta2 st, 7) 
1 
Again [= =-orL=?3,= = 
