in the length of the pendulum vibrating seconds. 345 
adjusted so as for its edges to coincide exactly with those of 
the extremity of the pendulum. The next step was to bring 
in a right line, the telescope, the extremity of the pendulum, 
and a white circle of the same diameter pasted on a black 
ground on the centre of the pendulum of the clock. For 
this purpose both pendulums being at rest, the telescope was 
slid laterally on its support* until a small particle of the disk 
was seen, and a mark was made on the support of the tele- 
scope with a pencil. The telescope was now slid in the 
opposite direction till an equal portion of the disk became 
visible, when another mark was made, and the telescope being 
placed so as to bisect these two marks, the centre of the object 
glass would evidently *be in the prolongation of a line joining 
the white disk and the extremity of the pendulum. 
The diaphragm was next brought by the circular horizontal 
movement of the telescope to correspond with the edges of the 
pendulum, and the divided arc for indicating the extent of the 
vibrations was placed so that its zero coincided with the 
extremity of the pendulum. 
The same thermometer which was used in my former ex- 
periments and for the loan of which I was indebted to the 
kindness of Dr. Wollaston, was suspended on the clock case 
near the middle of the pendulum, and every thing being thus 
arranged, the pendulum of the clock was put in motion, and 
the knife edge elevated by means of the Ys above the agate 
planes, to prevent any injury when not in use. 
A firm support for the transit instrument became the next 
object of attention, and for this purpose I tried a box nearly 
* The wooden support was placed so as for the telescope to be within the limits of 
the sliding adjustment. 
MDCCCXIX. Z Z 
