[ 459 
XXV. Experiments on the Length of the Seconds Pendulum at the Royal 
Observatory of Greenwich. By Captain Edward Sabine, of the Royal 
Regiment of Artillery, F.R.S. 
Read June 16, 1831. 
These experiments were made with the original coDvertible pendulum 
constructed by Captain Kater, and employed by him in 1817 in Portland 
Place, London. 
Prior to its employment in the present experiments I made the following 
alterations in the pendulum. 
1 . The tail pieces were removed altogether ; coincidences being observed 
by the bar itself (the ends being blackened for the purpose), and by a disc of 
corresponding diameter on the pendulum of the clock. 
2. The “ moveable weight” employed by Captain Kater was dispensed with 
altogether ; and the pendulum rendered convertible, within the limits of more 
exact adjustment by the slider, by filing away a small quantity of metal from one 
extremity of the bar. By this alteration there remained nothing moveable about 
the pendulum except the slider ; and that was so placed in these experiments, 
that a change in its position, of so great magnitude as the tenth of an inch, did 
not occasion an alteration of so much as the tenth of a second in the daily rate 
of the pendulum, when suspended with the weight below, which is the position 
in which the rate is determined. The slider was clamped to the bar, and was 
moved by a screw for slow motion, by which it could be adjusted with tolerable 
precision to the hundredth of an inch. The graduation on the bar, by which 
the place of the slider was regulated, was on the side of the bar next the observer, 
who could thus at all times assure himself that no change occurred in its posi- 
tion by the inversion of the pendulum. 
Neither of these alterations interfered with the distance between the knife 
edges, which was referred in 1817 to Sir George Shuckburgh’s scale by Cap- 
