$4 An Account of Experiments for determining the Length of 
other, when, if no light was perceptible between them in any 
position, it was inferred that the edge was a right line. They 
were then carefully finished on a plane green hone, giving them 
such an inclination as to make the angle on which the vibrations 
are per!»rmed about 120 degrees. 
Previously to the knife edges being hardened, each was tapped 
half way through, near the extremities, to receive two screws, 
which being passed through the knee pieces, drew the knife 
edges into close contact with them, the surfaces of both having 
been previcusly ground together to guard against any strain 
which might injure their figure. 
The Support, and other Apparatus. 
The support of the pendulum consists of a piece of bell metal 
six inches long, three inches wide, and three-eighths of an inch 
thick. An opening is made longitudinally through half the 
length of the piece, to admit the pendulum, and the bell metal 
is cast with a rectangular elevation on each side of the opening 
extending the whole length of the piece. Two plates of agate* 
were cemented to this elevated part, beds having been made to 
eceive them, in order that their surfaces might be in the same 
plane with that of the bell metal. The whole was then ground 
perfectly flat. A frame of brass is attached by two opposite 
screws, which serve as centres, to the sides of the elevated part 
of the support; and one end of this frame being raised or de- 
pressed by means of a screw, the pendulum when placed with its 
knife edge resting in Ys, at the other end of the frame, could be 
elevated entirely above the surface of the agate, or be gently 
lowered until the knife edge rested wholly upon it; and thus the 
knife edge was sure to bear always precisely on the same part of 
the agate plane, by elevating the Ys above its surface, placing 
the knife edge in them, and then letting down the whole gently 
by means of the screw, till the Ys were completely clear of the 
knife edge. The support was firmly screwed to a plank which 
will hereafter be described, 
To the kindness of Henry Browne, Esq. F.R.S., I am essen- 
tially indebted for the success of the experiments which form the 
subject of this paper. He most obligingly allowed me the use 
of his house, his excellent time-pieces, and transit instrument, 
assisting me with indefatigable zeal by his very accurate daily 
observations, and intermediate comparisons for determining the 
rate of the clock. The house is substantially built, and is situ- 
* Plates of hard steel were first tried, but were found to have suffered 
penetration by the knife edge. 
; ated 
