length of the pendulum vibrating seconds . 39 
tained by bringing the edge of the one in contact with the 
plane of the 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 performed 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 previously ground together to guard 
against any strain which might injure their figure. A plan 
of the pendulum is given in Plate IV. Fig. 1. 
The support , and other apparatus. 
The support of the pendulum is represented in Plate IV, 
Fig. 2. It 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 receive 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 repre- 
sented at Fig. 3, is attached by two opposite screws, which 
* Plates of hard steel were first tried, but were found to have suffered penetration 
by the knife edge. 
