the Pendulum vibrating Seconds in the Latitude of London. 98 
the knife edges. Four strong knees of hammered brass of the 
same width as the bar, six inches long and three quarters of an 
inch thick, are firmly screwed by pairs to each end of the bar, 
in such a manner, that when the knife edges are passed through 
the triangular apertures, their backs may bear steadily against 
the perfectly plane surfaces of the brass knees, which are formed 
as nearly as possible at right angles to the bar. The bar is cut 
of such a length, that its ends may be short of the extremities 
of the knee pieces about two inches. 
Two slips of deal 17 inches long, and of the same thickness 
as the bar, are inserted in the spaces thus left between the knee 
pieces, and are firmly secured there by pins and screws. These 
slips of deal are only half the width of the bar; they are stained 
black, and in the extremity of each a small whalebone point is 
inserted, for the purpose of indicating the extent of the are of 
vibration. 
A eylindrical weight of brass, three inches and a half diame- 
ter, one inch and a quarter thick, and weighing about two pounds 
seven ounces, has a rectangular opening in the direction of its 
diameter, to admit the knee pieces of one end of the pendulum. 
This weight being passed on the pendulum, is so thoroughly se- 
cured there hy means of a conical pin fitting an opening made 
through the weight and knee pieces, as to render any change 
of position impossible. A second weight, of about seven ounces 
and a half, is made to slide on the bar near the knife edge at the 
opposite end; and this weight may be fixed at any distance on 
the bar by two screws with which it is furnished. 
A third weight, or rather slider, of only four ounces, is move- 
able along the bar, and is capable of nice adjustment by means of 
a screw fixed to a clamp, which clamp is included in the weight. 
This slider is intended to nove near the centre of the bar. It 
has an opening, through which may be seen divisions, each 
equal to one-tyentieth of an inch, engraved on the bar; anda 
Jine is drawn on the edge of the opening to serve as an index to 
determine the distance of the slider from the middle of the bar. 
We now come to the most important part, the knife edges. 
These are made of that kind of steel which is prepared in India, 
and known by the name of wootz. Their form is triangular, 
and their length one inch and three quarters. Mr. Stodard was 
so obliging as to forge them for me: they were made as hard as 
possible, and tempered by immersing them merely in boiling 
water. 
The knife edges were ground on a plane tool, which neces- 
sarily ensured a perfectly straight edge. This was ascertained 
by bringing the edge of the one in contact with the plane of the 
other, 
