length of the pendulum vibrating seconds. 37 
paring the pendulum with a clock, at the distant intervals of 
24 hours ; but it will hereafter appear, that should any alte- 
ration in the form of the knife edge take place, it must 
become perceptible every ninth minute; in addition to which, 
I proposed to measure the distance of the knife edges both 
before and after the experiments, when any change would of 
course be immediately detected. 
Description of the pendulum employed. 
The pendulum constructed upon these principles is formed 
of a bar of plate brass, one inch and a half wide, and one 
eighth of an inch thick. Through this bar, two triangular 
holes are made, at the distance of 39,4 inches from each 
other, to admit the knife edges. Four strong knees of ham- 
mered 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 arc of vibration. 
