96 An Account of Experiments for determining the Length of 
firm as that which has been described, it became a point of con+ 
siderable importance to verify this by actual experiment. For 
this purpose 1 had recourse to a delicate and simple instrument 
invented by Mr. Hardy, clock-maker, the sensibility of which is 
such, that had the slightest motion taken place in the support, 
jt must have been instantly detected. This little instrument 
consists of a steel wire, the lower part of which inserted in the 
piece of brass which serves as its support, is flattened so as to 
form a delicate spring. On the wire, a small weight slides, by 
means of which it may be made to vibrate in the same time as 
the pendulum to which it is to be applied asa test, When thus 
adjusted, it is placed on the material to which the pendulum is 
attached ; and should this not be perfectly firm, its motion will 
be communicated to the wire, which in a little time wili accom- 
pany the pendulum in its vibrations. This ingenious contrivance 
appeared fully adequate to the purpose for which it was em- 
ployed, and afforded a satisfactory proof of the stability of the 
point of suspension. 
A firm triangular wooden stand, as high as the ball of the pen- 
dulum, was screwed to the floor at the distance of nine feet in 
’ front of the clock. This served as a support, to which was at- 
tached a small teleseope, magnifving about four times, which 
was capable of a horizontal motion en its axis, a vertical motion, 
and a motion at right angles to the line of sight. In the focus 
of the eye-glass was a diaphragm forming a perpendicular 
opening, the sides of which were parallel, and capable of being 
placed nearer, or further asunder. The. edges of this diaphragm 
were adjusted so as to form tangents to the horizontal diaméter 
of the white disk, and consequently to coincide with the edges 
of the slip of deal. When, therefore, both pendulums were at 
rest, nothing was visible through the telescope, excepting the 
divided are for ascertaining the extent of the vibrations, and 
which was seen through a horizontal opening made for that pur- 
pose in the top of the diaphragm. 
Method of determining the Number of Vibrations made by the 
Pendulum in twenty-four Hours. 
If both pendulums be now set in motion, the brass pendulum 
a little preceding that of the clock, the following appearances 
may be remarked. The slip of deal will first pass through the 
field of view of the telescope at each vibration, and will be fol- 
lowed by the white disk. But the distance between the centres 
of suspension and oscillation in the brass pendulum being rather 
the longer, the pendulum of the clock will gain upon it, the white 
disk will gradually approach the slip of deal, and at length, at a 
certain vibration, will be wholly concealed by it, The minute 
and 
