length of the pendulum vibrating seconds. 43 
which was capable of a horizontal motion on 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 perpen- 
dicular 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 hori- 
zontal diameter of the white disk, and consequently to coincide 
with the edges of the slip of deal. When, therefore, both pendu- 
lums were at rest, nothing was visible through the telescope, 
excepting the divided arc for ascertaining the extent of the 
vibrations, and which was seen through a horizontal opening 
made for that purpose in the top of the diaphragm. 
Method of determining the number of vibrations made by the 
pendulum in 24 hours. 
If both pendulums be now set in motion, the brass pendu- 
lum a little preceding that of the clock, the following ap- 
pearances may be remarked. The slip of deal will first pass 
through the field of view of the telescope at each vibration, 
and will be followed 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 second at which 
this total disappearance is observed, must be noted. The 
pendulums will now be seen to separate, and after a time will 
again approach each other, when the same phenomenon will 
take place. The interval between the two coincidences in 
seconds, will give the number of vibrations made by the pen- 
dulum of the clock ; and the number of oscillations of the 
