ame, in the centre of which the hands rotate, without any visible
connexion with the works of the clock.

Clocks have been made with a sort of balance wheel consisting of a
thread with a ball at the end which winds backwards and forwards
spirally round a rod. In others a swing or see-saw is attached to the
pendulum, or a ship under canvas is made to oscillate in a heavy sea. In
others the time is measured by the fall of a ball down an inclined
plane, the time of fall being given by the formula t = sqrt(2s/(g sin
a)), where s is the length of the incline and a the inclination. But
friction so modifies the result as to render experiment the only mode of
adjusting such a clock. Sometimes a clock is made to serve as its own
weight, as for instance when a clock shaped like a monkey is allowed to
slide down a rope wound round the going barrel. Or the clock is made of
a cylindrical shape outside and provided with a weighted arm instead of
a going barrel; on being put upon an incline, it rolls down, and the
fall supplies the motive power.

Clocks are frequently provided with chimes moved exactly like musical
boxes, except that the pins in the barrel, instead of flipping musical
combs, raise hammers which fall upon bells. The driving barrel is let
off at suitable intervals. The cuckoo clock is a pretty piece of
mechanism. By the push of a wire given to the body of the bird, it is
bent forward, the wings and tail are raised and the beak opened. At the
same time two weighted bellows measuring about 1 X 2 in. are raised and
successively let drop. These are attached to small wooden organ pipes,
one tuned a fifth above the other, which produce the notes. Phonographs
are also attached to clocks, by which the hours are called instead of
rung.

Clocks are also constructed with conical pendulums. It is a property of
the conical pendulum that if swung round, the time of one complete
revolution is the same as that of the double vibration of a pendulum
equal in length to the vertical distance of the bob of the conical
pendulum below its point of support. It follows that if the driving
force of such a pendulum can be kept constant (as it easily can by an
electric contact which is made at every revolution during which it falls
below a certain point) the clock will keep time; or friction can be
introduced so as to reduce the speed whenever the pendulum flies round
too fast and hence the bob rises. Or again by suitable arrangements the
bob m