Clock

—An instrument or machine automatically recording the pulsations of a pendulum and consequently measuring time by the movements of hands over a dial graduated to the hours and their subdivisions, also by striking the hours on a bell or chimes. The motive power is obtained from the descent of a raised weight which drives a train of wheels moving at different velocities to record the various intervals of time. The tendency to acceleration on the part of the train is periodically checked by an escapement, which checks the advance of the wheels at regular intervals; the pre ure of the gearing is transmitted to the pendulum, and gives to the latter sufficient force to overcome the resistances tending to bring it to rest. A pendulum is unsuitable for a portable time piece and is replaced by a balance wheel and spring, while a driving spring is substituted for the falling weight. Clocks are sometimes operated to a greater or le extent by electricity.