Pendulum

(pěnd'u-lum.) [From L. pendulus, hanging down.] (Dynamics.) A ponderous body so suspended that it may vibrate, or swing backwards and forwards, from some fixed point or axis of suspension. The vibrations of a pendulum are called its oscillations, and depend on the force of gravity. From the precision of its motions, it is employed in measuring time and space. The origin of the P. is traced to Galileo's observation of a hanging lamp in a church at Pisa continuing to vibrate long and with singular uniformity, after any accidental cause of disturbance. He was induced to investigate the laws of this phenomenon, which led to results in the highest degree important. A common clock is merely a pendulum with wheel work attached to it, to record the number of the vibrations; and with a weight or spring having force enough to counteract the retarding effects of friction and the resistance of the air. The wheels show how many swings or beats of the pendulum have taken place, because at every beat a tooth of the last wheel is allowed to pa . Now, if this wheel has sixty teeth, as is common, it will just turn round once for sixty beats of the pendulum, or seconds; and a hand fixed on its axis, and projecting through the dial-plate, will be the second-hand of the clock. The other wheels are so connected with this first, and the numbers of the teeth on them so proportioned, that one turns sixty times slower than the first, to fit its axis for carrying a minute-hand; and another, twelve times slower still, to fit its axis for carrying an hour-hand. The P. has been succe fully used to measure the force of gravity at different parts of the earth; the greater this is, the greater the length of a pendulum which will vibrate at agi ven rate. At Spitz bergen, the length of a pendulum vibrating seconds is 39-21469 English inches; at the Cape of Good Hope, 39-078. The pendulum is affected by temperature (see COMPENSATION PENDULUM). [s. 694]