Pendulum. The pendulum is a timekeeper, because the times of the vibrations are very near equal, whether it be moving much or little ; that is to say, whether the arc described by it be large or small. A common clock is merely a pendulam, with wheelwork 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 pendulum have taken place, because at every beat, a tooth of the last wheel is allowed to pass. 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 band fixed on its axis, projecting through the dial-plate, will bo the second-hand of the (dock. 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 to carry a minute-hand ; and another, by moving twelve times slower still, is fitted to carry the hour-hand.