PENDULUM—A body suspended by a rod or cord and free to swing to and fro; used in clocks to regulate the velocity with which the driving power moves the wheels and hence the hands. The isochronism of a pendulum vibrating in a cycloidal arc was first discovered by Galileo but he did not apply it to clocks. Most authorities credit Christian Huyghens with that adaptation to instruments for keeping time. The pendulum was first suspended by a silk cord and thus vibrated in a circular instead of cycloidal arc. "Huyghens' Checks" were an unsuccessful attempt to remedy this. Dr. Hooke succeeded in remedying it by suspending the pendulum by a flat ribbon of spring steel. [Illustration] PENDULUM, GRIDIRON—Invented by Harrison in 1726, and still with slight improvements an effective timekeeper. The rod of this pendulum is constructed of five steel and four brass rods so arranged that those which expand most are counteracted by those of less expansion, and the length of the pendulum remains constant. PENDULUM, MERCURIAL COMPENSATION—A pendulum having for a bob a jar of mercury which expands upward with the increase of temperature thus counteracting the lengthening of the rod from the same cause. Invented by Graham about 1720. With slight improvements still in use and keeps time very accurately. [Illustration] PENDULUM, TORSION—A pendulum vibrating by the alternate twisting and untwisting of an elastic suspension. The body is a horizontal disc weighted around its edges, and its suspension a steel or brass wire. The period of a torsion pendulum being much longer than a vibrating pendulum of the same length, the time of running is longer. Clocks fitted with torsion pendulums have run a year on one winding. PENDULUM SWING—The short ribbon of spring steel which suspends the pendulum of a clock. PENETRATION OF GEARING—The depth of intermeshing of the teeth of pinion and wheel. PHILLIPS SPRING—A balance spring with terminal curves after rules laid down by M. Phillips, an eminent French mathematician. A term seldom used though his curves are generally followed. [Illustration] PILLAR—The three or four short brass posts which keep the plates at their proper distance apart. In early days made in very artistic and elaborate shapes. Later they became plain straight cylindrical columns. PILLAR MODEL—A type of movement in which the works are hung between two plates supported and separated by posts or pillars and forming all the principal bearings of the movement. Only average adjustment is possible in this model. In this model the plate is sometimes cut away to imitate a "bridge model." The opposite extreme in construction to the "bridge model." PILLAR PLATE—The lower plate of a watch movement—the one nearest the dial—to which the pillars are solidly fixed, in a "pillar model."