OROLOGIERS—An obsolete form of horologers, a term not now in use but signifying men who constructed time-pieces. [Illustration] ORRERY—A planetarium; an instrument showing the relative motions, positions and masses of the sun and planets. It was so named from Lord Orrery, for whom the first modern planetarium was made in England. OSCILLATION—The movement back and forward of a pendulum or the swing of a balance spring. The vibration. OVERBANKING—Pushing of the ruby pin past the lever, caused by excessive vibration of the balance. In a cylinder escapement the turning back of the cylinder until an escape wheel tooth catches and holds it. In a chronometer escapement the second unlocking of the escape wheel from the same cause. OVERCOIL—The outermost coil of a Breguet spring which is bent back across the coil toward the center. PACIFICUS—Archdeacon of Verona, died about 850 A.D. It is claimed by some that he made a clock furnished with an escapement. (Bailly.) But this is not proved, and others believe it to have been merely a water-clock. PAD—The pallet of the anchor escapement for clocks. PAIR CASE—At one time watches were made with two or even three separate cases. The outer one of shagreen tortoise shell, or some other ornamental material was sometimes for the protection of the delicate enamel on the inner case. Sometimes as in the case of repeaters the inner case was pierced to emit the sound. Then the outer one served as dust protection to the works. PALLADIUM—A soft metal formerly used in alloy with copper and silver for the balance and balance spring of non-magnetizable watches. Too soft to be as serviceable as steel, it has been superseded by a platinum alloy. PALLET—Has different meanings, even among watchmakers. Generally, the part through which the escape wheel gives impulse to the balance or pendulum. PALLET STAFF—The arbor on which the pallet is mounted, and on which it turns. PALLET STONE—The jewel on the contact face of the pallet, where it is struck by the teeth of the escape wheel. PARALLAX—The apparent angular displacement of a heavenly body due to a change of the observer's position. PEDOMETER—An instrument which registers the number of paces walked—hence if properly adjusted to the length of step of the wearer it gives the distance traversed. PENDANT—The small neck and knob of metal connecting the bow of a watch case with the band of the case.