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 Too soft to be as serviceable as steel, it has been superseded by a platinum alloy.