MOMENT OF INERTIA—The resistance of a body in motion (or at rest) to a change in the velocity or direction of its motion. In a rotating body the sum of the products formed by multiplying the mass of each particle by the square of its distance from an axis. MONTH—An arbitrary division of the year, varying in the number of days it contains, according to the calendar in use. See _Calendar_. MORTISE—A slot or hole into which a tenon of corresponding shape is to be fitted. MOSELEY, C. S.—A pioneer in the field of designing and building automatic watchmaking machinery. He invented some of the most delicate and complicated tools and mechanisms used in watch manufacture. He was early connected with the Waltham Co., master mechanic for the Nashua Co., during its brief history; and later general superintendent of the Elgin National Watch Company. MOTION—The wheels that carry the hands: cannon pinion, horn wheel and minute wheel and pinion. MOTION WORK—The wheels in a watch which make the motion of the hour hand one twelfth as rapid as that of the minute hand. MOVEMENT—The watch or clock complete, without dial or case—the mechanism of the watch or clock. MUDGE, THOMAS—An English watchmaker of the 18th century. Born at Exeter in 1716, died 1794. In 1793 he received from Parliament three thousand pounds as a recompense for his improvements in chronometers. His work was celebrated for its excellence. NAME BAR—The bar which carries the upper end of the arbor of a watch barrel. NAVAL OBSERVATORY—The United States Naval Observatory at Washington, D. C. There is there a superlatively accurate clock from which the time is flashed electrically to all parts of the United States. NEUCHATEL—A town in the Jura Mountains' watch manufacturing district of Switzerland. A Cantonal Observatory at Neuchatel helps establish the reputation for the accuracy of Swiss watches. [Illustration] NON-MAGNETIC WATCH—A watch in which the quick-moving parts—lever, pallets, balance spring, etc., are made of some other metal besides steel—as aluminum bronze, invar, etc. NUREMBERG—A German city where Peter Henlein made the first watch. It was one of the chief clock centers of the 16th and 17th centuries and with Augsburg and Ulm supplied the markets of Europe with the first small clocks. NUREMBERG EGGS—Watches made in Nuremberg in the shape of eggs. If not the first watches at least very early examples. OBELISK—A square shaft with a pyramidal top. The ancient Egyptian obelisks are thought to have served as gnomons. [Illustration] OGIVE—A pointed arch—of the architectural type known as Gothic. OIL SINK—The cavity around the pivot hole in watch and clock plates, designed to hold a small particle of oil in contact with the pivot. ORMOLU—Gilt or bronzed metallic ware, or a fine bronze which has the appearance of being gilded. Used for ornamenting the cases of fine old clocks. OROLOGE—An obsolete form of horologe. See _Horologe_.