CUSTER, JACOB D.—(1809-1879.) A Pennsylvania clockmaker in 1831; he was one of the early makers of watches in America in 1840. However, his work was not important commercially, for he produced only about a dozen watches. A very ingenious man, who, it is said, made everything from a steam engine to his own shoes. He made hundreds of the clock movements which at that period were used to revolve the lanterns in lighthouses. CYCLE OF THE SUN—A period of twenty-eight years, after which the days of the week again fall on the same days of the month as during the first year of the former cycle. It has no relation to the sun's course but was invented for the purpose of finding out the days of the month on which the Sundays fall during each year of the cycle. Cycles of the sun date from nine years before the Christian era. CYCLOID—A curve generated by a given point in the circumference of a circle which is rolled along a straight line always in the same place. Example: The curve traced by any point in the rim of a wheel which travels in a straight line along a level road. CYLINDER ESCAPEMENT—See: _Escapement, Cylinder_. CYLINDER PLUGS—Plugs fitted into the ends of the cylinder of a cylinder escapement. Their outer extremities are formed into the pivots on which the cylinder rotates. DAMASKEEN—To decorate a metal by inlaying other metals or jewels, or by etching designs upon its surface. To be distinguished from snailing, with which it is often confounded. [Illustration] DAY—The time of one complete revolution of the earth on its axis. The actual length of this day is continually changing owing to the eccentricity of the earth's orbit and the angle of the ecliptic. The mean solar day is 24 hours. The sidereal day is 23 hours, 56 minutes, 4.099 seconds. DAY, NAUTICAL—The nautical day begins when the sun is on the meridian and eight bells are struck. The day is divided into "afternoon watch" or four hours, two "dog watches" of two hours each, then "middle watch," "night watch," "morning watch" and "forenoon watch," each of four hours, completing the day. DENISON, EDMUND BECKETT—Sir Edmund Beckett—Lord Grimthorpe. Born 1816. A lawyer by profession, and the inventor of the gravity escapement for turret clocks; also an authoritative writer on horological subjects. He designed and planned the Westminster clock said to be the best timekeeper of its kind in the world. Died 1905.