A part of the apparatus for measuring the rate of a ship's motion through the water, consisting of a flat piece of wood, usually in the form of a quadrant, loaded with lead at its circular edge to make it float upright. To it is attached the log-line, which runs freely from a reel, and is marked by knots at intervals of five fathoms. The number of knots run out during the running of the half-minute sand-glass tells the number of miles per hour which the vessel is making. (See Knot.)