Chip Log

A chip log, also called a common log or ship log, is a navigational instrument mariners used to estimate a vessel's speed through the water, made of a wooden board tied to a line, the log-line, knotted at regular intervals. A sailor threw the log over the stern, and because the log itself stayed roughly still in the water as the ship moved away, counting how many knots ran out through the sailor's hands in a fixed span of time gave the ship's speed. The first printed description of the method appears in William Bourne's 1574 book A Regiment for the Sea, and the practice is the direct source of the nautical term knot for a unit of speed, since sailors were literally counting knots on a rope to gauge how fast their ship was moving.