A Brief History of Nautical Logs to A.D. 1800

Extract

The name log, to denote a device for measuring a ship's speed or the distance she has made through the water, appears to have come down to us from the Saxon log—‘a hewn trunk or branch of a tree’. Harboard amplifies this by remarking that the log is so named from the word ligen (Dutch liggen=to lie down) because it lies, as it were, immovable. It therefore seemed appropriate that the primitive contrivance for measuring a ship's speed—a wooden board attached to a line—should be called a log. The principle of this—the so-called common log to distinguish it from more sophisticated contrivances—is simple: a floating object cast from a ship tends to remain stationary after entering the sea. The line attached to the log-ship is allowed to render freely as the vessel sails away after the log is hove from the vessel's stern into the sea. A specified length of line which runs out in a measured interval of time or, what became more common practice, a measured length of line which runs out in a specified interval of time, is equivalent—at least approximately—to the distance travelled by the vessel in that interval.