log + book

In the 1500s, sailors measured a ship's speed by throwing a wooden log — later a triangular piece of wood called a chip log — overboard from the stern and timing how quickly the ship moved away from it. The log was attached to a rope knotted at regular intervals. A sailor counted how many knots paid out in a fixed time (measured by a sandglass), and this gave the ship's speed in knots — a unit of measurement that persists in aviation and meteorology today. The results were recorded in the log book.

The log book quickly expanded beyond speed measurements. Captains began recording course, weather, sightings, disciplinary actions, births, deaths, and daily events. By the 1600s, the logbook was the official legal record of a voyage. Admiralty courts used logbooks as evidence in disputes over prizes, mutinies, and insurance claims. A captain's logbook was a legal document, and falsifying it was a court-martial offense.

The word log separated from its wooden origin by the 1700s. To log something meant to record it — any kind of record, not just maritime speed. The logbook became the log, and the log became a verb. Computer scientists in the twentieth century adopted the term wholesale: system logs, event logs, login (logging in to a system), logout. The piece of wood thrown from a Tudor galleon's stern became the root of modern computing vocabulary.

The physical chip log was made obsolete by the patent log in the nineteenth century and by electronic speed measurement in the twentieth. No ship has thrown a piece of wood overboard for speed measurement in over a hundred years. But the logbook endures, and the knot endures, and the verb to log endures. A piece of sixteenth-century maritime technology left a deeper mark on language than on navigation.