A log (or chip log) is a historical sailing instrument used to measure a ship's speed through the water by throwing a weighted wooden board attached to a knotted line overboard. Sailors counted how many knots passed over the stern in a set time (using a 28-second sandglass), giving rise to the unit "knots".

Key Aspects of the Log:
Purpose: To measure speed and estimate the distance traveled through water to aid in navigation, often via dead reckoning.

Components: A "log-ship" (a flat, quarter-circle, weighted piece of wood) attached to a long line with knots tied at specific, regular intervals (traditionally 7 fathoms or 42 feet).

The Process: The log was tossed over the stern, remaining relatively stationary in the water while the ship moved away. The knots passing through a sailor’s hand within 28 seconds (measured by a "log glass") determined the speed.

Logbook: The measurements derived from the chip log were recorded on a "log board," which was then transferred to a logbook, now simply called a ship's log or logbook.

Evolution: Later versions replaced the chip with a propeller-like rotator, known as a patent log.