[2] This nomenclature dates back to the days of sail, when sailors attached a piece of lumber (a "log" of wood) to a rope knotted at regular intervals off the stern of a ship.
Sailors counted the number of knots that passed through their hands in a given time to determine the ship's speed.
[1] The log-line attaches to the board with a bridle of three lines that connect to the vertex and to the two ends of the quadrant's arc.
A navigator who needed to know the speed of the vessel had a sailor drop the log over the ship's stern.
[3] A sailor threw a floating object overboard and used a sandglass to measure the time it took to pass between two points on deck.
With the introduction of the nautical mile as a standard unit of measure at sea in the 15th century, they began to mark the line at equal intervals proportional to the nautical mile and to the time interval used for measurement.
The sailor had to incorporate a number of considerations: Frequent measurements helped mitigate some of these inaccuracies by averaging out individual errors, and experienced navigators could determine their speed through the water with a fair degree of accuracy.
Modern navigation tools, such as GPS, report speed over ground, and in general do not give the same result as a log when currents are present.