The speed of the ship is ascertained by means of the log-line, which is a cord knotted at equal distances of 51 feet; 120 of these lengths are equal to a geographical mile. At one end of this line, the “log,” which is a piece of flat, light wood generally triangular in shape, weighted along one edge, is attached, much in the same way as a boy fastens his kite to the spring, so that it floats vertically,
with its flat surface presented to the ship. When thrown overboard, with the line allowed to pass over the stern freely, the log meets with so little resistance that theoretically it remains stationary. The number of knots in the cord being equal to the number of half minutes in an hour, it follows that as many “knots” of the line as pass over the stern of the vessel every half minute, so many geographical miles or knots are being “made” by the ship in an hour.
(Taken from an old training manual, as quoted in The Ocean Almanac by Robert Hendrickson, Doubleday.)