The "Log" was first described as being a piece of shaped wood with a weight attached at one end in order that it should float upright in the water. The log was attached by three short pieces of rope to the so-called "log line" and the log was cast astern of the vessel. The log line was knotted at regular intervals and the number of knots played out during a period of half a minute, again timed by a log glass, was counted. This operation provides the origin of the expression for describing the speed of a ship in "knots".
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