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More than 5000 entries on the history, culture and life of Britain (published in 1993 by Macmillan, now out of print)
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knot
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The unit of speed for a ship at sea. One knot is a speed of one nautical mile per hour (1.85kph/1.15mph). The name relates to a system introduced on British ships some time before 1633, the earliest recorded use of the term. It had long been the custom to assess a boat's speed by the amount of line pulled overboard by a floating object in a given time. The innovation first mentioned in 1633 was the tying of knots in the line at specific intervals to give a quicker and more accurate measurement. In the 20C the knot, already the international unit of speed at sea, was adopted also for aircraft.
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