Early ships calculated speed by using a log. It was a log of wood, weighted at one end, with the other end fixed to long piece of rope. The log, thrown over the stern of the ship, floated, and the rope was let out on as the ships sailed on. The speed of the ship could be calculated by seeing how much rope had been let out in a given time.
In later years, knots were tied at equal distances along the rope. A sailor counted how many knots passed through his hands in a certain time. This gave the speed of the ship. Sailors came to use the word knots to mean the speed of the ship.
Today, a knot has come to mean one nautical or sea mile per hour. A nautical mile equals 1.852km, a little more than a land mile. Imagine that a ship is sailing at a speed of 15 knots. This means that it is sailing at a speed of 15 nautical miles an hour or 28 kilometers an hour.
Logs are still used to show how fast a ship is travelling. But today, the logs used are special metals with flat blades around them. As the ship sails through the water, the metal rod rotates ad twists the rope round and round. The spinning rope works a device back on the ship that shows the actual speed.
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