Starting in the 14th century, and until the appearance of marine chronometers, hourglasses were the primary means used for marking the passage of time at sea. Although it could not be used to tell the time directly, it was used to mark and measure fractions of time. Generally, a ship's pilot and helmsman used several calibrated hourglasses for measuring various lengths of time, from half-minutes to hours. At sea, the time was regulated each day at noon, when the sun was at its highest point. A sundial was used for this purpose, and the hourglasses were then used to measure the passage of time during the day. The 30-minute hourglass was used to measure the length of watches. Each time the hourglass was turned, the helmsman would sound the ship's bell – one ring, or "bell", after the first half-hour, two after the second, and so on, until "eight bells" marked the end of a four-hour watch. Hourglasses were also used to measure a ship's speed. A wooden board (or log) attached to a line rope, was thrown off the stern of the vessel. The line had a number of knots tied in it at regular intervals. The speed of the ship was equal to the number of knots that passed over the stern in a given period of time, as measured by the hourglass. This is why, even today, the speed of a ship is given in "knots".
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