Sandglasses were used on board ships from the 14th century onwards to tell the time, but the hourglass itself has probably existed since antiquity. At first, there were only half-hour watches, the so-called watch hourglasses, which were used on ships to measure watch times. The coxswain or one of his mates was responsible for turning the watch sandglass, thus providing the time that had to be entered in the ship's logbook; the measurement of the watch began when the sun reached its highest point - the zenith - at noon, which was also the essential time reference point for navigation.
Watch-Sandglass aboard the Nonsuch a ships replica from the 17th century in the Museum of Manitoba (x)
At this point the ship's bell was struck eight times; after the first glass was emptied (after half an hour), the ship's bell was struck once, after another glass twice, and so on until four hours after noon, when it was struck eight times again.Then a new watch began, and the procedure was repeated, to check the half-hour glasses for correctness, hour or four-hour glasses were used to run next to it. From the 16th century onwards, half-minute glasses were also used to measure the speed of the ship in knots.
If one now takes the compass and the hourglass, as well as recording the speed measured with the log in the logbook, it enables the sailor to show his position on a map. Multiplying the speed by the time (measured with the sandglass) that the course was followed gives the distance travelled, with the compass indicating the direction of travel. This simple method is called dead reckoning (an abbreviation for estimated calculation).
As clocks became more and more mechanical and precise, the sandglasses became less and less popular from the mid of the 19th century onwards, until they were no longer used at the end of the 19th century.
