A half hour sand-glass. . .
Produced in: The Netherlands ca. 1790
Size: height 195mm.
In metal frame with five pillars with hexagonal decorated top and base and two glass ampoules joined by was and tread..
The element common to celestial position-finding, the calculation of speed for dead reckoning and even everyday life at sea is the time: hence ways of determining and keeping the time have been one of the major pre-occupations of seamen since the very beginning of scientific navigation. Counting the numbers of days or moons provides one elementary basis of measurement.
However, the hours of the day had to be divided and marked more precisely. Sand-glasses are simply clepsydras filled with sand. They are very early instruments for measuring the elapsed time and were to be found on board ship from the 14th century onwards. The two conical phials are separated by a disc perforated with a calibrated hole and bound together with a network of threads.
The very fine sand with which sand-glasses were filled was generally finely crushed eggshell or black powdered marble. On board ship the half-hour glass was turned on the dot of moon and than at each half hour until the next day. It soon became common to strike the watch bell whenever the glass was turned.<br />Sand-glass is running for 12 minutes.
Tread around glass ampoules probably replaced around 1920.
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