Glass clocks

In seafaring, the glass clock indicates the waking times by means of acoustic signals.

The term Glasen for the time calculation on seagoing ships comes from the glass hourglasses that were used to determine the time on board before the invention of the chronometer. These were a half-hour glass and a four-hour glass. The passing and turning of the half-hour glass was signaled by striking the ship's bell, with the number of strikes equal to the frequency of turning. This counting method went up to eight, i.e. corresponding to one watch, and then began again with the turning of the four-hour glass. The corresponding times are “eight glasses” (beginning of waking), “one glassing”, “two glasses”, – , “seven glasses” and again “eight glasses” (end of waking).

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