The Hourglass and the Ship's Bell
In seafaring, the Glasenuhr indicates the time by means of acoustic signals (Glasen). The term Glasen for timekeeping on seagoing ships derives from the glass hourglasses (hourglass) that were used to determine the time on board before the invention of the chronometer.
The rules for this were quite simple: originally, an hourglass was used on board to measure time. This required half an hour to run through. One run corresponded to one glass. After two runs, i.e. after one hour, there were accordingly two glasses. This run was signaled by the ship's bell. One stroke one glass, two strokes two glasses and so on. The day was divided into 6 x 4 periods, the changing of the guard on the ships always took place after eight bells, i.e. after four hours. Six bells was therefore one hour before the changing of the guard.
By the sound of the bell, the entire crew of the ship could be informed of the time without having to look at the hour glass or the glass clock.