A “glass” is a unit of time that was once used in seafaring and corresponds to half an hour. The word comes from the hourglass that was used to measure time before clocks became common on ships.
Each time the hourglass was turned, a signal was struck on the ship’s bell to indicate the time. This was called “striking the glass” — not to be confused with “raising a glass” for a toast. The signals range from one to eight strikes; eight glasses equal four hours, which was the length of a standard watch (a work shift on board).

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