Aboard Navy ships, bells are struck to designate the hours of being on watch. Each watch is four hours in length. One bell is struck after the first half-hour has passed, two bells after one hour has passed, three bells after an hour and a half, four bells after two hours, and so forth up to eight bells are struck at the completion of the four hours. Completing a watch with no incidents to report was "Eight bells and all is well."
The practice of using bells stems from the days of the sailing ships. Sailors couldn't afford to have their own time pieces and relied on the ship's bells to tell time. The ship's boy kept time by using a half-hour glass. Each time the sand ran out, he would turn the glass over and ring the appropriate number of bells.