sand-glass

sand-glass, an instrument for measuring the passage of time on board ship before suitable chronometers had been developed. Basically the sand-glass comprises two vacuum glass globes connected by a narrow neck, like an egg-timer. The sand ran from the top globe into the bottom one through the neck, emptying itself in a given interval of time. When it had all run through, the sand-glass was reversed and the process repeated. Four sizes of sand-glasses were manufactured for maritime use: half-minute, half-hour, hour, and four-hour glasses, but the two most in use were the half-minute and half-hour. The former, later changed to a 28-second glass, was used to estimate the speed of the ship with the aid of the ship's log. The latter was the main means of measuring the passage of time on board ship, the ship's bell being rung every time a half-hour glass emptied itself. A watch on board amounted to eight half-hour glasses, or four hours.