A means of measuring time on board ship before the development of reliable clocks for ships. It consists of two vacuum globes connected by a narrow neck, allowing sand to run from the top globe into the bottom one in a given period of time. Maritime sand-glasses were supplied in four sizes: half-minute, half-hour, hour, and four-hour. The half-minute glass was used to estimate the speed of the ship in conjunction with the log, by measuring the amount of line that would run out from the ship as she went through the water during the half-minute glass period. To announce time, the ship's bell was rung every time a half-hour glass emptied itself.