Sand clocks, or sand glasses, were used as timers for the chip log line, which was trolled aft of the vessel and timed to determine the vessel’s speed for the navigator. In use, a line knotted at regular intervals is attached to the ship log. After deploying the log, a sailor counted the number of knots passing through his hand over a given length of time as measured by a sand clock. Vessels of the period carried sand clocks timed to various increments such as 28 and 48 seconds, as well as minute glasses. These are two of five sand clocks so far found on the shipwreck. Image courtesy NOAA Okeanos Explorer Program.