The sand clock, or hourglass, is an instrument of medieval origin. It consists of two superposed glass bulbs connected by a small opening. One bulb is partly filled with fine sand, marble powder, eggshell powder, or similar material. Depending on the quantity of material and the neck width, the instrument indicates preset time intervals—5, 10, 15 or 30 minutes—when the top bulb empties out. For its convenience, the hourglass was used on ships to mark the time every half-hour. After eight turns—equivalent to four hours—the watch was changed.