Hourglass

An hourglass, also known as a sandglass or sand timer or sand clock, is a machine for the measurement of time. It consists of two glass bulbs located one above the other which are joined by a narrow tube. One of the bulbs is usually filled with fine sand which flows through the narrow tube into the bottom bulb at a given rate. Once all the sand has run to the bottom bulb, the device is inverted in order to measure another time period.