An hourglass, also known as a sanglass, sand timer or sand clock, is a device for the measurement of time. It consists of two glass bulbs placed one above the other which are connected 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 can be inverted in order to measure time again. The hourglass is named for the most frequently used sandglass, where the sands have a running time of one hour.