A small hour glass with sand consists of two glass spheres and a narrow connecting pipe, allowing sand to leak from the top to the bottom at a constant rate. Once the top glass is empty, turn it upside down and start the clock again. The sand watch is especially useful for ships at sea than a water clock because its timing is not affected by waves and its granular fillings (sand, powdered eggshells, and marble) are less susceptible to temperature changes. In fact, the sand clock hourglass sandglass was used to measure time, speed and distance on voyages until the 18th century.