This object is a clepsydra or water clock. The Greek word klepsydra means water thief, which describes the draining away of the water as time passed. Water clocks have been used for more than 3,000 years, initially in Egypt and later in Greece.

Unlike sundials, water clocks could be used when it was cloudy or at night, and were often used to measure small units of time. The simplest was a sinking bowl, which provided a standard, repeatable measurement of time. A small metal bowl with a hole in the bottom was placed into a larger container of water. The metal bowl would gradually fill with water until it sank - thus denoting the passage of a unit of time. This simple and effective method was used for timing the hiring of goods, or to tell speakers their time was up.

The water clock shown here is a stone container, with a hole in the bottom. As the water level changes a scale on the inside indicates the time. Obviously the volume of water affects the pressure, and hence the speed at which the water leaves the container. To compensate for this the sides of the stone vessel are sloped enabling the scale to be uniform.

Water clocks became increasingly sophisticated in the first half of the first century. An impressive example was built by a Chinese horologist, Su Sung in 1088 and made use of a water-driven escapement. Water clocks are easy to make, and a great topic for primary science covering floating, sinking, water pressure, scales and how they are interpreted, estimating, fair tests and repeatability of results.

Sophie Duncan is a physicist and programme manager with Science Year www.scienceyear.com