What Is a Water Clock?
A water clock, or clepsydra (Greek for “water thief”), measures time by the steady flow of water. It is one of the oldest timekeeping devices, used for thousands of years in Egypt, Babylon, Greece, Rome, China, India and the Islamic world. Unlike a sundial, it works at night and indoors.
How Does a Water Clock Work?
There are two basic designs, and the 3D model above combines them:
- Outflow water clocks are a vessel with a small hole near the bottom. As water drains out, the level drops past marks on the inside wall. The oldest known example, from the Temple of Karnak in Egypt around 1400 BC, has sloping sides. That is not decoration: water flows out faster when the vessel is full, so a wider top makes the level fall at a more even rate.
- Inflow water clocks collect the dripping water in a second vessel. A float rises with the water and moves a pointer along a scale. Around 270 BC, the Greek engineer Ctesibius of Alexandria improved this design by keeping the upper tank at a constant level, so the drip rate stayed steady, and linked the float to gears and pointers.
Later water clocks grew remarkably sophisticated. In 1088 the Chinese polymath Su Song built a giant water-driven astronomical clock tower that turned an armillary sphere and rang bells and gongs.
Using the Water Clock as a Timer
Set the minutes and seconds, or pick a preset, and press Start. The scale on the lower vessel is marked for your chosen duration, and a chime sounds when the float reaches the top. Drag the scene to look around. It makes a calm visual timer for classrooms, reading time or meditation, much like the sand timer and candle timer.
Make Your Own Water Clock
A simple water clock is a classic science project. Poke a small hole in the bottom of a plastic cup, hang it over a clear jar, and fill the cup with water. Use a stopwatch to mark the water level in the jar every minute. You will notice the marks get closer together as the cup empties and the flow slows, the same problem ancient engineers solved with sloped sides and constant-level tanks.