A water clock, or clepsydra, is a unique style of timepiece that measures time via the regulated flow of liquid (usually water, hence the name) into or out of a vessel (called inflow and outflow, respectively). Along with the sundial and the hourglass, the water clock is among the oldest known time-measuring devices.
The oldest surviving physical evidence of a water clock dates to roughly 1400 BCE. This water clock was used in the Temple of Amen-Re during the reign of Pharaoh Amenhotep III in Ancient Egypt. However, an inscription on the tomb of Egyptian court official Amenemhet identifying him as the inventor dates back at least two more centuries.
Even older evidence suggests that water clocks were used in astronomical calculations during the Old Babylonian period (circa 2000-1600 BCE). No physical examples from this period still remain, but records written on clay tablets have survived. The Babylonians measured time using temporal hours, which meant that, as seasons changed, the length of an hour fluctuated. As such, the amount of water that had to pass through these water clocks to mark each “hour” changed, as well.
Pints of Persia
Historical records of water clocks dating to 328 BCE describe the ancient Persians in what is now Iran using them to ensure the just and exact distribution of water from local wells for irrigation purposes. Water clocks were also used to calculate the holy days of pre-Islamic religions, such as the equinoxes and solstices.
A typical Persian water clock of this vintage consisted of a large pot full of water and a bowl with a small hole in its center. The bowl would be placed on top of the water, where it would start to slowly fill with water. Once full, it would sink into the pot. It was then retrieved and emptied, and the process would be repeated as necessary. Typically, the “manager” of the water clock would tally the number of cycles by placing a small stone in an unrelated jar for each time the bowl sank.