clepsidras

Clepsidras, or water clocks, are devices that measure intervals of time by the regulated flow or level of water. The term comes from the Greek klepsydra, formed from kleptein, “to steal,” and hudor, “water,” referring to water that slowly leaves a vessel.

Most common is the outflow clepsydra: a container releases water through a narrow orifice into a lower basin. As the upper vessel empties, marked scales or calibrated lines indicate elapsed time. In inflow designs, water is supplied at a controlled rate to maintain a constant level, or a siphon is used to regulate flow. Some early forms employed a float and scale to translate water movement into readable time marks.

Historically, clepsidras appear in ancient civilizations such as Egypt and Mesopotamia and were further developed in the Greek and Hellenistic worlds. They were used to time public activities such as speeches and legal proceedings and to divide daytime into hours that could vary with season. The technology spread to the Islamic world and medieval Europe, where improvements in flow control and vessel design increased usability. Clepsydras complemented sundials and other timekeeping methods, especially when sunlight or clear skies were unavailable.

Decline and legacy: with the rise of mechanical clocks from the late medieval and early modern periods, the practical use of clepsydras diminished. Nevertheless, they contributed to the history of timekeeping and provided early demonstrations of fluid dynamics and measurement principles.