klepsydra

Klepsydra, also spelled clepsydra, is an ancient timekeeping device that measures the passing of time by the regulated flow of water. The name derives from Greek kleptein, “to steal,” and hudor, “water,” often rendered as “water thief” or “water clock.”

Historically, water clocks appear in ancient Egypt and Mesopotamia and were later adopted and refined by the Greeks and Romans. In Hellenistic and Roman times, designers experimented with different vessel shapes, outflow orifices, and indicators to track hours. Heron of Alexandria (1st century CE) described sophisticated water clocks that used floats, gears, and scales to display the elapsed time, illustrating the concept’s integration with early engineering. Water clocks remained in use in various regions through the medieval period, especially where sundials or mechanical clocks were impractical.

Mechanism and operation typically involve a container that holds water and releases it through a small opening into a lower vessel or tank. The rate of flow, determined by the size of the aperture and stabilization of the source, sets the length of the time interval being measured. Some designs used graduated marks on the vessel or a float-driven pointer to indicate time. Variations could reset by refilling or transferring water between vessels.

Limitations include dependence on a constant flow, temperature-dependent viscosity, evaporation, and clogging, all of which affect accuracy. Today, klepsydras are studied as historical timepieces and engineering milestones, illustrating early attempts to quantify time before mechanical clocks.