waterklokken

Waterklokken, also known as water clocks or clepsydrae, are timekeeping devices that measure hours by the controlled flow or level of water. In their simplest form a vessel releases water through a small opening, while another container or scale records the passage of time as the water level falls. More advanced designs regulate the inflow of water into a graduated vessel or use a float to drive a pointer or gear.

There are two main types: outflow and inflow. In an outflow clepsydra, water drips or flows out of a vessel at a steady rate, and marks inside or on the vessel indicate time intervals as the level drops. In an inflow clepsydra, water is added at a constant rate into a measuring container, with the increasing level or a synchronized mechanism signaling elapsed time. Some implementations employ a siphon, a float, or a mechanical indicator to improve readability and consistency.

Historically, water clocks were used in several ancient and medieval cultures, including Egypt, Mesopotamia, Greece, India, and China, and continued to be employed in parts of Europe and the Islamic world for centuries. They served for religious observances, astronomy, navigation, and daily timekeeping before the spread of more accurate mechanical clocks.

Limitations of water clocks include sensitivity to changes in temperature and pressure, evaporation, leaks, and the need for a constant water supply and careful calibration. Their accuracy is generally modest by modern standards, but they remain historically significant as early instruments for measuring time. Today, surviving examples are often found in museums or used in demonstrations to illustrate early horology.