Leonardo da Vinci designed a water clock that used flowing water to measure the passage of time.
Water clocks, also called clepsydrae, measure time through the movement or accumulation of water. Leonardo’s studies investigated ways to regulate that flow and connect it to a visible indication of elapsed time. His interest in hydraulics made timekeeping a natural subject for mechanical experimentation.
Water clocks were much older than Leonardo. Ancient civilizations, including Egypt and Greece, used them, so Leonardo was refining and analyzing an established class of device rather than inventing the basic idea from nothing.
The accuracy of a water clock depends on maintaining a predictable flow. Changes in water level, pressure, temperature, or leakage can affect the result. For that reason, later clocks increasingly relied on mechanical escapements and pendulums.