There are two types of water clocks: inflow and outflow.
In an outflow water clock, a container is filled with water, and the water is drained slowly and evenly out of the container.
The Greeks and Romans advanced water clock design to include the inflow clepsydra with an early feedback system, gearing, and escapement mechanism, which were connected to fanciful automata and resulted in improved accuracy.
Further advances were made in Byzantium, Syria and Mesopotamia, where increasingly accurate water clocks incorporated complex segmental and epicyclic gearing, water wheels, and programmability, advances which eventually made their way to Europe.
Independently, the Chinese developed their own advanced water clocks, incorporating gears, escapement mechanisms, and water wheels, passing their ideas on to Korea and Japan.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade.
These early water clocks were calibrated with a sundial.
While never reaching a level of accuracy comparable to today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by more accurate pendulum clocks in 17th-century Europe.A water clock, also known as a clepsydra, uses a flow of water to measure time.If viscosity is neglected, the physical principle required to study such clocks is the Torricelli's law.A water clock or clepsydra (Greek κλεψύδρα from κλέπτειν kleptein, 'to steal'; ὕδωρ hydor, 'water') is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured.Water clocks, along with sundials and hourglasses, are likely to be the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick.Where and when they were first invented is not known, and given their great antiquity it may never be.