Water clock : An instrument or machine serving to measure time by the fall, or flow, of a certain quantity of water; a clepsydra.

Example:


Taxonomy
water clocks, Clocks, water clock, clepsydra, mechanism, History, timekeeping, Korean, Cambridge University Press, China, John, outflow, Astronomy, Science, inflow, clepsydrae, Babylonian, Islamic.

Defination from the web:
water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. ... The Greeks and Romans further advanced water clock design to include the inflow clepsydra with the earliest feedback system, gearing, and an ...