Greece: Two ancient Greek outflow water clocks (clepsydra) from the 5th century BCE, Ancient Agora Museum, Athens. Photograph by Marsyas (CC BY-SA 2.5)

A water clock or clepsydra is any timepiece by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount is then measured.

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Marsyas (CC BY-SA 2.5)

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Pictures From History