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In ancient times the sun was used to measure time during the day, but sundials weren't much help after dark, so peoples around the world invented clocks that used dripping water to mark the hours. In one kind of water clock, possibly invented by the Chaldeans, a vessel was filled with water that was allowed to escape through a hole. The vessel's inside was marked with graduated lines, and the time was read by measuring the level of the remaining water. The ancient Greeks called their water clocks "klepsydra" ("water thief"), which comes from "kleptein" ("to steal") and "hydōr" ("water"). English speakers stole "clepsydra" from the Greeks in the 16th century. Actual water clocks have become increasingly rare and we now use the word primarily in historical references.

: an instrument designed to measure time by the fall or flow of a quantity of water : water clock

Examples

The ancient Greeks were known to time political speeches with a clepsydra; when the water was gone, the oration was over.

"One of the earliest mechanisms to measure time ... was a clepsydra or water clock ... in which a vessel either filled or emptied at some slow, regular rate...." - From an article by David W. Ball in Spectroscopy, December 2006