clepsydra (n.)

"ancient Greek water-clock," 1640s, from Latinized form of Greek klepsydra, from stem of kleptein "to steal, to hide" (see kleptomania) + hydor "water," from suffixed form of PIE root *wed- (1) "water; wet."

A device for measuring time by the amount of water discharged from a terra cotta vessel into another through a small hole (it works on the same principle as the hourglass). It was used in classical Athens to measure smaller segments of time (probably 30 minutes or less) than the sundial (six minutes seems to have been a typical time to drain the vessel) and especially to regulate the allowed time for speeches in tribunals.