Clepsydra

Clepsydra, a compound Greek word from kleptein (to hide), and hudor (water), denoting a water-clock. Originally the instrument was simply a vessel containing water which was allowed to escape from a small orifice at a uniform rate, and whose sinking marked in a uniform manner the passage of time. An improvement on this provided a float which either marked the time by its sinking, or by compensatory action caused a balancing weight to rise and mark the time upon a graduated scale on the outside of the clock. Later the use of a dial and needle was introduced. Athens possessed an intermittent fountain somewhat of the nature of a water-clock.