资料来源 : Webster's Revised Unabridged Dictionary (1913)

Water clock \Wa"ter clock`\
   An instrument or machine serving to measure time by the fall,
   or flow, of a certain quantity of water; a clepsydra.

资料来源 : WordNet®

water clock
     n : clock that measures time by the escape of water [syn: {clepsydra},
          {water glass}]