water clock

from WordNet (r) 3.0 (2006)

water clock
    n 1: clock that measures time by the escape of water [syn:
         {water clock}, {clepsydra}, {water glass}]

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]