A water clock, especially as used in the ancient world.
1953, John Wyndham, The Kraken Wakes, page 124
"The dull, unflavoured drops from life's clepsydra".
2006, Thomas Pynchon, Against the Day, Vintage 2007, p. 856:
They sat among the choiring clepsydras of the evening garden, time elapsing in a dozen ways, allowing their cigars to go out, keeping a companionable silence.