The Clepsydra - the town clock
The large central tank would be filled, and as the water drained away through the outlet hole at the bottom, its falling level indicated the passing hours. The full tank took about 17 hours to empty (more than enough for the longest day: the Greeks divided daylight into twelve equal hours: a winter hour would be 45 minutes and a summer one 75). It was built between 350 and 300 B.C. and held about 1000 litres.