Sketch of the mechanism at the a model of al-Jazari’s goblet clepsydra (c.a. 1200). The sophisticated water-powered mechanism is hidden inside.
The Wikipedia has “A water clock or clepsydra (Greek κλέπτειν kleptein, 'to steal'; ὕδωρ hydor, 'water') is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured. Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick.“