Just to add to Webster 1913
's fine depiction of a water clock........
Water clocks were one of the first methods of measuring time that didn't depend on the observation of celestial bodies. Some of the oldest were found in tombs of the ancient civilizations buried around 1500 B.C. As Webster so finely notes, they were called clepsydra, that when translated means " water thief".
The ancient Greeks began using them around 325 B.C.. They were made of stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole cut in the bottom. Other clepsydra's that have been found were cylindrical or bowl shaped. These were also designed have water flow at a constant rate. Markings cut on the inside surface measured the passage of "hours" as the water level got higher. Another later version consisted of a metal bowl with a hole in the bottom. When placed in another container of water, the bowl would fill and sink in a certain time frame. Some of these were still in use in North Africa in this century.
More elaborate and mechanized water clocks were developed between 100 B.C and 500 A.D. by Greek and Roman horologists and astronomers. The added complexity was aimed at making the flow more constant by regulating the pressure of the water and providing fancier displays of the passage of time. Some water clocks rang bells and gongs, others opened doors and windows to show figures of people or moved pointers, dials, and astrological models of the universe.
An ancient Greek astrologer, Andronikos, oversaw the construction of the Tower of the Winds in Athens around the 1st century B.C. The structure was an octogon that showed scholars and marketplace shoppers both sundials and mechanical hour indicators. It featured a 24-hour mechanized clepsydra and indicators for the eight winds from which it the tower got its name, It also displayed the seasons of the year as well as astrological dates and periods.
In the Far East, mechanized astronomical/astrological clock making developed from 200 - 1300 A.D. Third century Chinese clepsydras drove various mechanisms that illustrated some the astological phenomena. One of the more elaborate structures was a 30 foot clock tower that incorporated a water driven escapement. It contained a bronze power driven armillary sphere for observation, an automatically rotating globe, and five front panels with doors that permitted the viewing of changing mannikins which rang gongs and held tablets indicating the hour or other special times of the day.
Since, at the time, the rate of the flow of water was hard to control accuarately, water clocks were replaced with other methods for measuring time.