A water clock or clepsydra is a device for measuring time by letting water regularly flow out of a container usually by a tiny aperture. Since the rate of flow of water is very difficult to control precisely, water clocks could never achieve high accuracy.

Water clocks were among the earliest chronometers that did not depend on the observation of celestial bodies. One of the oldest was found in the tomb of the Egyptian pharaoh Amenhotep I, buried around 1500 BC. Later named clepsydras ("water thieves") by the Greeks, who began using them about 325 BC, these were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. Other clepsydras were cylindrical or bowl-shaped containers designed to slowly fill with water entering at a constant rate. Markings on the inside surfaces measured the passage of "hours" as the water level reached them. These clocks were used to determine hours at night, but may have been used in daylight as well. Another version consisted of a metal bowl with a hole in the bottom; when placed in a container of water the bowl would fill and sink in a certain time. These were still in use in northern Africa in the 20th century.

More elaborate and impressive mechanized water clocks were developed between 100 BC and AD 500 by Greek and Roman horologists and astronomers. The added complexity was aimed at making the flow more constant by regulating the pressure, and at providing fancier displays of the passage of time. Some water clocks rang bells and gongs; others opened doors and windows to show little figures of people, or moved pointers, dials, and astrological models of the universe.

A Macedonian astronomer, Andronikos , supervised the construction of his Horologion , known today as the Tower of the Winds, in the Athens marketplace in the first half of the 1st century BC. This octagonal structure showed scholars and shoppers both sundials and mechanical hour indicators. It featured a 24-hour mechanized clepsydra and indicators for the eight winds from which the tower got its name, and it displayed the seasons of the year and astrological dates and periods. The Romans also developed mechanized clepsydras, though their complexity accomplished little improvement over simpler methods for determining the passage of time.

In eastern Asia, mechanized astronomical and astrological clock making developed from AD 200 to 1300. 3rd century Chinese clepsydras drove various mechanisms that illustrated astronomical phenomena.

Su Song's water clock tower

One of the most elaborate clock towers was built by Su Sung (蘇頌) and his associates in 1088. Su Sung's mechanism incorporated a water-driven escapement invented about 725. The Su Sung clock tower, over 30 feet tall, possessed a bronze power-driven armillary sphere for observations, an automatically rotating celestial globe, and five front panels with doors that permitted the viewing of changing manikins which rang bells or gongs, and held tablets indicating the hour or other special times of the day.

See also

• hourglass