Water Clock

A clepsydra is an ancient device for measuring time by the gradual flow of water. The Greeks named such vessels clepsydras, or water thieves, a term drawn from the Greek kleptein, to steal, and hydor, water. Water clocks were among the earliest timekeepers that did not depend on observing celestial bodies.

The Greek word klepsydra is formed from the terms for thief and water, and the device is accordingly rendered as a thief of water.

A water clock depends on a force that acts constantly and continuously over long periods, and in most designs that force is gravity. The simplest form is the outflow type: a vessel discharges water slowly and steadily, and lines marked on the interior register the passage of time. In the inflow type the upper vessel is kept constantly filled so that the level, and therefore the drain rate, stays constant, while a float in the lower vessel can drive a geared mechanism that displays the time on a dial. More sophisticated clocks poured water in and raised a floating drum that turned a cog, whose regulated movement could be measured.

The principal drawback of the device is that the flow rate decreases as the depth of water in the vessel decreases, which makes the remaining level an unreliable indicator of time. Because the rate of flow is very difficult to control accurately, a water clock could never achieve excellent accuracy. The rate of drainage also varies with temperature and humidity, and in cold climates the water could freeze, which limited the utility of water clocks in northern Europe.

Water clocks served as timekeeping instruments in ancient Egypt from at least the New Kingdom until the end of pharaonic history. The invention and establishment of the water clock in Egypt seems at first glance to be one of the best-documented developments in the history of ancient technology. Three-dimensional scanning of the fragmentary Egyptian clocks is indispensable for investigating their development and the knowledge of fluid dynamics that existed around 1500 BCE.

A fragment of a basalt clepsydra in the British Museum carries the registration number EA938 and shows on its exterior surface Philip Arrhidaeus offering to the deities Min, Sekhmet and an unidentified goddess. A basalt fragment of an Egyptian clepsydra shown by the Institute for the Study of the Ancient World measures 34 cm high and 30 cm wide and dates to about 320 BCE. The Karnak clock inscribed for Amenhotep III, the best-preserved Egyptian example, is made of calcite (alabaster) or travertine and has an external height of 34.5 cm, an upper diameter of 46 cm and a lower diameter of 26 cm.

The Greeks named these vessels clepsydras, or water thieves, and began using them about 325 BCE; they were stone vessels with sloping sides letting water drip at a nearly constant rate from a small hole near the bottom. Other clepsydras were cylindrical or bowl-shaped containers designed to fill slowly with water entering at a constant rate and marked inside to measure the passage of hours. Simple outflow klepsydras were known on a small scale as early as the 14th century BCE, and Vitruvius does not credit Ctesibius with their invention.

Ctesibius of Alexandria was the first to investigate the properties of the wind and of pneumatic power, applying his discovery of compressed air to hydraulic machines, to automata driven by enclosed water, and above all to water dials. In his water dial water flows evenly through a small perforation and raises an inverted bowl called the phellos or tympanum, connected to a rule and toothed drum wheels producing measured motion; hours are marked on columns or pillars, and a figure holding a wand rises from below to point at them through the day. The flow was regulated by a pair of fitted cones, one convex and one concave, which a rod drew together or apart to increase or diminish the stream. The first sophisticated water clocks are attributed to Ctesibius around 280 BCE, while Archimedes is credited with improving their accuracy.

The anaphoric dial turns a wheel painted with the zodiac, driven by a pliable brass chain carrying a phellos raised by water at one end and a counterpoise of sand at the other; as the phellos rises, the falling sand counterpoise turns the axis and the zodiac wheel, adjusting the hours to the seasons. At the winter solstice the tongue of the dial bears the full weight of the running water, so it passes with great velocity and the hours are shortened. The Romans invented a clepsydra made of a cylinder into which water dripped from a reservoir, with a float giving readings against a scale on the cylinder wall; siphons made such designs more accurate. More elaborate mechanized water clocks were developed between 100 BCE and 500 CE by Greek and Roman horologists and astronomers; some 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.

In the Athenian law courts a water clock known as the klepsydra fixed how long a single speech could last, about six minutes. The klepsydra was commonly used in the Athenian law courts from the end of the fifth century until at least the end of the fourth century BCE.

In suits involving sums from one thousand to five thousand drachmas the rebuttal speech received two measures of water, while the main argument was allowed seven.

Large public water clocks often measured a whole day, such as the clock in the 4th-century BCE agora of Athens, which held 1000 litres of water. The Agora water clock was built at the end of the 4th century BCE as a monumental outflow klepsydra, best understood by comparison with the better-preserved example at Oropos.

The Tower of the Winds in Athens, built by Andronicus in the 2nd century BCE, contained a large water clock in addition to nine sundials on its outer walls.

The stairstep water clocks developed in China around 200 BCE consisted of identical bowls arranged in a series of steps, an arrangement that maintained a nearly constant flow rate and a more uniform measure of time. Under the Han dynasty Chinese rulers sought advances in science and technology, and the Han used water clocks and sundials alongside paper and the seismograph.

Su Song's tower housed an armillary sphere, a celestial globe and an ensemble of wheels with figurines that gave aural and visual indications of the time of day, all driven by a water-powered large gear wheel.

Al-Jazari is remembered above all for his automaton designs, among them water-operated automatons, and for the Book of the Knowledge of Ingenious Mechanical Devices. A folio of that book showing the Elephant Clock is held in the Metropolitan Museum of Art, which dates it to the year 1315 CE.

In the elephant clock, every half hour the bird on the dome whistled, the man below dropped a ball into the dragon's mouth, and the driver hit the elephant with his goad.

In al-Jazari's Castle Clock a ball drops from the falcons' beaks into the vases, small doors open and close, a moon moves across the face, and musicians play every hour.

Medieval doctors used clepsydras to determine their patients' pulse rates, and the devices remained in service as late as the 16th century, when astronomers such as Tycho Brahe and Galileo employed them.

Working replicas of the giant Elephant Water Clock and the Castle Clock of al-Jazari are displayed in the technology gallery of the Museum of Science and Technology in Islam at KAUST. The Science Museum's scale model of Su Song's water-balance escapement was made by the museum workshops in 1965 and is listed among the museum's time measurement objects.

• about 1550–1069 BCE
  Earliest preserved Egyptian water clocks
  The earliest preserved water clocks in Egypt date to the New Kingdom.^{17Measuring time: hours · Digital Egypt for Universities, University College London ↗}
• 14th century BCE
  Compensating vessel design
  Egyptian specimens used a vessel with a smaller cross-sectional area at the bottom, which partly compensated for the decreased flow of water.^{1Clepsydra · Encyclopaedia Britannica ↗}
• end of the 4th century BCE
  Agora water clock
  The Agora water clock was built as a monumental outflow klepsydra, best understood by comparison with the better-preserved example at Oropos.^{11Notes on a Water Clock in the Athenian Agora · American School of Classical Studies at Athens (Hesperia) ↗}