A water clock or clepsydra (Greek κλέπτειν kleptein, 'to steal'; ὕδωρ hudor, '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. Where and when they were first invented is not known, and given their great antiquity it may never be. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, claim that water clocks appeared in China as early as 4000 BC. 
The Greeks and Romans further advanced water clock design to include the inflow clepsydra with an early feedback system, gearing, and escapement mechanism, which were connected to fanciful automata and resulted in improved accuracy. Further advances were made in Byzantium, Syria and Mesopotamia, where increasingly accurate water clocks incorporated complex segmental and epicyclic gearing, water wheels, and programmability, advances which eventually made their way to Europe. Independently, the Chinese developed their own advanced water clocks, incorporating gears, escapement mechanisms, and water wheels, passing their ideas on to Korea and Japan.
Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. These early water clocks were calibrated with a sundial. While never reaching a level of accuracy comparable to today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by more accurate pendulum clocks in 17th century Europe.
Regional development 
According to Callisthenes, the Persians were using water clocks in 328 BCE to ensure a just and exact distribution of water from qanats to their shareholders for agricultural irrigation. The use of water clocks in Iran, especially in Zeebad, dates back to 500BCE. Later they were also used to determine the exact holy days of pre-Islamic religions, such as the Nowruz, Chelah, or Yaldā - the shortest, longest, and equal-length days and nights of the years. The water clocks used in Iran were one of the most practical ancient tools for timing the yearly calendar. Water clocks, or Fenjaan, in Persia reached a level of accuracy comparable to today's standards of timekeeping. The fenjaan was the most accurate and commonly used timekeeping device for calculating the amount or the time that a farmer must take water from a qanat or well for irrigation, until it was replaced by more accurate current clock. Persian water clocks were a practical and useful tool for the qanat's shareholders to calculate the length of time they could divert water to their farm. The qanat was the only water source for agriculture and irrigation so a just and fair water distribution was very important. Therefore a very fair and clever old person was elected to be the manager of the water clock(MirAab), and at least two full-time managers were needed to control and observe the number of fenjaans and announce the exact time during the days and nights.
The fenjaan consisted of a large pot full of water and a bowl with a small hole in the center. When the bowl became full of water, it would sink into the pot, and the manager would empty the bowl and again put it on the top of the water in the pot. He would record the number of times the bowl sank by putting small stones into a jar.
The place where the clock was situated, and its managers, were collectively known as khaneh fenjaan. Usually this would be the top floor of a public-house, with west- and east-facing windows to show the time of sunset and sunrise. There was also another time-keeping tool named a staryab or astrolabe, but it was mostly used for superstitious beliefs and was not practical for use as a farmers' calendar. The Zeebad Gonabad water clock was in use until 1965 when it was substituted by modern clocks.
The oldest water clock of which there is physical evidence dates to c. 1417-1379 BCE, during the reign of Amenhotep III where it was used in the Temple of Amen-Re at Karnak. The oldest documentation of the water clock is the tomb inscription of the 16th century BCE Egyptian court official Amenemhet, which identifies him as its inventor. These simple water clocks, which were of the outflow type, were stone vessels with sloping sides that allowed water to drip at a nearly constant rate from a small hole near the bottom. There were twelve separate columns with consistently spaced markings on the inside to measure the passage of "hours" as the water level reached them. The columns were for each of the twelve months to allow for the variations of the seasonal hours. These clocks were used by priests to determine the time at night so that the temple rites and sacrifices could be performed at the correct hour. These clocks may have been used in daylight as well.
Water clock calculations by Nabû-apla-iddina.
|Size||H:8.2 cm (3.2 in)
W:11.8 cm (4.6 in)
D:2.5 cm (0.98 in)
|Present location||Room 55, British Museum|
In Babylon, water clocks were of the outflow type and were cylindrical in shape. Use of the water clock as an aid to astronomical calculations dates back to the Old Babylonian period (c. 2000 BCE–c. 1600 BCE).
While there are no surviving water clocks from the Mesopotamian region, most evidence of their existence comes from writings on clay tablets. Two collections of tablets, for example, are the Enuma-Anu-Enlil (1600–1200 BCE) and the MUL.APIN (7th century BC). In these tablets, water clocks are used in reference to payment of the night and day watches (guards).
These clocks were unique, as they did not have an indicator such as hands (as are typically used today) or grooved notches (as were used in Egypt). Instead, these clocks measured time "by the weight of water flowing from" it. The volume was measured in capacity units called qa. The weight, mana (the Greek unit for about one pound), is the weight of water in a water clock.
It is important to note that during Babylonian times, time was measured with temporal hours. So, as seasons changed, so did the length of a day. "To define the length of a 'night watch' at the summer solstice, one had to pour two mana of water into a cylindrical clepsydra; its emptying indicated the end of the watch. One-sixth of a mana had to be added each succeeding half-month. At equinox, three mana had to be emptied in order to correspond to one watch, and four mana were emptied for each watch of the winter solstitial night."
N. Kameswara Rao suggests that pots excavated from Mohenjodaro might have been used as water clocks; they are tapered at the bottom, have a hole on the side, and are similar to the utensil used to perform abhishekam (pour holy water) on shivalingam.
Ghati or Kapala (clepsydra or water clock) is referred to in Jyotisha Vedanga, where the amount of water that measures a nadika (24 minutes) is mentioned. A more developed form of the clepsydra is described in chapter xiii, 23 of the Suryasiddhanta.
At Nalanda, a Buddhist university, four hours a day and four hours at night were measured by a water clock, which consisted of a copper bowl holding two large floats in a larger bowl filled with water. The bowl was filled with water from a small hole at its bottom; it sank when completely filled and was marked by the beating of a drum at daytime. The amount of water added varied with the seasons and this clock was operated by the students of the university.
The description of a water clock in astrologer Varahimira's Pancasiddhantika (505) adds further detail to the account given in the Suryasiddhanta. The description given by mathematician Brahmagupta in his work Brahmasphutasiddhanta matches with that given in the Suryasiddhanta. Astronomer Lallacharya describes this instrument in detail. In practice, the dimensions were determined by experiment.
In China, as well as throughout eastern Asia, water clocks were very important in the study of astronomy and astrology. The oldest reference dates the use of the water-clock in China to the 6th century BCE. From about 200 BCE onwards, the outflow clepsydra was replaced almost everywhere in China by the inflow type with an indicator-rod borne on a float.
Huan Tan (40 BCE – 30 CE), a Secretary at the Court in charge of clepsydrae, wrote that he had to compare clepsydrae with sundials because of how temperature and humidity affected their accuracy, demonstrating that the effects of evaporation, as well as of temperature on the speed at which water flows, were known at this time. In 976, Zhang Sixun addressed the problem of the water in clepsydrae freezing in cold weather by using liquid mercury instead. Again, instead of using water, the early Ming Dynasty engineer Zhan Xiyuan (c. 1360-1380) created a sand-driven wheel clock, improved upon by Zhou Shuxue (c. 1530-1558).
The use of clepsydrae to drive mechanisms illustrating astronomical phenomena began with Zhang Heng (78-139) in 117, who also employed a waterwheel. Zhang Heng was the first in China to add an extra compensating tank between the reservoir and the inflow vessel, which solved the problem of the falling pressure head in the reservoir tank. Zhang's ingenuity led to the creation by Yi Xing (683–727) and Liang Lingzan in 725 of a clock driven by a waterwheel linkwork escapement mechanism. The same mechanism would be used by Su Song (1020–1101) in 1088 to power his astronomical clock tower, as well as a chain drive. Su Song's clock tower, over 30 feet (9.1 m) 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.
Today, in Beijing's Drum Tower an outflow clepsydra is operational and displayed for tourists. It is connected to automata so that every quarter-hour a small brass statue of a man claps his cymbals.
Greco-Roman world 
In Greece, a water clock was known as a clepsydra (water thief). The Greeks considerably advanced the water clock by tackling the problem of the diminishing flow. They introduced several types of the inflow clepsydra, one of which included the earliest feedback control system. Ctesibius invented an indicator system typical for later clocks such as the dial and pointer. The Roman engineer Vitruvius described early alarm clocks, working with gongs or trumpets.
A commonly used water clock was the simple outflow clepsydra. This small earthenware vessel had a hole in its side near the base. In both Greek and Roman times, this type of clepsydra was used in courts for allocating periods of time to speakers. In important cases, when a person's life was at stake for example, it was filled. But, for more minor cases, it was only partially filled. If proceedings were interrupted for any reason, such as to examine documents, the hole in the clepsydra was stopped with wax until the speaker was able to resume his pleading.
In the 4th century BCE, the clepsydra is known to have been used as a stop-watch for imposing a time limit on clients' visits in Athenian brothels. Slightly later, in the early 3rd century BCE, the Hellenistic physician Herophilos employed a portable clepsydra on his house visits in Alexandria for measuring his patients' pulse-beats. By comparing the rate by age group with empirically obtained data sets, he was able to determine the intensity of the disorder.
Between 270 BCE and 500 CE, Hellenistic (Ctesibius, Hero of Alexandria, Archimedes) and Roman horologists and astronomers were developing more elaborate mechanized water clocks. The added complexity was aimed at regulating the flow and at providing fancier displays of the passage of time. For example, some water clocks rang bells and gongs, while others opened doors and windows to show figurines of people, or moved pointers, and dials. Some even displayed astrological models of the universe. The 3rd century BCE engineer Philo of Byzantium referred in his works to water clocks already fitted with an escapement mechanism, the earliest known of its kind.
The biggest achievement of the invention of clepsydrae during this time, however, was by Ctesibius with his incorporation of gears and a dial indicator to automatically show the time as the lengths of the days changed throughout the year, because of the temporal timekeeping used during his day.
Also, a Greek astronomer, Andronicus of Cyrrhus, supervised the construction of his Horologion, known today as the Tower of the Winds, in the Athens marketplace (or agora) in the first half of the 1st century BCE. This octagonal clocktower 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.
Islamic and Arabic world 
In the medieval Islamic world (632-1280), the use of water clocks has its roots from Archimedes during the rise of Alexandria in Egypt and continues on through Byzantium. The water clocks by Al-Jazari, however, are credited for going "well beyond anything" that had preceded them.
In al-Jazari's 1206 treatise, he describes one of his water clocks, the elephant clock. The clock recorded the passage of temporal hours, which meant that the rate of flow had to be changed daily to match the uneven length of days throughout the year. To accomplish this, the clock had two tanks, the top tank was connected to the time indicating mechanisms and the bottom was connected to the flow control regulator. Basically, at daybreak the tap was opened and water flowed from the top tank to the bottom tank via a float regulator that maintained a constant pressure in the receiving tank.
The most sophisticated water-powered astronomical clock was Al-Jazari's castle clock, considered by some to be an early example of a programmable analog computer, in 1206. It was a complex device that was about 11 feet (3.4 m) high, and had multiple functions alongside timekeeping. It included a display of the zodiac and the solar and lunar orbits, and a pointer in the shape of the crescent moon which traveled across the top of a gateway, moved by a hidden cart and causing automatic doors to open, each revealing a mannequin, every hour. It was possible to re-program the length of day and night in order to account for the changing lengths of day and night throughout the year, and it also featured five musician automata who automatically play music when moved by levers operated by a hidden camshaft attached to a water wheel. Other components of the castle clock included a main reservoir with a float, a float chamber and flow regulator, plate and valve trough, two pulleys, crescent disc displaying the zodiac, and two falcon automata dropping balls into vases.
The first water clocks to employ complex segmental and epicyclic gearing was invented earlier by the Arab engineer Ibn Khalaf al-Muradi in Islamic Iberia c. 1000. His water clocks were driven by water wheels, as was also the case for several Chinese water clocks in the 11th century. Comparable water clocks were built in Damascus and Fez. The latter (Dar al-Magana) remains until today and its mechanism has been reconstructed. The first European clock to employ these complex gears was the astronomical clock created by Giovanni de Dondi in c. 1365. Like the Chinese, Arab engineers at the time also developed an escapement mechanism which they employed in some of their water clocks. The escapement mechanism was in the form of a constant-head system, while heavy floats were used as weights.
In 1434 during the Choson (or Joseon) Dynasty, Chang Yongsil (or Jang Young Sil), Palace Guard and later Chief Court Engineer, constructed the Jagyeongnu (self-striking water clock or striking clepsydra) for King Sejong. What made the Jagyeongnu self-striking (or automatic) was the use of jack-work mechanisms, by which three wooden figures (jacks) struck objects to signal the time. This innovation no longer required the reliance of human workers, known as "rooster men", to constantly replenish it. By 554, the water clock spread from Korea to Japan. Water clocks were used and improved upon throughout Asia well into the 15th century.
Modern water clock designs 
Only a few modern water clocks exist today. In 1979, French scientist Bernard Gitton began creating his Time-Flow Clocks, which are a modern-day approach to the historical version. His unique glass tube designs can be found in over 30 locations throughout the world, including one at Europa-Center's The Clock of Flowing Time in Berlin, Centre Commercial Milenis in Guadeloupe, the Giant Water Clock at The Children's Museum of Indianapolis in Indianapolis, Indiana, and the Shopping Iguatemi in Sao Paulo, Brazil.
Gitton's design relies on gravity powering multiple siphons; for example, after the water level in the minute or hour display tubes is reached, an overflow tube starts to act as a siphon and thus empties the display tube. Actual time keeping is done by a calibrated pendulum powered by a water stream piped from the clock's reservoir. The pendulum has a carefully constructed container attached to it; this measures the water that is then poured into the display system.
There are other modern designs of water clocks, including the Royal Gorge water clock in Colorado, the Woodgrove Mall in Nanaimo, British Columbia, in the Abbotsford Airport in Abbotsford, British Columbia, and the Hornsby Water Clock in Sydney, Australia.
Temperature, water viscosity, and clock accuracy 
The rate at which a fluid passes through an orifice depends, other things being equal, on the viscosity of the fluid. Approximately, the flow rate is inversely proportional to the viscosity. The viscosity depends on the temperature. Liquids generally become less viscous as the temperature increases. The reverse is true of gases. In the case of water, the viscosity varies by a factor of about seven between zero and 100 degrees Celsius. Thus, a water clock would run about seven times faster at 100°C than at 0°C. Water is about 25 percent more viscous at 20°C than at 30°C, and a variation in temperature of one degree Celsius, in this "room temperature" range, produces a change of viscosity of about two percent. Therefore, a water clock that keeps good time at some given temperature would gain or lose about half an hour per day if it were one degree Celsius warmer or cooler. To make it keep time within one minute per day would require its temperature to be controlled within 1⁄30°C (about 1⁄17° Fahrenheit). There is no evidence that this was done in antiquity, so ancient water clocks (unlike the modern pendulum-controlled one described above) cannot have been reliably accurate by modern standards.
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- Lewis, Michael (2000). "Theoretical Hydraulics, Automata, and Water Clocks". In Wikander, Örjan. Handbook of Ancient Water Technology. Technology and Change in History 2. Leiden. pp. 343–369 (356f.). ISBN 90-04-11123-9
- Noble, J.V. & de Solla Price, D. J. "The Water clock in the Tower of the Winds." American Journal of Archaeology, 72, 1968, pp. 345–355.
- Woodcroft, Bennet (translator). "The Pneumatics of Hero of Alexandria." London, Taylor Walton and Maberly, 1851.
- Vitruvius, P., The Ten Books on Architecture. (M.H. Morgan, translator) New York: Dover Publications, Inc., 1960.
- Indian water clocks
- Achar, N. "On the Vedic origin of the ancient mathematical astronomy of India." Journal of Studies on Ancient India, vol 1, 95-108, 1998.
- Fleet, J. F., "The ancient Indian water clock." Journal of the Royal Asiatic Society, 213-230, 1915.
- Kumar, Narendra "Science in Ancient India" (2004). ISBN 81-261-2056-8.
- Pingree, D. "The Mesopotamian origin of early Indian mathematical astronomy." Journal of the History of Astronomy, vol. 4, 1-12, 1973.
- Pingree, D. "The recovery of early Greek astronomy from India." Journal for the History of Astronomy, vol 7, 109-123, 1976.
- Japanese water clocks
- Kiyoyasu, Maruyma. "Hoken shakai to gijutsu - wadokei ni shuyaku sareta hoken gijutsu." Kagakushi kenkyu, September 1954, 31:16-22.
- Korean water clocks
- Hahn, Young-Ho and Nam, Moon-Hyon. "Reconstruction of the Armillary Spheres of Mid-Chosun: The Armillary Clocks of Yi Minchol." Hanguk Kwahaksa Hakhoeji (Journal of the Korean History of Science Society)19.1 (1997): 3-19. (in Korean)
- Hahn, Young-Ho, et al. "Astronomical Clocks of Chosun Dynasty: King Sejong's Heumgyonggaknu. Kisulgwa Yoksa (Journal of the Korean Society for the History of Technology and Industry) 1.1 (2000): 99-140. (in Korean).
- Hong, Sungook "Book Review: Korean Water-Clocks: "Chagyongnu", the Striking Clepsydra, and the History of Control and Instrumentation Engineering." Technology and Culture - Volume 39, Number 3, July 1998, pp. 553-555
- Nam, Moon-Hyon. "Chagyongnu: The Automatic Striking Water clock." Korea Journal, 30.7 (1990): 9-21.
- Nam, Moon-Hyon. Korean Water Clocks: Jagyongnu, The Striking Clepsydra and The History of Control and Instrumentation Engineering. Seoul: Konkuk University Press, 1995. (in Korean)
- Nam, Moon-Hyon. On the BORUGAKGI of Kim Don—Principles and Structures of JAYEONGNU. Hanguksa Yeongu (Studies on Korean History),101 (1998): 75-114 (in Korean)
- Nam, Moon-Hyon. Jang Yeong-Shil and Jagyeongnu - Reconstruction of Time Measuring History of Choseon Period. Seoul National University Press, 2002. (in Korean)
- Nam, Moon-Hyon and Jeon San-Woon. "Timekeeping Systems of Early Choson Dynasty." Proceedings of First International Conference on Oriental Astronomy, From Guo Shoujing to King Sejong, Seoul, October 6–11, 1993, Seoul, Yonsei University Press, 1997. 305-324.
- Needham, Joseph, Major, John S., & Gwei-Djen, Lu. "Hall of Heavenly Records: Korean Astronomical Instruments and Clocks, 1380-1780." Cambridge [Cambridgeshire] ; New York : Cambridge University Press, 1986. ISBN 0-521-30368-0
- Hyeonjong Shillock (Veritable Records of King Hyeonjong), 1669
- Jungjong Shillok (Veritable Records of King Jungjong), 1536.
- Sejong Shillock (Veritable Records of King Sejong), Chapter. 65, AD 1434 and Chapter. 80, AD 1438.
- Mesopotamian water clocks
- Brown, David R., Fermor, John, & Walker, Christopher B.F., "The Water Clock in Mesopotamia." Archiv für Orientforschung, 46/47 (1999/2000)
- Chadwick, R. "The Origins of Astronomy and Astrology in Mesopotamia." Archaeoastronomy. BULL. CTR ARCH. V. 7:1-4, P. 89, 1984. KNUDSEN Bibliographic Code: 1984BuCAr...7...89C
- Fermor, John, "Timing the Sun in Egypt and Mesopotamia." Vistas in Astronomy, 41 (1997), 157-167. Elsevier Science. doi:10.1016/S0083-6656(96)00069-4.
- Walker, Christopher and Britton, John. "Astronomy and Astrology in Mesopotamia." BMP, 1996 (especially pp. 42–67)
- Present-day water clocks
- Gitton, Bernard. "Time, like an everflowing stream." Trans. Mlle. Annie Chadeyron. Ed. Anthony Randall. Horological Journal 131.12 (June 1989): 18-20.
- Taylor, Robert. "Taiwan's Biggest Cuckoo Clock?: Recreating an Astronomical Timepiece". Sinorama Magazine. 3-15-2006
- Xuan, Gao. "Principle Research and Reconstruction Experiment of the Astronomical Clock Tower in Ancient China." Proceeding of the 11th World Congress in Mechanism and machine Science. August 18–21, 2003. Tianjin, China.
- Other topics on water clocks and related material
- Goodenow, J., Orr, R., & Ross, D. "Mathematical Models of Water Clocks." Rochester Institute of Technology
- Landels, John G. "Water-Clocks and Time Measurement in Classical Antiquity." Endeavour 3(1):32-37. 1979. ISSN 0160-9327
- Mills, A.A. "Newton’s Water Clocks and the Fluid Mechanics of Clepsydrae." Notes and Records of the Royal Society of London. 37(1):35-61. 1982. ISSN 0035-9149
- Neugebauer, Otto (1969) . The Exact Sciences in Antiquity (2 ed.). Dover Publications. ISBN 978-0-486-22332-2.
- Sarma, S.R., "Setting up the Water Clock for Telling the Time of Marriage." in Studies in the History of the Exact Sciences in Honour of David Pingree, éd. Ch. Burnett, J.P. Hogendijk, K. Plofker, M. Yano, Leiden-Boston, 2004, pp. 302–330.
- Snell, Daniel. "Life in the Ancient Near East, 3100-332 B.C.E." ISBN 0-300-07666-5.
- Non-English resources
- Bilfinger, Gustav, Die babylonische Doppelstunde: Eine chronologische Untersuchung (Wildt, Stuttgart, 1888).
- Borchardt, Ludwig. 1920. "Die Altägyptische Zeitmessung." (Old Egyptian time measurement). Berlin/Leipzig.
- Daressy, G., "Deux clepsydres antiques", BIE, serie 5, 9, 1915, pages 5–16
- Ginzel, Friedrich Karl, "Die Wassermessungen der Babylonier und das Sexagesimalsystem", Klio: Beiträge zur alten Geschichte, 16 (1920), 234-241.
- Planchon, "L'Heure Par Les Clepsydres." La Nature. pp. 55–59.
- Thureau-Dangin, François, "La clepsydre chez les Babyloniens [Notes assyriologiques LXIX]", Revue d’assyriologie et d’archéologie orientale, 29 (1932), 133-136.
- Thureau-Dangin, François, "Clepsydre babylonienne et clepsydre égyptienne", Revue d’assyriologie et d’archéologie orientale, 30 (1933), 51-52.
- Thureau-Dangin, François, "Le clepsydre babylonienne", Revue d’assyriologie et d’archéologie orientale, 34 (1937), 144.
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- Rees's Universal Dictionary article on Clepsydra, 1819
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