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{{short description|Timepiece in which time is measured by the flow of liquid into or out of a vessel}}
{{For|the individual water clock at The Children's Museum of Indianapolis|Water clock (Indianapolis)}}
[[File:AGMA Clepsydre.jpg|thumb|A display of two outflow water clocks from the Ancient Agora Museum in Athens. The top is an original from the late 5th century BC. The bottom is a reconstruction of a clay original.]]

A '''water clock''', or '''clepsydra''' ({{etymology|grc|''{{wikt-lang|grc|κλεψύδρα}}'' ({{grc-transl|κλεψύδρα}})|[[pipette]], water clock}}; {{etymology||''{{wikt-lang|grc|κλέπτω}}'' ({{grc-transl|κλέπτω}})|to steal||''{{wikt-lang|grc|ὕδωρ}}'' ({{grc-transl|hydor}})|water}}; {{lit| water thief}}), is a [[timepiece]] by which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel, and where the amount of liquid can then be measured.

Water clocks are some of the oldest time-measuring instruments.<ref>{{Harvnb|Turner|1984|p=1}}</ref> The simplest form of water clock, with a bowl-shaped outflow, existed in [[Babylon]], [[Ancient Egypt|Egypt]], and [[History of Iran#Classical antiquity|Persia]] around the 16th century BC. Other regions of the world, including [[History of India|India]] and [[History of China|China]], also provide early evidence of water clocks, but the earliest dates are less certain. Water clocks were used in [[ancient Greece]] and in [[ancient Rome]], as described by technical writers such as [[Ctesibius]] (died 222 BC) and [[Vitruvius]] (died after 15 BC).
{{TOCLIMIT|3}}

== Designs ==
[[File:1648 Eschinardi.jpg|thumb|Eschinardi's water clock (Reproduced from [[Francesco Eschinardi]], ''Appendix Ad Exodium de Tympano'')]]
A water clock uses the flow of water to measure time. If viscosity is neglected, the physical principle required to study such clocks is [[Torricelli's law]]. Two types of water clock exist: inflow and outflow. In an outflow water clock, a container is filled with water, and the water is drained slowly and evenly out of the container. This container has markings that are used to show the passage of time. As the water leaves the container, an observer can see where the water is level with the lines and tell how much time has passed. An inflow water clock works in basically the same way, except instead of flowing out of the container, the water is filling up the marked container. As the container fills, the observer can see where the water meets the lines and tell how much time has passed.
Some modern timepieces are called "water clocks" but work differently from the ancient ones. Their timekeeping is governed by a [[pendulum]], but they use water for other purposes, such as providing the power needed to drive the clock by using a [[water wheel]] or something similar, or by having water in their displays.

The [[Greeks]] and Romans advanced water clock design to include the inflow clepsydra with an early feedback system, gearing, and [[escapement]] mechanism, which were connected to fanciful [[Automaton|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 wheel]]s, and [[Program (machine)|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]].{{Citation needed|date=November 2011}}

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 a commonly used timekeeping device for millennia, until it was replaced by more accurate [[verge escapement]] mechanical clocks in Europe around 1300.<ref>{{cite journal |last1=Mills |first1=A. A. |title=Newton's Water Clocks and the Fluid Mechanics of Clepsydrae |journal=Notes and Records of the Royal Society of London |date=August 1982 |volume=37 |issue=1 |pages=35–61 |doi=10.1098/rsnr.1982.0004 |jstor=531476 |url=https://www.jstor.org/stable/531476 |access-date=18 June 2024|url-access=subscription }}</ref>

==Regional development==
===Egypt===
The oldest water clock of which there is physical evidence dates to c. 1417–1379 BC in the [[New Kingdom of Egypt]], during the reign of the pharaoh [[Amenhotep III]], where it was used in the [[Precinct of Amun-Re]] at [[Karnak]].{{sfn |Cotterell |Kamminga |1990 |pp=59–61}} The oldest documentation of the water clock is the tomb inscription of the 16th century BC Egyptian court official Amenemhet, which identifies him as its inventor.{{sfn |Cotterell |Kamminga |1990 |pp=59–61}}<ref>{{cite book | last = Berlev | first = Oleg | editor = Donadoni, Sergio | others = Trans. Bianchi, Robert ''et al.'' | title = The Egyptians | year = 1997 | publisher = The University of Chicago Press | location = Chicago | isbn = 0-226-15555-2 | page = 118 | chapter = Bureaucrats}}</ref> These simple water clocks, which were of the outflow type, were [[Stone vessels in Ancient Egypt|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 [[month]]s to allow for the variations of the seasonal hours. Priests used these clocks to determine the time at night so that the temple rites and sacrifices could be performed at the correct hour.<ref>{{Harvnb|Cotterell|Kamminga|1990|p.=59}}</ref>

===Babylon===
{{infobox artefact
|name = Clay tablet
|image =Water clock tablet.jpg
|image_caption =Water clock calculations by Nabû-apla-iddina.
|size = H:{{convert|8.2|cm|in|abbr=on}}<br />W:{{convert|11.8|cm|in|abbr=on}}<br />D:{{convert|2.5|cm|in|abbr=on}}
|writing = [[cuneiform]], [[Akkadian (language)|Akkadian]] 
|created = 600BC-500BC
|location = Room 55, [[British Museum]]
|id = {{British-Museum-db|29371|id=327283}}
}}
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 Empire]] (''c.'' 2000 – ''c.'' 1600 BC).<ref>{{cite book | last = Pingree | first = David | author-link = David Pingree | editor = Stephanie Dalley |editor-link=Stephanie Dalley| title = The Legacy of Mesopotamia | year = 1998 | publisher = Oxford University Press | location = Oxford | isbn = 0-19-814946-8 | pages = 125–126 | chapter = Legacies in Astronomy and Celestial Omens}}</ref> While there are no surviving water clocks from the Mesopotamian region, most evidence of their existence comes from writings on [[clay tablet]]s. Two collections of tablets, for example, are the ''[[Enuma Anu Enlil]]'' (1600–1200 BC) and the ''[[MUL.APIN]]'' (7th century BC).<ref>{{cite book | last = Evans | first = James | title = The History and Practice of Ancient Astronomy | year = 1998 | publisher = Oxford University Press | location = Oxford | isbn = 0-19-509539-1 | page = 15}}</ref> In these tablets, water clocks are used for payment of the night and day watches (guards).{{sfn|Neugebauer|1947}}

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.<ref name=neugebauer>{{Harvnb|Neugebauer|1947|pp=39–40}}</ref> The volume was measured in capacity units called ''qa''. The weight, ''mana'' or [[Mina (unit)|mina]] (the Greek unit for about one pound), is the weight of water in a water clock.{{Citation needed|date=December 2020}}

In 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 mana had to be added each succeeding half-month. At the [[equinox]], three mana had to be emptied in order to correspond to one watch, and four mana was emptied for each watch of the [[winter solstice|winter solstitial night]]."<ref name=neugebauer/>

===India ===
{{main|Hindu units of time}}
N. Narahari Achar and [[Subhash Kak]] suggest that water clocks were used in [[History of India|ancient India]] as early as the 2nd millennium BC, based on their appearance in the ''[[Atharvaveda]]'.<ref>{{cite journal| url = http://www.ejvs.laurasianacademy.com/ejvs0402/ejvs0402.txt| title = On the meaning of AV XIX. 53.3: Measurement of Time?| access-date = 2007-05-11| last = Achar| first = N. Narahari| date = December 1998| journal = Electronic Journal of Vedic Studies| archive-date = 2015-09-23| archive-url = https://web.archive.org/web/20150923234634/http://www.ejvs.laurasianacademy.com/ejvs0402/ejvs0402.txt| url-status = dead}}</ref><ref>{{cite book |last=Kak |first=Subhash |author-link=Subhash Kak |date=2003-02-17 |chapter=Babylonian and Indian Astronomy: Early Connections |title=History of Science, Philosophy & Culture in Indian Civilization |volume=1 Part 4 |editor-last=Pande |editor-first=G. C. |pages=847–869 |arxiv=physics/0301078 |bibcode=2003physics...1078K}}</ref>''
According to N. Kameswara Rao, pots excavated from the [[Indus Valley Civilisation]] site of [[Mohenjo-daro]] may 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 ''[[abhiṣeka]]'' (ritual water pouring) on [[lingam]]s.<ref>{{cite journal | first = N. Kameswara | last = Rao |date=December 2005 | title = Aspects of prehistoric astronomy in India | journal = Bulletin of the Astronomical Society of India | volume = 33 | issue = 4 | pages = 499–511 | url = http://www.ncra.tifr.res.in/~basi/05December/3305499-511.pdf | access-date =2007-05-11 | quote =It appears that two artifacts from Mohenjo-daro and Harappa might correspond to these two instruments. Joshi and Parpola (1987) lists a few pots tapered at the bottom and having a hole on the side from the excavations at Mohenjadaro (Figure 3). A pot with a small hole to drain the water is very similar to clepsydras described by Ohashi to measure the time (similar to the utensil used over the lingum in Shiva temple for abhishekam). |bibcode = 2005BASI...33..499R }}</ref>

The [[Jyotisha]], one of the six [[Vedanga]] disciplines, describes water clocks called ''ghati'' or ''kapala'' that measure time in units of ''nadika'' (around 24 minutes). A clepsydra in the form of a floating and sinking copper vessel is mentioned in the ''[[Sürya Siddhānta]]'' (5th century AD).<ref>"A copper vessel (in the shape of the lower half of the water jar) which has a small hole in its bottom and being placed upon clean water in a basin sinks exactly 60 times in a day and at night." – Chapter 13, verse 23 of the ''Sürya Siddhānta''.</ref> At [[Nalanda mahavihara]], an [[Ancient higher-learning institutions|ancient Buddhist university]], four-hour intervals were measured by a water clock, which consisted of a similar 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 filled and was marked by the beating of a drum in the daytime. The amount of water added varied with the seasons, and students at the university operated the clock.<ref>{{cite book | last = Scharfe | first = Hartmut | title = Education in Ancient India | url = https://archive.org/details/educationancient00scha | url-access = limited | year = 2002 | publisher = Brill Academic Publishers | location = Leiden | isbn = 90-04-12556-6 | page = [https://archive.org/details/educationancient00scha/page/n177 171]}}</ref>

Descriptions of similar water clocks are also given in the ''[[Pancha-Siddhantika|Pañca Siddhāntikā]]'' by the polymath [[Varāhamihira]] in the 6th century, which adds further detail to the account given in the ''Sūrya Siddhānta''.{{Full citation needed|date=July 2019}} Further descriptions are recorded in the ''[[Brāhmasphuṭasiddhānta]]'' by the mathematician [[Brahmagupta]] in the 7th century. A detailed description with measurements is also recorded by the astronomer [[Lalla]] in the 8th century, who describes the ''ghati'' as a hemispherical copper vessel with a hole that is fully filled after one ''nadika''.<ref>"A copper vessel weighing 10 palas, 6 angulas in height and twice as much in breadth at the mouth—this vessel of the capacity of 60 palas of water and hemispherical in form is called a ghati." This copper vessel, which was bored with a needle and made of 3 1/8 masas of gold and 4 angulas long, gets filled in one nadika."{{Full citation needed|date=July 2019}}</ref>

===China===
[[File:Clock Tower from Su Song's Book desmear.JPG|thumb|The water-powered mechanism of [[Su Song]]'s astronomical clock tower, featuring a clepsydra tank, [[waterwheel]], [[escapement]] mechanism, and [[chain drive]] to power an [[armillary sphere]] and 113 [[striking clock]] jacks to sound the hours and to display informative plaques]]

In [[ancient China]], as well as throughout East Asia, water clocks were very important in the study of [[astronomy]] and [[astrology]]. The oldest written reference dates the use of the water clock in China to the 6th century BC.<ref name=needham479>{{Harvnb|Needham|2000|p=479}}</ref> From about 200 BC onwards, the outflow clepsydra was replaced almost everywhere in China by the inflow type with an indicator-rod borne on a float(浮箭漏, fú jiàn lòu).<ref name=needham479/> The Han dynasty philosopher and politician [[Huan Tan]] (40 BC – AD 30), 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.<ref>{{Harvnb|Needham|1995|pp=321–322}}</ref> The liquid in water clocks was liable to freezing, and had to be kept warm with torches, a problem that was solved in 976 by the Chinese astronomer and engineer [[Zhang Sixun]]. His invention—a considerable improvement on Yi Xing's clock—used [[Mercury (element)|mercury]] instead of water. Mercury is a liquid at room temperature, and freezes at {{convert|-38.83|°C|°F|abbr=on|sigfig=3}}, lower than any air temperature common outside polar regions.{{sfn|Temple|1986|p=107}}<ref>{{Britannica | id=375837 | title=Mercury }}</ref> 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).<ref>{{Harvnb|Needham|1986|pp=510–511}}</ref>

The use of clepsydrae to drive mechanisms [[Armillary sphere|illustrating astronomical phenomena]] began with the Han Dynasty polymath [[Zhang Heng]] (78–139) in 117, who also employed a [[waterwheel]].<ref>{{Harvnb|Needham|2000|pp=30, 532}}</ref> 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.<ref name="needham479"/> Zhang's ingenuity led to the creation by the Tang dynasty mathematician and engineer [[Yi Xing]] (683–727) and [[Liang Lingzan]] in 725 of a clock driven by a waterwheel linkwork [[escapement]] mechanism.<ref>{{Harvnb|Needham|2000|pp=471, 490, 532}}</ref> The same mechanism would be used by the Song dynasty polymath [[Su Song]] (1020–1101) in 1088 to power his [[astronomical clock]] tower, as well as a [[chain drive]].<ref>{{Harvnb|Needham|2000|p=462}}</ref> [[Su Song]]'s clock tower, over {{convert|30|ft|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 [[mannequin]]s which rang bells or gongs, and held tablets indicating the hour or other special times of the day. In the 2000s, in [[Beijing]]'s [[Gulou and Zhonglou (Beijing)|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.<ref>{{cite web|url=http://commons.wikimedia.org/wiki/Image:CIMG1106_Clepshydra_in_Drum_Tower,_Beijing_august_2007.JPG|title=Clepsydra in the Drum Tower, Beijing, China|last=Ellywa|date=1 August 2007|via=Wikimedia Commons}}</ref>

===Persia===
[[File:Ancient water clock used in qanat of gonabad 2500 years ago.JPG|thumb|Ancient Persian clock]]

The use of water clocks in [[Greater Iran]], especially in the desert areas such as [[Yazd]], [[Isfahan]], [[Zibad]], and [[Gonabad]], dates back to 500 BC.<ref>{{cite web |first=G.H. |last=Rahimi |work=Tehran university science magazine |url=https://jihs.ut.ac.ir/article_69801_e14a6b2d0d61cd4df20e24ffd5b216a4.pdf |title=Water Sharing Management in Ancient Iran, with Special Reference to Pangān (cup) in Iran}}
</ref> Later, they were also used to determine the exact holy days of pre-Islamic religions such as [[Nowruz]] ([[March equinox]]), [[Mehregan]] ([[September equinox]]), [[Tirgan]] ([[summer solstice]]) and [[Yaldā Night]] ([[winter solstice]]) – the shortest, longest, and equal-length days and nights of the years. The water clocks, called ''pengan'' (and later ''fenjan'') used were one of the most practical ancient tools for timing the yearly calendar.<ref name=Qanat>{{cite web|url=http://www.aftabir.com/articles/view/science_education/technical/c3c1183387267p1.php|title=Conference of Qanat in Iran – water clock in Persia 1383|website=www.aftabir.com|language=fa}}</ref><ref name="japan.mfa.gov.ir">{{Cite web|url=https://japan.mfa.gov.ir/en/newsview/536035|title=Qanat is cultural and social and scientific heritage in Iran}}</ref> The water clock 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 more accurate current clocks replaced it.<ref>{{cite web|url=http://parssea.org/?p=2734|title= Water clock or Pengan in Iran, National conference 2004 Gonabad  |website=parssea.org|archive-url= https://web.archive.org/web/20170610061516/http://parssea.org/?p=2734 |archive-date= 2017-06-10 }}</ref><ref>{{cite web|url=http://vista.ir/article/265134|title= Qanat iscultural and social and scientific heritage in Iran |last=vista.ir}}</ref>

Persian water clocks were a practical, useful, and necessary tool for the qanat's shareholders to calculate the length of time they could divert water to their farms or gardens. The qanat was the only water source for agriculture and irrigation in arid area 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 or ''mir āb'', and at least two full-time managers were needed to control and observe the number of hours and announce the exact time of the days and nights from sunrise to sunset because shareholders usually divided between day and night owners.<ref name="amordadnews.com">{{cite news|url=http://amordadnews.com/neveshtehNamyesh.aspx?NId=6010|title=water clock in persia|work=amordadnews.com|url-status=dead|archive-url=https://web.archive.org/web/20140429163019/http://amordadnews.com/neveshtehNamyesh.aspx?NId=6010|archive-date=2014-04-29}}</ref>

The Persian water clock 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.<ref name="amordadnews.com"/> The place where the clock was situated and its managers were collectively known as the ''khane pengān''. 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. The [[Zibad]] water clock was in use until 1965,<ref name="japan.mfa.gov.ir"/> when it was replaced by modern clocks.<ref name=Qanat/>

===Greco-Roman world===
[[File:Clepsydra-Diagram-Fancy.jpeg|thumb|left|An early 19th-century illustration<ref>This engraving is taken from "Rees's Clocks, Watches, and Chronometers 1819–20. The design of the illustration was modified from Claude Perrault's illustrations in his 1684 translation of Vitruvius's Les Dix Livres d'Architecture (1st century BC), of which he describes Ctesibius's clepsydra in great length.</ref> of [[Ctesibius]]'s (285–222 BC) clepsydra from the 3rd century BC. The hour indicator ascends as water flows in. Also, a series of gears rotate a cylinder to correspond to the temporal hours.]]
[[File:Ctesibius' hydraulic clock.jpg|thumb|A modern reconstruction of [[Ctesibius]]' hydraulic clock (clepsydra), at the [[Museum of Ancient Greek Technology|Kotsanas Museum of Ancient Greek Archaeology]] in [[Athens]].]]
The word "[[wiktionary:κλεψύδρα#Ancient Greek|clepsydra]]" comes from the Greek meaning "water thief".<ref>{{cite book |last=Levy |first=Janey |year=2004 |title=Keeping Time Through the Ages: The History of Tools Used to Measure Time |publisher=Rosen Classroom |isbn=9780823989171 |page=[https://archive.org/details/keepingtimethrou0000levy/page/11 11] |quote=The Greeks named the water clock 'clepsydra' (KLEP-suh-druh), which means 'water thief'. |url=https://archive.org/details/keepingtimethrou0000levy/page/11 }}</ref> 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.<ref>{{Harvp|Goodenow|Orr|Ross|2007|p=7}}</ref> [[Ctesibius]] invented an indicator system typical for later clocks such as the dial and pointer.<ref name="John G. Landels 35">John G. Landels: "Water-Clocks and Time Measurement in Classical Antiquity", "Endeavour", Vol. 3, No. 1 (1979), pp. 32–37 (35)</ref> The [[Ancient Rome|Roman]] engineer [[Vitruvius]] described early alarm clocks, working with gongs or trumpets.<ref name="John G. Landels 35"/> 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, such as when a person's life was at stake, it was filled completely, but for more minor cases, only partially. 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.<ref>{{Harvnb|Hill|1981|p=6}}</ref>

====Clepsydrae for keeping time====
Some scholars suspect that the clepsydra may have been used as a stop-watch for imposing a time limit on clients' visits in [[Athens|Athenian]] brothels.<ref name="John G. Landels 32">{{cite journal|last=Landels|first=John G.|title=Water-Clocks and Time Measurement in Classical Antiquity|journal=Endeavour|volume=3|issue=1|year=1979|page=33|doi=10.1016/0160-9327(79)90007-3}}</ref> Slightly later, in the early 3rd century BC, 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.<ref name="John G. Landels 32"/>

Between 270 BC and AD 500, [[Hellenistic civilization|Hellenistic]] ([[Ctesibius]], [[Hero of Alexandria]], [[Archimedes]]) and [[Ancient Rome|Roman]] [[horology|horologists]] and [[astronomer]]s 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 [[bell (instrument)|bell]]s and [[gong]]s, while others opened doors and windows to show figurines of people, or moved pointers, and dials. Some even displayed [[astrology|astrological]] models of the universe. The 3rd century BC engineer [[Philo of Byzantium]] referred in his works to water clocks already fitted with an escapement mechanism, the earliest known of its kind.<ref>{{Harvnb|Lewis|2000|pp=356f.}}</ref>

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 BC. This [[octagon]]al [[clocktower]] showed scholars and shoppers both [[sundial]]s and a [[windvane]]. Inside it was a mechanized clepsydra, although the type of display it used cannot be known for sure; some possibilities are: a rod that moved up and down to display the time, a water-powered [[automaton]] that struck a bell to mark the hours, or a moving star disk in the ceiling.<ref>{{cite journal |last1=Noble |first1=Joseph V. |last2=de Solla Price |first2=Derek |title=The Water Clock in the Tower of the Winds |journal=American Journal of Archaeology |date=October 1968 |volume=72 |issue=4 |pages=345–355 |doi=10.2307/503828 |jstor=503828 |url=https://www.jstor.org/stable/503828 |access-date=19 June 2024|url-access=subscription }}</ref>

===Medieval Islamic world===
[[File:Al-jazari elephant clock.png|thumb|left|[[Al-Jazari]]'s [[Elephant clock|elephant water clock]] (1206).<ref>{{cite book | last = ibn al-Razzaz al-Jazari | author-link = al-Jazari | others = Translated and annotated by [[Donald Routledge Hill]] | title = The Book of Knowledge of Ingenious Mechanical Devices | year = 1974 | publisher = D. Reidel | location = Dordrecht | isbn = 969-8016-25-2}}</ref>]]

In the [[Islamic Golden Age|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 the Arabic engineer [[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 [[Regulator (automatic control)|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.<ref>{{Harvnb|al-Hassan|Hill|1986|pp=57–59}}</ref>

[[File:Clock of al Jazari before 1206.jpg|thumb|Water-powered automatic [[castle clock]] of [[Al-Jazari]], 12th century.]]

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.<ref name="Ancient Discoveries">{{cite web|title=Ancient Discoveries, Episode 11: Ancient Robots |publisher=[[History (U.S. TV channel)|History Channel]] |url=https://www.youtube.com/watch?v=rxjbaQl0ad8 |access-date=2008-09-06 |url-status=dead |archive-url=https://web.archive.org/web/20140301151115/https://www.youtube.com/watch?v=rxjbaQl0ad8 |archive-date=March 1, 2014 }}</ref> It was a complex device that was about {{convert|11|ft|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.<ref>Howard R. Turner (1997), ''Science in Medieval Islam: An Illustrated Introduction'', p. 184. [[University of Texas Press]], {{ISBN|0-292-78149-0}}.</ref><ref name=Hill2>[[Donald Routledge Hill|Routledge Hill, Donald]], "Mechanical Engineering in the Medieval Near East", ''Scientific American'', May 1991, pp. 64–69. ([[cf.]] [[Donald Routledge Hill]], [http://home.swipnet.se/islam/articles/HistoryofSciences.htm Mechanical Engineering] {{Webarchive|url=https://web.archive.org/web/20071225091836/http://home.swipnet.se/islam/articles/HistoryofSciences.htm |date=2007-12-25 }})</ref> 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.<ref name="Ancient Discoveries"/> 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 [[Automaton|automata]] dropping balls into vases.<ref>{{cite web|url=http://www.google.com.my/search?hl=en&q=two+falcon+automata+dropping+balls+into+vases&um=1&ie=UTF-8&tbo=u&tbs=bks:1&source=og&sa=N&tab=wp|title=two falcon automata dropping balls into vases – Google Search|website=www.google.com.my}}</ref>{{Unreliable source?|sure=y|reason=This is just a link to a google search for the preceding phrase"|date=March 2017}}

The first water clocks to employ complex segmental and [[epicyclic gearing]] was invented earlier by the [[Arab]] engineer [[Ibn Khalaf al-Muradi]] in [[Al-Andalus|Islamic Iberia]] c. 1000. His water clocks were driven by [[water wheel]]s, as was also the case for several Chinese water clocks in the 11th century.<ref name=Hassan/> Comparable water clocks were built in [[Damascus]] and [[Fes, Morocco|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 Dondi dell'Orologio|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.<ref name=Hassan>[[Ahmad Y Hassan|Hassan, Ahmad Y]], [http://www.history-science-technology.com/Articles/articles%2071.htm Transfer Of Islamic Technology To The West, Part II: Transmission Of Islamic Engineering], ''History of Science and Technology in Islam''</ref>

===Korea===

{{Main|Chang Yŏngsil#Water Clock}}

[[File:BoRuGak_Jagyeongnu.JPG|thumb|right|Chang Yŏngsil's self-striking water clock, the [[Borugak Jagyeongnu]]]]
In 718, [[Unified Silla]] established the system of clepsydra for the first time in Korean history, imitating the Tang Dynasty.<ref>{{cite book|first=Z.|last=Xu|year=2021|chapter=The Spread of Traditional Chinese Mathematics in the Sinosphere and Its Influence|title=The High Tide of Science and Technology Development in China: History of Science and Technology in China|volume=3|pages=413–426|doi=10.1007/978-981-15-7847-2_11|isbn=978-981-15-7846-5 |chapter-url=https://link.springer.com/content/pdf/10.1007/978-981-15-7847-2_11.pdf}}</ref> In 1434, during [[Joseon]] rule, [[Chang Yŏngsil]] ({{ko-hhrm|hangul=장영실|hanja=蔣英實}}), a palace guard and later chief court engineer, constructed the [[Borugak Jagyeongnu]] or self-striking water clock of Borugak Pavilion for [[Sejong the Great]].

What made his water clock self-striking (or automatic) was using jack-work mechanisms: three wooden figures or "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.{{Citation needed|date=December 2020}}

The uniqueness of the clock was its capability to announce dual-times automatically with visual and audible signals.<ref>{{cite book|last1=Koetsier|first1=Teun|last2=ceccarelli|first2=marco|title=Explorations in the History of Machines and Mechanisms: Proceedings of HMM2012|date=5 April 2012|publisher=Springer Science & Business Media|isbn=9789400741324|page=90|url=https://books.google.com/books?id=-x9NHOLSnNUC&pg=PA90|access-date=27 March 2017|language=en}}</ref> Chang developed a signal conversion technique that made it possible to measure analog time and announce digital time simultaneously as well as to separate the water mechanisms from the ball-operated striking mechanisms.<ref>{{cite book|last1=Koetsier|first1=Teun|last2=ceccarelli|first2=marco|title=Explorations in the History of Machines and Mechanisms: Proceedings of HMM2012|date=5 April 2012|publisher=Springer Science & Business Media|isbn=9789400741324|page=95|url=https://books.google.com/books?id=-x9NHOLSnNUC&pg=PA95|access-date=27 March 2017|language=en}}</ref> The conversion device was called ''pangmok'', and was placed above the inflow vessel that measured the time, the first device of its kind in the world.<ref>{{cite book|title=Fifty Wonders of Korea - Vol. 2|publisher=KSCPP|url=http://www.kscpp.net/LinkClick.aspx?fileticket=lILBCCXDCSk%3d&tabid=144&mid=528|archive-url=https://web.archive.org/web/20170327171319/http://www.kscpp.net/LinkClick.aspx?fileticket=lILBCCXDCSk%3d&tabid=144&mid=528|access-date=27 March 2017|archive-date=2017-03-27}}</ref> Thus, the Borugak water clock is the first hydro-mechanically engineered dual-time clock in the history of horology.<ref>{{cite book|last1=Ceccarelli|first1=Marco|title=Distinguished Figures in Mechanism and Machine Science: Their Contributions and Legacies|date=21 May 2014|publisher=Springer|isbn=9789401789479|page=111|url=https://books.google.com/books?id=yavIAwAAQBAJ&pg=PA111|access-date=27 March 2017|language=en}}</ref><ref>{{cite book|last1=Pisano|first1=Raffaele|title=A Bridge between Conceptual Frameworks: Sciences, Society and Technology Studies|date=30 June 2015|publisher=Springer|isbn=9789401796453|page=364|url=https://books.google.com/books?id=eUUPCgAAQBAJ&pg=PA364|access-date=27 March 2017|language=en}}</ref>

=== Japan ===
[[Emperor Tenji]] made Japan's first water clock called a {{Nihongo|Rokoku|漏刻}}. They were highly socially significant and run by {{ill|Rōkoku Hakase|lt=Doctors of Water Clock|ja|漏刻博士}}

==Temperature, water viscosity, and clock accuracy==
When viscosity can be neglected, the outflow rate of the water is governed by [[Torricelli's law]], or more generally, by [[Bernoulli's principle]]. [[Viscosity]] will dominate the outflow rate if the water flows out through a nozzle that is sufficiently long and thin, as given by the [[Hagen–Poiseuille equation]].<ref>{{Harvp|Goodenow|Orr|Ross|2007|p=6}}</ref> Approximately, the flow rate is for such design [[inversely proportional]] to the viscosity, which depends on the [[temperature]]. [[Liquid]]s generally become less viscous as the temperature increases. In the case of water, the viscosity varies by a factor of about seven between zero and 100 degrees Celsius. Thus, a water clock with such a nozzle would run about seven times faster at 100&nbsp;°C than at 0&nbsp;°C. Water is about 25 percent more viscous at 20&nbsp;°C than at 30&nbsp;°C, and a variation in temperature of one degree Celsius, in this "[[room temperature]]" range, produces a change of viscosity of about two percent.<ref>[[CRC Handbook of Chemistry and Physics]], page F-36</ref> Therefore, a water clock with such a nozzle 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 {{frac|30}}°C (about {{frac|17}}°F). There is no evidence that this was done in antiquity, so ancient water clocks with sufficiently thin and long nozzles (unlike the modern pendulum-controlled one described above) cannot have been reliably accurate by modern standards.  However, while modern timepieces may not be reset for long periods, water clocks were likely reset every day, when refilled, based on a sundial, so the cumulative error would not have been great.{{Citation needed|date=December 2020}}

==See also==
* [[Bernard Gitton]]
* [[History of timekeeping devices]]
* [[Hourglass]]

==Notes==
{{Reflist|2}}

=== Sources used ===
* {{Cite book | last1 =al-Hassan | first1 =Ahmad Y. | author-link =Ahmad Y Hassan | last2 =Hill | first2 =Donald R. | author2-link =Donald Routledge Hill | year =1986 | title =Islamic Technology: An Illustrated History | publisher =Cambridge University Press | isbn =0-521-26333-6 | oclc =13332728 | url-access =registration | url =https://archive.org/details/islamictechnolog0000hasa }}
* {{Cite book |last1=Cotterell |first1=Brian |last2=Kamminga |first2=Johan |title=Mechanics of pre-industrial technology: An introduction to the mechanics of ancient and traditional material culture |publisher=Cambridge University Press |year=1990 |isbn=0-521-42871-8 |oclc=18520966}}
* {{Cite book | last =Cowan | first =Harrison J. | year =1958 | title =Time and Its Measurement: From the stone age to the nuclear age | place =Ohio | publisher =The World Publishing Company | bibcode =1958tmfs.book.....C }}
* {{Citation
 | title = Mathematical Models of Water Clocks
 | url = http://www.nawcc-index.net/Articles/Goodenow-WaterClocks.pdf
 | last1 = Goodenow| first1 = Jennifer
 | last2 = Orr | first2 = Richard
 | last3 = Ross | first3 = David
 | publisher = Rochester Institute of Technology
 | year = 2007 
}}
* {{Cite book | last =Hill | first =D.R. | author-link =Donald Routledge Hill | year =1981 | title =Arabic Water–Clocks | place =Syria | publisher =University of Aleppo}}
* {{Cite book
 | last = Lewis
 | first = Michael
 | year = 2000
 | contribution = Theoretical Hydraulics, Automata, and Water Clocks
 | editor-last = Wikander
 | editor-first = Örjan
 | editor-link = Örjan Wikander
 | title = Handbook of Ancient Water Technology
 | series = Technology and Change in History
 | location = Leiden
 | volume = 2
 | pages = 343–369 (356f.)
 | isbn = 90-04-11123-9
}}
* {{cite book |last=Needham |first=Joseph |author-link=Joseph Needham |date=1986 |title=Science & Civilization in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering |location=Taipei |publisher=Caves Books}}
* {{Cite book |last=Needham |first=Joseph |author-link=Joseph Needham |year=1995 |title=Science & Civilisation in China: Volume 3, Mathematics and the Sciences of the Heavens and the Earth |publisher=Cambridge University Press |isbn=0-521-05801-5 |oclc= 153247126}}
* {{Cite book |last=Needham |first=Joseph |author-link=Joseph Needham |year=2000 |title=Science & Civilisation in China: Volume 4, Physics and Physical Technology, Part 2, Mechanical Engineering |publisher=Cambridge University Press |isbn=0-521-05803-1 |oclc= 153247141}}
* {{Cite journal | last=Neugebauer | first=Otto | author-link=Otto E. Neugebauer | year=1947 | title=Studies in Ancient Astronomy. VIII. The Water Clock in Babylonian Astronomy | journal=Isis | volume=37 | issue=1/2 | pages=37–43 | doi=10.1086/347965 | pmid=20247883 | s2cid=120229480 }} (Reprinted in {{cite book | last=Neugebauer | first=Otto |title=Astronomy and History: Selected Essays |year=1983 | pages=239–245}})
* {{cite book |title=The Genius of China: 3000 years of science, discovery and invention  |url=https://archive.org/details/geniusofchina3000temp/page/55 |url-access=limited  |last1=Temple |first1= Robert |year=1986 |publisher=Simon and Schuster |location=New York |page=55 |isbn=9780671620288 }}
* {{Cite book |last=Turner |first=Anthony J. |year=1984 |title=The Time Museum |volume =I: ''Time Measuring Instruments''; Part 3: ''Water-clocks'', ''Sand-glasses'', ''Fire-clocks'' |isbn=0-912947-01-2 |publisher=The Museum |location=Rockford, IL |oclc =159866762}}

==Bibliography==
{{main|Bibliography of water clocks}}

==External links==
{{wiktionary | clepsydra}}
{{Commons category|Water clocks}}
* [http://www.europa-center-berlin.de/en/the-sights/the-clock-of-flowing-time.html The Clock of Flowing Time in Berlin] {{Webarchive|url=https://web.archive.org/web/20190118081232/http://www.europa-center-berlin.de/en/the-sights/the-clock-of-flowing-time.html |date=2019-01-18 }}
* [https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks NIST: A Walk Through Time – Early Clocks]
* [http://www.marcdatabase.com/~lemur/dm-gitton.html#iw-time-flow-clocks Bernard Gitton's Time-Flow Clocks] {{Webarchive|url=https://web.archive.org/web/20110809073852/http://www.marcdatabase.com/~lemur/dm-gitton.html#iw-time-flow-clocks |date=2011-08-09 }}
* [https://japan.mfa.gov.ir/en/newsview/536035 Qanat is cultural, social and scientific heritage in Iran]
* [http://www.eternalegypt.org/EternalEgyptWebsiteWeb/HomeServlet?ee_website_action_key=action.display.element&story_id=19&module_id=5&language_id=1&element_id=60513 Egypt's Water Clock]
* {{Cite Americana|wstitle=Clepsydra |year=1920 |short=x}}
* [https://www.perseus.tufts.edu/GreekScience/Students/Jesse/CLOCK1A.html A Brief History of Clocks: From Thales to Ptolemy]
* [http://www.childrensmuseum.org/waterclock The Indianapolis Children's Museum Water Clock] {{Webarchive|url=https://web.archive.org/web/20120422125502/http://www.childrensmuseum.org/waterclock |date=2012-04-22 }}
* [http://www.landmarkclocks.com/page14.html Nanaimo, BC Water Clock] {{Webarchive|url=https://web.archive.org/web/20070811001650/http://www.landmarkclocks.com/page14.html |date=2007-08-11 }}
* [https://www.history-of-physics.com/2017/08/ancient-greece-water-clock-clepsydra-of_14.html Animation: Ctesibius Water Clock]
* [http://www.antique-horology.org/_editorial/clepsydra/ Rees's Universal Dictionary article on Clepsydra, 1819]
* [http://www.royalgorgebridge.com/WaterClock.aspx The Royal Gorge Bridge Water Clock] {{Webarchive|url=https://web.archive.org/web/20070501165809/http://www.royalgorgebridge.com/WaterClock.aspx |date=2007-05-01 }}
* {{Cite NIE|wstitle=Clepsydra |year=1905 |short=x}}
*  [http://www.muslimheritage.com/article/mechanical-water-clock-ibn-al-haytham The Mechanical Water Clock Of Ibn Al-Haytham]
* [https://maghzrayaneh.ir/services-computer computer servies on site] on clocks

{{Time Topics}}
{{Time measurement and standards}}
{{Authority control}}

[[Category:Water clocks| ]]
[[Category:Egyptian inventions]]
[[Category:Time measurement systems]]
[[Category:Timekeeping]]