Water clock

A **water clock** was a tool for measuring  time by the flow of liquid into or out of a vessel. Water clocks are one of the oldest time-measuring tools. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon, Egypt, and Persia around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks.

## Designs 
A water clock uses the flow of water to measure time. There are two types of water clocks: inflow and outflow. In an outflow water clock, a container is filled with water, and the water is drained slowly  out of the container. 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.
 Gonabad, zibad]]
## Regional development
### Persia
The simplest and more useable clock was a Persian water clock used by manager of the qanat . The use of water clocks in Persia or Greater Iran ,especially in desert area of Iran such as Yazd, Isfahan, and Gonabad,dates back to 500 BC. Later they were also used to say the exact time of holy days of pre-Islamic religions, such as telling  the time of the equal day and night ( *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. The water clock, or *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 clocks.  Persian water clocks were a practical and 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 (Kariz) was the only water source for agriculture and irrigation in arid areas so a just and fair water distribution was very important. A very fair and clever old person was elected to be the manager of the water clock called Mir Aab, and at least two full-time managers were needed to control and observe the number of Fenjans or Pengan hours and announce the exact time of the days and nights from sunrise to sunset because share holders usually were divided to the days owners and night owners. 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.

### China
'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. 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. 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.]]

#### 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 Athenian brothels. Slightly later, in the early 3rd century BC, the Hellenistic physician Herophilos had 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.]]

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 the Arabic engineer Al-Jazari, however, are credited for going "well beyond anything" that had preceded them. 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.
 of Al-Jazari, 12th century.]]

### Korea
In 1434 during the Joseon Dynasty, Jang Yeong-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. The uniqueness of the clock was its capability to announce dual-times automatically with both visual and audible signals. Jang 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.

## Notes
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## Other websites
* Qanat or Kariz tools https://japan.mfa.gov.ir/en/newsview/536035
* The Clock of Flowing Time in Berlin 
* NIST: A Walk Through Time - Early Clocks
* Bernard Gitton's Time-Flow Clocks 
* https://japan.mfa.gov.ir/en/newsview/536035 Qanat is cultural,social and scientific heritage in Iran
* Egypt's Water Clock
* A Brief History of Clocks: From Thales to Ptolemy 
* The Indianapolis Children's Museum Water Clock 
* Nanaimo, BC Water Clock