Editing Water clock (section)

===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(called fou chien lou,浮箭漏).<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>