Editing Zhang Heng (section)

===Extra tank for inflow clepsydra===
[[Image:Guardians of Day and Night, Han Dynasty.jpg|thumb|right|Han dynasty paintings on [[tile]]; being conscious of time, the Chinese believed in [[Chinese mythology|guardian spirits]] for the divisions of day and night, such as these two guardians here representing 11&nbsp;pm to 1&nbsp;am (left) and 5&nbsp;am to 7&nbsp;am (right)]]
The outflow [[Water clock|clepsydra]] was a timekeeping device used in China as long ago as the [[Shang dynasty]] (c.&nbsp;1600–c.&nbsp;1050&nbsp;BC), and certainly by the [[Zhou dynasty]] (1122–256&nbsp;BC).<ref name="needham 1986 volume 4 part 2 479">Needham (1986), Volume 4, Part 2, 479.</ref> The inflow clepsydra with an indicator rod on a float had been known in China since the beginning of the Han dynasty in 202&nbsp;BC and had replaced the outflow type.<ref name="needham 1986 volume 4 part 2 479"/> The [[Han Chinese]] noted the problem with the falling [[pressure head]] in the reservoir, which slowed the timekeeping of the device as the inflow vessel was filled.<ref name="needham 1986 volume 4 part 2 479"/> Zhang Heng was the first to address this problem, indicated in his writings from 117, by adding an extra compensating tank between the reservoir and the inflow vessel.<ref name="needham 1986 volume 4 part 2 479 footnote e"/><ref name="crespigny 2007 1050"/> Zhang also mounted two statuettes of a Chinese immortal and a heavenly guard on the top of the inflow clepsydra, the two of which would guide the indicator rod with their left hand and point out the graduations with their right.<ref name="needham 1986 volume 4 part 2 164">Needham (1986), Volume 4, Part 2, 164.</ref> Joseph Needham states that this was perhaps the ancestor of all [[Striking clock|clock jacks that would later sound the hours]] found in mechanical clocks by the 8th century, but he notes that these figures did not actually move like clock jack figurines or sound the hours.<ref name="needham 1986 volume 4 part 2 164"/> Many additional compensation tanks were added to later clepsydras in the tradition of Zhang Heng. In 610 the [[Sui dynasty]] (581–618) engineers Geng Xun and Yuwen Kai crafted an unequal-armed [[steelyard balance]] able to make seasonal adjustments in the pressure head of the compensating tank, so that it could control the rate of water flow for different lengths of day and night during the year.<ref name="needham 1986 volume 4 part 2 480">Needham (1986), Volume 4, Part 2, 480.</ref> Zhang mentioned a "jade dragon's neck", which in later times meant a siphon.<ref name="needham 1986 volume 3 320">Needham (1986), Volume 3, 320.</ref> He wrote of the floats and indicator-rods of the inflow clepsydra as follows:

{{Blockquote
|text=<poem>{{lang|zh|以銅為器，再疊差置，實以清水，下各開孔，以玉虯吐漏水入兩壺。右為夜，左為晝。| size=105%}}
Bronze vessels are made and placed one above the other at different levels; they are filled with pure water. Each has at the bottom a small opening in the form of a 'jade dragon's neck'. The water dripping (from above) enters two inflow receivers (alternately), the left one being for the night and the right one for the day.

{{lang|zh|蓋上又鑄金銅仙人，居左壺；為胥徒，居右壺。皆以左手抱箭，右手指刻，以別天時早晚。| size=105%}}
On the covers of each (inflow receiver) there are small cast statuettes in gilt bronze; the left (night) one is an immortal and the right (day) one is a policeman. These figures guide the indicator-rod (lit. arrow) with their left hands, and indicate the graduations on it with their right hands, thus giving the time.<ref name="needham 1986 volume 3 320"/></poem>
}}