Editing Zhang Heng (section)

===Water-powered armillary sphere===
[[Image:Clock Tower from Su Song's Book desmear.JPG|thumb|right|200px|The original diagram of [[Su Song]]'s (1020–1101) [[clock tower]], featuring an [[armillary sphere]] powered by a [[waterwheel]], [[escapement]] mechanism, and [[chain drive]]]]

Zhang Heng is the first person known to have applied hydraulic [[Power (physics)|motive power]] (i.e. by employing a [[waterwheel]] and [[Water clock|clepsydra]]) to rotate an [[armillary sphere]], an astronomical instrument representing the [[celestial sphere]].<ref name="needham 1986 volume 4 part 2 30">Needham (1986), Volume 4, Part 2, 30.</ref><ref name="morton 2005 70">Morton & Lewis (2005), 70.</ref><ref name="loewe 1968 107">Loewe (1968), 107.</ref> The Greek astronomer [[Eratosthenes]] (276–194&nbsp;BC) invented the first armillary sphere in 255&nbsp;BC. The Chinese armillary sphere was fully developed by 52&nbsp;BC, with the astronomer Geng Shouchang's (耿壽昌) addition of a permanently fixed [[equatorial ring]].<ref name="needham 1986 volume 3 343">Needham (1986), Volume 3, 343.</ref> In AD 84 the astronomers Fu An and [[Jia Kui (scholar)|Jia Kui]] added the ecliptic ring, and finally Zhang Heng added the [[Astronomical rings|horizon and meridian rings]].<ref name="crespigny 2007 1050"/><ref name="needham 1986 volume 3 343"/> This invention is described and attributed to Zhang in quotations by Hsu Chen and Li Shan, referencing his book ''Lou Shui Chuan Hun Thien I Chieh'' (Apparatus for Rotating an Armillary Sphere by Clepsydra Water). It was likely not an actual book by Zhang, but a chapter from his ''Hun I'' or ''Hun I Thu Chu'', written in 117 AD.<ref name="needham 1965 volume 4 part 1 486">Needham (1965), Volume 4, Part 1, 486.</ref> His water-powered armillary influenced the design of later Chinese water clocks and led to the discovery of the [[escapement]] mechanism by the 8th&nbsp;century.<ref name="needham 1986 volume 4 part 2 532"/> The historian [[Joseph Needham]] (1900–1995) states:

<blockquote>What were the factors leading to the first escapement clock in China? The chief tradition leading to Yi Xing (AD 725&nbsp;) was of course the succession of 'pre-clocks' which had started with Zhang Heng about 125. Reason has been given for believing that these applied power to the slow turning movement of computational armillary spheres and celestial globes by means of a water-wheel using clepsydra drip, which intermittently exerted the force of a lug to act on the teeth of a wheel on a polar-axis shaft. Zhang Heng in his turn had composed this arrangement by uniting the armillary rings of his predecessors into the equatorial armillary sphere, and combining it with the principles of the water-mills and hydraulic trip-hammers which had become so widespread in Chinese culture in the previous century.<ref name="needham 1986 volume 4 part 2 532">Needham (1986), Volume 4, Part 2, 532.</ref></blockquote>

Zhang did not initiate the Chinese tradition of [[hydraulic engineering]], which began during the mid Zhou dynasty (c.&nbsp;6th&nbsp;century&nbsp;BC), through the work of engineers such as [[Sunshu Ao]] and [[Ximen Bao]].<ref name="needham 1986 volume 4 part 3 271">Needham (1986), Volume 4, Part 3, 271.</ref> Zhang's contemporary, [[Du Shi]], (d.&nbsp;AD&nbsp;38) was the first to apply the motive power of waterwheels to operate the [[bellows]] of a [[blast furnace]] to make [[pig iron]], and the [[cupola furnace]] to make [[cast iron]].<ref name="needham 1986 volume 4 part 2 370">Needham (1986), Volume 4, Part 2, 370.</ref><ref>Wagner (2001), 75–76.</ref> Zhang provided a valuable description of his water-powered armillary sphere in the treatise of 125, stating:

<blockquote>The equatorial ring goes around the belly of the armillary sphere 91 and 5/19 (degrees) away from the pole. The circle of the ecliptic also goes round the belly of the instrument at an angle of 24 (degrees) with the equator. Thus at the summer solstice the ecliptic is 67 (degrees) and a fraction away from the pole, while at the winter solstice it is 115 (degrees) and a fraction away. Hence (the points) where the ecliptic and the equator intersect should give the north polar distances of the spring and autumn equinoxes. But now (it has been recorded that) the spring equinox is 90 and 1/4 (degrees) away from the pole, and the autumn equinox is 92 and 1/4 (degrees) away. The former figure is adopted only because it agrees with the (results obtained by the) method of measuring solstitial sun shadows as embodied in the Xia (dynasty) calendar.<ref name="needham 1986 volume 3 355-356">Needham (1986), Volume 3, 355–356.</ref></blockquote>

Zhang Heng's water-powered armillary sphere had profound effects on Chinese astronomy and mechanical engineering in later generations. His model and its complex use of gears greatly influenced the water-powered instruments of later astronomers such as [[Yi Xing]] (683–727), [[Zhang Sixun]] (fl.&nbsp;10th&nbsp;century), [[Su Song]] (1020–1101), [[Guo Shoujing]] (1231–1316), and many others. Water-powered armillary spheres in the tradition of Zhang Heng's were used in the eras of the [[Three Kingdoms]] (220–280) and [[Jin dynasty (266–420)]], yet the design for it was temporarily out of use between 317 and 418, due to invasions of northern [[Xiongnu]] nomads.<ref name="needham 1986 volume 4 part 2 483">Needham (1986), Volume 4, Part 2, 483.</ref> Zhang Heng's old instruments were recovered in 418, when [[Emperor Wu of Liu Song]] (r.&nbsp;420–422) captured the ancient capital of Chang'an. Although still intact, the graduation marks and the representations of the stars, Moon, Sun, and planets were quite worn down by time and rust.<ref name="needham 1986 volume 4 part 2 483"/> In 436, the emperor ordered Qian Luozhi, the Secretary of the Bureau of Astronomy and Calendar, to recreate Zhang's device, which he managed to do successfully.<ref name="needham 1986 volume 4 part 2 483"/> Qian's water-powered celestial globe was still in use at the time of the Liang dynasty (502–557), and successive models of water-powered armillary spheres were designed in subsequent dynasties.<ref name="needham 1986 volume 4 part 2 483"/>