Zhang Sixun (Chang Ssu-Hsun)

SUBJECT AREA: Horology

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b. fl. late 10th century

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Chinese astronomer and clockmaker who built the earliest recorded astronomical clock tower with a hydromechanical escapement.

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Most clepsydra clocks, such as that of al-Jarazi, measured time continuously by the constant flow of a liquid and most mechanical clocks measure time discontinuously by means of an escapement. The clepsydra clock devised by Zhang Sixun in 976 and completed in 979 was unusual as it incorporated an escapement. It consisted of a large wheel with buckets around its periphery. A constant stream of water was directed into one of the buckets until it reached a predetermined weight, this released the wheel, allowing it to rotate to a new position where the process was repeated (this mechanism may have been introduced by the Chinese astronomer and mathematician Zhang Heng in the second century). The water was later replaced by mercury to prevent freezing in winter. With suitable gearing the movement of the wheel was used to drive a celestial globe, a carousel for written time announcements and jacks for audible time signals. This clock has not survived and is known only from the work Hsin I Hsiang Fa Yao (New Armillary Sphere and Celestial Globe System Essentials), which was printed in 1172 and is ascribed to Su Song. This work also describes two similar but later astronomical clock towers with water-wheel escape-ments. Several models of the water-wheel escapement have been constructed from the description in this work.

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Further Reading

J.Needham (ed.), 1965, Science and Civilisation in China Vol. IV.2, Cambridge: Cambridge University Press: 38, 111, 165, 463, 469–71, 490, 524, 527–8, 533, 540.

J.H.Combridge, 1975, "The astronomical clocktowers of Chang Ssu-Hsun and his successors, A.D. 976 to 1126", Antiquarian Horology 9: 288–301.

J.Needham, Wang Ling and J.de Solla Price, 1986, Heavenly Clockwork. The Great Astronomical Clocks of Medieval China (2nd edn with supplement by J.H.Combridge), London (for a broader view of Chinese horology).

J.H.Combridge, 1979, "Clockmaking in China", in The Country Life International Dictionary of Clocks, ed. Alan Smith, London.

DV