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. Zhang mentioned a "jade dragon's neck", which in later times meant a siphon. He wrote of the floats and indicator-rods of the inflow clepsydra as follows: Water-powered armillary sphere Zhang Heng is the first person known to have applied hydraulic motive power (i.e. by employing a waterwheel and clepsydra) to rotate an armillary sphere, an astronomical instrument representing the celestial sphere. The Greek astronomer Eratosthenes (276–194 BC) invented the first armillary sphere in 255 BC. The Chinese armillary sphere was fully developed by 52 BC, with the astronomer Geng Shouchang's addition of a permanently fixed equatorial ring. In AD 84 the astronomers Fu An and Jia Kui added the ecliptic ring, and finally Zhang Heng added the horizon and meridian rings. 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. His water-powered armillary influenced the design of later Chinese water clocks and led to the discovery of the escapement mechanism by the 8th century. The historian Joseph Needham (1900–1995) states: What were the factors leading to the first escapement clock in China? The chief tradition leading to Yi Xing (AD 725 ) 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. Zhang did not initiate the Chinese tradition of hydraulic engineering, which began during the mid Zhou Dynasty (c. 6th century BC), through the work of engineers such as Sunshu