Zhang Sixun, although innovative, built upon the efforts of those before him. It was Han dynasty scientist and engineer Zhang Heng who invented the first hydraulic-powered armillary sphere. In addition, it was the Tang dynasty era Buddhistmonk and engineer Yi Xing who invented the first hydraulic-powered armillary sphere that incorporated the escapement mechanism. Yet Zhang Sixun applied some innovative ideas of his own in order for his hydraulic-powered armillary sphere to function. His astronomical armillary sphere and clock was much like that of the later statesman Su Song, incorporating the scoop-bearing driving-wheel and gearing, together with 19 clock jacks to report and sound the hours. His device also employed the use of liquid mercury in the closed circuit of the clepsydra and waterwheel instead of water, because water would freeze easily during winter, while mercury could assure smooth and continual function and time-keeping during the cold season. Later Ming dynasty clocks had the same concern in mind when they employed the use of falling sand grains to push the wheel drive. The later Su Song wrote that after Zhang's death, no one could replicate what he had achieved, much like with Su Song himself and his astronomical clock tower after his own death.
Historical texts
The later Song Dynasty historical text of the Song Shi records Zhang's work :
At the beginning of the Thai-Phing Hsing-Kuo reign-period the Szechuanese Chang Ssu-Hsun , a student in the Bureau of Astronomy, invented an astronomical clock and presented the designs to the emperor Thai Tsung, who ordered artisans of the Imperial Workshops to construct it within the Palace. On a kuei-mao day in the first month of the 4th year the elaborate machine was completed, and the emperor caused it to be placed under the eastern drum-tower of the Wen-Ming Hall.
The system of Chang Ssu-Hsun was as follows: they built a tower of three storeys more than ten feet in height, within which was concealed all the machinery. It was round the heavens, and square the earth. Below there was set up the lower wheel, lower shaft, and the framework base. There were also horizontal wheels, wheels fixed sideways, and slanting wheels ; bearings for fixing them in place, a central coupling device and a smaller coupling device ; with a main transmission shaft. Seven jacks rang bells on the left, struck a large bell on the right, and beat a drum in the middle to indicate clearly the passing of the quarter.
Each day and night the machinery made one complete revolution, and the seven luminaries moved their positions around the ecliptic. Twelve other wooden jacks were also made to come out at each of the hours, one after the other, bearing tablets indicating the time. The lengths of the days and nights were determined by the numbers of the quarters. At the upper part of the machinery there were the top piece, upper gear, upper linking device, upper ratchet pin, celestial gear-box, upper framework beam carrying bearings, and the upper connecting-chain. There were also the 365 degrees the sun, moon, and five planets; as well as the Purple Palace, the lunar mansions in their ranks, and the Great Bear, together with the equator and the ecliptic which indicated how the changes of the advance and regression of heat and cold depend upon the measured motions of the sun.
The motive power of the clock was water, according to the method which had come down from Chang Heng in the Han Dynasty through I-Hsing and Liang Ling-tsan in the Khai-Yuan reign period. But the bronze and iron had long gone to rust and could no longer move automatically. Moreover, as during winter the water partly froze and its flow was greatly reduced, the machinery lost its exactness, and there was no constancy between the hot and cold weather. Now, therefore, mercury was employed as a substitute, and there were no more errors...The images of the sun and moon were also attached high up and according to the old method they had been moved by human hand, but now success was attained in having them move automatically. This was a marvellous thing. Ssu-Hsun was considered the equal of the Thang clock-makers and was made Special Assistant in charge of the Armillary Sphere.
