... a tower of three storeys each over ten feet in height, within which was concealed all the machinery. It was round at the top to symbolize the heavens and square at the bottom to symbolize the earth. Below there was set up the lower wheel, lower shaft, and the framework base. There were also horizontal wheels, vertical wheels fixed sideways, and slanting wheels; bearings for fixing them in place; a central stopping device and a smaller stopping device [i.e. the escapement] 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 n-dddle to indicate clearly the passing of the quarter-hours. Each day and night [i.e. each 24 hours] 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 double-hours, one after the other, bearing tablets indicating the time. The lengths of the days and nights were determined by the varying numbers of the quarters passing in light or darkness. At the upper part of the machinery there were the top piece, upper gear-wheels, upper stopping device [escapement], upper anti-recoil ratchet pin, celestial ladder gear case [possibly the first chain drive in history, or otherwise this was invented shortly afterwards by Su Sung for his clock], upper beam of the framework, and the upper connecting-rod. There were also on a celestial globe the 365 degrees to show the movements of the sun, rnoon, and five planets; as well as the Purple Palace [north polar region], the lunar mansions in their ranks and the Great Bear; together with the equator and 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 ... But ... as during the 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 the cold weather. Now, therefore, mercury was employed as a substitute, and there were no more errors . . .

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