Chinese government official Su Song (1020-1101) was also a naturalist, cartographer, astronomer, horologist, and engineer. His greatest legacy was the clock tower he built in Kaifeng. In 1086 the emperor had ordered the construction of an "armillary clock" to keep time and track celestial bodies. A finished structure was completed in 1094, and consisted of three levels. The upper level contained a rotating armillary sphere that allowed astrological observations through sighting tubes; the middle level had a bronze celestial globe; and the lower level had mechanically timed manikins that would exit doors at fixed times of the day.
Perhaps most significant, however, was the clock's innovatory driving system. At the heart of the clock tower was the tian ti, or "celestial ladder." This is the oldest known endless power-transmitting chain drive. The chain transmitted the power from a water wheel to turn the armillary sphere and power the clock.
Drive belts had been present in China for approximately 1,000 years before Song's chain drive. However, such belts were primitive', haphazard affairs and lacked the precision necessary to drive a clock and armillary sphere. The chain links fitted sprockets and were not subject to stretching or slippage. Chain drive is now used in a wide range of mechanical devices, especially in vehicles such as bicycles, but it was not until several centuries after Song's innovation that Europeans independently discovered the technology.
The clock was captured and dismantled by invading Jurchens from Manchuria who overran Kaifeng in 1127. It is one of the great lost artifacts of the medieval age. However, Song's 1092 treatise (Essentials of a New Method for Mechanizing the Rotation of an Armillary Sphere and a Celestial Globe) survived, with illustrations and descriptions of the clock.
