Su Song was the engineer for a hydro-mechanical astronomical clock tower in medieval Kaifeng, which employed the use of an early escapement mechanism. The escapement mechanism of Su's clock tower had been invented by Buddhist monk Yi Xing and government official Liang Lingzan in 725 AD to operate a water-powered armillary sphere, although Su's armillary sphere was the first to be provided with a mechanical clock drive. Su's clock tower also featured the oldest known endless power-transmitting chain drive, called the "tian ti" (), or "celestial ladder", as depicted in his horological treatise. The clock tower had 133 different clock jacks to indicate and sound the hours. Su Song's treatise about the clock tower, "Xinyi Xiangfayao" (), has survived since its written form in 1092 and official printed publication in 1094. The book has been analyzed by many historians, such as Joseph Needham. The clock itself, however, was dismantled by the invading Jurchen army in 1127 AD, and although attempts were made to reassemble it, the tower was never successfully reinstated.