Su Song
Zhang Sixun, although innovative, built upon the efforts of those before him. It was Han dynasty scientist and engineer Zhang Heng (78-139 AD) who invented the first hydraulic-powered (i.e. with waterwheel and clepsydra) armillary sphere. In addition, it was the Tang dynasty era Buddhist monk and engineer Yi Xing (683-727 AD) 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 (1020-1101 AD), 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.