Zhang Heng
Although the astronomical armillary sphere (representing the celestial sphere) had existed in China since the 1st century BCE, the mathematician and court astronomer Zhang Heng (78–139 CE) provided it with motive power by using the constant pressure head of an inflow water clock to rotate a waterwheel that acted on a set of gears. Zhang Heng was also the first to address the problem of the falling pressure head in the inflow water clock (which gradually slowed the timekeeping) by setting up an additional tank between the reservoir and inflow vessel.