Su Song completes his astronomical clock tower

In 1092 the Song dynasty scholar-official Su Song finished a 12-metre-tall water-driven clock tower at the capital Kaifeng. It combined a huge armillary sphere, a celestial globe and a system of figures that appeared at doors to ring bells and show the time. Its heart was an escapement, a mechanism that released the power of the water wheel in measured steps, which makes it a forerunner of the mechanical clock. Su Song described the design, with drawings, in his book Xinyi Xiangfa Yao. The tower was dismantled by Jurchen invaders in 1127.

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