Water-Powered Armillary Sphere and Celestial Globe Tower
Su Song (original design); reconstruction by the museum (1020 to 1101) · original 1092; reconstruction 1993
A wooden tower three storeys high rises through the Human Cultures Hall, with a bronze armillary sphere on the roof, a celestial globe in the chamber below, and a pagoda of little doors on the front from which wooden figures emerge to strike bells and drums. This is the astronomical clock tower designed by the Song dynasty statesman Su Song and built in the capital Kaifeng in 1092. The original was lost within a few decades, but Su Song left a detailed illustrated treatise, and from it the museum built this full size working reconstruction, the only one of its kind anywhere in the world, when the hall opened in 1993.
The machine is driven by water. A steady stream fills small scoops on a great wheel, and an escapement, a device that releases the wheel one step at a time, lets it turn at a constant rate. That escapement is the heart of every mechanical clock, and Su Song's tower used it two centuries before clockwork appeared in Europe. The rotation drives the armillary sphere on the roof, which tracked the stars for observation, the celestial globe inside, which showed their positions, and the time telling figures in the front tower, which announced hours and night watches with placards, bells and drums.
Su Song was a scholar official, a pharmacologist and an ambassador as well as an engineer, and his tower was a state instrument for calendar making and imperial ceremony. The museum's reconstruction runs on a schedule so that visitors can watch the figures appear. A memorable detail is the chain drive that connects the water wheel to the sphere above, one of the earliest known uses of an endless power transmitting chain.
Did You Know? The tower's escapement let it keep time steadily two centuries before the first mechanical clocks in Europe, using nothing but flowing water.
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