In 1088 a Chinese civil servant finished building a tower about ten metres high in the imperial capital of Kaifeng. Inside it, a water wheel turned one step at a time, released by a mechanism that let through exactly enough water to fill a bucket and no more. The wheel drove three things at once: a bronze armillary sphere on the roof that tracked the sky, a celestial globe inside the building that showed the positions of the stars, and five tiers of small wooden figures that appeared at openings to strike bells and drums and hold up tablets announcing the hour.
Nothing comparable existed in Europe for another two hundred years. The man who built it, Su Song, considered it a minor part of his career.
Why an Emperor Pays for a Clock
The tower was not a public convenience. Chinese imperial astronomy was a political instrument, and time keeping was part of the mandate to rule.
The court needed an accurate calendar because the calendar was issued by the emperor and its correctness reflected on him. It needed predictions of eclipses, because an unpredicted eclipse was a bad omen and a predicted one was a demonstration of command. And it needed precise records of the positions of the planets at the moment of an imperial birth, because those records fed the astrological judgements that surrounded marriages and successions.
Su Song was commissioned in 1086 after a dispute in which two competing astronomical bureaus produced different times for an event, which was embarrassing. He was a bureaucrat of the classic Song type: an administrator, a diplomat, a pharmacologist who compiled a major work on materia medica, a poet, and an antiquarian, who happened also to be a superb engineer.
The Escapement
The central problem in any clock is the same. Some source of energy wants to run continuously, and time keeping requires it to advance in equal, counted steps. The device that converts one into the other is an escapement, and it is the thing that separates a clock from a mill.
Su Song’s solution works by weighing water. Each of the wheel’s buckets fills from a constant level tank. While a bucket is light, a locking lever holds the wheel. When the bucket reaches a set weight, it tips a balance lever, the lock is released, the wheel turns by exactly one bucket, and the next lock engages. The interval between steps is set by the flow rate, and the flow rate is held steady by the constant level tank.
The result advances in discrete, equal jumps. It is a genuine escapement, and it predates the European verge and foliot by roughly two centuries.
The design was not invented from nothing. A Buddhist monk named Yi Xing and an official named Liang Lingzan had built a water driven armillary sphere with a linkwork escapement in 725, and Zhang Sixun built another in 979 that used mercury instead of water so it would not freeze in winter. Su Song’s contribution was to gather the tradition, improve it, and, crucially, write it down.
The Chain
The tower contains one further piece of engineering that historians of technology single out. To drive the armillary sphere on the roof from the wheel below, Su Song used an endless chain drive, a loop of linked metal plates running over toothed wheels, which he called the celestial ladder.
It is the earliest known chain drive used to transmit power. The same arrangement, in essence, drives a bicycle.
The Book That Saved It
In 1092 Su Song presented the emperor with a treatise, the Xin Yi Xiang Fa Yao, explaining the machine. It contains around sixty illustrations, including exploded views of gear trains and detailed drawings of individual components, and it is among the earliest surviving technical manuals of its kind anywhere.
It also contains five star charts mapping about 1,460 stars, drawn in two different ways: polar projections for the regions near the celestial pole, and a cylindrical presentation for the equatorial band, in which the sky is unrolled into a rectangle. That second construction is the same basic idea, arrived at five centuries earlier, that Mercator used for the Earth in 1569, and it is used for the same reason: a rectangle is easier to draw, print and read than a sphere.
The star positions themselves were catalogued with the instruments in the tower, in a tradition running back to Hipparchus in the second century BC and independent of it. Chinese astronomers used equatorial coordinates, measuring position around the celestial equator, where the Greek tradition used the ecliptic. The equatorial system is the one modern astronomy uses.
What Happened to the Tower
It ran for about thirty five years. In 1127 the Jurchen armies of the Jin dynasty took Kaifeng, ending the Northern Song, and the clock tower was dismantled and carted north to the new capital at what is now Beijing.
It never worked again. The components were there, and nobody could get them back into adjustment. Su Song’s son was asked to rebuild it and could not manage it either, which tells you something about the difference between a drawing and a working machine.
The knowledge did not entirely vanish, but the tradition of monumental astronomical clocks in China faded over the following centuries, and later Chinese astronomy took a different direction. What survived was the book.
The Question Everyone Asks
Whether Su Song’s escapement travelled to Europe is unresolved and probably unresolvable. The first European mechanical clocks appear in the 1280s and 1290s, and they use a completely different escapement, a verge and foliot driven by falling weights, with no water anywhere in the mechanism.
Joseph Needham, whose vast Science and Civilisation in China made Su Song known outside sinology, thought some transmission of the idea was likely along the routes that carried other Chinese technologies west. Others consider independent invention more plausible, given how different the two mechanisms are.
The more useful observation is about what happened afterwards rather than what came first. The European clock was built by towns, guilds and churches, spread commercially, and was improved continuously by competing makers for six centuries, until Huygens applied the pendulum in 1656 and cut the daily error from minutes to seconds. Su Song’s tower was built once, for one emperor, in one capital, and when that capital fell there was nobody whose living depended on rebuilding it.
The difference is not in the engineering. It is in whether anyone other than the state had a reason to keep the thing running.
The Reconstruction
Because the treatise survived, the tower can be rebuilt, and it has been several times. A full scale working reconstruction stands in Taichung, Taiwan, and a large model is in the National Museum of Chinese History. They run.
Su Song’s own summary of the project, written for the emperor, is characteristic of the man and of the civil service he belonged to. He described the machine as an instrument for verifying the calendar, listed the officials who had worked on it, and asked that it be checked regularly against observation, since an instrument that is not compared with the sky will drift without anyone noticing.
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