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No. 120:
Su-sung's Clock

Today, we visit a wonderful Chinese clock. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Water clocks were used from remote antiquity until mechanical clocks finally replaced them -- only about 700 years ago. You might know them by their Greek name, clepsydra, which means "a stealer of water." That's because all water clocks -- one way or another -- used a steady flow of water to measure time. Second-century Egyptian engineers added a clever kind of self-regulation to water clocks. Their version was picked up by Arab artisans; and the Moors of medieval Spain finally brought it to a very high state of development in the West.

The Chinese, of course, had their own version of the water clock, and the Sung dynasty improved on it during the 11th century. Finally, in 1086, the emperor charged an official named Su-Sung to create what was to be the finest water clock up to that time. Su-sung put together a team that finished the clock 8 years later.

Most ancient clocks tried to do a lot more than just tell people how much time had passed. They tended to have bells and dials and displays of planetary motions. But it was hard for water clocks to do all these things, because a float indicator riding on a water surface didn't exert enough force to drive a lot of extra machinery.

Su-Sung got around that problem. His huge clock stood 40 feet high and was powered by a special water wheel. Buckets around its rim were filled, one at a time, by a steady flow of water. When each bucket was heavy enough to trip a mechanism, it fell forward -- carrying the bucket behind it into place under the water spout. And the process repeated. The weight of the buckets exerted enough force to activate all sorts of displays. Su-Sung's wonderful clock, with its tick-tock motion, was quite accurate. It looked a little like the mechanical clock which wasn't invented for another 200 years in Europe.

Su-Sung's clock was stolen when invading Tatars put an end to the Sung dynasty in 1126. The Tatars weren't able to get it running again, and the high art of Chinese clock-making completely disappeared. But even before the Tatar invasion, Taoistic reformers had come into power. They saw fancy clock-building as part of the older regime and did little to sustain it. Su-Sung's book on the operation of his clock didn't surface in the West until the 17th century. By then, of course, the Western mechanical clock was light-years ahead of it.

But the West didn't always hold a monopoly on time-keeping. It's sobering to know that the state of the art in the 11th century was, in fact, Su-Sung's marvelous water clock.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Temple, R., The Genius of China. New York: Simon & Schuster Inc., 1986, pp. 103-110.

This episode has been greatly revised as Episode 1580.


(From the 1832 Edinburgh Encyclopaedia)
Details of a European clepsydra (or water clock) operation