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No. 1580:
Su-Sung's Clock

Today, Su-Sung's wonderful 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.

When sixteenth-century Jesuit missionaries went to China, they found timekeeping in a deplorable state. Not even sundials were reliable! And the clocks they brought as gifts were seen only as playthings. Timekeeping was hardly on China's radar screen.

Of course, the purpose of all ancient clocks was not so much the simple telling of time as it was display. Old clocks typically had bells and dials, and they displayed planetary motions.

In the West, water clocks had evolved from remote antiquity until mechanical clocks finally replaced them seven hundred years ago. The Greek name for a water clock was clepsydra. That means "a stealer of water" because all water clocks depended on a steady flow of water to meter time. Greco-Egyptian engineers of the second century BC had added feedback control to regulate the water flow. That idea was carried forward by Arab artisans until the Moors of medieval Spain were building the finest clocks in the West.

The Chinese had also built water clocks for millennia, but without feedback control. In Western water clocks, a float on the surface of a steadily draining tank drove the displays. But float indicators exerted scant force for driving extra machinery. The Chinese, on the other hand, created a new kind of water-wheel-driven clock during the eighth to eleventh centuries. A steady inflow filled buckets around the rim, one at a time. As each bucket became heavy enough to trip a mechanism, it fell forward carrying the bucket behind into place under the water spout. That water wheel provided power to drive displays of lunar cycles, the movements of the heavens, and time as well.

Those clocks reached their apogee when the emperor of the Sung dynasty charged an official, Su-Sung, with creating the grandest clock that'd ever been built. Su-Sung assembled a team and finished the clock by 1092. It was huge -- forty feet high.

The tick-tock motion of the falling buckets has caused some historians to call it a mechanical clock. But it had nothing resembling the inertial escapement that began turning European clocks into precision instruments by 1300. Neither did it have the feedback control of Arab water clocks.

Invading Tatars stole the clock when they ended the Sung dynasty in 1126. They couldn't get it running again, and the high art of Chinese clockmaking disappeared. Even before the Tatar invasion, Taoistic reformers had come into power and let the great clock fall into disrepair. When Jesuits eventually brought Su-Sung's book on clockmaking back to Europe, it astonished the West -- even though the escapement clock was then light-years beyond it.

Su-Sung's clock seems to've been pretty accurate. Whether it reached the fifteen-minute-a-day accuracy of the best Western water clocks, we don't know. But, for a time, the Chinese were ahead of the West once again, with the grandest clock in the world.

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 is a greatly reworked version of Episode 120.

See also the Wikipedia page on Su Song.

Details of a European clepsydra (or water clock) operation

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