Today, an earth-shaken invention -- far older than we'd think. 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.

We face an odd problem when we look for the origins of our cleverest machines. So many were invented in China and never plowed into any mainstream of knowledge. So much spectacular human ingenuity was poured into the various Chinese royal courts and then forgotten. That's the way it was with seismography.

"China has always been plagued with earthquakes." observes historian Robert Temple. Some of them, like the great earthquake of 1556, were just awful. It killed 800,000 people. Down through the millennia, the Chinese have left very good records of those disasters.

So it was that Chang Heng, astronomer royal to the Han Dynasty, invented an accurate seismograph in AD 132 -- 1600 years before anyone in the West did. It was a large bronze urn with eight dragon heads gazing outward in eight directions. Each dragon held a ball in his mouth. Around the base of the urn, under each dragon, sat a frog with his mouth open.

A delicate inverted pendulum was hidden in the urn. The slightest seismic ripple moved it. The swinging pendulum tapped a mechanism that dislodged one of the balls. The ball fell from the mouth of the dragon into the mouth of the frog below. It landed with a great clang that announced the earthquake. Knowing which frog had been fed, you could tell the direction of the quake.

Members of the court thought the device had failed when the alarm sounded one day and they felt nothing. Doubt turned to astonishment a few days later. A messenger arrived from a town 400 miles away to report that it'd been savaged by an earthquake.

Chinese writings talked about Chang Heng's seismograph and ones like it until the Mongols overran China. After that, it vanished as though it had never been. The next seismograph was invented in France in 1703. But seismography really started up again -- and with pendulum devices -- only 130 years ago.

Chinese seismography was no flash in the pan from a one-time inventor. Chang Heng was brilliant. He described a round earth in infinite space. He invented lines of longitude and latitude. The Chinese called that, "throwing a net over the Earth."

And why is such brilliance so badly remembered? It's probably because people like Chang Heng were tied so tightly to their patrons. They didn't belong to the same loose, open communities of freely moving scholars that have diffused knowledge so effectively in the West.

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

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Temple, R., The Genius of China. New York: Simon & Schuster, 1986, pp. 162-166.