In 132 A.D., a Chinese inventor called Zhang Heng displayed his amazing earthquake-detection machine, or seismoscope, at the court of the Han Dynasty. Zhang's seismoscope was a giant bronze vessel, resembling a samovar almost 6 feet in diameter. Eight dragons snaked face-down along the outside of the barrel, marking the primary compass directions. In each dragon's mouth was a small bronze ball. Beneath the dragons sat eight bronze toads, with their broad mouths gaping to receive the balls.
The exact mechanism that caused a ball to drop in the event of an earthquake is not known. One theory is that a thin stick was set loosely down the center of the barrel. An earthquake would cause the stick to topple over in the direction of the seismic shock, triggering one of the dragons to open its mouth and release the bronze ball. The sound of the ball striking the toad's mouth would alert observers to the earthquake. This would give a rough indication of the earthquake's direction of origin, but it did not provide any information about the intensity of the tremors.
See http://asianhistory.about.com/od/asianinventions/a/SeismoInvention.htm
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