USGS:

The Chinese philosopher Chang Heng created the first seismoscope in the year A.D. 132. This was a sizable urn with eight dragon heads facing the eight cardinal directions of the compass on its outside. A toad with its mouth open toward the dragon was located beneath each dragon's head. One or more of the eight dragon-mouths would spew a ball into the toad's open mouth when there was an earthquake. Which of the dragons launched its ball depended on the direction of the shaking. According to reports, the sensor picked up an earthquake that occurred 400 miles distant but wasn't felt near the seismoscope. Since the seismoscope's inside is unknown, most theories presume that there is motion of some sort

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