Heng's creation was a heavy bronze vessel with nine dragons facing downward embedded into its outside. The dragons were spaced equidistant from one another on the vessel, and below each dragon, a detached frog looked upward as each frog held its mouth open.
Inside the vessel, a pendulum hung motionless until a tremor moved it. At this point, the pendulum's swing set the seismograph's internal levers in motion. This would trigger the release of a ball held in the mouth of the dragon facing the direction of the earthquake's epicenter. The ball would then fall into the mouth of the frog directly below it [source: Xinhua News Agency]. This first seismograph seems a bit basic, but it would be another 1,500 years before Western nations developed their own versions
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