In 132 CE the Han polymath Zhang Heng built the Houfeng Didong Yi, the first known seismoscope, a bronze vessel ringed by eight dragons each holding a ball over an open-mouthed toad. A pendulum mechanism inside released a single ball in the direction of a distant earth tremor, so officials could learn which province had been struck. In 138 it correctly registered a quake hundreds of kilometres to the west of the capital before any rider arrived with news. The original device is lost and known only through later texts and reconstructions.
It established that an instrument could detect and locate distant earthquakes, the conceptual ancestor of the seismology later used to assess seismic risk in city building.
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