history
Refers specifically to 候风地动仪 (Hòufēng dìdòngyí), built by Han astronomer Zhang Heng 张衡 (Zhāng Héng) in 132 CE. The bronze vessel had eight dragon heads holding balls; an earthquake would tip a ball into the matching toad's mouth below, showing the direction of the tremor - over 1,500 years before any Western equivalent.
