Zhang Heng presents the first seismoscope

Zhang Heng (78–139), a native of Nanyang, was one of the great scholars of the Eastern Han: an astronomer who catalogued some 2,500 stars and built a water-powered armillary sphere, a mathematician, an engineer and a poet. He twice served as Chief Astronomer (taishi ling), in 115–120 and 126–132. According to his biography in the Book of the Later Han, in the first year of the Yangjia era (132) he made the houfeng didong yi, an “instrument for observing the winds and the movements of the earth”. It was cast of fine bronze, eight chi (about two metres) across, with a domed lid, and was shaped like a wine vessel; eight dragons around it held bronze balls in their mouths, and below each sat a toad with its mouth open. When the earth shook, a mechanism inside, built around a central column, released one ball, which fell into a toad’s mouth and showed from which of eight directions the shock had come.

The biography tells that once a dragon dropped its ball although no tremor was felt in the capital, and the scholars of Luoyang doubted the device — until a few days later a courier brought news of an earthquake in Longxi, far to the northwest, in present-day Gansu. The source does not give the year of this incident; the English article on the seismometer suggests that it was the large earthquake in Gansu in 143, after Zhang Heng’s death. How the mechanism worked is unknown: Wang Zhenduo reconstructed it with a suspended pendulum in 1936 and with an inverted one in 1963, the Japanese seismologist Akitsune Imamura built another model in 1939, and modern replicas have not matched the accuracy and sensitivity that the chronicle describes.