Zhang Heng (78-139) was quite the Han dynasty renaissance man. Despite his fame as a poet of considerable talents, a celebrated scholar of the classics, and an official serving at court, Zhang’s greatest and best-known contributions actually came in the field of science and engineering.
He is credited with several accomplishments including a key feature in the clepsydra water clock, improving the Chinese calendar based on his observations and insights into astronomy, and using hydraulic power to rotate an armillary sphere, but Zhang Heng is perhaps most famous for inventing the world’s first seismometer, the 候风地动仪 houfeng didongyi.
According to records from the Han Dynasty, the years 96-125 C.E. were among the most seismically active in China’s history. In that three-decade span, there were 23 major earthquakes in the Han Empire, including several destructive tremblers. Zhang designed his device to help the court detect earthquakes and know (roughly) where in the empire the tremors occurred.
The eminent historian of science in China, Joseph Needham, described Zhang’s seismometer this way:
“It consisted of a vessel of fine cast bronze, resembling a wine-jar, and having a diameter of eight chi (Ed. note: 1 chi/zhi=8 inches). It had a domed cover, and the