The first seismoscope invented by Zhang Heng was presented in AD 132. Created during China’s Eastern Han dynasty, the device was called the Houfeng Didong Yi, often translated as “instrument for measuring seasonal winds and the movements of the Earth.”
The seismoscope was a large bronze vessel decorated with dragons and frogs. According to historical descriptions, an internal mechanism released a bronze ball from one of several dragon mouths when distant ground movement was detected. The ball fell into the corresponding frog’s mouth, indicating the earthquake’s direction.
Unlike a modern seismometer, Zhang Heng’s instrument did not record the strength or duration of an earthquake, and surviving accounts do not provide enough detail to reconstruct its mechanism with certainty. It was designed primarily to alert officials in the Han capital, Luoyang, that an earthquake had occurred elsewhere.
A common mix-up is calling it the first seismometer. “Seismoscope” is more precise: it indicated that an earthquake had happened and reportedly showed its direction, while modern seismometers measure and record seismic waves.