Zhang Heng invented the earliest known seismoscope for detecting distant earthquakes in ancient China.
Around 132 CE, Zhang Heng presented his device to the Han court. The instrument, called a houfeng didong yi, reportedly used a central pendulum or similar mechanism connected to eight dragon heads. When ground motion was detected, one dragon released a metal ball into a waiting toad’s mouth, indicating the direction of the disturbance.
The device did not record an earthquake’s magnitude, depth, or exact epicenter in the modern scientific sense. Its main purpose was to signal that an earthquake had occurred and suggest its direction, even when people near the instrument did not feel the shaking.
A common mix-up is calling Zhang Heng’s invention the first modern seismograph. Modern seismographs use continuously recording sensors and developed much later, but Zhang Heng’s seismoscope is an important early step in earthquake detection.