Seismometer

A seismometer is an instrument that responds to ground displacement and shaking caused by earthquakes, volcanic eruptions and explosions, typically paired with a timing and recording mechanism to produce a seismogram. The earliest known seismic detection device, the Houfeng Didong Yi, was built in Han dynasty China in the 2nd century AD by the mathematician and astronomer Zhang Heng; it was a seismoscope, a large bronze vessel ringed with dragon heads holding bronze balls that dropped to indicate the direction of a distant tremor, rather than a true recording instrument. The modern seismometer was developed in the 19th century, and the first horizontal pendulum seismometer, built by the team of John Milne, James Alfred Ewing and Thomas Gray while advising in Japan, recorded the first seismogram on 3 November 1880.