An instrument used to record seismic waves caused by earthquakes,
nuclear explosions, etc., and in prospecting; the record it produces is
called a seismogram. In its simplest form, it contains
a horizontal bar, pivoted at one end and with a recording pen at the other.
The bar, supported by a spring, bears a heavy weight. As the ground moves,
the bar remains roughly stationary owing to the inertia
of the weight, while the rest of the equipment moves. The pen traces the
vibrations on a moving belt of paper. Most modern seismographs are based
on the type developed by John Milne and
his colleagues in the 1880s.