From AMS Glossary
An instrument used to measure and record earthquake vibrations and other earth tremors.
It consists essentially of a heavy pendulum hung so that it can swing freely in one direction. A pen is attached to the end of the pendulum so that the pendulum's motion, relative to a recorder fixed solidly to the earth, may be determined. A seismological observatory requires three seismographs in order to measure the three component directions of the tremors and to fix approximately their origin. A study of the records of sensitive seismographs has shown that some of the tremors may be attributed to oceanographic and atmospheric phenomena.