SEISMOGRAPH

     A seismograph is used to measure the vibrations of the earth and to locate the source of earth vibrations. Phenomena capable of be­ing detected by a seismograph in­clude earthquakes, volcanic erup­tions, explosions such as bombs, powerful winds, violent storms at sea and even, in urban areas, traffic.

 Thanks to the seismograph whole populations can be warned and prepared for a variety of natural disasters. Seismic obser­vatories around the world, have provided information increasing our knowledge of the earth's structure.

     The seismograph operates on one of two basically simple mechanisms which are devices respon­sive to strain and pendulums. The earliest form of seismograph known was been used to measure the thick­ness of the ice sheets covering the polar regions of the earth and, when used in conjunction with artificial explosions created under special test conditions, seismographs can help geologists trace oil fields.