Seismographs are instruments used to measure and record ground motion, or seismic waves, caused by explosions and earthquakes. In the late 1800s, John Milne, an English mining engineer, developed the first precise seismometer, the sensor in a seismograph that detects and measures motion. Since then, seismograms, the data recorded by a seismograph, have helped seismologists predict much more than ground movement. These devices have also led to discoveries about the nature of the Earth's interior.

The process of using a tool to detect ground motion dates back to the ancient Han Dynasty when Chang Heng (78-139 A.D.), a Chinese astronomer and mathematician, invented the first seismometer in 132 A.D. Although the internal mechanics of this device are unknown, it probably used a pendulum that, when moved during an earthquake, would dislodge a bronze ball from the mouth of one of eight ornamental dragon's heads that adorned the...