Mary Ellen Sullivan, age 12, of Astoria, Long Island, N.Y. for her question;

What is a seismograph?

A seismograph records earthquake vibrations. Its base is a mass of concrete set deep in solid rock. When an earthquake trembles through the earth's crust, the concrete vibrates. A revolving drum and a sturdy post are fixed on top of the concrete and they vibrate also. A heavy lump of metal is suspended from the post. When the rest of the seismograph vibrates with an earthquake, the hanging metal remains steady.

The steady metal aims a pencil at the drum and writes a continuous line. An earthquake shakes the concrete and drum and the pencil jerks up and down. The jagged line is a record of the earthquake's vibrations.