Raymond Lope, age 14, of Philadelphia, Penna., for his question:

 What is a seismograph?

This is the instrument which records distant earthquakes. Each earthquake sets vibrations jogging through the solid earth. Some of the vibrations pass down, down towards the heavy core of the earth and some pass around and around the crust. The nature of a vibration changes as it passes through denser material and earthquake vibrations tell us a great deal about the materials in the deep central core of the earth.

The instrument gets its information from a suspended weight. It is set in a dense rock or massive bed of concrete which cuts out the surface vibrations of, say, a passing truck. The sensitive instrument is set to record only the vibrations that occur deep in the earth.