John Hook, age 9, of Williamsport, Pa., for his question: .

What is a seismograph?

This handsome word is borrowed from a very old word for earthquake. It belongs to the experts in earth science who make a special study of earthquakes. The seimo¬graph is a gadget made to check and measure all the shuddering shakes and quakes that happen in the earth.

Most earthquakes are so gentle that people do not notice them at all. But experts need to know about them because what is going on deep down in the earth's rocky crust may be important. Lots of little earth tremors may be a warning that a whopping big earthquake is getting ready to happen. Earthquake experts are called seismologists. And seismologists use instruments called seismographs to help them with their work. The best type of seismograph can report an earth tremor that is many, many times too weak for even a seismologist to notice.

There. are several different types of seismograph, and some of them are very big and bulky instruments. All of them are sensitive to the vibrating shudders that pierce through the ground when the earth shakes. To reveal its findings, the seismograph is fitted with a gadget which records these vibrations on paper. They look like a line that jogs up and down. The fierce vibrations from big earthquakes show high and low pointed peaks, very close together. The vibration picture is called a seismogram.

All seismographs are placed on massive slabs of granite embedded deep in the ground. The delicate measuring instruments are set on this solid platform. The finest type of seismogr4ph has a metal frame with a weight hanging at the end of a super¬sensitive spring. When the earth shakes, the frame shakes with it. But the weight tends to hang down in the same position without moving. The measuring job is done by comparing the movements of the metal frame with the non moving weight.

This comparison shows how much the earth shook. But we need to magnify the re¬port many times to detect slight shakes. Our super seismograph is fitted with a magnetic field and an assortment of electronic gadgets that make a slight shiver seem a million times bigger. A seismograph may specialize in measuring the vibrations down through the earth, or from east to west or from north to south. One of each type is kept at a seismology center. There are about 500 of these earthquake detecting centers around the world.

Seismographs are used to study more than earthquakes. The vibrations they record change as they pass through lighter and heavier materials. They have told earth scientists a lot about the hidden layers of material below the surface, right down to the very core of the earth. Seismographs also are used to test the thickness of ice fields and detect likely places to drill for oil. For these uses small dynamite explosions are set off to make vibrations, so the experts do not have to wait for a real earthquake.