Liz Cohen, age 14, of Utica, N.Y., for her question:

WHAT IS A CHRONOMETER?

A chronometer is a term used for any instrument that measures time. A watch, a clock and even a sundial can each be called a chronometer.

In a more specific sense, chronometer means a portable instrument made to measure time very accurately. In this classification, the sundial would have to go.

Chronometers are used for racing, by surveyors and by ships at sea.

Marine chronometers are so delicate that they are easily affected by outside influences. They must be kept away from the magnetic influence of compasses and the vibrations of the ship. Instruments of this type work best when they are horizontal.

Marine chronometers are hung in their cases on gimbals, which are devices that keep them horizontal whatever the ship's motion.