Jim Giddens, age 15, of Akron, Ohio, for his question:

WHAT IS A FOUCAULT PENDULUM?

A pendulum is an object that swings back and forth by gravity from a fixed point. A simple pendulum can be made by placing a weight on a hanging string. The path traveled by the weight is called the arc.

Some clocks use pendulums. A weighted rod is fastened to the clock's mechanism and it receives regular pushes to keep it swinging.

A Foucault pendulum is a device named for the French physicist and astronomer Jean Bernard Leon Foucault. It demonstrates visually the rotation of the earth.

Worldwide attention was attracted in 1851 when Foucault made his demonstration. From the dome of the Pantheon in Paris he fastened a 213 foot wire from which was fastened a 100 pound ball. The pendulum's bail, or bob, swings in one direction in space. But the rotation of the earth makes the pendulum's swing seem to change direction. The swing appears to change in a clockwise direction. The direction of the bob's swing actually stays the same but the rotation of the Earth causes the illusion of change.