Dolores Mendez, age 14, of Houston, Texas, for her question:

What is Foucault's pendulum?

In 1851, a large public gathering, including many celebrities, assembled in the lofty Pantheon building of Paris. The interested spectators were there to watch Jean Bernard Leon Foucault use a pendulum to prove that the earth rotates. Many were skeptical. But Foucault was certain his experiment would succeed because he had already tested and proved it in the basement of his home.
Way back in 395 B.C., Heraclitus taught his students that the earth rotated on its, axis. Aristotle and others, however, were sure that the starry heavens swung around' our little earth once every calendar day. Their viewpoint prevailed, and this misinformation was passed on for centuries. At last, in 1543, Copernicus suggested that the earth rotates daily on its axis and the Celestial Sphere around it is stationary. Thinking people began to re examine the long buried idea of Heraclitus, but most people were slow to accept it. Then in 1851, the French scientist Foucault proved it once and for all.
He used a pendulum and a simple explanation to demonstrate to a public gathering that the ground under their feet indeed was rotating with the spinning earth. The experiment depended upon no fancy figures and no reference points in the sky. The Foucault Pendulum made the earth itself prove its diurnal, or daily motion. Others could copy the experiment with equal success. And small, home made copies of the Foucault Pendulum will work success fully as long as the earth rotates.
Foucault gathered his audience under the great dome of the Paris Pantheon. A 200 food steel wire had been hung from a pivot point at the very top of the dome. At the other ends. of the wire was a 56 pound ball with a spike on the bottom. The bronze ball was suspended. above a circle of sand, just high enough to mark a furrow as it oscillated, swinging to and fro. Once it is set in motion, a freely suspended pendulum,.like this one, swings back and forth along a straight line. Even if the dome moved, it would not affect the direction of. the pendulum's swing, which is determined only by the force of gravity. The spike on the ball would continue to plow back and forth in the same furrow.
But if the earth were rotating, the floor would move with it. This would not make the freely suspended pendulum change its direction. But it would move the bed of sand below it and the spike would cut a different furrow with every swing. And so it did. It cut a curve of zig zagging furrows. Foucault's famous pendulum proved that the earth rotates and that it rotates towards the east. It was a grand experiment. But you can prove why it worked with a small gadget.
Cut a flat round board that you can slide around on a table. Attach two uprights supporting a horizontal bar. From the center of the bar, suspend a string weighted at the other end with a small ball, dust high enough to clear the base. Start your pendulum swinging and note the direction with markers in the room. Then gently turn the base and note that the pendulum still oscillates in the same direction. Now you know how Foucault used a pendulum to prove that the ground under our feet rotates with the rotating earth.

The Foucault experiment, however, presented a mystery to be solved later. Its furrows took 32 hours to complete a circle. When tested in London, the circle took 31 hours. Perhaps, then, 24 hours was nbt the earth's rotation time. Not at all. The earth rotates on its axis and different cities stand at different angles to this line through the globe. The equator is at right angles to the axis and here the pendulum can give a true 24 hour picture of the earth's rotation. Other latitudes show various distortions of this picture.