A demonstration of the rotation of the Earth.
The principle of the Foucault pendulum: After one quarter of a rotation (6 hours) the pendulum appears to swing perpendicular to its inital direction at the pole, giving it a rotation of 15° per hour; at the equator the pendulum is always aligned in the same direction. At intermediate positions the plane of oscillation rotates at a rate of less than 15° per hour and decreases from the pole towards the equator, where it reaches zero (no rotation).
A public demonstration Foucault pendulum in St. Petersburg.
Illustrations: public domain
The Foucault pendulum at the Musée des Arts et Métiers in Paris. To demonstrate the rotation of the pendulum's plane pegs are placed on the circle and knocked off as the pendulum turns with time.
Photo: © Hervé Marchebois, GNU Free Documentation license (Wikipedia)