Is it ensure, insure, or assure?
|Foucault pendulum |
A pendulum suspended from a long wire, set into motion, and sustained in motion over long periods. Due to the axial rotation of the earth, the plane of motion of the pendulum shifts at a rate and direction dependent on its latitude, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. At the poles the plane rotates once per day, while at the equator it does not rotate at all.