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"The Gyroscope is an instrument constructed by M. Foucault to make the rotation of the earth visible. The principle on which it proceeds is this– that, unless gravity intervene, a rotating body will not alter the direction in which its permanent axis points. In the gyroscope there is a rotating metallic disk, the middle point of whose axis is also the center of gravity of the machine. By this device the action of gravity is eliminated."—(Charles Leonard-Stuart, 1911)

Gyroscope

"The Gyroscope is an instrument constructed by M. Foucault to make the rotation of the earth visible.…