Page:Popular Astronomy - Airy - 1881.djvu/291



In the year 1851, a method of rendering certain effects of the earth's rotation visible to the eye was made known by M. Foucault, who had been led to discover it by considering how the rotation of the earth ought to affect the apparent motion of a pendulum vibrating freely at the earth's surface.

If a heavy body, as for example, a sphere of metal be suspended by a string from a point A, Figure 66, vertically above N, the North Pole of the earth, and allowed to hang freely, the motion of the earth about its axis ANS will twist the string, and so cause the sphere to rotate about its vertical diameter. If, now, the sphere be drawn aside to a point B and allowed to drop gently, it will begin to vibrate in the plane NAB, and as the rotation communicated to the sphere does not tend to withdraw it from that plane, it will continue constantly to move in it. A spectator near N, partaking of the earth's motion, changes his position with reference to this fixed plane: but being unconscious that he is moving himself, he attributes to the fixed plane a motion exactly similar