Though all the parts of the Earth complete their revolution in the same space of time, it is found that the rate of horizontal motion in Foucault’s pendulum varies with the latitude of the place where the experiment is made. At the pole, the pendulum would pass over 15 degrees in an hour, like the Earth itself, and complete its circuit in 24 hours. At Edinburgh, the pendulum would pass over 12¹⁄₂ degrees in an hour, and would complete its revolution in 29 hours 7 minutes. At Paris, the rate of motion is 11 degrees and 20 minutes per hour, and the revolution should be completed in 32 hours. [Illustration: FIG. 31.] Let the above figure represent a portion of the Earth’s surface near the north pole N. Suppose the pendulum to be set in motion at _m_, so as to vibrate in the direction _x y_, which coincides with that of the meridian _m_ N or _m r_. The Earth in the meantime is pursuing its easterly course, and the meridian line _m_ N has come in six hours into the position _n_ N. It has been hitherto supposed that the pendulum would now vibrate in the new direction _n_ N, assumed by the meridian, but thanks to M. Foucault, we now know that this is a mistake. The pendulum will vibrate in a plane _x n y_, parallel to its original plane at _m_, as will be manifest if the plane of vibration points to some object in absolute space, such as a star. While the meridian line _m_ N will in the course of 24 hours range round the whole circle of the heavens, and point successively in the direction _n_ N, _o_ N, _p_ N, _r_ N, _s_ N, _t_ N, and _u_ N, the pendulum’s plane of vibration _x y_, whether at _m_, at _n_, at _o_, at _p_, at _r_, at _s_, at _t_, or at _u_, will always be parallel to itself, pointing invariably to the same star, and were a circular table placed under the pendulum, its plane of vibration, while really stationary, would appear to perform a complete revolution.