Translated for the Journal of the Franklin Institute
Physical Demonstration of the Rotation of the Earth by Means of the Pendulum
By M.L. Foucault
The very numerous and important observations which have hitherto been made upon the pendulum, are especially relative to the time of its oscillations; those which I propose to relate to the Academy, have reference principally to the direction of the plane of oscillation, which being gradually displaced from east to west, gives a sensible proof of the diurnal motion of the terrestrial globe.
In order to succeed in justifying this interpretation of a constant result, I will neglect the earth's movement of translation, which is without effect upon the phenomenon which I wish to exhibit, and I will suppose the observer to have established at the pole a pendulum of the greatest simplicity: that is, a compound pendulum composed of a heavy, homogeneous, and spherical mass, suspended by a flexible thread from a point absolutely fixed. I will, moreover, suppose at first, that this point of suspension is exactly in the prolongation of the axis of rotation of the globe, and the solid masses which support it do not participate in the diurnal movement. If, under these circumstances, the mass of the pendulum is drawn aside from its position of equilibrium, and abandoned to the action of gravity without having any lateral impulse given to it, its centre of gravity will pass through the vertical, and by its acquired velocity will rise upon the other side of the vertical to a height nearly equal to that whence it came. Arrived at this point, its velocity dies out, changes its sign, and brings it back, causing it to pass again through the vertical to a point a little below its starting point. Thus a movement of oscillation is excited in an arc of a circle whose plane is clearly determined, to which the inertia of the mass gives an invariable posistion in space. If then these oscillations continue for a certain time, the motion of the earth, which does not cease turning from west to east, will become sensible by contrast with the immobility of the plane of oscillation, whose trace upon the ground will appear to have a motion comfortable to the apparent motion of the heavenly sphere; and if the oscillations could be continued for twenty-four hours, the trace of their plane would have executed in that time a complete revolution around the vertical projection of the point of suspension.