102 M. Foucault on the Physical Demonstfalion 



publication of the circumstantial details of his beautiful ex- 

 periment, which M. Foucault has promised to communicate, 

 we shall endeavour to give our readers a summary idea of it, 

 from the extract which has appeared in the Comptes Rendus 

 de r Academic des Sciences, for February the 3d, 1851. M. 

 Foucault remarks, first, that the movement of translation of 

 the earth may be discarded, as it exerts no influence upon the 

 phenomenon in question ; he then supposes an observer to 

 be transported to the pole, and there to set up a pendulum of 

 the utmost simplicity, i.e., a pendulum composed of a heavy, 

 homogeneous, spherical mass, suspended by a flexible wire to 

 an absolutely fixed point ; he supposes, moreover, that this 

 point of suspension lies exactly in the prolongation of the 

 axis of rotation of the globe, and that the solid pieces which 

 support it do not participate in the diurnal motion. If, under 

 these circumstances, the mass of the pendulum be moved 

 from its position of equilibrium, and it be left simply to the 

 action of gravity, an oscillatory movement is produced in the 

 direction of an arc of a circle, the situation of which is dis> 

 tinctly defined, and to which the inertia of matter ensures an 

 invariable position in space. If, then, these oscillations con- 

 tinue during a certain length of time, the motion of the earth, 

 which incessantly turns from the west towards the east, will 

 become sensible by contrast with the immobility of the plane 

 of oscillation, the trace of which upon the ground will appear 

 excited by a motion conformable to the apparent motion of the 

 celestial sphere ; and if the oscillations were capable of con- 

 tinuing for twenty-four hours, the trace of their plane would 

 perform during the same period an entire revolution about 

 the vertical projection of the point of suspension. 



These are the ideal conditions under which the motion of 

 rotation of the globe would become immediately evident to 

 observation. But in reality we are obliged to take a point 

 of support upon a moving surface ; the rigid attachments of 

 the upper extremity of the wire of the pendulum cannot be 

 withdrawn from the influence of the diurnal motion, and it 

 appeared at first sight that the motion communicated to the 

 wire and to the mass of the pendulum would alter the direc- 

 tion of the plane of oscillation. But M. Foucault has suc- 

 ceeded theoretically in ascertaining what has since been 



