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 LXXXIV. Intelligence and Miscellaneous Articles. 



PHYSICAL DEMONSTRATION OF THE EARTH's MOTION OF ROTA- 

 TION, BY MEANS OF THE PENDULUM. BY M. L. FOUCAULT. 



T 



truths which appear too incontestable for any one to venture to 

 call in question. Notwithstanding this, we have indirect proofs only of 

 its existence ; some of these are derived from the apparent movement 

 of the sun and of the vault of the heavens, others from the existence of 

 centrifugal force, and others from the flattened form of the terrestrial 

 globe at the poles, &c. To these proofs, M. Leon Foucault has just 

 added a new direct one, of such kind as to convince the most incre- 

 dulous, if any such still exist ; for he has succeeded in rendering the 

 rotation of the earth as evident to the sight as that of a spinning-top. 

 In anticipation of our publication of the circumstantial details of his 

 beautiful experiment, which M. Foucault has promised to commu- 

 nicate, w'e shall endeavour to give our readers a summary idea of it, 

 from the extract which has appeared in the Comptes Rendus de V Aca- 

 demic des Sciences for Fehruary the 3rd, 1851. M. Foucault remarks 

 first, that the movement of translation of the earth may be discarded, 

 as it exerts no influence upon the phsenomenon in question ; he thea 

 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 distinctly defined, and to which the inertia of matter ensures 

 an invariable position in space. If, then, these oscillations continue 

 during a certain length of time, the motion of the earth, which in- 

 cessantly 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 conform- 

 able to the apparent motion of the celestial sphere ; and if the oscil- 

 lations were capable of continuing for twenty- four hours, the trace 

 of their plane would jjcrform during the same period an entire revo- 

 lution 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 sup])ort upon a moving 

 surface ; the rigid attachments of the upjjcr extremity of the wire of 

 the pendulum cannot be withdrawn from the influence of the diurnal 

 motion, and it apj)ears at first sight, that the motion couununicated 

 to the wire and to the mass of the pendulum \\ouUl alter the direc- 

 tion of the plane of oscillation. But M. Foucault has succeeded 



