104 M. Foucault on the Physical Demonstration 



made, is a purely geometrical phenomenon, the explanation 

 of which can be given by simple geometry, as has been done 

 by M. Foucault. This was remarked by M. Poinsot at the 

 meeting of the Academy on the 25th of February, on suggest- 

 ing, in support of his opinion, a new experiment to be made 

 by M. Foucault. 



"We shall now shew the manner in which M. Foucault has 

 proceeded to determine the import and probable magnitude 

 of the reality of the phenomenon which he had so well anti- 

 cipated. We shall borrow the description of his experiment 

 from the extract which he has given of it in the Comptes 

 Rendus of the Academy. 



" In the vaulted roof a cellar, a strong piece of cast iron 

 was firmly imbedded to afford a support to the point of sus- 

 pension, which emanates from the centre of a small mass of 

 tempered steel, the free surface of which is perfectly hori- 

 zontal. The suspending wire consists of steel strongly 

 hardened by the action of the draw-plate ; its diameter varies 

 from j^ths to -^^ths of a millimeter ; it extends to the length 

 of two metres, and to its lower end is attached a sphere of 

 brass turned and polished, and which, moreover, was ham- 

 mered so that its centre of gravity should coincide with its 

 centre of form. This sphere weighed five kilogrammes, and 

 a sharp prolongation was fixed to it, apparently forming a 

 continuation of the suspending wire. 



" When it is wished to make the experiment, the first thing 

 to be done is to put an end to the torsion of the wire, and 

 the rotatory oscillations of the sphere. Then, for the purpose 

 of displacing it from its position of equilibrum, it is inclosed 

 in a noose of silk thread, the free extremity of which is at- 

 tached to some fixed point in the wall, at a small height above 

 the ground. According to the length given to this thread, 

 the displacement of the pendulum and the magnitude of the 

 oscillations which it maybe wished to communicate to it, are 

 arranged arbitrarily. In general, in my experiments, the 

 oscillations at the beginning comprised an arc of from fifteen 

 to twenty degrees. Before proceeding further, it is requisite 

 to deaden by some obstacle gradually withdrawn, the oscil- 

 lating motion still exercised by the pendulum while restrained 

 by the thread and suspending wire. As soon as the pendu- 



