576 Intelligence and Miscellaneous Articles. 



theoretically in ascertaining what has since been confirmed by expe- 

 riment, that provided the wire of the pendulum be round and homo- 

 geneous, it may be made to turn round upon itself with tolerable 

 rapidity in either direction, without sensibly influencing the position 

 of the plane of oscillation, so that the experiment which we have just 

 described would perfectly succeed at the pole. This remarkable in- 

 dependence of the plane of oscillation and of the point of suspension 

 is a mechanical phsenomenon dependent upon tlie inertia of matter, 

 which may be rendered evident in another form, by means of a very 

 simple experiment which led M. Foucault to the discovery. After 

 having fixed upon the arbor of a lathe and in the direction of its axis, 

 a round and flexible rod of steel, he set it in vibration by moving it 

 from its position of equilibrium and leaving it to itself. He thus 

 produced a plane of oscillation, which by the persistence of the visual 

 impressions, was clearly delineated in space ; and he remarked that 

 on turning round with the hand the arbor which formed the support 

 of this vibrating rod, the plane of oscillation was not disturbed, but 

 always retained the same direction in space. 



Returning to the pendulum, the ph?enomenon which is in its 

 greatest simplicity at the pole, becomes complicated, although con- 

 tinuing to exist, on descending towards our latitudes. In fact, in 

 proportion as we approach the equator, the plane of the horizon, 

 which at the pole was perpendicular to the axis of the earth, becomes 

 more and more oblique to it ; and the plumb-line, instead of turning 

 upon itself, describes a more and more open cone, the summit of 

 which is at the centre of the earth. The consequence is a retardation 

 of the apparent motion of the plane of oscillation, which vanishes at 

 the equator, previous to changing its direction in the other hemi- 

 sphere ; in fact, the angular displacement of the plane of oscillation is 

 equal to the angular motion of the earth in the same time multiplied by 

 the sine of the latitude. This motion of the plane of oscillation of a 

 simple pendulum, whereby it appears to turn around the vertical 

 line in the same direction as the stars, and which would cause it to 

 complete an entire revolution in twenty-four hours at the pole, and 

 a fraction of this revolution proportional to the sine of the latitude 

 of the plane where the experiment is made, is a purely geometrical 

 phsenomenon, the explanation of which can be given by simple geo- 

 metry, as has lieen done by M. Foucault. This was remarked by 

 M. Poinsot, at the meeting of the Academy on the 25th of Feljruarj^ 

 on suggesting in support of his opinion a new experiment to be 

 made by M. Foucault. 



We shall now show the manner in which M. Foucault has pro- 

 ceeded to determine the import and probable magnitude of the reality 

 of the phsenomenon which he had so well anticipated. 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 of a cellar, a strong piece of cast iron was 

 firmly imbedded to afford a support to the point of suspension which 

 emanates from the centre of a small mass of tempered steel, the free 

 surface of which is perfectly horizontal. The suspending wire con- 



