of the Earth's Motion of Rotation. 103 



confirmed by experiment, that provided the wire of the 

 pendulum be round and homogeneous, 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 de- 

 scribed would perfectly succeed at the pole. This remarkable 

 independence of the plane of oscillation and of the point of 

 suspension, is a mechanical phenomenon dependent upon the 

 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 posi- 

 tion 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 phenomenon which is in 

 its greatest simplicity at the pole becomes complicated, al- 

 though continuing to exist on descending towards our lati- 

 tudes. 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 

 hemisphere ; 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 



