130 ROTATION OF EAETH PROVED BY PENDULUM. 



being taken that the point of suspension be precisely in the sa 

 straight line with the vertical portion of the wire which passes 

 through the bar and forms the axis of rotation. The rotation of the 

 bullet will be better observed if one half of it be covered with black 

 varnish. If the pendulum is now started carefully, so as to make it 

 move as straight as possible, at least for some time, and the wire- 

 handle is turned, the thread and bullet will be observed to rotate, 

 while the plane of vibration is totally unaffected by the rotatio 

 The result will be the same, if the handle be turned in the opposi 

 direction. The experiment will always succeed if the precR.utio 

 indicated with reference to the point of suspension be attended to, 

 but it will fail if the latter through notbeing vertically below the wire 

 describes a circle, when the handle is turned. Still more beautiful 

 is the experiment if performed with the help of the centrifugal 

 apparatus described in the next article, by which the pendulum may 

 be made to rotate ten or twelve times in a second. 



This striking manifestation of the inertia of matter, 

 which, compels the pendulum to vibrate in the same 

 plane although its mass is rotating, has been applied b} ; 

 Foucault to prove the rotation of the earth by experi- 

 ment. 3 A, very Ipng pendulum, consisting of a wire frort 

 15 to 30 m long, and a bob of metal weighing from 10 1( 

 50 kgr , is suspended from the ceiling of a very lofty room 

 for instance, the interior of a church, in such a manne: 

 as to make it swing in the same arc, and to prevent al 

 lateral deflections. The earth rotates from west t< 

 east. The point of suspension rotates with the earth 

 Since the pendulum swings always in the same plane 

 the earth passes as it were underneath the swingin; 

 pendulum; but, as this motion cannot be observe<| 

 because we participate in it, we observe that the plan/ 

 of vibration of the swinging pendulum apparent! 

 rotates in the opposite direction ; namely, from eas' 

 through south, towards west. This rotation can onl 

 be observed after the lapse of some time; it is, then 



