20S PROCEEDINGS OF THE AMERICAN ACADEMY 



a hinge, working on a horizontal point, which allows the ring contain- 

 ing the disc to move in its own plane. This motion can, however, be 

 prevented by a revolving slide underneath. In some experiments the 

 slide is placed so as to prevent the motion on the hinge, and the arms 

 are balanced upon a fixed and pointed rod which is pushed into the 

 brass column. For this purpose there is a little cap under i, and a 

 counterpoise which slides on the opposite arm to balance the disc. 

 The top has less friction than Bohnenberger's or Fessel's apparatus. 

 Also in Fessel's machine the disturbing force is the whole weight of 

 the disc and ring, and not, as in Bohnenberger's machine, simply an 

 excess of weight on one side of the rotating body. Hence the pre- 

 cession is more rapid in the first than in the last. 



" If the disc is not rotating, it naturally drops down upon the hinge 

 7«, from its own weight. 



" But when the disc is made to rotate rapidly by means of the 

 thread, and then left free, it seems inditferent to gravity, and, instead of 

 dropping, it begins to revolve about the vertical axis. So that the axis 

 of the disc acquires a motion similar to the Precession of the Equi' 

 noxes in Astronomy. The motion of revolution is opposite in direction 

 to the rotation of the disc. When one of these motions is the great- 

 est, the other is the least. If the motion of revolution is increased 

 artificially, the disc appears lighter. If this motion is retarded, the 

 disc appears heavier. Reciprocally, if the gravity of the disc is arti- 

 ficially increased, the motion of revolution is greater. If the gravity 

 of the disc is artificially diminished, the motion of revolution is less. 

 This variation in the gravity of the disc is easily effected upon an 

 iron disc by means of a magnet. If the action of gravity is prevented 

 by the slide which confines the hinge, there is no motion round the 

 horizontal axis. 



" The following popular explanation is given of these peculiari- 

 ties of motion.* Place the disc in a vertical plane and make it 

 rotate. The tangential motion of each particle has a horizontal and 

 vertical component. As soon as the disc begins from its weight to 

 incline from its original vertical position, the horizontal components 

 still remain parallel to the new position, but the vertical components 

 do not. If the upright edge of the disc nearest to the eye is ascend- 

 ing, this edge is pushed to the left and the opposite edge to the right. 

 These two forces, resulting out of the deviation of the original vertical 



* Ann. Pogg., XC. p. 348. 



