NATURAL PHILOSOPHY. 193 



cist Pliicker, resulted in the construction of the apparatus, which during the 

 past year has excited much attention and discovery, being regarded by many 

 as an entirely new and inexplicable contrivance. 



" Upon a heavy base, stands a hollow brass column, supporting a steel pin, 

 terminating at the end in a point. At right angles to this pin are fastened 

 metallic arms. On one of these arms, and at the distance of two inches from 

 the pin, is fastened a vertical ring. Inside of this ring is placed a metallic disc, 

 loaded at the edge, and which turns, independently of the ring, upon an axis. 

 The motion is communicated by a thread wound upon the axis of the disc. 

 The metallic arms contain a hinge, working on a horizontal point, which allows 

 the ring containing the disc to move in its own plane. This motion can, how- 

 ever, 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 and a counterpoise which slides on the 

 opposite arm to balance the disc. The top has less friction than Bohnenber- 

 ger's or Fessel's apparatus. Also hi Fessel's machine the disturbing force is 

 the whole weight of the disc and ring, and not, as in Bohnenberger's machine, 

 simply an access 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 from its 

 own weight. 



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

 then left free, it seems indifferent to gravity, and, instead of dropping, it begins 

 to revolve about the vertical axis. So that the axis of the disc acquires a mo- 

 tion similar to the Precession of the Equinoxes in Astronomy. The motion of 

 revolution is opposite in direction to the rotation of the disc. "When one of 

 these motions is the greatest, the other is the least. If the motion of revolu- 

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

 the disc appears heavier. Eeciprocally, if the gravity of the disc is artificially 

 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 lunge, there 

 is no motion round the horizontal axis. 



"The folio wing popular explanation is given of these peculiarities of mo- 

 tion.* 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 ascending, this edge is pushed to the left and the opposite 

 edge to the right. These two forces, resulting out of the deviation of the ori- 

 ginal vertical components from parallelism with the disc, act as through a bent 

 lever to turn the whole disc round a vertical axis in a direction opposite to its 







Ann Po?g., XC. p. 848. 



9 



