OF ARTS AND SCIENCES. 209 



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

 lever to turn the whole disc round a vertical axis in a direction oppo- 

 site to its rotation. This can be shown experimentally by pressing 

 with the fingers upon these two parts of the edge. As soon as the 

 motion round the vertical axis begins, the horizontal components of 

 the original rotation no longer retain their parallelism with the disc. 

 But the tendency to preserve this parallelism, in other words, the 

 tendency of the disc to preserve unchanged its plane of rotation, gen- 

 erates forces which act on the top of the wheel to the left and on the 

 bottom of the wheel to the right. These forces, acting by leverage, 

 tend to lift the wheel, as may be seen by pressing in the same way 

 with the fingers. When friction is excluded, this uplifting force is an 

 exact balance of gravity, and the wheel neither rises nor falls. 



" The results of these experiments are remarkable, as showing how 

 differently gravity acts upon a body at rest and upon the same body 

 in motion. When it acts upon a body at rest, it tends to give it a 

 motion round a horizontal axis, but not about a vertical axis. When 

 a body is rotating in a vertical plane, gravity tends to give it no de- 

 scending motion round a horizontal axis, but simply to turn it upon a 

 vertical line. This apparent mechanical paradox is beautifully illus- 

 trated in the Precession of the Equinoxes. The disturbing influence 

 of the sun and moon, which represent the gravity to be considered in 

 this astronomical example, would make the equator drop down into 

 coincidence with the ecliptic, if the earth were not spinning on its axis, 

 and would make the precession an unknown phenomenon. But the 

 same forces, acting upon the rotating earth, move the line of equi- 

 noxes backward, and leave the obliquity essentially unchanged. It 

 follows, from the experimental illustration, as well as fromthe mathe- 

 matical theory, that, if the disturbing forces were greater, the preces- 

 sion would be greater; and if the earth's rotation were diminished, 

 ccEteris paribus, the precession would be increased. 



" 2. The Pohjtrop of Magnus consists of two rotating vertical discs, 

 arranged upon an axle, as the two wheels of a carriage. These discs 

 can be set in motion by cords wound upon the hub of each disc, the 

 free ends of the cords being attached to the same handle. The axle 

 which carries the discs is movable at its centre around a vertical and 

 also a horizontal axis, but either of these motions can be prevented at 

 pleasure. If both discs are made to rotate in the scane direction, or 

 if only one disc rotates, it^is not easy to turn the whole apparatus on 



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