148 AN EMPIEICAL STUDY OF GYRATING BODIES. 



directions, while the rotation of the axle is necessarily 

 the same in every part. 



With these facts in view we are prepared to attack the 

 problems which its movements offer. 



Why does a top rise % It is a real rise, for the center 

 of gravity becomes actually higher. It may start from 

 a great degree of obliquity, and rise to a vertical. 



I have already noted the fact that a top with a suffi- 

 ciently fine and well-centered point never rises. We 

 must, therefore, take one of a different form. 



We shall quickly see why. The most convenient 

 "point" that I have found is a truncated cone, that 

 marked c, fig. 25. The smaller diameter should not ex- 

 ceed one eighth of an inch ; one-twentieth is better. I place 

 this on my top, and set it going, the axis being inclined, 

 as shown in fig. 30. Since in all cases — save the impos- 

 sible one of no friction — the top gyrates around a point 

 between d, the point of support, and g, the foot of the 

 perpendicular let fall from the center of gravity, it fol- 

 lows that if, at any moment, c is coming towards you, d 

 is going from you. In other words, c and d, like the op- 

 posite sides of a wheel, move in opposite directions. 



It has been shown that the top gyrates in the same 

 direction that the under side moves, and being all one 

 piece, the cone, or "point," revolves on its own axis 

 (which is also the top's) in the same direction. Hence, 

 d, by its friction, pushes its end away from the observer, 

 and, of course, makes the other end, c, approach him 

 more rapidly than it otherwise would. Fig. 31 will 

 make this clearer. Here we have a wheel on the end of 

 a shaft, free to move horizontally around the support a, 

 the rim resting on the surface of the table. Now, if the 

 wheel is made to revolve in the direction of the small 

 arrows — the lower side coming towards us — the end, b, 

 on which the wheel is, will be made, by the friction on 

 the table, to recede from us, and the other end to ap- 



132 



