MAGIC TOP. 741 



quickly, and you strike it gently, it will scarcely turn out of its course. If, 

 on the contrary, it is moving slowly, and receives a violent shock, it will 

 run off almost exactly in the direction in which it has been struck. 



Now that which occurs in this example of a body tending to two 

 movements at the same time, is also produced when it is a question of 

 movements of rotation, so that if a force acts upon a body in rotation in 

 such a manner as to give it a movement of the same kind round another 

 axis, a third movement will be originated round a third axis, the direction 

 of which will be nearest to that in which the rotation is most rapid. Let 

 us apply this very simple principle to our top, and we shall see that magic 



Fig. 871. The magic top. 



has nothing whatever to do with these movements, which at first glance 

 appears so extraordinary. Having set it in motion, we rest it on its pivot, 

 its axis in a horizontal position ; we then find that we have two movements 

 before us ; first, that which we gave the top ourselves, and secondly, the 

 movement of rotation which occurs round a second axis equally horizontal 

 passing through the fulcrum and perpendicular line to the first. A move- 

 ment of rotation therefore originates round a third axis placed between the 

 two first, but whilst the real axis of the top, obeying this resultant move- 

 ment, takes up its new position, the law of gravity continuing to act, 

 displaces and moves it a little further, so that in endeavouring to reach its 

 centre of gravity, it turns round its fulcrum (fig. 871). From this explana- 



