THE THEORY OF DIABOLO 563 



the fact that upward forces due to this cause may be set up 

 which are two or three times as great as the weight of the ball. 

 Another example of this, but less easily followed, is the flight of a 

 spherical bullet from a bent smooth-bore barrel. If such a barrel 

 is curved in a horizontal plane so as to be suitable, according to 

 the story of our childhood, for shooting any one round a haystack, 

 the bullet, far from continuing the curve which it was compelled 

 to follow in the barrel, acquires therein a spin, owing to its 

 rolling along on the concave side, and this spin produces a 

 curvature when free opposite in kind to that of the barrel that 

 gave the rotation. Such a curved flight of a ball is well known 

 in games ; the light ball of lawn-tennis when well cut follows 

 a markedly curved course, and, best of all, the child's india- 

 rubber air-ball or balloon when cut by a sweeping stroke of 

 the hand curves rapidly upwards in its flight and provides the 

 material for an excellent and violent indoor winter game. 



Mysterious as some of these air-effects may be, they are 

 wholly eclipsed by the queer and apparently semi-contradictory 

 effects of the rotation of a body which are noticed when 

 anything is done to twist the axis of rotation into some 

 other direction. If a grown-up and otherwise well-educated 

 man had never seen a top spin, and, being familiar with 

 the fact that such a body will not stand up upon its point 

 when not spinning, and with the whole science of statics, 

 were suddenly presented with the phenomenon of a spinning- 

 top it is difficult to realise, familiar as we are with the 

 result from our earliest childhood, how great would be his 

 surprise. The fact that it should stand up at all would seem 

 sufficiently uncanny, but its gentle gyrations round the upright 

 position, more rapid as the rate of spin diminishes, would seem 

 to defy explanation. It is only universal familiarity with the 

 phenomenon that prevents surprise or any anxiety in the 

 majority of people to dissolve the mystery. It would be out 

 of place in an article such as this to prove from the simple laws 

 of motion propounded by Newton that a top should spin as it 

 does, or, more generally, how any spinning body will behave 

 when any forces are applied to it which would, if it were not 

 spinning, move the axis about which it happens to be turning 

 into some different direction. This subject is treated in a 

 charming manner by Professor Perry in his book on Spifming- 

 Tops and by Professor Worthington in his Dynamics of Rotation, 



