THE THEORY OF DIABOLO 565 



of it rises and it veers to the left ; on the other hand, a boomerang- 

 started with the same motion rises and comes back again. An 

 elongated bullet neither follows the trajectory, like an arrow, 

 nor maintains the direction of its axis, but conically screws 

 itself round a line slightly above the trajectory. Whenever a 

 spinning body is projected through the air which is in any 

 respect different in its conditions from other cases which have 

 been investigated, a new problem is almost certain to be found, 

 and it is not safe to jump to conclusions. 



In the case of the diabolo spool there are really no diffi- 

 culties, because the air-effects are barely noticeable, and they 

 are of the simple kind observed with golf-balls, tending to 

 deflect the spool to the right as it falls if it is spinning rapidly 

 with a right-hand spin. Ordinarily, in the draughty and eddying 

 enclosures called gardens attached to houses in London, this 

 deviation from a straight fall is not easy to detect, and it 

 certainly is of no importance in the game. If, however, the 

 spool were much lighter, and were ribbed or roughened like a 

 golf-ball, the air-effect would be increased. 



With an ordinary spool, the form of which it is unnecessary 

 to define, the centre of gravity is above the string support. It 

 is therefore unstable, and on whichever side it happens to 

 incline, on that side, if not spinning, it will fall. Gravity acts 

 in such a manner as to topple the axis of spin over towards a 

 vertical direction. With the usual right-hand spin the hands 

 are moved in such manner that the string is jerked tight when 

 the right hand is rising, and is loose when it is falling, and the 

 spool gets its support mainly while the string under it is moving 

 from left to right. The upper part, therefore, moves from right 

 to left. In order to correct the level of the axis, when for any 

 reason this ceases to be level, it is necessary to apply forces 

 twisting it not in the desired direction, but in a direction at 

 right angles thereto. Any child will tell you to put the left 

 hand forward if the far end is drooping. Putting the left hand 

 forward means the application of a twist on the axis, tending to 

 turn the far end to the right. This may be proved in a moment 

 by placing the spool on the ground with the string under it 

 and then gradually lifting it with the left hand forward. It will 

 then, not being in a condition of spin, immediately turn so that 

 the far end moves to the right. If the left hand is put forward 

 when the spool is spinning, the effect of the tendency to twist 



