570 SCIENCE PROGRESS 



like so much as i per cent., but when equality to this extent 

 is reached the spool becomes sufficiently unmanageable. 



When a stick has been properly adjusted the equality fails 

 if the stick is not centrally placed, and the spool can be spun 

 again quite easily. Now, however, being out of balance, the 

 heavy end in tending to fall actually goes sideways, and more 

 quickly so at starting, when the spinning is slow. With a right- 

 hand spin the heavy end, whichever way it is pointing, veers to 

 the right. This may, of course, be equally well shown with 

 any spool which is unbalanced, or which is made so by attaching 

 a nail or a lump of wax to one end. A stick too short to give 

 the spherical property when centrally placed may, nevertheless, 

 do this when pushed out of centre, and then, though the 

 combination will spin perfectly in the first position it is 

 impossible in the second. 



It might be thought that the ball in the game cup-and-ball, 

 which at least is a sphere in fact, should not maintain its axis 

 when spun with a view of being caught on the point. But here 

 there are two answers. The first is that though outwardly 

 a sphere the axial hole made for the point to enter destroys 

 its spherical properties. This may be the case to the extent 

 of an inequality in the moments of inertia of perhaps as much 

 as 5 per cent., and this is quite enough to give stability. I 

 have, however, turned a groove in the equator of a well-made 

 ball of this kind to such an extent as to bring the moments 

 of inertia the same, at least to a small fraction of i per cent. 

 I was rather surprised to find that I caught this sophisticated 

 ball the first time I tried it. The case, however, is not parallel 

 with the diabolo spool in this respect. While the tweaking of 

 the string on the spool is certain not to be exactly symmetrical 

 and free from disturbing influences, the axial string from which 

 the ball is suspended lifts it when spinning with a pull which 

 must be as truly axial as the turned work is round, and so the 

 disturbance, if any, must be so slight that the ball, in the short 

 time that it is free, has no time to take up an appreciably different 

 position before it is caught. All the time that it is on the string 

 it is held true, while, on the other hand, the spool is only 

 supported and tweaked by the string a very small fraction of the 

 whole time. 



Of all the appliances in the class-room for illustrating the 

 precessional properties of rotating bodies I do not know one 



