568 SCIENCE PROGRESS 



is put forward to cause one end to dip, the spool will then veer 

 to the right, and vice versa, for the end that dips is reversed with 

 a reversed direction of spin, and the action of the reversed 

 tendency to fall, combined with the reversal of spin, gives the 

 same direction of veering. This process was described by a 

 speaker at the Physical Society at a recent meeting where I was 

 showing a peculiar spool, to which I shall next refer. 



An ordinary fly-wheel or a disc is twice as effective as a 

 fly-wheel if it is mounted to spin in its own plane, as it would 

 be if it were made to spin on one diameter as an axis. Its 

 moment of inertia is said to be twice as great when spinning 

 as it does in practice as it would be if made to spin about a 

 transverse axis. On the other hand, a diabolo spool or an 

 elongated bullet is less effective as a fly-wheel when spinning 

 about its axis of rotational symmetry. In these cases the 

 moment of inertia is a minimum, while in the former it was 

 a maximum when spinning in the usual manner. Anything, 

 whatever shape it may have, must have some direction about 

 which the moment of inertia is a maximum or a minimum, and 

 it may have a single transverse axis about which it is corre- 

 spondingly a minimum or a maximum. 



If any body is set spinning about any axis and then liberated, 

 it will continue to spin about that axis and resist disturbance 

 only if the axis of spin is one of maximum or minimum moment 

 of inertia. If the body were spun about any inclined axis and 

 liberated, then, according to the position in the body of the 

 inclined axis and the relative values of the maximum and 

 minimum moments of inertia, the axis about which it there- 

 after is momentarily spinning will move within the. body 

 according to one curve and in space according to another, for 

 which an explanation must be sought in Maxwell's Collected 

 Works, vol. i. p. 248. If, on the other hand, the spinning 

 body were a uniform sphere, in which the moments of inertia 

 about all diametrical axes are alike, it would have no ten- 

 dency to move the axis of spin in any particular manner 

 within itself, and any disturbance would set this axis wandering 

 in the sphere, or, as Maxwell put it, the sphere has no dynamical 

 backbone. 



It is not necessary in order that a thing should have equal 

 moments of inertia in all directions that it should be spherical in 

 shape. A long round thing has a minimum moment of inertia 



