196 ANNUAL OF SCIENTIFIC DISCOVERY. 



powerfully to prevent its change of plane. A wheel so situated does not 

 'lie round loose,' but every portion of its run is very strongly pulled directly 

 from the centre, the effect of all which is to hold it very stiffly. The ' inertia,' 

 which tends to prevent the change of position of any mass, is .increased ten, 

 or perhaps one hundred fold when the mass is set in violent rotation. The 

 wreath in the play of ' Graces' is sent whirling in its flight, to diminish the 

 chances of its turning itself into an edgewise position ; and the obstinacy with 

 which a mass holds its relative position when very rapidly rotated, is availed 

 of in the rifle to keep the conical ball point foremost. The Minie ball is ha 

 form like a hay stack or sugar loaf, and the rifle is grooved so as to twist or 

 screw the ball around at the rate of one complete revolution in every three 

 or four feet of its path. In pitching quoits or pennies the same principle is to 

 a certain extent availed of, to steady the projectile in its flight through the 

 air ; freely suspended within rings like a mariner's compass, a whirling mass 

 will exhibit a strong desire to revolve constantly in the same plane, and thus 

 to keep its axis always in the same position. If, however, such a mass be 

 forcibly acted on by some exterior agency, and compelled to change its plane, 

 as if, for example, one extremity of the axis be pulled to one side, the mass 

 does not tend either to remain quietly where it is left, nor to resume its first 

 position, but leaps suddenly towards a new position at right angles to both 

 these. The immediate cause of this unexpected result is somewhat difficult of 

 popular explanation, but the fact exists, and from it directly results all the 

 peculiarities in the toy under notice. 



"The effort of the unsupported end of the shaft to fall to the earth induces 

 a horizontal twisting of the axial line, which, as one end is confined by the 

 support, results hi a revolution of the whole mass around that point. As the 

 tendency to fall is continuous, so the tendency to revolve is of the same con- 

 stant character, and this revolution, by continually changing the plane in a 

 horizontal direction, generates another twisting effort, tending to elevate one 

 end and depress the other. It chances that the end depressed by this force is 

 the supported one, while the other is correspondingly lightened, and so long 

 as the motion is sufficiently rapid to make this twisting force equal to the 

 whole weight to be supported, it will sustain itself apparently on nothing, to 

 the great delight of juveniles, and the profound amazement of graver thinkers. 

 The slow travelling motion of the whole around its support is vitally essential 

 to its operation, and it immediately displays its ordinary weight at both extre- 

 mities whenever this revolution is stopped. This motion is, in short, the effect 

 of its tendency to fall, and the cause of its tendency to climb upon its support, 

 so that the motions and forces described mutually cause and sustain each 

 other, and preserve of themselves their proper relations and intensities. 



"The same principles are, to some degree, called into action in the spinning 

 of a top, although the result is less remarkable and more familiar. The top 

 gams its steady equilibrium by changing its base in diminishing circles, but 

 is meanwhile preserved from falling immediately over by the agency first 

 alluded to, i.e. the great force with which it resists .any change of plane. 

 If the foot of a top accidentally drops into a depression which retains it, the 

 mass does not fall, but gyrates rapidly around at an inclination which dirni- 



