EQUILIBRATION. 499 



" wabbling." These two subordinate motions, variable in 

 their proportions to each other and to the chief motion, are 

 commonly soon brought to a close by separate processes of 

 equilibration. The momentum which carries the top bodily 

 along the table, resisted somewhat by the air, but mainly by 

 the irregularities of the surface, shortly disappears ; and the 

 top thereafter continues to spin on one spot. Meanwhile, in 

 consequence of that opposition which the axial momentum 

 of a rotating body makes to any change in the plane of rota- 

 tion, (so beautifully exhibited by the gyroscope,) the " wab- 

 bling " diminishes; and like the other is quickly ended. 

 These minor motions having been dissipated, the rotatory 

 motion, interfered with only by atmospheric resistance and 

 the friction of the pivot, continues some time with such uni- 

 formity that the top appears stationary: there being thus 

 temporarily established a condition which the French 

 mathematicians have termed equilibrium mobile. It is true 

 that when the axial velocity sinks below a certain point, 

 new motions commence, and increase till the top falls; but 

 these are merely incidental to a case in which the centre of 

 gravity is above the point of support. Were the top, having 

 an axis of steel, to be suspended from a surface adequately 

 magnetized, all the phenomena described would be dis- 

 played, and the moving equilibrium having been once ar- 

 rived at, would continue until the top became motionless, 

 without any further change of position. Now the 



facts which it behoves us here to observe, are these. First, 

 that the various motions which an aggregate possesses are 

 separately equilibrated : those which are smallest, or which 

 meet with the greatest resistance, or both, disappearing first ; 

 and leaving at last, that which is greatest, or meets with least 

 resistance, or both. Second, that when the aggregate has a 

 movement of its parts with respect to each other, which en- 

 counters but little external resistance, there is apt to be es- 

 tablished an equilibrium mobile. Third, that this moving 

 equilibrium eventually lapses into complete equilibrium. 



