Mechanical Illustrations of the Planetary Motions. 313 



so as to approach, within any degree of closeness, the exceed- 

 ingly slow precessional movement of the earth's equator. 



We have thus succeeded in obtaining, in the model, the 

 precise motion of which we were in quest ; but if it can also 

 be established that that motion is not only the same as the 

 corresponding motion of the earth, but arises from the same 

 cause, every object will have been attained that can possibly 

 be desired in a model. To establish that, however, will draw 

 us into somewhat abstruse and lengthened theoretical consi- 

 derations, to which a patient attention must be requested, 

 since they are absolutely indispensable not only to a right 

 appreciation of this particular instrument, but to the eluci- 

 dation of other parts of the subject. 



The first point to be ascertained, then, is — what physical 

 cause produces the conical motion of the axis, either in the 

 instrument before us, or in the common spinning-top ? and 

 that question throws us back upon another, — what prevents 

 a spinning-top from falling 1 — in what way does its motion 

 keep it in an erect position % 



A popular notion is that the standing of a top is due to its 

 centrifugal force. The fallacy of that idea is very well ex- 

 posed by Dr Arnott. He shows that (since the force acts 

 equally on all sides of the axis) if the axis is placed upright, 

 the centrifugal force can have no tendency to incline it to one 

 side more than to another, and can have no more effect in 

 doing so when the axis is inclined. The inclination of the top 

 can have no effect in changing the direction of the centrifugal 

 force, which will still act perpendicularly to the axis, and equal- 

 ly on all sides, neither accelerating nor retarding the fall. 



Dr Arnott having shown the fallacy of the opinion that 

 centrifugal force is the cause, substitutes, in its place, ano- 

 ther equally fallacious. " While the top," to use his own 

 words, " is perfectly upright, its point, being directly under 

 its centre, supports it steadily, and, although turning so ra- 

 pidly, has no tendency to move from the place ; but, if the 

 top incline at all, the side of the peg, instead of the very 

 point, comes in contact with the floor, and the peg then be- 

 comes a little wheel or roller, advancing quickly, and, with 

 its touching edge, describing a curve, as a skater does, until 



