Mechanical Illustrations of the Planetary Motions. 311 



a simultaneous revolution of that axis itself, carrying the 

 earth with it in the opposite direction, that we naturally 

 look around for any illustration that can be given of it more 

 satisfactory and more natural than turning the model with 

 the hand. About the same time my attention was also more 

 particularly directed to the same point by meeting with a 

 remark in Sir John Herschel's excellent volume on Astrono- 

 my. " A child's peg-top," he says, " or te-totum, exhibits, in 

 the most beautiful manner, the whole phenomenon," of the 

 precession of the equinoxes, " in a manner calculated to give 

 at once a clear conception of it as a fact, and a considerable 

 insight into its cause as a dynamical effect." So far well ; 

 but this objection comes in the way — an objection which, of 

 course, the writer just quoted did not overlook — that, in all 

 ordinary tops and te-totums, the motion in question is in the 

 contrary direction to that which we are required to illustrate 

 in the planets, the conical revolution of the axis being, in the 

 former, in the same direction with the rotation, while, in the 

 latter, it is in the opposite direction. 



I observed, however, that in tops which have short pegs, 

 this motion — the conical motion of the axis — is slower than 

 in those which have long ones ; and, in fact, the shorter the 

 peg, the slower the revolution. It therefore occurred to me 

 that, if we could lower the centre of gravity till it coincided 

 with the centre of motion, this movement would cease alto- 

 gether, and the top would continue to spin with its axis 

 pointing permanently in any direction in which it might be 

 placed. I also concluded that, if we 

 still further extended the same change 

 which gradually annihilated the posi- 

 tive motion, it would re-appear nega- 

 tive, or in the opposite direction. 

 With that view I had an instrument 

 constructed of the form shewn in the 

 annexed cut, consisting of a wooden 

 ball hollowed out in its lower part, so 

 as to admit the support upon which 

 it rests to be raised above the centre 

 of gravity of the ball, and with a screw 



