. Edinburgh New Philosophical Journal. 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 beauti
. Edinburgh New Philosophical Journal. 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 ; 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
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