. Edinburgh New Philosophical Journal . 324 James Elliot on certain a long cycle of years, again increasing. In the case of the model, however, the obliquity continues to diminish, an up- right position of the axis being constantly approached, but, as I have attempted to demonstrate in a previous note, never attained. The reason of the difference it is not easy to per- ceive. The next piece of apparatus is intended to exhibit the Re- trogradation of the Moon's Nodes. That phenomenon is similar to the precession of the equi- noxes, both in its description and in its cause. In the model, we have
. Edinburgh New Philosophical Journal . 324 James Elliot on certain a long cycle of years, again increasing. In the case of the model, however, the obliquity continues to diminish, an up- right position of the axis being constantly approached, but, as I have attempted to demonstrate in a previous note, never attained. The reason of the difference it is not easy to per- ceive. The next piece of apparatus is intended to exhibit the Re- trogradation of the Moon's Nodes. That phenomenon is similar to the precession of the equi- noxes, both in its description and in its cause. In the model, we have the same similarity: we have the plane of the moon's orbit occupying the same place which the earth's equator occu- pied in the first experiment. Like the plane of the equator, it is inclined to the plane of the ecliptic, the two points in which it cuts that plane, called its nodes, corresponding exactly to the equinoxes in the other case. When the line of the nodes is in the same line with the centres of the earth and sun, we have eclipses of the sun and moon ; when otherwise, the moon passes its change and full without eclipses of either luminary. The line of the moon's nodes revolves in a direc- tio contrary to that of the moon's revolution round the earth. There is the same difficulty of conveying a clear idea of this motion by mere words to students of astronomy, that there is in the precession of the equinoxes; but all the difficulty disappears when we can actually show the move- ment, and that not under the constraint of wheels and bars? but under the impulse of gravitation and inertia. The cause of the peculiar motion in the model is similar to that which produces the corresponding changes of the direction of the plane of the moon's orbit itself, but is not precisely identical with it, inasmuch as, in the model, the force of attraction acts upon the whole plane, while, in the reality, it acts only upon the moon whilst moving in that plane. But the difference is more in
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