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 appearance than in 

 kind, since we may conceive the solid orbit in the model to 



