Mechanical Illustrations of the Planetary Motions. 325 



consist of a multitude of moons; and, since the effect upon 

 each must be the same, the result will be the same, as in the 

 case of a single moon. The effect will not even be magnified, 

 since the inertia is increased in the same proportion in which 

 the attraction is increased. 



Another application of the same model is, to exhibit and 

 illustrate the various kinds of Perturbation which one planet 

 exercises on another's orbit. As in illustrating the moon's 

 motion, we use, in this case also, instead of the planet itself, 

 the plane of its orbit, or, more correctly speaking, a solid 

 disc of iron in the position of that plane. The place of the 

 disturbing planet is supplied by a magnet. 



Before entering on the astronomical application of the ex- 

 periment, it may be curious to observe the peculiar way in 

 which the disc is affected by the magnet. When the disc is 

 at rest, if we bring the magnet near it, either above or below, 

 it is immediately attracted by the magnet and brought into 

 collision with it. But if the disc is made to rotate with suf- 



ficient rapidity, and the magnet is again brought near it, as 

 before, it now seems no longer to be attracted by the magnet, 

 but rather appears to evade it : you might almost be per- 

 suaded that the magnet had a repulsive effect upon the disc* 

 But it evades it in different ways (at least apparently), ac- 



* The peculiar fact of the rotating iron disc evading the direct action of the 

 magnet, was first shown to me by a friend, but with no perception, on his part, 

 either of its cause, or of its connexion with astronomy. 



