Mechanical Illustrations of the Planetary Motions, 323 



Therefore the axis, AB, will be drawn in towards the line 

 CS, while no such tendency would arise in the sphere, and 

 consequently no conical revolution of the axis. 



These two objections being set aside, it will readily be 

 perceived that the tendency of the line AB to fall into the 

 line OS, exactly corresponds to the tendency of the top to 

 fall, or to the tendency of the revolving ball of the model, 

 when its centre of gravity is below the point of the pivot, to 

 bring its axis into a vertical position, and, consequently, that 

 the same results must follow. The conical motion produced in 

 the axis of the ball corresponds to that which goes on in the 

 axis of the earth, not only in its existence but also in its cause. 



Thus, then, we have two motions of the model agreeing 

 with two corresponding motions of the earth, — viz., the Ro- 

 tation and the Precession of the Equinoxes, which is identical 

 with the conical motion of the axis. If we place the instru- 

 ment, while in motion, upon a stand, and suspend the stand 

 by a cord, from a great height, we may then, as is well known, 

 exhibit the Elliptical Orbit, and also the Progression of the 

 Apsides. 



If we next load the sphere, on one side, very slightly, by 

 any means, we obtain an illustration of the Nutation of the 

 Earitis Axis, the axis making a multitude of minute conical 

 revolutions round the circumference of the greater conical 

 revolution. Neither are the causes very different in the 

 model and in that which it represents; for the moon's at- 

 traction does to the earth what the little weight does to the 

 model : it loads it on one side. The periods of the motions, 

 however, are different in the two ; for, in the earth, the pe- 

 riod is a lunar month ; in the model, a day. This cannot 

 easily be avoided ; although, if desired, the period might be 

 obtained strictly correct by means of a revolving magnet, re- 

 presenting the moon, acting upon the iron circle which forms 

 the equator of the model earth. 



The next motion illustrated by the apparatus is the gra- 

 dual Diminution of the Obliquity of the Equator to the Eclip- 

 tic. In the case of the earth itself, Laplace has computed 

 that the diminution is not permanent, but confined within 

 certain limits both of time and of extent, the obliquity, after 



