THE SMITHSONIAN INSTITUTION. 185 



of ages, the poles of the equator must likewise perform a revolution 

 around the poles of the eclii)tic at the same slow rate. 



While the rotasco]ie exhibits the precession as in the last experi- 

 ment, you will perceive, if your attention is given to the pole of the 

 spheroid, that it describes a circle ariniiid the pole of the ecliptic, or 

 the pivot at the top of the frame. For many years past and to come, 

 the conspicuous star in the extremity of the tail of the Litle Bear is 

 nearly enough in the direction of the earth's axis to he called the pole- 

 star. But the time will come when the little feljow will not be held 

 so unceremoniously by the end of his tail, and wliirled round every 

 day without touching feet to the ground, as he now is. He will re- 

 tire from his dizzy position in the north, and evei-y twenty- four hours 

 will go to rest and rise again, like most other animals. In 13,000 

 years from this time, the Little Bear will rise in the northeast, cul- 

 minate over our heads — I should say over the heads of our success- 

 ors — and set in the northwest ; wlnle the beautiful Harp will take 

 its station in the northern watch-tower, furnishing a far more bril- 

 liant pole-star (Alpha Lyne) than the one which ive enjoy. 



The retrograde motion of the moon's nodes is explained in the 

 same manner as the precession. The sun is the disturbing body, 

 always in the plane of the ecliptic, while the moon's path about the 

 earth is inclined to the ecliptic about five degrees. A smnll compo- 

 nent of the difference of the sun's action on the earth and moon is 

 employed to press the moon towards the plane of the ecliptic. The 

 two revolutions thus impressed on the moon cause it to revolve in an 

 intermediate direction. Recurring to the experiment by which I 

 illustrated the fact of this retrograde motion, a moment's attention, 

 in view of what has been presented on compound rotations, will suf- 

 fice for understanding the reason of it. The wooden ring represent- 

 ing the ecliptic as heretolbre, the lamp in its centre the earth, and 

 the brass ball the moon, we must imagine the sun at the distance of 

 some five hundred feet in the extension of the wooden ring. Now, 

 as I carry the ball around the ring obliquely while it is above, and 

 tending, by its inertia and gravity of the earth, to go forv>rard in its 

 orbit, the distant sun exerts a small force to depress it into the plane 

 of the ring, and it therefore goes hehveen, and passes the plane at 

 an earlier point than if the sun had not acted ; tliat is, the node has 

 moved backward. At every semi-revolution the same canse is in ope- 

 ration, and the effect is, therefore, perpetually produced on each node. 

 But this retrogradation is far more rapid than the precession, prin- 

 cipally because the moon is not attached (as the equatorial ring of 

 the earth is) to a mass vastly larger than itself, to which the motion 

 must be communicated. 



Before leaving the subject, I will use the rotascope to perform two 

 experiments which strikingly illustrate the general lav/ of compound 

 rotations. 



From the ceiling there is suspended a strong wire, on the lower 

 end of which is a cord, or rather a bundle of cords, about two feet 

 long, terminating in a hook. I take the spheroid and rings from 

 the frame, by raising the pivot-screw at the top, and hang the outer 



