350 Sir I S A A C N E W T O N’s Book IV.

ring would go backward in the fame manner as the nodes ofthe orbit of any one planet revolving there. Suppose then thisring to adhere to the earth ; and its nodes would still go back-ward, but with a much flower motion, because the ringmust move the whole earth,* to which it is supposed toadhere. The elevation of the equatorial parts of the earthhas the fame effect as such a ring would'have ; only the mo-tion of the nodes of the equator, or of the equinoctial points,is flower, because the accumulated parts of the earth, above aspherical figure, are diffused over its surface, and have a lesseffect than if they were all collected in the plane of the e-quator, in the form of a ring. The moon has a greater forceon this ring than the fun, because of her less distance from theearth ; and they both contribute to produce the retrograde mo-tion of the equinoctial points : the motion, however, producedby both is so flow, that those points will not finish a revo-lution in less than 25000 years. Our author has determinedthe quantity of this motion, from its causes, and finds it, fromthe theory, to be perfectly consonant with the observations ofastronomers.

There is another effect of the action of the fun and moonon this ring, which is too small to be sensible in astronomical'observations : their action on the ring, makes its inclinationto the ecliptic to decrease and increase, by turns, twice everyyear.

CHAP.