344 ' Zir I r S A A C 'M W T O ‘N’s Book IV.

primary towards the fun, (represented by ad) as ^ to oras AL to A S x ff = A B.

Thus we arrive at the fame conclusion which Sir IsaacNewton, more briefly, derived from an analysis of the motionsof the satellite ; that while the satellite gravitates towards theprimary, if, at the fame time, it be acted on by the fame solarforce as the primary, and with a parallel 'direction, it will re-volve about the primary, in the fame manner as if this lastwas at rest, and there was no solar action. These two forces,the gravitation towards the primary, and a force equal and pa-rallel to the gravitation of the primary towards the fun, areexactly sufficient to account for the compounded motion of thesatellite in its path, however complex a curved-line it may ap-pear to be. Nor is there any perturbation of the satellite’smotion, but what arises from the inequality of the gravity ofthe satellite, and os the primary towards the fun, or fromtheir not acting in parallel lines. If we should suppose themto move about their common center of gravity, while this kcarried round the fun, or if we suppose the orbits to be ellip-tical, the conclusions will still be found consonant to what wasmore briefly deduced by this great author.

CHAP. VI.

Of the figure of the earth , and the preceffio7i of the equinoxes .

i.TF the earth was fluid, and had no motion on its axis,the equal gravitation of its parts towards each otherwould give it a'figure exactly spherical, the columns from thesurface to the center mutually sustaining each other at equalheights from it. But, because of the diurnal rotation of theearth on its axis, the gravity of the parts at the equator is di-minished