Chap. 2. PHILOSOPHY. 175

6. But now, in these demonstrations some very minute in-equalities in the motion of the planets are neglected; which isdone with a great deal of judgment; for whatever be theircause, the effects are very inconsiderable, they being so exceed-ing small, that some astronomers have thought fit wholly to passthem by a . However the excellency of this philosophy, whenin the hands of so great a geometer as our author, is such, thatit is able to trace die least variations of things up to their causes.The only inequalities, which, have been observed common toall the planets, are the motion of the aphelion and the nodesThe transverse axis of each orbit does not always remain fix-ed, but moves about die fun with a veiy stow progressive-motion: nor do the planets keep constantly the fame plane,but change them, and the lines in which those planes inter-sect each other by insensible degrees. The first of theseinequalities, which is the motion of the aphelion, may be ac-counted for, by supposing the gravitation of the planets to-wards the fun to differ a little from the forementioned re-ciprocal duplicate proportion of the distances; but the se-cond, which is the motion of the nodes, cannot be account-ed for by any power directed towards the fun; for no suchcan give the planet any lateral impulse to divert it from theplane of its motion into any new plane, but of necestity mustbe derived from some other center. Where that power islodged, remains to be discovered. Now it is proved, asshall be explained in the following chapter, that the threeprimary planets Saturn, Jupiter, and the earth, which havesatellites revolving about them, are endued with a power of

* Street, id Astron. Carolin,-

' causing.