I must before I leave this, add one Consideration more, That if we shall upon these Considerations think it reasonable, thus to consider the _Common Center of Gravity of the Earth and Moon_; it may as well be thought reasonable, that the like Consideration should be had of _Jupiter_ and his four _Satellites_, which according to the Complication of their several motions, will somewhat change the position of _Jupiter_, as to that _Common center of Gravity_ of all these Bodies; which yet, because of their smallness, may chance to be so little, as that, at this distance, the change of his apparent place may not be discernable. And what is said of _Jupiter_, is in the like manner to be understood of _Saturne_ and his _Satelles_, discovered by _Hugenius_: For all these _Satellites_ are to their _Principals_, as so many Moons to the Earth. And I do very well remember, in the Letters forecited, Mr. _Horrocks_ expresseth some such little inequalities in _Saturnes_ motion, of which he could not imagine what account to give, as if (to use his Expression) this crabbed _Old Saturn_ had despised his _Youth_. Which, for ought I know, might well enough have been accounted for, if at that time the _Satelles_ of _Saturn_ had been discovered, and that Mr. _Horrocks_ had thought of such a notion as the _Common Center of Gravity_ of _Saturn_ and his _Companion_, to be considerable, as to the guiding of his motion. You have now, in obedience to your Commands, an Account of my thoughts, as to this matter, though yet immature and unpolished: What use you will please to make of them, I shall leave to your prudence, &c. * * * * * _An _APPENDIX_, written by way of Letter to the _Publisher_; Being an answer to some Objections, made by several Persons, to the precedent Discourse._ I Received yours; and am very well contented, that _objections_ be made against my _Hypothesis_ concerning _Tydes_: being {282} proposed but as a conjecture to be examined; and, upon that Examination, rectified, if there be occasion; or rejected, if it will not hold water.