But this, though it might furnish us with the foundation of a _Menstrual_ Period of Accelerations and Retardations in the compound motion of several parts of the Earths surface; yet I am not at all inclined to admit this as a _true Hypothesis_, for divers Reasons, which if not demonstrative, are yet so consonant to the general Systeme of the World, as that we have no good ground to disbelieve them. For 1. The Earth being undeniably the greater Body of the two (whereof there is no doubt to be made) it cannot be thought probable, that this should be carried about by the Moon, lesser than it self: The contrary being seen, not onely in the _Sun_, which is bigger than any of the Planets, which it carryes about; but in _Jupiter_, bigger than any of his _Satellites_; and _Saturne_, bigger than his. 2. As the _Sun_ by it's motion about it's own Axis, is with good reason judged to be the _Physical_ cause of the _Primary_ Planets moving about it; So there is the like reason to believe, that _Jupiter_ and _Saturne_ moving about their Axes, are the Physical cause of their _Satellites_ moving about them, which motion of _Jupiter_ hath been of late discover'd, by the help of a _fixed_ Spot discern'd in him; and we have reason to believe the like of _Saturne:_ Whether _Venus_ and _Mercury_ (about whom no _Satellites_ have been yet observed) be likewise so moved; we have not yet the like ground to determine: But we have of _Mars_; from {271} the Observations of Mr. _Hook_ made in _February_ and _March_ last, and by him communicated to the _Royal Society_, and since Printed in the _Transactions_, published _Apr._ 2. 1666. consonant to the like observations of _Jupiter_, made by him in _May._ 1664, and since communicated to the same _Society_; and then published in the _Transactions_, of _March._ 6. then next following. Now that the Earth hath such a motion about its own _Axis_ (whereby it might be fitted to carry about the Moon) is evident by its _Diurnal_ motion. And it seems as evident that the Moon hath not; because of the same side of the Moon alwaies turned towards us; which could not be, if the Moon carried the Earth about: Unlesse we should say, that it carries about the Earth in just the same Period, in which it turnes upon its own Axis: Which is contrary to that of the Sun carrying about the Planets: the shortest of whose Periods, is yet longer than that of the Suns moving about its own Axis. And the like of _Jupiter_, shorter than the Period of any of his _Satellites_; if at least the Period of his conversion about his Axis, lately said to be observed, prove true. (Of _Saturn_ we have not yet any Period assigned; but it's likely to be shorter, than that of his _Satelles_.) And therefore we have reason to believe, not that by the Moons motion about its Axis the Earth should be carried by a contemporary Period (whereby the same face of the Moon should be ever towards us;) but that by the Earths revolution about its Axis in 24.