And then as to _Gassendus_, in his discourse _De AEstu Maris_; I find him, after the relating of many other Opinions concerning the Cause of it, inclining to that of _Galilaeo_, ascribing it to the Acceleration & Retardation of the Earths motion, compounded of the Annual and Diurnal; And moreover attempting to give an account of the _Menstrual Periods_ from the Earths carrying the Moon about it self, as _Jupiter_ doth his _Satellites_; which together with them is carryed about by the _Sun_, as one Aggregate; (and that the Earth with its Moon is to be supposed in like manner to be carried about by the Sun, as one Aggregate, cannot be reasonably doubted, by those who entertain the _Copernican Hypothesis_, and do allow the same of _Jupiter_ and his _Satellites_.) But though he would thus have the Earth and Moon looked upon as two parts of the same moved Aggregate, yet he doth still suppose (as _Galilaeo_ had done before him) that the line of the Mean Motion of this Aggregate (or, as he calls, _motus aequabilis et veluti medius_) is described by the _Center_ of the _Earth_ (about which Center he supposeth both its own revolution to be made, and an Epicycle described by the Moons motion;) not by another Point, distinct from the Centers of both, about which, as the {288} common Center of Gravity, as well that of the Earth, as that of the Moon, are to describe several Epicycles. And, for that Reason fails of giving any clear account of this _Menstrual_ Period. (And in like manner, he proposeth the Consideration as well of the Earths _Aphelium_ and _Perihelium_ as of the _AEquinoctial_ and _Solstitial_ Points, in order to the finding a Reason of the _Annual_ Vicissitudes; but doth not fix upon any thing, in which himself can Acquiesce: And therefore leaves it _in medio_ as he found it.) It had been more agreeable to the Laws of _Staticks_, if he had, (as I do,) so considered the _Earth_ and _Moon_ as two parts of the same movable, (not so, as he doth, _aliam in Centro et sequentem praecise revolutionem axis, aliam remotius ac velut in circumferentia_, but,) so, as to make neither of them the Center, but both out of it, describing Epicycles about it: Like as, when a long stick thrown in the Air, whose one end is heavyer than the other, is whirled about, so as that the End, which did first fly foremost, becomes hindmost; the proper line of motion of this whole Body is not that, which is described by either End, but that, which is described by a middle point between them; about which point each end, in whirling, describes an Epicycle. And indeed, in the present case, it is not the Epicycle described by the Moon, but that, described by the Earth, which gives the _Menstrual_ Vicissitudes of motion to the Water; which would, as to this, be the same, if the Earth so move, whether there were any Moon to move or not; nor would the Moons Motion, supposing the Earth to hold on its own course, any whit concern the motion of the Water. [Illustration]