Now though what _Galilaeo_ answers to a like Objection in his _Hypothesis_; (that its possible there may be some small difference, which _Astronomers_ have not yet been so accurate, as to observe) might here perhaps serve the turn; Yet my answer is much otherwise; to wit, that such difference hath been observed and hath very much puzzeled _Astronomers_ to give an account of. About which you will find Mr. _Horrocks_ (in some of his Letters, whereof I did formerly, upon the Command of the _Royal Society_, make an _Extract_) was very much perplexed; and was fain, for want of other relief, to have recourse to somewhat like _Keplers_ amicable _Fibres_, which did according to the several positions of the Moon, accelerate or retard the Moon's motion; which _amicable Fibres_ he had no affection to at all (as there appears) if he could any other waies give account of those little inequalities; and would much rather (I doubt not) have embraced this Notion of the Common Center of Gravity, to salve the _Phaenomenon_, had it come to his mind, or been suggested to him. And you find, that other _Astronomers_ have been seen to bring in (some upon one supposition, some upon another) some kind of _Menstrual AEquation_, to solve the inequalities of the Moon's motion, according to her _Synodical_ Revolution, or different Aspects (of New-moon, Full Moon, &c.) beside what concerns her own _Periodical_ motion. {281} For which, this consideration of the _Common Center of Gravity of the Earth and Moon_, is so proper a remedy (especially if it shall be found precisely to answer those _Phaenomena_, which I have not Examined, but am very apt to believe) that it is so far from being, with me, an Objection against it, that it is one of the reasons, which make me inclinable to introduce it.