* * * * * _Munday_, _August_ 6. 1666. * * * * * The Contents. _An essay of Dr. _John Wallis_, exhibiting his _Hypothesis_ about the _Flux and Reflux of the Sea_, taken from the consideration of the _Common Center of Gravity of the Earth and Moon_; together with an _Appendix_ of the same, containing an _Answer_ to some _Objections_, made by severall Persons against that _Hypothesis_. Some Animadversions of the same _Author_ upon Master _Hobs'_s late Book, _De Principiis & Ratiocinatione Geometrarum_._ * * * * * _An Essay of Dr. _John Wallis_, exhibiting his _Hypothesis_ about the _Flux and Reflux of the Sea_._ How abstruse a subject in Philosophy, the _Flux and Reflux of the Sea_ hath proved hitherto, and how much the same hath in all Ages perplexed the Minds even of the best of _Naturalists_, when they have attempted to render an Account of the Cause thereof, is needless here to represent. It may perhaps be to more purpose, to take notice, that all the deficiencies, found in the _Theories_ or _Hypotheses_, formerly invented for that End, have not been able to deterre the Ingenious of _this_ Age from making farther search into that Matter: Among whom that Eminent Mathematician Dr. _John Wallis_, following his happy _Genius_ for advancing reall Philosophy, hath made it a part of his later Inquiries and Studies, to contrive and deduce a certain Hypothesis concerning that _Phaenomenon_, taken {264} from the Consideration of the _Common Center of Gravity of the Earth and Moon_, This being by several Learned Men lookt upon, as a very rational Notion, it was thought fit to offer it by the Press to the Publick, that other Intelligent Persons also might the more conveniently and at their leisure examine the _Conjecture_ (the Author, such is his Modesty, presenting it no otherwise) and thereupon give in their sense, and what Difficulties may occur to them about it, that so it may be either confirm'd or laid aside accordingly; As the _Proposer_ himself expressly desires in the Discourse, we now, without any more _Preamble_, are going to subjoyn, as it was by him addressed, by way of Letter, from _Oxford_ to Mr. _Boyle, April_ 25. 1666. and afterwards communicated to the _R. Society_, as follows: You were earnest with me, when you last went from hence, that I would put in writing somewhat of that, which at divers times, these three or four years last past, I have been discoursing with your self and others concerning the _Common Center of Gravity of the Earth and Moon_, in order to salving the _Phaenomena_ as well of the _Seas Ebbing and Flowing_; as of some perplexities in _Astronomical Observations_ of the _Places_ of the Celestial Bodies.