erplexed 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.

How much the World, and the great Bodies therein, are manag'd according to
the _Laws of Motion_, and _Statick Principles_, and with how much more of
clearness and satisfaction, many of the more abstruse _Phaenomena_ have been
salved on such Principles, within this last Century of years, than formerly
they had been; I need not discourse to you, who are well versed in it. For,
since that _Galilaeo_ and (after him) _Torricellio_, and others, have
applied _Mechanick_ Principles to the salving of _Philosophical_
Difficulties; _Natura