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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 expresly 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 Phœnomena 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 Phænomena 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 Galilæo, and (after him) Torricellio, and others, have applied Mechanick Principles to the salving of Philosophical difficulties; Natural Philosophy is well known to have been rendered more intelligible, and to have made a much greater progress in less than an hundred years, than before for many ages.
The Seas Ebbing and Flowing, hath so great a connexion with the Moons motion, that in a manner all Philosophers (whatever other Causes they have joyned with it) have attributed much of its cause to the Moon; which either by some occult quality
