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 Phænomenon, taken
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 Phænomena 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,
or particular influence, which it hath on moyst Bodies, or by some Magnetick vertue, drawing the water towards it, (which should therefore make the Water there highest, where the Moon is vertical) or by its gravity and pressure downwards upon the Terraqueous Globe (which would make it lowest where the Moon is vertical) or by whatever other means (according to the several Conjectures of inquisitive persons,) hath so great an influence on, or at least a connexion with, the Sea's Flux and Reflux, that it would seem very unreasonable, to seclude the consideration of the Moons motion from that of the Sea: The Periods of Tides (to say nothing of the greatness of them near the New moon and Full moon) so constantly waiting on the Moon's motion, that it may be well presumed, that either the one is governed by the other, or at least both from some common cause.
But the first that I know of, who took in the consideration of the Earth's motion, (Diurnal and Annual) was Galilæo; who in his Systeme of the World, hath a particular discourse on this subject: Which, from the first time I ever read it, seemed to me so very rational, that I could never be of other opinion, but that the true Account of this great Phænomenon was to be referred to the Earths motion, as the Principal cause of it: Yet that of the Moon (for the reasons above mentioned) not to be excluded, as to the determining the Periods of Tides, and other circumstances concerning them. And though it be manifest enough, that Galilæo, as to some particulars, was mistaken in the account which there he gives of it; yet that may be very well allowed, without any blemish to so deserving a person, or prejudice to the main Hypothesis: For that Discourse is to be looked upon onely as an Essay of the general Hypothesis; which as to particulars was to afterwards adjusted, from a good General History of Tides; which it's manifest enough that he had not; and which is in a great measure yet wanting. For were the matter of Fact well agreed on, it is not likely, that several Hypotheses should so far differ, as that one should make the Water then and there at the Highest, where and when the other makes it at the Lowest; as when the Moon is Vertical to the place.