"the motion of the comets, unless the ce"lestial spaces be entirely cleared of that " fictitious matter; or if they will have the "heavens filled with a fluid matter, they "must suppose it void of any vis inertiæ ; "if it has no vis inertia, it can have no "force to communicate motion; if it has "no force wherewith to communicate mo"tion, it can have no force to produce any "change in one or more bodies; and if it "has no faculty wherewith to produce any "change of any kind, it has no manner "of efficacy: therefore certainly this bypothefis may be juftly called ridiculous, "and unworthy a philofopher; fince it is

altogether without foundation, and does "not in the least serve to explain the na"ture of things." Thus fays the ingenious Mr Cotes in his preface. What would he have faid, had he lived to see an hypothefis pronounced by him to be ridiculous, and unworthy a philofopher, adopted by a celebrated Newtonian and mathematician, to free Sir Ifaac Newton from the imputation of making gravity a property of matter? And in the edition of his Optics 1706, Sir

Ifaac fays, "That feigned and imaginary

"matter with which the heavens are fill

ed, is by no ways useful for explaining "the phenomena of nature; fince the mo"tions of the planets and comets, by the "means of gravity, are better explained " without it; and gravity has not, as yet, "been explained by it." In the edition of 1719 by Clarke, and his own of 1721, this fame query is differently worded; and he talks of filling the heavens with an exceedingly rare ethereal medium. So then, in 1706, this rare ethereal medium was of no use towards the explanation of the phenomena of nature, and ferved only "to disturb and retard the motion of the heavenly bodies, and make the frame of nature languish:" but, in 1719 and 1721, this rejected medium is again taken into fervice, to explain gravity, and the phenomena of nature, which before were bet→ ter explained without it; and now a denfe medium is only excepted againft; but, at the fame time, he makes this medium fo exceedingly rare, as not to anfwer his purpose. Now, in what light can we view

these

thefe felf-contradictions, but as ftudied evafions to the objections made by Mr Leibnitz and others, and to what he faw his Principia fo juftly liable? For in the dif pute between Mr Leibnitz and Dr Clarke, this medium is denied to be fufficient for the explanation of the phenomena of nature; and Mr Leibnitz, by way of triumph, is called upon to fhew how it acts.

INDEED, in the last edition of his Optics, he talks much, by way of query, of an ethereal medium; and one is sometimes tempted to think him inclined to make it the cause of gravity. But then, as he was confcious of the neceffity of two mediums, the one denfer, the other rarer; or rather of a plenitude of matter, alternately put into different conditions, to perform what he saw was performed in the universe; he abfolutely denies the existence of fuch a medium. In fhort, he would fill the celestial spaces with a rare, unrefifting, inactive medium; and yet fuppofe that this medium may fuffice to impel the planets with all that power which he calls gravity. H

So

So that gravity thus explained, amounts to no more than gravity unexplained, or confidered as a property of matter; and this ethereal medium is no better than the materia fubtilis of Duns Scotus. For if nei

ther Sir Ifaac Newton, nor any of his followers, have demonftrated the laws of this ethereal medium, and proved to fatisfaction how it acts upon matter fo as to be the cause of gravity, it is a mere gratis dictum to talk of its being the cause of gravity; fince if this medium have no refiftance, no vis inertia, it is neither fit for motion, or to be the cause of motion; fince bodies, according to their laws, continue in motion by the vis inertia alone, after the vis impreffa has left them; and if it have vis inertia, then it remains for them to fhew how that power is overcome. Now, if this medium, affigned for the cause of gravity, be infufficient for that purpofe; or, to use Sir Ifaac Newton's own words, gravity has not as yet been explained by it; gravity can be accounted only a virtue, difpofition, affection, or property of matter; or the farthest they can go, is to call it an effect

of

of fome unknown or unexplained cause. For to say that gravity is caused by the impulfes of an ethereal medium, and yet neither to be able to tell us what this medium really is, or how it acts, is to tell us nothing; and is a difingenuity not becoming a philofopher. But how are we to distinguish this medium from that which he rejects? for one egg is not more like another, than his fubtile ethereal medium which is now introduced to explain gravity, and that feigned and imaginary matter which he has expelled out of the celeftial spaces, to make way for the regular and lafting motions of the planets and comets. Befides, when this his opinion of the agency of an ethereal medium, as delivered in his Optics, was urged by Mr H. and his friends, as a conceffion of motion by impulse of the fluid of the air in its different conditions of light and fpirit; then the Newtonians made light of his queries, as what we might not rely upon, and from which we could not argue, or draw any conclufions with certainty; and chose to stick by gravity unexplained, as a quality or property belonging to every par