Conic-Sections, which are fometimes very Eccentric, and fometimes nearly Circles; one may reasonably afk, how it comes to pafs, that thefe Vortices remain entire, and have fuffered no Manner of Perturbation in so many Ages from the Actions of the conflicting Matter. Certainly, if thefe fictitions Motions are more compounded, and more hard to be accounted for, than the true Motions of the Planets and Comets, it ferves to no Purpose to admit them into Philofophy; fince every Caufe ought to be more fimple than its Effect. Allowing Men to indulge their own Fancies, fuppofe any Man fhould affirm, that the Planets and Comets are furrounded with Atmospheres like our Earth; which Hypothefis feems more reasonable than that of Vortices. Let him then affirm, that these Atmospheres by their own Motion move about the Sun, and defcribe Conic-Sections, which Motion is much more eafily conceived, than that of the Vortices penetrating each other. Laftly, that the Planets and Comets are carried about the Sun by thefe Atmospheres of theirs, and then applaud his own Sagacity in difcovering the Caufes of the Cæleftial Motions. He that rejects this Fable, muft alfo reject the other; for two Drops of Water are not more alike, than this Hypothefis of Atmospheres, and that of Vortices.

GALILEO has fhewn, that when a Stone projected moves in a Parabola, its Deflexion into that Curve from its Rectilinear Path is occafioned by the Gravity of the Stone towards the Earth, that is, by an occult Quality. But now fomebody, more cunning than he, may come to explain the Caufe after this Manner. He will fuppofe a certain fubtile Matter, not difcernable by our Sight, our Touch, or any other of our Senfes, which fills the Spaces that are near, and contiguous to the Superficies of the Earth; and that this Matter is carried with different Directions, and various, and often contrary Motions, defcribing Parabolic Curves. Then fee how eafily he may account for the Deflexion of the Stone above fpoken of The Stone, fays he, floats in this fubtile Fluid, and following its Motion, can't chufe but defcribe the fame Figure. But the Fluid moves in Parabolic Curves; and therefore the Stone must move in a Parabola of Course. Would not the Acuteness of this Philofopher be thought very extraor

dinary,

dinary, who could deduce the Appearances of Nature from mechanical Caufes, Matter and Motion, fo clearly, that the meanest Man may understand it? Or indeed fhould we not fmile to fee this new Gallileo taking fo much Mathematical Pains to introduce occult Qualities into Philofophy, from whence they have been fo happily excluded? But I am ashamed to dwell fo long upon Trifles.

THE Sum of the Matter is this: The Number of the Comets is certainly very great; their Motions are perfectly regular; and obferve the fame Laws with thofe of the Planets. The Orbits, in which they move, are ConicSections, and thofe very eccentric. They move every Way towards all Parts of the Heavens, and pafs through the Planetary Regions with all poffible Freedom; and their Motion is often contrary to the Order of the Signs. Thefe Phænomena are moft evidently confirmed by Aftronomical Obfervations, and cannot be accounted for by Vortices. Nay indeed, they are utterly irreconcileable with the Vortices of the Planets. There can be no Room for the Motions of the Comets, unless the Cæleftial Spaces be entirely cleared of that fictitious Matter.

FOR if the Planets are carried about the Sun in Vortices; the Parts of the Vortices, which immediately furround every Planet, must be of the fame Denfity with the Planet, as was fhewn above. Therefore all the Matter contiguous to the Perimeter of the Orbis Magnus, muft be of the fame Denfity with the Earth. But that which lies between the Magnus Orbis, and the Orb of Saturn, must have either an equal or greater Denfity. For to make the Conftitution of the Vortex permanent, the Parts of lefs Denfity muft lie near the Centre, and thofe of greater Denfity muft go farther from it. For fince the periodic Times of the Planets are in the Sefquiplicate Ratio of their Diftances from the Sun, the Periods of the Parts of the Vortices muft alfo preferve the fame Ratio. Thence it will follow, that the centrifugal Forces of the Parts of the Vortex must be reciprocally as the Squares of their Distances. Thofe Parts therefore, which are more remote from the Centre, endeavour to recede from it with lefs Force; whence, if their Denfity be deficient, they muft yeild to the greater Force with which the Parts that

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lie nearer the Centre endeavour to afcend. Therefore the denfer Parts will afcend, and thofe of less Denfity will defcend; and there will be a mutual Change of Places, till all the fluid Matter in the whole Vortex be fo adjufted and difpofed, that, being reduced to an Equilibrium, its Parts become quiefcent.

IF two Fluids of different Denfity be contain'd in the fame Veffel; it will certainly come to pass, that the Fluid of greater Denfity will fink the loweft; and by a like Reafoning it follows, that the denfer Parts of the Vortex, by their greater centrifugal Force, will afcend to the higheft Places. Therefore all that far greater Part of the Vortex, which lies without the Earth's Orb, will have a Denfity, and by Confequence a Vis Inertia, anfwering to the Bulk of the Matter, which cannot be less than the Dénfity and Vis Inertia of the Earth. But from hence will arife a mighty Refiftance to the Paffage of the Comets, and fuch as can't but be very fenfible; not to fay, enough to put a Stop to, and abforb their Motions entirely. But now it appears, from the perfectly regular Motion of the Comets, that they fuffer no Refiftance that is in the leaft fenfible; and therefore that they meet with no Matter of any kind, that has any refifting Force; or, by Confequence, any Denfity, or Vis Inertia. For the Refiftance of Mediums arifes, either from the Inertia of the Matter of the Fluid, or from its Want of Lubricity. That which arifes from the Want of Lubricity is very fmall, and is fcarce obfervable in the Fluids commonly known, unless they be very tenacious like Oil and Honey. The Refiftance we find in Air, Water, Quick-Silver, and the like Fluids that are not tenacious, is almost all of the first kind; and cannot be diminished by a greater Degree of Subtilty, if the Denfity and Vis Inertia, to which this Refiftance is proportional, remains: As is most evidently demonftrated by Sir ISAAC NEWTON, in his noble Theory of Refiftances, in the Second Book of his Phil. Nat. Princ. Math.

LODIES, in going on through a Fluid, communicate their Motion to the ambient Fluid by little and little, and by that Communication lofe their own Motion, and by lofing it are retarded. Therefore the Retardation is proportional to the Motion communicated; and the commu

nicated

nicated Motion, when the Velocity of the moving Body is given, is as the Denfity of the Fluid; and therefore the Retardation or Refiftance will be as the fame Denfity of the Fluid; nor can it be taken away, unless the Fluid, coming about to the hinder Parts of the Body, reftore the Motion loft. Now this cannot be done, unless the Impreffion of the Fluid on the hinder Parts of the Body be equal to the Impreflion of the Fore Parts of the Body on the Fluid; that is, unless the relative Velocity, with which the Fluid pushes the Body behind, is equal to the Velocity with which the Body pushes the Fluid; that is, unless the abfolute Velocity of the recurring Fluid be twice as great as the abfolute Velocity with which the Fluid is driven forwards by the Body, which is impoffible. Therefore the Refiftance of Fluids, arifing from their Vis Inertia, can by no Means be taken away. So that we must conclude, that the Cæleftial Fluid has no Vis Inertia, because it has no refifting Force; that it has no Force to communicate Motion with, becaufe it has no Vis Inertie; that it has no Force to produce any Change in one or more Bodies, because it has no Force wherewith to communicate Motion; that it has no Manner of Efficacy, because it has no Faculty wherewith to produce any Change of any kind. Therefore certainly this Hypothefis may be justly called ridiculous, and unworthy a Philofopher; fince it is altogether without Foundation, and does not in the least ferve to explain the Nature of Things.

THOSE, Who Wou'd have the Heavens filled with a fluid Matter, but fuppofe it void of any Vis Inertie, do indeed in Words deny a Vacuum, but allow it in Fact. For fince a fluid Matter of that kind can no ways be diftinguished from empty Space; the Difpute is now about the Names, and not the Nature of Things. If any are fo fond of Matter, that they will by no Means admit of a Space void of Body; let us confider, where they must come at laft.

FOR either they will fay, that this Conftitution of a World, every where full, was made fo by the Will of GoD to this End, that the Operations of Nature might be af fifted every where by a fubtle Æther pervading and filling all Things; which cannot be faid however, fince we have fhewn, from the Phenomena of the Comets, that this ther is of no Efficacy at all; or they will fay, that it became

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fo by the fame Will of GoD, for fome unknown End which ought not to be faid, because for the fame Reafon a different Conftitution may be as well fuppofed; or, laftly, they will not fay, that it was caufed by the Will of God, but by fome Neceffity of its Nature. Therefore they will at at fink into the Mire of that infamous Herd, who dream that all Things are governed by Fate, and not by Pio idence; and that Matter exifts by the Neceffity of its Nature always, and every where, being infinite and eternal. But, fuppofing thefe Things, it must be also every where uniform; for Variety of Forms is entirely inconfiftent with Neceffity. It must be alfo unmoved 1; for if it be neceffarily moved in any determinate Direction, with any determinate Velocity, it will, by a like Neceffity, be moved in a different Direction, with a different Velocity; but it can never move in different Direc tions, with different Velocities; therefore it must be unmoved. Without all doubt, this World, fo diverfify'd with that Variety of Forms and Motions we find in it, could arife from nothing but the perfectly Free Will of God directing, and prefiding over all.

FROM this Fountain it is, that thofe Laws, which we call the Laws of Nature, have flowed; in which there appear many Traces indeed of the most wife Contrivance, but not the leaft Shadow of Neceffity. These therefore we must not feek from uncertain Conjectures, but learn them from Obfervations and Experiments. He who thinks to find the true Principles of Phyfics, and the Laws of natural Things by the Force alone of his own Mind, and the internal Light of his Reafon, muft either fuppofe that the World exifts by Neceffity, and by the fame Neceffity follows the Laws propofed, or, if the Order of Nature was eftablished by the Will of GoD, that himself, a miferable Reptile, can tell what was fitteft to be done. All found and true Philofophy is founded on the Appearances of Things; which, if they draw us, never fo much against our Wills, to fuch Principles as do moft clearly manifeft to us the most excellent Counsel and fupreme Dominion of the All-wife and Almighty Being thofe Principles are not therefore to be laid afide, because fome Men may perhaps diflike them. They may call 'em, if they please, Miracles or occult Qualities; but Names maliciously