tive, quality; and, tho' it be allowed that the vis inertiæ of bodies is only apparently inactive, yet bodies at reft do not, in confequence of being poffeffed of it, actually refift other bodies, unless thofe other bodies are moved, or preffed against them: fo that their actual resistance is the effect of motion. Every phyfical fubject, whether confidered as fomething diftinct from, or as a compound of, its properties, is called a fubftance; and every phyfical fubftance muft have dimenfions; this fubftance then must describe more space when it acts than when it does not; that is, it must move for if it neither wholly or partially changes places, it is totally inactive, and if it doth, it moves.

Again, fuppofe it could be proved, that a perfect folid, lying at reft, fhould refift another solid striking against it: this is no proof of the exiftence of any active principle in the former; the active part of the refiftance appertaining to, and arifing from, the body ftriking, or preffing, against it. Hence, if there exift in Nature a perfect folid body, it is, doubtless, perfectly inactive; and, tho' in moft bodies that come under phyfical experiment, we find not only a paffive but an active refiftance, yet it is more reasonable to fufpect this to be the effect of the inteftine motion of the conftituent parts of fuch bodies, than to conceive fuch body has a power of refiftance in itself, without any fuch motion.

It is true, Mr. Colden fays, that the property of resistance is fo very different from that of moving, that they can in no manner be conceived as the effects of the fame agent or cause and again, that nothing of motion enters the conception of refiftance., But let me afk him, by what means he obtained his idea of refiftance. Let us fuppofe a ball of ivory, or marble, made to vibrate with fuch velocity, that its vibrations fhould be imperceptible; and that, at the end of each vibration, it fhould ftrike the palms of the hands, placed at twelve inches diftance from each other, fhould we not conceive that we held a piece of ivory or marble between our hands, twelve inches long? and would not the sensation occafioned by the motion of fuch a ball, be exactly the fame as that of the refiftance occafioned by our holding fuch a piece of marble or ivory endwife between our hands? Doubtlefs it would, as certainly as that a lighted ftick turned fwiftly round, appears to be a hoop of fire; or, as a fwiftly vibrating chord appears to be as thick as the length of its vibrations.

See Principles of Action, page 11.

In

In Mr. Colden's treatise, we are further told, that there exifts another kind of Matter, endowed with a principle of felf-motion, or a tendency to move in every direction; which always takes effect on the fide of the leaft refiftance *. Now, as experiment does not furnish us with any idea of the active resistance of a substance, independent of motion, so neither can we form any phyfical idea of the felf-motion of such fubftance, or of the tendency of it to move in all directions; without fuppofing fuch a tendency the effect of some prior motion.

I do not pretend to deny the existence of bodies capable of actual refiftance, or fuch whofe parts are propense to move in the direction of the leaft refiftance. A thousand experiments ferve to prove the exiftence of elaftic bodies actually refifting in every direction; but it does not follow that fuch elafticity is a primary effential property, owing to no mechanical caufe. To fay that refiftance or motion is effential to this, or that, kind of matter, which fome how or other re fifts and moves, is no explanation of the phenomena of the refiftance and motion of bodies.

It is not, however, an eafy task to explain the feveral pearances in Nature, folely from the principles of matter and motion: but, as a mechanical explication would be infinitely more fatisfactory than a metaphyfical one, it is requifite that method fhould be purfued much farther than it has yet been done. For I cannot conceive we are under the neceffity of fuppofing the existence of two or three different kinds of elementary matter, till it be fhewn that the properties by which they are diftinguished, are not, or cannot be, the mechanical effects of one kind of matter, variously put in motion,

I will not take upon me, notwithstanding, to fay, there exists in Nature an abfolutely, and in every fenfe, inert, impenetrable substance, fuch as the primary elements of bodies have been supposed to be. I fubfcribe, in a great degree, to the Berkleian fyftem, and believe the contrary: but, with refpect to all physical reasoning, it is exactly the fame thing whether it does or not. For, agreeing with Mr. Colden, that the property or quality of any thing is nothing else but the action of that thing +; I fay, the vis inertia, elasticity, gravity, and all the other active properties of bodies, arife only from the different modes of the motion of the component parts of fuch bodies; the primary elements, confidered

See Principles of Action, page 12. + See Ibid. page 3.

therefore

therefore as divefted of these properties, muft also have no mechanical action, i. e. motion or principle of motion left.

In this cafe, matter would confist only of folidity and mobility; that is, would only take up a certain quantity of fpace, and be liable to be put in motion. It may be faid, indeed, that the cause of its folidity, or that caufe in confequence of which any one part of matter excludes every other part of it the fame place, is an agent. It may be fo; and I conceive it is; but its agency is of so different a nature to that of mechanical action, that no experiment can affift us in the investigation of it. It should be esteemed, therefore, an object of abftract metaphyfical reasoning, and ought never to be ranked in the clafs of physical agents.

It is a mistake to think the refiftance of bodies proves the impenetrability of the primary matter, or that such impenetrability proceeds from a fimilar cause to the refiftance of bodies. The firft elements may be perfectly inert, in a mechanical fenfe, and yet be notwithstanding impenetrable; their impenetrability being, in fact, a neceffary confequence of their being homogeneous; for, even fuppofing every one of them to be agents, and their mode of action what it will, a fimilar agent, acting with the fame degree of power, in every one of them, how fhould one be penetrable by the other? And, by the way, let it be obferved, that the only proof we can have of the impenetrability of body is, that it is impenetrable to other bodies. The abfolute impenetrability of matter is, therefore, in all probability, a mere chimera: at leaft, whether it be or not, no experiment we can make can poffibly determine. With regard to all mechanical reafoning, however, it is, and must be confidered, as both inert and impenetrable; and if, from the motion of fuch elements, may be deduced the feveral properties of bodies, with the general and particular phenomena of Nature, it is furely needlefs to attribute them to the agency of powers, of whose mode of action we can form no competent idea.

With refpect to Mr. Colden's application of his Principles, in explaining the cause of Gravitation, and the Motion of the Planets, it is certainly ingenious, and, perhaps, very near the truth. The exiftence of an elaftic medium, or Ether, in the space between all bodies, is undoubtedly true; and there is all the reafon to believe, that Gravitation is the effect of the action of fuch bodies on that Ether, and of the reaction of that Ether on bodies: but it must not be concluded, as before obferved with regard to refifting bodies, that the Elafticity of that Ether is owing to any effential quality in

3

its

its component parts, inexplicable on mechanical principles. On the contrary, it were not, perhaps, difficult to fhew in what manner an elastic body would neceffarily be formed by the most fimple motion given to the primary elements of matter, such as I have above reprefented them, i. e. only as inert, moveable, and impenetrable.

It is well known, that in order to be moveable, whatever be their form, they muft lie at fome little distance from each other, or, in confequence of their impenetrability, they could not move ‡.

Suppose then a motion given to a number of contiguous particles, lying as it were in a plane, in a direction perpendicular to the faid plane; it is evident, that as thefe particles move forward in parallel directions, thofe lying at reft before them must be difplaced by their motion. But, unless the direction of the moving particles fhould pafs juft through the center of every other lying before them, they would ftrike or prefs them unequally on the fides, juft as they should strike them more or lefs obliquely: thofe particles only which should lie in a line perpendicular to the center of the moving plane, and therefore would be impelled with equal force on both fides, being carried directly forward. These alfo, being impelled with the greatest force, would, in confequence of the univerfal principle of action and reaction, retard the motion. of the moving particles in the center of the plane, while those toward the extremities of the faid plane would move fafter than thofe in the middle, in confequence of meeting with less refistance; the particles lying at reft, in the direction of the moving ones, on the outfide of the plane, being fooner difplaced than those lying in the way of the middle of that plane. Now, the particles defcribing, or moving in, this imaginary plane, being moved in every part of it with an equal degree of force, and refifted by a force unequal in every part, those particles will not continue to move forward as they fet out, in parallel directions, but tend to a certain. point, or focus, perpendicular to the center of the plane.

That is, not one among another, in the fame quantity of space they should defcribe while at reft.

REV. Dec. 1759.

Mm

For,*

a

b

18+

a

d

For, let the Circles a, b, c, d, e, f, g, b, in the annexed Diagram, reprefent the particles lying in the diameter of the faid plane; and being moved, b fetting out in the parallel directions a a, b b, &c. it is plain, that as, for the reafons before given, the particles a and b will move fafter than all the others, and d and e flower; and that the reft will move more or less flow in proportion to their diftance from the center; fo, for the fame reafon, the fide of each particle tohward the center will move flower than the other fide of it, and therefore the direction of all those

80+...

f

g

particles, will be changed; and, instead of moving in parallels, as in the above Diagram, will move toward a point, as in the following,

As the motion should be continued, alfo, the inequality of refistance increafing, the focus, or point, to which they would tend, would approach nearer and nearer, till it fhould arrive in the center of all the moving particles; which at that inftant forming a fphere, and ftriking together, in directions to their common center, would vibrate back again in the oppofite direction, all in right lines from that center.

It may be objected, indeed, that these particles ftriking against each other in oppofite directions, their whole motion

fhould