municates, and thence loses nothing by the communication.

Air and fire, by the established laws of Nature, have a motion of their own, prior to the motion of the machine. The motion does not observe the law of projectiles, but is contrary to it. The motion of projectiles is from more to less: the motion of air and fire is from less to more; as when the fire of a spark increases to the fire of a whole city : or, the motion first excited in a little corn of gun-powder, gives fire to the whole charge of the mine; in which cases, the motion is not diminished, but increased by communication. It has been said, that if the resistance of a fluid, arising from its vis inertiæ, is diminished, the quantity of matter must be diminished: but there appears to be another way of diminishing the resistance; for, let the fluid in question act as an impelling cause to a moving body, and its resistance, with respect to the same body, becomes of no account: it is in a manner annihilated, and that without removing a single particle of matter. The like is true of all floating bodies; where the motion of the fluid conspires with the motion of the body: for, when this happens, the body moves with the velocity

velocity of the fluid, and loses nothing by communication. The same will hold in all other cases of the kind. In fluids which have a motion of their own, bodies under proper circumstances will not lose motion, but acquire it. If the matter of the heavens has a motion of its own, the planets will thence derive their revolutions, instead of being retarded by it.

As it was a practice with many writers in the last age to deny the existence of any fluid matter more subtile than air, I thought it necessary to consider their arguments. This gave occasion to a report, that I had revived the subtile matter of Descartes, whose hypothesis of the Vortices had long been out of repute. But this was not true: I had, indeed, insisted on the reality of a subtile matter; but had rejected that of Descartes, and had taken occasion to advance what I thought an insuperable objection, viz. that he had ascribed a motion to it not verified by any one instance in Nature, so far as I could then perceive. I find (though I could scarcely think it possible under the present state of philosophy), that there are some learned men to this day, who are strongly disaffected to all subtile mediums. The

learned

any

learned Spaniard, Feyjoo, several of whose pieces have lately made their appearance in an English translation, offers a mechanical reason, to prove that all subtile matters must be impotent and useless. The more fluid any matter is (says he), the less is its impulsive force. A stream of water gives a less violent shock to a wall, than a solid body of equal dimensions; and air, a much less than water: a subtile matter more fluid than air would impress a weaker impulse; and if there is matter infinitely fluid, its impulse must totally vanish: therefore, no body can be moved by the impulse of a subtile matter. This looks fair; but, when the case is truly stated, the author's reasoning may be inverted. Fluid matter can move with greater velocity in proportion to its fluidity: water flows faster than oil, and air faster than water. A stream of air will give a more violent impulse to a leaden bullet than any stream of water of the same dimensions, from its velocity: whence a body of infinite fluidity will have infinite velocity; and from its infinite velocity its force will be infinite. This conclusion is contradictory in terms to that of the learned Spaniard; but it is much nearer to the truth as it is in nature. Nobody doubts

doubts but that the matter of lightning is more fluid than the matter of air; yet, from its velocity, we see it shatter stone-buildings, and rend oak-trees, with a force incomparably greater than a column of air of the same dimensions. A friend of mine at Cambridge, learned in philosophy, expresses the same mean opinion of a subtile medium; and thinks that, if admitted, it would not lessen the difficulties in philosophy. But Newton certainly did not reason in this manner: he thought, that gravity, with the impulse of a subtile medium, was a lesser difficulty in philosophy, than gravity without impulse: why, else, did he propose the former to supply the defects which had been objected to the latter? It is asked, whether it is easier to conceive a medium of so great rarity and elasticity diffused through all the planetary regions, than to conceive gravity without impulse? I must say, I think it is; because it will always be easier to conceive gravity with impulse, than gravity without it, even under all the disadvantages of that rarity in the medium, which it is now no longer necessary to suppose. But to these disadvantages it is added, that if we introduce the agency of one subtile medium into nature,

VOL. IX.

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we

1

we must account for the power of that one, by multiplying other subtile matters without end. But the consequence is not necessary: for, if we keep within the bounds of nature and experiment, the proper province of physical disquisition, we shall soon come to a ne plus ultra. Thus, for example-a barrel of gun-powder blows a house up; and we find that gun-powder is a solid composition of nitre, sulphur and charcoal. How must we account for its effect? If we go beyond the solid matters of the composition, the minerals and the charcoal, shall we be obliged to multiply subtile matters without end? Surely there can be no occasion for this.. Accordingly we find, when we examine farther, that the explosive force of gun-powder is from a blast of air in the nitre, expanded by fire in the sulphur; and that these are more suddenly kindled and opened by the mediation of charcoal. Thus we understand of gun-powder what philosophy inquires after: we discover, that its force is no quality of the solid materials; but that these comprehend quiescent air, excited to action, and expanded by fire. Gun-powder, with fire and air to give it force, is to me a much lesser difficulty in philosophy, as well as a

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