There was a time when it was the fashion, in opposition to the French system of Descartes, to banish all subtile matter out of the world, to make room for qualities in a vacuum: but later observations have reinstated a subtile medium, which is now triumphant in every branch of philosophy: and the learned in general seem more inclined than formerly to make use of it, for the explanation of the most difficult appearances in nature, not excepting even gravity itself. The inquiry is rational, and deserves to be promoted, though it may bear hard upon some popular prejudices pretty deeply rooted: for so long as actio in distans, be it in bodies in particles or in atoms, keeps its place in philosophy, it will be an insuperable bar to all improvements; because the most useful experiments toward the advancement of physical knowledge are those made upon the elements, all of which tend to shew us how matter interposes to produce such changes and motions as we observe in bodies distant from each other. How does the sun act upon the fruits of the earth, but by the mediation of its light? How do the clouds water the earth, but by the mediation of the air? How does the chemist produce so many changes in na

tural bodies, but by the mediation of fire? In short, wherever distant bodies are found to affect each other, there is always something to mediate, whether we see it or not; and where this mediation is no farther to be traced, there philosophy ends, and the fictions of imagination begin; which are all of equal value, whatever name you call them by, be it sympathy, antipathy, attraction, repulsion, cohesion, elasticity, antiperistasis, or any other, ancient or modern. Nothing is intelligible but the action of matter upon matter; and though we may affect to soar above this principle in theory, we are always obliged to descend to it when we come to practice. The experiments usually made to illustrate the doctrine of central forces, are very ingenious and elegant, and will captivate the attention of students who are máthematically inclined; but there is this error running through them all, that the moving body is connected by a line with the centre of its motion; which cannot therefore be accommodated to motion in a vacuum, where no line of communication is supposed, but is objected to upon principle, lest it should hinder the effect. And there is this farther defect in those experiments, that

the

the centrifugal force, or force in the tangent, being consequential to the artificial revolution of a whirling body, cannot be used as a cause of the motion; because it is the nature of all causes to be prior to the effect, but here it is posterior: the body is never disposed to fly off in a tangent, till it has acquired its revolution. Such a force therefore can never be applied to account for any of the celestial motions; because it comes to this absurdity, that there is nothing to account for the motion but the motion itself, or its consequence, which is the same thing.

Circulation of Matter necessary to be supposed.

If we are permitted to argue from analogy, which is the best and safest rule we can follow, it is most reasonable to suppose that all the lasting motions of the world depend upon a circulation of matter. It is evident to sense that this principle prevails in the human frame; and our view of its mechanism has been greatly enlarged by the discovery of the circulation of the blood. Where this motion begins, or where it ends, it is hard to say. The heart is the centre

of it; but

whether

whether the influx of the venal blood, or the efflux of the arterial, is first in order, the anatomist will find it very difficult to determine. Nothing can give without receiving; but where the giving and receiving depend on each other, as in a circle, the motion is perpetuated. Nothing is lost in the heart by what it conveys to the arteries, be-. cause it receives as much by the veins; and, so long as the machine is in order, what it receives by the veins cannot overcharge the heart, and occasion a stagnation, because it is discharged as fast by the arteries. In like manner, all the rivers run into the sea, and yet the sea is not full; and on the other hand, all the rivers come out of the sea, and yet the sea is not exhausted. What it receives it sends off in vapours from its surface, or by percolation through subterraneous passages, which, as so many vessels, communicate with the grand reservoir of waters. But this circulation is eminently carried on by a changing of the water into a new form, and a regeneration of it into its primitive form again. It goes off from the surface of the ocean in the form of a rare, invisible, expanded vapour, perfectly dissolved in the air as in a menstruum, and being for some time suspended

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suspended in that state, it is afterwards condensed into mists and clouds, then gathered into drops when it falls, which drops are all assembled into one fluid mass, and in this form it returns to the place from whence it came, to take its turn once more in the common course of evaporation, and be circulated again and again to the great promptuary of the world.

Such a Circulation is an undoubted Fact.

This change of matter into a different form, with the subsequent regeneration of it into its primitive form, is one of the great secrets of nature, whereby the world is kept from decaying, either with respect to its matter, or its motion. The source which returns upon itself can never fail; and it is of little moment to consider which is first in order, whether the vapour or the water; for the vapour will never want water to supply it, and the water will never want the return of vapour to keep its stores undiminished. How this order of things contributes to the support of the earth and its productions, is not before us now, it being sufficient here to observe, that, by means of a circulation in

matter,