artificial force or violence, contrary to the nature of the medium in which it moves, the parts of the medium, by endeavouring to recover their natural ftate, will refift the motion of the body till the equilibrium is reftored, and the body is at rest. Such will neceffarily be the cafe of all violent motions; it is foon deftroyed by refiftance, though the time in which it is deftroyed may differ from a variety of circumftances.

But on the other hand, if the motion of the body arifes from the motion of the medium in which it moves, then the refifting nature of the medium is no longer an objection to the motion of the body, neither can it be, for it is the cause of it's motion; and it is abfurd to fuppofe, that the cause of a motion can refift the motion it caufes. No inference therefore from the refiftance of mediums, can lead us to the neceffity of a vacuum. A vacuum is only neceffary when a motion is proposed, which is independent of the action of every medium; but nature knows of no fuch motion.

A variety of motions may be exhibited, for whofe production the prefence of a refifling medium is abfolutely neceffary; and they fhew, that fo far from a vacuum being neceffary to the continuance of motion in any space, the motion is promoted and occafioned by a refifting medium. That bypothetical train of reafoning which leads us to conclude, that if lefs matter were in the fpace, the motion would be more free, and continue much longer, is as unphilofophical, as it would be, if in order to enable a man to run faster, we fhould rid him of the incumbrance of his boots and fpurs, by cutting off his legs.

Air is, you know, a refifting medium, yet inftead of retarding the motion of the lamp ma

chine, which I before fhewed you, by it's refiftance preferves that motion; and if the motion be at laft difcontinued, it does not arife from defect or irregularity of the caufe, but to the imperfection of the materials. If the materials which are acted upon would but continue in the fame ftate, the motion would be unretarded as long as air and fire, which are the caufes thereof, fubfift in the world. In this experiment the caufes are not artificial and violent, as in the central force machine, but fuch as are fupplied by nature itself, in it's regular mode of action; which both begins and continues the motion. What is performed by the agents in nature, in the one cafe, may certainly be done in others. The planets may be carried round in their orbits by the fame means. The heavens may be filled throughout with an etherial fluid, not infinitely rarified, unrefifting, and impotent, but denfe and continuous in it's parts.

The writers in favour of the mechanical fyftem urge, that their opponents have no notion or means of refolving their axioms, or relative laws of motion, to mechanifm, but confider them merely as laws; another word, as they use it, for ultimate, fpiritual, unmechanical power. As the penetration of fome amongst them has carried them fo far as to fuppofe an impelling etherial medium for maintaining attraction, gravitation, &c. &c. it is rather furprising that they could not perceive, that the fame medium was neceffary for fupporting their laws of motion, rest, refiftance, &c. for the difficulty does not lie in accounting for gravitation, or any particular kind of motion, but in finding powers to produce and maintain motion in general. If thefe are mechanical, it is cafy to fuppofe, that the contriver may have adjusted the mechanifm fo as to produce the particular tendencies. But if they are unmecha

nical, you may call them laws, properties, or any other name, either with or without a meaning. How detrimental it is to the increase of knowledge in the powers and agency of nature, to have the moft curious productions of thefe powers reduced to unintelligible laws, characterized by words without meaning, and which render their inventors no wifer than the moft heedlefs and unattentive!

Without infrumental, or fecond caufes, there can be no regular course of nature; and without a regular course, nature could never be understood. The order and courfe of things, and the experiments we daily make, fhew that there is a mind, that governs and actuates this mundane fyftem, as the proper real agent and caufe; the inferior and inftrumental cause seems to be fire; with refpect to attraction, it cannot produce, and in that fenfe account for the phenomena, being itfelf one of the phenomena produced and to be accounted for. What is faid of forces refiding in bodics, whether attracting or repelling, it can only be confidered as a mathematical hypothefis, not as any thing real and exifting in nature.

The mechanical agency of the elements accords with the defcriptions and állusions of the facred fcriptures, The heathens were in fome degree acquainted therewith, When this doctrine was in their hands, a principle of intelligence was afcribed to the active elements, and they were taken for the Gods who govern the world. But with those who are taught that the TRUE GOD is diftinct from, and above the world of matter, though virtually prefent by a providential infpection and fuperintendance, it ferves only to enlarge and exalt their ideas by fetting before them the vifible evidence of divine wifdom, which with fo exquifite a contrivance, and fuch fimplicity of defign, hath

adapted

adapted phyfical caufes to the production of their refpective effects.

We have now to confider, 4thly, the mathematical principles of philofophy. The celeftial motions have been thought to refemble thofe exhibited to us in the phenomena of magnetifm and electricity; thefe and the celeftial bodies feem to act upon each other at a distance, without any observed intervening impulfe. Accordingly many philofophers, both ancient and modern, have imagined that the planets are influenced by caufes fimilar to thofe of thefe more familiar phenomena. But these philofophers had formed no accurate notions of the agency of the causes of the motions from which they attempted to derive an explanation; neither had they examined attentively the circumstances of the motions which they attempted to explain. At laft, SIR ISAAC NEWTON Contented himself with an inveftigation of the laws obferved in the agency of the caufes of the celestial motions, difcovered that thefe laws were the fame with those observed in the agency of the causes of the motion of common heavy bodies, and from this discovery gave a theory of mathematical aftronomy. We are indebted, however, to KEPLER, for the generalization of the facts, which form the basis of the mathematical theory.

KEPLER'S LAWS.

Kepler's first law is, that the planets, in revolving round the fun, defcribe equal areas in equal

times.

Kepler's fecond law is, that the orbits defcribed by the planets are ellipfes, having the fun, or the primary planets, in the focus.

Kepler's third law is, that the fquares of the

VOL. IV.

S

periodital

periodical times of the planets are as the cubes of their mean diftances from the fun. That is, as the fquare of the time which a planet A takes to revolve in it's orbit, is to the time which any other planet B takes to run through it's orbit; fo is the cube of the mean distance of A from the fun, to the cube of the mean diftance of B from the fun.

OF DEFLECTING FORCES.*

In confequence of the inertia of matter, all motion is confidered as equable and rectilineal, and as being in a strait line with the direction of the moving force; and as preferving this direction until it be hindered or put out of it's way by fome extrinfic cause.

If therefore a body moves in a curve, that curvature muft proceed from fome external force continually acting upon the body; and whenever that force ceafes to act, the body will move forward in a right line, touching the curve in that point where the body is at the inftant of time when the force ceafes to act.

When you observe a change in the direction of any motion, you may infer the action of a force, whofe direction croffes that of the former motion. This may be called a deflecting force.

The change of direction is measured by the angle contained between the former and the new direction.

When the motion of a body is curvilineal, the deflection is continual, and you may infer the continual action of a deflecting force. On the other hand, the continual action of a deflecting force produces a curvilineal motion.

In

*Profeffor Robifon's Outlines of Mechanical Philofophy, page 34 to 107.