Sidor som bilder
PDF
ePub

England, it therefore burst forth through the mist by which it was enveloped, with resplendent lustre ; and by the indefatigable labours of this truly great man, Sir Isaac Newton, the Copernican system is established so firmly, that there can be no fear on the minds of the lovers of truth, that this doctrine will be ever upset; which is to say, that the Sun is in the center of our system, and that the planets revolve round that center, each in his respective orbit, and that the earth, as well as the rest of the planets, revolves round their own centers diurnally. Thus we are arrived at an epocha so much wanted, which sets at rest that long contested question, and furnishes us with the necessary foundation on which to build future speculations; without which it would be impossible to proceed. However, much yet remains to be done.

1. What are the means by which this wonderful Machinery of Nature is supported and kept in the exact uniformity which their different motions display?

2. By what means are Light and Heat caused? 3. What causes that regularity of the winds within the Tropics, and irregularity without?

4. By what means is it that all Heavy Bodies incline towards the Earth, and Lighter Ones move from it?

5. What occasions the Tides?are such weighty questions, that require such answers that the most simple scholar may comprehend them agreeably to the ways of nature, unembarrassed.And several other phenomena of minor consideration, which have never yet been satisfactorily accounted for. It may be said in respect to the first question that Sir Isaac Newton has fully éstablished, that projectile and centripetal forces are the powers which keep the planetary system in motion, which opinion is founded on the following reasons, as given to us by Maclaurin in his preface

to Sir Isaac Newton's System of the World. "There "remained one thing farther to be known, which "was, the cause which retains the planets in those "regular motions they are found to revolve with. "For since projected bodies, impelled by a single "force, move by their own nature uniformly along "the right lines in which the forces that impel them "are directed without any possibility of deviating " from those rectilinear paths; it follows that bodies like the planets, that move in curve lines, must be "acted on by more forces than one, or by some "compound force, drawing them out of the right "lines they naturally tend to describe. The ques"tion then was, of what nature that force is, which "is diffused through so vast an extent as that of "the planetary system, and which seems to be "simple and uniform by its uniform effects above" mentioned, of the proportionality of the areas and "times; and the sesquialteral ratio of the distances "and periods, said above to be collected from the "observations.

"To this end the author did not frame hypo"theses as other philosophers used to do, but set "himself to examine the phenomena themselves, "by mathematical reasoning. And first of all "he found that bodies describing areas propor"tional to the times by radii drawn to any given point, are impelled by two forces; by one of "which they continually endeavour to fly off in the tangents of the orbits they describe, and by the other, which continually impels them to the given "point, they are withheld from doing so, and "retained in those curvilinear paths.

[ocr errors]
[ocr errors]

"Since then the planets by radii drawn to the "sun, describe areas proportional to the times, it "follows that the compound force which keeps "them in their regular motions according to that "law, is compounded of two forces; one of which impels them according to the tangents of their

orbits, and the other impels them towards the "sun; the first of which we may call the projectile " and the other the centripetal force.

66

"Now by this discovery we have made a great "advance, having found, first, that this phenomenon "of the motions of the planets may be accounted "for by two such forces; and, secondly, that the "centripetal force is directed to one fixed and given place, the sun's center; and not to any "uncertain and wandering places; which would "have made our future speculations very intricate, if "it had proved so. What the cause is of this force, "we do not yet pretend to determine; our business "is, since such a force is found to exist, to search "into the properties and proportions of that force, "before we think of enquiring into the cause of it."

In this work it will be pointed out how projectiles, particularly those of globular figure, require no more than one force to cause them to perform a circular motion, from which it is impossible to deviate; and (contrary to the whole of Maclaurin's reasoning on this point) that nothing but a second force could cause any of the planets to go off in a tangent, so that Maclaurin's grand discovery of the two points boasted of are of no consequence; because it will appear very clearly, that although the force which impels the planets through their orbits, are not directed to any central point, yet their paths are most regular, and undeviating; than which no motion can be more so. But admitting existence to such double forces, even this knowledge does not develope the nature of those powers, it merely teaches by mathematical calculations what strength is required from two powers acting on a substance at cross angles, to make it perform a revolution between these two powers, but how these powers are generated, we are not acquainted with, and how the planets are supported in space through which they move, is another problem not yet solved; the

[ocr errors]

doctrine of Gravity does not teach any thing, because here again we are engaged with a term, the explänation of which we are not informed, it is said to be a tendency which one body has on another; and acts in a certain ratio according to size and distance, ask what it is that acts? the answer is, Gravity, what Gravity is? is unknown; thus we are amused with an unknown qualification to answer the first question of Astronomy to a child of nature. That such bulky globes as the planets are, should be kept in their different situations by a power of which the intellectual mind canuot frame any notion, and at the vast distances that they are situated from each other, and also without that support being allowed necessary in space, is a kind of philosophy that cannot command implicit confidence. Experience tells us that a feather will fall with the greatest velocity in the exhausted receiver of an airpump, it must therefore be very reasonable to suppose that such vast bodies as the planets are, could not maintain the positions which they are known to do, without support.

The second question has been more ably handled by Sir Isaac Newton, Boerhaave, and other philosophers, yet as they are not agreed, and as the modern Phlogiston seems to differ therefrom, that matter cannot be said to be settled.

"The matter of the sun, or of light, the phlo"giston, fire, the sulphurous principle, the inflam"mable matter, are all of them names by which "the element of fire is usually denoted. But it "should seem, that an accurate distinction hath not

yet been made between the different states in "which it exists; that is, hetween the phenomenon "of fire actually existing as a principle in the "composition of bodies, and those which it exhibits "when existing separately and in its natural state : "nor have proper distinct appellations been assigned "to it in those different circumstances." Macquer.

"The various theories respecting heat considered "as matter and a component part of bodies are not sufficiently grounded on decisive facts to admit of a cursory discussion.--The late Dr. Black of "Edinburgh, Professor Wilcke of Stockholm, Dr. "Irwin of Glasgow, and Dr. Crawford of London, "are among the leading names of philosophers who "invented and illustrated this excellent theory, and "it is sincerely to be wished that some cotemporary "writer would settle their respective claims before "the lapse of time shall have rendered it difficult.

"Notwithstanding these various researches into "the nature and principles of light and heat, and "the very able discussions of the phlogistic and "antiphlogistic writers on chemistry, it is to be "lamented that no absolute decision has yet been "made on this long controverted subject. After "the ingenious Lavoisier had successfully combated "and refuted the phlogistic system of former che

mists, M. Buequet and Macquer, endeavoured to "restore the same doctrine under a different form. "M. Buequet in his latter course of lectures, "explains upon this principle the greater part of "the phenomena of combustion, calcination, and "reduction of metallic calces; but it does not

afford adequate reasons for the flame which is "produced by bodies in a state of ignition, nor "the rapid motion, and other changes that at"tend it. M. Macquer, though well aware of "the influence of the modern discoveries on che"mical theory, advanced an opinion that they "do not entirely overthrow the phlogistic doctrine " of Sthal. And he has found means to unite the "pneumatic doctrine of the moderns with that of "phlogiston, by considering this principle as light "fixed in bodies; after having shown that pure "light such as is emitted by the sun, may be re"garded as the true matter of fire, and that by "admitting it as fixed in bodies it constitutes the

« FöregåendeFortsätt »