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ation, the Laws of the whole planetary fyftem, which, on finding their truth confirmed by repeated facts and continued experience, he called the MATHEMATICAL PRINCIPLES OF ASTRONOMY."

From thefe Principles or general Laws of Motion, by the fame geometrical calculations, he deduced the ftupendous theory of the elliptical orbits of the planets both primary and fecondary, of the spaces through which they pass, of the different velocity with which they move, both in respect of each other and of themselves in the different stages of their ethereal journey, of their relative times, and refpective fituations; and crowned this amazing system of the heavens by his new philosophy of the motion of the comets, of the moon, and of the sea.

For the mathematical foundation of his Aftronomy, as the effects were the same, he affigned the fame or fimilar Forces existing in nature, as the efficient caufes both of

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Principia tradidi a Mathematicis recepta et Experientia multiplici confirmata. Newtoni Præf. in Princip.

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Mechanical and Celestial motion." And, indeed, both in the act of deriving his Principles from the projectile phænomena, and afterwards for the purpose of applying them to the planetary, it was neceffary to analyze the elliptical motion of the heavenly bodies into a compound of two fimple motions in right lines produced by the action of these dif

"See his Regula Philofophandi prefixed to the third book of his Principia.

Having founded his Aftronomy on the Analogy between the phænomena of Projectile and Planetary motion, from the phænomena he pushed the Analogy to the forces as the efficient causes of both. Defpiciamus,' fays Mr. Cotes in his preface, qualis fit in terreftribus natura Gravitatis, ut ' deinde tutius progrediamur ubi ad corpora cœleftia lon'giffime a fedibus noftris remota perventum fuerit;' which is more fully explained in the following words; Videamus 'jam comparatione inftituta inter Planetarum Vires Centri'petas et Vim Gravitatis, annon ejufdem forte fint generis. 'Ejufdem vero generis erunt, fi deprehendantur hinc et inde 'leges eædem, eædemque affectiones.'

• The projective impulfe given to a ftone being the cause of its affent in the air, and its own weight that of its descent to the earth, he adopted Projection and Gravitation or the Centrifugal and Centripetal Forces, in their compound operation both on the Projectile and Planet, as the causes of their fimilar motions: and these Forces he made the Mechanical foundation of his Principia. Mechanica rationalis 'erit fcientia motuum qui a Viribus quibufcunque refultant, accurate propofita ac demonftrata.' And, after his PrinciL 2

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ferent Forces: and this might also be useful for the purposes of teaching and demonftration; just as we find it neceffary, in all parts of fcience, to separate what in nature is infeparable, for the convenience and affistance of the understanding. The planetary motions may, however, be fimple and uncompound, as they most probably are, (for no experiments can be tried in these distant regions;) and the Aftronomy of Newton, which is only

pia were formed, these and fimilar Forces are the subjects upon which they were to operate in the demonftration of other phænomena. Nos ea tractamus quæ ad Gravitatem, "Levitatem, Vim Elafticam, Refiftentiam fluidorum et ejus• modi Vires, feu attractivas seu propulfivas, spectant; et ea propter hæc noftra tanquam Philosophiæ Principia Mathe*matica propofuimus. Omnis enim Philofophiæ difficultas ❝in co verfari videtur, ut a Phænomenis motuum inveftige

mus Vires Naturæ, deinde ab his viribus demonftremus ' phænomena reliqua.' This is professedly the defign and conduct of his great aftronomical work. Et huc fpectant Propofitiones generales quas libro primo et fecundo pertrac'tavimus. In libro autem tertio exemplum hujus rei propo'fiumus per explicationem fyftematis mundani. Ibi enim 'ex phænomenis cœleftibus, per Propofitiones in libris pri'oribus mathematice demonstratas, derivantur Vires Gravi*tatis quibus corpora ad Solem et Planetas fingulas tendunt: ' deinde ex his Viribus, per Propofitiones etiam Mathematicas, deducuntur Motus Planetarum, Cometarum, Lunæ et Maris.' Newtoni Præf. in Princip.

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the application of his Mathematical Principles to their mensuration from their Analogy to projectile motions, did not at all require that the Forces of Projection and Gravitation, however useful their fuppofition might be, fhould be affigned as their real exiftent Causes. It is fufficient for the Analogy, on

Sir Ifaac Newton is, doubtlefs, the firft Aftronomer in the world; and his great merit confifts in the application of Geometry to the phænomena of Motion, which he refolved into two general powers or caufes, for the purpose of applying his Mathematical calculations with effect ta discover the Principles of Motion in general, and by them to demonstrate the Phanomena of the Solar fyftem -A ftupendous exertion of the human mind. But men who have gone far, have fometimes, in the laudable zeal for knowledge and the perfection of their science, been carried beyond the bounds of truth in attempting to go farther. It was fufficient for the purpose of Aftronomy, (which differs from Phyfics properly understood, inasmuch as it confifts in the calculation of Effects, whatever the Caufes may be,) to fuppofe the existence of the Forces affigned for, whether a combination of the two Forces be the real cause of the celestial motions, does not at all affect the truth and utility of the calculations founded upon them, which would demonftrate the Phænomena equally as well upon the hypothesis, and be equally convincing when confirmed by Experience, which would crown the science.

Thus a Geometrician can work with imaginary Forces, as well as with real ones; and, as an Aftronomer, in calculating the celeftial motions, he had nothing to do with

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which the whole philofophy is founded, that the Phænomena of motion are known, from

investigating the celeftial caufe: for he himself declares that his Principles are Mathematical, not Phyfical (p. 356.) And Dr. Berkeley has well obferved, that what is faid of Forces, whether attracting or repelling, is to be regarded only as a Mathematical hypothefis, and not any thing exifting in nature' [Siris]. Though Sir Ifaac may be totally wrong as to the Caufes, he may be totally right as to the Laws, of celeftial motion, afcertained by figures and confirmed by facts.

To the fame Phænomena as Effects his philofophy attributes the fame exiftent Caufes, by a Maxim founded more in the authority of antiquity than in the truth of things. To inveftigate and affign the true caufes of things is a much more hazardous task than philofophers are willing to allow. Because the Curve defcribed by a Planet is of the fame fpecies, and the motion fimilar to those of a Projectile, to infer that therefore the Cause is actually the fame, is a conclufion more than the ANALOGY, on which his Aftronomy is founded, either requires or warrants: for the curves of both are the fame with that produced by the fimple act of cutting a piece of wood of a certain fhape in a certain direction, and it cannot be therefore argued that the causes are the fame.

That the First Caufe of all Motion, Projectile or Aftronomical, inceffantly acting in his providential care, according to general laws, is one and the fame, is most true: But, whether he may not move the grander wheels of his material fyftem by a more immediate act of his omnipotent mind, whilst he produces the fame apparent effects or fimilar motions on this earth by the operation of fecond caufes: Or, whether he does not produce the heavenly motions by fe

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