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and Tails deriv'd from the fame, especially after their Perihelia, and thofe fubject to fuch Mutations, pass thro' fo much Cold and Darkness near their Aphelia, and fo much Light and Heat near their Perihelia, as imply them defign'd for very different Purposes from the Planets; and indeed, as to their outward Parts, in their prefent State they are plainly uninhabitable. Yet by paffing through the Planetary Regions in all Planes and Directions, they fully prove thofe Spaces to be deftitute of Refiftance or folid Matter, and feem fit to cause vaft Mutations in the Planets, particularly in bringing on them Deluges and Conflagrations, according as the Planets pafs through their Atmospheres, in their Defcent to, or Afcent from the Sun; and fo feem capable of being the Inftruments of Divine Vengeance upon the wicked Inhabitants of any of thofe Worlds; and of burning up, or perhaps, of purging the outward Regions of them in order to a Renovation. This, I mean, feems likely to be their Ufe in the present State; tho' indeed they do withal feem at present Chaos's or Worlds in Confufion, but capable of a Change to Orbits nearer Circular, and then of fettling into a State of Order, and of becoming fit for Habitation like the Planets; but thefe Conje&tures are to be left to farther Enquiry, when it pleafes the Divine Providence to afford us more Light about them. However, in my Solar Syftem I have defcrib'd the Orbits of all the Comets that Dr. Halley has put into his Catalogue, and that in the Order of their Nearness to the Sun, at their Perihelia, and as they are in their proper Planes, without any Reduction to the Ecliptick. They are in Number 21; for tho' he has 24 there fet C 4

down,

down,(all which are accordingly number'd there) yet because he scarcely doubts that three of them are the fame Comet, and gueffes that two more are alfo the fame, in both which Cafes I fully agree with him; the real Number will then be but 21. The former of these two (which alfo feems to have appeared before his Catalogue begins, Anno Domini 1456.) was feen in 1531, 1607, and 1682, whofe Period therefore is 75 or 76 Years, and whofe Return is to be expected in 1758. The latter of them appeared Anno Dom. 1532; and probably the same again in 1661, whose Period therefore being about 129 Years, it is to be expected again in 1789. The most eminent of them all appear'd in the 44th Year before the Chriftian Æra; as alfo A. D. 531, or 532; and A. D. 1106; and laftly, A. D. 1680, 1681, when I faw it; and fo has made within the Limits of our present Hiftories, three periodical Revolutions, in about 575 Years apiece. The middle Distance of the former from the Sun's Center must be 1458, 900.000 of Miles, and its longer Axis twice fo long, and fo its Aphelion Distance near four times as great as the Distance of Saturn; and its greatest Distance to its least as about 60 to 1; and therefore its greatest Light and Heat to its Jeaft as about 3600 to 1. The middle Distance of the fecond must be about 2025,000.000 of Miles, and its longer Axis twice fo long, and fo its Aphelion Distance between 5 and 6 times as great as the Distance of Saturn; and its greatest Distance to its leaft, as more than 100 to 1, and therefore its greatest Light and Heat to its leaft, as more than ten thousand to one. The middle Distance of the last must be about

5.600,000.000. Miles; and its longer Axis twice fo long; and fo its Aphelion Distance about 14 times as great as the distance of Saturn; and its greateft diftance to its leaft as above 20.000 to 1. and fo its greatest Light and Heat to its least as above 400,000.000 to 1.

As to the Fixed Stars, they are vaftly remote from this our Planetary and Cometary Syftem, but may perhaps every one be the Center of another fuch like Syftem. Dr. Hook and Mr. Flamfteed think they have difcover'd their annual Parallax, and that it is about 47′′, which will imply them to be about 700.000.000.000 of Miles diftant from the Sun; or, according to an exact Calculation in the like Cafe, farther than a Bullet fhot out of a Musket would go in 5000 Years. But of fuch vaft and numberless Systems, if fuch they are, we know very little: Only so much we know of the Planetary and Cometary Worlds, and of the Probability of vaftly more among the Fixed Stars (to fay nothing of the noblest or invifible Parts of the Creation, nor of the particular Phænomena here below) as is fufficient to make us cry out with the Pfalmift, O Lord, how manifold are thy Works! In Wisdom haft tho made them all! Pfal. civ. 24.

PART

PART III.

The Truth of the foregoing SYSTEM briefly Demonftrated.

I

N order to let the Reader fee the Certainty of our prefent Syftem of Aftronomy, and to prepare the way for his entire Satisfaction, as to the noble Inferences that fhall hereafter be drawn from the fame, I fhall now attempt, not only to prove the foregoing Syftem, in all its Parts, to be very probable, and fo preferable to any other Hypothefis; but to Demonftrate it to be really true and certain ; and this after fo familiar a manner, that ordinary Mathematicians may eafily apprehend the Force of each Argument, and fee the Evidence for the feveral Conclufions all along. Now the Propofitions I fhall here Demonstrate are thefe:

I. That the Diurnal Motion, or that which occafions the Succeffion of Day and Night, and the apparent rifing and fetting of the Sun, Moon, and Stars, in the space of 24 Hours, which we

call

call a Day, belongs to the Earth, and not to the Heavens.

II. That the Annual Motion, or that which occafions the Succeffion of Summer and Winter, and the apparent Motion of the Sun through the Ecliptick in the Space of 365 Days, which we call a Year, belongs to the Earth, and not to the Sun.

III. That there is an Univerfal Power of Gravity acting in the whole Syftem; whereby every Body, and part of a Body, Attracts, and is Attracted by every other Body and part of a Body through the whole Syftem; that this Power of Gravity is greater in greater Bodies, and leffer in leffer; and this in the exact proportion of fuch their Magnitude: That it is alfo greater when the Bodies are nearer, and leffer when they are farther off; and this in the exact duplicate proportion of fuch their nearnefs: That this Power is the fame in all Places, and at all Times, and to all Bodies: And that, lastly, this Power is entirely Immechanical, or beyond the Power of all material Agents whatsoever.

IV. That the Orbits, Revolutions, Diftances, Quantities of Matter, Denfities, Gravity on the Surfaces, Revolutions about their Axes, Quantities of Light and Heat, &c. above fet down, concerning the Sun and Planets, both Primary and Secondary, with thofe of the Comets alfo, are true and certain; with an Account of the Ways whereby we difcover every one of thofe Particulars.

PROP.

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