We have here exhibited to the Readers View the Seven Primary Planets, in their true Proportions, as in the Table page 54; Jupiter has its Belts; and in one of them that Spot by whose Revolution several times his Diurnal Period was difcovered. Saturn has alfo here Its Ring, in its true Proportion

Besides thefe we have here reprefented an Eclipfe of one of Jupiters Planets, both at its Immersion ate and its Emersi= on at f. The former of which is alone visible to us from Jupiters Conjunction with the Sun to its Opposition; or while the Earth pabes from e, by a and P to 0: As is the latter visible alone from that Planets Opposition to us Conjunction; or while the Earth pabes from o, by dand bto c. (Jupiters

Sun

Body still hiding one of those appearances from us.) But what Earth's:
is here peculiarly remarkable is this, that in the former cafe
the Immerwoh is still seen so much too soon, by our
going to meet the last Rays before; and the Emersion
so much too late, by our going from the first Rays after
the Eclipte, as implys the motion of Light to be aty rate
of 180,000 miles in one Second of time; or to be half a quar
ster of an Hour in coming 81,000,000 miles from the Sun to
the Earth.
1.Senex feulp

Orbit

N. B. This Ring cafts a mighty Shadow upon large Regions of Saturn's Body, which removes from one Part to another, and caufes great Diversity as to Light and Darkness thereon: And, what is probably to them more strange, they hardly know what it is that causes thofe Varieties: For though we stand conveniently to fee what it is, and know it to be a mighty Ring encompaffing Saturn's Body; yet is it not fo eafy for any in Saturn to difcover it. They must naturally imagine it to be in the Heavens above them; and have, as it feems, no way but by Aftronomical Obfervations and Parallax, to find out what it is: Juft as we find out the Distance and Motions of the Moon, or of any other Heavenly Body. Whether this Ring, or indeed Saturn it felf, revolve about its Axis, we are not yet affur'd: Nor are we certain whether this Ring be Solid or Fluid. When once our Glaffes can fhew us any permanent Spots in the Body and Ring of Saturn, we fhall be able to determine whether there be fuch a diurnal Motion or not; but till that Time we must be content with Conjectures: For which, if there be room, I fhould certainly suppose the

See the Account of Co

mets to

Mathe

Ring to be Solid, and both to have fuch a Motion; nor only from its Conveniency for any Creatures that may be thereon, but from parity of Reafon, and the general Cafe of the rest of the Heavenly Bodies: Not one of which are yet known to be deftitute of fuch a Motion; although fome of them have not yet afforded us an Opportunity of certainly determining the

fame.

(15.) The Number of the Comets is very confiderable; to be fure much greater than of wards the the vifible Planets. Our famous Dr. Halley, by end of my fearching into the Hiftories of them, and comparing the Obfervations made about them, has Philofo- given us a Catalogue or Table of fo many as he could find well enough defcrib'd to afford Foundation for determining their Orbits; which Table I have elsewhere given the Reader.

matick

phy, at

large. Ibid. P 415.

P. 494. 465.

This Number is 24, all which have appear'd within the last 400 Years, in these Parts of the World. He has also obferv'd, that out of these 24, Three had their Orbits and Circumftances fo very like, and the Intervals of appearing fo nearly equal; and that twiceTwo others had their Orbits and Circumftances fo very like alfo, that he justly concluded it exceeding probable that the former Three were one and the fame; and in fome Measure fo, that the latter Two were feverally alfo one and the fame Comet; returning the ift after 75, the 2d after 81, and the last after 129 Years. Sir Ifaac Newton alfo difcover'd,and in the new Edition of his Principia, Published his Discovery, that the last most eminent Comet of 1680, 1681, towards the end of its Appearance, bent its Courfe fo much inward from a

Para

Parabolick Line, as to fhew its real Trajectory to be Elliptical; and this in fuch Proportion, that its Period of returning must be more than 500 Years. After which Discovery I my self, for feveral Reasons, fuppofing this to be the fame Comet that caufed the Deluge, did accordingly guess the Period to be either 575, or 504 Years; according as it had made either Seven or Eight Revolutions fince that Time and drew up Tables upon both those Hypothefes, when the fame Comet must have appear'd afterwards, in order to fearch whether they did fo or not; but not having either Hevelius's or Luvienetz's Hiftories of Comets then by me, I could not immediately confirm my Hypothesis any further. But in a little time I found, that Sir Ifaac Newton, and Dr. Halley, had compleated what I wanted; and had discovered that just such a Comet had appear'd the 44th Year before the Chriftian Era, which was the Year that Julius Cæfar was flain; as alfo, Anno Dom 531, or 532; and again A. D. 1106; and laftly, A. D. 1680, 1681, and this ftill, after the forementioned Interval of about 575 Years; and that they accordingly did justly conclude it to be the very fame Comet that appeared in thofe feveral Years. So that we have the Orbits of only 21 Comets, and the Periods at the most of only four of them yet known.

N. B. The firft of thefe Three appear'd probably 1304, and 1456, but more certainly 1531, 1607, and 1682; and will by Confequence appear again 1758, and 1832, and fo every 75 Years afterwards. The Second of them appear'd probably 1475, 1556, and 1718, and may not improbably be expected 1799. The third appeared A. D. 1582, and not improbably 1661,

and