the Comet, or of the little Ball E at the End of it; or the Meaning of the large Circle of Stars, though I partly guefs at them too.

Cleon. Thefe Things will all be easy to Euphrofyne when the Machine is in Motion.-Obferve, when I turn the Winch, the brazen Comet moves, and with a very unequal Pace in its elliptic Orbit, about the focal Sun at S. That when it is neareft the Sun (in what is called the Perihelion at B) it moves very quick,-and, on the contrary, at its greateft Distance (or Aphelion at A) it moves extremely flow; and confequently, that in its Return to the Sun, its Motion is always accelerated, and conftantly retarded, as it recedes from the folar Focus.

Euphrof. All this I obferve and understand with Pleafure. But now for the Reafon of it, Cleonicus; if I must be a Philofopher, let me be a rational one, or none at all. Why moves the Comet with fuch an unequal and variable Motion? When it is got fo far off, what brings it round again, or why does it not keep on increasing its Distance from the Sun, and with a decreafing Motion arrive at laft fomewhere at a State of Reft?

Cleon. Why, truly, my Euphrofyne, these are very reasonable Queries, but they are more eafily afked, than their Solutions are to be understood, till you have made a little further Progrefs in the Elements of Aftronomy. I can only fay at prefent, that one Reafon of the Comet's Motion, is the Power of Gravity, by which it tends towards the Sun in every Place; and because this Power is greatest when the Distance is leaft, the Comet will there of course move quickeft; and at greater Diftances, where the Force of Gravity is lefs, the Motion will be retarded, and become flower in Proportion. But there are other Caufes co-operating to produce this Sort of Motion, and when they are explained by Experiments hereafter to be made, you will fee the Reason why the Comets muft neceffarily return, and move in elliptic Orbits.

Euphrof. I am content to ftay till then, though I fear it will be a pretty while firft.-But, Cleonicus, is not the Ufe of that Wire to keep the Comet in the Groove, and to oblige it, at the fame Time, to move about the Sun ? Cleon. It is just the Thing you mention ;-and farther, you obferve the Part of the Wire beyond the Comet, grows

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grows gradually longer as the Comet approaches the Sun, and fhorter as it returns from the Sun; and that therefore it does, in fome Measure, not unaptly resemble the Comet's Tail?

Euphrof. Indeed it will serve for that Purpose very well, and naturally enough reprefents the Rifing and Encrease of that wonderous Exhalation, as the Comet approaches to the Sun, and its gradual Diminution as it retreats from the fame. But does not that little Ball at the End of the Wire, answer fomewhat like the fame Purpofe as the Bead in the ftellated Planetarium for the apparent Motion of Venus?

Cleon. In part it does, but not altogether. Here you are to fuppofe an Eye placed in the Sun, and viewing the revolving Comet in its Orbit; the apparent Motion of the Comet in this Cafe, would be fhewn by the fmall Ball among the Stars in the Sky, as that Ball appears, to fuch an Eye, to move by the Stars on the Circle IK. And the Inequality of the apparent Motion of the Comet among the Stars, is much greater than that of its true or real Motion in its Orbit.

Euphrof. Can you make this Matter intelligible to fuch a Novice as I am, think you, Cleonicus?

Cleon. I make no Doubt of it; for firft, you obferve that the true Velocity of the Comet is flower in its Aphelion at A, than in the Perihelion at B, and it is just so much flower as the Distance is greater, viz. As SA is greater than SB. But, fecondly, when the Comet is at B, in its nearest Distance, it is flower than the small Ball upon the ftarry Circle at L, in Proportion as the Distance of the Comet SB, is lefs than the Distance of the Ball SL. Therefore, upon the Whole, the Velocity of the Comet at its greatest Distance at A feen from the Sun, or upon this Inftrument, is as much less than the apparent Velocity of the fame Comet among the Stars at L, when the Comet is nearest of all to the Sun at B, in Proportion as the Square of the greatest Distance SA, is to the Square of the leaft SB.

Euphrof. If I do not nicely understand your Proportion of Squares, and fuch Things, you must excuse me, at prefent. I can plainly enough fee the Difference of the Motion in thofe two Places is prodigiously great, and

an extremely pleafing Phænomenon it is.-But fay, Cleonicus, how will the Comet appear to move to an Eye on the Earth, to us for Inftance, when the next comes round?

Cleon. The Comet always appears to us to move in a great Circle through the Heavens; to go forwards, and backwards at Times; to rife before, and fet after the Sun; and fuch other Phænomena as we obferved of the Planets, which, were it neceffary, we might explain in the fame Manner as was then done; but of these Things I fhall give you a more particular Account when the next Comet appears, and illuftrate each Phænomenon by Experiments. on the Comet itfelf.

Euphrof. Truly, you almoft make me wifh for the Comet's Return; the Thing which before filled me with Apprehenfion and Horror, you have now rendered not only not dreadful, but even defirable.

Cleon. It is always one good Effect that Philofophy has, to deliver the Mind from the Infamy of Ignorance, and those base Sentiments and flavish Fears that continually subject it to unneceffary Pain and Anxiety. In fhort, it gives the ingenious and liberal Mind a Pleasure in viewing those Phænomena of Wonder-working Nature, which vulgar and fuperftitious Souls conftrue into direful Omens and Prodigies of Fate; and fcare themselves, and their unthinking Neighbours, with Notions of divine Wrath and Judgment much oftner than there is Occafion for.

Euphrof. You speak Truth undoubtedly; yet our Frame will be fomewhat fhaken at fuch amazing and unusual Appearances.-I think you faid the next Comet will appear in the Winter of the Year 1757; I fuppofe that is fhewn by the Motion of the Hand H, of the Circle GH, divided into 75 equal Parts, over which it moves, I fee, in one Revolution of the Comet.

Cleon. That is the very Cafe; for you obferve when the Comet is nearest the Sun at B, the Index H of the Circle of Years, points to the Beginning of that Circle, or O; after which, as the Comet revolves, the Index points to 10, 20, 30, &c. fhewing the refpective Places of the Comet for every ten Years of the Period. And fince in 1682, this Comet was in its Perihelion at B, it