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Cleon. There are no less than three which offer themfelves this Evening to the View, but one of them fi. e. Mercury) is now fet; the bright Evening-ftar, which you fee low in the western Hemisphere, is the Planet Venus; and that pale-faced Planet which you fee yonder, South by Eaft, is Saturn; and all the rest are fixed Stars.

Euphrof. So the Evening-far, that every body takes fo much Notice of, is a Planet, I find; pray, Cleonicus, how am I to know a Planet from a Star at any Time?

Cleon. Very eafily, Sifter, two ways; one is, that every fixed Star twinkles, but a Planet never does.

Euphrof. Indeed! pray, what can be the Reafon of that, Cleonicus?

Cleon. The Stars are only lucid Points to Appearance, and therefore any opake Atom, or Particle, floating in the Air, is fufficient to eclipfe, or caufe a momentary Occultation of them; and this, joined with the Agitation or Tremours of the Air, will caufe that conftant Twinkling in the Stars which you fee: But the Flanets, though small, are ftill bigger than thofe Motes to the Eye, and therefore their Light will fuffer little or no Diminution.

Euphrof. Very good; and what is the fecond Thing that diftinguishes a Planet from a Star?

Cleon. This, my Euphrofyne, is the proper Criterion or Characteristic of a Planet, namely, a Planet is always in Motion from one Part of the Heavens to another, and from thence it is called a Planet, which in Greek fignifies a wandering Star; now the other Stars are all fixed, and keep the fame Distances conftantly from each other.

Euphrof. Dear Cleonicus, you have thoroughly prepared me to diftinguish the Planets from the Stars. I have one more Queftion to ask you, and that shall be all for To-Night; What is the Sky or Firmament? And why does it appear always of that equally round Form?

Cleon. The Sky, my Euphrofyne, is not any Thing real, but only the apparent Boundary of our Sight; Space is every Way infinitely extended; but our Sight is terminated all around us, at a small but equal Distance; and this is the only Reafon why the Firmament appears of a perfectly spherical, concave Figure; for the Surface of a Sphere is every where equally diftant from

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the Center, and every Man's Eye is the Center of his View; could he fee farther one Way than another, the Heavens would appear of an irregular Form, and quite different from that beautiful Hemifphere, or azure Canopy, which conftantly prefents itself to our View.

Euphrof. Well, Cleonicus, how happy would it be if every Walk I took for the future were to prove fo delightful and inftructive as this! But fee, the Candles are lighted, and Supper, I fuppofe, on the Table; we are at Home juft in Time.

Cleon. We are fo; we will now folace ourselves with the Refreshments of kind Nature, and To-morrow reaffume this Subject; for many Things remain for my Euphrofyne yet to understand.

DIALOGUE IV.

A View of the demonftrative Proofs of the FALSITY of the PTOLOMAIC HYPOTHESIS, as represented

TH

in a NEW PLANETARIUM.*

Euphrofyne.

HIS Morning, Cleonicus, I remember you told me we fhould be employed in a more particular Confideration of the Ptolomaic Syftem; pray, in what Refpect do you mean?

Cleon. I mean, my Euphrofyne, to point out to you thofe particular Phanomena of the Heavens, which are eafy to be obferved by yourself, or any one, and which do of themfelves feverally evince the abfolute Falfity of the aforefaid Syftem; and you'll not be difpleased to have it easily in your Power to refute an Hypothefis in half an Hour, which has been univerfally and ftrenuously maintained for Ages paft.

The Reader, through this whole Dialogue, is to fuppofe, that the Ptolomaic Syftem is reprefented by a new Planetarium with all the planetary Bodies, together with the Sun, revolving about the Earth at Reft in the Center; which Machine is reprefented in a general Manner in Plate II.

Euphrof. Is this poffible, Cleonicus?-This will be charming indeed!-This will be doing great Things, fure enough.

Cleon. You will foon be fatisfied it is poffible, by means of this Machine you fee here, which is contrived for this very Purpose; for by it you will fee both the Ptolemaic and Copernican Syftem reprefented, and by comparing the Appearances in each of thefe with thofe of the Heavens, you will obferve fuch a palpable Abfurdity and Repugnance in the one, and fuch a perfect Confistency and Agreement in the other, as will leave no Room for the leaft Doubt, that one is fpurious and falfe, and the other the genuine and true Syftem of the World.

Euphrof. Very good, my Cleonicus; but, pray, what do you call this curious Inftrument ?

Cleon. It is called a PLANETARIUM; because it exhibits. a just View of the Planetary Syftem, at least what relates to the Number, Order, and annual Motions of the primary Planets, all in the fame Periods of Time as in the Heavens; for the Wheel-work is calculated to a Minute of Time.

Euphrof. Very good, Cleonicus; I fee the fmall Globe of the Earth in the Center, and about it five round Ivory Balls, which, I fuppofc, represent the five Planets; and a little Brass Ball for the Sun, in the third Place from the Center; am I right, Cleonicus?

Cleon. You have a very juft Notion of the Thing; you are perfectly right, Sifter. You farther obferve, two Circles on the Surface of the Machine, one containing the 12 Signs of the Ecliptic, and the other the Calender of the Months and Days adapted to it, so that any Planet may be placed in that Part of the Ecliptic, or rather Zodiac, which it poffeffes for any Day of the Year, and fo the whole Syftem may be adjusted for any given Time.

Euphrof. Well; and fo by winding up the Machine, you put all the Planets, together with the Sun, in Motion about the central Earth, and thus you reprefent to me Ptolomy's Syftem.

Cleon. I do ;-and now you see them all in Motion.And the first Thing you fee is an Argument which proves this Syftem falfe, and that is,-that the two Planets next the Earth, Mercury and Venus, can never be feen beyond the Sun in this Syftem.

VOL. I.

C

Euphrof. I plainly fee it is impoffible; because their Orbits are both contained within the Orbit of the Sun. But are they feen at any Time to go beyond the Sun, Cleonicus? Cleon. Yes, juft as often as on this Side of it; as we fhall have an Opportunity of obferving foon.

Euphrof. Such an Appearance will give me great Pleafure, and be an undeniable Argument of the Falfity of this System.

Cleon. But you will find a fecond Proof of the same Thing more flagrant, if poffible, than the firft.-For you fee the Sun, there, in the Weft, Sifter;-and if you look. here, you will fee the Planet Mercury in the Eaft.-And, again, there you fee the Planet Venus in the South.Now these are such Aspects and Positions of the Planets as never were seen by any Man.

Euphrof. If I take this Affair right, you mean to fhew me, that if this were the true Syftem, I might fee (juft after Sun-fet, in a clear Evening) the Planets Mercury and Venus at any Distance from the Sun from Wel to Eaft, don't you, Cleonicus?

Cleon. That is the very Thing intended, my Euphrofyne; but you will eafily recollect, that inftead of feeing thefe Planets in every Part of the. Heavens in a Star-light Evening, you feldom fee them at all; nay, one of them, Mercury, fo feldom, that you never yet obferved it in the leaft; and the other, Venus, or the Evening-ftar, appears very rarely, being generally near the Sun; and when fartheft, not more than about 47°, that is, you never faw the faid Planet farther from the setting-Sun than about Southwest; you never faw it in the South, or Eastern Parts of the Hemifphere of an Evening, which Phænomenon does. therefore inconteftably prove the Abfurdity of this System.

Euphrof. I am convinced of it thoroughly; I do not know that ever I faw the Planet Mercury; and as to Venus, I never faw her but in the Circumftances you mention. But what is your next Argument against this Hypothesis ?

Cleon. The Earth being fuppofed the Center, the Planets. will be in every Part of their Orbits at an equal Distance, and therefore will appear equally big at all Times; but this is contrary to all Appearances; for the apparent Magnitude of all the Planets is conftantly variable: Nay, fo great is the Difference in Mars, that at one Time he appears as

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large as Jupiter, and at another Time fo fmall, that you cannot diftinguish him from a fixed Star, but by his red Afpect.

Euphrof. If this be the Cafe, it must certainly be an indifputable Argument against the vulgar Hypothefis.

Cleon. You will hereafter be fully fatisfied, by your own Experience, that is really the Cafe; at prefent, that Planet is not in a Situation to be observed, but he foon will.-I fhall mention next another irrefragable Argument to the fame Purpose.You fee the Planets in this Machine move with an uniform and equal Velocity in every Part of their Orbits.

Euphrof. I do fo; they move neither fafter nor flower in one Part than another.

Cleon. But this is not the Cafe of the Planets in the Heavens; for they all appear to move there with very unequal Paces; fometimes they are very flow in Motion, fometimes very quick, and fometimes they appear stationary for fome Time, or without any Motion at all. This you may be as easily convinced of as you can defire, in a few Weeks Time in the Planet Venus, which is now an Evening-star, and very flow in moving from one fixed Star to another; but fome Time hence, you will obferve, she has no Motion at all for feveral Days, then after that, you'll fee her move very fast back again towards the Sun, and be foon loft in his Blaze. In all which Obfervations, I fhall take particular Care to affift you.

Euphrof. I fhall be obliged to you, Cleonicus; and greatly pleased to obferve fuch curious Appearances of the Planets; efpecially as they will afford me fuch ample Conviction of the Falfity of this Syftem, which fhews this, and every Planet moving conftantly with the fame even Pace.

Cleon. There remains yet one more Obfervation to prove this Syftem a moft grofs and abfurd Hypothefis, and that is-you obferve, that a Spectator on the Earth, in the Center, would view all the Planets moving one and the fame Way continually, or from West to Eaft; don't you, my Euphrofyne?

Euphrof. Yes, I do very plainly; I am certain, from what I now fee, that the Planets muft neceffarily appear to move as they really do, fince we are fuppofed to be in the Center of their Motions.

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