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an extremely pleasing Phænomenon it is.—But say, Cleonicus, how will the Comet appear to move to an Eye on the Earth, to us for Instance, 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 l'imes ; to rise before, and set after the Sun; and such other Phänomena as we observed of the Planets, which, were it necessary, we might explain in the same Manner as was then done; but of these Things I shall give you a more particular Account when the next Comet appears, and illustrate each Phænomenon by Experiments.on the Comet itself.
Euphrof. Truly, you almost make me wish for the Comet's Return; the Thing which before filled me with Apprehension and Horror, you have now rendered not only not dreadful, but even desirable.
Cleon. It is always one good Effect that Philosophy has, to deliver the Mind from the Infamy of Ignorance, and those base Sentiments and Pavilh Fears that continually subject it to unnecessary Pain and Anxiety. In Thort, it gives the ingenious and liberal Mind a pleasure in viewing those Phänomena of Wonder-working Nature, which vulgar and superstitious Souls conftrue into direful Omens and Prodigies of Fate; and scare themselves, and their unthinking Neighbours, with Notions of divine Wrath and Judgment much oftner than there is Occasion for.
Euphrof. You speak Truth undoubtedly; yet our Frame will be somewhat maken at such amazing and unusual Appearances.-I think you said the next Comet will appear in the Winter of the Year 1757 ; I suppose that is mewn by the Motion of the Hand H, of the Cir. cle GH, divided into 751 equal Parts, over which it moves, 'I fee, in one Revolution of the Comet.
Cleon. That is the very Case ; for you observe 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 0; after which, as the Comet revolves, the Index points to 10, 20, 30, &c. Newing the respective Places of the Comet for every ten Years of the Period. And since in 1682, this Comet was in its Perihelion at B, it
culations of the larger Cælestial Bodies of the System, viz. the Primary Planets and Comets; the Order of our Method, I presume, brings us now to those of the Secondary Clafs.
Cleonicus. 'Yes, as we have considered the Sun the Center of the Great System, about which the Chorus of the largest Planets move;, so now we shall proceed to consider the Bodies of some of those Planets as the Center of a lesser System of Secondaries : And of these we have but three, viz. the Earth, Jupiter, and Saturn.
Euphrof. To the Earth you asign one Moon; to piter, four; and to Saturn, five, if I remember right.
Cleon. 'Tis very right: Of these we will discourse in Order, beginning with the Moon pertaining to the Earth ; which as it is the nearest of all the heavenly Bodies to us, so it appears the most perfect, either by the Eye or Glass; and exceeds even the Sun itself in its apparent Magnitude. No Wonder, therefore, Moses should call it a great Light.
Euphrof. I remember Milton's Description of the Creation of the Moon, which is very beautiful. Having described that of the Sun, he says,
Less bright the Moon,
Cleon. To your Description from Milton, I shall add that of Mr. Cowley, which has a peculiar Beauty and Sweetness.
He smooth'd the rough-cast Moon's imperfect Mold,
Euphrof. This is a sweet Description indeed; but let us have a little Philosophy with the Poetry. I have a great many Questions to ask about the Moon; and pray, in the first Place, what Distance may she be from the Earth?
Cleon. The true Distance of the Moon from the Earth is always variable; being sometimes greater, sometimes less; but when nearest, she is distant from us about two hundred and twenty thousand Miles.
Euphrof. Well, the next Question of Course is of her Magnitude.
Cleon. The Moon is in Diameter two thousand one hundred and seventy-fide Miles. Wherefore the Moon is about fifty Times less than the Earth.
Euphrof. But as I remember, she is near as big as the Planet Mercury.
Cleon. Yes, she is so; but Mercury being above one hundred and forty Times farther from us, is the Reason why he appears so small, and the so large, in Comparison of each other.
Euph:of. What is the precife Time of her Revolution al out the Earth ?
Cleon. She moves from Weft to East, in about 27 Days, seven Hours, and 43 Minutes.
Euphros. I think there is no other apparent Motion of the Moon but that each Day from East to West, which, I presuine, you impute to the daily Motion of the Earth about its Axis.
Cleon. You are right in what you say last; but even the Moon's periodical Motion about the Earth, is apparent enough, if well observed : For, suppose the Moon were this Night just by any fixed Star, if you observe her Tomorrow Night at the same Time, you will see her at a considerable Distance from that Star towards the East; and the second Night twice as far Eastward ; and so on, till she has performed a Revolution, and comes in Conjunction with the Star again; advancing each Day a very sensible Distance Eastward.
Euphrof. This is an Observation I shall be very well pleased with, and will prove it the first Opportunity. But, pray, Cleonicus, has the Moon any other Motion but that about the Earth now mentioned ?
Cleon. Yes, she moves about her own Axis too.
Cleon. Just in the same Time as she moves round the Earth.
Euphrof. Indeed! Why the Moon has something very surprizing and peculiar in her Motion.
Cleon. She has so; her Days and Months are of an equal Length; which can be said of no other Body in the Heavens that we know of.
Euphrof. But do you prove this by any Method that I i can understand ?
Cleon. Yes, very eafily; for confidering the Moon as a Globe revolving round the Earth as a Center, having its Superficies all over variegated with light and dark Parts, it will from thence be easy to conceive, that if different Parts of the Superficies be at any Time turned towards the Earth, it will by that Means be very difcernible to a Spectator at the Earth.
Euphrof. Undoubtedly it would, from the different Appearance of those Parts or Spots.
Cleon. But you know this would necessarily be the Case, were the Moon to be viewed in different Parts of her Orb, supposing the had no other Motion.
Euphrof. I conceive it would; for all the Part, or Hemisphere, turned towards us in one Part, would be quite hid from us in the opposite Part; and all the Parts visible in the first Case, must be now invisible.
Cleon. Very good, my Euphrosyne : But since you observe that in whatsoever Part of the Orbit the Moon is in, the same Face, or the same Appearance of Spots is always observed, without any sensible Variation, the must of Necessity have such a Motion about her own Axis as turns every Moment so much of her Superficies from our View, as is turned to us by her periodical Motion; that is, she must move in the same Time about her Axis as she does about the Earth.
Euphros. Your Reasoning is certainly very just, and amounts to a Demonstration. But why must it be so ? Why, Cleonicus, was it not permitted us to see the other Half of the Moon's Surface ?
Cleon. The Almighty only knows ! Any other diurnal Motion but what it has would have gratified our Curiosity with that pleasing Prospect, from which we are now eternally excluded. But as it may not only be satisfactory, but also instructive to see this Matter represented in the Planetarium, from the fame Cause as produced in the Heavens, viz. the Power of Attraction-Therefore, you observe, on the central Part I place the Earth, and at a Distance, on a proper Stem, I suspend an Ivory Ball by a String, so that it is free to move any Way.-But now the Machine is in Motion.—You observe the 'Ball which represents the Moon keeps moving about the central Earth, but ever with the same Part towards it, while the Earth itself is constantly turning all Parts of its Surface towards the Moon
Euphrof. I can't but observe it with equal Curiosity and Wonder. But you know my inquisitive Temper, and you must shew me the Reason of the Thing before I Thall be satisfied.
Cleon. I am never better pleased than to find such a Disposition in Pupils.—You are soon convinced of the Reason of this Phænomenon—I take the Earth off from a small Stem of Steel, which is an artificial Magnet, and there is a piece of a natural Magnet within the Ball or Moon, and the Power of Magnetism here causes the