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first Oval LM, and is moved by it always at unequal Distances from the Center, or rather Focus , and therefore the Motion must needs be very unequable, while that of the circular Wheels is constantly the fame. Whence by this Contrivance, an equal Motion, you fee, produces a very unequal one ; which is a Sort of mechanic Paradox.

Euphrof. Truly it is, Cleonicus, and could never have thought which Way it was to be done ; and I cannot but admire the singular Structure, and curious Mechanism of the Parts-To see an uniform Motion in the circular Wheels- A different Motion in every different Part of the first oval Wheel, and yet in the same Part the Motion is uniformly and always the same - And laftly, to see so very unequal a Motion in every Part of the second Oval ; I say, to observe so many variable and diffimilar Effects in so small and simple an Instrument; and at the same Time explanatory of the most unusual and extraordinary Phænomena of the System, is what instructs and entertains me beyond any Thing I could have imagined.—But I fear I have kept you too long upon a Subject, which, however new and engaging to me, may in fome Degree prove tedious to you, who understand it so well,

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The Young GENTLEMAN and LADY'S

PHIL OS O P H Y.

PART

II.

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DIALOGUE I.
Of the MOON.

Euphrosyne.
UR last Conversation, Cleonicus, concluded our Spe-

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 confidered the Sun tñe 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, Yupiter, and Saturn.

Euphrof. To the Earth you allign one Moon; to 12piter, 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 moft 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.

Euphros. I remember Milton's Description of the Creation of the Moon, which is very beautiful. Having described that of the Sun, he says,

- -Leji

Less bright the Moon,
But opposite in levelld Wef was set
His Mirror, with full Face borrowing her Light
From him, for other Light she needed none
In that Aspect, and fill that Distance keeps
Till Night; then in the East her Turn the shines,
Revolu'd on Heav'n's great Axle ; and her Reign
With thousand lesser Lights dividual holds,
With thousand thousand Stars, that then appear'd
Spangling the Hemisphere.-

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-caf Moon's imperfect Mold,
And comb'd her beamy Locks with sacred Gold;
Be thou, said he, Queen of the mournful Night,
And as he spoke, me rose o'er-clad with Light;
With thousand Stars attending on her Train,
With her they rise, with her they set again.

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 hun. dred 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 to small, and the fo large, in Comparison of each other.

Euphrof. What is the precife Time of her Revolution al out the Earth?

Cleon. She moves from Wef 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 presume, you impute to the daily Motion of the Earth about its Axis.

Cleon. You are right in what you say laft; but even the Moon's periodical Motion about the Earth, is apparent enough, if well observed : For, fuppose 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 confiderable Distance from that Star towards the East; and the second Night twice as far Eastward ; and so on, till fe has performed a Revolution, and comes in Conjunction with the Star again; advancing each Day a very sensible Distance Eastward.

Euphrof. This is an Obfervation 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, the moves about her own Axis too.
Euphrof. In what Time?

Cleon. Just in the same Time as she moves round the Earth.

Euphros. Indeed! Why the Moon has something very surprizing and peculiar in her Motion.

Cleon. She has fo; 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 can understand ?

Cleon. Yes, very easily; for considering 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 dircernible 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, fuppofing 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 juft, 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 same 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 fame Part towards it, while the Earth itself is constantly turning all Parts of its Surface towards the Moon.

Euphros. 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 hall be satisfied.

Cleon. I am never better pleased than to find such a Difpofition in Pupils. You are foon 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

Moon

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