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DIALOGUE XVI.
On the Use of the COMETARIUM.

Euphrosyne.
CINCE you gave me the Lecture on Comets, you

have filled my Head with such odd kind of Ideas, that I scarcely know whether I hope or fear most to see a Comet ; but, dear Cleonicus, since that is shortly to be the Case, and a Comet we must behold, if your Astronomical Prediction is to be regarded, I think I may as well take Courage, and resolve to attend the important Event undauntedly.

Cleon. Fortitude, my Euphrofyne, is an excellent Virtue; and here I must admonith you to speak with more Reverence of astronomical Predictions, or else you may chance, one Day or other, to be accosted in the Pontifical Stile, and be told, That Astronomers only have Infallibility on their Side; that their Prophecies are sacred and certain Truths ; that they must be believed, or that an implicit Faith in all they say is your highest Duty, and absolutely necessary to your future Reputation; that

Euphrof. Hold, Cleonicus, too much of this kind of Denunciation does not found well after a liberal Education ; you are now talking to a Woman, and they are frail Creatures, God knows; if we should be guilty now and then of a little astronomical Infidelity, we hope it may be looked upon as a venal Crime, and for which we may be entitled to an Indulgenca.

Cleon. You are quite on the right Side of the Question, Sister; the Ladies are sure to be forgiven, with or without Confesion. Now we are on this Subject, what will you think of a Prophesy or Prediction of a Heathen, the celebrated Seneca, in the following Words?

“ I cannot affent to our Philosophers, nor think the " Comets are Fires suddenly kindled, which appear “ a-while, and are then extinguished; but I reckon them « among the eternal Works of Nature. --- And why “ should we wonder that Comets (such a rare Spectacle " in the World) should not yet be restricted by certain “ Laws; nor have the Times of their appearing or dir

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56 appearing known, as they take their Courses through “ such prodigious Intervals of Space.- The Tiine will “ come when a Day shall bring to light, and the Dili“ gence of a future Age discover, those Things which so now lie hid.

" The Time will come in which Posterity will won“ der that we were ignorant of Things so very plain.-- A Person Mall one Day arise, who fall demonstrate 6 into what Regions the Comets wander, why they move fo

Separately from the rest of the Planets, and how large, and what kind of Bodies they are.”—

I say, dear Sister, what can you think of all this, when if you could red Sir Isaac's Principia, and there see every Particular of this whole Prediction fulfilled to a Tittle, and no other Man besides Sir Isaac did so much as even to attempt the arduous Task?

Euphrof. Think! why what could one think less than that he prophesied of that very great Man.-But tho' I am not able to understand the Writings of that Philosopher, yet, if I remember right, you once told me, that you could make the Manner of the Comet's Motion intelligible by a proper Instrument, as well as those of the Planets.

Cleon. I did so; the Instrument I mean is called the COMETARIUM, and which I fall now spend one Quarter of an Hour in explaining to you.--Here is the Machine.

Euphrof. And a beautiful one it is; I can almost tell the Use of it by its very Appearance ; the Brass-ball C, is the Comet, I dare say; and the long oval Groove, the Orbit in which it is to move about the Sun, which I observe is engraved at one End of it.

Cleon. Very right, my Euphrolyne ; this Instrument fhews the Motion of the Comet of the Year 1682, whose Period is 757 Years.

Euphrol. Then that Period, or those Years, I suppose, are represented by the Circle FG, and the Hand H, pointing out the particular Year of the Period, for any Position of the Comet in its Orbit : am I right, Cleonicus?

Cleon. So very good is your Apprehension, that it almost makes a Description of the Instrument unnecessary.

Euphrof. Not fo, indeed; for I can scarce tell what Use that long Wire is of that goes through the Body of

the 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 guess at thein too.-

Cleon. These Things will all be easy to Euphrosyne when the Machine is in Motion.-Observe, 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 nearest 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 slow ;-and consequently, that in its Return to the Sun, its Motion is always accelerated, and constantly retarded, as it recedes from the solar Focus. Euphrof. All this I observe and understand with Pleasure.—But now for the Reason of it, Cleonicus ; if I must be a Philosopher, let me be a rational one, or none at all. Why moves the Comet with such an unequal and variable Motion ?-When it is got so far off, what brings it round again, or why does it not keep on increasing its Distance from the Sun, and with a decreasing Motion arrive at last somewhere at a State of Rest?

Cleon. Why, truly, my Euphrosyne, these are very reasonable Queries, but they are more easily asked, than their Solutions are to be understood, till you have made a little further Progress in the Elements of Astronomy. I can only say at present, that one Reason 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 least, the Comet will there of course move quickest ; and at greater Distances, where the Force of Gravity is less, the Motion will be retarded, and becoine flower in Proportion. But there are other Causes co-operating to produce this sort of Motion, and when they are explained by Experiments hereafter to be made, you will see the Reason why the Comets muft necessarily return, and move in elliptic Orbits.

Euphrof. I am content to stay till then, though I fear it will be a pretty while first.-But, Cleonicus, is not the Use of that Wire to keep the Comet in the Groove, and to oblige it, at the same Time, to move about the Sun?

Cleon. It is just the Thing you mention ;-and farther, you observe the Part of the Wire beyond the Comet,

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there felt when the every Place, pay, by whiche Comet's

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

Euphrof. Indeed it will serve for that Purpose very well, and naturally enough represents the Rising and Encrease of that wonderous Exhalation, as the Comet approaches to the Sun, and its gradual Diminution as it retreats from the same. But does not that little Ball at the End of the Wire, answer somewhat like the same Purpose 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 suppose an Eye placed in the Sun, and viewing the revolving Comet in its Orbit; the apparent Motion of the Comet in this Case, would be shewn by the small Ball among the Stars in the Sky, as that Ball appears, to such 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 such a Novice as I am, think you, Cleonicus ?

Cleon. I make no Doubt of it; for first, you observe that the true Velocity of the Comet is slower in its Aphelion at A, than in the Perihelion at B, and it is just so much slower as the Distance is greater, viz. As SA is greater than SB. But, secondly, when the Comet is at B, in its nearest Distance, it is flower than the small Ball upon the starry Circle at L, in Proportion as the Distance of the Comet SB, is less than the Distance of the Ball SL. Therefore, upon the Whole, the Velocity of the Comet at its greatest Distance at A seen from the Sun, or upon this Inftrument, is as much less than the apparent Velocity of the same 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 greates Dif:ance SA, is to the Square of the leasi SB.

Euphrof. If I do not nicely understand your Proportion of Squares, and such Things, you must excuse me, at present.-I can plainly enough see the Difference of the Motion in those two Places is prodigiously great, and

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