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• present Theory, that one would be apt to think he had presaged all • that later Experience and Oblervations have discovered.". We thell now Examine
what M. Maupertuis himself fays of Comete. from his own Kņowledge in Astronomy and Geometry,
• In order to itlustrate this Subject, as Comets fòrm 'a Part of our • general Solar System, the Author gives us a short View of it, in a • clear and distinct Light. We shall only take that Part of it, which is
necessary for understanding the Subject in Question.
• We will suppose the general Sykem of the World, known to moff • of our Readers, even those of the fair Sex, many of whom have aço • quired the Knowledge of the abftruleft Parts of natural Philosophy . and Phyfick.
• The Planets then describe Ellipses about the Sun, who does not fill • the Centre of their Orbits, but is placed in a Point we call the Foeus, " i. e, which is nearer the Extremity of one part of the Curve than the • other, and is so much nearer in Proportion to the Elongation of the • Ellipfis. On this Account it is, that the Planets in certain Parts or • Points of their Orbits, which we term their Peribelion, approach • nearer the Sun, than in other Points, which are called their Aphelion. • But as the Ellipses which our Planets, viz. Mercury, Venus, the Earth, • Mars, Jufiter and Saturn, describe around the Sun, differ but a little • from Circles, we never lose sight of them in the Course of their peri• cdical Revolutions.
• Comets differ from Planets only in this, that in their Courses round • the Sun, who is their common Focus, they form Ellipses extremely . oblong of various Degrees. Consequently they take up in their Re • volutions a much longer Space of Time than the Planets, and this in • Proportion to the Length of their Ellipses.
• This Elongation of the Comers with regard to their Orbits, joined • to the Slowness of their Revolutions, has prevented, hitherto, our com • ing to any Certainty as to the exad Time of their Returns. We never • lose light of the Planets, because they never go beyond such a Dil.
cance; whereas the Comets are only visible to us during that fort part
of their Course which lies near the Earth, the reft being fpent in the most diftant Regions of the Heavens, during which they are
entirely loft to us. So that when a new Comet appears, we have no • other way to know it again, than by examining what Comets have
appeared before at fuch certain Periods of time, and comparing their • Course with that of the new Comer, if we can make any certain Ob• servations upon
it. • It is by this Method, that we have probable ground to believe, that the Comet of 1682 finishes its periodical Revolution in 75 Years, for • we find in 1607 there appear'd a Comet, which seem'd to have the • fame Mction, another in 1531, and a fourth in 1456, so that it is • likely all these Comets were one and the fame.
This will be yet more certain of it should re appear in 1757 or 1758.
M. Maupertuis inftances the Example of two other Comets, whole Revolutions have been calculated, but the Proofs do not feem so frong.
It is only from Observation and Time that we can hope to bring the Theory of Comets co Perfection, but as to the Course of any particular
Comet, we may be more certain. Subject to the fame Laws that direct the other planetary Bodies, no sooner a Comet appears and has mark'd its Orbit in any known Point of the Heavens, but we may by Theory determine its Progress, and with regard to those Comets that have hitherto been carefully observed, the Eyent has verify'd the Calculations
Of this we have a remarkable Proof in the Comet of 1664. Mr Cefini after a few Observations exactly foretold the Rout it held, which we Thall relate in the Words of our Aubor himself.
• He (M. Caffini) was fo fyre of his System, in respect to Comets, that after his first two Observations taken the 17th and 18th Nights in
December, he boldly mark'd out, to the Queen, on the celestial Člobe, • the Course it would take; after a fourth Obieryation, made on the 22d, • he asserted it was not yet in its nearest Proximity to the Earth, which • on the 23d, he predicted would happen on the 29th, and thoʼat that • Time it exceeded the Moon in Swiftness, and seem'd likely to make • the Tour of the Heavens in a thort Time, he asserted that it would
stop in Aries from which it was then but two Signs diftant, and after • resting there, would move in a retrograde Direction to that which it • had held. Many laugh'd at there Prediâions, and were incredulous • enough to maintain that the Comet would cheat the Philosopher, but
when they saw.it fulfil his Calculation, they were glad to recant their • Error,
We have already remark'd with our Author how much Comets are fallen into Disesteem with respect to their Infuence, in return M. Maxpertuis is willing to make them fome Recompense for this Loss by giving them. Credit another way. According to him, as physical Causes, they are capable of producing the most important Events. brated Mr Halley, who has calculated the Orbit's of 24 Comets from astronomical Oblervations, has concluded from thence, that they move in all Directions, fome in the Order of the Signs, others in a retrograde Course, their Circles interseding the Earth's Orbit in all Points, and having nothing in common, but being described around the Sun as their mutual Focus.
• In such a variety of Mocions, it is easy (says our Author) to fee • how possible it is for a Comet to meet some Planet, or even our Earth • in its way, and produce the most terrible Effects. The bare Ap• proach of two such valt Bodies would occasion violent Alterations in ! their Motions, either by the Attraction they would exert towards each • other, or by the Compression of their Atmospheres. The leaft that I could happen in this Case, would be no less than to change the Situa• tion of the Axis and Poles of our Globe. That Part of the Earth, • that pow lies under the Equator, would be turn'd to the Poles, and • vice versa.
Nor have we less to fear from the Tails of Comets than their Bodies themselves. These are probably only immense Torrents of fiery Exhalations or Vapours, which the solar Heat forces from their Bodies. Now a Comet may país so near the Earth, as to overwhelm us in this igneous Train, which it draws after it, or in an Atmosphere of the fame Nature which surrounds it.
The Motion of the Earth may otherwise, by these means, be so chang'd, as to convert it into a Comet itself. Another Accident which if not so fatal. would be at leaft as humbling to our terreftrial Planete, would be, if fome large Comet, by coming too near it should force it from its Orbit, and subject it to make its Revolution round it. In this case the Earth becoming the Satellite of this Comet, would be constrain'd to attend her to the Extremities of the Heavens. By the same Means a Comet might rob us of the Moon, and happy if we escape fo cheap. The worst would be if a Comet by its shock should crush us to Pieces.
Thus we see that M. Triffotin had been less ridiculous if he had plac'd a Comet instead of a World in these Lines Moliere makes him lpeak, Scene 3. Act 4. of the (Femal Pedants] learned Women.
Je viens vous annoncer une grande' nouvelle,
• If our Earth (proceeds M. Maupertuis) has as yet fuffer'd none of • these dreadful Catastrophies, it has at least felt some Effe&s of the • Force of Comets. The Marks of petrify'd Fishes and Shells, found • in Places vastly diftant from the Sea, and even on the Tops of • Mountains, are incontestable Marks of some of these Events.
An ingenious English Author (Mr Whifton) advances, that the natural Cause which God made use of to produce the Deluge was the fame Comet which appear'd in 1680, which approached so near the Sun that it received a Heat 28 Millions of Times greater than what the Earth had from the Sun. In tracing back from the Appearance of this. Comet in 1680, he finds another appear'd in 1106, another in 535 ar 2, and another at the Death of Julius Cæfar 44 Years before Chrift. All these Comets, which he concludes the same, keep a Period of 575 Years, and the 7th Period upwards, from 1680, falls exactly in with the Year of the Deluge. Mr Whifton thinks that the mutual Attraction of the Earth and this Comet, changed the figure of the former, and, by lengthening it towards the latter, broke the external Crut or Surface of the Earth, and made way for the Waters contained in the great Abyss to over flow the Earth. The same Author imagines that a Comet, perhaps the fame in its Return from the Sun, will occalion the Conflagration that is to consume the Earth at the Day of Judgment.
Some may call these Ideas of Mr Whiffon Vigilantis Somnia, or learned Reveries. The Reader may see what M. Mairon has said on this Subje& in his Treatise on the Aurora Borealis.
According to M. Maupertuis, these physical Powers of the Planets demand some Respect to be paid them, yet we don't find they are very formidable at prelent, except to such Philosophers as Mr Whifton. Were they really so, it is impossible to perswade the Bulk of Mankind, that Bodies só minute to the naked Eye, can ever produce such great Events. Men easily swallow Things contrary to Reason, but it is hard
to persuade them their Senses deceive them. M. Maupertuis however thinks proper to dispell our Fears on this Occasion by these Reflexions.
The Duration of Life, says he, is so short, and the Knowledge we have from Experience, that no such Accident has happened for so many thousand Years, are sufficient to prevent our Dread of being either the Witnesses or Vi&ims of such a Calamity. Terrible as Thunder is, its Fall is less to be apprehended by each Individual from the small Space he occupies in the Extent where it descends. In like manner the fhort Portion of Time we enjoy in that Eternity in which these great Events happen, annihilates to us the Danger, tho' it does not alter the Nature of Things.
M. Maupertuis is not satisfy'd only with comforting his fair Correfpondent, and dispelling her Apprehensions. He finishes his Letter yec more agreeably by giving her Hopes ; he tells her that a Comet by a flight Shock
To alter the Axis of our Globe, as to procure us a conAtant Spring, or, by gently changing its Orbit, make it describe an exact Circle round the great Source of Light and Heat. We have observed a large Comet may deprive' us of the Moon, but if it be small, it may in its turn serve us as second Moon. Perhaps the one we enjoy at preSent was originally a Comet, who by approaching too near us, was press’d into our Service.
To conclude, even the Tail of Comecs is a Plunder may enrich us. We may happen, without any ill 'Consequences, to appropriate one of these as a Ring, like that of Saturn, which (according to our Author in his Treatise upon the Planets) is only the Remnant of a broken Comet's Tail.
In this Letter M. Maupertuis has abridgʻd the Theory of Comets in a Manner fo agreeable, clear and easy, that the Piece appears to be the Work of a Philosopher, who is equally known for his Genius and Politeness, as for his profound Knowledge as an Astronomer and Mathematician.