An essay on comets, their nature, the laws of their motions, the cause and magnitude of their atmosphere, and tails; with a conjecture of their use and design.

AN ESSAY on COMETS.

'TIS an admirable Work of our GOD, that the many Globes in the Universe are placed at such Distances, as to avoid all violent Shocks upon one another, and every thing wherein they might prove a prejudice to one another.

Even Comets too, move so as to serve the Holy Ends of their Creator! COMETS, which are commonly called Blazing Stars, ap∣pear unto later Observations to be a sort of Excentrical Planets, that move periodically about the Sun.

Sir Isaac Newton, from whom 'tis a difficult thing to dissent in any thing that belongs to Philosophy, concludes, that the Bodies of Comets are solid compact, fixed, and durable even like those of the other Planets.

He has a very critical Thought upon the Heat, which these Bodies may suffer in their Transits near the Sun. A famous one, in the Year 1680, passed so near the Sun, that the Heat of the Sun in it must be twenty-eight thousand time as intense as it is in England at Midsummer; where∣as the Heat of boiling Water, as he tried, is but little more than the dry Earth of that Island, exposed unto the Midsummer-Sun: and the Heat of red-hot Iron he takes to be three or four times as great as that of boil∣ing Water. Wherefore the Heat of that Comet in its Perihelion was near two thousand times as great as that of red-hot Iron. If it had been an Aggregate of nothing but Exhalations, the Sun would have render'd it invisible. A Globe of red-hot Iron, of the Dimensions of our Earth, would scarce be cool, by his Computation, in Fifty Thousand Years. If then this Comet cooled an hundred times as fast as red-hot Iron, yet, since his Heat was Two Thousand times greater than that of red-hot Iron, if you suppose his Body no greater than that of this Earth, he will not be cool in a Million of Years.

The Tails of Comets, which are longest and largest just after their Pe∣rihelions, he takes to be a long and very thin Smoke, or a mighty Train of Vapours, which the ignited Nucleus, or the Head of the Comet, emits from it. And he easily and thoroughly confounds the silly Notion of their being only the Beams of the Sun, shining thro' the Head of the Star.

The Phaenomena of the Tails of Comets depend upon the Motion of their Heads, and have their Matter supplied from thence.

There may arise from the Atmosphere of Comets, Vapours enough to take up such immense Spaces as we see they do. Computations made of and from the Rarity of our Air, which by and by issue in Astonishments, will render this Matter evident.

That the Tails of Comets are extremely rare, is apparent from this; the Fixed Stars appearing so plainly thro' them.

The Atmosphere of Comets, as they descend towards the Sun, is very sensibly diminished by their vast running out, that they may afford Mat∣ter to produce the Blaze. Hevelius has observed, that their Atmosphere is enlarged, when they do not so much run out into Tail.

Sir Isaac Newton has an Apprehension, which is a little surprizing, That those Vapours which are dilated, and go off in the Blazes of Comets, and are diffused thro' all the Celestial Regions, may by little and little, by their own proper Gravity, be attracted into the Planets, and become intermin∣gled with their Atmospheres. As to the Constitution of such an Earth as ours, it is necessary there should be Seas; thus, for the Conservation of the Seas, and Moisture of the Planets, there may be a necessity of Comets, from whose condensed Vapours, all that Moisture, which is consumed in Vegetations and Putrefactions, and so turned into dry Earth, may by de∣grees be continually supplied, repaired, and recruited. Yea, he has a sus∣picion, that the Spirit, which is the finest, the most subtle, and the very best part of our Air, and which is necessarily requisite unto the Life and Being of all Things, comes chiefly from Comets. If this be so, the Ap∣pearance of Comets is not so dreadful a thing, as the Cometomantia, gene∣rally prevailing, has represented it.

Mr. Cassini will thus far allow bad Presages to Comets, That if the Tail of a Comet should be too much intermingled with our Atmosphere, or if the Matter of it should, by its Gravity, fall down upon the Earth; it may induce those Changes in in our Air, whereof we should be very sensible.

Bernoulli, in his Systema Cometarum, supposes, That there is a Primary Planet, revolving round the Sun in the space of four Years and 157 Days; and at the Distance of 2,583 Semidiameters of the Or∣bis Magnus. This Primary Planet, he supposes, either from his mighty

Distance, or his minute Smallness, to be not visible unto us; to have several Satellits moving round him, tho' none descending so low as the Orbit of Saturn; and that these becoming visible to us, when in their Perigaeon, are what we call Comets.

Seneca's Prediction, That a Time should come, when our Mysteries of Comets should be unfolded, seems almost accomplished. However Seneca has not obliged us with the Phaenomena observed by him, which en∣couraged this Prediction.

No Histories of Comets were of service to the Theory of them, until Nicephoras Gregoras, a Constantinopolitan Astronomer, described the Path of a Comet in 1337.

All that considered Comets until Tycho Brabe, consider'd them as no o∣ther than Vapours below the Moon.

Anon, the sagacious Kepler improving on Tycho's Discoveries, came at a true System of Comets, and found, that they moved freely through the Planetary Orbs, with a Motion that is not much different from a Rectilinear one.

The incomparable Hevelius went on, and tho' he embraced the Keplerian Hypothesis, of the Rectilinear Motion of Comets, yet he was aware, That the Path of a Comet was bent into a curve Line towards the Sun.

At last the illustrious Sir Isaac Newton arrives with Demonstrations, That all the Phaenomena of Comets would naturally follow from the Keplerian Principles. He shewed a Method of delineating the Orbits of Comets geometrically; which caused Admiration in all that considered it, and comprehended it.

The most ingenious Dr. Halley has made Calculations, upon which he ventures to foretell the Return of Comets; but he observes, that some of them have their Nodes pretty near the annual Orb of the Earth. I will transcribe the Words he concludes with: 'What may be the Consequences of so near an Appulse, or of a Contact, or lastly of a Shock of the Celestial Bodies, (which is by no means impossible to come to pass) I leave to be discussed by the Studious of Physical Matters.'

The Sentiments of so acute a Philosopher as Dr. Cheyne upon Comets, deserve to be transcribed. Says he,

There is a Species of Heavenly Bodies, call'd COMETS, which re∣volve about the Sun; in very Oblong Elliptick Orbits, approaching to Parabolick Curves. The Times of their Periodical Revolutions are very long, since in three or four Thousand Years, we have not positively de∣termin'd the Returns of above one or two; however, its certain, that like our Planets, they do move in a recurring Orbit, that the Sun is in one of the Foci of this Orbit, that by a Ray from the Sun, they describe e∣qual

Area's in equal Times, that the same Law of Gravitation obtains in them, which does in the Planets; that their Peroidical Times are cer∣tain and invariable, and that their Motions are regular, only their Course in their Orbits is not determin'd one way, but in differently some of 'em move one way, others another: They are also about the same Bulk with the Planets, generally speaking; and like them are compact solid Bodies, but surrounded with a vastly large thin Fluid, intermixt with several gros∣ser Particles, and composing an irregular unequally dispos'd and uncer∣tainly agitated Mass; which is call'd its Atmosphere, whose Diameter, is ten or fifteen Times as long as that of its Body. Besides which, it has a long lucid Train, which is rais'd in its Approach to the Sun, by the Heat thereof, and extends sometimes to four hundred Thousand Miles above its Body. It is always opposite to the Sun, because it is the thiner part of its Atmosphere, extremely rarified by his Rays; and so rare that the Stars may be seen through it. This Tail accompanies it through its Course overall the Planetary Regions, encreasing in its Approach to the Sun, and lessen∣ing in its Recess. These Comets sometimes come so near the Sun, as to be heated to such a Degree that they cannot become cool again, in many Thousands of Years. This with its violent Motion in a Curve, which comes near to a streight Line, after it has pass'd its Perihelium, and the irregular Disposition of the confus'd Mass of its Atmosphere, makes it an un∣fit Habitation for Animals, that are not in a State of Punishment, so far as we can conceive of the Nature of Animals. And therefore some have tho't it design'd, to supply the Expence of Fluids in the Sun and the Planets. But as I have before hinted, this does not seem so very probable, because Nature always supplies constant and regular Expences after a constant and regular Manner. Now the Returns of these Bodies are so irregular and uncertain, and we so little feel the Effects of these Returns (which of Necessity must be felt, if these frightful Bodies made the Reimburse∣ment mentioned, since the Sun and Planets as recruited all at once, or in a very short time, of all the loss they have suffer'd in their Fluids for many Years before,) that I am afraid no such benign Influences, are to be expected from them. I readily grant, that there may be some Clouds of Vapours sweep'd off the Tails of these Comets, by the Sun and Planets as they approach them. But then it is uncertain of what Nature these Vapours are, for every Vapour will not become a Fluid, unless its Parts be of such a determin'd Figure and Size as the Nature of Fluids require. Now it's certain, that Heat will raise any Body into a Vapour, provided it be sufficient to dissolve the Union of its Parts, and increase their Bulk to a necessary Dimension: Moreover its hardly accountable how the Sun shou'd draw from thence only the Fluid of Light, the Earth, that of Wa∣ter,

the other Planets their proper Fluids; or if they were suppos'd to draw all promiscuously, how these Effects can answer the Design, since Wa∣ter in the Sun, which wanted only more of the Fluid of Light, wou'd be as improper a Guest, as Fire on the Earth which wanted only more Water. I think it is more probable, that these frightful Bodies are the Ministers of Divine Justice, and in their Visits, lend us Benign or Noxious Vapours, according to the Designs of Providence: that they may have brought, and may still bring about, the great Catastrophes of our System, by raising of Tides, changing the Figures, and the Positions of the Planets, and the very Nature of the Orbits themselves: and that they may be the Habi∣tation of Animals in a state of Punishment; which, if it did not look too national, there are many Arguments to render not improbable. But as for the Diminution of the Quantity of Fluids in the Sun and Planets, it is certainly so small and so inconsiderable (tho' still it be someting) that it will never be sensible in any finite Number of Years, and so needs not so magnificent of Light, I do believe it may be demonstrated, that if all the Fluid which the Sun loses in a Year were brought into a solid Form, it would not equal a Grain of Sand. Now what is that to the vast Body of the Sun? And as for the Fluids on our Globe, tho' some of them be reduc'd into a solid Form, yet upon Examination it will be found that most of the same is again dissolv'd into a Fluid; for in all Animals and Vegetables, the Matter is plain; and as for Minerals and Metals, I doubt much, whether there be very many new Productions or Generations of that kind, since their first Production. So that though there be some loss of Fluids on our Globe, yet there is store enough to supply all the Uses of Life and Vegetation, for any finite number of Years; and by the Frame, and Make of this our System, it does not seem design'd to last for ever, without some considerable Changes.

As to the Comets, I have little more to say about them, than what I have already said, their Nature, Orbits, Motions and Situations, have been so lately determin'd, (indeed it is but of late, that the Astronomy of the Planets themselves, and their Satellits has been brought to any toler∣able Perfection, and much later since final Causes have been cultivated, with that Care becoming so noble and about 'em extant, so few of 'em, that we know of, have visited us twice, that we have scarce any solid Foundation to build our Reasoning upon. Only, as I have before hint∣ed, these blazing Stars seem not design'd for the Habitation of Animals in a State of Happiness, nay even scarce of Animals not under a State of Punishment; they may be the first Rudiments of Planets, not as

yet brought into our System, or rather the Ruins of some banish'd thence, to wander in these long Eccentrick Orbits, through the World. But most likely they are the Ministers of Divine Justice, sending baneful Steams, from their long Trains, upon the Planets, they come nigh; and if what is commonly said of 'em by Astrologers be true, they seldom visit us without some such direful Salutation. However, from them we may learn that the Divine Vengeance, may find a Seat for the Punishment of his disobedient Creatures, without being put to the Expense of a new Creation.'

When I see a vast Comet, blazing and rolling about the unmeasura∣ble Aether, I will think;

'Who can tell, but I now see a wicked World made a fiery Oven in the Time of the Anger of GOD! The Lord swallowing them up in his Wrath and the Fire devouring them!

What prodigious Mischief and Ruin might such a Ball of Confusion bring upon our sinful Globe, if the Great GOD order its Approach unto us.

How happy they, that are in the Favour and Friendship of that Glori∣ous Lord, who knows how to deliver the Pious out of Distresses, and reserve the Unjust for a Punishment of a Day of Judgment.

—Si fractus illabatur Orbis, Impavidum ferient Ruine.