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from that Part towards which
COM
wards obliquely, ftitl receding the fmoaking Body proceeds.
7thly, ' That Declination will be flill the lealt near the 1 Nucleus of the Comet-, and when the Comet is neareft the TSun :' Bccaufe the Vapour afcends more fwittly near the Head of the Canet, than in the higher Extremity of its Tail ; and when the Comet is at a lefs diftance from the Sun, than when at a greater. „
Hthly, ' The Tail is brighter, and better defin d in its 1 convex Part, than in its concave :' Bccaufe the Vapour in the convex Part, which goes firft, being fomewhat nearer and denfer, reflects the Light more copioufly.
9thly, ' The Tail mult appear broader towards the high- ( er Extremity of the Comer, than towards the Head :' Be- caufe, the Vapour in a free Space perpetually rarefies and di- lates. - , lothly, ' The Tails muft be tranfparcnt :' Becaufe conlilt- ing of infinitely thin Vapour, l$c.
Thus accurately does the Hypothecs tally to the Pheno- mena.
Chafes of Comets. The Nuclei, which we alfo occasionally call the Heads and 'Bodies of Comets, view'd thro' a Telefcope, fhew a very different Pace from thofe of the Fix'd Stars, or Pla- nets. Sturmius tells us, that obferving the Comet of itf8o with a Telcfcope, it appear'd like a Coal dimly glowing, or a rude Mafs of Matter iilumin'd with a duiky fumid Light, lefs fenfiblc at the Extremes than in the middle ; rather than as a Star which appears with a round Diik, and a vivid Light. Hevelius ubfervd of the Comet of 1661, that its Nucleus was of a yellowifh Colour, bright and confpicuous, but with- out any glittering Light : in the middle was a denfe ruddy Nucleus, almolt equal to Jupiter, incompafs'd with a much fainter thinner Matter. 5 Feb. its Head was fomewhat big- ger and brighter, of a gold-colour ; but its Light more dufky than the reft of the Stars: Here, the Nucleus appear'd divided in.o feveral Parts. 6 Feb, the Dilk was leffen'd ; the Nu- clei flill exifted, tho lefs than before : one of 'em, on the lower Part of the Diik, on the left, much denfer and bright- er than the reft; its Body round, and reprefenting a very lucid little Star: TJ» Nuclei flill incompafs'd with another kind of Matter, to Feb. the Head fomewhat more obfeure, and the Nuclei more confufed, but brighter at top than bot- tom. 13 Feb. the Head diminifh'd much, both in Magni- tude and brightnels. 2 March its roundnefs a little im- pair'd, its Edges lacerated, &c. 28 March very pale and exceeding thin ; its Matter much difpers'd ; and no diftinct Nucleus at all appearing.
H'eigelius, wfeoTaw the Comet of 1664, the Moon, and a little Cloud iilumin'd by the Sun at the fame time ; obferv'd, that the Moon, thro' the Telefcope, appear'd of a continued lu- minous Surface ; but the Comet very different ; being per- fectly like the little Cloud in the Horizon, iilumin'd by the Sun. From thefe Obfervations it was, that Hevelius conclu- ded Comets to be like MacuU, or Spots form'd out of the fo- lar Exhalations.
The Length of the Tails of Comets is various : That of i<S8o, according to Sturmius, about the 20th of November > was but fmall, at moll not exceeding 20 Degrees in length: in a little time it grew to a length of do Degrees 3 after which it dwindled very fenfibly.
Formation of the lails of Comets. Sir Tfaac Neivto??. fliews, that the Atmofpheres of Comets will furnifh Vapour fufficient to form their Tails : This he argues from- that wonderful Rarefaction obferv'd in our Air, at a diftance from the Earth : a Cubic Inch of common Air, at the diftance of half the Earth's Diameter, or 4000 Miles, would expand it felf fo as to fill a fpace larger than the whole Region of the Stars. Since then the Coma, or Atmofphere of a Comet is ten times higher than the Surface of the Nu- cleus, counting from the Centre thereof; the Tail, amend- ing much higher, mult needs be immenfely rare : fo that 'tis no wonder the Stars fhould be vifible thro' it.
Now, the Afcent of Vapours into the Tail of the Comet, he fuppofes occafion'd by the Rarefaction of the Matter of the Atmofphere at the Time of the Perihelion. Smoke,'tis obferv'd, afcends the Chimney by the Impulfe of the Air wherein it floats 5 and Air, rarefy'd by heat, afcends by the Diminu- tion of its fpecific Gravity, taking up the Smoke along with it : Why then mould not the Tail of a Comet be fuppos'd to be rais'd after the fame manner by the Sun ? For the Sun- Beams don't act on the Mediums they pafs thro', any other- wife than by Reflection and Refraction.
The reflecting Particles, then, being warm'd by the Ac- tion, will again warm the vEther wherewith they are com- pounded ; and this, rarefy'd by the Heat, will have its fpe- cific Gravity, whereby it before tended to defcend, dimi- nifli'd by the Rarefaction ; fo as to afcend, and to carry along with it thofe reflecting Particles, whereof the Tail of the Comet is compos'd.
This Afcent of the Vapours will be promoted by their cir- cular Motion round the Sun ; by means whereof, they will
endeavour to recede from the Sun, while the Sun's Atmo, fphere, and the other Matters in the celeftial Spaces, are ei! ther at reft, or nearly fo ; [as having no Morion but what they receive from the Sun's Circum rotation.
Thus are the Vapours rais'd into the Tails of Comets in the neighbourhood of the Sun, where the Orbits are molt curve ; and where the Comets being within the denfer At- mofphere of the Sun, have their Tails of the greateft length. The Tails thus produced, by preferving that Motion, and at the fame time gravitating towards the Sun, will move round his Body in Ellipfes, in like manner as their Heads ■ and by this means, will ever accompany, and freely adhere to their Head. In effect, the Gravitation of the Vapours to- wards the Sun, will no more occafion the Tails of the Co- mers to forfake their Heads, and fall down towards the Sun, than the Gravitation of their Heads will occafion them to fall off from their Tails : but by their common Gravitation, they will either fall down together to the Sun, or be tog e ! ther fufpended, or retarded. This Gravitation, therefore, does not at all hinder, but that the Heads and Tails of Co- mets may receive and retain any pofition towards each other, which either the abovemenfion'd Caufes, or any other, may occafion.
The Tails, therefore, thus produced in the Perihelion of Comets, will go off along with their Head into remote Re- gions ; and either return thence, together with the Comets^ after a long Series of Years ; or, rather, be there loft, and vanifh by little and little, and the Comet be left bare ; till at its return, defcending towards the Sun, fome little fliort Tails be gradually and flowly produe'd from the Heads ; which af- terwards, in the Perihelion, defcending down into the Sun's Atmofphere, will be immenfely incrcas'd.
The Vapours thus dilated, rarefy'd, and diffus'd thro' all the celeftial Regions, the fame Author obferves, may pro- bably, by little and little, by means of their own Gravity, be attracted down to the Planets, and become intermingled with their Atmofpheres.
He adds, that for the Confervation of the Water, and Moitture of the Planets, Comets feem abfolutely requifite ; from whofecondens'd Vapours and Exhalations, all thatMoif- ture which is fpent in Vegetations and Putrefactions, and turn'd into dry Earth, &c. may be refupply'd and recruited. For all Vegetables grow, and increafe wholly from Fluids ; and, again, as to their greateft part, turn, by Putrefaction, into Earth again; an earthy Slime being perpetually precipitated to the bottom of putrefying Liquors. Hence, the quantity of dry Earth muft continually increafe, and theMoifture of the Globe decreafe, and at laft be quite evaporated; if they have not a continual Supply from fome part or other of the Uni- verfe. And I fufpecr, adds our great Author, that the Spirit, which makes the fineft, fubtileft, and beft parr of our Air, and which is abfolutely requifite for the Life and Being of all Things, comes principally from the Comets.
On this Principle, there feems to be fome Foundation for the popular Opinion of Prefages from Comets : fince the Tail of a Comet thus intermingled with our Atmofphere may pro- duce Changes veryfenfible in Animal and Vegetable Beings. M. Facto has fuggefted, that fome of the Comets having their Nodes fo very near the annual Orbit of the Earth; fhould the Earth happen to be found in that Part next the Node, ,at the Time of a Comet's paffing by ; as the apparent Motionof the Comet, will be incredibly fwift, fo its Parallax will become very fenfible ; and the Proportion thereof to that of the Sun will be given : whence, fuch Tranfits of Comets will afford the beft Means of determining the diftance of the Earth and Sun.
The Comet of 1472, v.g. had a Parallax above twenty times greater than the Sun's : and if that of i<5iS had come down in the beginning of March to its defcending Node, it would have been much nearer the Earth, and its Parallax much more notable. But, hitherto, none has threaten'd the Earth with a nearer Appulfe than that of io~8o : For, by Calculation, Dr. Halley finds, that November 11, iH. tfMin. P. M. that Comet was not above oneSemidiameter of tho Earth, to the Northwards of the Way of the Earth ; at which time, had the Earth been in that part of its Orbit, the Comet would have had a Paraliax equal to that of the Moon : What might have been the confequence of fo near an Ap- pulfe, a Contaft, or laftly, a Shock of the celeftial Bodies ? Motion of Ccmets. If their Paths be fuppofed directly parabolic, as fome have imagin'd, it would follow, that being impell'd towards the Sun by a centripetal Force, they defcend as from Spaces in- finitely diftant ; and by their Falls acquire fuch a Velocity, as that they may again run off into the remoteft Regions ; ftiu moving upwards, with fuch a perpetual Tendency as never to return. But the frequency of their Appearance, and their degree of Velocity, which does not exceed what they might acquire by their Gravity towards the Sun ; feem to put it paft doubt that they move, Planet-like, in elliptic Orbits, tho thofe exceedingly eccentric ; and fo return again, tho af- ter very long Periods. See Ellipsis.
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