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ASTRONOMY.
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at the distance of the Sun ircm us, was about 1', hence its diame-ter was about 3 times that of tht; Earth ; at another time the dia-meter of the nucleus was nearly equal to Jupiter.
. (183.) Some comet', however, from their apparent magnitude.and distance, have been supposed to be nmen larger than theMoon , or even equal in magnitude to some of the primary planets ;and soin: have imagined that, by an interposition of these bodiesbetwixt the Earth and Sun , we might account for those darknesseswhich canuot be derived from any interposition of the Moon .Such are tho e mentioned by llero-fotu=, lib. 7. cap. 37, and lib.9, cap. 70 ; likewise the eclipse mentioned by Dicn, which hap-pened a little before the death of Augustus ; and it is ohservablethat Seneca saw a comet that year. Some have even attemptedto account in this manner for the darkness which happened whenour Saviour was upon the cross ; and indee 1 it is certain (thoughwe hv no means think it credible that this was the case at the timejust referred to) that were a comet in its perigee to come betweenthe Earth and Sun , and to be moving the same way with theEarth , it must cause a darkness much more intense, as well as ofmore considerable duration, than wiiat could take place in anysolar eclipse.
(184.) The tails of comets have given rise to various conjec-tures ; though it is acknowledged by all, that they depend on theSun some way or other, as they are always turned from him. DesCartes and his followers were of opinion, that the tail of a cometwas owing to the refraction of its head : blit if this were the case,the planets and principal fixed,tars would have tails also ; for the raysfrom them pass through the same medium as the light from the comets,
(185.) Sir Isaac Newton was of opinion, that the tail of a cometis a very thin vapour which the head sends out by reason of itsheat: that it ascends from the Sun just as smoke does from theEarth ; that as the ascent of smoke is caused by the rarefactionof the air wherein it is entangled, causing such air to ascend andcarry the smoke up with it; so the Sun ’s rays acting upon thecoma or atmosphere of the comet, do by rarefaction and refrac-tion heat it: that this heated atmosphere heats, and thus rarefiesthe ether that is involved therein ; and that the specific gravity,with which such ether tends to the Sun , is so diminished by ttsrarefaction, and that it will now ascend from him by its relativelightness, and carry with it the reflecting particles whereof thetail is composed: Though the immensely large tails of somecomets seem to require a great quantity of matter to producetheiri, they afford no objection to this solution ; for every day’sexperience shews what a great quantity of smoke is produced froma very little wood or coal ; and Newton lias demonstrated, that acubic inch of air equally rarefied with that at the distance of asemidiameter from the Earth ’s surface, would fill all the planetaryregions to the orbit of Saturn and beyond.
(186.) Marian supposed the tails of the comets to be formedout of the luminous matter whereof the Sun ’s atmosphere consists,to extend as far as the orbit of the Earth , and to furnish mattertor the aurora borealis. De La Lande joins the two last opinionstogether. Part of the matter which forms the tails of comets hesupposes to arise from their own atmosphere rarefied by heat, andpushed forward by the force of the light streaming from the Sun ;and also that a comet passing through the Sun ’s atmosphere isdrenched therein, and carries away some of it.
(1S7.) Rowning objects to Newton ’s theory, that it can hardlybe supposed the thin vapour of the tail should go before the moresolid body of the comet, when its motion is sometimes so extremelyswift, as that of some of the comets is said to be after the rate(as New ton calculated the motion of the comet of 1680 to be)of no less than 880,000 miles an hour. He, therefore, supposesthe atmosphere of the comet to extend every way round it as faras the tail reaches ; and that the part of it which makes the tail isdistinguished from the rest, so as to fall, thick upon that part of theatmosphere which goes before the comet in its progress along itselliptic orbit. An insuperable objection to this is the immensemagnitude of the atmospheres, as it must now be supposed, toaccount for the vast lengths of the tails of some comets, whichhave been said to measure above 80 millions cf miles.
(183.) M. Euler thinks there is a great aflinity between the tailsof the comets, the zodiacal light, and the aurora borealis ; and thatthe common cause of them all is the action of the Sun ’s light onthe rtino pheres of the comets, of the Sun, and of the Earth . Hesupposes, that the impulse of the rays of light, on the atmosphere
of comets, may dri ve some of its finer particles far beyond thelimits of that atmosphere, and that this force of impulse combinedwith that of gravity towards the comet, would produce a tail,which would always be in .opposition to the Sun , if the cometdid not move. l?ut the motion of the comet in its orbit, and aboutits axis, must vary the position and figure of the tail, giving it acurvature and deviation from the right line joining the centres »fthe Sun and comet, which deviation will be greater as the orbit ofthe comet has tiie greater curvature, and as its motion is the morerapid. The velocity of the comet in its perihelion may be sogreat that the force of the solar raj s may produce a new tail be'fore the old one can follow. '1 he possibility of this was con-firmed by the comet of 1744, which had several tails while it wasin its perihelion.
(189.) The resemblance between the tails of comets and theaurora borealis, which is commonly considered as an electricalpluenomenon, has suggested an opinion that the tails of cometsarc streams of electric matter. Hamilton, in a small treatise, in-titled, “• Conjectures on the Nature of the Aurora Borealis , andon the Tails of Comets,” supposes that the comets are of use to.bring back the electric fluid to the planets, which is continuallydischarged from the higher regions of their atmospheres.
(190.) The near approach of some comets to the Sun subjectsthem to intense and inconceivable degrees of heal. Newton cal-culated that the heat of the comet of 1680 must have been near‘3000 times as great as that of red-hot iron. The calculation isfounded upon this principal, that the heat of the Sun falling uponany body at different distances is reciprocally as the squares ofthose distances ; but it may lie observed, that the effect of the heatof the Sun upon all bodies near our Earth depends very much oilthe constitution of those bodies, and of the air that surrounds them.
(191.) Mr. Whiston, from Flamsteed’s measure of its apparentdiameter, concluding the nucleus of the comet to be about tentimes as big as the Moon, or equal to a 4th part of the Earth ,attributes the universal deluge to the near approach thereof. Hisopinion was, that the Earth passing through the atmosphere of thecomet, attracted therefrom great part of the water of the flood ;that the nearness of the comet raised a great tide in the subterra-neous waters, so that the outer crust of the Earth was changedfrom a spherical to an oval figure ; that this could not be donewithout making fissures and cracks in it, through which the watersforced themselves, by the hollow of the Earth being changed intoa less capacious form; carrying along with them trees, animals,fishes, Sec. the bones, shells, and other relics which are oftenfound at great depths in the Earth . The same comet, he thought,would probably, coming near the Earth when heated in an im-mense degree in its perihelion, be the instrumental cause of thatgreat catastrophe, the general conflagration, foretold in the sacredwritings, and conjectured from ancient tradition.
(192.) The analogy discovered by Kepler, between the perio-dical times of the planets and their distances from the Sun , takesplace also in the comets. In consequence of this the mean dis-tance of a comet?from the Sunlnay be found by comparing itsperiod with the time of the Earth's revolution round the Sun .Thus the period of the comet that appeared in 1531, 1607, 1682,and 1759, being about 76 years, its mean distance from the Sun may be found by this proportion: As 1, the square of one year,the Earth 's periodical time, is to 5776 the square of 76, thecomet’s periodical time ; so is 1000000, the cube of 100, theEarth’s mean distance from the Sun , to 5776000000, the cubeof the comet’s mean distance. The cube root of this last numberis 1794; the mean distance itself in such parts as the mean distanceof the Earth from the Sun contains 100. If the perihelion distanceof the comet, 58, be taken from 3588, double tbe mean distance,we shall have the aphelion distance, 3530, of such parts as the dis-tance of the Earth contains 100; which is a little more than 35times the distance of the Earth from the Sun . By a like method,the aphelion distance of the comet of 1680 conies out 138 timesthe mean distance of the Earth from Ihe Sun , supposing its periodto be 575 years; so that this comet, in its aphelion, goes more than7 times the distance from the Sun that Ilerschel does. Euler com-putes the orbit of this comet from three of Flamsteed's observa-tions taken near together, compared with a fourth taken at somedistance from the other three, and from thence concludes theperiod to be a little m ore than 170 years.
(193.) The celebrated Dr. Halley examined with great care the'ob-
servatiuBS