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ASTRONOMY.

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(heir orbits are not vet determined ; nor can we as yet find whe-ther they have their periods, or whether they return again in arertain order.”—“ 'I he time will come, continues he, whereinthese things which are now hid trom us will be discovered; whichobservation, and the diligence of after-ages, will find out. For itis not one age that is sufficient for so great matters: The time willbe when posterity will wonder that we were ignorant of things soplain ; one will . arise who will demonstrate in what regions ofspace the comets wander, why they recede so far from the otherplanets, how great and what sort of bodies they are.” See Dr.Keil’s Astronomy, Led . 17.

(173.) In the dark and superstitions ages, comets were held to be theforerunners of every kind of calamity, and w ere supposed to havedifferent degrees of malignity according to the shape they as-sumed ; from whence also they were differently denominated.Thus, some were said to be bearded, some hairy ; some to re-present a beam, sword, or spear; others a target, &c. ; whereasmodern astronomers acknowledge only one species of comets,and account for their different appearances, from their differentsituations and distances from the Sun ami Earth .

(174.) Kepler, in other respects a very great genius, indulgedthe most extravagant conjectures, not only concerning comets,but the whole system of nature. The planets he imagined to behuge animals, who swam round the Sim by means of certain finsacling upon the ethereal fluid as those of fishes do on the water:and agreeably to this motion, he imagined the comets to be mon-strous and uncommon animals generated in the celestial spaces;and he explained how the air engendered them by ail animal faculty !

(173.) John Bodin, a learned Frenchman of the lOth century,entertained an opinion, if possible, still more ridiculous, liemaintained that comets “ are spirits which having lived on theEarth innumerable ages, and being at last arrived on the confinesof death, celebrated their last triumph, or are recalled to the fir-mament like shining stars ! This is follow ed by famine, plague,&c. because the people destroy the governors and chiefs who ap-pease the wrath of God !!!”

(17(3.) Others have denied even the existence of comets, andmaintained that they were only false appearances occasioned by theretraction or reflection of light. 1 he first rational conjectureamong the moderns we meet w ith is that of James Bernonilli, whoimagined them to be the satellites of some yerv distant planet, whichwas invisible to us on account of its distance, as were also thesatellites, unless w hen in a certain part of their course.

(177.) The first astronomer who restored the comets to theirtrue rank in the creation, was Tycho Brahe . Before his time se-veral had been observed with toierable exactness by Regiomonta-nus , Appian, Fabricius, and others; yet they thought them belowthe Moon. But Tycho being provided with much better instru-ments, set himself with great diligence to observe the famous co-met of 1577; and from many careful observations deduced that ithad no sensible diurnal parallax; and therefore.was not only far abovethe regions of our atmosphere, hut much higher than the Moon .But though few have come so near the Earth as to have any diurnalparallax; all of them have wdiat may he called an annual parallax;that is, the revolution of the Earth in her orbit, causes theirapparent motion to he very different from what it would heif viewed from the Sun : and this shews them to he much nearerthan the fixed stars, which have no such parallax. Kepler, thedisciple of Tycho , was also very attentive to the motions of thecomets, and found that they did not move in straight lines, as hadbeen supposed. He shew ed that their paths were concave towardsthe Sun , and supposed them to move in parabolic trajectories.

(178.) That comets are not only above the air, but also beyond theMoon , is plain; because comets seen from distant places, are ob-served to he at the same distance from a tixed star which is nearthem. As for example, the comet which Tvcho Brahfi observedat l.'raniburg, was likewise seen by llagecius at Prague in Bohe-mia at the same time; which two places differ six degrees in lati-tude, and are nearly under the same meridian, anil both measuredtin* distance of this comet from the star called the vulture; that is,how much it was below it towards the horizon, for both the vul-ture and comet were in the same vertical circle, and both obser-vers found their distances the same, and consequently they bothviewed tiie comet in the same point of the heavens ; which couldnot he, unless it had been further from us than the Moon .

^ 179.) The figures of comets asc observed to he very different, for

some of them throw forth beams like hair every w ay roun i them,and these are called hairy comets. Others again have a lon£ beard,or rather a fiery tail, opposite to tire region in which the bun is seen ;and they are called bearded, or comets with tails. 1 he tails of co-mets olten appear bent, (see Plate XXIV. fig. 9,) owing to theresistance of the aether ; which, though extremely small, mayhave a sensible effect on so thin a v apour as file tail consists of.This bending is seen only when the Earth is not in the plane ot theorbit of the comet continued. When that plane pusses through theeye of the spectator, the tail appears straight. '1 heir magnitudehas also been observed to be very different; many of them with-out the hair, appear no bigger than stars of the lirst magnitude.But some authors have given us an account of others which weremuch greater: such was that which appeared in the time of theemperor Nero , which, as Seneca relates, w as not inferior in mag-nitude to the Sun itself. In like manner, the comet which IIr-velius observed, in the year 1652, did not seem to he less thanthe Moon , though it had not so bright a splendor; for it had apale and dim light, and appeared with a dismal aspect. Mosthave a dense and dark atmosphere surrounding their bodies, whichweakens and blunts the Sun ’s rays; hut within it appears the nu-cleus or solid body of the comet, which, when the clouds are dis-persed, gives a splendid and brisk light. The particulars in whichcomets differ from planets are, that they move in various direc-tions-, some the same way with the planets, others the contrary ;neither are their motion' confined within the zodiac, their orbits ad-mitting of any inclination to the ecliptic w hatever. And the eccen-tricity of their orbits is so very great, that some of the comets per-form the greatest part of their motion in curves approaching al-most to right lines. Hence it is, that when they are in their peri-helia, their motion is immensely swifter than when they are intheir aphelia. This will better appear from fig. 9. l’l. XXIV.which is a delineation of about the 80th part of the orbit of thecomet seen by a great number of astronomers in different parts ofEurope , from the 4tli to the 25th of November, 1680, in the morn-ing, apparently descending almost in a straight line from the region ofthe taxed stars toward the Sun. It was hid in the Sun ’s rays till ithad passed its perihelion, and in its return was seen in the eveningfrom the 12th of December to the 9th of March following. Ofall the orbits of the comets, yet observed, this is the narrowestand most eccentric : its nearest approach to the Sun was within asixth part of the Sun ’s diameter, and its velocity in the perihelionwas upwards of a million of miles in an hour. There is a greatdiff erence in the eccentricities of the comelary orbits. Eig. 2. re-presents one les' eccentric : let S he the Sun , RET the orbit ofthe Earth , the comet is seen in its approach at F, in its perihelionat P, where its velocity is to its velocity at the aphelion A as ASto SP ; let the areas PSD and ASB be equal, then will the areasPD and AI? be described in the same time.

(180.) It is now the general opinion of astronomers, that co-mets are opaque bodies enlightened by the Sun. Hevelius , in alarge work, wherein he gives the opinions of various authors onthis subject, mentions some who were of the same sentimentswith himself, that comets were so far transparent as to let the lightof the Sun pass through them, which formed their tails. Sir IsaacNewton was of opinion, that they are quite opaque ; and in con-firmation of this, he observes, that if a comet be'seen in tw o partsof its orbit, at equal distances from the Earth , hut at unequal dis-tances from the Sun, it always shines brightest in that nearest the Sun .

(181.) Tycho Brahfi, found the. distance of the comet of 1377to he about 210 semidiameters of the Earth , or about 8-10000miles distant from us, its apparent diameter being seven minutes ;whence he concluded, that the true diameter of the comet was tothat of the Earth as 3 to 14. Hevelius , from the parallax and ap-parent diameter of the head of the comet in 1652, computed itsdiameter to be to that of the Earth as 52 to 100. By the samemethod he found the diameter of the head of the comet of 1664to be at one time 12 semidiameters of the Earth , and at anothernot much more than 5: and the diameter of the nucleus of thecomets of 1661 and 1665 to he only about a tenth part of thatof the Earth ; and Cysatus makes the true diameter ol the cometof 1618 to be about the same size.

(182.) But the diameter of the atmosphere is often 10 or 15times as great as that ot the nucleus, the former, in the comet of1682, was found by Ilamsteed to be 2’ when the diameter of thenucleus was 11 or yj'' The diameter of the comet of J74-1, when

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