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peor 7 Sank v a —— . E a «o> FW 52, ‘ eet | < ig THE COMETS:————— A DESCRIPTIVE TREATISE UPON THOSE BODIES. WITH A CONDENSED ACCOUNT OF THE NUMEROUS MODERN DISCOVERIES RESPECTING THEM; AND A TABLE OF ALL THE CALCULATED COMETS, From the Earliest Ages to the Present Time. BY J. RUSSELL HIND, FOREIGN SECRETARY OF THE ROYAL ASTRONOMICAL SOCIETY OF LONDON; CORRESPONDING MEMBER OF THE NATIONAL INSTITUTE OF FRANCE, ETC, ETC. AND FORMEBLY OF THE ROYAL OBSERVATORY, GREENWICH. LONDON: JOHN W. PARKER AND SON, WEST STRAND. MDCCCLIT, LONDON: SAVILL AND EDWARDS, PRINTERS, _ CHANDOS STREET. PREFACE. THERE is perhaps no branch of astronomy in which our knowledge has progressed more rapidly of late years than that relating to the Comets, but I believe there is not at present any special descriptive work upon the subject. In treatises on the science of astronomy. generally, the space allotted to the Comets is usually very limited, and many interesting points connected with their history are neces- sarily passed over unnoticed. I have endea- voured, in the following pages, to supply the want of an elementary treatise upon these bodies, and to place before the reader a popular account of the discoveries which have been made in the cometary department of astronomy in modern times, and a brief description of the most remarkable comets recorded in history. 263255 vi PREFACE, The facts which are here collected together are, for the most part, scattered through many works of a purely scientific character, usually in foreign languages, and not known or easily accessible to the general reader. The Cata- logue of the elements of the Orbits of Comets, with the accompanying Notes, will, it is hoped, be acceptable to the young student, as pre- senting in a small compass the results of a vast amount of time and labour which various astronomers have expended upon them. J. Russerx Huinp. Grove-road, St. John’s Wood, London, November, 1852. CONTENTS. Cuaptrer I. Of Comets in general—Their number—Duration of visibility—Length of their apparent tracks in the heavens—Their nuclei and tails—Apparent di- mensions. CHAPTER Il. Of the real dimensions of comets—Phases observed in some of them—Their physical constitution— Chances of collision with the earth Cuaprer IIT. Of the paths of comets in space—Elements of their orbits—Uses of a table of elements—Effects of planetary attraction upon their movements Cuaprer IV. The comet of Halley CHAPTER V. Comets of short period—The comet of Encke. PAGE 17 27 35 58 viii CONTENTS. Cuarrer VI. PAGE. Comets of short period (continued)—The comet of Biela ; ‘ 5 : 3 ; : - wl Cuarter VII. Other comets of short period—Faye’s, De Vico’s, Brorsen’s, D’ Arrest’s : é ; : . 80 Cuarter VIII. Periodical comets (continued)—Of six comets, pro- bably with short periods—Lexell’s comet in 1770 —Comets which appear to revolve round the sun in about seventy-five years—Others to which longer periods have been assigned. : op ae ifs Carter IX. A brief notice of some of the most remarkable comets that have been observed from the earliest times © to the present day . : : ; , . 102 CHAPTER X. On the expected Great Comet . . i : . 116 A Catalogue of the Orbits of all the Comets hitherto computed, B.c. 370 to a.p. 1852 : 5 . 125 Notes to the Catalogue of Cometary Orbits. . 139 THE COMETS. CHAPTER I. OF COMETS IN GENERAL—THEIR NUMBER—DURA- TION OF VISIBILITY—LENGTH OF THEIR APPARENT TRACKS IN THE HEAVENS—THEIR NUCLEI AND TAILS—-APPARENT DIMENSIONS. Comets are nebulous-looking bodies, moving in orbits of great eccentricity, and visible to us only about the time of their nearest approach to the sun, or, as it is termed by astronomers, the time of perihelion passage. Unlike the planets, they move in every possible di- rection, and are seen as well near the poles as about the equatorial or zodiacal regions of the heavens. In the dark ages of superstition and ignorance, these ' bodies were regarded as omens of evil to men in general, and the sudden appearance of a great comet excited the utmost consternation, and on more than one occasion influenced the progress of sublunary affairs. One which became visible in the spring of 837 in the southern parts of the heavens, so alarmed Louis I. of France, that he ordered the building of churches and monasteries, in the hope of appeasing the B 2 THE COMETS. wrath of heaven, which was considered to be manifested by its appearance. Again, in June, 1456, when the successes of the Turks under Mahomet IT. excited the utmost fear at Rome for the safety of Christendom, a great comet (known at the present day as Halley’s) was observed, and the Pope, Calixtus III., regarding it as a sign of approaching evils, caused prayers to be said in the churches, and the bells to be rung every day at noon, as a warning to the inhabitants to supplicate the divine aid and forgiveness. Those who neglected this call were subject to severe punishment. Hence arose the custom, still prevalent in Roman Catholic countries, of ringing the cathedral bells at noon. A cen- tury later, or in 1556, the appearance of a fine comet is said to have induced Charles V. of Austria to cede — the imperial crown to his son Ferdinand. The his- | torians of these ages, in speaking of comets, fre- quently describe them as of ‘horrible aspect,’ or as celestial monsters of prodigious magnitude, fearful and terrible stars.) While they were beheld with such feelings of dread and superstition, it will hardly be expected that many useful accounts of their move- ments amongst the stars would be transmitted to us, and it accordingly happens that but very little information available to astronomers at the present day, is to be found in European chronicles. The Chinese astronomers, though they looked upon comets without any fears of their malignant, agencies, had a very fanciful opinion respecting them, which never- ‘theless led to the frequent observation of the position of these bodies in the heavens, the results of which have been found most valuable in modern times, THEIR NUMBER. 3 The number of comets from the Christian era to the middle of the present century, of which there is any trustworthy record, is rather over 600, including, besides the comets mentioned by European historians, all those observed in China, according to the annals of the various dynasties reigning in that country. But as the telescope was not much employed in searching out these objects before the middle of the eighteenth century, the real number of comets which have visited these parts of space must be very much larger, probably amounting to not less than 3000, since it would appear, from the experience of the past century, that only one in every five becomes sufficiently conspicuous to the naked eye to attract general atten- tion. The Chinese, it is true, were very diligent in their celestial observations, yet many comets must have escaped their notice, since they had no telescopes to assist them. A considerable number of comets have doubtless appeared in past times in the southern heavens. only, so as to be invisible in Europe, and allowing for this, we shall probably be within reason- able limits when we assume that upwards of 4000 have approached the sun within the orbit of Mars since the commencement of the Christian era. The following table exhibits the number of suffi- ciently authenticated comets in each century :— Comets observed in Comets observed in Century. Europe and China. } Century. Europe and China. " * etal. 21021 Wiles arc eee We atbrehiat (2).kor ee Wallop tac starr Simao FE ogee 2a vy, 44 2,7 1) Oem re ees Sha) H5) J. ASE ROTI ae Sen MN EXSY. sn can dos ae haters, «4 HG Mee Se Me hee Bo 4 Tue COMETS. Comets observed in Comets observed in Century. Europe and China. | Century. Europe and China. XGieee 8 Te Bs te OO OVA, ee SL Bleu, (ote Yau) VO CVA eae me Teo SCTE. * Perce onde) XV acest aoe SS IESES OSS MRP eet) XIX. (first half) . 80 PSOY tags Us 27 Giving a total of 607. The duration of a comet’s visibility varies from a few days to more than a year, but it most usually happens that it does not exceed two or three months. Much depends on the apparent position of a comet in reference to the sun’s place, on its distance from the earth, and particularly on its intrinsic brightness, or rather, we should say, on its capability of reflecting the sun’s rays. Some of these bodies have been observed at a far greater distance from the earth and sun than others. The remarkable comet of 1729 never approached nearer to the sun than fowr times the earth’s mean distance from that luminary, yet it was seen without any great optical aid for nearly six months. That discovered by Colla, in May, 1847, was also followed till its distance from the earth was three-and-a-quarter times the distance of our globe | from the sun, and another found in the same year by Mauvais was visible in the large telescopes at several observatories in this country, till it had receded to nearly three times that distance. In both these in- _ stances the comet’s distance from the sun was equally great. Many of these bodies, on the contrary, fade away even in the most powerful instruments we possess, before they have attained anything like a distance equal to the diameter of the earth’s orbit. DURATION OF VISIBILITY. 5 Amongst those comets which have continued longest in sight, we may mention the following :— A.D. 60 (in China) . 6 5 . 135 days. 64 (according to Seneca) 180 days or more. 247 (in China) . ‘ : - 156 days. 1729 : ; ; 5 9 ES er 1773 Shes lb A male een a 1807 . ’ : . . ye LOo Miss 1811 : : 5 : : nO LOE Ess 1815 . . . : . be ES > as 1825 f ; Bip : Aras iteys By, 1835 c : , : , ec Oues 1844 (Mauvais’ comet) . 3 . 246 ,, 1847 (Colla’s comet) : “0 = 2401, 1847 (Mauvais’ comet) . ‘ LOOM 5h The first three comets in the above list were of course observed without the assistance of a telescope, the others have all been followed with optical aid. That of 1835 was predicted, and therefore discovered much sooner than it is probable it otherwise would have been. In several cases the comets have not remained visible throughout the whole period, but after disappearing for some days or weeks in the sun’s rays, have become conspicuous again on the other side : in these instances we have given the interval between the earliest and latest observations as sufficient for our present purpose. It would appear that out of the many hundred comets recorded in history, only six have been observed so long as eight months, and these six instances have all occurred during the present century. There are some few comets which have only been seen once, unfavourable weather 6 THE COMETS. preventing further observation. In these cases we cannot ascertain any particulars respecting their distances, or real paths in space. The lengths of the apparent tracks of comets in the heavens, during the time of visibility, varies greatly, being dependent on the distance of the body from the earth, and on the direction of its real movement, in reference to that of our globe. The comet of 1729 which, as already remarked, was always very remote from us, traversed an are of only 15° during the six months it continued in view, while that of 1769 passed through an are of 240° in longitude; and another in 1825, which moved extremely slow at first, (the change of right ascension and declination in ten days being little over two degrees), subsequently quickened its rate of progress, and traversed an arc of 225°. It is possible that a comet may be so situated in reference to the earth, as to appear to pass through no less than 140° in the space of an hour, or 178° in a day ; but the combination of circumstances neces- sary to produce this enormous change in the apparent positions, has never yet taken place: the real motion of the comet must be retrograde or contrary to that of the earth in its orbit, it must be in opposition to the sun, and in perihelion, moving in the plane of the ecliptic, and at a distance from us not exceeding that of the moon. If a comet be moving in the same di- rection as the earth, the distance of the two bodies being small, its apparent position may scarcely change in the course of the day, and if it be advancing towards the earth, the same, or nearly the same place THEIR FIGURES. * 7 in the heavens, may be preserved for many weeks, as in the case of the comet of 1825, to which we have just alluded. When a comet is conspicuous to the naked eye, it consists, in nearly every case, of a roundish and more or less condensed mass of nebulous matter, termed the head, from which issues, in a direction opposite to that nf the sun, a train of a lighter kind of nebulosity, called the tail. Sometimes the centre of the head is occupied by a starlike point or nucleus ; at others by a well-defined planetary disk, while in by far the greater number of instances, it exhibits nothing more than a higher degree of condensation of the nebulous matter, which always has a confused appearance in the telescope. Occasionally a low magnifying power will afford evidence of the existence of a stellar nucleus, but, on applying higher magnifiers, this appearance vanishes, the light towards the centre being merely of greater intensity than at the borders, without coming up suddenly to a point, so as to resemble a star. Telescopic comets are generally destitute of a tail, and appear most commonly as roundish nebu- losities, strongly condensed towards the centre, but without any decided nucleus. There are exceptions, however, to this rule, as we shall presently find. The same comet may put on at different times of its visi- bility, every variety of figure and general appearance from the dim nebulous spot hardly discernible in the telescope to the ‘cometa terribilis,’ ‘horrendze mag- nitudinis,’ the tailed and hairy stars which spread so much alarm amongst our forefathers, before science showed the groundlessness of such fears. These 8 ¢ Tne CoMETs. changes in the aspect of the same comet, are caused by variation in its distance from the earth and sun, by its position in respect to the former, and possibly also in some degree by actual change in the form of the comet itself. The envelope, mentioned by astronomers in their observations of large comets, consists of a border of light surrounding the head on the side near the sun, and passing round in each direction, so as to form the commencement of the tail. The Coma is the nebulosity which surrounds a highly condensed or planetary nucleus. Some comets have attained such an extraordinary degree of splendour, as to be distinctly visible at noon- day, or to render the stars dim by contrast, and cast sensible shadows at night. A comet mentioned by Diodorus Siculus, was so brilliant as to cast shadows during the night, as strongly marked as those formed by the moon. Another, which appeared, according to Seneca, shortly before the war of Achaia, B.c. 146, was as large as the sun, and dissipated the darkness of the nights: its disk was red andfiery. The comets ‘recorded by Justin, at the time of the birth and accession of Mithridates, rivalled the sun in splendour. One observed in February, 1106, was seen close to the sun all over Europe. That of March, 1402, was visible day and night in the circumpolar regions of the heavens, in Germany and Italy; and another grand comet in the summer of the same year, was bright enough to be discerned before sun-set. The second comet of 1618, was distinctly seen in full daylight by Marsilius, in Bohemia. The comet of UsvuaL CoLour. 9 March, 1843, by far the most imposing of the present century, was discovered in various parts of the world, two or three degrees only from the sun’s limb: it must have far exceeded the planet Venus in bril- liancy. Instances are on record where comets have become visible during an eclipse of the sun, a circum - stance indicative of great intrinsic splendour, since stars of the second magnitude are not very readily dis- cerned on such occasions. Philostorgius says, that on the 19th of July, 418, when the sun was eclipsed, and stars were visible, a great comet, in the form of a cone, was discovered near that luminary, and was afterwards observed during the nights. The comet of 1744 was observed with the telescope at noon-day, and some persons were able to follow it without optical aid for a considerable time after sunrise. The first comet of 1847, discovered by the author, was seen for some hours about two degrees from the sun’s limb, with a powerful telescope, and is the last instance of the kind on record. In most comets the head is white, or has a pale watery appearance. Occasionally, however, the more condensed part has a dull reddish or yellowish tinge, or more rarely inclines to green. Out of forty-nine instances, in which the colours of comets were re- corded by the Chinese, we find twenty-three were white, twenty bluish, four red or reddish-yellow, and two greenish. Planetary nuclet in comets, are of rare occurrence, and it is pretty certain that they consist of nothing more than nebulous matter in a very high degree of condensation, and are not to be regarded as solid BS 10 Tue ComeETS. bodies. Itis questionable whether any comet has yet exhibited during the whole of its visibility, a well defined disk of the same magnitude and appearance : changes of form and apparent diameter are observed, which clearly indicate a gaseous nature, or at any rate are conclusive against the idea of solidity. The nucleus of the great and celebrated comet of 1680 is described by some observers as resembling ‘a burning coal—red and glowing; and the same simile is used by Struve, in reference to the appear- ance of Halley’s comet during the autumn of 1835. Some comets are pale and livid, as that of 1652,— stated by Comiers and Hevelius to be equal to the moon in apparent size. Others, like that of 1744, are bright and dazzling. The head of the fine comet of December 1618, is said to have been composed of several bright points or nuclei: Cysat observed three or four on the 8th of that month,.and on the 20th the head seemed to be a cluster of small stars. Hevelius mentions something analogous in the appearance of the comet of 1661 and 1665; such phenomena are to be attributed to -changes in the gaseous envelope of the nucleus. We are not aware that they have been remarked in any of the larger comets of recent times. Though, as we have seen, the nuclei of comets have occasionally attained so great a degree of brightness as to be discernible in full daylight, or to cast shadows at night, it is their tails or trains which give them so imposing an appearance in the heavens, and which have excited so much astonishment in all ages. But very few of the brighter comets have been observed Tue TAILS oF COMETS. 11 | without an appendage of this kind, though the telescopic class rarely appear otherwise than as roundish nebulosities. In some comets the tail has been observed as a long narrow ray of light, somewhat brighter near the head, and gradually fading away into darkness. In others a dark line has divided it into two branches, and instances are on record where two tails, evidently distinct from each other, have been remarked. Other comets have bushy, fan-shaped tails, compared by the ancient observers to the train of a peacock. Not unfrequently the appearance of the tail will vary greatly on successive evenings, or even during the same night. As a general rule, the tail of a comet is turned from the sun, forming a prolongation of the radius vector, or of the line joining the sun and comet. This remarkable fact was first noticed by Peter Apian of Ingoldstadt, who found it the case with the trains of five conspicuous comets which he observed between the years 1531 and 1539. Exceptions do, however, occur. The tail of the comet of 1577 deviated 21° from the line joining the sun and comet, and that of the comet of 1680, nearly 5°. In some few instances where more than one tail has been observed, the second has extended towards the sun, as was the case with a comet discovered in October, 1851. That which appeared in January, 1824, exhibited two tails, both distinctly visible to the naked eye, forming with each other an angle varying from 138° to 170°: one tail was in the usual direction, and appeared shorter but brighter than the second, which streamed off from the nucleus towards the sun. A fine comet, visible in 12, THE COMETS. the southern hemisphere early in 1845, had, in adui tion to a lengthy tail turned from the sun, a second narrow ray of light, forming an angle of about 140° with the other. A comet mentioned by Diodorus Siculus, and ob- served by Aristotle in the year 371 Bc, exhibited a train 60° long. Another, in the year 133 after the Christian era, was attended by a tail 50° long and 2° broad, according to the Chinese observations. The comet of 178 had an appendage of equal length. That of 390, observed in Europe and China, is said to have had a train of the enormous length of 100°. The grand comet of 837, mentioned by nearly all the historians of the time, was remarkable for the length of its tail: on April 10th it extended 50° from the head, and was divided near the extremity into two branches; next day its length was 60°, but only one ray was visible, and on the 14th it had increased to 80°, Another great comet in 1264 was accompanied by a train fully 100° long, agreeably to the Chinese description, while European contemporaries tell us, when the head was just clear of the eastern horizon, the tail stretched past the mid-heaven westward, which seems to indicate an extent of more than 90°. The comet of 1362 is said to have had a train 100° long, as observed in China: in Europe it was de- scribed as a ‘very great and brilliant star.’ The comet of 1456, known as Halley’s, which created so much alarm amongst the Turks, was accompanied by a tail 60° in length. The great one, which appeared in the winter of 1618, exhibited a tail no less than 104° long, according to Longomontanus, the Danish Tue Tarts or Comets. 13 astronomer. Kepler had observed it fully 70° in length a day or two previous. The famous comet of 1680, which was attentively watched by Newton and Halley, was attended by a train 90° long. That ob- served in the southern hemisphere in 1689, had a tail more than 60° in length, and was two hours and a half in rising; and in another which appeared six years afterwards, the tail was nearly of equal mag- nitude. The comet of 1769 was attended by a long and conspicuous tail, but observers differ considerably in their estimations of its apparent length, even on the same day, which can only be attributed to the state of our atmosphere at the various stations. This circumstance had been previously noticed in reference to the comet of 1680, but it was more marked in the present case. Thus on the 9th of September, 1769, Maskelyne at Greenwich considered the length of the tail 43°; at Paris it was Judged to be 55°; at the Isle of Bourbon, it was traced 60° and more by La Nux, while Pingré saw it fully 75° long, being at sea at the time, between Teneriffe and Cadiz. Two days after- wards, this astronomer found it 90° long, while La Nux traced it 97° from the nucleus. The only comet of the present century which has been accompanied by a tail of very great length, is that of 1843, its average extent was about 45°, but on one occasion a narrow ray of light attained the enormous distance of 65° from the head. It would lead us far beyond the limits of the pre- sent work, were we to particularize all the varied phenomena which have been observed in the tails of comets, but there is one singular appearance in the 14 THE COMETS. trains of great, comets which we must not pass over in silence. It consists of apparent vibrations or co- ruscations, similar to the pulsations peculiar to the Aurora Borealis. These vibrations commence at the head, and appear to traverse the whole length of the tail in a few seconds of time. The cause was long supposed to be connected with the nature of the comet itself, but Olbers pointed out that such appear- ances could only be attributed to the effects of our own atmosphere. The reason is this: the various portions of the tail of a large comet must often be situated at widely different distances from the earth, so that it will frequently happen that light would re- quire several minutes longer to reach us from the extremity of the tail than from the end near the nucleus. Hence, if the coruscation were caused by some electrical emanation from the head of the comet, travelling along the tail, even if it occupied only one second in passing over the whole distance, several minutes must necessarily elapse before we could see it reach the end of the tail. This is contrary to ob- servation, the pulsations being almost instantaneous. Gregory of Tours mentions a comet in January 582, the train of which resembled the smoke of a distant conflagration, a description which may perhaps have some relation to these vibrations or coruscations. The first distinct mention of the phenomenon is to be found, we believe, in the Chinese Annals in reference to the comet of 615, the tail of which was between 50° and 60° long, and during the night had a kind of libration to and fro. Kepler says the tail of the comet of 1607 was short one moment, but extended itself in the CURVATURE OF THE TAIL. 15 twinkling of an eye. Longomontanus, in describing the immense tail of the comet of 1618, states that it had ‘an enormous vibration,’ and Father Cysat says it appeared as though it had been agitated by the wind ; the same phenomenon was remarked by Kepler, Wendelin, and other observers of this splendid comet. Hevelius noticed similar undulations in the tails of the comets of 1652 and 1661, and Pingré says he had observed them in the train of that which was visible in 1769. The pulsations were very distinctly seen in the tail of the grand comet of March 1843, and they have been remarked in a greater or less degree in other cases. Another curious phenomenon occasionally observed in the tails of comets is a curvature of the extremity, or of a greater or less portion, so as to give the whole train the form of a sabre. The ancient historians fre- quently make use of this simile in describing the aspect of these bodies. The comet which appeared about the time of the Battle of Salamina, a.p. 479, was of this form, as also those observed at Constan- tinople in 912, 1340, 1402, 1456, and many others. The Chinese compare the tail of the comet of 1232 to the tusk of an elephant. The grand comets of 1264, 1618, and 1689, exhibited curved tails 80° or 100° in length, the latter being particularly described by an observer in the southern hemisphere as having a strik- ing resemblance to ‘a great sabre.’ Pingré noticed a curvature in the tail of the comet of 1769, the convexity towards the north, and at times a second small arc was formed near the extremity, turned in the opposite direction; the same appearances were 16 THE CoMETSs. remarked by La Nux at the Isle of Bourbon. The train of the last great comet in 1843 was very sensibly bent downwards towards the horizon at the end of March, when it became conspicuous in this country, and the smaller comet of December and January 1844-5 exhibited the same phenomenon. THE COMETS. 17 CHAPTER II. OF THE REAL DIMENSIONS OF COMETS—PHASES OB- SERVED IN SOME OF THEM—THEIR PHYSICAL CON- STITUTION —- CHANCES OF COLLISION WITH THE EARTH. HAVING described the apparent dimensions of various comets we must now say a few words relative to their real magnitude. When the distance of one of these bodies is known, and we have observed the angular diameter subtended by the nucleus or head, it becomes a very easy matter to ascertain the true diameter in the same way that we find the dimensions of the sun, moon, and planets. But as it never happens that the borders of the nebulosity are sharply defined, and but rarely so as regards the nucleus, our determinations of their real dimensions are necessarily open to a good deal of uncertainty. One thing, however, is quite certain, that the cometic atmosphere surrounding the nucleus varies greatly in extent in different comets, and even in the same body at different epochs during its visibility. The actual length of the tail of a comet may be computed by trigonometry, when we know the distance of the nucleus from the earth and its position in respect to the sun, always assuming that 18 THE CoMETS. the tail is projected from the sun in the line joming that luminary and the head of the comet. The following were the real diameters in English miles of the nuclei of some of the more remarkable comets, which have been satisfactorily measured within the last century :— Miles, The comet of 1815, discovered by Olbers , : . 5800 The great comet of 1825 , 7 ‘ ; . 5100 The great comet of March, 1843. ; ; ; . 6000 The first comet of 1780 , , . 4270 The first comet of 1847, aeserared. by the acthoe 5 . 8500 The fine comet of July, 1819 : : . 8280 Second comet of 1811, measured by Herschel , . 2640 The great comet of 1807, measured by Herschel. . 538 The celebrated comet of 1811, measured by Herschel . 428 The second, of 1798, according to Schréter and Harding 125 The comet of 1805, known as Biela’s . é » «Onto AN The comet which was visible to the naked eye in June 1845 had a bright planetary-looking nucleus, which must have been nearly 8000 miles in diameter, or about equal to that of the earth. On the 24th of January, 1836, Mr. Maclear, at the Cape of Good Hope, saw a well-defined disk within the head of the comet of Halley, which, from the apparent diameter assigned, could not have been less than 97,000 miles in breadth, but in the previous autumn the very same comet had exhibited a brilliant ‘kernel,’ or nucleus, varying on different dates from 250 to 1000 miles in diameter. A more striking proof of the variable dimensions of a comet could hardly be adduced. The coma, or atmosphere enveloping the nucleus, REAL DIMENSIONS. 19 or more condensed part of a comet, is subject to the same variation, as the subjoined results will show :— Diameter of Head. The grand comet of 1811 . . 1,125,000 miles. Comet of Halley, 1836 . . . 357,000 ,, Comet of Encke, 1828 .   year . . 18,000, During the autumn of 1811, or while the splendid comet of that year was visible to the naked eye, the real diameter of the head appears to have differed but little from 1,000,000 miles, or more than four times the distance which separates the moon from the earth. No other on record has surpassed this. In most in- stances the visible atmosphere of a comet is less than 100,000 miles in diameter, and but very rarely exceeds 200,000. It must be remembered, however, that the nebulosity may extend much further than we can trace it from the earth, and consequently that our estimates of the true magnitudes are very likely to be under-rated. a It has been remarked above, that the real dimen- sions of comets are found to vary greatly at different dates during their visibility. Contrary to what we might have expected, there is no doubt that many of these bodies contract as they approach the sun, and dilate on receding from that luminarv. We have the 20 THE CoMETs. strongest proofs of this in the case of Encke’s comet, which is one of short period, and has been repeatedly observed, as we shall notice more at length presently. This comet arrived at its least distance from the sun on the 10th of January, 1829, and the following were its actual dimensions on the various dates of observa- tion during its descent towards the sun :— 1828. Diameter in Miles. Distance from Sun. October 28 . . 312,000 . . 146 November’ a7) .9 : 257,000; \. <5 1:32 November 80 . . 119,000 . . O97 December 7 . . 79,000 . . 0°85 December 14 . . 46,000 . . 0-72 December 24 . . 14,000 . . O54 The observations of the same comet ten years later furnish another instance. It was at its least distance from the sun on the 19th of December, 1838, and found to have had the following diameters on different days preceding the arrival at perihelion. 1838. Diameter in Miles. Distance from Sun. October 9 AS.) 281/000). ~~ 1°42 October 25 .. 120,500°. . 1°19 November 6 . . 79,000 . . 1:00 November 13 . . 74,000 . . 0°88 November 16 . . 63,000 . . 0°83 November 20 . . 55,500 . . O76 November 23. . 88,500 . . 0°71 November 24 . . 30,000 . . 0:69 December 12 . . 6,600 . . 0°39 December 14 . . 5,400 . . 0:36 December 16 . . 4,250 . . 035 December 17 . . 3,000 . . 034 The nebulosity surrounding the nucleus of the great comet of 1807 expanded as the distance from the sun REAL DIMENSIONS. 21 increased, The measures of Schréter give the fol- lowing diameters, expressed as before in English miles. The comet was nearest to the sun on September 19th :— 1807. Diameter in Miles. Distance from Sun. October 20 . . 117,900 . . 0°92 October 21 . . 125,800 . . 0:93 October 22 . . 188,500 . . 0°94 October 23 . . 138,400 . . 0°96 October 25 . . 156,200 . . 0°99 October 31 . . 159,000 . . 1:08 November 3 . . 198,400 . . 1:12 Kepler first remarked this singular contraction and expansion of the nebulosity surrounding the nucleus of a comet as it approaches and recedes from the sun respectively, in the case of the fine comet which ap- peared in the year 1618. The tails of comets in some cases extend only a few hundred thousand miles from the nucleus, while in others they are projected to the astonishing distance of one hundred or one hundred and fifty millions of miles, or even more. The train of the first comet of 1847 was 5,000,000 miles in length; of the beautiful comet of 1744, 19,000,000; of the comet of 1769, about 40,000,000. The third of 1618 had a tail more than 50,000,000 miles in length, when it crossed the plane of the earth’s orbit about the 25th of November, and it was subsequently of greater extent. The great comets of 1680 and 1811. had trains considerably more than 100,000,000 miles long ; and the second of the latter year was accompanied by a tail 130,000,000 miles in length. Even these comets, however, were surpassed by the grand one which attracted so much 22 THE CoMETS. attention in 1843, and which exhibited a brilliant train that on different dates was found to attain the enormous distances of 150, 180, and 200 millions of miles from the head! If such a comet had been in the plane of the ecliptic, and close to the sun, the train would have extended far beyond the orbits of the Earth and Mars, terminating amongst those of the minor planets. Yet this wonderful appendage was formed in less than three weeks. It has frequently been remarked, that, if comets shine by reflected light, and there be anything like solidity in the nucleus, we ought at certain times to observe phases similar to those presented by the moon in the course of her revolution round the earth. There are some few instances on record where such appearances have been noticed. We cannot place much reliance on the cometary nature of the object mentioned in the Chinese Annals as having been observed in the northern heavens in October 684, which resembled a half moon, or of that recorded by the historians of the ninth century, at the beginning of the reign of Leo the Armenian in 813, said to have been like two moons joined together. Something very closely analogous to a phase was, however, ob- served in the head of the fine comet of 1744, and in another large one of the same century (1769), an observer expressly states that he had seen the nucleus first as a thin crescent, and subsequently as a half- moon, in the course of the comet’s descent towards the sun. The nucleus of the comet of 1819 pre- sented a very distinct crescent, according to Nicholas Cacciatore, of Palermo, Pd PHYSICAL CONSTITUTION. 23 Concerning the physical constitution of comets we have but a very imperfect knowledge at present. Sir John Herschel regards them as masses of thin vapour, capable of reflecting the solar rays from their internal as well as external parts—an inference which is ren- dered necessary, in order to account for all the pheno- mena revealed by telescopes. It is certain that stars of a very faint class have been repeatedly seen through comets of from fifty thousand to one hundred thou- sand miles in diameter, and, in the majority of cases, not the least perceptible diminution of the star’s brightness took place. There are one or two instances on record, where astronomers have been convinced of a sensible increase of brilliancy when a star has been viewed through the cometic vapour. In addition to a remarkable observation of this kind by Piazzi, at Palermo, during the appearance of the grand comet of 1811, we may mention a more recent one by Pro- fessor Reslhuber of Kremsmiinster, in reference to a star seen through the denser part of a comet disco- vered by M. Brorsen in March 1846, and which, under ordinary circumstances, belonged to the eighth class, or was just beyond unassisted vision. Professor Struve made some interesting observations during the transit of the comet of Biela over a small _ star, on November 6th, 1832. The brightness of the ‘star was not in the least diminished by the interven- tion of the comet, and its light suffered no perceptible 24 THE CoMETs. refraction, a point established by a continuous series of measures with the micrometer. That comets shine by a reflected light is a conclu- sion which few astronomers will dispute. It is evi- dent from telescopic observation of the degree of brilliancy at different distances, and in various posi- tions of a comet, with respect to the sun and eart , and M. Arago has established the fact from experi- ments with a polariscope, during the visibility of the famous comet of Halley in the autumn of 1835 Still the variation in the intensity of light is not um versally such, as should follow if the comet mere. reflected the sun’s rays, under certain permanen. conditions, and we are under the necessity of looking .to physical causes inherent in the body itself for an explanation of some few observations which appear irreconcilable with the theory of reflected solar light. The first comet of 1780 was closely examined by Dr, Olbers, the eminent astronomer of Bremen. He found it attained its greatest brightness on the 8th of November, thirteen days subsequent to its discovery, whereas, according to the law of reflected light, it should have become gradually fainter since the first observation ; and, supposing the comet self-luminous, the intensity of light should have increased each day until November 26, when the maximum would take place : yet, in the interval between the 8th and 26th of that month, it grew rapidly less. The comet dis- covered by Dr. Galle, of Berlin, on the 25th of Janu- ary, 1840, presented similar anomalies. Assuming that it had no light of its own, it should have appeared twice as bright on the 23rd of February as on the 21st PHYSICAL CONSTITUTION. o5 of March, yet at the latter date Professor Plantamour found the intensity of light had imereased in the pro- portion of more than two to one. Such variations are probably to be attributed to changes in the physi- al constitution of the comet, due to the action of the sun. 