1828, 1832, and at regular intervals up to the present time this little filmy object has returned to its perihelion, or nearest distance to the Sun. At these times it has always been eagerly sought for, and its orbit has been computed, so that its motion in space is now known with nearly the same accuracy as that of an ordinary minor planet. There are several other comets of short period whose orbits are tolerably well known, as those discovered originally by MM. Faye, De Vico, Brorsen, Winnecke, and Biela, all of which are faint telescopic objects. That of Biela is remarkable for having divided itself into two comets, since which it appears to have been entirely dissolved. This comet had faithfully returned, according to prediction, on several occasions, but in 1865, when its reappearance was expected, no trace of it could be seen even in the most powerful telescopes. Halley's comet, whose period is nearly seventy-seven years, has been so well observed, both in 1758-9 and 1835-6, that its return in 1912 may be looked upon as a matter of certainty.

Although the periodical comets are so peculiarly interesting to the astronomical observer, yet it cannot be denied that the popular interest rests entirely in favour of that class which has a more sensational appearance. The astronomer also takes this opportunity to observe the varying forms of the nucleus, and to determine, if possible, the nature of its physical composition. We have remarked in a preceding chapter that the spectroscope has enabled Mr. Huggins to observe that the spectrum of a small comet, discovered in 1868, is coincident with that produced by the combustion of carbon. How such a material should compose the head of a comet is difficult of explanation, and must remain for the present a matter for future research. On the next appearance of a brilliant comet, it is expected that the analysis of its light by means of the spectroscope will add considerably to our present imperfect knowledge of the constitution of these bodies, which have thus been proved to shine by their own light, and not by the reflected light of the Sun.

The most remarkable comets which have appeared during the present century are those of 1811, 1843, 1858, and 1861. That of 1811 was situated in a very favourable position for observation, having been always above the horizon of Great Britain during several months. It was first detected by M. Flauguergues on the 26th of March, 1811, in the south of France, and it remained visible until August, 1812. It was at its greatest brilliancy about October, 1811. At this time the length of the train of nebulous light was upwards of one hundred millions of miles, and its breadth about fifteen millions. It was remarked by M. Burckhardt that the tail was not immediately connected with the comet, but that it formed, at some little distance from the nucleus, a wide belt, which girded the nucleus in the same manner as the rings of Saturn. The colour of the head appeared to be greenish, or bluish green, giving rather a curious effect. Sir William Herschel noticed that there seemed to be an accumulation of matter towards the Sun in the head of the comet, the diameter of which was about one hundred and twenty-seven thousand miles. By viewing the comet with a nightglass of low power, Sir W. Herschel found the tail was generally inclosed at the sides

COMETS.

by two condensed streams of light. Being very near the Milky Way, a comparison of the comet with a portion of that luminous stratum where no stars can be seen by the naked eye, showed that the two were of equal relative brightness.

The comet of 1811 was supposed to have had a considerable influence on the weather of that year, and upon the bountiful harvest and vintage, which appear to have been almost universal. Even now we often hear a reference made to the celebrated comet year, and wines have been sold at unusually high prices on account of some favourable influence produced by this popular comet. Other comets of later years have, however, been reputed to have had similar influences on the weather and the fruitfulness of agricultural produce. It is not always agreeable to express any doubt of the truth of some of the most popular beliefs in the supposed influences of the heavenly bodies on the atmosphere of the Earth. But it is nevertheless a fact, determined from several long series of observations, that neither the Moon, planets, stars, nor comets, give out sufficient heat to affect the most delicate mercurial thermometer yet constructed, and that any supposed influence which they may individually have upon the weather is only a popular fallacy resting on no real foundation. It is a well-known meteorological fact that the summer of 1811 was more than usually hot; but this coincidence with the appearance of a great comet can only be assumed to be accidental. The following lines by Hogg, the Ettrick Shepherd, refer to the appearance of the magnificent comet of 1811:

"Stranger of heaven, I bid thee hail;
Shred from the pall of glory riven,
That flashest in celestial gale,

Broad pennon of the King of heaven.

"Whate'er portends thy front of fire,

And streaming locks so lovely pale;
Or peace to man, or judgments dire,
Stranger of heaven, I bid thee hail."

Those who witnessed the great comet of 1843 generally agree that it was the finest seen in the present century. Its full brilliancy was not visible in northern latitudes, where its luminous train first attracted attention while the nucleus was below the horizon. The tail was at that time, March 17th, about forty degrees in length, stretching from the horizon over the south-western sky as far as the constellation Lepus. Great excitement was caused by the sudden apparition of this long train of nebulous light, which at first was taken by some observers as belonging to the zodiacal light, which is generally most visible in the west soon after sunset at this time of the year. Throughout the southern hemisphere this comet presented a splendid appearance in the first days of March. Although the nucleus was not of great magnitude, yet it was extremely bright, and distinctly coloured; according to some observers, of a golden hue, similar to that of Venus; and according to others it was tinged with red. The disk had a well-defined planetary appearance, and was estimated to be about 4,500 miles in diameter, or rather more than half the size of

the Earth. Sir Thomas Maclear, Government Astronomer at the Cape of Good Hope, observes that "of the casual observatory phenomena, the grand comet of March takes precedence; and few of its kind have been so splendid and imposing. I remember that of 1811; it was not half so brilliant as the late one. Immersed in the ravines of the Cedarberg, with high and precipitous ranges on each side of me, I made strenuous efforts to reach the Snewberg station, to command a view of the sudden visitor. Those unacquainted with the character of the Cedarberg cannot form a conception of the difficulties I had to encounter. For seventeen days we toiled on, tantalised every evening by seeing a portion of the tail over the mountain tops, and sometimes a sight of its bright head, as openings in the mountains permitted." The intrinsic lustre of the nucleus was so great, that the comet was seen in full daylight, when at a short distance from the Sun, by several observers in different parts of the world. Mr. Hind remarks that, "according to the most trustworthy calculations, the perihelion distance was only 538,500 miles; therefore the centre of the comet would be distant from the surface of the Sun's globe less than 96,000 miles!"

The great comet of 1858, known as Donati's comet, was visible to the naked eye in northern latitudes during at least six weeks, and, as a telescopic object, much longer. It was first seen by M. Donati, of Florence, on the 2nd of June, when it was very faint. For some time it gave no indication of becoming the splendid object which it presented from the middle of September till the second week in October. On the 23rd of September, the Moon being nearly full, the head of the comet appeared about equal to a star of the first magnitude, and was clearly visible half an hour after sunset. The greatest length of the train was observed on October 10th, when the main stream of light could be distinguished through an arc of sixty degrees, equal to about fifty millions of miles, the breadth of the widest part being about ten millions. Although the comet of 1858 has been surpassed by others in magnitude, yet the intensity of its nucleus will bear comparison with any previously observed comet. The nebulosity surrounding the head of the comet of 1811 was considerably larger; but including all the attractive features exhibited by Donati's comet, the popular interest evinced by the appearance of the two appears to have been about equal. It was fortunate that during the time when the comet of 1858 was near the star Arcturus, which was enveloped in the tail on October 5th, the Moon was absent from our evening skies. The effect of moonlight would have been to cut off at least two-thirds of the train, and a large proportion of the brightness of the remaining third. The tail was subject to constant changes of form. It was sensibly curved, and it had the appearance of an ostrich feather being gently carried through the air by the hand. At one time the train was bifurcated, issuing from the head in two unequal streams, forming its two sides, a dark space being left between them behind the nucleus. This dark space is delineated in most of the later telescopic drawings of the comet. On the 5th of October, when at its greatest brilliancy, the extremity of the train reached the two southernmost stars in the tail of Ursa Major, the head being

COMETS.

in Boötes a few degrees south of Arcturus. Between the 16th and 20th of October, the comet ceased to be observed in Europe and North America. It was first seen in the southern hemisphere in the second week of October, and telescopic observations were continued at the Cape of Good Hope till the 4th of March, 1859.

The telescopic appearance of Donati's comet was remarkable in consequence of the continual changes which the nucleus and envelope were undergoing. As seen in one of the telescopes at the Royal Observatory on October 2nd, at 7 p.m., the nucleus presented a bright stellar point surrounded by an annulus of light, about four times its diameter, considerably fainter than the nucleus, but brighter than the general envelope, and about two-thirds complete. The Astronomer Royal divided the head into the following parts:-"1st. The parabolic envelope and inclosed illumination. 2nd. A brighter flat circular disk laid upon the inclosed illumination, just touching the parabola at its vertex. 3rd. A still brighter, flat, circular disk, concentric with the last, about one-fourth of its diameter; no bright ring. 4th. The nucleus, concentric with the last, about one-third of its diameter, well-defined, and looking very hard. From the nucleus a dark shadow diverged, cutting off the light of the circular disks, and everything except the nucleus itself." On October 3rd, several of these appearances had considerably changed, the matter composing the nucleus and envelope being evidently in a continual state of local excitement.

The orbit of this comet has been most accurately computed from the numerous observations made in Europe and North America. It seems to show that this is a periodical comet, but how far it goes into space in the interval during its absence from us no one can tell. It is, however, expected that in two thousand years hence, if nothing unforeseen happens to it, it will return once more to its perihelion passage around the Sun, when it will again adorn our skies. Meanwhile, many years will probably elapse before the remembrance of this beautiful celestial visitor will be erased from the memories of all those of the present generation who were so fortunate as to witness it.

The comet of 1861, although less attractive than the three just described, was a very interesting object. It was discovered in Europe low down in the north on the evening of June 30th, by several people. M. Secchi first took the immense train of light for the smoke of fireworks, which they often have at Rome on the festival of St. Peter and St. Paul; but he soon perceived that the light continued, when he judged that it must be a comet, although the head was too near the horizon to be well observed. About midnight on the same day it was seen at the Royal Observatory, Greenwich. The telescopic appearance was generally observed to be subject to similar changes to those noticed in former comets; the nucleus was, however, somewhat elliptical, its light nearly equalling that of Saturn. It was well defined on all sides, but on the side nearest the Sun it was less sharp, jets of reddish light starting from its edge in the form of a fan. It is a remarkable fact that on the morning of June 29th, the Earth was very near the extremity of the tail; and Mr. Hind has

suggested that a peculiar illumination of the sky which he noticed about this time was possibly "attributable to the commingling of the matter forming the tail of the comet with the Earth's atmosphere."

Although the great comet of 1861 was not visible in Europe till the evening of the last day in June, yet a young amateur astronomer, Mr. Tebbutt, of Windsor, New South Wales, saw it so early as May 13th. The telescopic appearance of this comet was remarkable for the numerous jets of light thrown out from the nucleus. An example of this is exhibited in the woodcut on page 281. These jets of light were very distinct, and often varied in form in short intervals of time, even while the eye of the observer was directed to them. The Rev. T. W. Webb, referring to the different envelopes which were exterior to the nucleus and jets of light, remarks: -“The number and complex arrangement of these luminous veils on June 30th, as seen in the comet eye-piece, with a field of about 52′ and a power of 27, produced a singular as well as magnificent effect; it was as though a number of light hazy clouds were floating around a miniature full Moon. Portions of six could be more or less distinctly traced. The innermost of these was very narrow and short, faint on the left hand, but brighter on the right, in the inverted view, and coming up close to the nucleus. The second was a parabolic arc in which the nucleus stood." On the next night of observation, July 2nd, the envelopes had considerably changed, but they still retained their general form.

From a comparison of some drawings of the comets of 1858 and 1861 made at the Royal Observatory, the comet of 1858 is the more attractive of the two with regard to their general appearance; but that of 1861 is the more interesting as a telescopic object, so far as relates to the continual and fantastic changes in the form of the nucleus and the numerous jets emanating from it.