SECTION IV.

DIRECTION OF THE TAILS OF COMETS.

Direction of the tail opposite to the sun; discovered by Apian; the Chinese astronomers were acquainted with this law-Deviations in some comets-Variable aspect of the tail according to the relative positions of the comet, the earth, and the sun.

IN respect to the direction of cometary tails let us call attention to an important point-to a general phenomenon which was remarked by the ancients in the very earliest times. Seneca refers to it in the following line:

Comæ radios solis effugiunt.

The coma of comets fly the rays of the sun. According to Edward Biot the Chinese astronomers had observed, since the year 837, this constant direction of cometary tails from the sun. 'In Europe,' says Lalande, Apian was the first to perceive that the tails of comets were always opposite to the sun; this rule was afterwards confirmed by Gemma Frisius, Cornelius Gemma, Fracastoro. and Cardan. Nevertheless, Tycho Brahé did not believe it to be very general or well demonstrated; but the fact itself is beyond a doubt.'

Pingré observes with truth that the direction of the tail is not always strictly opposite to the sun. He instances the comet of 1577, whose tail was deflected as much as 21° towards the south, and the great comet of 1680, when the

a

deflection was about 45°. On both these occasions, however, the comet and the earth occupied the same relative positions in the heavens. The deviation is less in proportion as the tail is more inclined to the orbit; viz., considering only the portion of the tail in the neighbourhood of the nucleus, the deviation is less in proportion as the comet draws near to its perihelion ; and it takes place towards the region of the heavens last quitted by the comet in its course.

It results, therefore, from this law that the tail of a comet sometimes follows and sometimes precedes that body in its course. It follows the comet before the perihelion

passage, and precedes it when the perihelion has been passed. Moreover, tails have very frequently a more or less considerable degree of curvature, and this curvature appears more marked in proportion as the earth is removed from the orbit of the comet. If the earth be situated in the plane of the comet's orbit, the curvature is nil,* and the tail is rectilinear, or at

The two tails of the great comet of 1861 at first appeared to offer an exception to this law. On the 30th of June, on which day the earth passed through the plane of the comet's orbit, the two tails, projected the one upon. the other, appeared to form but one, wider in the first third of its length, reckoning from the nucleus; but both were rectilinear. But M. Valz and Mr. Bond, from observations made by the former and by Father Secchi, discovered, as they believed, that the two tails presented a slight deviation to the east of the plane of

least appears so; this is no doubt an effect of perspective, and the curvature then takes place in the plane of the orbit. It is more marked in those portions of the tail that are furthest from the nucleus; from which it follows that if we draw radii vectores from the sun to the several positions of the comet, the tail always presents its convex side to these lines, as may be seen in fig. 24.

There is yet another conclusion to be drawn from these facts, which is, that if the earth occupies such a position with reference to the comet and the sun that the comet is in opposition to the latter, its tail, being likewise opposite to the sun, is situated behind the nucleus, and is consequently invisible. It is then only the breadth of the tail that is seen, and it appears to surround the nucleus as a coma, thus increasing only the extent of the cometary atmosphere. This fact may serve to explain the absence of tails in some comets, which, from their nearness to the earth, we should have expected to have been so provided.

the orbit. This would render still more difficult the theory of the formation of tails. But, if we adopt the conclusions of M. Faye in this respect, the deviation existed in appearance only, and this difficulty would be removed. This is a point well deserving the attentive consideration of all future observers of cometary phenomena.

Comets with double tails, one of which is opposite to the sun and the other directed towards that luminary, appear likewise to follow the law stated above. Olbers says of the comet of 1823: 'On the 23rd of January the earth passed through the orbit of the comet; on this day not the least deviation could be discerned between the direction of the abnormal tail and the prolonged axis of the other tail.' 'Thus,' says M. Faye, in citing this passage, 'the two tails of the comet were projected, each on the prolongation of the other, which shows that tails directed towards the sun have, like the others, their axes situated in the plane of their orbit.'

SECTION V.

NUMBER OF TAILS.

Double tails of comets; comets of 1823, 1850, and 1851-Tails multiple, fan-shaped, rectilinear, curved-Variable number of tails belonging to the same comet; comets of Donati, of 1861 and of Chéseaux.

GENERALLY a comet has but one tail, which varies considerably in form or size, or, at all events, appears to do so. Sometimes these changes take place very rapidly, but still, as a rule, the tail consists of one luminous train. Nevertheless, examples may be adduced of double and even multiple tails. The comets of 1807 and 1843 were furnished with double tails, or, what comes to the same thing, single tails formed of two branches of very unequal length. It was the same with the comet of 1823, about which Arago gives the following details:

'On the 23rd of January, 1824, the comet, in addition to its ordinary tail opposite to the sun, had another which was directed towards the sun, so that it resembled somewhat the great nebula of Andromeda. The first tail appeared to include a space of about 5°, but the length of the second was scarcely 4°. Their axes formed between them a very obtuse angle of nearly 180° (fig. 25). In the close vicinity of the comet the new tail was hardly to be seen. Its maximum brightness occurred at a distance of 2° from the nucleus. During the

first few days in February the tail opposite to the sun was alone visible; the other had disappeared, or had become so faint that the best telescopes in the clearest weather failed to show any trace of it.'

The comets of 1850, I., and 1851, IV. (figs. 26 and 27), exhibited the same phenomenon of two unequal tails, the shorter of which was directed towards the sun.

The observations and the drawings of Messier show that the great comet of 1769 had, if not a multiple tail, at least

lateral jets of light, resembling secondary tails, proceeding from the nucleus, but much smaller and less extended than the principal tail, and making unequal angles with the latter: all the tails were rectilinear.

Donati's comet exhibited, in 1858, a similar peculiarity. In addition to the principal tail, remarkable for its extent, its curvature, and brilliancy, there appeared first one and then two luminous trains, much fainter, apparently rectilinear, and nearly tangential to the limiting curves of the great tail. The figures of Plates VII. and IX. give a