rarely) presents a radiated appearance. Arago remarked that this nucleus is generally eccentrically placed in the head, lying towards the margin nearest the Sun. Eddie noticed that the nucleus of Fabry's comet of 1886 was of a ruddy brown colour. Both the size and the brilliancy of the object progressively increase; the coma, or cloud-like mass around the nucleus, becomes less regular; and a tail begins to form, which becomes fainter as it recedes from the body of the comet. This tail increases in length so as sometimes to spread across a large portion of the heavens; sometimes there are more tails than one, and occasionally the tail is much narrower in some parts than in others. The comet approaches the Sun in a curvilinear path, which frequently differs but little from a right line. It generally crosses that part of the heavens in which the Sun is situated so near the latter body as to be lost in its rays; but it emerges again on the other side, frequently with increased brilliancy and increased length of tail. The phenomena of disappearance are then not unlike those which marked the original appearance but in the reverse order. In magnitude and brightness comets exhibit great diversity: at rare intervals one appears which is so bright as to be visible in the daytime; but the majority are quite invisible to the naked eye and need more or less optical assistance. These latter are usually called telescopic comets. The appearance of the same comet at different periods of its return is so varying that we can never certainly identify a given comet with any other by any mere physical peculiarity of size or shape until its "elements" have been calculated and compared. It is now known that "the same comet may, at successive returns to our system, sometimes appear tailed, and sometimes without a tail, according to its position with respect to the Earth and the Sun; and there is reason to believe that comets in general, for some unknown cause, decrease in splendour in each successive revolution "." Fig. 180 represents the comparative diameters of the heads of 4 well-known comets as measured on particular occasions. The ↳ Month. Not., vol. xlvi. p. 456, June 1886. с Smyth, Cycle, vol. i. p. 235. woodcut is drawn to scale, but it must not be inferred that there is any permanence in the sizes here indicated. The periods of comets in their revolutions vary greatly, as also do the distances to which they recede from the Sun. Whilst the orbit of Encke's comet is contained within that of Jupiter, the orbit of Halley's extends beyond that of Neptune. Some comets indeed proceed to a much greater distance than this, whilst others are supposed to move in curves which do not, like the ellipse, return into themselves. In this case they never come back to the Sun. These orbits are either parabolic or hyperbolic. The density, and also the mass, of comets is exceedingly small, and their tails consist of matter of such extreme tenuity that even small stars are visible through them-a fact first recorded by Seneca. That the matter of comets is exceedingly rare is sufficiently proved by the fact that they have at times passed very near to some of the planets without disturbing in any appreciable degree the motions of the said planets. Thus the comet of 1770 (Lexell's) in its advance towards the Sun, became entangled amongst the satellites of Jupiter, and remained near them for 4 months, without in the least affecting them so far as we know. It can therefore be shown that this comet's mass could not have been so much as that of the Earth. The same comet also came very near to the Earth on July 1-its distance from it at 5h on that day being about 1,400,000 milesso that had its matter been equal in quantity to that of the Earth it would, by its attraction, have caused our globe to move in an orbit so much larger than it does at present that it would have increased the length of the year by 2h 47m, yet no sensible alteration took place. The comet of 837 remained for 4 days within 3,700,000 miles of the Earth without any untoward consequences. Very little argument, therefore, suffices to show the absurdity of the idea of any danger happening to our planet from the advent of any of these wandering strangers. Indeed, instead of comets exercising any influence on the motions of planets, there is the most conclusive evidence that the converse is the case that planets influence comets. This fact is strikingly exemplified in the history of the comet of 1770, just mentioned. At its appearance it was found to have an elliptical orbit, requiring for a complete revolution only 5 years; yet although this comet was a large and bright one, it had never been observed before, and has moreover never been seen since; the reason being that the influence of the planet Jupiter, in a short period, completely changed the character of its path. "Du Séjour has proved that a comet, whose mass is equal to that of the Earth, which would pass at a distance of 37,500 miles only, would extend the length of the year to 367a 16h 5m, and could alter the obliquity of the ecliptic to the extent of 2°. Notwithstanding its enormous mass and the smallness of its distance, such a body would then produce upon our globe only one kind of revolution,—that of the calendar d." Fig. 181 will illustrate, almost without the necessity of any written description, the influence of Jupiter on the group of periodical comets which have come within its reach. These comets, arranged in the order of their aphelion distances, are as follows: And it is probable that some other comets ought now to be added to this list; e.g., Finlay's (1886, vii.), Wolf's (1884, iii.), and Denning's (1881, v.). A comet may move in either an elliptic, parabolic, or hyperbolic orbit; but for reasons with which mathematical readers are acquainted, no comet can be periodical which does not follow an elliptic path. In consequence, however, of the comparative d Arago, Pop. Ast., vol. i. p. 642, Eng. ed. |