SECTION I.

COMETS PHYSICALLY CONSIDERED.

The physical or chemical constitution of a celestial body; nature of the question involved; explained by reference to the earth-A cometary problem.

WHAT is meant by the physical or chemical constitution of a celestial body, or of any luminary whatever, whether star or sun, planet or moon; or, as we are treating of comets only, what is meant by the physical or chemical constitution of a comet?

We here have presented for our consideration a question the nature of which is easily explained and not less easily understood; but it is one that the best-informed of astronomers would find it difficult to answer in its full integrity.

By comparison with the bodies that we see on the surface of the earth and with the terrestrial globe itself, considered as a whole, we shall proceed to explain what is meant by the physico-chemical constitution of a comet.

The earth is a globe, more accurately, a spheroid, whose form and dimensions are perfectly defined and well known, at all events as far as concerns its solid crust, the atmosphere that surrounds it, and the rocks and strata near its surface. By direct observation we are acquainted with the solid crust to the depth of many hundred feet, and the atmosphere to a height of several miles. Induction has supplied us with a knowledge concerning atmospheric strata to which man has been unable

to ascend, and depths in the earth to which he has not yet penetrated. The mean density of the earth, its mass and weight, and the relation of its mass to that of the principal members of the solar system, are known.

What are comets from these various points of view? Are they globes similar to our earth, illuminated like it by the sun, or do they shine by their own light? Have they a solid or liquid nucleus, surrounded by a vaporous atmosphere, or are they gaseous masses, collections of particles more or less condensed? Has any certain estimate been formed of their masses, or the density of the matter of which they are composed? As regards their movements we know that they do not differ from other members of the celestial group of which we form a part, and that the same universal force, the same laws govern them. Coming probably from the depths of space, of distinct origin therefore, and of very different aspect to the planets and their satellites, we may not apply to both the lines of Ovid:

-Facies non omnibus una

Nec diversa tamen qualem decet esse sororum.

Comets are, from all these points of view, their movements alone excepted, conspicuously different from the earth and the rest of the planets. In physical constitution they appear to be quite dissimilar-chemically speaking, are they equally unlike? That is to say, is the matter of which they are composed formed of unknown elements, or of elements identical with those of which the planets themselves are constituted?

All these questions possess a high degree of scientific interest. Nor are they less important if we view them in their relation to the superstitious beliefs which for so long a time made comets formidable to the world-beliefs which, having changed in form perhaps more than in substance, are still to a certain extent current even in our enlightened century. Although not susceptible of proof, the habitability of the planets is a thesis

that has long been maintained and is still maintained with very considerable probability. More than this, in the last century it was supposed, and some savants even of our time believe, that comets have likewise their inhabitants. Are comets indeed habitable? We are urged by an instinct of invincible curiosity to put such questions to ourselves; and if it appears next to impossible to return positive replies, at least we are not forbidden to examine the probability of each. But, if we would not abandon ourselves to vain and profitless conjectures, it is clear that we must, in the first place, acquaint ourselves with what science has to communicate, not respecting this problem, which may be considered as extra-scientific, but upon the physical and chemical conditions which observation and experiment show to be compatible with the existence of human beings, as far as they are known to us.

We shall, therefore, examine what is known of the constitution of comets at the present day; and we shall begin with the study of their aspect and external form.

SECTION II.

COMETARY NUCLEI, TAILS, AND COMÆ.

Comæ and tails-Classification of the ancients according to apparent external form; the twelve kinds of comets described by Pliny-The 'Guest-star' of the Chinese-Modern definitions: nucleus, nebulosity or atmosphere; tails.

WHAT is the distinctive sign of a comet by which it is universally known, by which it is distinguished from all other celestial bodies? Everyone answers at once, it is the train of luminous vapour, the nebulosity of more or less length, which accompanies it or at least surrounds it; in other words, the tail and the coma

This is what the etymology implies, the word comet signifying long-haired or hairy. Armed with its tail, which appears brandished in the heavens like an uplifted sword or a flaming torch, the precursor of some untoward event, a comet is everywhere recognised on the instant of its appearance; it needs no passport signed by astronomers to prove its identity. But should the tail be absent, should no appendage or surrounding nebulosity distinguish the celestial visitor on its apparition, for the world at large it is no comet, but simply an ordinary star like any other.

Nevertheless, there are tailless comets. The comet of 1585 was equal to Jupiter in size, but less brilliant; its light was dull. It had neither beard nor tail, and it might have been compared to the nebula in Cancer (Pingré). Lalande

observes that the comets of 1665 II. and 1082

exhibited discs as round, clear, and well defined as that of Jupiter himself, without tail, beard, or coma.' We are here speaking of comets visible only to the naked eye; of telescopic comets a great number are destitute of tail, and it very often happens that they are simple nebulosities, in the midst of which a faint nucleus is but just discernible, sometimes nothing but a luminous condensation at the centre. Moreover, from the presence or absence of a tail at one time of the apparition, we cannot infer that the same is true at another. Thus, the above-mentioned comet of 1682 (no other than Halley's comet), which Cassini observed to be without tail on August 26, had developed one of 30° in length by the 29th of the same month. And as regards the comet of 1585, twelve days after its apparition, a slender and hardly perceptible ray was seen to issue from it, a hand's breadth or more in length.' It likewise often happens that the tail which has been invisible to the naked eye is readily perceived in the telescope; instances of this we shall meet with as we proceed. All that we have here to bear in mind is, that the distinctive sign of a comet, astronomically speaking, is not to be sought in the tail, the coma, or in any of the variable appendages which may surround the star during its apparition. The elements of its orbit, its large eccentricity, great inclination, direction (oftentimes retrograde), &c., constitute the true points of difference between a comet and the planets. We have already called attention to these differences, and need, therefore, only allude to them here.

It is clear that up to the sixteenth century, before the employment of the telescope in astronomical observations, the accounts given of cometary apparitions can refer only to comets seen by the naked eye. The strange forms of their tails, their beards and comæ, attracted the attention alike of the multitude and the learned. The ancients, who have not always clearly