revolution is only about six years and three quarters.* These two bodies experience greater changes of temperature than the planets, though they are never subject to the same extremes of heat and cold as the other comets. The light and heat which Halley's comet receives when nearest to the sun, is 3600 times greater than it experiences when at the farther end of its orbit. The great comet which appeared in the year 1680, according to the computation of Newton came 166 times nearer to the sun than the earth is, and must have experienced a degree of heat twenty-seven million five hundred and fifty-six thousand times greater than we do in summer -a heat far beyond any that can be produced artificially, and sufficient to convert into vapour the greater number of terrestrial substances. At the other end of its orbit, the sun could have little more influence on the comet than the fixed stars have on the earth it must, therefore, have been exposed to the temperature of space, which is 90° below the freezing point of Fahrenheit's thermometer. These enormous variations of temperature, which cause part of the substance of comets to change successively from a solid state, probably of congelation, to the liquid and gaseous form, show that they are of an organisation totally unlike that of any of the other celestial bodies with which we are acquainted, and that they are probably destined to fulfil very different parts in the economy of the universe. However, circumstances might be mentioned, which may possibly temper these violent alternations.

The planets are compact solid bodies, with sharply defined discs, which, as far as we know, never vary in size; whereas all the comets that have come to our system within the period of astronomical observation seem to be loose aggregates of gaseous or nebulous matter, more or less condensed towards the centre,' and subject to remarkable changes of magnitude-forcing upon us the inference of a necessary connexion between an attenuated texture and a remarkable elongation of orbit. What fixed relation exists between the extent of the orbits and the physical structure of the comets revolving in them remains to be seen; but there certainly is a gradation in comets, from such as consist of a mass of highly attenuated vapour, to those which actually have or indicate some approach to a solid form.

Vast numbers of comets are only visible with telescopes, and many of these are like round cloudy spots or films of vapour, frequently without the smallest appearance of tails. Those which came into view in the years 1795, 1797, 1798, and the little comet of 1804, the diameter of whose head was about 5000 miles, were of this nature, being merely globular masses of vapour somewhat condensed towards the centre. In general, however, a comet

* This comet was discovered by M. Biela, an officer in the Austrian service.

resembles

resembles a planet surrounded by a luminous atmosphere, which is drawn out into a tail on the side opposite to the sun.

The ball of vapour which forms the head of a comet is sometimes of uniform brightness, and is occasionally so transparent, that the smallest stars may be seen through it; this fact was observed in the days of Seneca, who takes notice of it as a distinction between a star and a comet. He says, Nunquam apparet stella per stellam: acies nostra non potest per medium sidus exire, ut per illud editiora prospiciat. Per cometen autem non aliter quam per nubem ulteriora cernuntur, ex quo apparet, illum non esse sidus sed lenem ignem et tumultuarium.' Olbers perceived a very small star through the head of the comet of 1796, and he saw a smaller star shining with undiminished lustre through the very centre of that of 1802. Stars were likewise seen through the heads of the comets of 1774 and 1825, by MM. Montaigne and Valz. Encke's comet passed also centrally over a small double star in 1795, and in its revolution in 1828 it passed over several minute stars without perceptibly diminishing their light. Sir John Herschel saw a whole cluster of very small stars almost through the centre of Biela's comet. They would have been entirely hid by the slightest fog; and yet their light must have passed through 50,000 miles of the cometic matter. No effects of the refraction of light have ever been noticed, consequently that matter must be infinitely less dense than our atmosphere. This shows how much the substance of these comets must be dilated, though it cannot be concluded that they have no solid part, since it may have been beyond the line of visionand yet we can scarcely suppose that to have happened in all the instances recorded. The head, however, is frequently more condensed, and sometimes sufficiently opaque to eclipse the stars it passes over, and the brilliancy gradually increases towards the centre, as if the comet were formed of layers of increasing density.

An extremely small brilliant point, which is called the nucleus, has sometimes been seen with the naked eye in the centre of the head, supposed to have been the solid part or actual body of the comet. It has generally, however, been so minute and ill-defined in its edges, that little reliance can be placed on the calculations made of its magnitude. From various measurements, Sir William Herschel computed the diameter of the apparently solid part of the comet of 1807 to have been 538 miles, while Schröter made it 997 miles. These two great astronomers differed still more with regard to the comet of 1811: one estimated the small brilliant speck in the head to have a diameter of 2637 miles, while the other made it only 570. It may, therefore, be concluded, that what seemed to be. the solid part of that comet had no fixed demarcation. This point,

so brilliant to the naked eye, has sometimes appeared to be merely an indeterminate increase of light, when viewed with a telescope, and it is said, in some instances, to have required excellent telescopes to bring it into view. We know by experience, that bodies attract one another more or less powerfully, exactly in proportion to their density—that is, to the number of material particles they contain: now, comets are powerfully disturbed in their motions round the sun by the attraction of the earth and planets, but they never have had the smallest effect in disturbing the motions of any of the bodies of the solar system, though they have passed so near the planets that they must have altered their course and velocity very materially if their mass had been of any magnitude.

The comet of 1770 was very brilliant, and the diameter of its head was supposed to be thirteen times as great as that of the moon; yet it passed twice through the system of Jupiter's satellites, without producing the smallest disturbance in their motions, though it remained four months each time within the sphere of Jupiter's attraction. In July, 1770, this comet passed about six times the distance of the moon from the earth, without affecting our tides; La Place computed that, if its mass had been equal to that of the earth, it ought to have lengthened our sidereal year by two hours and forty-seven minutes, but as no change has taken place in the length of the year, he concluded that its mass could not have amounted to the five thousandth part of that of the earth. Moreover, the moon always turns nearly the same face to us, because the time of her revolution about her axis is exactly equal to the time of her revolution round the earth. The smallest attraction of a foreign body would destroy this equality; and as comets have come very near the moon, it may be concluded that their masses must have been too small to destroy this delicate equilibrium.

There is always an envelope of light, consisting of a nebulous or gaseous matter, like a luminous haze or atmosphere of very great tenuity, sometimes in connexion with the nucleus, but oftener separated from it by an obscure space, beyond which there is a succession of alternate bright and dark rings, caused by a series of envelopes of vapour, like the coats of an onion of greater or less density, alternately reflecting more or less of the sun's rays. There are often three or four of these bright envelopes suspended round the nucleus, varying in distance as well as in thickness; but they are frequently incomplete on the side opposite to the sun.

The heat comets are exposed to in their passage round the sun changes the substances at their surface into vapour, and is, in all probability, the cause of their tails, and of the nebulous envelopes surrounding their nuclei-a probability borne out by the circum

stance

stance of their tails never attaining their greatest length till after the comet has emerged from the sun's rays in its passage round him. Sir W. Herschel says, with regard to the first comet of 1811

In every instrument in which I have examined the comet, I perceived a very faint or rather darkish interval surrounding the head, wherein the gradually diminishing light of the central brightness was lost. This can only be accounted for by admitting a transparent elastic atmosphere to envelope the head. Its transparency I had an opportunity of examining on the 18th September, when I saw three very small stars of different magnitudes within the compass of it: and its elasticity may be inferred from the circular form under which it was always seen; for being surrounded by a certain bright equidistant envelope, we can only account for the equality of the distance, by admitting the interval between the envelope and the head to be filled with an elastic atmospheric fluid.'-Phil. Trans. 1812.

Sir William's observations led to the following theory of the formation of the luminous coatings already described. When a comet comes within a certain distance from the sun, a quantity of vapour rises from its surface and remains suspended in its atmosphere; as the comet approaches still nearer to the sun, this first envelope becomes more rarefied, and ascends higher in the atmosphere, where it remains suspended, while another layer or envelope rises at the surface, and being more dense than the first, maintains a lower station. In this manner a succession of these coatings is formed, amounting occasionally to three or four; the internal one being always more dense than that immediately above it, and consequently capable of reflecting more light-corresponding exactly with observation for the comet of 1744, which approached to within a fifth part of the earth's distance from the sun, had a double envelope three weeks before it arrived at its shortest distance from him, and a week after it had passed that point it had acquired another, and these three layers were brighter in proportion as they were near to the centre of the comet. The envelopes are always brighter on the side next to the sun, because of his direct light, and also because we look through a greater depth of matter at that part than elsewhere. During the time the sun is volatilizing the matter of the comet, the light of the nucleus is often obscured by the haze which this occasions, but as soon as the envelope is completed it shines forth with its wonted lustre. Sometimes, as the comets retreat from the sun, these coats seem to be more or less condensed, and fall down in vapour on the surface of the nucleus. Indeed, during their whole course round the sun, the aspect of comets is in a state of perpetual change. The size of the coatings shows their atmosphere to be very extensive. The nebulous matter surrounding the

head

head of the comet of 1799 was about twenty-one thousand miles thick, but it was far surpassed by the luminous coatings of the comets of 1807 and 1811, which were-one thirty thousand and the other twenty-five thousand miles-more than three times the diameter of the earth. When a comet has a tail, these luminous coatings completely surround the head on the side next to the sun like a hemispherical cap, but they diverge all round on the opposite side, and constitute the tail, which, consequently, has the singular property of being hollow; and as the line of vision passes through a greater depth of nebulous matter at the sides of the tail than in its centre, they reflect a greater quantity of light, which makes the lateral edges of the tail look like two luminous streams, uniting at a short distance from the head, and leaving a dark shade between them which does not extend through the whole length of the tail. The extremity of the tail is generally ten or twelve times broader than the head, which gives the whole the figure of a cone or sugar-loaf. Nothing can give a stronger proof of the extreme tenuity of the tails, than that stars have been seen through them, though they are often many millions of leagues thick.

When a comet first appears, it shines with a very faint light and has little or no tail; its brilliancy increases, and the tail becomes longer as it approaches the sun, and both acquire their utmost splendour during its passage round him. By the time the comet emerges from the sun's rays, the tail has attained its greatest length; it then gradually diminishes, and the light of the comet grows feeble in proportion as its distance from the sun increases, till at last it becomes too dim to be visible. As the tail is almost always in a direction opposite to the sun, it follows the comet in approaching him, and precedes it in its return.

Comets which come nearest to the sun generally have the longest tails; there are no doubt some exceptions: for example, the comet of 1811, which had so splendid a tail, never came so near the sun as the earth is: but it cannot be supposed that bodies which differ so much in external character should have the same conformation, and therefore they must yield more or less to the evaporating force of the sun's heat, according to their different densities. It is, probably, for this reason that comets vary so much in the length of their tails; many have no tail, as that of 1804; the second comet of 1811 had a short and faint one, while the first comet of that year had a tail forty-four millions of leagues long, and those which came to our system in the years 1618, 1680, and 1769, had tails which extended over 104, 90 and 97 degrees of space, so that when the heads of these comets were set, the extremities of their tails were still over head in the zenith. They sometimes consist of several branches, separated by dark intervals: