The comet's tail seems, in reality, to be a thin oblong case of vapour, formed out of the cometic substance by the increasing intensity of the sunshine, and enclosing the denser portion of that substance at one end. The diverging streams which it displays upon the sky are merely the retiring edges of the rounded case, where the greatest depth of luminous matter comes into sight. As the comet nears the sun, much of its substance is vaporised for the construction of this envelope; but as it goes off again into remoteness, the vaporous envelope is once more condensed. The tail may then be seen to flow back towards the head, out of which it was originally derived.
But here, again, a difficulty presents itself. The comet's tail is believed by most of the illustrious astronomers of the day, to be the body converted into vapour by solar influence. If it be so, the vaporising process must be a much more subtile one than any that could be performed in our alembics, for the comet's substance is already all vapour before the distillation commences. The faintest stars have been seen shining through the densest parts of comets without the slightest loss of light, although they would have been effectually concealed by a trifling mist extending a few feet from the earth's surface. Most comets appear to have bright centres—nuclei, as they are called; but these nuclei are not solid bodies, for as soon as they are viewed by powerful telescopes, they become as diffused and transparent as the fainter cometic substance. Comets are properly atmospheres without contained spheres; enormous clouds rushing along in space, and bathed with its sunshine, for they have no light excepting sunlight. They become brighter and brighter as they get deeper within the solar glare, and dimmer and paler as they float outwards from the same. The light of the comet only differs from the light of a cloud that is drifted across the cerulean sky of noon, in the fact, that it is reflected from the inside as well as the outside of the vaporous substance. The material illuminated reflects light, and is permeated by light, at once. In this respect it resembles air as much as cloud—the blueness of the sky is the sunlit air seen through the lower and inner strata of itself. In the same way, the whiteness of the comet is sunlit vapour seen through portions of itself. The sunbeams pass as readily through the entire thickness of the cometic substance as they do through our own highly permeable atmosphere.
The belief in the comet's surpassing thinness and lightness is not a mere speculative opinion. It rests upon incontrovertible proof. In 1770, Lexell's Comet passed within six times the moon's distance of the earth, and was considerably retarded in its motion by the terrestrial attraction. If its mass had been of equal amount with the earth's mass, its attraction would have influenced the earth's movement in a like degree in return, and the earth would have been so held back in its orbitual progress in consequence, that the year would have been lengthened to the extent of three hours. The year was not, however, lengthened on that occasion by so much as the least perceptible fraction of a second; hence it can be shewn, that the comet must have been composed of some substance many thousand times lighter than the terrestrial substance. Newton was of opinion, that a few ounces of matter would be sufficient for the construction of the largest comet's tail.
Light as the comet's substance is, it is not, however, light enough to escape the grasp of the sun's gravitating attraction. When the mass of thin vapour is rushing through the obscurity of starlit space, so far from the sun that the solar sphere looks but the brightest of the stellar host, it feels the influence of the solar mass, remote as it is, and is constrained to bend its course towards it. Onwards the thin vapour goes, the sun waxing bigger and bigger with each stage of approach, until at last the little star has become a fiery globe, filling up half the heavens with its vast proportions, and stretching from the horizon to the zenith of the visible concave. The great comet of 1680 came in this way from a region of space where the sun looked but half as wide as the planet Mars in the sky, and where the solar heat was imperceptible, the surrounding temperature being 612 degrees colder than freezing water, into another in which the sun filled up 140 times greater width of the sky than it does with us, and where the heat was some hundred times higher than the temperature of boiling water. It was then only 880,000 miles away from the solar surface, and would have fallen to it in three minutes, in obedience to its attraction, if the impetus of its motion in a different direction had been on the instant destroyed or arrested. But this impetus proved too great for the attraction, light as the material of the moving body was. When the comet has approached comparatively near to the grand source of attraction, the speed of its accelerating motion has become so excessive, that it is able to withstand the augmented solicitation it is subjected to, and move outwards in a more direct course. It goes, however, slower and slower, and curving its journey less and less, until at last its motion in remote obscurity is again so sluggish, that the sun's attraction is once more predominant, and able to recall the truant towards its realms of light. Such is the history of the comet's course.
Thin comet vapours drift through space, sustained by exactly the same influences that uphold dense planetary spheres. They are supported in the void by the combined effects of motion and attraction. Their own impetus strives to carry them one way, while the sun's attraction draws them another, and they are thus constrained to move along paths that are intermediate to the lines of the two impulses. Now, when bodies are driven in this way by two differently acting powers, they must travel along curved lines, if both the driving forces are in continued operation, for a new direction of motion is then impressed on them at each succeeding instant. There are three kinds of curved lines along which bodies thus doubly driven may move: the circular curve, which goes round a central point at an unvarying equal distance, and returns into itself; the elliptical curve, which returns into itself by a route that is drawn out considerably in one direction; and the hyperbolic curve, that never returns into itself at all, but has, on the other hand, a course which sets outwards each way for ever. The parabolic curve, as it is called, is a line partaking of the closeness of the ellipse on the one hand, and the openness of the hyperbola on the other. A parabola is an ellipse passing into a hyperbola; or, in other words, it is a part of an ellipse whose length, compared with its breadth, is too great to be estimated, and is consequently deemed to be endless for all practical purposes.
In most instances, comets move in space, about the sun, in ellipses so very lengthened, that their paths seem to be parabolas as long as the cloudy bodies are visible in the sky. Two of them, Ollier's Comet and Halley's, are known to return into sight after intervals of seventy-four and seventy-six years, during which they have visited portions of space a few hundred millions of miles further than the orbit of Neptune. Six comets travel in elliptical orbits that are never so far from the sun as the planet Neptune, and return into visibility in short periods that never exceed seven or eight years. These interior comets of short period seem to be regular members of our world-system in the strictest sense. Their paths, although more eccentric, are all contained in planes that nearly correspond with the planes of the planetary orbits, and they travel in these paths in the same general direction with their planetary brethren in every case. The planetoid comets of short period are—Encke's, De Vico's, Brorsen's, D'Arrest's, Biela's, and Fage's. The comet of 1843 is half suspected to belong to the group, and to be also a periodic body, revisiting our regions punctually at intervals of twenty-one years.
The comet's motions strikingly illustrate the almost absolute voidness of space. If the thin vapour experienced any resistance while moving, its free passage would be checked, although that resistance was many thousand times less than the one the hand feels when waved in the air. It is found, however, that Encke's Comet does indicate the presence of some such resistance. It goes slower and slower with each return, and contracts the dimensions of its elliptical journey progressively. But it must be remembered, that this is one of the close comets that never gets well out of the solar domain in which our neighbouring planets float. The resisting medium which opposes its journey may be merely an ethereal solar atmosphere surrounding the sun, as our air surrounds the earth, but spreading to distances of millions instead of tens of miles. On the other hand, it must be remembered also that starlight passes through universal space, and is everywhere spread out therein, and that it is hardly possible to think of starlight as an existence without some sort of material reality. Some physicists believe that Encke's Comet, with its retarded motions, will some day fall into the sun; while others fancy that such a consummation can never take place, because successive portions of its substance will be thrown off by the tail-forming process with each perihelion return; so that long before the cometic mass could reach the sun, it will have been altogether dissipated into space, and nothing will be left to accomplish the final state of the fall.
The great peculiarity of cometic paths, as compared with the planetary ones, is, that they consist of ellipses of very much more eccentric proportions; and that, therefore, the bodies moving in them, go alternately to much greater and less distances from the sun than the planets do. It must not be imagined, however, that all comets revolve about the sun even in the most lengthened ellipses. Three at least—the comets of 1723, 1771, and 1818—are known to have moved along hyperbolic paths instead of parabolic or elliptical ones. These comets, therefore, can make but one appearance in our skies. Having once shewn themselves there, and vanished, they are lost to us for ever. They are but stray and chance visitors to the domains of our sun, and refuse to submit themselves, with the more regular members of their fraternity, to the regulation-arrangements of our system, or to appear punctually at the systematic roll-call therein instituted. They are the true free-wanderers of the Infinite, passing from shore to shore of immensity, and presenting themselves, for short and uncertain intervals, to star after star. When they flit through our skies, they shew themselves in all possible positions, and move along all possible directions. They sometimes, however, yield too much to temptation, and have to suffer the penalty of a short imprisonment in consequence. Lexell's Comet, for instance, rushed in its hyperbolic path too near to Jupiter, and was caught in the attraction of its mass, and made to dance attendance on the sun through two successive elliptical revolutions. At the end of the second, the influence that had impounded the comet came, however, into play oppositely, and restored it again to its wandering life and hyperbolic courses. Its cloudy form has not presented itself amongst our stars since 1770, when its visit was thus strangely received by Jupiter.
Twenty-three comets were seen by the naked eye during the sixteenth century, 12 were seen in the seventeenth, 8 in the eighteenth, and 9 in the first half of the nineteenth. This does not, however, give anything like an adequate idea of the number of comets really in existence. When Kepler was asked how many comets he thought there were, he answered: 'As many as there are fishes in the sea.' And modern science seems determined, that the sagacious German shall not be at fault even in this predication. Two or three fresh telescopic comets are now usually found out every year. In 1847, 178 comets were known to be moving in parabolic orbits, and therefore to be in some way permanent connections of our world-system. Lalande has enumerated 700 comets, but Arago believes that not less than 7,000,000 exist, which fall at some time or other within the reach of our sun's influence.