Читать книгу: «Chambers's Edinburgh Journal, No. 450», страница 3

CLOUDS OF LIGHT

In March of the year 1843, a remarkable beam of light shot suddenly out from the evening twilight, trailing itself along the surface of the heavens, beneath the belt stars of Orion. That glimmering beam was the tail of a comet just whisked into our northern skies, as the rapid wanderer skirted their precincts in its journey towards the sun. To the watchful eyes of our latitudes, the unexpected visitant presented an aspect that was coy and modest in the extreme; its head, indeed, was scarcely ever satisfactorily in sight. But it dealt far otherwise with the more favoured climes of the south. At the Cape of Good Hope, it was seen distinctly in full daylight, and almost touching the solar disk; and at night appeared with the brilliancy of a first-class star, with a luminous band flowing out from it to a distance some hundred times longer than the moon's face is wide. Few persons who caught a glimpse of that shining tail, either as it fitfully revealed itself in our heavens, or as it steadily blazed upon the opposite hemisphere of the earth, were led to form adequate notions of the magnificence of the object they were contemplating. No one, unaided by the teaching of science, could have conceived that the streak of light, so readily compressed within the narrow limits of an eye-glance, stretched out 170 millions of miles in length.

The comet comes from regions of unknown remoteness, and rushes, with continually increasing speed, towards our own source of warmth and light—the genial sun. When it has reached within a certain distance of this object, it appears, however, to overshoot the mark of its desire, as if too ardent in the chase, and then sways round with fearful impetus, beginning reluctantly to settle out into space again, and moving with less and less velocity as it goes, until its misty form is once more withdrawn by distance from human sight. When the comet of 1813 swept round the sun in this way, it was so near to the shining surface of the solar orb, that it must have been rushing for the time through a temperature forty-seven thousand times higher than any which the torrid region of the earth ever feels. Such heat would have been twenty-four times more than enough to melt rock-crystal. The overburdened sense experiences a feeling of relief in the mere knowledge, that the comet passed this fiery ordeal as the lightning's flash might have done. In two short hours, it had shifted its place from one side to the other of the solar sphere. In sixty little minutes, it had moved from a region in which the heat was forty thousand times greater than the fiercest burning of the earth's torrid zone, into another, in which the temperature was four times less. The comet might well have a glowing tail as it came from such a realm of fire. Flames that were colder by many hundred times, would make the dull black iron shine with incandescent brightness.

As, however, it is the comet's nature to guard its ornamental appendages with jealous care, it may be conceived that this tail of 170 million miles might prove a somewhat troublesome travelling companion in so rapid a journey. Comets always turn their tails prudentially out of harm's way as they whisk through the neighbourhood of the solar blaze. In whatever direction these bodies may be moving, they are always seen to project their caudal beams directly from the sun. Imagine the case of a rigid straight stick, held by one end in the hand, and brandished round through a half-circle. The outer end of the stick would move through a considerable sweep. If the stick were 170 million miles long, the extent of the sweep would be not less than 500 million miles! Through such a stupendous curve did the comet of 1843 whirl its tail in two little hours as it rounded the solar orb. It is hardly possible to believe, that one and the same material substance could have been subjected to the force of such motion without being shattered into a myriad fragments. Sir John Herschel very beautifully suggests, that the comet's tail, during this wonderful perihelion passage, resembled a negative shadow cast beyond the comet, rather than a substantial body; a momentary impression made upon the luminiferous ether where the solar influence was in temporary obscuration. But this suggestion can only be received as an ingenious and expressive hint; it cannot be taken as an explanation. There is as much difficulty, as will be presently seen, in the way of admitting that comets have shadows of any kind, as there would be in compassing the idea that bodies of enormous length can be whirled round through millions of miles in the minute. The truth is, the comet's tail is yet an unguessed puzzle, and vexes even the wits of the wise. It keeps grave men seated on the horns of a dilemma, so long as their attention is fixed upon its capricious charms.

The comet's tail is always thrown out away from the sun, just as the shadow of an opaque body in the same position would be. But this is not all that can be said of it. It is not only cast away from the sun: it is really cast by the sun—shadow-like, although not of the nature of shadow. It only appears when the comet gets near to the sun's effulgence, and is lost altogether when that body gets far from the great source of mundane light and heat. It is raised from the comet's body, by the power of sunshine, as mist is from damp ground. When Halley's Comet of 1682 approached the fierce ordeal of its perihelion position, the exhalation of its tail was distinctly perceived. First, little jets of light streamed out towards the sun, as if bursting forth elastically under the influence of the scorching blaze; very soon these streams were stopped, and turned backwards by the impulse of some new force, and as they flowed in this fresh direction, became the diverging streaks of the tail. Not only a vapour-forming power, but also a vapour-drifting power, is brought into play in the process of tail formation; and this latter must be some occult agent of considerable interest in a scientific point of view, as well as of considerable importance in a dynamic one, for it is a principle evidently antagonistic to the great prevailing attribute of gravitation, so universally present in matter. The comet's tail is the only substance known that is repelled instead of being attracted by the sun.

The repulsive power to which the development of the comet's tail is due, is one of extraordinary energy. The comet of 1680 shot out its tail through something like 100 million miles in a couple of days. Most probably, much of the matter that is thus thrown off from the cometic nucleus is never collected again, but is dissipated into space, and lost for ever to the comet. The tail of the comet of 1680 was seen in its greatest brilliancy soon before the solar approach; this was, however, an exception to the general rule. Comets nearly always have the finest tails, and present altogether the most beautiful appearance, immediately after they have been in the closest proximity to the sun.

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.