twice as near, exposed to a heat of proportionate greater intensity. Sir John Herschel computed that the heat received by its surface during the passage of the perihelion was equal to that which would be received by an equal portion of the earth's surface, if it were exposed to the influence of 47,000 suns, placed at the common distance of the actual sun. It is difficult to conceive how a flimsy body could have resisted such a temperature so as not to have been entirely dissipated. This comet is also remarkable as one of the few on record which have been visible in broad daylight. Comets have been observed to undergo remarkable transformations on their approach to the perihelion, breaking up into two or more separate parts, evidently owing to the action of the sun. Seneca reports an instance of dissolution of this kind; Hevelius witnessed another; and Biela's comet exhibited the strange phenomenon on the occasion of its third re-appearance, in the year 1846. It then separated into two distinct comets, which moved side by side, in distinct and independent orbits, as long as visible. One of these objects was a little fainter than the other, but both had tails, and exhibited the distinctive features of a comet. The change of constitution seems to have taken place very suddenly. It was first observed in Europe on the 15th of January, by Mr Challis of Cambridge, and M. Wickmann of Konigsberg; but it was afterwards found to have been seen on the 12th by Lieutenant Maury, at the Observatory of Washington, in the United States. The separation has probably been permanent, and become more decided. In 1852, on the occasion of the fourth re-appearance, Biela's comet was preceded by another, a fainter object, which may have been the companion separated from it, removed to a greater distance. The annexed cut represents some of the various appearances presented by Halley's comet in 1835 in different parts of its orbit-a, b, c, in approaching the sun; d, e, in retreating. It is probable that a great comet, revolving round the sun in the long period of nearly three centuries, was observed in the years 975, 1264, and 1556. This is the opinion of Gauss of Gottingen, Mr Hind, and other astronomers. Assuming the comets of those years to be three apparitions of the same body, its return to the sun may be anticipated after the lapse of a corresponding interval. The wanderer is now overdue, as full three centuries have elapsed since the last period; but as its time of revolution may be lengthened by planetary perturbations, it will be looked for till the year 1860. In 1264, the comet was visible many months, and very conspicuous. All Europe regarded it as intimating the death of Pope Urban IV. "Its apparition," says Thierri, "boded his illness, and its disappearance his death, as events have proved." Historians relate that it was last seen on the night of the pontiff's decease. In 1556, the comet was also a striking object, and similarly interpreted as signifying terrestrial changes. It was noticed that "the tail was always at first turned towards Spain." The emperor Charles V. was greatly alarmed at its appearance; and looked upon it as a sign of his approaching death. He is said to have been induced by the impression to cede the imperial crown to his son. The leading features of the chief cometary appearances of modern times have now been sketched. There are various inquiries which naturally suggest themselves with reference to these bodies. What is their physical constitution? What their origin and office in the system? Are they inherently luminous, or dependent upon the solar glory, shining like the planets by virtue of his light? Have they any terrestrial influence? Is there a chance of our globe coming into actual collision with them; and supposing collision, what would be its probable effects? Upon most of these points we have no certain knowledge. Herschel and Schroeter thought the comet of 1811 a self-luminous body, but in opposition to this opinion Cassini is quoted as having descried the comet of 1744 showing a phase. On the very day, says Arago, that any comet shall appear with a distinct phase, all doubts will have ceased. At present however no satisfactory evidence is possessed of such an appearance being observed. Upon the question of physical constitution, it is pretty certain that the great majority of these bodies, and most probably all of them, are entirely gaseous—simple aggregations of vapour. The evidence to this effect is various. The comet of 1770 passed twice through the system of Jupiter; and calculation shows, that had it been of one of the satellites in mass, it would have sensibly affected that system. Yet there was not the slightest derangement of the planes of motion, or of the periods of revolution, by its intrusion among the satellites. The same body also passed at that time at no very great distance from the earth. In fact it approached us nearer than any other that has visited our terrestrial sky. Had it possessed a quantity of solid matter equal to that of the earth, it would then have shortened the length of our year by one ninth of a day; or had it been of the earth in mass, it would have appreciably altered its length to a degree that must long ago have been observed. But not the least perturbation was caused by its close proximity. These are sufficient proofs of the smallness of its mass, even allowing it to have had any solid matter at all, which may be reasonably suspected. Through the very centre of Biela's comet in 1832 a group of stars of the sixteenth magnitude was very distinctly seen by Sir John Herschel. While admitting that many comets are mere agglomerations of vapour, some hold to the opinion that where there is a nucleus remarkable for its vivacity of light, there is a solid and opaque body. But several facts declare against this supposition. Instances have occurred of stars being visible through a strongly defined nucleus. In 1618, the nucleus of the comet of that year is described as having dissolved into several detached parts; that of 1661 observed by Hevelius, changed also from a globular figure, and entirely disappeared; and the most extraordinary transformations are suddenly effected in the constitution of these objects. It is most probable that a comet is altogether a gaseous body, and has no solid matter whatever. Sir John Herschel remarks, that "whenever powerful telescopes have been turned on them, they have not failed to dispel the illusion which attributes solidity to that more condensed part of the head, which appears to the naked eye as a nucleus; though it is true that in some a very minute stellar point has been seen, indicating the existence of a solid body." Mr. Airy also states, that "on the physical constitution of comets we have learnt nothing, except that they appear to be wholly gaseous." These views of the constitution of cometary bodies show the fallacy of apprehending those consequences from a shock with them, of which terrific pictures have been drawn, and the impossibility of those events being produced by collision, which have been assigned to it, such as the deluge of Noah, the depression of the Caspian Sea and its neighbourhood, with the formation of the small telescopic planets out of a comet-stricken orb. "It is easy to represent," says Laplace, "the effect of such a shock upon the earth; the axis and motion of rotation changed; the waters abandoning their ancient position to precipitate themselves towards the new equator; the greater part of men and animals drowned in a universal deluge, or destroyed by the violence of the shock given to the terrestrial globe; whole species annihilated; all the monuments of human industry reversed; such are the disasters which a shock of a comet would produce. We see then," he observes, referring 66 to this cause some singular facts in geology, explained, "why the ocean has abandoned the highest mountains, on which it has left incontestable marks of its former abode. We see why the animals and plants of the south may have existed in the climates of the north, where their relics and impressions are still to be found. Lastly, it explains the short period of the existence of the moral world, whose earliest monuments do not go much further back than three thousand years. The human race, reduced to a small number of individuals, in the most deplorable state, occupied only with the immediate care for their subsistence, must necessarily have lost the remembrance of all sciences and of every art; and when the progress of civilisation has again created new wants, everything was to be done again, as if mankind had been just placed upon the earth." When this was the language of a philosopher of such high repute, the cockneys and belles of Paris might well tremble at the announcement of a comet. "Popular terrors," said a professor there upon a recent occasion, are productive of serious consequences. Several members of the Academy may still remember the accidents and disorders which followed a similar threat, imprudently communicated to the Academy by M. Delande in May 1773. Persons of weak minds died of fright, and women miscarried. There were not wanting people, who knew too well the art of turning to their advantage the alarm inspired by the apprehended comet, and places in Paradise were sold at very high prices. The announcement of the comet of 1832 may produce similar effects, unless the authority of the Academy apply a prompt remedy; and this salutary intervention is at this moment implored by many benevolent persons." The possibility of collision with one of these vagrant cruisers in space may indeed be soberly entertained, as they move in all imaginable directions, penetrate within the interior of the planetary orbits, and often pass between Mercury and the sun. But a calculation of probabilities shows, that of 281,000,000 of chances, there are 280,999,999 that are favourable to one unfavourable. The probability, therefore, of such an event happening in the experience of any individual of the human race is no greater than it would be with reference to his drawing one black ball, supposing it in an urn with 280,999,999 white balls. As to the near approach of a comet producing any great terrestrial change, such as deflecting our globe from its orbit by attraction, and scampering off with it as a satellite, we have plain warrant to treat the assumption as romance. The case of Lexel's lost comet intruding in the system of Jupiter without disturbing it, but being itself twisted into a new path, may lead us to allow of some approach to fellowship with perfect safety. That comet advanced to within six times the distance of the moon from us, yet it neither raised our tides a jot, nor added a single second to our year, though the diameter of its head was estimated to be thirteen times that of our satellite. We, indeed, were not quite so passive upon that occasion, for the action of the earth upon the comet increased the time of its revolution by two days. Even should an instance of actual contact occur, there seems no more reason to infer physical convulsion from the attack of a gaseous body, than in the case of a squadron of clouds assailing the sides and summit of a mountain. In all probability the only effect would be a change of temperature, with some peculiar atmospheric phenomena, yet compatible with a full security to human life and happiness. That the orbital course and rotation of our planet would be affected; that the pole and the equator would exchange places; that the ocean would leave its present bed, and the dry land be submerged; that any consequence would follow beyond a temporary alteration of climate, we have not only no authority to suppose, but strong grounds to deny. The surmise has been entertained that, in the year 1837, our globe experienced some cometary entanglement; and nothing more likely than that repeatedly, since the Creation, the terrestrial surface has received a brush. No trifling service has been rendered to mankind by science, that now these bodies are divested of those attributes of terror with which they were identified in ages past, when regarded as the heralds of political misfortune, or portending fatal physical events. When tidings came across the seas, brought by merchant and monk, that William the Norman was preparing to contest the possession of his territories with Harold, a comet, flaring in the heavens, raised misgivings in the Saxon mind as to the issue of the event, and unnerved him for the struggle when all his vigour was most required. An after chronicle relates how a star with three long tails appeared in the sky, how the learned declared that such stars appeared only when a kingdom wanted a new king, and how the said star was called a "comette." So, in 1618, a similar object was believed, in France, to foreshow another Bartholomew massacre; in Holland, to predict the death of Barneveldt; at Vienna its fiery aspect was viewed as symbolic of destruction to the Bohemian heretics; while, in England, it was connected with coming wars, and the death of James's queen. It is no slight advantage to the moderns, that they can gaze upon such objects without anticipating disaster, and regard them as controlled by those laws to which their own world is obedient. CHAPTER VI. ROM every region of the globe, and in all ages of time within the range of history, exhibitions of apparent instability in the heavens have been observed, when the curtains of the evening have been drawn. Suddenly, a line of light arrests the eye, darting like an arrow through a varying extent of space, and in a moment the firmament is as sombre as before. The appearance is exactly that of a star falling from its sphere, and hence the popular title of shooting star applied to it. The apparent magnitudes of these meteorites are widely different, and also their brilliancy. Occasionally, they are far more resplendent than the brightest of the planets, and throw a very perceptible illumination upon the path of the observer. second or two commonly suffices for the individual display, but in some instances it has lasted several minutes. In every climate it is witnessed, and at all times of the year, but most frequently in the autumnal months. As far back as records go, we meet with allusions to these swift and evanescent luminous travellers. Minerva's hasty flight from the peaks of Olympus to break the truce between the Greeks and Trojans, is compared by Homer to the emission of a brilliant star. Virgil, in the first book of the Georgics, mentions the shooting stars as prognosticating weather changes: "And oft, before tempestuous winds arise, The seeming stars fall headlong from the skies, Various hypotheses have been framed to explain the nature and origin of these remarkable appearances. When electricity began to be understood, this was thought to afford a satisfactory explanation, and the shooting stars were regarded by Beccaria and Vassali as merely electrical sparks. When the inflammable nature of the gases became known, Lavoisier and Volta supposed an accumulation of hydrogen in the higher regions of the atmosphere, because of its inferior density, giving rise by ignition to the meteoric exhibitions. While these theories of the older philosophers have been shown to be untenable, there is still great obscurity resting upon the question, though we have reason to refer the phenomena to a cause exterior to the bounds of our atmosphere. Upon this ground, the subject assumes a strictly astronomical aspect, and claims a place in a treatise on the economy of the solar system. The first attempt accurately to investigate these elegant meteors was made by two university students, afterwards Professors Brandes of Leipsic, and Benzenberg of Dusseldorf, in the year 1798. They selected a base line of 46,200 feet, somewhat less than nine English miles, and placed themselves at its extremities on appointed nights, for the purpose of ascertaining their average altitude and velocity. Out of twenty-two appearances identified as the same, they found 7 under 45 miles 9 between 45 and 90 miles above 90 miles 1 above 140 - The greatest observed velocity gave twenty-five miles in a second. A more extensive plan was organised by Brandes in the year 1823, and carried into effect in the neighbourhood of Breslau. Out of ninety-eight appearances, the computed heights were, The velocities were between eighteen and thirty-six miles in a second, an average velo. city far greater than that of the earth in its orbit. The rush of luminous bodies through the sky of a more extraordinary kind, though a rare occurrence, has repeatedly been observed. They are usually discriminated from shooting stars, and known by the vulgar as fire-balls; but probably both proceed from the same cause, and are identical phenomena. They have sometimes been seen of large volume, giving an intense light, a hissing noise accompanying their progress, and a loud explosion attending their termination. In the year 1676, a meteor passed over Italy about two hours after sunset, upon which Montanari wrote a treatise. It came over the Adriatic Sea as if from Dalmatia, crossed the country in the direction of Rimini and Leghorn, a loud report being heard at the latter place, and disappeared upon the sea towards Corsica. A similar visitor was witnessed all over England in 1718, and forms the subject of one of Halley's papers to the Royal Society. Sir Hans Sloane was one of its spectators. Being abroad at the time of its appearance, at a quarter past eight at night, in the streets of London, his path was suddenly and intensely illuminated. This, he apprehended at first, might arise from a discharge of rockets; but found a fiery object in the heavens, moving after the manner of a falling star, in a direct line from the Pleiades to below the girdle of Orion. Its brightness was so vivid, that several times he was obliged to turn away his eyes from it. The stars disappeared, and the moon, then nine days old, and high near the meridian, the sky being very clear, was so effaced by the lustre of the meteor as to be scarcely seen. It was computed to have passed over three hundred geographical miles in a minute, at the distance of sixty miles above the surface, and was observed at different extremities of the kingdom. The sound of an explosion was heard through Devon and Cornwall, and along the opposite coast of Bretagne. Halley conjectured this and similar displays to proceed from combustible vapours aggregated on the outskirts of the atmosphere, and suddenly set on fire by some unknown cause. But since his time, the fact has been established, of the actual fall of heavy bodies to the earth from surrounding space, which requires another hypothesis. To these bodies the term aërolites is applied, signifying atmospheric stones, from anp, the atmosphere, and λíos, a stone. |