Part 11 (2/2)

But of this remarkable double event nothing was known until more than a month later, when the fact of its past occurrence emerged from the calculations of Olbers.[274] Nor had the comet itself been generally visible previous to the first days of July. Several observers, however, on the publication of these results, brought forward accounts of singular spots perceived by them upon the sun at the time of the transit, and an original drawing of one of them, by Pastorff of Buchholtz, has been preserved. This undoubtedly authentic delineation[275] represents a round nebulous object with a _bright_ spot in the centre, of decidedly cometary aspect, and not in the least like an ordinary solar ”macula.” Mr. Hind,[276] nevertheless, showed its position on the sun to be irreconcilable with that which the comet must have occupied; and Mr. Ranyard's discovery of a similar smaller drawing by the same author, dated May 26, 1828,[277] reduces to evanescence the probability of its connection with that body. Indeed, recent experience renders very doubtful the possibility of such an observation.

The return of Halley's comet in 1835 was looked forward to as an opportunity for testing the truth of floating cometary theories, and did not altogether disappoint expectation. As early as 1817, its movements and disturbances since 1759 were proposed by the Turin Academy of Sciences as the subject of a prize ultimately awarded to Baron Damoiseau. Pontecoulant was adjudged a similar distinction by the Paris Academy in 1829; while Rosenberger's calculations were rewarded with the gold medal of the Royal Astronomical Society.[278]

They were verified by the detection at Rome, August 6, 1835, of a nearly circular misty object not far from the predicted place of the comet. It was not, however, until the middle of September that it began to throw out a tail, which by the 15th of October had attained a length of about 24 degrees (on the 19th, at Madras, it extended to fully 30),[279] the head showing to the naked eye as a reddish star rather brighter than Aldebaran or Antares.[280] Some curious phenomena accompanied the process of tail-formation. An outrush of luminous matter, resembling in shape a partially opened fan, issued from the nucleus _towards_ the sun, and at a certain point, like smoke driven before a high wind, was vehemently swept backwards in a prolonged train. The appearance of the comet at this time was compared by Bessel,[281] who watched it with minute attention, to that of a blazing rocket. He made the singular observation that this fan of light, which seemed the source of supply for the tail, oscillated like a pendulum to and fro across a line joining the sun and nucleus, in a period of 4-3/5 days; and he was unable to escape from the conclusion[282] that a repulsive force, about twice as powerful as the attractive force of gravity, was concerned in the production of these remarkable effects. Nor did he hesitate to recur to the a.n.a.logy of magnetic polarity, or to declare, still more emphatically than Olbers, ”the emission of the tail to be a purely electrical phenomenon.”[283]

The transformations undergone by this body were almost as strange and complete as those which affected the brigands in Dante's _Inferno_. When first seen, it wore the aspect of a nebula; later it put on the distinctive garb of a comet; it next appeared as a star; finally, it dilated, first in a spherical, then in a paraboloidal form, until May 5, 1836, when it vanished from Herschel's observation at Feldhausen as if by melting into adjacent s.p.a.ce from the excessive diffusion of its light. A very uncommon circ.u.mstance in its development was that it lost all trace of tail _previous_ to its arrival at perihelion on the 16th of November. Nor did it begin to recover its elongated shape for more than two months afterwards. On the 23rd of January, Boguslawski perceived it as a star of the sixth magnitude, _without measurable disc_.[284] Only two nights later, Maclear, director of the Cape Observatory, found the head to be 131 seconds across.[285] And so rapidly did the augmentation of size progress, that Sir John Herschel estimated the actual bulk of this singular object to have increased forty-fold in the ensuing week.

”I can hardly doubt,” he remarks, ”that the comet was fairly evaporated in perihelio by the heat, and resolved into transparent vapour, and is now in process of rapid condensation and re-precipitation on the nucleus.”[286] A plausible, but no longer admissible, interpretation of this still unexplained phenomenon. The next return of this body, which will be considerably accelerated by Jupiter's influence, is expected to take place in 1910.[287]

By means of an instrument devised to test the quality of light, Arago obtained decisive evidence that some at least of the radiance proceeding from Halley's comet was derived by reflection from the sun.[288]

Indications of the same kind had been afforded[289] by the comet which suddenly appeared above the north-western horizon of Paris, July 3, 1819, after having enveloped (as already stated) our terrestrial abode in its filmy appendages; but the ”polariscope” had not then reached the perfection subsequently given to it, and its testimony was accordingly far less reliable than in 1835. Such experiments, however, are in reality more beautiful and ingenious than instructive, since ignited as well as obscure bodies possess the power of throwing back light incident upon them, and will consequently transmit to us from the neighbourhood of the sun rays partly direct, partly reflected, of which a certain proportion will exhibit the peculiarity known as polarisation.

The most brilliant comets of the century were suddenly rivalled if not surpa.s.sed by the extraordinary object which blazed out beside the sun, February 28, 1843. It was simultaneously perceived in Mexico and the United States, in Southern Europe, and at sea off the Cape of Good Hope, where the pa.s.sengers on board the _Owen Glendower_ were amazed by the sight of a ”short, dagger-like object,” closely following the sun towards the western horizon.[290] At Florence, Amici found its distance from the sun's centre at noon to be only 1 23'; and spectators at Parma were able, when sheltered from the direct glare of mid-day, to trace the tail to a length of four or five degrees. The full dimensions of this astonis.h.i.+ng appurtenance began to be disclosed a few days later. On the 3rd of March it measured 25, and on the 11th, at Calcutta, Mr. Clerihew observed a second streamer, nearly twice as long as the first, and making an angle with it of 18, to have been emitted in a single day.

This rapidity of projection, Sir John Herschel remarked, ”conveys an astounding impression of the intensity of the forces at work.” ”It is clear,” he continued, ”that _if we have to deal here with matter, such as we conceive it_--viz., _possessing inertia--at all_, it must be under the dominion of forces incomparably more energetic than gravitation, and quite of a different nature.”[291]

On the 17th of March a silvery ray, some 40 long and slightly curved at its extremity, shone out above the sunset clouds in this country. No previous intimation had been received of the possibility of such an apparition, and even astronomers--no lightning messages across the seas being as yet possible--were perplexed. The nature of the phenomenon, indeed, soon became evident, but the wonder of it did not diminish with the study of its attendant circ.u.mstances. Never before, within astronomical memory, had our system been traversed by a body pursuing such an adventurous career. The closest a.n.a.logy was offered by the great comet of 1680 (Newton's), which rushed past the sun at a distance of only 144,000 miles; but even this--on the cosmical scale--scarcely perceptible interval was reduced nearly one-half in the case we are now concerned with. The centre of the comet of 1843 approached the formidable luminary within 78,000 miles, leaving, it is estimated, a clear s.p.a.ce of not more than 32,000 between the surfaces of the bodies brought into such perilous proximity. The escape of the wanderer was, however, secured by the extraordinary rapidity of its flight. It swept past perihelion at a rate--366 miles a second--which, if continued, would have carried it right round the sun in _two hours_; and in only eleven minutes more than that short period it actually described half the _curvature_ of its...o...b..t--an arc of 180--although in travelling over the remaining half many hundreds of sluggish years will doubtless be consumed.

The behaviour of this comet may be regarded as an _experimentum crucis_ as to the nature of tails. For clearly no fixed appendage many millions of miles in length could be whirled like a brandished sabre from one side of the sun to the other in 131 minutes. Cometary trains are then, as...o...b..rs rightly conceived them to be, emanations, not appendages--inconceivably rapid outflows of highly rarefied matter, the greater part, if not all, of which becomes permanently detached from the nucleus.

That of the comet of 1843 reached, about the time that it became visible in this country, the extravagant length of 200 millions of miles.[292]

It was narrow, and bounded by nearly parallel and nearly rectilinear lines, resembling--to borrow a comparison of Aristotle's--a ”road”

through the constellations; and after the 3rd of March showed no trace of hollowness, the axis being, in fact, rather brighter than the edges.

Distinctly perceptible in it were those singular aurora-like coruscations which gave to the ”tresses” of Charles V.'s comet the appearance--as Cardan described it--of ”a torch agitated by the wind,”

and have not unfrequently been observed to characterise other similar objects. A consideration first adverted to by Olbers proves these to originate in our own atmosphere. For owing to the great difference in the distances from the earth of the origin and extremity of such vast effluxes, the light proceeding from their various parts is transmitted to our eyes in notably different intervals of time. Consequently a luminous undulation, even though propagated instantaneously from end to end of a comet's tail, would appear to us to occupy many minutes in its progress. But the coruscations in question pa.s.s as swiftly as a falling star. They are, then, of terrestrial production.

Periods of the utmost variety were by different computators a.s.signed to the body, which arrived at perihelion, February 27, 1843, at 9.47 p.m.

Professor Hubbard of Was.h.i.+ngton found that it required 533 years to complete a revolution; MM. Laugier and Mauvais of Paris considered the true term to be 35;[293] Clausen looked for its return at the end of between six and seven. A recent discussion[294] by Professor Kreutz of all the available data gives a probable period of 512 years for this body, and precludes its hypothetical ident.i.ty with the comet of 1668, known as the ”Spina” of Ca.s.sini.

It may now be asked, what were the conclusions regarding the nature of comets drawn by astronomers from the considerable amount of novel experience acc.u.mulated during the first half of this century? The first and best a.s.sured was that the matter composing them is in a state of extreme tenuity. Numerous and trustworthy observations showed that the feeblest rays of light might traverse some hundreds of thousands of miles of their substance, even where it was apparently most condensed, without being perceptibly weakened. Nay, instances were recorded in which stars were said to have gained in brightness from the process![295] On the 24th of June, 1825, Olbers[296] saw the comet then visible all but obliterated by the central pa.s.sage of a star too small to be distinguished with the naked eye, its own light remaining wholly unchanged. A similar effect was noted December 1, 1811, when the great comet of that year approached so close to Altair, the _lucida_ of the Eagle, that the star seemed to be transformed into the nucleus of the comet.[297] Even the central blaze of Halley's comet in 1835 was powerless to impede the pa.s.sage of stellar rays. Struve[298] observed at Dorpat, on September 17, an all but central occultation; Glaisher[299]

one (so far as he could ascertain) absolutely so eight days later at Cambridge. In neither case was there any appreciable diminution of the star's light. Again, on the 11th of October, 1847, Mr. Dawes,[300] an exceptionally keen observer, distinctly saw a star of the tenth magnitude through the exact centre of a comet discovered on the first of that month by Maria Mitch.e.l.l of Nantucket.

Examples, on the other hand, are not wanting of the diminution of stellar light under similar circ.u.mstances;[301] and we meet two alleged instances of the vanis.h.i.+ng of a star behind a comet. Wartmann of Geneva observed the first, November 28, 1828;[302] but his instrument was defective, and the eclipsing body, Encke's comet, has shown itself otherwise perfectly translucent. The second case of occultation occurred September 13, 1890, when an eleventh magnitude star was stated to have completely disappeared during the transit over it of Denning's comet.[303]

From the failure to detect any effects of refraction in the light of stars occulted by comets, it was inferred (though, as we know now, erroneously) that their composition is rather that of dust than that of vapour; that they consist not of any continuous substance, but of discrete solid particles, very finely divided and widely scattered. In conformity with this view was the known smallness of their ma.s.ses.

Laplace had shown that if the amount of matter forming Lexell's comet had been as much as 1/5000 of that contained in our globe, the effect of its attraction, on the occasion of its approach within 1,438,000 miles of the earth, July 1, 1770, must have been apparent in the lengthening of the year. And that some comets, at any rate, possess ma.s.ses immeasurably below this maximum value was clearly proved by the undisturbed parallel march of the two fragments of Biela's in 1846.

But the discovery in this branch most distinctive of the period under review is that of ”short period” comets, of which four[304] were known in 1850. These, by the character of their movements, serve as a link between the planetary and cometary worlds, and by the nature of their construction, seem to mark a stage in cometary decay. For that comets are rather transitory agglomerations, than permanent products of cosmical manufacture, appeared to be demonstrated by the division and disappearance of one amongst their number, as well as by the singular and rapid changes in appearance undergone by many, and the seemingly irrevocable diffusion of their substance visible in nearly all. They might then be defined, according to the ideas respecting them prevalent fifty years ago, as bodies unconnected by origin with the solar system, but encountered, and to some extent appropriated, by it in its progress through s.p.a.ce, owing their visibility in great part, if not altogether, to light reflected from the sun, and their singular and striking forms to the action of repulsive forces emanating from him, the penalty of their evanescent splendour being paid in gradual waste and final dissipation and extinction.

FOOTNOTES:

[Footnote 241: _Allgemeine Geographische Ephemeriden_, vol. iv., p.

287.]

[Footnote 242: _Astr. Jahrbuch_, 1823, p. 217. The period (1,208 days) of this body is considerably shorter than that of any other known comet.]

[Footnote 243: ”Sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi et denique mori.”--_De Cometis_, Op., vol. vii., p.

110.]

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