A Popular History of Astronomy During the Nineteenth Century Part 47

The Biela meteors of 1885 did not merely gratify astronomers with a fulfilled prediction, but were the means of communicating to them some valuable information. Although their main body was cut through by the moving earth in six hours, and was not more than 100,000 miles across, skirmishers were thrown out to nearly a million miles on either side of the compact central battalions. Members of the system were, on the 26th of November, recorded by Mr. Denning at the hourly rate of about 130; and they did not wholly cease to be visible until December 1. They afforded besides a particularly well-marked example of that diffuseness of radiation previously observed in some less conspicuous displays.

Their paths seemed to diverge from an area rather than from a point in the sky. They came so ill to focus that divergences of several degrees were found between the most authentically determined radiants. These incongruities are attributed by Professor Newton to the irregular shape of the meteoroids producing unsymmetrical resistance from the air, and hence causing them to glance from their original direction on entering it. Thus, their luminous tracks did not always represent (even apart from the effects of the earth's attraction) the true prolongation of their course through s.p.a.ce.

The Andromedes of 1872 were laggards behind the comet from which they sprang; those of 1885 were its avant-couriers. That wasted and disrupted body was not due at the node until January 26, 1886, sixty days, that is, after the earth's encounter with its meteoric fragments. These are now probably scattered over more than five hundred million miles of its...o...b..ts;[1230] yet Professor Newton considers that all must have formed one compact group with Biela at the time of its close approach to Jupiter about the middle of 1841. For otherwise both comet and meteorites could not have experienced, as they seem to have done, the same kind and amount of disturbance. The rapidity of cometary disintegration is thus curiously ill.u.s.trated.

A short-lived persuasion that the missing heavenly body itself had been recovered, was created by Mr. Edwin Holms's discovery, at London, November 6, 1892, of a tolerably bright, tailless comet, just in a spot which Biela's comet must have traversed in approaching the intersection of its...o...b..t with that of the earth. A hasty calculation by Berberich a.s.signed elements to the newcomer seeming not only to ratify the ident.i.ty, but to promise a quasi-encounter with the earth on November 21. The only effect of the prediction, however, was to raise a panic among the negroes of the Southern States of America. The comet quietly ignored it, and moved away from instead of towards the appointed meeting-place. Its projection, then, on the night of its discovery, upon a point of the Biela-orbit was by a mere caprice of chance. North America, nevertheless, was visited on November 23 by a genuine Andromede shower. Although the meteors were less numerous than in 1885, Professor Young estimated that 30,000, at the least, of their orange fire-streaks came, during five hours, within the range of view at Princeton.[1231]

Bredikhine estimated the width of the s.p.a.ce containing them at about 2,700,000 miles.[1232] The antic.i.p.ation of their due time by four days implied--if they were a prolongation of the main Biela group, the nucleus of which pa.s.sed the spot of encounter five months previously--a recession of the node since 1885 by no less than three degrees. Unless, indeed, Mr. Denning were right in supposing the display to have proceeded from "an a.s.sociated branch of the main swarm through which we pa.s.sed in 1872 and 1885."[1233] The existence of separated detachments of Biela meteors, due to disturbing planetary action, was contemplated as highly probable by Schiaparelli.[1234] Such may have been the belated flights met with in 1830, 1838, 1841, and 1847, and such the advance flight plunged through in 1892. A shower looked for November 23, 1899, did not fall, and no further display from this quarter is probable until November 17, 1905, although one is possible a year earlier.[1235]

The Leonids, through the adverse influence of Jupiter and Saturn, inflicted upon mult.i.tudes of eager watchers a still more poignant disappointment. A dense part of the swarm, having nearly completed a revolution since 1866, should, travelling normally, have met the earth November 15, 1899; in point of fact, it swerved sunward, and the millions of meteorites which would otherwise have been sacrificed for the illumination of our skies escaped a fiery doom. The contingency had been forecast in the able calculations of Dr. Johnstone Stoney and Dr.

A. M. W. Downing,[1236] superintendent of the Nautical Almanac Office; but the verification scarcely compensated the failure. Nor was the situation retrieved in the following years. Only ragged fringes of the great tempest-cloud here and there touched our globe. As the same investigators warned us to expect, the course of the meteorites had been not only rendered sinuous by perturbation, but also broken and irregular. We can no longer count upon the Leonids. Their glory, for scenic purposes, is departed. The comet a.s.sociated with them also evaded observation. Although it doubtless kept its tryst with the sun in the spring of 1899, the attendant circ.u.mstances were too unfavourable to allow it to be seen from the earth.[1237] By an almost fantastic coincidence, nevertheless, a faint comet was photographed, November 14, 1898,[1238] by Dr. Chase, of the Yale College Observatory, close to the Leonid radiant, whither a "meteorograph" was directed with a view to recording trails left by precursors of the main Leonid body. A promising start, too, was made on the same occasion with meteoric researches from sensitive plates.[1239] Indeed, Schaeberle and Colton[1240] had already, in 1896, determined the height of a Leonid by means of photographs taken at stations on different ridges of Mount Hamilton; and Professor Pickering has prosecuted similar work at Harvard, with encouraging results. Everything in this branch of science depends upon how far they can be carried. Without the meteorograph, rigid accuracy in the observation of shooting stars is unattainable, and rigid accuracy is the _sine qua non_ for obtaining exact knowledge.

Biela does not offer the only example of cometary disruption. Setting aside the unauthentic reports of early chroniclers, we meet the "double comet" discovered by Liais at Olinda (Brazil), February 27, 1860, of which the division appeared recent, and about to be carried farther.[1241] But a division once established, separation must continually progress. The periodic times of the fragments will never be identical; one must drop a little behind the other at each revolution, until at length they come to travel in remote parts of nearly the same orbit. Thus the comet predicted by Klinkerfues and discovered by Pogson had already lagged to the extent of twelve weeks, and we shall meet instances farther on where the r.e.t.a.r.dation is counted, not by weeks, but by years. Here original ident.i.ty emerges only from calculation and comparison of orbits.

Comets, then, die, as Kepler wrote long ago, _sicut bombyces filo fundendo_. This certainty, antic.i.p.ated by Kirkwood in 1861, we have at least acquired from the discovery of their generative connection with meteors. Nay, their actual materials become, in smaller or larger proportions, incorporated with our globe. It is not, indeed, universally admitted that the ponderous ma.s.ses of which, according to Daubree's estimate,[1242] at least 600 fall annually from s.p.a.ce upon the earth, ever formed part of the bodies known to us as comets. Some follow Tschermak in attributing to aerolites a totally different origin from that of periodical shooting-stars. That no clear line of demarcation can be drawn is no valid reason for a.s.serting that no real distinction exists; and it is certainly remarkable that a meteoric fusillade may be kept up for hours without a single solid projectile reaching its destination. It would seem as if the celestial army had been supplied with blank cartridges. Yet, since a few detonating meteors have been found to proceed from ascertained radiants of shooting-stars, it is difficult to suppose that any generic difference separates them.

Their a.s.similation is further urged--though not with any demonstrative force--by two instances, the only two on record, of the tangible descent of an aerolite during the progress of a star-shower. On April 4, 1095, the Saxon Chronicle informs us that stars fell "so thickly that no man could count them," and adds that one of them having struck the ground in France, a bystander "cast water upon it, which was raised in steam with a great noise of boiling."[1243] And again, on November 27, 1885, while the skirts of the Andromede-tempest were trailing over Mexico, "a ball of fire" was precipitated from the sky at Mazapil, within view of a ranchman.[1244] Scientific examination proved it to be a "siderite," or ma.s.s of "nickel-iron"; its weight exceeded eight pounds, and it contained many nodules of graphite. We are not, however, authorised by the circ.u.mstances of its arrival to regard the Mazapil fragment of cosmic metal as a specimen torn from Biela's comet. In this, as in the preceding case, the coincidence of the fall with the shower may have been purely casual, since no hint is given of any sort of agreement between the tracks followed by the sample provided for curious study, and the swarming meteors consumed in the upper air.

Professor Newton's inquiries into the tracks pursued by meteorites previous to their collisions with the earth tend to distinguish them, at least specifically, from shooting-stars. He found that nearly all had been travelling with a direct movement in orbits the perihelia of which lay in the outer half of the s.p.a.ce separating the earth from the sun.[1245] Shooting-stars, on the contrary, are entirely exempt from such limitations. The Yale Professor concluded "that the larger meteorites moving in our solar system are allied much more closely with the group of comets of short period than with the comets whose orbits are nearly parabolic." They would thus seem to be more at home than might have been expected amid the planetary family. Father Carbonelle has, moreover, shown[1246] that meteorites, if explosion-products of the earth or moon, should, with rare exceptions, follow just the kind of paths a.s.signed to them, from data of observation, by Professor Newton.

Yet it is altogether improbable that projectiles from terrestrial volcanoes should, at any geological epoch, have received impulses powerful enough to enable them, not only to surmount the earth's gravity, but to penetrate its atmosphere.

A striking--indeed, an almost startling--peculiarity, on the other hand, divides from their congeners a cla.s.s of meteors identified by Mr. Denning during ten years' patient watching of such phenomena at Bristol.[1247] These are described as "meteors with stationary radiants," since for months together they seem to come from the same fixed points in the sky. Now this implies quite a portentous velocity.

The direction of meteor-radiants is affected by a kind of _aberration_, a.n.a.logous to the aberration of light. It results from a composition of terrestrial with meteoric motion. Hence, unless that of the earth in its...o...b..t be by comparison insignificant, the visual line of encounter must shift, if not perceptibly from day to day, at any rate conspicuously from month to month. The fixity, then, of many systems observed by Mr. Denning seems to demand the admission that their members travel so fast as to throw the earth's movement completely out of the account. The required velocity would be, by Mr.

Ranyard's calculation, at least 880 miles a second.[1248] But the aspect of the meteors justifies no such extravagant a.s.sumption. Their seeming swiftness is very various, and--what is highly significant--it is notably less when they pursue than when they meet the earth. Yet the "incredible and unaccountable"[1249] fact of the existence of these "long radiants," although doubted by Tisserand[1250] because of its theoretical refractoriness, must apparently be admitted. The first plausible explanation of them was offered by Professor Turner in 1899.[1251] They represent, in his view, the c.u.mulative effects of the earth's attraction. The validity of his reasoning is, however, denied by M. Bredikhine,[1252] who prefers to regard them as a congeries of separate streams. The enigma they present has evidently not yet received its definitive solution.

The Perseids afford, on the contrary, a remarkable instance of a "shifting radiant." Mr. Denning's observations of these yellowish, leisurely meteors extend over nearly six weeks, from July 8 to August 16; the point of radiation meantime progressing no less than 57 in right ascension. Doubts as to their common origin were hence freely expressed, especially by Mr. Monck of Dublin.[1253] But the late Dr.

Kleiber[1254] showed, by strict geometrical reasoning, that the forty-nine radiants successively determined for the shower were all, in fact, comprised within one narrowly limited region of s.p.a.ce. In other words, the application of the proper correction for the terrestrial movement, and the effects of attraction by which each individual shooting-star is compelled to describe a hyperbola round the earth's centre, reduces the extended line of radiants to a compact group, with the cometary radiant for its central point; the cometary radiant being the spot in the sky met by a tangent to the orbit of the Perseid comet of 1862 at its intersection with the orbit of the earth. The reality of the connection between the comet and the meteors could scarcely be more clearly proved; while the vast dimensions of the stream into which the latter are found to be diffused cannot but excite astonishment not unmixed with perplexity.

The first successful application of the spectroscope to comets was by Donati in 1864.[1255] A comet discovered by Tempel, July 4, brightened until it appeared like a star somewhat below the second magnitude, with a feeble tail 30 in length. It was remarkable as having, on August 7, almost totally eclipsed a small star--a very rare occurrence.[1256] On August 5 Donati admitted its light through his train of prisms, and found it, thus a.n.a.lysed, to consist of three bright bands--yellow, green, and blue--separated by wider dark intervals. This implied a good deal. Comets had previously been considered, as we have seen, to shine mainly, if not wholly, by reflected sunlight. They were now perceived to be self-luminous, and to be formed, to a large extent, of glowing gas.

The next step was to determine what _kind_ of gas it was that was thus glowing in them; and this was taken by Sir William Huggins in 1868.[1257]

A comet of subordinate brilliancy, known as comet 1868 ii., or sometimes as Winnecke's, was the subject of his experiment. On comparing its spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by electricity, he found the agreement exact. It has since been abundantly confirmed. All the eighteen comets tested by light a.n.a.lysis, between 1868 and 1880, showed the typical hydro-carbon spectrum[1258] common to the whole group of those compounds, but probably due immediately to the presence of acetylene. Some minor deviations from the laboratory pattern, in the shifting of the maxima of light from the edge towards the middle of the yellow and blue bands, have been experimentally reproduced by Vogel and Ha.s.selberg in tubes containing a mixture of carbonic oxide with olefiant gas.[1259] Their illumination by disruptive electric discharges was, however, a condition _sine qua non_ for the exhibition of the cometary type of spectrum. When a continuous current was employed, the carbonic oxide bands a.s.serted themselves to the exclusion of the hydro-carbons. The distinction has great significance as regards the nature of comets. Of particular interest in this connection is the circ.u.mstance that carbonic oxide is one of the gases evolved by meteoric stones and irons under stress of heat.[1260] For it must apparently have formed part of an aeriform ma.s.s in which they were immersed at an earlier stage of their history.

PLATE II.

[Ill.u.s.tration: Great Comet.

Photographed, May 5, 1901, with the thirteen-inch Astrographic Refractor of the Royal Observatory, Cape of Good Hope.]

In a few exceptional comets the usual carbon-bands have been missed. Two such were observed by Sir William Huggins in 1866 and 1867 respectively.[1261] In each a green ray, approximating in position to the fundamental nebular line, crossed an otherwise unbroken spectrum.

And Holmes's comet of 1892 displayed only a faint prismatic band devoid of any characteristic feature.[1262] Now these three might well be set down as partially effete bodies; but a brilliant comet, visible in southern lat.i.tudes in April and May, 1901, so far resembled them in the quality of its light as to give a spectrum mainly, if not purely, continuous. This, accordingly, is no symptom of decay.

The earliest comet of first-cla.s.s l.u.s.tre to present itself for spectroscopic examination was that discovered by Coggia at Ma.r.s.eilles, April 17, 1874. Invisible to the naked eye till June, it blazed out in July a splendid ornament of our northern skies, with a just perceptibly curved tail, reaching more than half way from the horizon to the zenith, and a nucleus surpa.s.sing in brilliancy the brightest stars in the Swan.

Bredikhine, Vogel, and Huggins[1263] were unanimous in p.r.o.nouncing its spectrum to be that of marsh or olefiant gas. Father Secchi, in the clear sky of Rome, was able to push the identification even closer than had heretofore been done. The _complete_ hydro-carbon spectrum consists of five zones of variously coloured light. Three of these only--the three central ones--had till then been obtained from comets; owing, it was supposed, to their temperature not being high enough to develop the others. The light of Coggia's comet, however, was found to contain all five, traces of the violet band emerging June 4, of the red, July 2.[1264] Presumably, all five would show universally in cometary spectra, were the dispersed rays strong enough to enable them to be seen.

The gaseous surroundings of comets are, then, largely made up of a compound of hydrogen with carbon. Other materials are also present; but the hydro-carbon element is probably unfailing and predominant. Its luminosity is, there is little doubt, an effect of electrical excitement. Zollner showed in 1872[1265] that, owing to evaporation and other changes produced by rapid approach to the sun, electrical processes of considerable intensity must take place in comets; and that their original light is immediately connected with these, and depends upon solar radiation, rather through its direct or indirect electrifying effects, than through its more obvious thermal power, may be considered a truth permanently acquired to science.[1266] They are not, it thus seems, bodies incandescent through heat, but glowing by electricity; and this is compatible, under certain circ.u.mstances, with a relatively low temperature.

The gaseous spectrum of comets is accompanied, in varying degrees, by a continuous spectrum. This is usually derived most strongly from the nucleus, but extends, more or less, to the nebulous appendages. In part, it is certainly due to reflected sunlight; in part, most likely, to the ignition of minute solid particles.

FOOTNOTES:

[Footnote 1188: _Month. Not._, vol. xix., p. 27.]

[Footnote 1189: _Mem. de l'Ac. Imp._, t. ii., 1859, p. 46.]

[Footnote 1190: _Harvard Annals_, vol. iii., p. 368.]

[Footnote 1191: _Ibid._, p. 371.]

[Footnote 1192: _Month. Not._, vol. xxii., p. 306.]

[Footnote 1193: Stothard in _Ibid._, vol. xxi., p. 243.]

[Footnote 1194: _Intell. Observer_, vol. i., p. 65.]

[Footnote 1195: _Comptes Rendus_, t. lxi., p. 953.]

[Footnote 1196: _Smiths. Report_, 1881 (Holden); _Nature_, vol. xxv., p.

94; _Observatory_, vol. xxi., p. 378 (W. T. Lynn).]

[Footnote 1197: _Ueber den Ursprung der von Pallas gefundenen Eisenma.s.sen_, p. 24.]

[Footnote 1198: Arago, _Annuaire_, 1836, p. 294.]

[Footnote 1199: Humboldt had noticed the emanation of the shooting stars of 1799 from a single point, or "radiant," as Greg long afterwards termed it; but no reasoning was founded on the observation.]

[Footnote 1200: _Am. Journ. of Sc._, vol. xxvi., p. 132.]

[Footnote 1201: _Annuaire_, 1836, p. 297.]

[Footnote 1202: _Ann. de l'Observ._, Bruxelles, 1839, p. 248.]

[Footnote 1203: _Ibid._, 1837, p. 272.]

[Footnote 1204: _Astr. Nach._, Nos. 385, 390.]

[Footnote 1205: _Am. Jour. of Sc._, vol. x.x.xviii. (2nd ser.), p. 377.]

[Footnote 1206: _Ibid._, vol. x.x.xviii., p. 61.]

[Footnote 1207: _Month. Not._, vol. xxvii., p. 247.]

[Footnote 1208: _Am. Jour. of Sc._, vol. xliii. (2nd ser.), p. 87.]

[Footnote 1209: Grant, _Month. Not._, vol. xxvii., p. 29.]

[Footnote 1210: P. Smyth, _Ibid._, p. 256.]