FOOTNOTES:

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

employed, the carbonic oxide bands asserted 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 circumstance 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 mass in which they were immersed at an earlier stage of their history.

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 latitudes 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-class lustre to present itself for spectroscopic examination was that discovered by Coggia at Marseilles, 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 surpassing in brilliancy the brightest stars in the Swan. Brédikhine, Vogel, and Huggins[1263]were unanimous in pronouncing 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. Thecompletehydro-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 alsopresent; but the hydro-carbon element is probably unfailing and predominant. Its luminosity is, there is little doubt, an effect of electrical excitement. Zöllner 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 circumstances, 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:[1188]Month. Not., vol. xix., p. 27.[1189]Mém. de l'Ac. Imp., t. ii., 1859, p. 46.[1190]Harvard Annals, vol. iii., p. 368.[1191]Ibid., p. 371.[1192]Month. Not., vol. xxii., p. 306.[1193]Stothard inIbid., vol. xxi., p. 243.[1194]Intell. Observer, vol. i., p. 65.[1195]Comptes Rendus, t. lxi., p. 953.[1196]Smiths. Report, 1881 (Holden);Nature, vol. xxv., p. 94;Observatory, vol. xxi., p. 378 (W. T. Lynn).[1197]Ueber den Ursprung der von Pallas gefundenen Eisenmassen, p. 24.[1198]Arago,Annuaire, 1836, p. 294.[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.[1200]Am. Journ. of Sc., vol. xxvi., p. 132.[1201]Annuaire, 1836, p. 297.[1202]Ann. de l'Observ., Bruxelles, 1839, p. 248.[1203]Ibid., 1837, p. 272.[1204]Astr. Nach., Nos. 385, 390.[1205]Am. Jour. of Sc., vol. xxxviii. (2nd ser.), p. 377.[1206]Ibid., vol. xxxviii., p. 61.[1207]Month. Not., vol. xxvii., p. 247.[1208]Am. Jour. of Sc., vol. xliii. (2nd ser.), p. 87.[1209]Grant,Month. Not., vol. xxvii., p. 29.[1210]P. Smyth,Ibid., p. 256.[1211]Hind,Ibid., p. 49.[1212]Reproduced inLes Mondes, t. xiii.[1213]Comptes Rendus, t. lxiv., p. 96.[1214]Astr. Nach., No. 1,626.[1215]Ibid., No. 1,632.[1216]Month. Not., vol. xxxviii., p. 369.[1217]Schiaparelli,Le Stelle Cadenti, p. 54.[1218]Ueber Feuer-Meteore, p. 406.[1219]Astr. Nach., No. 347 (Mädler); see also Boguslawski,Die Kometen, p. 98. 1857.[1220]Nature, vol. vi., p. 148.[1221]A. S. Herschel,Month. Not., vol. xxxii., p. 355.[1222]Astr. Nach., Nos. 1,632, 1,633, 1,635.[1223]Nature, vol. vii., p. 122.[1224]A. S. Herschel,Report Brit. Ass., 1873, p. 390.[1225]Humboldt,Cosmos, vol. i., p. 114 (Otté's trans.).[1226]Month. Not., vol. xxxiii., p. 128.[1227]Even this was denied by Bruhns,Astr. Nach., No. 2,054.[1228]Am. Jour., vol. xxxi., p. 425.[1229]Month. Not., vol. xlvi., p. 69.[1230]In Schiaparelli's opinion, centuries must have elapsed while the observed amount of scattering was being produced.Le Stelle Cadenti, 1886, p. 112.[1231]Astr. and Astroph., vol. xi., p. 943.[1232]Bull. de l'Acad. St. Petersbourg, t. xxxv., p. 598. 1894.[1233]Observatory, vol. xvi., p. 55.[1234]Le Stelle Cadenti, p. 133;Rendiconti dell' Istituto Lombardo, t. iii., ser. ii., p. 23.[1235]Denning,Memoirs Roy. Astr. Soc., vol. liii., p. 214; Abelmann,Astr. Nach., No. 3,516.[1236]Proc. Roy. Soc., March 2, 1899;Nature, November 9, 1899.[1237]Berberich,Astr. Nach., No. 3,526.[1238]Elkin,Astroph. Jour., vol. ix., p. 22.[1239]Elkin,Astroph. Jour., vol. x., p. 24.[1240]Pop. Astr., September, 1897, p. 232.[1241]Month. Not., vol. xx., p. 336.[1242]Revue des deux Mondes, December 15, 1885, p. 889.[1243]Palgrave,Phil. Trans., vol. cxxv., p. 175.[1244]W. E. Hidden,Century Mag., vol. xxxiv., p. 534.[1245]Amer. Jour. of Science, vol. xxxvi., p. i., 1888.[1246]Revue des Questions Scientifiques, January, 1899, p. 194; Tisserand,Bull. Astr., t. viii., p. 460.[1247]Month. Not., vol. xlv., p. 93.[1248]Observatory, vol. viii., p. 4.[1249]Denning,Month. Not., vol. xxxviii., p. 114.[1250]Comptes Rendus, t. cix., p. 344.[1251]Month. Not., vol. lix., p. 140.[1252]Bull. de l'Acad. St. Petersb., t. xii., p. 95.[1253]Publ. Astr. Pac. Soc., vol. iii., p. 114.[1254]Month. Not., vol. lii., p. 341.[1255]Astr. Nach., No. 1,488.[1256]Annuaire, Paris, 1883, p. 185.[1257]Phil. Trans., vol. clviii., p. 556.[1258]Hasselberg,Mém. de l'Ac. Imp. de St. Pétersbourg, t. xxviii. (7th ser.), No. 2, p. 66.[1259]Scheiner,Die Spectralanalyse der Gestirne, p. 234. Kayser (Astr. and Astroph., vol. xiii., p. 368) refers the anomalies of the carbon-spectrum in comets wholly to instrumental sources.[1260]Dewar,Proc. Roy. Inst., vol. xi., p. 541.[1261]Proc. R. Soc., vol. xv., p. 5;Month. Not., vol. xxvii., p. 288.[1262]Keeler,Astr. and Astrophysics, vol. xi., p. 929; Vogel,Astr. Nach., No. 3,142.[1263]Proc. Roy. Soc., vol. xxiii., p. 154.[1264]Hasselberg,loc. cit., p. 58.[1265]Ueber die Natur der Cometen, p. 112.[1266]Hasselberg,loc. cit., p. 38.

[1188]Month. Not., vol. xix., p. 27.

[1189]Mém. de l'Ac. Imp., t. ii., 1859, p. 46.

[1190]Harvard Annals, vol. iii., p. 368.

[1191]Ibid., p. 371.

[1192]Month. Not., vol. xxii., p. 306.

[1193]Stothard inIbid., vol. xxi., p. 243.

[1194]Intell. Observer, vol. i., p. 65.

[1195]Comptes Rendus, t. lxi., p. 953.

[1196]Smiths. Report, 1881 (Holden);Nature, vol. xxv., p. 94;Observatory, vol. xxi., p. 378 (W. T. Lynn).

[1197]Ueber den Ursprung der von Pallas gefundenen Eisenmassen, p. 24.

[1198]Arago,Annuaire, 1836, p. 294.

[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.

[1200]Am. Journ. of Sc., vol. xxvi., p. 132.

[1201]Annuaire, 1836, p. 297.

[1202]Ann. de l'Observ., Bruxelles, 1839, p. 248.

[1203]Ibid., 1837, p. 272.

[1204]Astr. Nach., Nos. 385, 390.

[1205]Am. Jour. of Sc., vol. xxxviii. (2nd ser.), p. 377.

[1206]Ibid., vol. xxxviii., p. 61.

[1207]Month. Not., vol. xxvii., p. 247.

[1208]Am. Jour. of Sc., vol. xliii. (2nd ser.), p. 87.

[1209]Grant,Month. Not., vol. xxvii., p. 29.

[1210]P. Smyth,Ibid., p. 256.

[1211]Hind,Ibid., p. 49.

[1212]Reproduced inLes Mondes, t. xiii.

[1213]Comptes Rendus, t. lxiv., p. 96.

[1214]Astr. Nach., No. 1,626.

[1215]Ibid., No. 1,632.

[1216]Month. Not., vol. xxxviii., p. 369.

[1217]Schiaparelli,Le Stelle Cadenti, p. 54.

[1218]Ueber Feuer-Meteore, p. 406.

[1219]Astr. Nach., No. 347 (Mädler); see also Boguslawski,Die Kometen, p. 98. 1857.

[1220]Nature, vol. vi., p. 148.

[1221]A. S. Herschel,Month. Not., vol. xxxii., p. 355.

[1222]Astr. Nach., Nos. 1,632, 1,633, 1,635.

[1223]Nature, vol. vii., p. 122.

[1224]A. S. Herschel,Report Brit. Ass., 1873, p. 390.

[1225]Humboldt,Cosmos, vol. i., p. 114 (Otté's trans.).

[1226]Month. Not., vol. xxxiii., p. 128.

[1227]Even this was denied by Bruhns,Astr. Nach., No. 2,054.

[1228]Am. Jour., vol. xxxi., p. 425.

[1229]Month. Not., vol. xlvi., p. 69.

[1230]In Schiaparelli's opinion, centuries must have elapsed while the observed amount of scattering was being produced.Le Stelle Cadenti, 1886, p. 112.

[1231]Astr. and Astroph., vol. xi., p. 943.

[1232]Bull. de l'Acad. St. Petersbourg, t. xxxv., p. 598. 1894.

[1233]Observatory, vol. xvi., p. 55.

[1234]Le Stelle Cadenti, p. 133;Rendiconti dell' Istituto Lombardo, t. iii., ser. ii., p. 23.

[1235]Denning,Memoirs Roy. Astr. Soc., vol. liii., p. 214; Abelmann,Astr. Nach., No. 3,516.

[1236]Proc. Roy. Soc., March 2, 1899;Nature, November 9, 1899.

[1237]Berberich,Astr. Nach., No. 3,526.

[1238]Elkin,Astroph. Jour., vol. ix., p. 22.

[1239]Elkin,Astroph. Jour., vol. x., p. 24.

[1240]Pop. Astr., September, 1897, p. 232.

[1241]Month. Not., vol. xx., p. 336.

[1242]Revue des deux Mondes, December 15, 1885, p. 889.

[1243]Palgrave,Phil. Trans., vol. cxxv., p. 175.

[1244]W. E. Hidden,Century Mag., vol. xxxiv., p. 534.

[1245]Amer. Jour. of Science, vol. xxxvi., p. i., 1888.

[1246]Revue des Questions Scientifiques, January, 1899, p. 194; Tisserand,Bull. Astr., t. viii., p. 460.

[1247]Month. Not., vol. xlv., p. 93.

[1248]Observatory, vol. viii., p. 4.

[1249]Denning,Month. Not., vol. xxxviii., p. 114.

[1250]Comptes Rendus, t. cix., p. 344.

[1251]Month. Not., vol. lix., p. 140.

[1252]Bull. de l'Acad. St. Petersb., t. xii., p. 95.

[1253]Publ. Astr. Pac. Soc., vol. iii., p. 114.

[1254]Month. Not., vol. lii., p. 341.

[1255]Astr. Nach., No. 1,488.

[1256]Annuaire, Paris, 1883, p. 185.

[1257]Phil. Trans., vol. clviii., p. 556.

[1258]Hasselberg,Mém. de l'Ac. Imp. de St. Pétersbourg, t. xxviii. (7th ser.), No. 2, p. 66.

[1259]Scheiner,Die Spectralanalyse der Gestirne, p. 234. Kayser (Astr. and Astroph., vol. xiii., p. 368) refers the anomalies of the carbon-spectrum in comets wholly to instrumental sources.

[1260]Dewar,Proc. Roy. Inst., vol. xi., p. 541.

[1261]Proc. R. Soc., vol. xv., p. 5;Month. Not., vol. xxvii., p. 288.

[1262]Keeler,Astr. and Astrophysics, vol. xi., p. 929; Vogel,Astr. Nach., No. 3,142.

[1263]Proc. Roy. Soc., vol. xxiii., p. 154.

[1264]Hasselberg,loc. cit., p. 58.

[1265]Ueber die Natur der Cometen, p. 112.

[1266]Hasselberg,loc. cit., p. 38.

RECENT COMETS(continued)

The mystery of comets' tails had been to some extent penetrated; so far, at least, that, by making certain assumptions strongly recommended by the facts of the case, their forms can be, with very approximate precision, calculated beforehand. We have, then, the assurance that these extraordinary appendages are composed of no ethereal or supersensual stuff, but of matter such as we know it, and subject to the ordinary laws of motion, though in a state of extreme tenuity.

Olbers, as already stated, originated in 1812 the view that the tails of comets are made up of particles subject to a force of electrical repulsion proceeding from the sun. It was developed and enforced by Bessel's discussion of the appearances presented by Halley's comet in 1835. He, moreover, provided a formula for computing the movement of a particle under the influence of a repulsive force of any given intensity, and thus laid firmly the foundation of a mathematical theory of cometary emanations. Professor W. A. Norton, of Yale College, considerably improved this by inquiries begun in 1844, and resumed on the apparition of Donati's comet; and Dr. C. F. Pape at Altona[1267]gave numerical values for the impulses outward from the sun, which must have actuated the materials respectively of the curved and straight tails adorning the same beautiful and surprising object.

Thephysicaltheory of repulsion, however, was, it might be said, still in the air. Nor did it even begin to assume consistency until Zöllner took it in hand in 1871.[1268]It is perfectly well ascertained that the energy of the push or pull produced by electricity depends (other things being the same) upon thesurfaceof the body acted on; that of gravity upon itsmass. The efficacy of solar electrical repulsion relatively to solar gravitational attraction grows, consequently, as the size of the particle diminishes. Make this small enough, and itwill virtually cease to gravitate, and will unconditionally obey the impulse to recession.

This principle Zöllner was the first to realise in its application to comets. It gives the key to their constitution. Admitting that the sun and they are similarly electrified, their more substantially aggregated parts will still follow the solicitations of his gravity, while the finely divided particles escaping from them will, simply by reason of their minuteness, fall under the sway of his repellent electric power. They will, in other words, form "tails." Nor is any extravagant assumption called for as to the intensity of the electrical charge concerned in producing these effects. Zöllner, in fact, showed[1269]that it need not be higher than that attributed by the best authorities to the terrestrial surface.

Forty years have elapsed since M. Brédikhine, director successively of the Moscow and of the Pulkowa Observatories, turned his attention to these curious phenomena. His persistent inquiries on the subject, however, date from the appearance of Coggia's comet in 1874. On computing the value of the repulsive force exerted in the formation of its tail, and comparing it with values of the same force arrived at by him in 1862 for some other conspicuous comets, it struck him that the numbers representing them fell into three well-defined classes. "I suspect," he wrote in 1877, "that comets are divisible into groups, for each of which the repulsive force is perhaps the same."[1270]This idea was confirmed on fuller investigation. In 1882 the appendages of thirty-six well-observed comets had been reconstructed theoretically, without a single exception being met with to the rule of the three types. A further study of forty comets led, in 1885, only to a modification of the numerical results previously arrived at.

In the first of these, the repellent energy of the sun is fourteen times stronger than his attractive energy;[1271]the particles forming the enormously long straight rays projected outward from this kind of comet leave the nucleus with a mean velocity of just seven kilometres per second, which, becoming constantly accelerated, carries them in a few days to the limit of visibility. The great comets of 1811, 1843, and 1861, that of 1744 (so far as its principal tail was concerned), and Halley's comet at its various apparitions, belonged to this class. Less narrow limits were assigned to the values of the repulsive force employed to produce the second type. For the axis of the tail, it exceeds by one-tenth (= 1·1) the power of solargravity; for the anterior edge, it is more than twice (2·2), for the posterior only half as strong. The corresponding initial velocity (for the axis) is 1,500 metres a second, and the resulting appendage a scimitar-like or plumy tail, such as Donati's and Coggia's comets furnished splendid examples of. Tails of the third type are constructed with forces of repulsion from the sun ranging from one-tenth to three-tenths that of his gravity, producing an accelerated movement of attenuated matter from the nucleus, beginning at the leisurely rate of 300 to 600 metres a second. They are short, strongly bent, brush-like emanations, and in bright comets seem to be only found in combination with tails of the higher classes. Multiple tails, indeed—that is, tails of different types emitted simultaneously by one comet—are perceived, as experience advances and observation becomes closer, to be rather the rule than the exception.[1272]

Now what is the meaning of these three types? Is any translation of them into physical fact possible? To this question Brédikhine supplied, in 1879, a plausible answer.[1273]It was already a current surmise that multiple tails are composed of different kinds of matter, differently acted on by the sun. Both Olbers and Bessel had suggested this explanation of the straight and curved emanations from the comet of 1807; Norton had applied it to the faint light tracks proceeding from that of Donati;[1274]Winnecke to the varying deviations of its more brilliant plumage. Brédikhine defined and ratified the conjecture. He undertook to determine (provisionally as yet) the several kinds of matter appropriated severally to the three classes of tails. These he found to be hydrogen for the first, hydro-carbons for the second, and iron for the third. The ground of this apportionment is that the atomic weights of these substances bear to each other the same inverse proportion as the repulsive forces employed in producing the appendages they are supposed to form; and Zöllner had pointed out in 1875 that the "heliofugal" power by which comets' tails are developed would, in fact, be effective just in that ratio.[1275]Hydrogen, as the lightest known element—that is, the least under the influence of gravity—was naturally selected as that which yielded most readily to the counter-persuasions of electricity. Hydro-carbons had been shown by the spectroscope to be present in comets, and were fitted by their specific weight, as compared with that of hydrogen, to form tails of the second type; while the atoms of iron were just heavy enough to compose those of the third, and, from the plentifulness of their presence in meteorites, might be presumed to enter, in no inconsiderable proportion, into the mass of comets.These three substances, however, were by no means supposed to be the sole constituents of the appendages in question. On the contrary, the great breadth of what, for the present, were taken to be characteristically "iron" tails was attributed to the presence of many kinds of matter of high and slightly different specific weights;[1276]while the expanded plume of Donati was shown to be, in reality, a whole system of tails, made up of many substances, each spreading into a separate hollow cone, more or less deviating from, and partially superposed upon the others.

Yet these felicities of explanation must not make us forget that the chemical composition attributed to the first type of cometary trains has, so far, received no countenance from the spectroscope. The emission lines of free, incandescent hydrogen have never been derived from any part of these bodies. Dissentient opinions, accordingly, were expressed as to the cause of their structural peculiarities. Ranyard,[1277]Zenker, and others advocated the agency of heat repulsion in producing them; Kiaer somewhat obscurely explains them through the evolution of gases by colliding particles;[1278]Herz of Vienna concludes tails to be mere illusory appendages produced by electrical discharges through the rare medium assumed to fill space.[1279]But Hirn[1280]conclusively showed that no such medium could possibly exist without promptly bringing ruin upon our "dædal earth" and its revolving companions.

On the whole, modern researches tend to render superfluous the chemical diversities postulated by Brédikhine. Electricity alone seems competent to produce the varieties of cometary emanation they were designed to account for. The distinction of types rests on a solid basis of fact, but probably depends upon differences rather in the mode of action than in the kind of substance acted upon. Suggestive sketches of electrical and "light-pressure" theories of comets have been published respectively by Mr. Fessenden of Alleghany,[1281]and by M. Arrhenius at Stockholm.[1282]Although evidently of a tentative character, they possess great interest.

Brédikhine's hypothesis was promptly and profusely illustrated. Within three years of its promulgation, five bright comets made their appearance, each presenting some distinctive peculiarity by which knowledge of these curious objects was materially helped forward. The first of these is remembered as the "Great SouthernComet." It was never visible in these latitudes, but made a short though stately progress through southern skies. Its earliest detection was at Cordoba on the last evening of January, 1880; and it was seen on February 1, as a luminous streak, extending just after sunset from the south-west horizon towards the pole, in New South Wales, at Monte Video, and the Cape of Good Hope. The head was lost in the solar rays until February 4, when Dr. Gould, then director of the National Observatory of the Argentine Republic at Cordoba, caught a glimpse of it very low in the west; and on the following evening, Mr. Eddie, at Graham's Town, discovered a faint nucleus, of a straw-coloured tinge, about the size of the annular nebula in Lyra. Its condensation, however, was very imperfect, and the whole apparition showed an exceedingly filmy texture. The tail was enormously long. On February 5 it extended—large perspective retrenchment notwithstanding—over an arc of 50°; but its brightness nowhere exceeded that of the Milky Way in Taurus. There was little curvature perceptible; the edges of the appendage ran parallel, forming a nebulous causeway from star to star; and the comparison to an auroral beam was appropriately used. The aspect of the famous comet of 1843 was forcibly recalled to the memory of Mr. Janisch, Governor of St. Helena; and the resemblance proved not merely superficial. But the comet of 1880 was less brilliant, and even more evanescent. After only eight days of visibility, it had faded so much as no longer to strike, though still discoverable by the unaided eye; and on February 20 it was invisible with the great Cordoba equatoreal pointed to its known place.

But the most astonishing circumstance connected with this body is the identity of its path with that of its predecessor in 1843. This is undeniable. Dr. Gould,[1283]Mr. Hind, and Dr. Copeland,[1284]each computed a separate set of elements from the first rough observations, and each was struck with an agreement between the two orbits so close as to render them virtually indistinguishable. "Can it be possible," Mr. Hind wrote to Sir George Airy, "that there is such a comet in the system, almost grazing the sun's surface in perihelion, and revolving in less than thirty-seven years. I confess I feel a difficulty in admitting it, notwithstanding the above extraordinary resemblance of orbits."[1285]

Mr. Hind's difficulty was shared by other astronomers. It would, indeed, be a violation of common-sense to suppose that a celestial visitant so striking in appearance had been for centuries back an unnoticed frequenter of our skies. Various expedients, accordingly, were resorted to for getting rid of the anomaly. The most promisingat first sight was that of the resisting medium. It was hard to believe that a body, largely vaporous, shooting past the sun at a distance of less than a hundred thousand miles from his surface, should have escaped powerful retardation. It must have passed through the very midst of the corona. It might easily have had an actual encounter with a prominence. Escape from such proximity might, indeed, very well have been judged beforehand to be impossible. Even admitting no other kind of opposition than that dubiously supposed to have affected Encke's comet, the result in shortening the period ought to be of the most marked kind. It was proved by Oppolzer[1286]that if the comet of 1843 had entered our system from stellar space with parabolic velocity it would, by the action of a medium such as Encke postulated (varying in density inversely as the square of the distance from the sun), have been brought down, by its first perihelion passage, to elliptic movement in a period of twenty-four years, with such rapid diminution that its next return would be in about ten. But such restricted observations as were available on either occasion of its visibility gave no sign of such a rapid progress towards engulfment.

Another form of the theory was advocated by Klinkerfues.[1287]He supposed that four returns of the same body had been witnessed within historical memory—the first in 371b.c., the next in 1668, besides those of 1843 and 1880; an original period of 2,039 years being successively reduced by the withdrawal at each perihelion passage of 1/1320 of the velocity acquired by falling from the far extremity of its orbit towards the sun, to 175 and 37 years. A continuance of the process would bring the comet of 1880 back in 1897.

Unfortunately, the earliest of these apparitions cannot be identified with the recent ones unless by doing violence to the plain meaning of Aristotle's words in describing it. He states that the comet was first seen "during the frosts and in the clear skies of winter," setting due west nearly at the same time as the sun.[1288]This implies some considerable north latitude. But the objects lately observed had practicallynonorth latitude. They accomplished their entire courseabovethe ecliptic in two hours and a quarter, during which space they were barely separated a hand's-breadth (one might say) from the sun's surface. For the purposes of the desired assimilation, Aristotle's comet should have appeared in March. It is not credible, however, that even a native of Thrace should have termed March "winter."

With the comet of 1668 the case seemed more dubious. The circumstances of its appearance are barely reconcilable with the identity attributed to it, although too vaguely known to render certainty one way or the other attainable. It might however, be expected that recent observations would at least decide the questions whether the comet of 1843 could have returned in less than thirty-seven, and whether the comet of 1880 was to be looked for at the end of 17-1/2 years. But the truth is that both these objects were observed over so small an arc—8° and 3° respectively—that their periods remained virtually undetermined. For while the shape and position of their orbits could be and were fixed with a very close approach to accuracy, the length of those orbits might vary enormously without any very sensible difference being produced in the small part of the curves traced out near the sun. Dr. Wilhelm Meyer, however, arrived, by an elaborate discussion, at a period of thirty-seven years for the comet of 1880,[1289]while the observations of 1843 were admittedly best fitted by Hubbard's ellipse of 533 years; but these Dr. Meyer supposed to be affected by some constant source of error, such as would be produced by a mistaken estimate of the position of the comet's centre of gravity. He inferred finally that, in spite of previous non-appearances, the two comets represented a single regular denizen of our system, returning once in thirty-seven years along an orbit of such extreme eccentricity that its movement might be described as one of precipitation towards and rapid escape from the sun, rather than of sedate circulation round it.

Thegeometricaltest of identity has hitherto been the only one which it was possible to apply to comets, and in the case before us it may fairly be said to have broken down. We may, then, tentatively, and with much hesitation, try aphysicaltest, though scarcely yet, properly speaking, available. We have seen that the comets of 1843 and 1880 were strikingly alike in general appearance, though the absence of a formed nucleus in the latter, and its inferior brilliancy, detracted from the convincing effect of the resemblance. Nor was it maintained when tried by exact methods of inquiry. M. Brédikhine found that the gigantic ray emitted in 1843 belonged to his type No. 1; that of 1880 to type No. 2.[1290]The particles forming the one were actuated by a repulsive force ten times as powerful as those forming the other. It is true that a second noticeably curved tail was seen in Chili, March 1, and at Madras, March 11, 1843; and the conjecture was accordingly hazarded that the materials composing on that occasion the principal appendage having become exhausted, those of the secondary one remained predominant, andreappeared alone in the "hydro-carbon" train of 1880. But the one known instance in point is against such a supposition. Halley's comet, the onlygreatcomet of which the returns have been securely authenticated and carefully observed, has preserved its "type" unchanged through many successive revolutions. The dilemma presented to astronomers by the Great Southern Comet of 1880 was unexpectedly renewed in the following year.

On the 22nd of May, 1881, Mr. John Tebbutt of Windsor, New South Wales, scanning the western sky, discerned a hazy-looking object which he felt sure was a strange one. A marine telescope at once resolved it into two small stars and a comet, the latter of which quickly attracted the keen attention of astronomers; for Dr. Gould, computing its orbit from his first observations at Cordoba, found it to agree so closely with that arrived at by Bessel for the comet of 1807 that he telegraphed to Europe, June 1, announcing the unexpected return of that body. So unexpected that theoretically it was not possible before the year 3346; and Bessel's investigation was one which inspired and eminently deserved confidence. Here, then, once more the perplexing choice had to be made between a premature and unaccountable reappearance and the admission of a plurality of comets moving nearly in the same path. But in this case facts proved decisive.

Tebbutt's comet passed the sun, June 16, at a distance of sixty-eight millions of miles, and became visible in Europe six days later. It was, in the opinion of some, the finest object of the kind since 1861. In traversing the constellation Auriga on itsdébutin these latitudes, it outshone Capella. On June 24 and some subsequent nights, it was unmatched in brilliancy by any star in the heavens. In the telescope, the "two interlacing arcs of light" which had adorned the head of Coggia's comet were reproduced; while a curiousdorsal spineof strong illumination formed the axis of the tail, which extended in clear skies over an arc of 20°. It belonged to the same "type" as Donati's great plume; the particles composing it being drivenfromthe sun by a force twice as powerful as that urging themtowardsit.[1291]But the appendage was, for a few nights, and by two observers perceived to be double. Tempel, on June 27, and Lewis Boss, at Albany (N.Y.), June 26 and 28, saw a long straight ray corresponding to a far higher rate of emission than the curved train, and shown by Brédikhine to be a member of the (so-called) hydrogen class. It had vanished by July 1, but made a temporary reappearance July 22.[1292]

The appendages of this comet were of remarkable transparency.Small stars shone wholly undimmed across the tail, and a very nearly central transit of the head over one of the seventh magnitude on the night of June 29, produced—if any change—an increase of brilliancy in the object of this spontaneous experiment.[1293]Dr. Meyer, indeed, at the Geneva Observatory, detected apparent signs of refractive action upon rays thus transmitted;[1294]but his observations remain isolated, and were presumably illusory.

The track pursued by this comet gave peculiar advantages for its observation. Ascending from Auriga through Camelopardus, it stood, July 19, on a line between the Pointers and the Pole, within 8° of the latter, and thus remained for a lengthened period constantly above the horizon of northern observers. Its brightness, too, was no transient blaze, but had a lasting quality which enabled it to be kept steadily in view during nearly nine months. Visible to the naked eye until the end of August, the last telescopic observation of it was made February 14, 1882, when its distance from the earth considerably exceeded 300 million miles. Under these circumstances, the knowledge acquired of its orbit was of more than usual accuracy, and showed conclusively that the comet was not a simple return of Bessel's; for this would involve a period of seventy-four years, whereas Tebbutt's comet cannot revisit the sun until after the lapse of two and a half millenniums.[1295]Nevertheless, the twin bodies move so nearly in the same path that an original connection of some kind is obvious; and the recent example of Biela readily suggested a conjecture as to what the nature of that connection might have been. The comets of 1807 and 1881 are, then, regarded with much probability as fragments of a primitive disrupted body, one following in the wake of the other at an interval of seventy-four years.

Imperfect photographs were taken of Donati's comet both in England and America;[1296]but Tebbutt's comet was the first to which the process was satisfactorily applied. The difficulties to be overcome were very great. The chemical intensity of cometary light is, to begin with, extraordinarily small. Janssen estimated it at 1/300000 of moonlight.[1297]Hence, if the ordinary process by which lunar photographs are taken had been applied to the comet of 1881, an exposure of at leastthree dayswould have been required in order to get an impression of the head with about a tenth part of the tail. Butby that time a new method of vastly increased sensitiveness had been rendered available, by which dry gelatine-plates were substituted for the wet collodion-plates hitherto in use; and this improvement alone reduced the necessary time of exposure to two hours. It was brought down to half an hour by Janssen's employment of a reflector specially adapted to give an image illuminated eight or ten times as strongly as that produced in the focus of an ordinary telescope.[1298]

The photographic feebleness of cometary rays was not the only obstacle in the way of success. The proper motion of these bodies is so rapid as to render the usual devices for keeping a heavenly body steadily in view quite inapplicable. The machinery by which the diurnal movement of the sphere is followed, must be especially modified to suit each eccentric career. This, too, was done, and on June 30, 1881, Janssen secured a perfect photograph of the brilliant object then visible, showing the structure of the tail with beautiful distinctness to a distance of 2-1/2° from the head. An impression to nearly 10° was obtained about the same time by Dr. Henry Draper at New York, with an exposure of 162 minutes.[1299]

Tebbutt's (or comet 1881 iii.) was also the first comet of which the spectrum was so much as attempted to be chemically recorded. Both Huggins and Draper were successful in this respect, but Huggins was more completely so.[1300]The importance of the feat consisted in its throwing open to investigation a part of the spectrum invisible to the eye, and so affording an additional test of cometary constitution. The result was fully to confirm the origin from carbon-compounds assigned to the visible rays, by disclosing additional bands belonging to the same series in the ultra-violet; as well as to establish unmistakably the presence of a not inconsiderable proportion of reflected solar light by the clear impression of some of the principal Fraunhofer lines. Thus the polariscope was found to have told the truth, though not the whole truth.

The photograph so satisfactorily communicative was taken by Sir William Huggins on the night of June 24; and on the 29th, at Greenwich, the tell-tale Fraunhofer lines were perceived to interrupt the visible range of the spectrum. This was at first so vividly continuous, that the characteristic cometary bands could scarcely be detached from their bright background. But as the nucleus faded towards the end of June, they came out strongly, and were more and more clearly seen, both at Greenwich and at Princeton, to agree, not with the spectrum of hydro-carbons glowing in a vacuum tube, but with that of the same substances burning in a Bunsen flame.[1301]It need not, however, be inferred that cometary materials are really in a state of combustion. This, from all that we know, may be called an impossibility. The additional clue furnished was rather to the manner of their electrical illumination.[1302]

The spectrum of the tail was, in this comet, found to be not essentially different from that of the head. Professor Wright of Yale College ascertained a large percentage of its light to be polarized in a plane passing through the sun, and hence to be reflected sunlight.[1303]A faint continuous spectrum corresponded to this portion of its radiance; but gaseous emissions were also present. At Potsdam, on June 30, the hydro-carbon bands were indeed traced by Vogel to the very end of the tail;[1304]and they were kept in sight by Young at a greater distance from the nucleus than the more equably dispersed light. There seems little doubt that, as in the solar corona, the relative strength of the two orders of spectra is subject to fluctuations.

The comet of 1881 iii. was thus of signal service to science. It afforded, when compared with the comet of 1807, the first undeniable example of two such bodies travelling so nearly in the same orbit as to leave absolutely no doubt of the existence of a genetic tie between them. Cometary photography came to its earliest fruition with it; and cometary spectroscopy made a notable advance by means of it. Before it was yet out of sight, it was provided with a successor.

At Ann Arbor Observatory, Michigan, on July 14, a comet was discovered by Dr. Schaeberle, which, as his claim to priority is undisputed, is often allowed to bear his name, although designated, in strict scientific parlance, comet 1881 iv. It was observed in Europe after three days, became just discernible by the naked eye at the end of July, and brightened consistently up to its perihelion passage, August 22, when it was still about fifty million miles from the sun. During many days of that month, the uncommon spectacle was presented of two bright comets circling together, though at widely different distances, round the North pole of the heavens. The newcomer, however, never approached the pristine lustre of its predecessor. Its nucleus, when brightest, was comparable to the star Cor Caroli, a narrow, perfectly straight ray proceeding from it to a distance of 10°. This was easily shown by Brédikhine to belong to the hydrogen type of tails;[1305]while a "strange, faint second tail, or bifurcation of the first one," observed by Captain Noble, August 24,[1306]fell into the hydro-carbon class of emanations. It was seen, August 22 and 24, by Dr. F. Terby of Louvain,[1307]as a short nebulous brush,like the abortive beginning of a congeries of curving trains; but appeared no more. Its well-attested presence was significant of the complex constitution of such bodies, and the manifold kinds of action progressing in them.

The only peculiarity in the spectrum of Schaeberle's comet consisted in the almost total absence of continuous light. The carbon-bands were nearly isolated and very bright. Barely from the nucleus proceeded a rainbow-tinted streak, indicative of solid or liquid matter, which, in this comet, must have been of very scanty amount. Its visit to the sun in 1881 was, so far as is known, the first. The elements of its orbit showed no resemblance to those of any previous comet, nor any marked signs of periodicity. So that, although it may be considered probable, we do notknowthat it is moving in a closed curve, or will ever again penetrate the precincts of the solar system. It was last seen from the southern hemisphere, October 19, 1881.

The third of a quartette of lucid comets visible within sixteen months, was discovered by Mr. C. S. Wells at the Dudley Observatory, Albany, March 17, 1882. Two days later it was described by Mr. Lewis Boss as "a great comet in miniature," so well defined and regularly developed were its various parts and appendages. Discernible with optical aid early in May, it was on June 5 observed on the meridian at Albany just before noon—an astronomical event of extreme rarity. Comet Wells, however, never became an object so conspicuous as to attract general attention, owing to its immersion in the evening twilight of our northern June.

But the study of its spectrum revealed new facts of the utmost interest. All the comets till then examined had been found (with the two transiently observed exceptions already mentioned) to conform to one invariable type of luminous emission. Individual distinctions there had been, but no specific differences. Now all these bodies had kept at a respectful distance from the sun; for of the great comet of 1880 no spectroscopic inquiries had been made. Comet Wells, on the other hand, approached its surface within little more than five million miles on June 10, 1882; and the vicinity had the effect of developing a novel feature in its incandescence.

During the first half of April its spectrum was of the normal type, though the carbon bands were unusually weak; but with approach to the sun they died out, and the entire light seemed to become concentrated into a narrow, unbroken, brilliant streak, hardly to be distinguished from the spectrum of a star. This unusual behaviour excited attention, and a strict watch was kept. It was rewarded at the Dunecht Observatory, May 27, by the discernment of what had never before been seen in a comet—theyellow ray of sodium.[1308]By June 1, this had kindled into a blaze overpowering all other emissions. The light of the comet was practically monochromatic; and the image of the entire head, with the root of the tail, could be observed, like a solar prominence, depicted, in its new saffron vesture of vivid illumination, within the jaws of an open slit.

At Potsdam, the bright yellow line was perceived with astonishment by Vogel on May 31, and was next evening identified with Fraunhofer's "D." Its character led him to infer a very considerable density in the glowing vapour emitting it.[1309]Hasselberg founded an additional argument in favour of the electrical origin of cometary light on the changes in the spectrum of comet Wells.[1310]For they were closely paralleled by some earlier experiments of Wiedemann, in which the gaseous spectra of vacuum tubes were at once effaced on the introduction of metallic vapours. It seemed as if the metal had no sooner been rendered volatile by heat, than it usurped the entire office of carrying the discharge, the resulting light being thus exclusively of its production. Had simple incandescence by heat been in question, the effect would have been different; the two spectra would have been superposed without prejudice to either. Similarly, the replacement of the hydro-carbon bands in the spectrum of the comet by the sodium line proved electricity to be the exciting agent. For the increasing thermal power of the sun might, indeed, have ignited the sodium, but it could not have extinguished the hydro-carbons.

Sir William Huggins succeeded in photographing the spectrum of comet Wells by an exposure of one hour and a quarter.[1311]The result was to confirm the novelty of its character. None of the ultra-violet carbon groups were apparent; but certain bright rays, as yet unidentified, had imprinted themselves. Otherwise the spectrum was strongly continuous, uninterrupted even by the Fraunhofer lines detected in the spectrum of Tebbutt's comet. Hence it was concluded that a smaller proportion of reflected light was mingled with the native emissions of the later arrival.

All that is certainly known about theextentof the orbit traversed by the first comet of 1882 is that it came from, and is now retreating towards, vastly remote depths of space. An American computer[1312]found a period indicated for it of no less than 400,000 years; A. Thraen of Dingelstädt arrived at one of 3617.[1313]Both are perhaps equally insecure.

We have now to give some brief account of one of the most remarkable cometary apparitions on record, and—with the single exception of that identified with the name of Halley—the most instructive to astronomers. The lessons learned from it were as varied and significant as its aspect was splendid; although from the circumstance of its being visible in general only before sunrise, the spectators of its splendour were comparatively few.

The discovery of a great comet at Rio Janeiro, September 11, 1882, became known in Europe through a telegram from M. Cruls, director of the observatory at that place. It had, however (as appeared subsequently), been already seen on the 8th by Mr. Finlay of the Cape Observatory, and at Auckland as early as September 3. A later, but very singularly conditioned detection, quite unconnected with any of the preceding, was effected by Dr. Common at Ealing. Since the eclipse of May 17, when a comet—named "Tewfik" in honour of the Khedive of Egypt—was caught on Dr. Schuster's photographs, entangled, one might almost say, in the outer rays of the corona, he had scrutinized the neighbourhood of the sun on the infinitesimal chance of intercepting another such body on its rapid journey thence or thither. We record with wonder that, after an interval of exactly four months, that infinitesimal chance turned up in his favour.

On the forenoon of Sunday, September 17, he saw a great comet close to, and rapidly approaching the sun. It was, in fact, then within a few hours of perihelion. Some measures of position were promptly taken; but a cloud-veil covered the interesting spectacle before mid-day was long past. Mr. Finlay at the Cape was more completely fortunate. Divided from his fellow-observer by half the world, he unconsciously finished, under a clearer sky, his interrupted observation. The comet, of which the silvery radiance contrasted strikingly with the reddish-yellow glare of the sun's margin it drew near to, was followed "continuously right into the boiling of the limb"—a circumstance without precedent in cometary history.[1314]Dr. Elkin, who watched the progress of the event with another instrument, thought the intrinsic brilliancy of the nucleus scarcely surpassed by that of the sun's surface. Nevertheless it had no sooner touched it than it vanished as if annihilated. So sudden was the disappearance (at 4h. 50m. 58s., Cape mean time), that the comet was at first believed to have passedbehindthe sun. But this proved not to have been the case. The observers at the Cape had witnessed a genuine transit. Nor could non-visibility be explained

PLATE III.

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