Part 49 (1/2)
[Footnote 1259: Scheiner, _Die Spectrala.n.a.lyse 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.]
[Footnote 1260: Dewar, _Proc. Roy. Inst._, vol. xi., p. 541.]
[Footnote 1261: _Proc. R. Soc._, vol. xv., p. 5; _Month. Not._, vol.
xxvii., p. 288.]
[Footnote 1262: Keeler, _Astr. and Astrophysics_, vol. xi., p. 929; Vogel, _Astr. Nach._, No. 3,142.]
[Footnote 1263: _Proc. Roy. Soc._, vol. xxiii., p. 154.]
[Footnote 1264: Ha.s.selberg, _loc. cit._, p. 58.]
[Footnote 1265: _Ueber die Natur der Cometen_, p. 112.]
[Footnote 1266: Ha.s.selberg, _loc. cit._, p. 38.]
CHAPTER XI
_RECENT COMETS_ (_continued_)
The mystery of comets' tails had been to some extent penetrated; so far, at least, that, by making certain a.s.sumptions strongly recommended by the facts of the case, their forms can be, with very approximate precision, calculated beforehand. We have, then, the a.s.surance 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.
The _physical_ theory of repulsion, however, was, it might be said, still in the air. Nor did it even begin to a.s.sume consistency until Zollner 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 the _surface_ of the body acted on; that of gravity upon its _ma.s.s_. 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 it will virtually cease to gravitate, and will unconditionally obey the impulse to recession.
This principle Zollner was the first to realise in its application to comets. It gives the key to their const.i.tution. 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 a.s.sumption called for as to the intensity of the electrical charge concerned in producing these effects. Zollner, 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. Bredikhine, 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 cla.s.ses. ”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 princ.i.p.al tail was concerned), and Halley's comet at its various apparitions, belonged to this cla.s.s. Less narrow limits were a.s.signed to the values of the repulsive force employed to produce the second type. For the axis of the tail, it exceeds by one-tenth (= 11) the power of solar gravity; for the anterior edge, it is more than twice (22), 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 cla.s.ses.
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 Bredikhine 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. Bredikhine defined and ratified the conjecture. He undertook to determine (provisionally as yet) the several kinds of matter appropriated severally to the three cla.s.ses 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 Zollner 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 ma.s.s of comets. These three substances, however, were by no means supposed to be the sole const.i.tuents 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 a.s.sumed to fill s.p.a.ce.[1279] But Hirn[1280] conclusively showed that no such medium could possibly exist without promptly bringing ruin upon our ”daedal earth” and its revolving companions.
On the whole, modern researches tend to render superfluous the chemical diversities postulated by Bredikhine. 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.
Bredikhine's hypothesis was promptly and profusely ill.u.s.trated. 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 Southern Comet.” It was never visible in these lat.i.tudes, 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 astonis.h.i.+ng circ.u.mstance connected with this body is the ident.i.ty 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.