Again, the central pa.s.sage of an enormous spot on September 9, 1898, synchronised with a sharp magnetic disturbance and brilliant aurora;[479] and the coincidence was substantially repeated in March, 1899,[480] when it was emphasised by the prevalent cosmic calm. The theory of the connection is indeed far from clear. Lord Kelvin, in 1892,[481] p.r.o.nounced against the possibility of any direct magnetic action by the sun upon the earth, on the ground of its involving an extravagant output of energy; but the fact is unquestionable that--in Professor Bigelow's words--"abnormal agitations affect the sun and the earth as a whole and at the same time."[482]

The nearer approach to the event of September 1, 1859, was photographically observed by Professor George E. Hale at Chicago, July 15, 1892.[483] An active spot in the southern hemisphere was the scene of this curiously sudden manifestation. During an interval of 12m.

between two successive exposures, a bridge of dazzling light was found to have spanned the boundary-line dividing the twin-nuclei of the spot; and these, after another 27m., were themselves almost obliterated by an overflow of far-spreading brilliancy. Yet two hours later, no trace of the outburst remained, the spot and its attendant faculae remaining just as they had been previously to its occurrence. Unlike that seen by Carrington, it was accompanied by no exceptional magnetic phenomena, although a "storm" set in next day.[484] Possibly a terrestrial a.n.a.logue to the former might be discovered in the "auroral beam" which traversed the heavens during a vivid display of polar lights, November 17, 1882, and shared, there is every reason to believe, their electrical origin and character.[485]

Meantime M. Rudolf Wolf, transferred to the direction of the Zurich Observatory, where he died, December 6, 1893, had relaxed none of his zeal in the investigation of sun-spot periodicity. A laborious revision of the entire subject with the aid of fresh materials led him, in 1859,[486] to the conclusion that while the _mean_ period differed little from that arrived at in 1852 of 11.11 years, very considerable fluctuations on either side of that mean were rather the rule than the exception. Indeed, the phrase "sun-spot period" must be understood as fitting very loosely the great fact it is taken to represent; so loosely, that the interval between two maxima may rise to sixteen and a half or sink below seven and a half years.[487] In 1861[488] Wolf showed, and the remark was fully confirmed at Kew, that the shortest periods brought the most acute crises, and _vice versa_; as if for each wave of disturbance a strictly equal amount of energy were available, which might spend itself lavishly and rapidly, or slowly and parsimoniously, but could in no case be exceeded. The further inclusion of recurring solar commotions within a cycle of fifty-five and a half years was simultaneously pointed out; and Hermann Fritz showed soon afterwards that the aurora borealis is subject to an identical double periodicity.[489] The same inquirer has more recently detected both for aurorae and sun-spots a "secular period" of 222 years,[490] and the Kew observations indicate for the latter, oscillations accomplished within twenty-six and twenty-four days,[491] depending, most likely, upon the rotation of the sun. This is certainly reflected in magnetic, and perhaps in auroral periodicity. The more closely, in fact, spot-fluctuations are looked into, the more complex they prove. Maxima of one order are superposed upon, or in part neutralised by, maxima of another order;[492] originating causes are masked by modifying causes; the larger waves of the commotion are indented with minor undulations, and these again crisped with tiny ripples, while the whole rises and falls with the swell of the great secular wave, scarcely perceptible in its progress because so vast in scale.

The idea that solar maculation depends in some way upon the position of the planets occurred to Galileo in 1612.[493] It has been industriously sifted by a whole bevy of modern solar physicists. Wolf in 1859[494]

found reason to believe that the eleven-year curve is determined by the action of Jupiter, modified by that of Saturn, and diversified by influences proceeding from the earth and Venus. Its tempting approach to agreement with Jupiter's period of revolution round the sun, indeed, irresistibly suggested a causal connection; yet it does not seem that the most skilful "coaxing" of figures can bring about a fundamental harmony. Carrington pointed out in 1863, that while, during _eight successive periods_, from 1770 downwards, there were approximate coincidences between Jupiter's aphelion pa.s.sages and sun-spot maxima, the relation had been almost exactly reversed in the two periods preceding that date;[495] and Wolf himself finally concluded that the Jovian origin must be abandoned.[496] M. Duponchel's[497] prediction, nevertheless, of an abnormal r.e.t.a.r.dation of the maximum due in 1881 through certain peculiarities in the positions of Ura.n.u.s and Neptune about the time it fell due, was partially verified by the event, since, after an abortive phase of agitation in April, 1882, the final outburst was postponed to January, 1894. The interval was thus 13.5 instead of 11.1 years; and it is noticeable that the delay affected chiefly the southern hemisphere. Alternations of activity in the solar hemispheres were indeed a marked feature of the maximum of 1884, which, in M. Faye's view,[498] derived thence its indecisive character, while sharp, strong crises arise with the simultaneous advance of agitation north and south of the solar equator. The curve of magnetic disturbance followed with its usual strict fidelity the anomalous fluctuations of the sun-spot curve. The ensuing minimum occurred early in 1889, and was succeeded in 1894 by a maximum slightly less feeble than its predecessor.[499]

It cannot be said that much progress has been made towards the disclosure of the cause, or causes, of the sun-spot cycle. No external influence adequate to the effect has, at any rate, yet been pointed out.

Most thinkers on this difficult subject provide a quasi-explanation of the periodicity in question through certain a.s.sumed vicissitudes affecting internal processes;[500] Sir Norman Lockyer and E. von Oppolzer reach the same end by establishing self-compensatory fluctuations in the solar atmospheric circulation; Dr. Schuster resorts to changes in the electrical conductivity of s.p.a.ce near the sun.[501] In all these theories, however, the course of transition is arbitrarily arranged to suit a period, which imposes itself as a fact peremptorily claiming admittance, while obstinately defying explanation.

The question so much discussed, as to the influence of sun-spots on weather, does not admit of a satisfactory answer. The facts of meteorology are too complex for easy or certain cla.s.sification. Effects owning dependence on one cause often wear the livery of another; the meaning of observed particulars may be inverted by situation; and yet it is only by the collection and collocation of particulars that we can hope to reach any general law. There is, however, a good deal of evidence to support the opinion--the grounds for which were primarily derived from the labours of Dr. Meldrum at Mauritius--that increased rainfall and atmospheric agitation attend spot-maxima; while Herschel's conjecture of a more copious emission of light and heat about the same epochs has recently obtained some countenance from Savelieff's measures showing a gain in the strength of the sun's radiation _pari pa.s.su_ with increase in the number of spots visible on his surface.[502]

The examination of what we may call the _texture_ of the sun's surface derived new interest from a remarkable announcement made by Mr. James Nasmyth in 1862.[503] He had made (as he supposed) the discovery that the entire luminous stratum of the sun is composed of a mult.i.tude of elongated shining objects on a darker background, shaped much like willow-leaves, of vast size, crossing each other in all possible directions, and possessed of unceasing relative motions. A lively controversy ensued. In England and abroad the most powerful telescopes were directed to a scrutiny encompa.s.sed with varied difficulties. Mr.

Dawes was especially emphatic in declaring that Nasmyth's "willow-leaves" were nothing more than the "nodules" of Sir William Herschel seen under a misleading aspect of uniformity; and there is little doubt that he was right. It is, nevertheless, admitted that something of the kind may be seen in the penumbrae and "bridges" of spots, presenting an appearance compared by Dawes himself in 1852 to that of a piece of coa.r.s.e straw-thatching left untrimmed at the edges.[504]

The term "granulated," suggested by Dawes in 1864,[505] best describes the mottled aspect of the solar disc as shown by modern telescopes and cameras. The grains, or rather the "floccules," with which it is thickly strewn, have been resolved by Langley, under exceptionally favourable conditions, into "granules" not above 100 miles in diameter; and from these relatively minute elements, composing, jointly, about one-fifth of the visible photosphere,[506] he estimates that three-quarters of the entire light of the sun are derived.[507] Janssen agrees, so far as to say that if the whole surface were as bright as its brightest parts, its luminous emission would be ten to twenty times greater than it actually is.[508]

The rapid changes in the forms of these solar cloud-summits are beautifully shown in the marvellous photographs taken by Janssen at Meudon, with exposures reduced at times to 1/100000 of a second! By their means, also, the curious phenomenon known as the _reseau photospherique_ has been made evident.[509] This consists in the diffusion over the entire disc of fleeting blurred patches, separated by a reticulation of sharply-outlined and regularly-arranged granules. The imperfect definition in the smudged areas may be due to agitations in the solar or terrestrial atmosphere, unless it be--as Dr. Schemer thinks possible[510]--merely a photographic effect. M. Janssen considers that the photospheric cloudlets change their shape and character with the progress of the sun-spot period;[511] but this is as yet uncertain.

The "grains," or more brilliant parts of the photosphere, are now generally held to represent the upper termination of ascending and condensing currents, while the darker interstices (Herschel's "pores") mark the positions of descending cooler ones. In the penumbrae of spots, the glowing streams rushing up from the tremendous sub-solar furnace are bent sideways by the powerful indraught, so as to change their vertical for a nearly horizontal motion, and are thus taken, as it were, in flank by the eye, instead of being seen end-on in mamelon-form. This gives a plausible explanation of the channelled structure of penumbrae which suggested the comparison to a rude thatch. Accepting this theory as in the main correct, we perceive that the very same circulatory process which, in its spasms of activity, gives rise to spots, produces in its regular course the singular "marbled" appearance, for the recording of which we are no longer at the mercy of the fugitive or delusive impressions of the human retina. And precisely this circulatory process it is which gives to our great luminary its permance as a _sun_, or warming and illuminating body.

FOOTNOTES:

[Footnote 405: _Mem. R. A. S._, vol. xxi., p. 157.]

[Footnote 406: _Ibid._, p. 160.]

[Footnote 407: _Month. Not._, vol. xxi., p. 144.]

[Footnote 408: _Le Soleil_, t. i., pp. 87-90 (2nd ed., 1871).]

[Footnote 409: See _ante_, p. 58.]

[Footnote 410: _Observations at Redhill (1863)_, Introduction.]

[Footnote 411: _Month. Not._, vol. x.x.xvi., p. 142.]

[Footnote 412: _Cape Observations_, p. 435, _note_.]

[Footnote 413: _Month. Not._, vol. x., p. 158.]

[Footnote 414: _Rosa Ursina_, lib. iii., p. 348.]

[Footnote 415: _Observations at Redhill_, p. 8.]

[Footnote 416: _Op._, t. iii., p. 402.]

[Footnote 417: _Rosa Ursina_, lib. iv., p. 601. Both Galileo and Scheiner spoke of the _apparent_ or "synodical" period, which is about one and a third days longer than the _true_ or "sidereal" one. The difference is caused by the revolution of the earth in its...o...b..t in the same direction with the sun's rotation on its axis.]

[Footnote 418: _Rosa Ursina_, lib. iii., p. 260.]

[Footnote 419: Faye, _Comptes Rendus_, t. lx., p. 818.]

[Footnote 420: _Ibid._, t. xii., p. 648.]

[Footnote 421: _Proc. Am. a.s.s. Adv. of Science_, 1885, p. 85.]

[Footnote 422: _Observations at Redhill_, p. 221.]

[Footnote 423: _Am. Jour. of Science_, vol. xi., p. 169.]

[Footnote 424: _Month. Not._, vol. xix., p. 1.]

[Footnote 425: _Vierteljahrsschrift der Naturfors. Gesellschaft_ (Zurich), 1859, p. 252.]

[Footnote 426: Lockyer, _Chemistry of the Sun_, p. 428.]

[Footnote 427: Maunder, _Knowledge_, vol. xv., p. 130.]

[Footnote 428: _Month. Mon._, vol. l., p. 251.]

[Footnote 429: Maunder, _Knowledge_, vol. xvii., p. 173.]

[Footnote 430: _Astr. Nach._, No. 1,315.]

[Footnote 431: As late as 1866 an elaborate treatise in its support was written by F. Coyteux, ent.i.tled _Qu'est-ce que le Soleil? Peut-il etre habite?_ and answering the question in the affirmative.]

[Footnote 432: The subsequent researches of Plucker, Frankland, Wullner, and others, showed that gases strongly compressed give an absolutely unbroken spectrum.]

[Footnote 433: _Comptes Rendus_, t. lx., pp. 89, 138.]

[Footnote 434: _Ibid._, t. c., p. 595.]

[Footnote 435: _Bull. Meteor. dell Osservatorio dell Coll. Rom._, Jan.

1, 1864, p. 4.]

[Footnote 436: _Quart. Jour. of Science_, vol. i., p. 222.]

[Footnote 437: _Ann. de Chim. et de Phys._, t. xxii., p. 127.]

[Footnote 438: _Phil. Trans._, vol. clix., p. 575.]

[Footnote 439: _Les Mondes_, Dec. 22, 1864, p. 707.]

[Footnote 440: _Comptes Rendus_, t. lx., p. 147.]