Page images

In a later paper (communicated to the Royal Society in 1801) Sir William Herschel records the results of further observations. He draws special attention to certain characteristic features of the Sun's surface. These are, first, corrugations, which he regards as elevations and depressions causing the mottled appearance of the Sun; secondly, nodules, or smaller elevations in the corrugations themselves over which they are distributed as bright spots; punctulations, or dark spaces between the nodules; and pores, or 'darkercoloured places in the punctulations. He also enters into many particulars as to the behaviour of spots, pores, corrugations, nodules, and so on. To this valuable paper, as to the other, from which I have quoted, I would invite the attention of every reader

Jupiter, Saturn, and Uranus, regard the primnry orbs round which they travel as mere attractire centres, to keep together their orbits, to direct their revolution round the Sun, and to supply them with reflected light in the absence of direct illumination. 'Ought we not,' he asks, 'to condemn their ignorance, as proceeding from want of attention and proper reflection? It is very tr^e that the Earth and those other planets that have satellites about them, perform all the offices that have been named for the inhabitants of these little globes; but to us, who live upon one of these planets, their reasonings cannot but appear very defective, when we see what a magnificent dwelling-place the Earth affords to numberless intelligent beings. These considerations ought to make the inhabitants of the planets wiser than we have supposed those of their satellites to be. We surely ought not, like them, to say, 'The Sun (that immense globe, whose body would much more than fill the whole orbit of the Moon) is merely an attractive centre to us. From experience we can affirm that the performance of the most salutary offices to inferior planets is not inconsistent with the dignity of superior purposes; and in consequence of such analogical reasonings, assisted by telescopic views, which plainly favour the same opinion, we need not hesitate to admit that the Sun is richly sto'ed with inhabitants.'

interested in solar physics. Here I shall only quote two observations bearing on the periodicity of the disturbances which affect the Sun's surface. We have seen that in 1671 Cassini had for a long time noticed the absence of Sun-spots. But on July 5, 1795, Sir William Herschel remarked that the Sun presented an appearance far more remarkable, and such, he remarks, as differed wholly 'from what he had ever seen before. There was not a single opening in the whole disc; there were no ridges or nodules, and no corrugations.' On December 9, 1798, he noticed that a similar state of things prevailed.

We may sum up as follows the views of Sir William Herschel as to the general constitution of the solar globe and surface :—He supposed the Sun to be an opaque globe surrounded by a luminous envelope. He considered that this envelope is neither fluid nor gaseous, but consists rather of luminous clouds floating in a transparent atmosphere. Beneath this layer or envelope of luminous clouds he conceived that there floats in the same atmosphere a layer of opaque clouds, rendered luminous on the outside by the light which they receive from the outer layer. These opaque clouds protect, according to this theory, the solid and relatively unilluminated nucleus of the Sun. When openings are formed in the same region in both layers of clouds, we see the body of the Sun as a dark spot. If the apertures are equally large, the spot will be uniformly dark; but if, as more commonly happens, the outer aperture is the greater, the dark nucleus of the spot will seem to be surrounded by a dusky border. If the upper layer alone is perforated, a dusky spot -without any dark central portion makes its appearance. Herschel supposed that those spots in which both layers are broken through, are caused by an uprush of some highly elastic gas breaking its way through the lower layer, and then, after expansion, removing the upper self-luminous clouds.

AVe shall see that while all the facts observed by Herschel have been confirmed, and while his reasoning, so far as it relates to observed facts, has been abundantly justified, some of his hypotheses have been disproved by recent observations.

I pass on next to the researches of Sir John Herschel, recorded in that store-house of valuable facts, the ' Results of Astronomical Observations at the South Cape.'

Sir John Herschel's observations led him to pay particular attention to a feature of the solar surface which had been first noted by GaHeo. The spots are confined to two definite zones, extending about 35°* on each side of the equator; an intermediate zone to a distance of some 8° on either side of the solar equator being ordinarily free from spots. Fig. 46 serves to indicate the regions where spots occur, and also (where

* If wo may trust an observation of La Hire's (which, however, Mr. Carrington, than whom no higher authority can be cited, is disposed to reject), a spot has been seen as far as "O3 from the Sun's equator. In 1846, Dr. Peters, of Altona, saw a spot 50° 55' from the equator, while Carrington and Capocci have each seen spots about l!>° from that circle.

Fin. -16.

the darkest zones are shown) those regions in
spots occur most frequently
and attain the greatest dimen-
sions. The Sun is so placed
in the four views as to show
the way in which the spot-
belts are actually presented—
I. early in December; II early
in March; III. early in June;
and IV. early in September.
The actual dates are those
indicated under the several fi-
gures. I shall have to discuss
further on those researches of
Carrington and others into
the laws of the Sun's rota-
tion and the position of the
solar equator * on which these

* It is surprising that in Loekyer's Elementary Lessons of Astronomy, not only are pictures admitted in which the effects of the Sun's inclination are altogether exaggerated, but the author actually states that in September' and March (corresponding to figs. 3 and 4) the paths of the spots are observed to be sharply curved. This is the more astonishing, because when that book was published, Mr. lockyer had long been an observer of solar phenomena; and the slight nature of the curvature is a peculiarity which can hardly escape notice, even though observations are continued only for a few days in September or in March.

which dates and the presentations shown in fig. 46 are founded.


It was to the explanation of this peculiarity that Sir John Herschel directed his chief attention. He remarks that the very existence of these zones ' at once refers the cause of spots to fluid circulations, modified, if not produced, by the Sun's rotation, by reasoning of the very same kind whereby we connect our own system of trade and anti-trade winds with the Earth's rotation. Having given any exciting cause for the circulation of atmospheric fluids from the poles to the equator and back again, or vice versa, the effect of rotation will necessarily be to modify those currents as our tradewinds and monsoons are modified, and to dispose all those meteorological phenomena (on a great scale) which accompany them as their visible manifestations, in zones parallel to the equator with a calm equatorial zone interposed.' Thus far, be it observed, Sir John Herschel is dealing with observed facts, and pointing to almost inevitable conclusions. He passes on (following in this respect, as in so many others, the procedure of his father) to hypothetical considerations, in dealing with the question—Whether any cause of atmospheric circulation can be found in the economy of the Sun, ' so far as we know and can comprehend it?'

He is thus led to the inquiry, whether a transparent atmosphere extends beyond the luminous surface of the Sun. He mentions the deficiency of light at the borders of the visible disc of the Sun, remarking that this feature is so obvious that he is surprised it should

« PreviousContinue »