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.

We 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 Galleo. 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 we 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 70' 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 45° from that circle.

the darkest zones are shown) those regions in which

spots occur most frequently

and attain the greatest dimensions. The Sun is so placed in the four views as to show the way in which the spotbelts are actually presentedI. 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 figures. I shall have to discuss further on those researches of Carrington and others into the laws of the Sun's rotation and the position of the solar equator on which these

* It is surprising that in Lockyer'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.

FIG. 46.

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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

ever have been controverted.

6

He mentions in corro

boration the extraordinary phenomenon of the rosecoloured solar clouds witnessed during the total eclipse of July 8, 1842, which must have floated in, and been sustained by, an exterior transparent atmosphere." And he suggests that this atmosphere must extend to some distance beyond the visible disc, because the darkening of the solar disc is not limited to the immediate neighbourhood of the edge but extends some distance within the disc.t

Assuming the existence of such an atmosphere, the rotation of the Sun would cause the outer surface of the atmosphere to take up an oblately spheroidal figure, the least axis of which would correspond with the polar axis of the Sun. Consequently, the equatorial portions of this envelope must be of a thickness different from that of the polar, density for density, so that a different obstacle must be thereby opposed to the escape of heat from the equatorial and the polar regions of the Sun. The former therefore ought,

*When Sir John Herschel thus wrote, less thoughtful astronomers were questioning whether these prominences belong to the Sun, whether they may not be lunar mirages, or phenomena of the Earth's atmosphere, or finally, whether they have any existence at all.

This argument, however, is not strictly sound. The extension of the darkening over the disc indicates shallowness rather than depth. This is easily seen, if we consider that were the atmosphere indefinitely deep, the luminosity of the disc would be uniform. In the actual case, of course, the atmosphere is not uniformly dense; but still the reasoning is analogous, and the extension of the darkening over the disc implies shallowness rather than the reverse; though not the same degree of shallowness as would follow in the case of an atmosphere of uniform density.

according to this reasoning, to be habitually maintained at a different temperature from the latter.' The spots,' adds Sir John Herschel,' would come, on this view of the subject, to be assimilated to those regions on the Earth's surface in which, for the moment, hurricanes and tornadoes prevail-the upper stratum being temporarily carried downwards, displacing by its impetus the two strata of luminous matter beneath (which may be conceived as forming an habitually tranquil limit between the opposite upper and under currents), the upper of course to a greater extent than the lower, and thus wholly or partially denuding the opaque surface of the Sun below. Such processes cannot be unaccompanied by vorticose motions, which, left to themselves, die away by degrees and dissipate-with this peculiarity, that their lower portions come to rest more speedily than their upper, by reason of the greater resistance below, as well as the remoteness from the point of action, which lies in a higher region, so that their centre (as seen in our water-spouts, which are nothing but small tornadoes) appears to retreat upwards. Now, this agrees perfectly with what is observed during the obliteration of the solar spots, which appear as if filled in by the collapse of their sides, the penumbra closing in upon the spot, and disappearing after it.'

With all deference to one who is as high an authority in meteorological and thermological questions (which are both involved in this matter) as in astronomical matters, I must venture to point out what appears to me a flaw in the reasoning by which an excess of heat