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Herschel then states that he regards the spots as regions where the atmosphere is free from lucid clouds, the faculæ as regions where such clouds are more numerous than elsewhere. The penumbra being generally depressed about half-way between the level of the nucleus and that of the photosphere, must of course be fainter than other parts. No spot favourable for taking measures having lately been on the Sun,' he adds, I can only judge from former appearances that the regions in which the luminous solar clouds are formed, adding thereto the elevation of the faculæ, cannot be less than 1,843, nor much more than 2,765 miles in depth. It is true that in our atmosphere the extent of the clouds is limited to a very narrow compass; but we ought rather to compare the solar ones to the luminous decompositions which take place in our aurora borealis, or luminous arches, which extend much farther than the cloudy regions. The density of the luminous solar clouds, though very great, may not be exceedingly more so than that of our aurora borealis. For if we consider what would be the brilliancy of a space two or three thousand miles deep filled with such corruscations as we see now and then in our atmosphere, their apparent intensity, when viewed at the distance of the Sun, might not be much inferior to that of the lucid solar fluid.'

his descriptions, one is very apt to forget, when he is describing mere hypotheses, that he is not discussing established conclusions; and to this probably is due the fact that some of his warmest admirers do him the injustice of insisting as earnestly on views which he put forward simply as hypotheses, as though they had been enunciated by him as legitimate deductions from observed facts.

From the luminous atmosphere of the Sun, Herschel proceeds to the opaque body, which he surmises to be of great solidity, on account of the power it exerts upon the planets. From the phenomena of those dark spots which have been repeatedly seen in the same place,

and otherwise denote inequalities in their level,' he suggests that the Sun's surface is diversified with mountains and valleys.'

Then follows that remarkable passage which every student of astronomy knows by heart; but which yet (even though we may not accept-as I confess I do not-the opinions suggested in it) will well bear repetition :

The Sun, viewed in this light, appears to be nothing else than a very eminent, large, and lucid planet, evidently the first, or, in strictness of speaking, the only primary one of our system, all others being truly secondary to it. Its similarity to the other globes of the solar system with regard to its solidity, its atmosphere, and its diversified surface; the rotation upon its axis, and the fall of heavy bodies, lead us on to suppose that it is most probably also inhabited, like the rest of the planets, by beings whose organs are adapted to the peculiar circumstances of that vast globe. Whatever fanciful poets might say in making the Sun the abode of blessed spirits, or angry moralists devise in pointing it out as a fit place for the punishment of the wicked, it does not appear that they had any other foundations than mere opinion and vague surmise; but now I think myself authorised, upon

astronomical principles, to propose the Sun as an inhabitable world, and am persuaded that the foregoing observations, with the conclusions I have drawn from them, are fully sufficient to answer every objection that may be made against it.'

Herschel proceeds to consider the objection founded on the great heat which here at a distance of so many millions of miles we receive from the Sun, and the tremendous nature of the heat which consequently (one would suppose) must affect the imagined inhabitants of the Sun. Our admiration for the greatest astronomer of modern times must not cause us to lose sight of the fact that the reasoning at this stage of the inquiry is founded on inexact notions of the nature and laws of heat-though not such as in his day could have been unfavourably commented upon by most physicists. He remarks that the Sun's rays are the cause of the production of heat by uniting with the matter of fire which is contained in the substances that are heated." He then instances the snow-covered summits of lofty mountains, and the cold experienced by aeronauts ; and he concludes, that we have only to admit that on the Sun itself, the elastic fluids composing its atmosphere, and the matter on its surface, are of such a nature as not to be capable of any excessive affection from its own rays, which seems indeed to be proved by their copious emission.'*

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*After noting other possible objections, Sir William Herschel-who did not disdain at times to be as imaginative and fanciful in theorising as he was exact and scrupulous in observing-proceeds to consider the possibility that the inhabitants of the Moon and of the satellites of

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 primary orbs round which they travel as mere attractive 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 true 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 stored 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

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