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.'* * 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 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, theResults 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 Galileo. 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 dimen- 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 *In some works on astronomy pictures are admitted in which the effects of the Sun's inclination are altogether exaggerated. FIG. 46. IV. Sept. 5. I. Dec. 6. II. March 6. III. June 5. |