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distances separating these dotted lines, and must needs therefore be such' as are shown in the figure. The shape of the depression would also be as shown. We see, then, that to an eye watching a depression of this sort as it rotated from the position A' to the position c', changes of shape would be shown which correspond exactly with those recognised by Dr.' Wilson.

This is the proper place to point out, however,

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that Dr. Wilson's observations were insufficient to demonstrate that a spot is a region which lies as a whole beneath the solar surface. They show only that the nucleus of the spot he observed lay at a lower level than the penumbra. It is evident from fig. 43 that appearances precisely corresponding to those observed by Wilson would be seen if spots were caused by a double layer of clouds in the solar atmosphere, the lower opaque, the upper semi-transparent and extending on all sides beyond the limits covered by the lower. Such an interpretation has indeed been put forward in recent times by Kirchhoff, and is still maintained, I believe, by Sporer.

Wilson's physical interpretation of the occurrence of these solar depressions need not detain us here. They have not been regarded as so successful as his geometrical analysis of the observed phenomena. It is only just to add that he himself did not attach equal weight to them; for in answer to objections urged by Lalande to his theory that the spots are depressions, "Wilson wrote thus in 1783:—' Whether their first production and subsequent numberless changes depend upon the eructation of elastic vapours from below, or upon eddies or whirlpools commencing at the surface, or upon the dissolving of this luminous matter in the solar atmosphere, as clouds are melted and again given out by our air, or, if the reader pleases, upon the annihilation and reproduction of parts of this resplendent covering, is left for theory to guess at.'

Passing also over the theories of Bode, which differed in no important respect from those of Wilson, let us turn now to the observations and theories of the greatest observational astronomer the world has ever known—Sir William Herschel. I propose to treat at considerable length what he has advanced upon the subject of the Sun, partly because of the great value of his work, but partly also to reclaim for him many discoveries which have been assigned to later observers. What these are I need not in every instance particularise; but it seems to me essential that the original observation of facts of so much interest and importance, only determinable by long and patient scrutiny of the Sun, should be assigned to their proper place.

Let me begin by quoting the fine passage in which Sir William Herschel speaks of the central luminary of our system.

'Among the celestial bodies,' he says, 'the Sun is certainly the first which should attract our notice. It is a fountain of light that illuminates the world! it is the cause of that heat which maintains the productive power of nature, and makes the Earth a fit habitation for man! it is the central body of the planetary system; and what renders a knowledge of its nature still more interesting to us, is that the numberless stars which compose the universe, appear by the strictest analogy to be similar bodies. Their innate light is so intense, that it reaches the eye of the observer from the remotest region of space, and forcibly claims his notice.'

Next let us hear his summary of the theories which had been put forward respecting the physical constitution of the Sun. I may note in passing that the opening remarks are as applicable in the present day as when Herschel wrote them :—' I should not wonder,' says the great astronomer, 'if we were induced to think that nothing remained to complete our knowledge; and yet it will not be difficult to show that we are still very ignorant, at least with regard to the internal constitution of the Sun. The various conjectures which have been formed on this subject are evident marks of the uncertainty under which we have hitherto laboured. The dark spots in the Sun, for instance, have been supposed to be solid bodies revolving very near its surface. They have been conjectured to be the smoke of volcanoes, or the scum floating upon an ocean of fluid matter. They have also been taken for clouds. They were explained to be opaque masses, swimming in the fluid matter of the Sun; dipping down occasionally. It has been supposed that a fiery liquid surrounded the Sun, and that, by its ebbing and flowing, the highest parts of it were occasionally uncovered, and appeared under the shape of dark spots; and that by the return of this fiery liquid they were again covered, and in that manner successively assumed different phases. The Sun itself has been called a globe of fire, though perhaps metaphorically. The waste it would undergo by a gradual consumption, on the supposition of its being ignited, has been ingeniously calculated. And in the same point of view its immense power of heating the bodies of such comets as draw very near to it has been assigned.' 'In supporting,' he proceeds, ' the ideas I shall propose in this paper with regard to the physical constitution of the Sun, I have availed myself of the labours of all these astronomers, but have been induced thereto only by my own actual observation of the solar phenomena, which, besides verifying those particulars that had been already observed, gave me such views of the solar regions as led to the foundation of a very rational

system. For having the advantage of former observations, my latest reviews of the body of the Sun were immediately directed to the most essential points; and the work was by this means facilitated and contracted into a pretty narrow compass.'

'In the year 1779,' he begins, ' there was a spot on the Sun which was large enough to be seen with the naked eye. By a view of it with a 7-feet reflector, charged with a very high power, it appeared to be divided into two parts. The largest of the two on April 19 measured 1' 8"-06 in diameter, which is equal in length to more than 31,000 miles. Both together must certainly have exceeded 50,000. The idea of its being occasioned by a volcanic explosion, violently driving away a fiery fluid, which on its return would gradually fill up the vacancy, and thus restore the Sun, in that place, to its former splendour, ought to be rejected on many accounts. To mention only one, the great extent of the spot is very unfavourable to that supposition. Indeed, a much less violent and pernicious cause may be assigned, to account for all the appearances of the spot.

'The Earth is surrounded by an atmosphere composed of various elastic fluids. The Sun, also, has its atmosphere, and if some of the fluids which enter into its composition should be of a shining brilliancy while others are merely transparent, any temporary cause which may remove the lucid fluid will permit us to see the body of the Sun through the transparent ones. If an observer were placed on the Moon, he would see the

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