The volume, solid contents, geometrically speaking, or bulk of the Sun exceeds that of the Earth 1,405,000 times: in other words, it would take that number of Earths to make one globe of the same magnitude as the Sun. This is 600 times greater than the bulk of all the planets at présent known to exist.

The mass of the sun, or the quantity of matter it contains as measured by weight, and known by its attractive power, exceeds that of the Earth 356,000 times, and is 740 times greater than the masses of all the known planets put together.

The period of the rotation of the sun upon its axis, which takes place in the same direction as that of the Earth, or from west to east, is about 25 days 8 hours; the excess over the 25 days is uncertain in exact amount, but actually exists. It is probable that the distinct spherical strata or shells of which the sun consists differ slightly in their periods of rotation, whence, in part, the uncertainty seems to arise.

The north end of the axis, or North Pole of the Sun leans seven and a half degrees from a perpendicular to the Earth's path.

The diameter of the sun is nearly four times greater than that of the Moon's orbit around the Earth, which is 238,000 miles, or nearly 30 times the diameter of the Earth; so that if the Earth were placed in the centre of the Sun, the moon would revolve at a depth within the sun of more than 187,000 miles from its surface.

Perhaps another kind of familiar illustration may be useful. Nothing which assists the mind to comprehend so wonderful a subject as that before us can rightly be considered undignified. A Railway Train at the average speed of thirty miles in an hour, continuously maintained, would arrive at the Moon in eleven months, but would not reach the Sun in less than about 352 years, so that, if such a train had been started in the year 1512, the third year of the reign of King Henry the Eighth, it would reach the Sun in 1864. When arrived, it would be rather more than a year and a half in reaching the sun's centre, three years and a quarter in passing through the sun, supposing it was tunnelled through, and ten years and one-eighth in going round it. How great these dimensions are may be conceived from the statement that the same train would attain the centre of the Earth in five days and a half, pass through it in eleven days, and go round it in thirty-seven days.

The first subject of recent investigation we have to notice is that of the Form or Figure of this luminary.

The polar and equatorial diameters of the sun's disc as observed, have been supposed to differ, though by a very small quantity only, and either has been believed to exceed the other. The observations of Maskelyne and Littrow make the figure of the sun that of a prolate spheroid, having the polar diameter greater than the equatorial; while those of Carlini and Bianchi make it an oblate spheroid, like that of the earth, the polar diameter being less than the equatorial. Observations of equal excellence and calculations of equal accuracy, instituted by equally competent astronomers, having these opposed but complementary results, it would seem a rational and satisfactory conclusion that the form of the disc is a true circle; and since we

observe the sun in a direction nearly at right angles with its axis, and nearly coinciding with the plane of its equator, it would seem equally satisfactory to conclude that it must be a solid of revolution, and the visible disc being sensibly a perfect circle, that the figure of the sun must be sensibly a perfect sphere. But doubts as to the true figure of the sun having been lately raised, they have been met by reference to the measures of Schlüter and Wichmann, which, according to Dr. Winnecke, "determine the question for the present position of the science of observation," giving perfect equality of the diameters. Still, however, "it has been proposed lately to prepare an apparatus for the purpose of examining whether the sun's disc is really circular, and, in particular, for ascertaining whether the diameters nearly perpendicular to the ecliptic are equal to those nearly parallel to the ecliptic." The Astronomer-Royal, however, has finally shown, that no new information could be given by such apparatus regarding the measures of the sun's diameters, in any degree comparable to that which we already possess in the Greenwich Observations, made from the year 1836 to the present time; and the evidence afforded by which is growing every day, giving at present a horizontal diameter exceeding the vertical only by 0"1; that is, by the 13th part of the entire angular or apparent diameter of the sun. This, it will be easily believed, Mr. Airy is correct in saying is a quantity smaller than we can answer for in these or in any other methods of observation." Therefore, again, we have a right to say that the disc is a perfect circle, and to conclude the truth to be that the sun is a perfect sphere; being indeed the only body of the solar system having that figure, and the only known example of a perfect sphere in nature, though it would seem possible that the stars -the distant suns of other systems—may agree with our sun in this respect.

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The phenomena of the perspective projection of the sun on the visual plane on the retina of the eye, in fact-called his disc,-the visible sun, and the perfection of his figure, are reciprocally connected; we infer the latter from observations on the former, and we find the exact circularity of the disc again to be the consequence of the perfect sphericity of his figure. But not only is the disc geometrically perfect in the equality of its diameters, it is bounded also by a mathematically perfect curved line: though we know that there are great inequalities in the surface of the sun, the contour of his disc is continuous and unbroken. The "Luminous Prominences" seen in total eclipses of the sun, when the solar illumination of our atmosphere is locally nearly extinguished by the intervention of the moon, and which have attracted so much attention of late years, do not impair the uniformity of this outline, but are apparently based upon or exist beyond it. Preparatory, however, to inquiring into the cause of this simplicity and exactness of visible contour, we must briefly review the phenomena presented for observation within or upon the disc, from which all our notions of the physical constitution of the sun are obtained; and especially those which have been recently discovered or investigated.

The mottled or curdled appearance which astronomers who have

viewed the sun through powerful telescopes have described as characterising its entire surface-the surface, that is, of the photosphere, or outer luminous envelope of the sun, and the brighter portions of which have been termed facula-is described by Mr. Warren De la Rue, from his large photographs, as arising from the fact that the visible photosphere is entirely composed of an undulating mass of waves like the surface of the sea agitated by wind; a description which is in harmony both with the accounts and the inferences of other observers. But according to the observations of Mr. James Nasmyth, communicated to the British Association in 1862, the entire luminous surface consists of an aggregation of greatly elongated, lenticular-shaped masses, resembling in figure Willow-leaves, and some forms of Diatomaceæ, interlacing with and superposed on one another in every possible direction. They are said by Mr. Nasmyth to be "arranged without any approach to symmetrical order in the details, but rather (if the term may be used) in a sort of regular random scattering." He found them also to be in constant motion relatively to one another, sometimes approaching, sometimes receding; and sometimes assuming a new angular position with respect to each other, by one end of a lenticular mass either maintaining a fixed distance from, or approaching one end of its neighbour, while at the other ends they retired from each other. They were observed to shoot in comparatively narrow streams across the enormous chasms we call the spots of the sun, "bridging" them over in well-defined bands or lines, and appearing to adhere to one another by lateral attraction. The appearance resulting from the combination of simultaneous motions which they present, is compared to that of a dense shoal of fish, which they also individually resemble in form. The spots also exhibit the extremities of these leaf-like bodies pointing inwards, and fringing the sides of the cavern far down into the abyss; sometimes by crowding in on the edges they close it in, and frequently at length coliterate it. These bodies are stated to be of various dimensions, some being as large in superficial area as all Europe, and some even as the entire surface of the Earth. In general their length exceeds their breadth nine or ten times, at the middle or widest part; the former measuring not less than 1,000 miles, by a breadth of about 100 miles, while their height or thickness, doubtless very great, is as yet entirely unknown: the entire bulk of a "lenticular body," therefore, immensely exceeding that of the greatest group or range of mountains on the earth. All these dimensions may be regarded as proportionate to the grandeur of the scale on which solar phenomena are carried on; but the greater ones probably apply to lateral aggregations of the elementary lenticular objects.

Mr. Dawes had previously found that the interior edge of the penumbra, or less dark border of the spots, frequently appears extremely jagged; the bright ridges on its surface, which are usually directed nearly towards the centre of the spot, being seen projected to irregular distances on to the cloudy stratum (a region of the spots between the penumbra and the dark nucleus, and first observed by Mr. Dawes, as will be noticed hereafter), "and looking much like a piece of coarse thatching with straw, the edge of which has been left

untrimmed." This was evidently an observation of the lenticular bodies fringing a spot.

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Mr. Nasmyth's description of the surface of the sun agrees substantially with the descriptions of astronomers in general, though derived from observations of a special nature, and given in more familiar, but not less definite language. The aspect of a shoal of fish is that of the tumultuous agitation which the whole surface had often been stated to exhibit. His account also agrees with the character ascribed by various telescopic observers to individual portions of the disc. As a general statement, it may be said that a lenticular body is a facula, or more correctly, perhaps, that a facula is its brilliant crest; an association of them is described as consisting of faculæ, and as a mass, or billow, or mountain of photospheric matter." Instances of this will appear in certain remarkable observations of the solar spots we shall presently cite; and the concurrence of different observers in giving essentially the same description of the phenomena establishes the accuracy of all, and the general identity of the objects described; though, on the other hand, the terms designating the brighter elements of the sun's surface are employed in a convertible and not quite definite manner. But this account of the surface of the photosphere is that of an ocean of fluid matter in flames-understanding by flame incandescent gaseous matter, without reference to the cause of the temperature rendering it incandescent. The lens-shaped bodies described by Mr. Nasmyth would appear to be the perspective projections of flexuous sheets of flame, seen in what, if relating to objects on the surface of the earth, we should term a bird's-eye view-a view, that is, in this case, looking down on the apices and crests of the flames, and seeing in this direction their sides and the form of their bases. We shall find in the sequel how well this interpretation agrees with the results of the observations and inductions of other astronomers who have given particular attention to the sun. Sir John F. W. Herschel, it will be remembered, had long ago (in 1833) assimilated the photosphere to "a luminous medium intermixed, but not confounded, with a transparent and nonluminous atmosphere, either floating as clouds in our air, or pervading it in vast sheets and columns like flame, or the streamers of our northern lights, directed in lines perpendicular to the surface."

The Rev. F. Howlett, one of the telescopic observers who have of late years devoted themselves to the examination of the sun, alluding to the doubts which have been entertained whether the solar spots are really cavities or not, and to the evidence on the subject afforded by the new scientific art of Celestial Photography, remarks on the result of its application to the sun: "It proves, surely, in agreement with the best eye observations, that the faculæ undoubtedly are raised billows or mountains, as it were, of photospheric matter, the crests of which (as Padre Secchi pointed out to me) are necessarily not very visible when situated in the more central parts of the disc, where they are seen through the least perspective depth of the solar atmosphere, and where, in consequence, all similarly luminous photospheric matter, whether raised or otherwise, appears nearly alike - brilliant; but which faculæ naturally exhibit themselves in far

stronger relief towards the marginal portions of the disc, because that the rays which emanate from their lofty crests have to traverse, in their passage to our eyes, a far smaller amount of solar atmosphere than those rays have which emanate from their bases, and from the mean photospheric level above which the faculæ rise;" and which latter rays, consequently, suffer a greater comparative amount of diminution of brightness by the absorptive action of the atmosphere. Mr. Howlett noticed, on the 22nd of June, 1860, a facula which appeared to extend, in a wavy continuous streak, at least 150,000 miles on the sun's surface-a length more than equal to two-thirds of the moon's distance from the earth; compared to this the greatest range of terrestrial mountains would have been diminutive indeed.

The Rev. Professor Selwyn, in the Report of the British Association for 1862, describing several photographic "autographs" of the sun, and among them one in which is seen a group of spots 118,000 miles in length, also mentions that in two autographs of the 4th of August in that year, "where the great spot (20,000 miles in diameter) appears on the edge, a very distinct notch is seen, and the sun appears to give strong evidence that the spots are cavities; but eye observations and measurements by the Rev. F. Howlett and others," it is added, "tend to show that this evidence is not conclusive, for there was still a remaining portion of photosphere between the spot and the edge." In these autographs, "the facula are clearly shown, and seem to prove that the tropical regions of the sun are highly agitated, and that immense waves of luminous matter are thrown up, between which appear the dark cavities of the spots, whose sloping sides form the penumbræ."

The view of the nature of the photosphere taken by Sir J. Herschel, and cited above, is doubtless in reality the summary result of his own observations on the sun and of his inductions from those of other astronomers. But he thus describes the immediate suggestion of it to his mind, beginning with a statement of the telescopic aspect of the sun which agrees with the account of it given by other observers :

"The part of the sun's disc not occupied by spots is far from uniformly bright. Its ground is finely mottled with an appearance of minute, dark dots, or pores, which, when attentively watched, are found to be in a constant state of change. There is nothing which represents so faithfully this appearance as the slow subsidence of some flocculent chemical precipitate in a transparent fluid, when viewed perpendicularly from above: so faithfully, indeed, that it is hardly possible not to be impressed with the idea of a luminous medium intermixed," &c., proceeding as above. That in the photosphere we have both a luminous medium and an atmosphere distinct from it, and that it is pervaded by vast sheets and columns like flame, there is much evidence to show, as will appear from many observations stated in this paper. But Mr. Dawes, so far back as 1852, by means of his solar eye-piece, using a very small field, but with large telescopic aperture and high power, was enabled "to examine, very closely, the pores and mottled structure of the sun's bright surface," his conclusion being "that the apparent rapid fluctuation of the porous struc