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sun upon the axis is in the same direction as the motion of the planets round the sun, that is to say, from west to east.

SPOTS ON THE SUN..

One of the earliest fruits of the invention of the telescope was the discovery of the spots upon the sun, and the examination of these has gradually led to a knowledge of the physical constitution of the centre of our system.

When we submit a solar spot to telescopical examination, we discover its appearance to be that of an intensely black irregularly-shaped patch, edged with a penumbral fringe, the brightness of the general surface of the sun gradually fading away into the blackness of the spot. When a spot is watched for a considerable time, it is found to undergo a gradual change in its form and magnitude; at first increasing gradually in size, until it attains some definite limit of magnitude, when it ceases to increase, and soon begins, on the contrary, to diminish; and its diminution goes on gradually, until at length the bright sides closing in upon the dark patch, it dwindles first to a mere point, and finally disappears altogether. The period which elapses between the formation of the spot, its gradual enlargement, subsequent diminution, and final disappearance, is very various. Some spots appear and disappear very rapidly, while others have lasted for weeks and even for months. The magnitudes of the spots are in proportion to the magnitude of the sun itself. At the distance of the sun, a spot, the magnitude of which would be barely visible, must have a diameter of four hundred and sixty miles, and an area of one hundred and sixty-six thousand square miles, which is, therefore, the smallest space on the surface of the sun which would be distinctly seen. Among the many spots which have been recorded, one was observed by Mayer, the area of which was about fifteen hundred millions of miles square, or about thirty times the surface of the earth.

Spots have been occasionally seen on all parts of the sun, but that region on which they are found generally to prevail, is one which corresponds with the tropical parts of the earth, that is, a space extending about thirty degrees on either side of the solar equator.

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PHYSICAL CONSTITUTION OF THE SUN.

What are the spots? Two, and only two, suppositions have been proposed to explain them. One supposes them to be scoriæ, or dark scales of incombustible matter floating on the general surface of the sun. The other supposes

them to be excavations in the luminous matter which coats the sun, the dark part of the spot being a part of the solid non-luminous nucleus of the sun. In this latter supposition it is assumed that the physical constitution of the sun is a solid non-luminous globe, covered with a coating of a certain thickness of luminous matter. This latter supposition has been in a great measure demonstrated by continued and accurate observations on the spots.

That the spots are excavations, and not mere black patches on the surface, is proved by the following observations: If we select a spot which is at the centre of the sun's disk, having some definite form, such as that of a circle, and watch the appearance of the same spot when, by the motion of the sun upon its axis it is carried toward the edge, we find, first, that the circle becomes an oval. This, however, is what would be expected even if the spot were a circular patch, inasmuch as a circle seen obliquely is foreshortened into an oval. But we find that as the spot moves toward the side of the sun's limb, the black patch gradually disappears, the penumbral fringe on the inside of the spot becomes invisible, while the penumbral fringe on the outside of the spot increases in apparent breadth, so that when the spot approaches the edge of the sun, the only part that is visible is the external penumbral fringe. Now this is exactly what would occur if the spot were an excavation. The penumbral fringe is produced by the shelving of the sides of the excavation, sloping down to its dark basis. As the spot is carried toward the edge of the sun, the height of the inner side is interposed between the eye and the bottom of the excavation, so as to conceal the latter from view. The surface of the inner shelving side also takes thé direction of the line of vision or very nearly, diminishes in apparent breadth, and ceases to be visible, while the surface of the shelving side next the edge of the sun becomes nearly perpendicular to the line of vision, and, consequently, appears of its full breadth.

In short, all the variations of appearance which the spots undergo, as they move across the sun's disk, changing their distances and positions with regard to the sun's centre, are exactly those changes of appearance which would be produced by an excavation, and not at all those which a dark patch on the solar surface would undergo.

It may be considered then as proved, that the spots on the sun are excavations; and that the apparent blackness is produced by the fact that the part constituting the dark portion of the spot is either a surface totally destitute of light or by comparison so much less luminous than the general surface of the sun as to appear black. This fact combined with the appearance of the penumbral edges of the spots have led to the supposition, which appears scarcely to admit of doubt, that the solid, opaque nucleus, or globe of the sun, is invested with two atmospheres, that which is next the sun being like our own, nonluminous, and the superior one being that in which alone light and heat are evolved; at all events, whether these strata be in the gaseous state or not, the existence of two such, one placed above the other, the superior one, being luminous, seems to be exempt from doubt.

By observing the magnitude of the spots, and the rate at which they increase. and diminish, the velocity of their edges has been ascertained, and this velocity has been found to be such as can scarcely be attributed to matter except in the gaseous form.

We are not warranted in assuming that the black portion of the spots are

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had access to the original papers containing this investigation, we can only speak of it from the imperfect information supplied by that report. It would seem from it that Arago reasons in the following manner: There are two states in which light is capable of existing; the ordinary state, and the state of polarization. It has been proved by Fourier, that all bodies rendered incandescent by heat, which are in the solid or liquid state, emit polarized light; while bodies which are gaseous, when rendered incandescent, invariably emit light in its ordinary state. Thus the physical condition of a body may be distinguished when it is incandescent, by examining the light which it affords. There are polariscopic instruments by which we are enabled to distinguish these different states of light. On applying these tests to the direct light of the sun, it has been found to be in the unpolarized, or ordinary condition. Hence it has been inferred by Arago, that the matter from which this light proceeds must be in the gaseous state. It will doubtless be readily understood that gas, when incandescent, is that which is commonly called flame. If Arago's reasoning, then, be rightly reported, and his observations correct, it follows that the globe of the sun is a solid, opaque, non-luminous orb, invested with an ocean of flame.

Certain observations made by Bouguer, led that astronomer to suppose that the sun is surrounded by an atmosphere of considerable extent above the surface of the luminous coating. The ground of this supposition was the impression that the splendor of the sun's light near the borders of the disk was less than near the centre; an effect which could not be produced if the luminous coating had nothing above it imperfectly transparent. On the contrary, the brightness toward the borders, owing to the obliquity of the direction of the surface to the line of vision would be greater, inasmuch as a greater extent of luminous, surface would be comprised within the same visual angle. The more accurate observations, however, of Arago, made with delicate polariscopic instruments disprove this by showing that the brightness is the same on all parts of the sun's disk

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L

ECLIPSES.

Lunar and Solar Eclipses.-Their Causes.-Shadow of the Earth.-And Moon.-Magnitude of Eclipses. When they can happen.-Central Solar Eclipse.-Great Solar Eclipse described by Halley.-Ecliptic Limits.

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