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again. The luminous matter, whatever it be, is never at rest, but is evidently tumbling and weltering in endless agitation.
The large spots noticed by Galileo are continually changing, They break out, perhaps, all of a sudden, and immediately begin to alter their shape; sometimes closing up at a rapid and steady rate till they disappear, sometimes dividing into several smaller spots before they vanish. They may last for days or weeks, but they are never permanent.
When one of the spots is closely examined, it is seen to have in the centre a perfectly black speck, called the shadow, or umbra, and round about the shadow a rim of faint light, called the penumbra, or partial shadow. Not unfrequently there is a ring, of more than ordinary brightness surrounding the whole, as if the luminous matter had been excavated from the hollow of the spot, and accumulated on its brink.
It was the existence of the spots, as landmarks on the surface of the sun, that first showed that his body was not at rest, but had a slow whirl, as if about an axle, performing an entire turn or revolution in about twenty-five days. Moreover, the change in appearance in any one of the spots, as it moved' round with the sun towards his edge, there to disappear, led to curious suppositions about them. When near the edge of the sun, the penumbra on one side disappeared, and left no margin between the central blackness and the brilliant outer ring. When the same spot reappeared, about twelve days after, at the opposite edge, the penumbra on the other side was wanting; namely, that on the side farthest from the sun's edge. This appearance would exactly arise if the spot were a deep cavity, with walls of light or of luminous matter sloping or shelving towards the bottom. When the spot lay about the middle of the sun's face, we would look directly into the cavity, and its shelving sides all round would be distinctly seen. But by the revolution of the sun, the opening of the cavity would be, as it were, turned some
and while the walls on one side would still be seen, the opposite wall would be hidden. If the spot were near the sun's edge, it is precisely the penumbra next the edge that should be visible.
This discovery led Sir William Herschel and others to speculate upon the constitution of the sun. It seems likely that the great mass of his matter forms a round, solid globe, which is perfectly dark of itself. On this globe there is a transparent atmosphere, or ocean of clear air, whose depth is immense-perhaps upwards of a thousand miles. If there were no more about him than the solid mass and this aërial covering, he would be not unlike the earth. But within the clear atmosphere there floats huge masses of luminous matter, or blazing cloud, of such quantity and extent, as for the most part to cover and overshadow the whole enormous surface of the sun. This floating fire is eternally in commotion, so as to be sometimes excited to
unusual glare, at other times to break up, and leave great holes or openings in the air, and down through these we see the black, naked body of the sun. Such openings are the spots, and the little black points, or pores, are merely smaller openings, not exceeding perhaps a few hundreds of miles in breadth, while the larger openings measure thousands, and even tens of thousands of miles across. The perpetual movement and tumult of the fire-clouds will sometimes cause a sort of blow-up here and there, and leave a huge rent in the floating matter, which, however, will immediately dart forth on all sides to close it up again.
Sir William Herschel imagined two different kinds of floating cloud in the transparent atmosphere-the one above the source of the heat and light, which it gives forth, and which are reflected by an inferior region of opaque cloud, which is thereby made of a dazzling whiteness. This inferior cloud he would make the umbra of the spots, it being less broken up than the superior fire stratum. The stormy commotion above might extend itself to this matter floating in the lower depths, and cause it to break up in the same way.
What, however, seems most certain, is the threefold composition of the sun-the dark solid body, the deep clear atmosphere, and the floating luminous masses in incessant motion and change. What the mode of action is that creates for us a perpetual stream of heat and light, we are utterly unable to discover, or even to imagine. All that we know of the production of heat tends to show that by no process that takes place in the earth is it absolutely created. If a fire really created heat, we might suppose that a vast combustion, a tremendous eternal bontire, was sustained all over the sun's ground; but combustion merely evolves heat that was previously latent in the bodies burning, and this heat must have been itself originally derived from some other source.
It is the same with every other mode of causing heat artificially : it cannot be shown that in any case heat is created where none had ever been laid up. There is no original source of heat but the sun itself, and therefore there is nothing to compare his agency with :' it is to us an unfathomable mystery. We may hope to know the deepest secrets of animal life, because it is within our reach; but the remote power that supplies us with unceasing warmth, which is constantly expending, and yet never exhausted, we can hardly expect to understand. Even the nature of the solar beam itself, compounded of at least two distinguishable elements—its heating and lighting rays—is very difficult to be investigated.
Of all the bodies that float in the sky, the moon is the nearest to us, and therefore the best seen. Although appearing about as big as the sun, it is really a mere atom in comparison : the sun, measured across, exceeds the moon 400 times, but happening to
be about 400 times the distance of the moon, the apparent breadths of the two come to be equal.
A telescope of ordinary power applied to the moon, enables us to discover the character of her surface. Unlike the sun, she is not the source of her own light, but is lighted up by the solar rays as the earth is; consequently her brilliancy is nothing else but a reflection of these rays, like the brilliancy of the clouds in our sky, or of the sea and the green fields on a summer day.
Light is reflected from the surfaces that it falls on in two very different ways. The one way is by minor reflection, the other is by being absorbed into the surface, and emitted as from a new centre. In the first case, the rays are not broken up at all; they are sent back as they came, and each ray received is a single ray in being reflected. The effect of mirrored rays, likewise, is to show, not the face of the reflecting surface, but the face of the body that first sent out the rays. Thus, when the sun or moon falls on the surfaces of water, their image is reflected unbroken; and though we are looking at the water, we see not its surface, but the faces of the luminaries themselves. When a surface reflects light in this way, it can show us nothing of its own character, except indirectly. If it is very smooth, it will reflect the image of a pure and perfect shape; and therefore the shape observed will be a test of the smoothness. If it is wavy, it will repeat the image on every wave, and make a long streak of images, as we see when the moon shines on a ruffled lake. The manner of the reflection will therefore tell us what the outline of the surface is; it will also tell us if it is a good reflecting surface or not; but beyond these points it gives no information.
The second kind of reflection is the reflection that we really see bodies by. The light falling on a cloud tinges the matter of the cloud with light; this it sends out in all directions, and thereby shows the cloud itself, and not the sun that lighted it. The cloud is not a mirror reflecting rays, but a substance actuated with the power of emitting rays from itself as a centre; it is kindled up with the light-giving energy, and every point of it diffuses a shower of light all around, as if it were a point of the sun's own surface. By this kind of light, we have a distinct view of the cloud's own body, and no reference whatever to the object that originally supplied the light. Such reflection, or rather secondary emission and diffusion, affords us the means of seeing bodies fully and directly, in their whole extent, and with their proper shape and colour, and all the peculiarities of their surface.
The light of pure reflection being unbroken and undivided, is always strongest. A reflected image of the sun is more brilliant than the whitest cloud in the sky. Where the water is reflecting the sun, its own proper surface-light, its light of visi
bility, is quenched. In viewing pictures, the surface is invisible where reflected rays are coming to our eyes.
From a distant object like the moon, the rays of pure reflection must be what chiefly come to the earth. The rays of emission from the surface will be so much feebler than these, that they can only be seen in spots where no reflection happens at the time. Hence the rays that would show us the colour of the moon's surface, that would tell whether it is green, or blue, or red, or brown, cannot be expected to strike our eyes with distinctness through such a distance. The real luminous rays must be rays of reflection, and by these we can infer only the forms and the reflecting power of the surface, as we can infer the rippling of water from the alternation of lights and shadows.
If the moon were a plane surface of one uniform quality, like a vast ocean or a sandy desert, she would be evenly lighted all
But the mere naked eye shows that is not the case: we can see ridges of strong light, and spaces of a darker hue. These ridges must be of better reflecting material than the dark plains; as if they were of a clear crystallised surface, while the other were crumbled powder or sand, or some material of a dullish hue.
Telescopic appearance of the Moon. A bright speck with a shadow beside it, if the position of the shadow correspond to the direction of the sun at the time, is a decided indication of a prominence or mountain whose top catches and reflects the sun's rays, and throws a shadow at its
base. Now such appearances are abundant in the moon. They are of course totally distinct from those permanent differences of brilliancy in different spots, which can only be accounted for by a difference of substance or quality in the several tracks of surface. The shadows of the mountains are best seen in the ragged edge of light and darkness when the moon is a crescent. If the surface or ground were even and smooth, this edge would be a perfect unbroken line of a slightly-curved form. But the mountains give it a ragged appearance. Within the dark surface specks of light are seen, as of mountains whose base has gone out of the reach of illumination, but whose high tops still catch the rays; just as a high mountain in the earth has its summit lightened when the sun has set to the plains below. Again, mountains within the enlightened half throw a premature shadow on the ground that they conceal from the sun, which becomes black through their presence before its time. Such shadows are made known by dark inroads in the illumined edge. We have thus light points within the line of darkness, and dark indentations in the lightened face; and these are the proofs of the mountainous character of the surface.
It is possible to guess at the height of the mountains, by the extent of their shadows. The higher a mountain is, the longer will its top remain lightened after it has passed into the dark side. The extreme distance, therefore, of a bright speck from the edge of illumination will serve to show the height of the reflecting summit. By the requisite measurements and calculations, many of the heights have been estimated; but astronomers do not agree as to the exact results. Sir John Herschel says the highest of the mountains is about 1 miles, or 9000 feet. But others have maintained that some of the lunar peaks are loftier than the highest mountains of the earth.
The lunar mountains, which are scattered abundantly over her whole surface, are of several different kinds. 1. A great number are single or isolated heights, or sugar-loaf elevations
, rising out of a wide plain, and terminating in sharp peaks. 2: Many are formed in chains or ridges, like most of our terrestrial mountains. 3. There is a class of formations of the crater species, or great cavities surrounded by mountain walls. These are of various dimensions ; in some the cavity is broad, and in others it is contracted, and surrounded by steep rocky walls, rugged and cleft. Sometimes a conical peak starts up in the middle, giving it all the character of a volcano. The entire character of the surface has always struck observers as resembling the volcanic regions of our earth; and appearances have sometimes been seen as of volcanic fires in action.
There is no decided trace of an atmosphere in the moon. She has no clouds, and no such action upon stars at her edge as an atmosphere would have. The extended plains are in no case covered with water: she has no seas. It is uncertain if any kind