450,830,245,547 square miles; or more than 12,200 times as many as the surface of the earth. If we should imagine, as some have done, that the sun is a habitable globe; and allow it to be as thickly peopled as the world we live in, it must contain at least 8,570,000,000,000 inhabitants; equal to the population of about 25,000 empires such as China, or 650,000 such countries as the United States.

These calculations are startling, but there are others which are scarcely less so. The size of the sun, as above stated, is pretty well known, nor can there be room for any considerable mistake about the size of the planets. Its diameter is 111 times greater than that of the earth, and its magnitude 1,300,000 greater. It is even many times greater than the combined mass of all the planets.

To illustrate more fully the comparative magnitude of the earth and sun, let us suppose that as many earths as would equal the sun in bulk were laid together, side by side, in a circle in close contact. They would fill a circumference more than eighteen times as great as the earth's orbit, and nearly equal to the orbit of Herschel, the most distant planet yet known in our system! If these globes were placed in close contact, instead of in a single ring as above, they would fill a circle of nearly ten millions of miles in diameter!

Such is the general brilliancy of the sun-that for many ages its surface was supposed to be one uniform and uninterrupted blaze of light. But when we view it with a telescope of even moderate magnifying power, furnished with a piece of dark or smoked glass, to interrupt a portion of its rays, we perceive, occasionally, a number of dark spots upon its surface, of various magnitudes and forms. These are sometimes so large as to be distinguishable by the naked eye.

From the invention of telescopes in the year 1690, to the present time, such spots have been frequently observed on the face of the sun, sometimes, only a few together, or perhaps one alone. It should be observed that there are bright spots observable, as well as dark ones. All these spots, as they vary in number, vary considerably in size. Sometimes they can hardly be seen; at others, they are so large as to exceed, in size, the whole earth. Still they are rarely large enough to be distinguished without the aid of a telescope.

There has been much speculation about the nature ol these spots. From their great changes in size, shape, &c., some have supposed they were dissolved from time to time, and afterwards formed anew. Others that they are a kind of cloud; and, if they were so, the sun [must have an atmosphere. Those who believed the sun to be an immense body of liquid fire, supposed that the spots were the eminences of large masses of thick matter, sometimes bright and sometimes dark, which by the irregular agitation of the fluids, sometimes swim upon the surface, and at other times sink and disappear.

Dr. Herschel viewed the subject differently. He supposed the sun to be a solid body, with mountains and valleys, like our earth, but larger, in about the same proportion as the sun's size exceeds that of the earth; that it is surrounded by a lucid atmosphere, by the decomposition of which light is emitted; that this atmosphere is sometimes so transparent in certain places, as to allow the body of the sun to be seen through it; that a dark spot in the sun is a part of its surface thus perceived; and that the bright spots are more copious mixtures of such fluids as decompose each other. In other words, he thought that some, at least, of the dark spots, are nothing but the projection of mountains above the surface of the shining fluid, and the bright spots, elevated parts of the sun, on which luminous matter is condensed.

He supposed also that the rays of the sun are by no means hot in themselves, but only produce heat by acting on other substances. In short, he believed the sun to be a great central planet, formed of similar materials to those which compose the rest of the system; and that it is even peopled with inhabitants of some sort or other. It is said, however, that Dr. Herschel entertained different views on this point, at different periods of his life.

Sometimes a very singular appearance attends the sun, just before or just after sunset, especially in the beginning of March. It is called the zodiacal light. It is of a whitish color, but so faint and thin, that stars may be seen through it. The shape of this light spot is that of a cone, with its base towards the sun; the top reaching from 45 to 125 degrees above him. In the torrid zone, this light is almost constantly visible. Its cause is unknown.

171

THE MOON.

E moon, though it appears to be almost as large as the sun, is four thousand and eighty-nine times smaller. The diameter of the sun, according to Sir John Herschel, is eight hundred and eighty-two thousand miles; that of the moon, only two thousand one hundred and eighty. The reason why the moon appears so large, compared with the sun, is, because it is three hundred times nearer to us. The distance from the centre of the earth, to the centre of the moon, is only two hundred and thirty-seven thousand miles; which is less than ten times the earth's circumference; while from surface to surface of the two bodies, is only about two hundred and thirty-two thousand. The moon revolves round the earth, once in twenty-nine days, twelve hours, forty-four minutes, and nearly three seconds. The path in which it appears to travel, is called its orbit. This orbit is, of course, four hundred and seventy-four thousand miles in diameter; or only a little more than one half of the diameter of the sun's body. So that if the centre of the sun were placed where the centre of the earth now is, the sun is so large, that it would much more than fill the whole of the moon's orbit. In other words, it is a great deal farther from the centre of the sun to its surface-almost twice as far-as from us to the

moon.

As the moon, like the earth, has nò light of its own, it only sends out light to us by reflection from the part of it which the sun shines upon. The sun always shines upon just half of the moon at once; as it does upon half of our earth. If there are inhabitants upon it, they must of course, have day and night like ourselves; though_astronomers tell us that their day and night are much longer than ours. Now if the half of the moon which the sun shines upon, were constantly turned towards us, we should always have what is called a full moon. But as it often happens, that only a part of its enlightened surface is towards us, we have all the changes of new moon, first quarter, full moon, and last quarter.

The physical constitution of the moon is better known to us than that of any other heavenly body. By the aid of telescopes, we discern inequalities on its surface, which

can be no other than mountains and valleys, for this plain reason, that we see the shadows cast by the mountains, in the exact proportion as to length, which they ought to have, when we take into account the inclination of the sun's rays to that part of the moon on which they are seen. The convex outline of the part which is turned towards the sun is always circular and very nearly smooth, but the opposite border of the enlightened part, which if the moon were a perfect sphere, ought to be an exact and sharply defined ellipse, is always observed to be extremely rugged, and indented with deep recesses and prominent points. The mountains, near this rugged edge, cast long black shadows, as they should evidently do, when we consider that the sun is in the act of rising or setting to the parts of the moon so circumstanced. But as the enlightened edge advances beyond them, i. e. as the sun to them gains altitude, their shadows shorten; and at the full moon, when all the light falls in our line of sight, no shadows are seen on any part of her surface.

The heights of these mountains have many of them been calculated. The highest are about one and three quarters English miles in perpendicular height. They are therefore somewhat higher, in proportion to the moon's diameter, than the Himmaleh mountains are to that of our globe.

The lunar mountains generally present a striking uniformity and singularity of appearance. They are wonderfully numerous, occupying by far the greater portion of the surface, and are almost universally of an exactly circular or cup shaped form. Most of the larger ones have, however, a flat bottom within, from which rises centrally, a small, steep, conical hill. They appear, in short, like volcanoes, and have the true volcanic character: and in some of the principal ones, decisive marks of volcanic stratification, arising from successive deposits of ejected matter, may be clearly traced with powerful telescopes. What is, moreover, extremely singular in the geology of the moon is, that although nothing having the character of seas can be traced, yet there are large regions perfectly level, and apparently of a decidedly alluvial character. The large dusky spots, which were formerly thought to be seas, are found upon close examination, to present appearances wholly incompatible with the supposition of deep water. The moon turns on its axis, but not in twenty-four

hours, as the earth does. It is as long in doing this, as it is in going round the earth; and thus it is that we have the same side of this luminary always towards us.

The moon has no clouds, and, it is thought, no atmosphere. Had it any, it could not fail to be perceived in the occultations of stars, and the phenomena of solar eclipses. Hence its climate must be very extraordinary. For a fortnight, there must be an unmitigated and burning sunshine; while for the next fortnight a degree of cold and frost prevails, equal, at least, to that of our polar winters. It is also estimated that the light of the moon is only one three hundred thousandth part of that of the sun. Such a condition of things, must require different inhabitants-if it has inhabitants, at all, from those on our own globe. We are justified in this conclusion, not only from the remarkable condition of the climate, but from the want of an atmosphere of air. It seems impossible that any forms of life, analogous to those on earth, can exist there. No appearance which indicates vegetation, or the slightest variation of surface, such as we should expect from a change of season, or from human art or labor, can any where be discovered. Telescopes must yet be greatly improved, before we can expect to see signs of inhabitants, as manifested by edifices, or any kind of changes on its surface.

If there be inhabitants in the moon, resembling ourselves, the earth must present to them the extraordinary appearance of a bright body, much larger than the moon appears to us, always standing still in the sky, while the stars must seem to pass slowly beside and behind it. It will appear clouded with variable spots, and belted with equatorial and tropical zones corresponding to our tradewinds; but it may be doubted whether, in their perpetual change, the outlines of our seas and continents can ever be clearly discovered.

COMETS.

ONE of the most common effects attributed to these bodies, is an influence over the temperature of our seasons. It would be easy to expose such an error, by showing upon general physical principles, that there is no reason what