ASTRONOMY.

CHAPTER I.

THE SOLAR SYSTEM.

THE sun, his attendant planets, and comets, constitute the solar system.

SECTION 1.-OF THE SUN.

The sun is an object pre-eminent in the solar system. The great source of light and heat, it diffuses its rays to every part of an immense sphere, giving life and motion to innumerable objects. Like its divine Author, while it controuls the greatest, it does not overlook the most minute. According to the Copernican system, it is the centre of all the planetary and cometary motions, all the planets and comets revolving round it in different periods, and at different distances. The sun is considered in the lower focus of the planetary orbits. Strictly, if the focus of Mercury's orbit be considered in the centre of the sun, the focus of Venus' orbit will be in the common centre of gravity between Mercury and the sun; the focus of the earth's orbit, in the common centre of gravity of Mercury, Venus and the sun; and thus of the other planets. The foci of all the orbits, however, except those of Saturn and Herschel, will not be sensibly removed from the centre of the sun. Nor will the foci of Saturn and Herschel be sensibly different from the common centre of gravity between Jupiter and the sun.

The sun, though stationary in respect to surrounding objects, is not destitute of motion. It turns on its axis from

west to east, making a revolution in 25d. 15h. 16m., or, according to some, in 25d. 10h. The sun is globular, its diameter being 883,246 miles. The sun's rotation is demonstrated from the revolution of its spots; and its globular form, from its always appearing a flat, bright circle, whatever side is presented to an observer.

The physical construction of the sun has been an object of much inquiry. Considering the sun a globe of fire, some say, "The sun shines, and his rays collected by concave mirrors, or convex lenses, burn, consume, and melt the most solid bodies, or else convert them into ashes or gas; wherefore, as the force of the solar rays is diminished by their divergency, in a duplicate ratio of the distances reciprocally taken, it is evident their force and effect are the same, when collected by a burning lens or mirror, as if we were at such distance from the sun, where they were equally dense. The sun's rays, therefore, in the neighbourhood of the sun produce the same effects, as might be expected from the most vehement fire consequently, the sun is of a fiery substance." There seems force in this reasoning. It would lead us to conclude, that however antiquated the opinion may be, that the sun is a globe of fire, its surface must resemble a vast combustion. But, if heat come from the sun, why is it always cold in the upper regions of the air, though nearer the sun, than the surface of the earth? and why are the tops of lofty mountains covered with perpetual snow, even under the equator? It may be answered, that animal heat is generated in the lungs from the oxygen of the atmosphere; that air is a bad conductor of heat, and of course a good defence against cold, or rather preservative of heat, preventing its escape from the body.. The more dense the air, therefore, the warmer in any situation.

The density is considered as decreasing in a geometrical proportion, upwards from the surface of the earth. If the decrease be not always thus proportioned, yet it is certain, that the air becomes very rare in high regions, as fully tested

by experiment on the tops of lofty mountains. Hence, the supply of heat from the oxygen of the atmosphere, and the security against cold, or the preservation of heat from the non-conducting power of the air, are greatly diminished. This must affect sensation, and in some degree the thermometer. But this is not the only cause, perhaps not the principal cause, why high regions of the air are cold. Chemists assert, that all bodies, even those to us the most frigid, radiate heat. Hence, on the common surface of the earth, not the great mass of the globe only, but thousands of other bodies, with which we are surrounded, supply us with heat. But the elevated observer on the top of Chimborazo or Himmaleh, is retired, at least in some measure, from the influence of the earth, and of the bodies on its surface. He must exhaust his own treasure of heat, while, except immediately from the sun, he receives next to nothing in return.

The most elevated height, to which human beings can ascend, though very considerable in regard to the height of the atmosphere, is not worthy of consideration, when compared with the distance of the sun. What are four or five miles to ninety-five millions?

It must, however, be conceded, that besides the powerful attraction of the sun, incompatible with its being a mass of flame only, the spots on its surface are conclusive, that, at least in part, it must be composed of other matter.

The hypothesis of Dr. Herschel, respecting the sun, deserves some detail, on account of its ingenuity,' and the eminence of its author. Rejecting the names, spots, nuclei, penumbræ, faculæ, and luculi, he adopts the terms, openings, shallows, ridges, nodules, corrugations, indentations, and pores. Openings, he says, are those places, where by the accidental removal of the luminous clouds of the sun, its own solid body may be seen; and this not being lucid, the openings, through which we see it, may, by a common telescope, be mistaken for mere black spots.

Shallows are extensive and level depressions of the luminous solar clouds, generally surrounding the openings to a con

siderable distance. As they are less luminous than the rest of the sun, they seem to have some distant, though very imperfect resemblance to penumbra, which occasioned their being called so formerly.

Ridges are bright elevations of luminous matter, extended in rows of an irregular arrangement.

Nodules are also bright elevations of luminous matter, but confined to a small space. These nodules and ridges, on account of their being brighter than the general surface of the sun, and also differing a little from it in colour, have been called faculæ and luculi.

Corrugations he calls that very particular and remarkable unevenness or asperity, which is peculiar to the luminous solar clouds, and extends all over the surface of the globe of the sun. As the depressed parts of the corrugations are less luminous than the elevated ones, the disk of the sun has a mottled appearance.

Indentations are the depressed or low parts of the corrugations; they also extend over the whole surface of the luminous solar clouds.

Pores are very small holes or openings about the middle of the indentations.

That the appearances, which have been called spots in the sun, are real openings in the luminous clouds of the solar atmosphere, he evinces by a number of observations. His next series of observations is adduced to prove, that the appearances, which have been called penumbræ, are real depressions or shallows. These are followed by others, alleged to show, that ridges are elevations above the general surface of the luminous clouds of the sun; that nodules are small but highly elevated luminous places; that corrugations consist of elevations and depressions; that the dark places of the corrugations are indentations; and that the low places of indentations are pores. Hence he infers, that the several phenomena, above enumerated, could not appear, if the shining matter of the sun were a liquid; since, by the laws of hydrostatics, the openings, shallows, indentations, and