Page images
PDF
EPUB

volving in infinite space, is an object deserving the assiduous care of the well-informed teacher.

What constitute the solar system? How may the solar system be represented? What may supply the place of an orrery?

SECTION II. Of the Sun.

The Sun is the great source of light and heat to the bodies of the solar system. It is an object pre-eminent; of inconceivable utility and grandeur. Diffusing its rays to an immense distance, and filling a sphere of incomprehensible extent, it gives life and motion to innumerable objects. In some humble measure it resembles its Divine Author. The most minute beings are not overlooked; the greatest are subject to his control.

The Sun is considered in the lower focus of the planetary orbits. But if the centre of the Sun be considered the focus of Mercury's orbit, the common centre of gravity between Mercury and the Sun will be the focus of Venus's orbit; and the common centre of gravity between Mercury, Venus, and the Sun will be the focus of the Earth's orbit. Thus the attraction of the planets nearest the Sun will in a small degree affect the foci of those more remote. Except the foci of Saturn and Herschel, however, those of all the orbits 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.

Though stationary in respect to surrounding objects, the Sun is not destitute of motion. It revolves on its axis from west to east in 25 d. 15 h. 16 m., or, according to some authors, in 25 d. 10 h. The Sun's rotation is known from the revolution of its spots.

The form of the sun is globular. This is demonstrable from its always appearing a flat, bright circle, whatever side is presented to the observer. The diameter

of the Sun is 883,246 miles; its circumference 2,774,897 miles. The Sun is 1,364,115 times larger than the Earth. Thus, surpassing in greatness the globe we inhabit more than 13 million times, it swells beyond our conception. Some imperfect idea of the immense magnitude of the Sun may be formed by one or two computations. A celestial courier, passing at the rate of forty miles a day, would be about 190 Julian years in circumambulating the Sun. If the Sun were a hollow globe, and the Earth placed at its centre, the Moon, at its present distance from the Earth, 240,000 miles, might revolve uninterrupted, being but little more than half way from the centre to the circumference of the Sun. Such a hollow globe might, therefore, contain within itself a brilliant system of revolving worlds.

The physical construction of the Sun has excited much inquiry and speculation. From time immemorial, an opinion seems to have prevailed, that the Sun was 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 he force of the solar rays is diminished by their diverging, 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 a distance from the Sun, where they were equally dense. The Sun's rays, therefore, in the neighborhood of the Sun, produce the same effects as might be expected from the most vehement fire; consequently, the Sun is a fiery substance." The force of this reasoning would lead us to conclude, that, however antiquated or repudiated 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, or the moving cause of heat originate in that luminary, 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 un

der the equator? The reply is, 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 is, therefore, the warmer is any situation.

The density of the atmosphere is considered as decreasing in a geometrical proportion upwards from the surface of the Earth. If the decrease be not always thus proportioned, it is well ascertained by experiments on the tops of lofty mountains, that the air becomes very rare in high regions. 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. According to chemists, 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 other bodies innumere, with which we are surrounded, supply us with heat. But the elevated observer on the top of Chimborazo or Himmaleh is retired, in some measure, above the influence of the Earth, and the bodies on its surface. He must exhaust his own treasure of heat, while, except immediately from the Sun, he can receive next to nothing in return. It may be added, that heat, or caloric, is by very many considered a fluid put in action by the Sun's rays. If so, it may be confined near the surface of the Earth, or be far short of the atmosphere in height. On the modern theory of caloric, therefore, elevation must greatly diminish, rather than increase the heat.

The highest elevation to which human beings can ascend, though quite a proportion in regard to the height of the atmosphere, vanishes, when compared with the distance of the Sun. What are four or five miles in comparison to ninety-five millions! No mountain is so

elevated, no balloon can ascend so high, as to make any perceptible difference in respect to the distance of the Sun.

In regard to the ancient theory, it is worthy of notice, that the powerful attraction of the Sun is incompatible with its being a mass of flame only, and the spots on its surface are conclusive, that in part, at least, it must be composed of other matter.

The celebrity of Dr. Herschel, and the ingenuity of his hypothesis respecting the Sun, make this hypothesis deserve some particular consideration. Rejecting the terms spots, nuclei, penumbra, facule, and luculi, he adopts 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 considerable extent. Being less luminous than the rest of the Sun, they seem to have some very imperfect resemblance to penumbræ, which occasioned them formerly to be so called.

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

Nodules are also elevations of luminous matter, but confined in extent to a small space. Those ridges and nodules, being brighter than the general surface of the Sun, and slightly differing from it in color, have been called luculi and faculæ.

Corrugations are a remarkable unevenness or asperity, peculiar to the luminous clouds, extending over the whole apparent surface of the Sun. The depressed parts of the corrugations being less luminous than those more elevated, the disk of the Sun has a variegated or mottled" appearance.

[ocr errors]

Indentations are the low or depressed parts of the corrugations.

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

By a number of observations he would evince, that the appearances, called spots, in the Sun, are real openings in the luminous clouds of the solar atmosphere.

His next series of observations is adduced to prove, that the appearances, which have been called penumbræ, are real depressions, or shallows. Following these are others, alleged to show, that ridges are elevations above the luminous solar clouds; that nodules are small but highly elevated luminous places; that corrugations consist of elevations and depressions; that indentations are dark places of the corrugations; and that pores are the low places of indentations. He hence infers, that the several phenomena, above enumerated, could not appear, if the Sun's shining matter were a liquid; since, by the laws of hydrostatics, the openings, shallows, indentations, and pores, would instantly be filled up; and ridges and nodules could not preserve their elevation a single moment. But many openings have been known to last during a whole revolution of the Sun; and elevations large in extent have continued for several days. Much less can this shining matter be an elastic fluid of an atmospheric nature; because this would be still more ready to assume a level by filling up the low places. It must therefore exist in the manner of luminous, empyreal, or phosphoric clouds, suspended in the higher regions of the solar atmosphere.

"It appears highly probable," says Dr. Brewster," and consistent with other discoveries, that the dark solid nucleus of the Sun is the magazine, from which its heat is discharged, while the luminous or phosphorescent mantle, which that heat freely pervades, is the region whence its light is generated." The high authority of these men does not free their hypotheses from objection. If the spots are openings only in the luminous clouds of the Sun, why are they stationary for so long a time, except as they partake of the Sun's rotation; and why should heat be emitted from the dark body of the Sun, and not

« PreviousContinue »