The most probable opinion is, that the body of the sun is opake, like one of the planets; that it is surrounded by an atmosphere of considerable depth; and that the light is sent off from a luminous stratum of clouds, floating above or outside the atmosphere. This theory accords best with his density, and with the phenomena of the solar spots.

The

321. Of the temperature of the sun's surface, Dr. Herschel thinks that it must exceed that produced in furnaces, or even by chemical or galvanic processes. By the law governing the diffusion of light, he shows that a body at the sun's surface must receive 300,000 times the light and heat of our globe; and adds that a far less quantity of solar light is sufficient, when collected in the focus of a burning-glass, to dissipate gold and platina into vapor. The same writer observes that the most vivid flames disappear, and the most intensely ignited solids appear only as black spots on the disk of the sun, when held between him and the eye. From this circumstance he infers, that however dark the body of the sun may appear, when seen through its spots, it may, nevertheless, be in a state of most intense ignition. It does not, however, follow, of necessity, that it must be so. contrary is, at least, physically possible. A perfectly reflective canopy would effectually defend it from the radiation of the luminous regions above its atmosphere, and no heat would be conducted downward through a gaseous medium increasing rapidly in density. The great mystery, however, is to conceive how so enormous a conflagration (if such it be) can be kept up from age to age. Every discovery in chemical science here leaves us completely at a loss, or rather seems to remove further from us the prospect of explanation. If conjecture might be hazarded, we should look rather to the known possibility of an indefinite generation of heat by friction, or to its excitement by the electric discharge, than to any actual combustion of ponderable fluid, whether solid or gaseous, for the origin of the solar radiation.

821. Sun's temperature? Dr. Herschel's idea? What reasoning against his opinion? What mystery?

322. The Zodiacal Light is a faint nebulous light, resembling the tail of a comet, or the milky way, which

seems to be reflected from the regions about the sun, and is distinguishable from ordinary twilight. Its form is that of a pyramid or cone, with its base toward the sun, and inclined slightly to the ecliptic. It seems to surround the sun on all sides, though at various depths, as it may be seen in the morning preceding the sun, as well as in the evening following him; and the bases of the cones,

where they meet at the sun, are much larger than his diameter.

323. Of the nature of this singular phenomenon, very little is positively known. It was formerly thought to be the atmosphere of the sun. Prof. Nichol says: "Of this, at least, we are certain-the zodiacal light is a phenomenon precisely similar in kind to the nebulous atmospheres of the distant stars, &c." Sir John Herschel remarks that it is manifestly of the nature of a thin lenticularly-formed atmosphere, surrounding the sun, and extending at least beyond the orbit of Mercury, and even of Venus. He gives the apparent angular distance of its vertex from the sun, at from 40° to 90°; and the breadth of its base, from 8° to 30°. It sometimes extends 50° westward, and 70° east of the sun at the same time.

324. The form of this substance surrounding the sun, and which is sufficiently dense to reflect his light to the

322. What is the zodiacal light? Its form? When seen?

823. Nature of this light? Former opinion? Prof. Nichol's remark? Dr. Herschel's? Its extent from the sun?

324. Form of this light? How situated with respect to sun's axis, &c.? (Illustrate by diagram.)

earth, seems to be that of a lens; or rather that of a huge wheel, thickest at the center, and thinned down. to an edge at the outer extremities. Its being seen edgewise, and only one-half at a time, gives it the appearance of two pyramids with their bases joined at It is an interest

the sun.

ing fact, stated by Prof.
Nichol, that this light or
nebulous body lies in the
plane of the sun's equator.
A line drawn through its
transverse
from one apex of the
mids to the other, would
cross the axis of the sun

diameter, or

pyra

at right angles. This fact

FORM, EXTENT, ETC., OF THE ZODIACAL
LIGHT.

would seem to indicate a revolution of this curious substance with the sun upon his axis.

Let A, in the above cut, represent the sun, BB his axis; then CC will represent the extent, and D D the thickness of this curious appendage.

325. In regard to its atmospheric character, Dr. Dick observes that this opinion now appears extremely dubious; and Prof. Olmsted refers to La Place, as showing that the solar atmosphere could never reach so far from the sun as this light is seen to extend.

Another class of astronomers suppose this light, or rather the substance reflecting this light, to be some of the original matter of which the sun and planets were composed a thin nebulous substance in a state of condensation, and destined either to be consolidated into new planetary worlds, during the lapse of coming ages, or to settle down upon the sun himself as a part of his legitimate substance. This theory will be noticed again when we come to speak of Nebula and Nebulous Stars.

326. After all the observations that have been made,

325. Remark of Dr. Dick respecting its atmospheric character? Olmsted and La Place? What other theory?

and the theories that have been advanced, it must be admitted that the subject of the zodiacal light is but imperfectly understood. Prof. Olmsted supposes it to be a nebulous body, or a thin vapory mass revolving around the sun; and that the meteoric showers which have occurred for several years in the month of November, may be derived from this body. This is the opinion of Arago, Biot, and others.

The best time for observing the zodiacal light is on clear evenings, in the months of March and April. It may be seen, however, in October, November, and December, before sunrise; and also in the evening sky.

THE SUN'S MOTION IN SPACE.

327. Although, in general terms, we speak of the sun as the fixed center of the system, it must not be understood that the sun is absolutely without motion. On the contrary, he has a periodical motion, in nearly a circular direction, around the common center of all the planetary bodies; never deviating from his position by more than twice his diameter. From the known laws of gravitation, it is certain that the sun is affected in some measure by the attraction of the planets, especially when many of them are found on the same side of the ecliptic at the same time; but this would by no means account for so great a periodical motion.

328. In addition to the motion above described, the sun is found to be moving, with all his retinue of planets and comets, in a vast orbit, around some distant and hitherto unknown center. This opinion was first advanced, we think, by Sir William Herschel; but the honor of actually determining this interesting fact belongs to Struve, who ascertained not only the direction of the sun and solar system, but also their velocity.

326. Is this subject well understood as yet? Prof. Olmsted's theory! When the best time for observing the zodiacal light?

327. Is the sun really stationary? What motion? How affected by planets?

328. What other motion? Who first advanced the opinion that he had such a motion? Who demonstrated it? Toward what point is the sun and

The point of tendency is toward the constellation Hercules, right ascension 259°, declination 35°. The ve locity of the sun in space is estimated at 8 miles per second, or 28,000 miles per hour. Its period is about 18,200,000 years; and the arc of its orbit, over which the sun has traveled since the creation of the world, amounts to only about th part of his orbit, or about 7 minutes an arc so small, compared with the whole, as to be hardly distinguishable from a straight line.

000

329. With this wonderful fact in view, we may no longer consider the sun as fixed and stationary, but rather as a vast and luminous planet, sustaining the same relation to some central orb that the primary planets sustain to him, or that the secondaries sustain to their primaries. Nor is it necessary that the stupendous mechanism of nature should be restricted even to these sublime proportions. The sun's central body may also have its orbit, and its center of attraction and motion, and so on, till, as Dr. Dick observes, we come to the great center of all -to the THRONE OF GOD!

Professor Mädler, of Dorpat, in Russia, has recently announced as a discovery that the star Alcyone, one of the seven stars, is the center around which the sun and soiar system are revolving.

CHAPTER XI.

MISCELLANEOUS REMARKS UPON THE SOLAR SYSTEM.

NEBULAR THEORY OF THE ORIGIN OF THE SOLAR SYSTEM.

330. Ir was the opinion of La Place, a celebrated French astronomer, that the entire matter of the solar system, which is now mostly found in a consolidated

solar system tending? Its velocity? Period of revolution? Amount of its progress since the creation of the world?

329. How, then, should the sun be considered? How extend the analogy? What further recent discovery, and by whom?

330. State the "nebular theory" of the origin of the solar system? Who first started this theory?