atmosphere, almost always filled with luminous clouds, occasionally opening and disclosing the dark mass within.

337. The speculations of Laplace were different. He imagined the solar orb to be a mass of fire, and the violent effervescences and explosions seen on its surface, to be occasioned by the eruption of elastic fluids, formed in its interior, and the spots to be enormous caverns, like the craters of our volcanoes. Others have conjectured that these spots are the tops of solar mountains, which are sometimes left uncovered by the luminous fluid in which they are immersed.

338. Among all the conflicting theories that have been advanced, respecting the physical constitution of the Sun, there is none entirely free from objection. The prevailing one seems to be, that the lucid matter of the Sun is neither a liquid substance, nor an elastic fluid, but that it consists of luminous clouds, floating in the Sun's atmosphere, which extends to a great distance, and that these dark spots are the opaque body of the Sun, seen through the openings in his atmosphere. Herschel supposes that the density of the luminous clouds need not be greater than that of our Aurora Borealis, to produce the effects with which we are acquainted.

339. The similarity of the Sun to the other globes of the system, in its supposed solidity, atmosphere, surface diversified with mountains and valleys, and rotation upon its axis, has led to the conjecture that it is inhabited, like the planets, by beings whose organs are adapted to their peculiar circumstances. Such was the opinion of the late Dr. Herschel, who observed it unremittingly, with the most powerful telescopes, for a period of fifteen years. Such, too, was the opinion of Dr. Elliot, who attributes to it the most delightful scenery; and, as the light of the Sun is eternal, so, he imagined, were its seasons. Hence he infers that this luminary offers one of the most blissful habitations for intelligent beings of which we can conceive.

387. Laplace's speculations? What other opinions? 338. Is there a satisfactory theory of the physical nature of the Sun? State the prevailing one? Herschel's supposition? 839. What conjecture in regard to the inhabitants of the Sun, and upon what founded? Who held to this idea?

CHAPTER III. LA

THE PRIMARY PLANETS-MERCURY AND VENUS.

340. MERCURY is the nearest planet to the Sun that has yet been discovered, and with the exception of the asteroids, is th smallest. Its diameter is only 3,140 miles. Its bulk, therefore, is about sixteen times less than that of the Earth. It would require more than twenty millions of such globes to compose a body equal to the Sun.

Here the student should refer to the diagrams, exhibiting the relative magnitudes and distances of the Sun and Planets, Map I. And whenever this subject recurs in the course of this work, the student should recur to the figures of this Map, until he is able to form in his mind distinct conceptions of the relative magnitudes and distances of all the planets. The Sun and planets being spheres, or nearly so, their relative bulks are estimated by comparing the cubes of their diameters: thus, the diameter of Mercury being 3,140 miles, and that of the Earth 7,912; their bulks are as the cube of 3,140, to the cube of 7,912, or as 1 to 16, nearly.

341. Mercury revolves on its axis from west to east in 24 hours, 5 minutes, and 28 seconds; which makes its day about 10 minutes longer than ours. It performs its revolution about the Sun in a few minutes less than 88 days, and at a mean distance of nearly 37,000,000 of miles. The length of Mercury's year, therefore, is equal to about three of our months.

The rotation of a planet on its axis, constitutes its day; its revolution about the Sun constitutes its year.

342. Owing to the dazzling brightness of Mercury, the swiftness of its motion, and its nearness to the Sun, astronomers have made but comparatively few discoveries respecting it. When viewed through a telescope of considerable magnifying power, it exhibits at different periods all the various phases of the Moon; except that it never appears quite full, because its enlightened hemisphere is never turned directly towards the Earth, only when it is behind the Sun, or so near to it as to be hidden by the splendor of its beams. Its enlightened hemisphere being thus always turned towards the Sun, and the opposite one being always dark, prove that it is an opaque body, similar to the Earth, shining only in the light which it receives from the Sun.

343. Mercury is not only the most dense of all the planets, but receives from the Sun six and a half times as much light and

340. Subject of Chapter III.? Size and position of Mercury? What map illustrates this subject? 341. State the time of Mercury's revolution upon his axis? How does this compare with the Earth? His period of revolution around the Sun ? said of discoveries upon Mercury, his phases, &c.? What proof that he is opaque ?

342. What

heat as the Earth. The truth of this estimate, of course, depends upon the supposition that the intensity of solar light and heat at the planets, varies inversely as the squares of their distances from the Sun.

In this diagram the light is seen passing in right lines, from the sun on the left toward the several planets on the right. It is also shown that the surfaces A, B, and C receive equal quantities of light, though B is four times, and C nine times as large as A; and as the light falling upon A is spread over four times as much surface at B, and nine times as much at C, it follows that it is only one-ninth as intense at C, and one-fourth at B, as it is at A. Hence the rule, that the light and heat of the planet is, inversely, as the squares of their respective distances.

The student may not exactly understand this last statement. The square of any number is its product, when multiplied by itself. Now suppose we call the distances A, B, and C, 1, 2, and 3 miles. Then the square of 1 is 1; the square of 2 is 4; and the square of 3 is 9. The light and heat, then, would be in inverse proportion at these three points, as 1, 4, and 9; that is, four times less at B than at A, and nine times less at C. These amounts we should state as 1, 4, and one-ninth.

344. This law of analogy, did it exist with rigorous identity at all the planets, would be no argument against their being inhabited; because we are bound to presume that the All-wise Creator has attempered every dwelling-place in his empire to the physical constitution of the beings which he has placed in it.

From a variety of facts which have been observed in relation to the production of caloric, it does not appear probable, that the degree of heat on the surface of the different planets depends on their respective distances from the Sun. It is more probable, that it depends chiefly on the distribution of the substance of caloric on the surfaces, and throughout the atmospheres of these bodies, in different quantities, according to the different situations which they occupy in the solar system; and that these different quantities of caloric are put into action by the influence of the solar rays, so as to produce that degree of sensible heat requisite to the wants, and to the greatest benefit of each of the planets. On this hypothesis, which is corroborated by a great variety of facts and experiments, there may be no more sensible heat experienced on the planet Mercury, than on the surface of Herschel, which is fifty times farther removed from the Sun.

345. The rotation of Mercury on its axis, was determined from the daily position of its horns, by M. Schroeter, who not only discovered spots upon its surface, but several mountains in its southern hemisphere, one of which was 10 miles highnearly three times as high as Chimborazo, in South America.

344.

343. His density, and light and heat? Upon what rule is this estimate based? Would not this law of analogy make against the doctrine that the planets are inhabited? Is it probable that this law does prevail? Upon what may the relative heat of the planets depend? 345. How was his diurnal revolution determined, and by whom? What said of his surface? What observation respecting mountains in general?

It is worthy of observation, that the highest mountains which have been discovered in Mercury, Venus, the Moon, and perhaps we may add the Earth, are all situated in their Bouthern hemispheres.

346. During a few days in March and April, August and September, Mercury may be seen for several minutes, in the morning or evening twilight, when its greatest elongations happen in those months; in all other parts of its orbit, it is too near the Sun to be seen by the naked eye. The greatest distance that it ever departs from the Sun, on either side, varies from 16° 12', to 28° 48', alternately.

The distance of a planet from the Sun, as seen from the Earth (measured in degrees), is called its elongation. The greatest absolute distance of a planet from the Sun is denominated its aphelion, and the least its perihelion.

347. The revolution of Mercury about the Sun, like that of all the planets, is performed from west to east, in an orbit which is nearly circular. Its apparent motion, as seen from the Earth, is, alternately, from west to east, and from east to west, nearly in straight lines; sometimes directly across the disc of the Sun, but at all other times either a little above or a little below it.

Were the orbits of Mercury and Venus in the same plane with that of the Earth, they would cross the Sun's disc at every revolution; but as one-half of each of their orbits is above, and the other half below the ecliptic, they generally appear to pass either above or below the Sun.

B

E

Let the right line A, joining the Earth and the Sun in the above diagram, represent the plane of the ecliptic. Now when an interior planet is in this plane, as shown at A, It may appear to be upon the Sun's disc; but if it is either above or below the ecliptic, as shown at B and C, it will appear to pass either above or below the Sun, as shown at D and E.

For the relative position of the planets' orbits, and their inclination to the plane of the ecliptic, see 1, of the Atlas. Here the dotted lines continued from the dark lines, denote the inclination of the orbits to the plane of the ecliptic, which inclination is marked in figures on them. Let the student fancy as many circular pieces of paper intersecting each other at the several angles of inclination marked on the Map, and be will be enabled to understand more easily what is meant by the "inclination of the planets' orbits."

348. Being commonly immersed in the Sun's rays in the evening, and thus continuing invisible till it emerges from them in the morning, Mercury appeared to the ancients like two distinct stars. A long series of observations was requisite, before they

846. When may Mercury be seen? Why not at other times? How far does it depart from the Sun on either side? What is meant by the elongation of a planet? Its aphelion and perihelion? 847. In what direction do the planets revolve around the Sun ? What is the apparent motion of Mercury? Do they ever cross the Sun's disc? Why Lot at every revolution? 848. How was Mercury regarded by the ancients?

recognized the identity of the star which was seen to recede from the Sun in the morning with that which approached it in the evening. But as the one was never seen until the other disappeared, both were at last found to be the same planet, which thus oscillated on each side of the Sun.

349. Mercury's oscillation from west to east, or from east to west, is really accomplished in just half the time of its revolution, which is about 44 days; but as the Earth, in the mean time, follows the Sun in the same direction, the apparent elongations will be prolonged to between 55 and 65 days.

350. The passage of Mercury or Venus directly between the Earth and the Sun, and apparently over his disc, is called a Transit. A transit can never occur except when the interior planet is in or very near the ecliptic. The Earth and the planet inust be on the same side of the ecliptic; the planet being at one of its nodes, and the Earth on the line of its nodes.

This cut represents the ecliptic and zodiac, with the orbit of an interior planet, his nodes, &c. The line of his nodes is, as shown, in the 16° of 8 and the 16° of m. Now if the earth is in 8, on the line L N, as shown in the cut, when Mercury is at his ascending node (8), he will seem to pass upward over the Sun's face, like a dark spot, as represented in the figure. On the other hand, if Mercury is at his descending node (8), when the earth is in the 16° of m, the former will seem to pass downward across the

disc of the Sun.

351. As the nodes of his orbit are on opposite sides of the ecliptic, and are passed by the Earth in May and November, it follows that all transits of Mercury must occur in one or the other of these months. They are, therefore, called the Node months. As is shown in the diagram, the Earth passes the

349. In what time is the oscillation of Mercury from east to west What is the apparent time, and why? 350. What is a transit? What are the nodes of a planet's orbit? The line of the nodes?

really accomplished? When do they occur? 851. What are the