rarely visible to the naked eye, frequently pass over his disc, from east to west, in the period of nearly fourteen days.

These spots are usually surrounded by a penumbra, or less deeply shaded border, and that, by a margin of light more brilliant that that of the Sun. A spot when first seen on the eastern edge of the Sun, appears like a line which progressively extends in breadth, and increases its apparent velocity, till it reaches the middle, when it begins to contract, and to move less rapidly, till it ultimately disappears at the western edge. In some rare instances, the same spots re-appear on the east side, and are permanent for two or three revolutions. But, as a general thing, the spots on the Sun are neither permanent nor uniform. Sometimes several small ones unite into a large one; and, again, a large one separates into numerous small ones. Some continue several days, weeks, and even months, together; while others appear and disappear, in the course of a few hours. Those spots that are formed gradually, are, for the most part, as gradually dissolved;

whilst those that are suddenly formed, generally vanish as quickly.

331. It is the general opinion, that spots on the Sun were first discovered by Galileo, in the beginning of the year 1611; though Scheiner, Harriot, and Fabricius, observed them about the same time. During a period of 18 years from this time, the Sun was never found entirely clear of spots, excepting a few days in December, 1624: at other times, there were frequently seen twenty or thirty at a time, and in 1625, upwards of fifty were seen at once. From 1650 to 1670, scarcely any spots were to be seen; and, from 1676 to 1684, the orb of the Sun presented an unspotted disc. Since the beginning of the eighteenth century, scarcely a year has passed, in which spots have not been visible, and frequently in great numbers. In 1799, Dr Herschel observed one nearly 30,000 miles in breadth.

A single second of angular measure, on the Sun's disc, as seen from the Earth, corresponds to 462 miles; and a circle of this diameter (containing therefore nearly 220,000 square miles) is the least space which can be distinctly discerned on the Sun as a visible area, even by the most powerful glasses. Spots have been observed, however, whose lincar diameter has been more than 44,000 miles; and, if some records are to be trusted, of even still greater extent.

DR. DICK, in a letter to the author, says: "I have for many years examined the solar spots with considerable minuteness, and have several times seen spots which were not less than the one twenty-fifth part of the Sun's diameter, which would make them about 22,192 miles in diameter, yet they were visible neither to the naked eye, nor through an opera glass magnifying about three times. And, therefore, if any spots have been visible to the naked eye-which we must believe, unless we refuse respectable testimonythey could not have been much less than 50,000 miles in diameter."

38 Who first saw them? When? How was it for the next 18 years? How in 1625? From 1650 to 1670? From 1676 to 1684? How since the beginning of the eighteenth century? Dr. Herschel's measurements? Dr. Dick's remarks and conclusion?

332. The apparent direction of these spots over the Sun's diso is continually varying. Sometimes they seem to move across it in straight lines, at others in curve lines. Sometimes the spots seem to move upward, as they cross from east to west, while at other times they incline downward, while the curve lines are sometimes convex towards one pole of the Sun, and sometimes towards the other.

333. All these phenomena are owing to the fact that the axis of the Sun is inclined to the ecliptic, so that viewing him from different points in the Earth's orbit, the apparent direction of the spots must necessarily vary. The following diagrams may serve to illustrate :

Let E F represent the plane of the ecliptic. In March, the spots describe a curve, which is convex to the south, as shown at A. In June, they cross the Sun's disc in nearly straight lines, but incline upward. In September, they curve again, though in the opposite direction; and in December, pass over in straight lines, inclining downward. The figures B and D show the inclination of the Sun's axis.

The following diagram will serve still further to illustrate the cause of the change of direction of the solar spots.

SOLAR SPOTS OBSERVED FROM DIFFERENT POINTS.

Let the student imagine himself stationed upon the earth at A, in March, looking upon the sun in the center, whose north or upper pole is now inclined toward him. The spots will then curve u noard. Three months afterward-viz., in June-the earth will be

.

332. In what general direction do these spots move? What variations? is the cause of these varying phenomena ?

888. What

at B; when the sun's axis will incline to the left, and the spots seem to pass upward to the right. In three months longer, the observer will be at C, when the north pole of the sun will incline from him, and the spots seem to curve upward; and in three months longer, he will be at D, when the axis of the sun will incline to the right, and the spots seem to incline downward.

334. From the regularity with which these spots revolve, it is concluded, with good reason, that they adhere to the surface of the Sun and revolve with it. They are all found within 30° of his equator, or within a zone 60 in width.

335. The apparent revolution of a spot, from any particular point of the Sun's disc, to the same point again, is accomplished in 27 days, 7 hours, 26 minutes, and 24 seconds; but during nat time, the spot has, in fact, gone through one revolution, together with an arc, equal to that described by the Earth in ner orbit in the same time; which reducest he time of the Sun's actual rotation on his axis, to 25 days, 9 hours, and 36 minutes.

Let S represent the sun, and A the earth in her orbit. When she is at A, a spot is seen upon the Gisc of the sun at B. The sun revolves in the direction of the arrows, and in 25 days 10 hours the spot comes round to B again, or opposite the star E. This is a sidereal revolution.

During these 25 days 8 hours, the earth has passed on in her orbit some 25°, or nearly, to C, which will require nearly two days for the spot at B to get directly toward the earth, as shown at D. This last is a synodic revolution. It consists of one complete revolution of the sun upon his axis, and about 27° over.

336. The part of the Sun's disc not occupied by spots, is far from being uniformly bright. Its ground is finely mottled with an appearance of minute dark dots, or pores, which, attentively watched for several days in succession, are found to be in a constant state of change.

It

What the physical organization of the Sun may be, is a question which astronomy, in its present state, cannot solve. seems, however, to be surrounded by an ocean of inexhaustible flame, with dark spots of enormous size, now and then floating upon its surface. From these phenomena, Sir W. Herschel supposed the Sun to be a solid, dark body, surrounded by a vast

334. Are these spots supposed to adhere to the body of the Sun? On what part of the Sun are they found? 835. What is their time of apparent revolution? The actual time? How arrived at? 836. What said of the part of the Sun about his poles? Of What does it seem to be? How did Sir W. Hersche

his physical organization? regard it?

atmosphere, almost always filled with luminous clouds, occasion. ally 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 divers ded 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.

337. Laplace's speculations? What other opinions? 838. Is there a satisfactory theory of the physical nature of the Sun? State the prevailing one? Herschel's suppc. sition? 839. What conjecture in regard to the inhabitants of the Sun, and upon That founded? Who held to this idea?

CHAPTER III.

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 the smallest. Its diameter is about 3,000 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 8,140 miles, and that of the Earth 7,912; their bulks are as the cube of 8,140, 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 Jonstitutes 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 tl:s compare with the Earth? His period of revolution around the Sun? 342 What euid of discoveries upon Mercury, his phases, &c.? What proof that he is opaque?