In the opinions of the Egyptian sages, in those of Pythagoras, Philolaus, Aristarchus, and Nicetas, he recognized his own earliest conviction that the Earth was not the center of the universe. His attention was much occupied with the speculation of Martinus Capella, who placed the Sun between Mars and the Moon, and made Mercury and Venus revolve round him as a center, and with the system of Appollonius Pergous who made all the planets revolve around the Sun, while the Sun and Moon were carried around the Earth in the center of the universe.

321. The examination, however, of various hypotheses, by Copernicus, gradually expelled the difficulties with which the subject was beset, and after the labor of more than thirty years, he was permitted to see the true system of the universe. The Sun he considered as immovable, in the center of the system, while the Earth revolved around him, between the orbits of Venus and Mars, and produced by its rotation about its axis all the diurnal phenomena of the celestial sphere. The other planets he considered as revolving about the Suu, in orbits exterior to that of the Earth. (See the Relative Distances of the Planets' Orbits, Map I. of the Atlas.)

Thus, the stations and retrogradations of the planets were the necessary consequence of their own motions, combined with that of the Earth about the Sun. He said that "by loug observation, he discovered that, if the motions of the planets be compared with that of the Earth, and be estimated according to the times in which they perform their revolutions, not only their several appearances would follow from this hypothesis, but that it would so connect the order of the planets, their orbits, magnitudes, and distances, and even the apparent motion of the fixed stars, that it would be impossible to remove one of these bodies out of its place without disordering the rest, and even the whole of the universe also."

322. Soon after the death of Copernicus, arose Tycho Brahe, born at Knudstorp, in Norway, in 1546. Such was the distinetion which he had attained as an astronomer, that when, dissatisfied with his residence in Denmark, he had resolved to remove, the King of Denmark, learning his intentions, detained him in the kingdom, by presenting him with the canonry of Rothschild, with an income of 2,000 crowns per annum. He added to this sum a pension of 1,000 crowns, gave him the island of Huen, and established for him an observatory at an expense of about 200,000 crowns. Here Tycho continued, for twenty-one years, to enrich astronomy with his observations.

His observations upon the Moon were important, and upon the planets numerous and precise, and have formed the data of the present generalizations in astronomy. He, however, rejected the system of Copernicus; considering the Earth as immovable in the center of the system, while the Sun, with all the planets and comets revolving around him, performed his revolution around the earth, and, in the course of twenty-four hours, the stars also revolved about the central body. This theory was not so simple as that of Copernicus, and involved the absurdity of making the Sun, planets, &c., revolve around a body comparatively insignificant.

321. How was Copernicus led to discover the true system of astronomy? What is that system? Does it account for the stations and retrogradations of the planets? 322 What distinguished astronomer next arose? What said of his detention in Denmark His observations? His theory

323. Near the close of the 15th century, arose two men, who wrought most important changes in the science; Kepler and Galileo, the former a German, the latter an Italian. Previous to Kepler, all investigations proceeded upon the supposition that the planets moved in circular orbits which had been a source of much error. This supposition Kepler showed to be false. He discovered that their orbits were ellipses. The orbits of their secondaries or moons he also found to be the same curve. Ile next determined the dimensions of the orbits of the planets, and found to what their velocities in their motions through their orbits, and the times of their revolutions, were proportioned; all truths of the greatest importance to the science.

324. While Kepler was making these discoveries of facts, very essential for the explanation of many phenomena, Galileo was discovering wonders in the heavens never before seen by the eye of man. Having improved the telescope, and applied it to the heavens, he observed mountains and valleys upon the surface of our Moon; satellites or secondaries were discovered revolving about Jupiter; and Venus, as Copernicus had predicted, was seen exhibiting all the different phases of the Moon, waxing and waning as she does, through various forms.

Many minute stars, not visible to the naked eye, were described in the Milky-Way; and the largest fixed stars, instead of being magnified, appeared to be small brilliant points, an incontrovertible argument in favor of their immense distance from us. All his discoveries served to confirm the Copernican theory, and to show the absurdity of the hypothesis of Ptolemy.

325. Although the general arrangement and motions of the planetary bodies, together with the figure of their orbits, had been thus determined, the force of power which carries them around in their orbits, was as yet unknown. The discovery of this was reserved for the illustrious Newton, though even his discovery was in some respects anticipated by Copernicus, Kepler and Hooke. By reflecting on the nature of gravity—— that power which causes bodies to descend toward the center of the earth-since it does not sensibly diminish at the greatest distance from the center of the earth to which we can attain, being as powerful on the loftiest mountains as it is in the deepest caverns, he was led to imagine that it might extend to the Moon, and that it might be the power which kept her in her orbit, and caused her to revolve around the Earth. He was next led to suppose that perhaps the same power carried the

828. What two noted astronomers next arose? What did Kepler discover? 824, Galileo and his discoveries? What theory did they serve to establish? 825 What gat discovery next made, and by whom? How led to it? Successive steps?

primary piasets around the Sun. By a series of calculations, he was enabled at length to establish the fact, that the same force which determines the fall of an apple to the Earth, carries the moons in their orbits around the planets, and the planets aud comets in their orbits around the Sun.

To recapitulate briefly: The system (not hypothesis, for much of it has heen established by mathematical demonstration) by which we are now enabled to explain with a beautifu' simplicity the different phenomena of the Sun, planets, moons, and comets, is, that the Sun is the central body in the system: that the planets and comets move round him in elliptical orbits, whose planes are more or less inclined to each other, with velocities bearing to each other a certain ascertained relation, and in times related to their distances; that the moons, or secondaries, evolve in like manner about their primaries, and at the same time accompany them in their motion around the Sun; all meanwhile revolving on axes of their own; and that these revolutions in their orbits are produced by the mysterious power of attraction. The particular mode in which this system is applied to the explanation of the different phenomena, will be exhibited as we proceed to consider, one by one, the several bodies above mentioned.

326. These bodies, thus arranged and thus revolving, constitute what is termed the Solar System. The planets have been divided into two classes, primaries and secondaries. The latter are also termed moons, and sometimes satellites. The primaries are those that revolve about the Sun, as a center. The secondaries are those which revolve about the primaries. There have been discovered to this date (1854), thirty-five primary plauets, viz.: Mercury, Venus, the Earth, Mars, Flora, Clio, Vesta, Iris, Metis, Eunomia, Psyche, Thetis, Melpomene, Fortuna, Massilia, Lutetia, Calliope, Thalia, Hebe, Parthenope, Irene, Egeria, Astræa, Juno, Ceres, Pallas, Hygeia, Jupiter, Saturn, Uranus, Neptune, and four other Asteroids, whose names and places have not yet been determined. Mercury is the nearest to the Sun, and the others follow in the order in which they are named. The seventeen small planets from Flora to Hygeia, inclusive, were discovered by means of the telescope, and, because they are very small, compared with the others, are called Asteroids. Neptune, also, is a telescopic planet, though much larger than any of the Asteroids.

There have been discovered twenty secondaries. Of these, the Earth has one, Jupiter four, Saturn eight, Herschel six, and Neptune one. All these, except our Moon, as well as the Asteroids and Neptune, are invisible to the naked eye.

Map I. of the Atlas, "exhibits a plan of the Solar System," comprising the relative magnitudes of the Sun and Planets; their comparative distances from the Sun, and from each other; the position of their orbits, with respect to each other; the Earth and the Sun; together with many other particulars which are explained on the map. There, the

Describe the Copernican theory? 326. What do the bodies mentioned constitute? How are the planets divided? Describe each? What number of primaries? Name them in order from the Sun? Which are the Asteroids? Which telescopic? How many secondary planets? How distributed? Are they visible to the naked eye? What said

first and most prominent object which claims attention, is the representation of the Sun's circumference, with its deep radiations, bounding the upper margin of the map. It is apparent, however, that this segment is hardly one-sixth of the whole circumference of which it is a part. Were the map sufficiently large to admit the entire orb of the Sun, even upon so diminutive a scale as there represented, we should then see the Sun and Planets in their just proportions-the diameter of the former being 112 times the diameter of the Earth.

It was intended, originally, to represent the Earth upon a scale of one inch in diameter and the other bodies in that proportion; but it was found that it would increase the map to four times its size; and hence it became necessary to assume a scale of half an inch for the Earth's diameter, which makes that of the Sun 56 inches, and the other bodies, as represented upon the map.

The relative position of the Planets' orbits is also represented, on a scale as large as the sheet would permit. Their relative distances from the Sun as a center, and from each other, are there shown correctly. But had we wished to enlarge the dimensions of these orbits, so that they would exactly correspond with the scale to which we have drawn the planets, the map must have been nearly two miles in length. "Hence," says Sir John Herschel," the idea that we can convey correct notions on this subject, by drawing circles on paper, is out of the question."

To illustrate this-Let us suppose ourselves standing on an extended plane, or field of ice, and that a globe 4 feet 8 inches in diameter is placed in the center of the plane, to represent the Sun. Having cut out of the map the dark circles representing the planets, we may proceed to arrange them in their respective orbits about the Sun, as follows:

First, we should take Mercury, about the size of a small currant, and place it on the circumference of a circle 194 feet from the Sun; this circle would represent the orbit of Mercury, in the proper ratio of its magnitude. Next, we should take Venus, about the size of a rather small cherry, and place it on a circle 862 feet from the Sun, to represent the orbit of Venus. Then would come the Earth, about the size of a cherry, revolving in an orbit 500 feet from the Sun. After the Earth we should place Mars, about the size of a cranberry, on a circle 762 feet from the Sun. Neglecting the Asteroids, some of which would not be larger than a pin's head, we should place Jupiter, hardly equal to a moderate-sized melon, on a circle at the distance of half a mile (2601 feet) from the Sun; Saturn, somewhat less, on a circle nearly a mile (4768 feet) from the Sun; Herschel, about the size of a peach, on the circumference of a circle nearly 2 miles (9591 feet) from the Sun; and last of all Neptune, a little larger than Herschel, and on à circle of nearly 3 miles (15,366 feet) from the Sun.

To imitate the motions of the planets in the above-mentioned orbits, Mercury must describe its own diameter in 41 seconds; Venus, in 4 minutes 14 seconds; the Earth, in 7 minutes; Mars, in 4 minutes 48 seconds; Jupiter, in 2 hours 56 minutes; Saturn, ir 8 hours 18 minutes; Herschel, in 12 hours 16 minutes; and Neptune, in 23 hours 25 min. Many other interesting subjects are embraced in Map I.; but they are either explained on the map, or in the following chapters, to which they respectively relate.

CHAPTER II.

THE SUN-HIS DISTANCE, MAGNITUDE, &o.

327. THE Sun is a vast globe, in the center of the solar systera, dispensing light and heat to all the planets, and governing all their motions. It is the great parent of vegetable life, giving warmth to the seasons, and color to the landscape. Its rays are the cause of various phenomena on the surface of the earth and in the atmosphere. By their agency, all winds are pro

of Map I.! Its scale f Remark of Dr. Herschel? does he furnish? 827. Subject of Chapter 11.? B.G.

What illustrations of the Solar System
Describe the Sun?

duced, and the waters of the sea are made to circulate in vapor through the air, and irrigate the land, producing springs and rivers.

328. The Sun is by far the largest of the heavenly bodies whose dimensions have been definitely ascertained. Its diameter is about 889,000 miles. Consequently, it contains a volume of matter equal to fourteen hundred thousand globes of the size of the Earth. Of a body so vast in its dimensions, the human mind, with all its efforts, can form no adequate conception.

THE SUN AND THE MOON'S ORBIT.

2030004240.000

240,000

480.000

03.000

Were the Sun a hollow sphere, perforated with a thousand openings to admit the twinkling of the luminous atmosphere around it-and were a globe as large as the Earth placed at its center, with a satellite as large as our Moon, and at the same distance from it as she is from the earth, there would be present to the eye of a spectator on the interior globe, a universe as splendid as that which now appears to the uninstructed eye -a universe as large and extensive as the whole creation was conceived to be in the infancy of astronomy.

The mean distance of the Moon from the Earth is 240,000 miles, consequently the average diameter of her orbit is 480,000 miles; and yet, were the Sun to take the place of the Earth, he would fill the whole orbit of the Moon, and extend 200,000 miles beyond it in every direc tion! To pass from side to side through his center, at railroad speed (30 miles an hour), would require nearly three and a half years,

and to traverse his vast circumference nearly eleven years. Here let the student refer to Map I., where the Relative Magnitudes of the Sun and Planets are exhibited. Let him compare the segment of the Sun's circumference, as there represented, with the entire circumference of the Earth. They are both drawn upon the same scale. The segment of the Sun's circumference, since it is almost a straight line, must be a very small part of what the whole circumference would be, were it repre. sented entire. Let the student understand this diagram, and he will be in some measure able to conceive how like a mere point the Earth is, compared with the Sun, and to form in his mind some image of the vast magnitude of the latter.

329. The next thing which fills the mind with wonder, is the distance at which so great a body must be placed, to occupy, apparently, so small a space in the firmament. The Sun's mean

distance from the Earth is twelve thousand times the Earth's diameter, or a little more than 95,000,000 of miles. We may derive some faint conception of such a distance, by considering that the swiftest steamboats, which ply our waters at the rate of 200 miles a day, would not traverse it in thirteen hundred years; and, that a cannon ball, flying night and day, at the rate of 16 miles a minute, would not reach it in eleven years.

330. The Sun, when viewed through a telescope, presents the appearance of an enormous globe of fire, frequently in a state of violent agitation or ebullition; dark spots of irregular form,

838. His magnitude? Diameter? Compared with the Earth? What illustration given? What reference to the Map? 329. Distance of the Sun? What illustration given 880. How does the Sun appear through a telescope? Describe these spots?