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 descried in the milky-way; and the largest fixed stars, instead of being magnified, appeared to be small brilliant points, an incontrovertible argument in favour 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.

Although the general arrangement and motions of the planetary bodies, together with the figure of their orbits, had been thus determined, the force or power which carries them around in their orbits, was as yet unknown. The discovery of this was reserved for the illustrious Newton.* By reflecting on the nature of gravity-that power which causes bodies to descend towards the centre of the earth-since it does not sensibly diminish at the greatest distance from the centre 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 primary planets 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 and comets in their orbits around the Sun.

To recapitulate briefly: the system, (not hypothesis, for much of it has been established by mathematical demonstration,) by which we are now enabled to explain with a beautiful 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 othert a certain ascertained relation, and in times related to their distances; that the moons, or secondaries, revolve in like manner, about their primaries, and at the same time accompany

*The discovery of Newton was in some measure anticipated by Copernicus, Kepler, and Hooke.

The orbits or paths of the planets were discovered by tracing the course of the planets by means of the fixed stars.

What was the discovery of Newton? How was he led to make it? Recapitulate briefly the system by which we are enabled to explain the different celestial phenomena.

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.

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 which revolve about the Sun, as a centre. The secondaries are those which revolve about the primaries. There have been discovered eleven primaries; namely, Mercury, Venus, the Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Herschel; of which, Mercury is the nearest to the Sun, and the others follow, in the order in which they are named. Vesta, Juno, Ceres, and Pallas, were discovered by means of the telescope, and, because they are very small, compared with the others, are called asteroids. There have been discovered, eighteen secondaries. Of these, the Earth has one, Jupiter four, Saturn seven, and Herschel six. All these, except our Moon, as well as the asteroids, are invisible to the naked

eye.

Plate 1, 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 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 adinit 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 4 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 centre, 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 4 miles in length. Hence, says Sir John Herschel, "the idea that

What is meant by the Solar System? Into what two classes have the planets been divided? Define a primary planet. Define a secondary planet. How many primary planets have been discovered? What are their names, and what the order of their distance from the sun? Which of them were discovered by means of the telescope? Why are these termed asteroids? How many secondaries have been discovered? How are they distributed among the primaries? Which of the primaries and secondaries are invisible to the naked eye?

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 centre 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 362 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 cranber ry, 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 cirle nearly a mile (4769 feet) from the Sun; and last of all, we should place Herschel, about the size of a peach, on the circumference of a circle nearly 2 iniles (9591 feet) from the Sun.

To imitate the motions of the planets, in the abovementioned 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, in 3 hours 13 minutes; and Herschel, in 2 hours 16 minutes.

Many other interesting subjects are embraced in Plate 1; but they are either explained on the map, or in the following Chapters, to which they respectively relate.

CHAPTER XIX.

THE SUN.

The sun is a vast globe, in the centre of the solar system, 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 colour to the landscape. Its rays are the cause of various vicissitudes on the surface of the earth and in the atmosphere. By their agency, all winds are produced, and the waters of the sea are made to circulate in vapour through the air, and irrigate the land, producing springs and rivers.

The Sun is by far the largest of the heavenly bodies whose dimensions have been ascertained. Its diameter is something more than 887 thousand 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

Mention some of the effects produced by the Sun. What is its magnitude compared with that of the other heavenly bodies whose dimensions have been estimated? What is its diameter 7 How much larger is the Sun than the Earth?

form no adequate conception. The whole distance between the Earth and the Moon would not suffice to embrace one third of its diameter.

Here let the student refer to Plate 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 represented 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 An his mind some image of the vast magnitude of the latter.

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 centre, 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 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 millions 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.

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

These spots are usually surrounded by a penumbra, and that, by a margin of light, more brilliant than 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, till it reaches the middle, when it begins to contract,

What is the whole distance between the Earth and the Moon, compared with the diameter of the Sun? Give some illustration to enable us to conceive of the magnitude of the Sun. What is the distance of the Sun from the Earth? Give some illustration to enable us to conceive of the distance. What is the appearance of the Sun when viewed through a telescope? In what time do the spots seen on the Sun pass across the disc? In what direction do they move? Describe their appearance.

In some

and ultimately disappears, at the western ehe east side,

rare instances, the same spots re-appear on

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.

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 linear 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 testimony-they could not have been much less than 50,000 miles in diameter."

The apparent motion of these spots over the Sun's surface, is continually varying in its direction. Sometimes they seem to move across it in (straight lines, at others in curve lines. These phenomena may be familiarly illustrated in the following manner.

Do the same spots ever re-appear on the east side? Are the spots generally permanent and uniform? Describe their irregularities? Who, is it generally supposed, first discovered spots on the Sun? Who else observed them about the same time? What was the breadth of the one seen by Dr. Herschel in 1799 ? In what direction do the spots on the Sun appear to move?