occasion of such unworthy fears, but to forewarn men of their occurrence; and by predicting the time, place, and circumstances of the phenomenon, to disarm it of its terror.

There is, however, nothing perhaps so surprising in this science, as that it makes us acquainted with methods, by which we can survey those bright fields on which it is employed, and apply our own familiar measures to the paths which are there traced, and to the bodies that trace them; that we can estimate the form, and dimensions, and inequalities of objects so immense, and so far removed from the little scene of our labours. What would be the astonishment of an inhabitant of one of those bodies, of Jupiter for instance, to find that, by means of instruments of a few feet in length, and certain figures and characters still smaller, all of our own invention, we had succeeded in determining the magnitude and weight of this great planet, the length of its days and nights, and the variety of its seasons, that we had watched the motions of its moons, calculated their eclipses, and applied them to important domestic purposes? What would be our astonishment to learn, that an insect, one of those for instance which serve sometimes to illuminate the waters of the ocean, though confined by the exercise of its properorgans and locomotive powers, to the sphere of a few inches, had, by artificial aids of its own contriving, been able to extend its sphere of observation to the huge monsters that move about it; that it had even attempted, not altogether without success, to fathom the depth of the abyss, in which it occupies so insignificant a place, and to number the beings it contains?

The first use of the telescope, about the commencement of the seventeenth century, opened a new and most brilliant era in the science of astronomy. The defect of the natural organ with respect to the objects of this science had never been recognised. We had gazed upon them without comprehending what we saw. We had cast a vacant eye over the splendid pages of this volume, as children amuse themselves with a book which they are unable to read. We had caught here and there a capital letter, or a picture, but we had failed to distinguish those smaller characters on which the sense of the whole depended. It is not the least of the advantages of this wonderful instrument, that it has taught us the importance of those means of improvement and enjoy

THE SOLAR SYSTEM.

91

ment, which are placed within the reach of our own ingenuity and skill. No one surely would have dreamed of procuring such an aid to the natural sight, any more than of creating a new sense. It would have seemed like changing the law of our being, and the condition in which we are placed. We have, by means of this instrument, emerged, as it were, from a prison. The mind has effected its enlargement, as an insect bursts its little tenement, and flutters through the free air, and over the gay fields.

Another change in this science, of the first importance, was wrought by the genius of Kepler, who died in the year 1630. But the last and most important of all the revolutions that have taken place in it, is that achieved by Newton. There is no other instance of so signal a change in the opinions and pursuits of the philosophic world. It may be compared to those great and rapid conquests, by which new boundaries and new laws have been given to states and kingdoms, and new directions to the industry and active employments of men; with this difference, however, that these have been made by violence, and with the aid and co-operation of others, while the revolution in the sciences effected by Newton, was the silent, solitary work of an individual.

QUESTIONS.-1. What is astronomy? 2. What is said of the improved state of this branch of knowledge? 3. What may be regarded as most surprising in it? 4: What is said of the first use and importance of the telescope? 5. What is said of Kepler? 6. Of Newton ? INOTE. Newton died March 1727, aged 85.]

LESSON 41.

The Solar System.

Or bit, the path in which a celestial body moves. Car'dinal, one of the chief officers in the church of Rome. Inquisition, a court established for the detection of heresy. THE true solar system consists of the sun and an unknown number of opaque bodies, which revolve round the sun, and some of which at the same time revolve round others. Those which revolve round the sun only, are called primary planets and comets. Those which revolve round a primary planet, at the same time they are revolving round the sun,

are called secondary planets, moons, or satellites: The number of comets is unknown. The sun is the centre of the system, and the eleven primary planets, at different distances, and in different times, move round him, from west to east, in the following order, Mercury, Venus, the Earth, Mars, Vesta, Juno, Pallas, Ceres, Jupiter, Saturn, and Herschel or Uranus. The Earth has one moon, Jupiter four, Saturn seven, and Uranus six. Venus and Mercury being nearer to the sun than our earth, are called inferior planets, and all the rest, which are without the earth's orbit, are called superior planets; some astronomers distinguish them by the terms interior and exterior, which seem preferable. The planets are retained in their orbits by the united operation of the centripetal force, by which a body is attracted to the centre of gravity, and the centrifugal force, by which it endeavours to persevere in a straight line. These two powers, mutually balancing each other, compel them to make their respective revolutions. The time of performing their revolutions round the sun is called their year, and the time of performing their revolution on their axis, their day. The axis of a planet is an imaginary line conceived to be drawn through its centre, about which it revolves, and the extremities of this line, terminating on opposite points of the planet's surface, are called its poles.

The first material step in improving the science of astronomy was the establishment of the present arrangement of the sun and planets by Copernicus, who died in the year 1543. This doctrine, it is true, was held by Pythagoras, but it was now presented in a new and stronger light, with its leading features more fully and distinctly unfolded. It is remarkable, that in so many instances, it should have exposed its authors and defenders to persecution. Pythagoras, we are told, made it known only to a select few; but one of his disciples, who had the courage to teach it publicly, was obliged to flee in order to escape the odium it excited. Copernicus meditated upon the subject for many years, before he undertook to give his thoughts to the world, and scarcely surviving the publication of his work, he left to others to receive the shock that awaited those who espoused it. Galileo could not resist the accumulated evidence, that presented itself to his enlarged and philosophic mind, in favour of this refined scheme, and was accordingly destined

to bear the whole weight of indignation that was ready to burst upon the disturbers of a prejudice so old and so deeply rooted. He was arrested, and seven cardinals clothed with the authority of the church sat in judgment upon him, and sentenced him to the prison of the Inquisition for opinions, which they pronounced false in philosophy, heretical, and contrary to the word of God. After a year's confinement he was liberated, but continuing his discoveries, and apparently. persevering in his opinions, he was imprisoned a second time. After being made to abjure what were deemed his errors, and to do penance for his offences, he was again restored to liberty. Indignant at the cruelty of this treatment, and the bigotry and blindness of his persecutors, he yet continued his pursuits; but in silence and fear. His excessive application, and the constant use of his telescope, together with frequent exposure to the air by night, had such an effect upon him, that he lost his sight. He died in 1642, at the age of seventy-eight.

QUESTIONS. 1. Of what does the solar system consist? 2. What are primary planets?-secondary planets? 3. What is the order in which the eleven primary planets move round the sun? 4. What planets have moons, and how many have each? 5. What are interior and exterior planets? 6. How are the planets retained in their orbits? 7. Define year, day, axis, poles. 8. What was the first material step in the progress of astronomy? 9. What is remarkable with respect to the true doctrine of the solar system? 10. What course did Copernicus adopt? 11. What is said of Galileo? 12. Explain Engr. IV.

LESSON 42.

The Sun.

Spheroid, a body approaching to the form of a sphere, but not exactly round.

Elliptical, oval,-an ellipse is produced from the section of a
cone by a plane cutting both its sides, but not parallel to the
base. All the planets move round the sun in elliptical orbits,
and the sun itself is situated in one of the foci of each ellipse:
that focus is called the lower focus. See the Earth's orbit in
fig. 40.

GREAT Source of day! best image here below
Of thy Creator, ever pouring wide,

From world to world, the vital ocean round;

On nature, write with every beam, His praise.
Soul of surrounding worlds!—

'Tis by thy secret, strong, attractive force,
As with a chain indissolubly bound,

Thy system rolls entire; far from the bourn
Of utmost Herschel, wheeling wide his round
Of eighty years; to Mercury whose disk
Can scarce be caught by philosophic eye,
Lost in the near effulgence of thy blaze.

THOMSON.

The sun is a fountain of light that illuminates the world; it is the cause of that heat which maintains the productive power of nature, and makes the earth a fit habitation for man. The figure of the sun is a spheroid, higher under the equator than about the poles; and his diameter is computed to be nearly nine hundred thousand miles. His solid bulk is more than a million of times larger than that of the earth. The sun has two motions; the one is a periodical motion, in an elliptical or very nearly circular direction, round the common centre of all the planetary motions; the other is a revolution upon its axis, which is completed in about twenty-six days. That the sun has a rotation round his axis is made evident by the spots seen on his surface. Some of these spots have made their first appearance near the edge or margin of the sun, and have been seen some time after on the opposite edge; whence, after a stay of more than thirteen days, they have re-appeared in their first place, and taken the same course over again. These spots were entirely unknown before the invention of telescopes, though they are sometimes of sufficient magnitude to be discerned by the naked eye. Some have been so large, as by computation to be capable of covering the continents of Asia and Africa, the whole surface of the earth, or even five times its surface. The sun has commonly been considered a globe of fire; but this has been doubted by modern astronomers.. The celebrated Herschel considers the sun as a most magnificent habitable globe, surrounded by a very extensive atmosphere, which consists of elastic fluids that are more or less lucid and transparent; and of which the lucid ones furnish us with light. The appearances, called spots in the sun, he considers as real openings in the luminous clouds of the solar atmosphere.