the earth; for an eclipse of one of these satellites appears to us to take place sixteen minutes sooner, when the earth is in the part of her orbit nearest Jupiter, than when in the part farthest from him. Hence light is sixteen minutes in crossing the earth's orbit, and of course eight minutes in coming from the sun. An observer on Jupiter, with eyes constructed like ours, could never see Mercury, Venus, the Earth, or Mars, for, on account of the immense distance, they are always immersed in the sun's rays.

QUESTIONS.-1. What is the diameter of Jupiter?-distance from the sun?-time of revolution round the sun ?-diurnal rotation? 2. Describe the telescopic appearance of Jupiter. 3. What is the posi tion of his axis, and the consequence of that position? 4. What is said of Jupiter's moons? 5. Of what use are their eclipses? 6. To what discovery have they led ?-how?

LESSON 52.

Saturn and Uranus.

Anomaly, irregularity, deviation from rule.

Hypothesis, a supposition, a system formed under some principle not proved.

SATURN though not so brilliant as Jupiter, is a very conspicuous planet. It shines with a pale light, and the degree of heat and light is eighty times less than at the earth. It revolves round the sun in little less than thirty years, at the mean distance of nine hundred millions of miles. It turns on its axis in little more than ten hours, and its diameter is seventy-nine thousand miles.

Saturn, as seen through a good telescope, is a beautiful object, having seven moons, a double ring, and appearances similar to the belts of Jupiter. The ring is one of the greatest anomalies in our system. It is a thin, broad, opaque, circular body, encompassing the planet without touching it, like the wooden horizon of an artificial globe. Although the phenomenon is usually termed the ring, yet it consists of two, entirely detashed from each other and from the body of the planet, one exactly without or beyond the other. Stars have been seen through the vacancy between them, and also between the inner ring and the planet. Concern

ing the nature and uses of the ring there have been various hypotheses. Dr. Herschel thinks it not less solid than the body of Saturn itself, and it is observed to cast a strong shadow upon it. The light of the ring is generally brighter than that of the planet, which has been attributed to its situation above the region of mists and clouds. Both the planet and the ring perform their rotations about the same common axis, and in nearly the same time. The ring disappears twice in every revolution of the planet round the sun; that is, once in fifteen years, and Saturn appears quite circular for nine months together. Some have supposed that the use of the ring is to collect, refract, and transmit the rays of the sun to the body of the planet.

The planet Uranus, or Herschel, completes a revolution round the sun in about eighty-four years. On account of its distance from the earth, which is eighteen hundred millions of miles, its diurnal rotation has never been determined. Heat and light at Uranus are about three hundred and sixty times less than at the earth. It is scarcely visible to the naked eye, although its diameter is thirty-five thousand miles. Astronomers formerly considered it as a star, but on the 13th of March, 1781, Dr. Herschel discovered it to be a planet.

QUESTIONS.-1. How far is Saturn from the sun? 2. What degree of light and heat has it? 3. How often does it revolve round the sun? 4. On its own axis? 5. What is the appearance of Saturn as seen through a telescope? 6. Describe the ring. 7. What is said concerning the nature and uses of the ring. 8. In what time does Uranus complete a revolution? 9. At what distance from the sun? 10. What is the diameter of Uranus? 11. Degree of heat and light? 12. When and by whom discovered? [NOTE. Saturn's inner ring is distant from its body 21,100 miles. The breadth of the inner ring is 20,000 miles. The outer ring is distant from the inner ring 2,839 miles, and the breadth of the outer ring is 7,200 miles. Uranus is the name which has been given to the dus, on the continent of Europe.]

planet Herschel, or Georgium Si

LESSON 53.

Comets.

Hast thou ne'er seen the comet's flaming flight?
Th' illustrious stranger passing, terror sheds

On gazing nations, from his fiery train

Of length enormous; takes his ample round
Through depths of ether; coasts unnumbered worlds
Of more than solar glory; doubles wide

Heaven's mighty cape, and then revisits earth, From the long travel of a thousand years. YOUNG. Besides the primary and secondary planets, there are other bodies which revolve round the sun, and consequently make a part of the solar system. These are called comets, and appear occasionally in every part of the heavens. They are solid, opaque bodies, generally distinguished by a lucid train or tail, issuing from that side which is turned away from the sun. Most of them move in very elliptical orbits; at one time coming very near the sun, even nearer than Mercury, and again receding to a distance far beyond the orbit of Uranus. The train is so transparent, that the fixed stars may be seen through it, and sometimes it extends to an immense distance in the heavens. The farther it reaches, the broader it seems to become, and at times it is divided into rays.

That

Viewed through a telescope, comets appear full of spots and inequalities, and a vapour frequently renders it impossible to observe their figure. In a clear sky, however, the solid body of a comet often reflects a splendid light. part of astronomy relating to comets is still imperfect, for the opinion once prevailed, that they were only meteors generated in the air, like those we see in a clear night, vanishing in a few moments, and no care therefore was taken to observe or record their phenomena with accuracy.

The number of comets belonging to the solar system is unknown. More than five hundred have appeared since the commencement of the christian era. The orbits of ninetyeight comets, up to the year 1808, have been calculated; but of all the comets the periods of only three are known with any degree of certainty, being found to return at intervals of seventy-five, one hundred twenty-nine, and five hundred and seventy-five years; and of these that which appeared in 1680'is the most remarkable. This comet, which will not appear again till the year 2225, at its greatest distance, is about eleven thousand two hundred millions of miles from the sun, while its least distance from the centre of the sun is about four hundred ninety thousand miles. In that part of its orbit nearest the sun, it flies, according to Newton, with

a velocity of eight hundred eighty thousand miles an hour; but according to calculations made since the days of Newton, its motion has been computed to be one million two hundred forty miles an hour.

The comet of 1758 was looked for with great interest by astronomers, because its return had been predicted. But it is worthy of remark, that what, in this century, excited only the curiosity of astronomers and mathematicians, had been regarded four revolutions before, in 1456, with feelings of horror. Its long train spread consternation over all Europe, already terrified at the success of the Turkish arms, which had just destroyed the great empire. Pope Callixtus, on this occasion, ordered a prayer, in which the comet and the Turks were included in the same anathema.

QUESTIONS.-1. What are comets? 2. How do they move? 3. What is said of the train of a comet? 4. How do comets appear through a telescope? 5. What is said of the number of comets? 6. What is known of the orbits of comets? 7. What is said of the comet of 1680? 8. What is worthy of remark with respect to the comet of 1758? [NOTE. The comet of 1758 is expected to return in 1834.]

own.

LESSON 54.

The Fixed Stars.

Neb'ula, (plural, nebulæ,) a cloud of obscure light in the heavens; some nebulæ consist of clusters of telescopic stars, others appear as luminous spots of different forms. Sir'ius, the dog-star. THOSE luminous bodies which always appear in the heavens at the same distance from each other, are called fixed stars; because, with the exception of a few, which, in a course of years, appear to change their places, it has not been discovered, that they have any proper motion of their When viewed through a telescope they appear as points of small magnitude; they must be at such an immense distance, therefore, as to be invisible to the naked eye, if they borrowed their light; as is the case with the satellites of Jupiter and Saturn, although they appear of very distinguishable magnitude through a telescope. The stars are probably suns, around each of which revolve primary and secondary planets, as about our sun. They are distinguishable from the planets by their twinkling,

The magnitudes of the fixed stars appear to be different from one another, which difference may arise either from a diversity in their real magnitudes, or distances; or from both these causes acting together. The difference in the apparent magnitude of the stars is such as to admit of their being divided into six classes, the largest being called stars of the first magnitude, and the least which are visible to the naked eye, stars of the sixth magnitude. Stars that cannot be seen without the help of glasses are called telescopic stars. The number of stars, visible at any one time to the naked eye, is about one thousand; but Dr. Herschel, by his skilful improvements of the reflecting telescope, has discovered that the whole number is great beyond all conception. Upon viewing the heavens during a clear night, we discover a pale irregular light, and a number of stars whose mingled rays form the luminous tract called the milky-way. The stars themselves are at too great a distance to be perceived by the naked eye; and among those which are visible with a telescope there are spaces apparently filled with others in immense numbers. Many whitish spots or tracts (called nebula) are visible in different parts of the heavens, which are supposed to be milky-ways at an inconceivable distance.

The distance of these remoter bodies is so vast and measureless, that we can hardly speak of it except in relation to the inconceivable swiftness of light. The rays by which they are now made visible to the eye of the astronomer, the rapid motion of which might circle the earth while one is pronouncing a syllable, have been darting forward for thousands and ten thousands of years to reach us. All the events and revolutions, which history records, have taken place during their progress. They commenced their career, it has been computed, at a period of such remote antiquity, that, compared with it, the date of that time, when God gave the earth to man for a habitation, is but of yesterday.

Dr. Herschel has calculated that the distance of the remotest nebulæ, exceeds that of the nearest fixed star at least three hundred thousand times. Upon this fact, he thus remarks; a telescope with a power of penetrating into space, like my forty feet one, has also, as it may be called, a power of penetrating into time past. To explain this we must consider that from the known velocity of light, it may be proved that when we look at the star called Sirius, the