same meridian opposite the moon; we are fifty minutes in overtaking her, and the tides are retarded for the same reason that the moon rises later on one day than on the ceding.

pre

The tides, though constant, are not equal; but are greatest when the moon is in conjunction with the sun or in opposition to it, or at the time of new and full moon; and least, when in quadrature to it. This increase and diminution constitute the spring and neap tides. The attraction of the sun does not raise tides; its only effect is to increase or diminish those of the moon. The tides are highest when both the luminaries are in the equator, and the moon at the least distance from the earth. This happens at the time of the equinoxes. The tide is at the greatest height, not when the moon is in the meridian, but some time afterwards, because the force by which the moon raises the tide continues to act after it has passed the meridian. The regular tides are greatly affected by strong winds. Continents also stop them in their course from east to west, and in narrow rivers they are frequently very high and sudden, from the resistance of the banks. The advantages arising from tides are great. By agitating the waters of the ocean they preserve them in a state of purity. Aided by their means, ships of the largest burden sail up rivers against their natural course, and convey into the interior of countries those productions which stimulate the industry and promote the happiness of nations.

QUESTIONS.-1. What are the tides? 2. How are they occasioned? 3. How does it appear that the moon produces high water in two places at the same time? 4. How do you account for the tide being fifty minutes later every day? 5. What are spring and neap tides? 6. What is the effect of the sun's attraction? 7. When are tides highest? 8. What produces irregularity in tides? 9. What advantages arise from tides? 10. Look at figures 42 and 43, and explain the tides.

LESSON 49.

Eclipses.

An'nular, having the form of a ring, from annulus, a Latin word for ring.

THE earth being an opaque body enlightened by the sun, necessarily projects a shadow into the regions of space in a

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contrary direction. When it so happens that the moon, in the course of her revolution about the earth, falls into this shadow, she loses the sun's light, and appears to us eclipsed. If we suppose two straight lines drawn from the opposite parts of the solar disk, touching the surface of the earth on opposite sides; these lines will represent the limits of the shadow, and as the sun is much larger than the earth, they will meet at a point and cross each other behind the earth, and the shadow will thus take the form of a cone. The moon can come within the shadow of the earth only when it is full, or in opposition to the sun. But the moon is not eclipsed every time it is full, because its orbit does not coincide with the plane of the earth's orbit, one half being about five degrees and a third above it, and the other half as much below it; and unless the full moon, therefore, happen in or near one of the nodes, that is, in or near the points in which the two orbits intersect each other, she will pass above or below the shadow of the earth, in which case there can be no eclipse. If the moon be within twelve degrees from the node, at the time when she is full, there will be a partial or total eclipse, according as a part, or the whole of her disk falls within the earth's shadow. As the shadow is considerably wider than the moon's diameter, an eclipse of the moon lasts sometimes three or four hours. It is by knowing exactly at what distance the moon is from the earth, and of course the width of the earth's shadow at that distance, that eclipses are calculated with the greatest accuracy, for many years before they happen. Lunar eclipses are visible over every part of the earth that has the moon at that time above the horizon; and the eclipse appears of the same magnitude to all from the beginning to the end. That faint reddish

colour, which the moon exhibits in the midst of an eclipse, is supposed to proceed from the rays of light, which are refracted by the earth's atmosphere, and fall upon the surface of the moon.

An eclipse of the sun is caused by an interposition of the moon between the sun and the earth. This can happen only at the new moon, or when the moon at her conjunction is near one of her nodes; for unless the moon is in or near one of her nodes, she cannot appear in the same plane with the sun, or seem to pass over his disk. In every other part of her orbit she will appear above or below the sun. If the

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ECLIPSES OF THE SUN.

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moon be in one of her nodes, she will, in most cases, cover the whole disk of the sun and produce a total eclipse; if she be any where within about sixteen degrees of a node, a partial eclipse will be produced. When a bright luminous ring appears round the dark body of the moon during an eclipse of the sun, it is called an annular eclipse. This kind of eclipse is occasioned by the moon being at her greatest distance from the earth at the time of an eclipse; in which situation, the vertex or point of the cone of the moon's shadow does not reach the surface of the earth. total eclipse of the sun is a very curious and uncommon spectacle; and total darkness cannot last more than three or four minutes. Of one that was observed in Portugal more than one hundred and fifty years ago, it is said that the darkness was greater than that of night;—that some of the largest stars made their appearance ;—and that birds were so terrified that they fell to the ground. A very remarkable total eclipse took place in New England June 16, 1806. The day was clear; several stars were visible; the birds were greatly agitated; and a gloom spread over the landscape. The first gleam of light, contrasted with the previous darkness, seemed like the usual meridian day.

QUESTIONS.-1. What is an eclipse of the moon? 2. Describe the earth's shadow. 3 When does an eclipse of the moon happen? 4. Why is she not eclipsed at every full moon? 5. How near a node must she be in order to be eclipsed? 6. How long may an eclipse of the moon last? 7. From the knowledge of what circumstances aro lunar eclipses calculated? 8. Over what part of the earth are they visible? 9. What is the cause of an eclipse of the sun? 10. When does an eclipse of the sun happen? 11. Why can it not happen at other times? 12. When will the moon produce a total eclipse of the sun? 13. Partial? 14. When is an eclipse of the sun called annular?-why? 15. What occasions this kind of eclipses? 16. How long may a total eclipse of the sun last? [NOTE. The diameters of the sun and moon are supposed to be divided into 12 equal parts, called digits. They are said to have as many digits eclipsed as 12th parts involved in darkness.] 17. Look at fig. 45. and illustrate an eclipse of the moon. 18. At fig. 44. and illustrate an eclipse of the sun.

MARS, VESTA, JUNO, AND PALLAS.

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LESSON 50.

Mars, Vesta, Juno, Pallas, and Ceres.

Eccentric, deviating from the centre.

Eccentricity, the distance between the centre of an ellipse and the focus.

MARS, the first of the exterior planets, is distinguished from the rest by the redness of its colour, which has been attributed to the density of his atmosphere. He revolves round the sun in about two years, at the mean distance of one hundred and forty-four millions of miles, and turns on his axis in a little less than twenty-five hours. The time of his diurnal rotation was discovered by means of a large spot seen on his surface, when in that part of his orbit which is opposite to the sun and the earth. The telescopic appearance of Mars is exceedingly variable; but the predominant brightness of his polar regions, leads to the supposition, that, like those of the earth, they are covered with perpetual snow. The proportion of light and heat, received at Mars from the sun, is less than one half of that enjoyed by the earth.

The planet next to Mars in the solar system is Vesta. It shines with a pure and white light, and is visible in a clear evening without the aid of a telescope. It revolves round the sun, in about three years and eight months, at the mean distance of two hundred and twenty-three millions of miles. Vesta was first discovered by Dr. Olbers, of Bremen, in Lower Saxony, March 29, 1807.

Juno was discovered by Mr. Harding, near Bremen, September 1, 1804. It completes its revolution in about four years and four months, at a mean distance from the sun of about two hundred and fifty-three millions of miles. It is distinguished from all the other planets by the great eccentricity of its orbit; and the effect of this is such that it passes over one half of its orbit in half the time that it employs in describing the other half. From the same cause its greatest distance from the sun is double the least distance, the difference between the two distances being about one hundred and twenty-seven millions of miles.

Pallas was discovered by Dr. Olbers, March 28, 1802. It completes its revolution in about four years and seven months, and its orbit is nearly as eccentric as that of Juno.

Its mean

distance from the sun is two hundred and sixty-three millions of miles. Its atmosphere seems to be dense and cloudy.

The planet Ceres was discovered by Piazzi, at Palermo, in Sicily, January 1, 1801. It is apparently surrounded by a dense atmosphere, and is of a ruddy appearance. Its mean distance from the sun, and its revolution in its orbit is nearly the same as that of Pallas. These newly discovered planets exhibit various changes in appearance and size; so that their real magnitude has not been ascertained with certainty.

From some irregularities, observed in the motions of the old planets, some astronomers had been led to suppose, long before the discovery of the four new planets, that a planet existed between the orbits of Mars and Jupiter. Dr. Ölbers, before he made his last discovery, conceived that these small celestial bodies were merely the fragments of a larger planet, which had been burst asunder by some internal convulsion, and that several more might be discovered. With the intention, therefore, of detecting other fragments of the supposed planet, he examined, thrice every year, the little stars in certain constellations, till his labours were crowned with success by the discovery of the new planet Vesta. The opinion, that these four small planets have been separated from one original planet, by some convulsion in nature, has been maintained by Dr. Brewster with much ingenuity and plausibility. He supposes, moreover, that the phenomena of the meteoric stones, which have fallen on the earth from the atmosphere, may have been occasioned by the bursting of this planet,

QUESTIONS.-1. By what is Mars distinguished from the rest of the planets? 2. In what time does Mars revolve round the sun? 3. At what mean distance? 4. What is the time of his diurnal rotation, and how was it discovered? 5. What is the telescopic appearance of Mars? 6. Proportion of light and heat? 7. What is the appearance of Vesta? 8. When, where, and by whom were each of the new planets discovered? 9. What is the distance of each from the sun? 10. By what is Juno distinguished from all the other planets? 11. What supposition did some astronomers make before the discovery of the new planets? 12. What was the conjecture of Dr. Olbers, and to what did it lead? 13. To what does Dr. Brewster think the phenomena of meteoric stones may be attributed?