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ELEMENTARY GEOGRAPHY.

GEOGRAPHY is the science which describes the surface of the earth, or of that member of the solar system which forms the abode of man. It consists of three principal parts—-viz., Mathematical, Physical, and Political Geography.

Mathematical Geography treats of the form, motions, and size of the earth; of its relation to the other orbs that with it constitute the solar system; and of the exact position of places on its surface.

Physical Geography treats of the configuration of the earth's crust; the materials of which it is composed; and the effects of climate on its living inhabitants.

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Political Geography treats of the artificial divisions of the land into states and empires, their extent, population, government, religion, language, and material resources.

MATHEMATICAL GEOGRAPHY.

THE SOLAR SYSTEM.-The earth on which we live is only one of the members of a large family of similar bodies, collectively called the Solar System, because the sun (in Latin, sol) is by far the largest body belonging to it, and because all the other members of the system revolve around him, deriving from him their light and heat.

So far as known at present, the solar system consists of 141 distinct bodies, besides the great central luminary, and a vast number of comets which move around him with inconceivable velocity and in very eccentric orbits. All these are united in one beautiful and harmonious whole by the mysterious power of gravitation-a power, doubtless, which directly emanates from the divine hand that formed them, and which completely regulates all the parts of this, stupendous piece of mechanism. The 141 bodies above referred to may be regarded as forming three distinct classes. We have, first, 9 large planets-viz., Vulcan, Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, all of which

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revolve around the sun in nearly circular orbits, in well-defined periods, and at curiously regulated distances from the sun and from each other. Next, we have 114 planetoids, or small planetary bodies, situated between the orbits of Mars and Jupiter, and supposed by some to be the fragments of a large disrupted planet; and, finally, we have 18 satellites or moons, performing elliptical orbits around certain of the larger planets, and not immediately around the sun. One of these moons belongs to the earth, 4 to Jupiter, 8 to Saturn, 4 to Uranus, and 1 to Neptune. Only a very few of this large number of bodies were known to the ancients-viz., the Earth, Moon, Mercury, Venus, Mars, Jupiter, Saturn, and a few of the more remarkable comets. All the remainder have been discovered since the invention of the telescope, in the beginning of the seventeenth century. To Galileo, the Italian astronomer, belongs that high honour, and he had his brilliant reward; for the very first glance he gave to the heavens through his newly-invented instrument, he discovered the four satellites of Jupiter. Uranus was discovered by Sir William Herschel in 1781; Neptune, by Adams and Leverrier, in 1846; and all the planetoids during the present century. Every year, indeed, is adding new members to the system, as instruments are improved and the number of observers multiplied. The following table exhibits in detail the principal facts which modern science has ascertained in regard to the different members of

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RELATION OF THE SOLAR SYSTEM TO THE UNIVERSE.-The Solar System forms but a very small part of the material universe. When we survey the heavens at night, we behold a multitude of luminous objects called fixed stars, which by their paler and more silvery light, and still more by their twinkling, are easily distinguished from such of the planets as can be seen by the naked eye, all of which shine with a yellowish, mellow, and steady ray.

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Number of the Stars.-The fixed stars appear to the spectator to be very different sizes, and this difference enables the astronomer to arrange them into classes, and thus to form some estimate of their number. Those visible to the naked eye are divided into six classes. The brightest are said to be of the first magnitude, and are few in number; those of an inferior degree of brightness, of the second magnitude; and so on, down to those of the sixth magnitude, or the smallest that can be seen without a telescope. The total number of stars visible to the naked

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eye, in a clear moonless night, is 5000. It is only at the equator, however, that so large a number can be seen; for there only has the spectator the opportunity of seeing the whole heavens without altering his position. Should he take up his position at either of the poles, no more than half the starry firmament can ever pass in review before him; while at all intermediate positions, the number of stars visible in any one night will depend on the latitude of the place. The telescope vastly extends our power of vision, and astronomers are familiar with stars of the sixteenth degree of magnitude. No fewer than 13,000 stars belong to the seventh magnitude alone; while the total number of stars visible by means of the best telescopes is estimated at 500,000,000,000, or half a million of millions!

Distance of the Stars.-The distance of the fixed stars from our sun is as inconceivable as their number, and till recently there were no reliable facts on which to build any probable calculation. In the year 1838, however, the parallax (or angle subtended by the diameter of the earth's orbit, as seen from a star) was measured in the case of three of them, viz., Alpha Centauri, 61 Cygni, and Alpha Lyra, whose respective parallaxes were ascertained to be one second, one-third of a second, and a quarter of a second. The major diameter of the earth's orbit being about '185,930,000 miles, a parallax of one second will give a distance of twenty billions of miles, or 7000 times the distance of Neptune from the sun. This, then, is the probable distance of our sun from the nearest fixed star,- -a distance so great that light, which travels at the rate of 192,000 miles per second, would require 31 years to traverse it. The distance of the star 61 Cygni, its parallax being only onethird of a second, will be three times as great, and of Alpha Lyra four times. The distance of 12 fixed stars is now approximately determined. Magnitude of the Stars.-The actual magnitude of even the nearest of the fixed stars is still very uncertain; though it is the general belief of astronomers, founded on careful calculations, that many of them are greatly larger than our sun. Were our luminary and Alpha Centauri, for example, to exchange places, it is calculated that we should enjoy 2 times more light than we do at present, while not only would the sun cease to be visible to the naked eye, but no telescope yet invented could give us any idea of his size; and hence, it is argued, that star is 2 times larger than our luminary. The illuminating power of Sirius iš 147 times greater than that of the sun, and that star is therefore supposed to be 147 times larger than our luminary. But considerable uncertainty attaches to this method of determining the magnitude of the heavenly bodies; for the amount of light which reaches our world from a star does not depend on its magnitude alone, but also on its distance.

Motion of the Stars.-The so-called "fixed stars" are, in reality, all in motion, and no fixed point-no object absolutely at rest-can be met with in the material universe. The power of gravitation, which binds together the numerous members of the solar system, appears to be equally operative among the most distant objects in space. The relative distances of the fixed stars, and, consequently, the configuration of the constellations, are imperceptibly but daily altering. Of all the bright stars known to the ancients, not one has kept its place unchanged. While some vary only the twentieth part of a second annually, others vary from seven to eight seconds. Even our own sun, so long regarded as stationary, is now ascertained to be in rapid motion through space, carrying our earth and the other planets in his train, and traversing

daily a distance of 422,000 miles-a space nearly equal to his own radius. Sir W. Herschel, three-quarters of a century ago, arrived at the conclusion that the sun was moving in the direction of the constellation Her cules; but science has not yet determined whether this motion be in a straight line or in a curve, and if the latter, what that point in space is around which he is revolving. Dr Mädler, of Dorpat, indeed, hazards the conjecture that the star Alcyone, the brightest orb in the beautiful constellation Pleiades, is the grand central point in the heavens around which our system is moving. The distance of our sun from that centre he calculates at 31,000,000 times the distance of the earth from the sun -a distance so great that light would not traverse it in less than 500 years, and requiring 18,000,000 years for one complete revolution! As yet, however, this can be regarded only as a conjecture.

THE EARTH.-Having thus briefly treated of the relation of the earth to the solar system, and of the latter to the stellar universe, we now proceed to consider more particularly the earth itself.

Form and Size of the Earth.-In common with all the other planets, the form of the earth is that of a sphere somewhat flattened or compressed at the poles, like an orange; or, it is the form which a perfect sphere of semi-fluid consistency would assume were it made to rotate around its own axis with the same rapidity as the earth does. Such a form is called an oblate spheroid.* The larger or equatorial diameter exceeds the polar diameter by 26 miles-the former being 7925 and the latter 7899 miles. In round numbers, the radius or semi-diameter may be stated at 4000 miles; the diameter at 8000 miles; the circumfer ence at 25,000 miles; the area or superficial content at 197,000,000 square miles; and the volume or solid content at 260,000,000,000 cubic miles.+

Motions of the Earth.-The earth has three distinct motions:-1st, That referred to above, in accompanying the sun through space, with a velocity of 420,000 miles per day, or 17,500 miles per hour; 2d, Its annual or orbitual motion round the sun in 365 days, 5 hours, 48 minutes; and, 3d, Its diurnal motion round its own axis in 24 solar hours, the equatorial parts moving at the rate of more than 1000 miles per hour.

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Among the numerous proofs of the spherical form of the earth, the following may be mentioned:-1. A much greater extent of the earth's surface is visible from the top of a mountain than from a plain near the level of the sea. 2. As the mariner nears the land, he first sees the tops of the mountains, and, on approaching nearer, discerns the lower grounds. 3. In cutting for a canal, it is found that allowance must be made for a dip of about eight inches per mile, in order to keep the water at a uniform level. 4. In travelling to any considerable distance, either north or south, new stars come to view in the direction in which the traveller is advancing, while others disappear in the direction from which he is receding. 5. Many navigators who have sailed constantly in one general direction, whether due east or due west, have returned to the port from which they set out. 6. The shadow which the earth casts on the moon during an eclipse is always circular. 7. All the other members of the solar system are spherical.

The circumference of a circle is found by multiplying its diameter by 3.1416, or by 34, and its area by multiplying the circumference by half the radius. The area of a globe is equal to the convex area of its circumscribing cylinder; while its solid content is equal to two-thirds of the solid content of the circumscribing cylinder. More briefly, the area of a sphere = diam.2 × 3.1416; while its solid rad. x area

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