urth, the planet which we inhabit, the third from the sun and the fifth in size. It is attended by one satellite. See MOON. SHAPE. To an observer whose view is not obstructed, any part of the earth presents itself as a circular and horizontal expanse, on the circumference of which the heavens appear to rest. Accordingly, in remote antiquity the earth was regarded as a flat, circular body, floating on the water. But even in antiquity the spherical form of the earth began to be suspected. It is only on this supposition that we can explain why the horizon of vision grows wider and wider the higher the position we choose, why the tops of towers and mountains at a distance become visible before the bases, why the hull of a ship disappears first as she sails away, and why, as we go from the poles toward the equator, new stars become visible. Besides these proofs there are many others, such as the circular shadow of the earth seen on the moon during an eclipse, the gradual appearance and disappearance of the sun, and, lastly, the fact that since 1519 it has been regularly circumnavigated. The earth is not, however, an exact sphere, but is very slightly flattened at the poles, so as to have the form known as an oblate spheroid. In this way the polar diameter, or diameter from pole to pole, is shorter than the diameter at right angles to this--the equatorial diameter. The most accurate measurements make the polar diameter about 27 miles less than the equatorial, the equatorial diameter being found to be 7,925.6 miles, and the polar, 7,899.14. SURFACE LINES. The earth is regarded as divided into halves--the northern and the southern hemisphere--by the equator, an imaginary line passing around it midway between the poles. In order to indicate with precision the position of places on the earth, additional lines are imagined to lie on the surface in such a manner that those of one set all pass through both poles, while those of the other are drawn parallel to the equator. The former are called meridians, the latter parallels of latitude, and by them we can tell the latitude and longitude, and thus the exact position of any place on the earth's surface. DENSITY. Many experiments by various methods have been made in order to determine the average density of the earth, that is, the quantity of matter it contains, and from them it has been calculated that the density of the earth is about five and one-half times that of water. MOTIONS. The earth, in common with the other planets, moves around the sun, completing its revolution in about 365 1/4 days, our common year; or exactly 365 days, 9 hours, 9 minutes and 9 3/10 seconds, making the sidereal year. The orbit of the earth is an ellipse, with the sun in one of its foci. Hence, the earth is not equally distant from the sun in all parts of the year, but is about 3,000,000 miles nearer at one time than at another, its least distance, according to recent calculations, being 89,897,000 miles, its greatest 92,963,000, and the mean distance, or half the length of the long axis of the orbit, 91,430,000 miles. From this it may be calculated that the velocity of the earth in its orbit is about 17 miles a second. In the winter of the northern hemisphere the earth is nearest the sun and in summer farthest from it; for the difference in the summer and winter temperature is not occasioned by the greater or less distance of the earth from the sun, but by the more or less oblique direction of the sun's rays. The passage of the earth around its orbit causes the sun to appear to describe a similar orbit in the heavens; and hence it is that at one time of the year one group of stars is seen in the neighborhood of the sun at sunrise and sunset and at another time another group. This apparent path of the sun is the ecliptic, and corresponds with what would be the path of the earth as seen from the sun; and the groups of stars through which the sun successively passed form the zodiac. The earth's daily motion about its own axis takes place, according to mean time, in twenty-three hours, fifty-six minutes and four and one-tenth seconds. The axis on which the earth performs its rotation is inclined towards the plane of its path about the sun at an angle of 23 1/2 deg (which angle also represents that between the plane of the ecliptic and the plane of the earth's equator), the sun ascends in the heavens from March 21 to June 21 (the summer solstice), about 23 1/2 deg above the equator toward the north pole, and descends again toward the equator from June 21 to September 23; it then sinks till December 21 (the winter solstice), about 23 1/2 deg below the equator toward the south pole, and returns again to the equator by March 21. This arrangement is the cause of the seasons and of the inequality of day and night attending them. For all countries lying beyond the equator, day and night are equal only twice in the year (at the equinoxes). At the summer solstice the north pole of the earth is turned toward the sun, and the south pole away from it, and for 23 1/2 deg about the north pole there is a period of longer or shorter duration during which the sun is continually above the horizon for more than twenty-four hours, while round the latter is an equal extent of surface within which the sun for similar periods is below the horizon. The reverse condition holds at the winter solstice. The circles bounding these regions are called respectively the arctic and the antarctic circle, and the regions themselves the polar or frigid zones. Through-out a region extending to 23 1/2 deg on each side of the equator the sun is directly overhead at every point in succession twice in the year. The circles which bound this region are called the tropics, that in the northern hemisphere being the tropic of Cancer, that in the southern the tropic of Capricorn, while the region between is the torrid zone. The regions between the tropics and the polar circles are respectively the north and south temperate zones. SURFACE. The surface of the earth contains over 196,000,000 square miles, of which scarcely a third part is dry land, the remaining two-thirds being water. The land is arranged into masses of irregular shape and size, the greatest connected mass being in the eastern hemisphere. The chief masses receive the name of continents, detached masses of smaller size forming islands. The surface of the land is variously diversified, exhibiting mountains, valleys, plains, plateaus and deserts. The water area of the earth is divided into oceans, seas, bays, gulfs and sounds, while rivers and lakes may be regarded as features of the land surface. The great phenomena of the oceans are currents and tides. INTERIOR. From the evidence furnished by volcanoes, hot springs and the sinking of mines, it is known that the earth has a high internal temperature of its own. Taking the average of the various observed rates of increase, this temperature seems to increase 1 deg F. for every sixty feet of descent. Assuming this to continue, the rocks at a depth of two miles would be as hot as boiling water, and at a depth of fifty miles the heat would be such as at the surface would melt every known solid. This being so, various theories as to the internal condition of the earth have been proposed: 1, That a thin envelope or crust surrounds a molten interior. It can be shown, however, that as tides must be produced in such a molten mass the cool outer crust would be unable to withstand the enormous force of these unless it were about 2,000 miles thick. 2, That the interior is solid, with spaces here and there filled with liquid or gaseous material. This theory assumes that there are within the earth enormous cavities filled with molten rock, which escapes, when local pressure is removed, in the form of volcanic outbursts. 3, That the earth consists of a thin crust, a large solid nucleus, and a liquid film between the nucleus and the crust; the temperature at the center being not much greater than comparatively near the surface. 4, That the earth is solid to the center, but any part may become liquid if local pressure is removed. We know that if the pressure on a solid be increased the melting point is correspondingly raised; now the pressure at the center of the earth, or even at the depth of fifty or one hundred miles, must be something enormous, and probably is so great as to keep the rocks there permanently in a solid condition. notwithstanding the heat. This last theory is considered the most probable. On the supposition of its correctness, volcanoes might be explained by supposing that at certain points here and there pressure is removed by the elevations of portions of the earth's surface which are constantly taking place, and that this allows the rocks to liquefy. Water may then soak down to these liquid rocks, and, being converted into steam, produce various volcanic phenomena. The earth is believed to have condensed and solidified from a gaseous or nebular condition, and to have once had a far higher temperature than now. If such were the case the outer surface, losing heat by radiation, would be the first part to cool quickly; while the interior, losing its heat by conduction, would not cool so rapidly, and therefore would naturally have a higher temperature than the portion at the surface. This is what all observations indicate the condition of the earth to be, and the shape of the earth also indicates that it must once have been in a fluid state. ATMOSPHERE. The earth is surrounded by an atmosphere which moves with it and is really a part of it. Upon the surface of the earth proper this atmosphere presses with a force of 15 pounds to the square inch. The atmosphere has a thickness of from 50 to 60 miles, but it is so much denser below that half of its bulk is within 2 1/2 miles of the surface. MAGNETISM. Another feature that the earth as a whole presents is magnetism. When a magnetic needle is balanced on a point it remains at rest in one position only, pointing then nearly due north and south. This can be explained only on the supposition that the earth acts as a great magnet. It has, in fact, two poles--a north and a south magnetic pole--which are not very far from, but by no means coincident with, the geographical poles. The north magnetic pole was located exactly in 1905. There is also a neutral line or magnetic equator, which does not greatly diverge from the geographical equator. The earth acts upon all magnets as they act upon one another, and it is for this reason that they point north and south.