GEOGRAPHY

(Yunanca köken — orijinale bakınız.) The description of the earth. The etymology of the term indicates the nature and extent of this department of knowledge, which, in fact, embraces every thing relating to the circumstances and condition, natural or artificial, of the globe which we inhabit. The immense variety of subjects comprehended in the science of Geography are usually arranged under three great divisions, — Mathematical, Physical, and Political Geography. Math etna tical Geography. — This division of geography has for its object the determination of the form -and dimensions of the earth, its relations with the celestial bodies, the relative positions and distances of places on its surface, and the representation of the whole or portions of its surface on globes or maps. In order to describe the earth the first element that must be ascertained is its general form. That the surface of the earth is convex, is apparent even to the senses. Let a spectator stand on the sea shore, and attend to the succe ive appearances of a ship proceeding to sea. First, the lower part becomes invisible; then the whole hull; and, lastly, the masts and rigging, the lower parts disappearing first. Now this phenomenon can only be explained by the convexity of the portion of the water between the ship and the spectator; and when the curvature of the sea is recognized, the conclusion can scarcely be avoided that the land, abstracting from its local inequalities, participates in the same curvature, and that the earth is a round body. The same conclusion is drawn from the gradual coming on of day and night, 400 GEOGRAPHY. and the deplacement of the stars in proceeding towards the north or south; for if the earth were flat, the sun, in appearing above the horizon, would illumine the whole of its surface at the same instant; and on the supposition of its being either flat or cylindrical, the pole star would maintain the same elevation, whereas to the traveller who advances northward it gradually appears more elevated, and to him who advances southward more depre ed. All these appearances are observed in the same manner on whatever part of the earth we are placed; and from them alone the globular form of the earth might be inferred, even if the fact had not been placed beyond doubt by the voyages of circumnavigators. As str on omers in modern times have indeed discovered that it is not a perfect sphere, being flattened in a slight degree at two opposite points (which are the poles of rotation); but the deviation from perfect sphericity is so small, that for all the purposes with which geography is concerned it may be neglected without sensible error. See Figure of the Earth. In order to determine the relative positions of points on a sphere in the most commodious manner, mathematicians refer them to two great circles; that is to say, circles formed by the intersection of the surface of the sphere by a plane pa ing through its centre. The earth is a body which revolves about an axis of rotation; the position of this axis, therefore, with respect to the celestial constellations, determines one great circle, namely, the equator, or the circle which is equally distant from the two poles of rotation, and divides the globe into two opposite hemispheres. The equator, therefore, being marked out by nature, o; by the apparent revolutions of the celestial bodies arising from the real diurnal rotation of the earth, is a circle to which all places are most conveniently referred: and it has this property, that it never undergoes any change of place on the sphere; for astronomers have proved that since the date of the earliest observations, the poles of rotation, and consequently the equator, have always maintained the same invariable position on the earth's surface. The distance of a place from the equator cannot be directly measured; but by means of astronomical observations we can determine that distance in aliquot parts of the earth's circumference, that is, in degrees of a great circle. It is not, however, sufficient to know how many degrees a place is distant from the equator. It is nece ary, also, in order to distinguish it from all other places, to know how it is situated with respect to a certain meridian^ or great circle perpendicular to the equator, and consequently pa ing through the jtoles. In reference to the diurnal rotation of the earth, all meridians are in precisely the same circumstances; the choice is consequently entirely arbitrary, and geographers, as well as astronomers, generally select that which pa es through the capital (rf their own country as the circle to which they refer all other places. The equator and the a umed meridian thus form the co-ordinates of the sphere. The distance of any place from the equator, measured on the arc of the meridian, is the latitude of the place; and its distance from the a umed meridian, measured on a circle parallel to the equator; is its longitude; and when the latitude and longitude of any place are both known, the position of the place itself is entirely determined. The latitude of a place is ascertained by observing the height of the pole above its horizon; and the longitude of one place in respect of another, by the interval of time which elapses between the pa age of any celestial body over their respective meridians. See Latitude and Longitude. Besides the rotatory motion about its axis, the earth has also a motion of revolution about the sun, which is completed in the course of one year. The first of these motions causes the succe ion of day and night; the second the vici itudes of the seasons, and the inequality of the length of the natural days. These two last phenomena depend on two circumstances: — 1 st, that the axis of the earth is not perpendicular to the plauL' of the ecliptic, or the plane in which the annual revolution is performed; and 2d, that the extremities of this axis, or the two poles of rotation, continue to be directed to the same points of the celestial sphere during the whole time of the revolution. Hence it follows that the plane of the equator, though invariable in respect of absolute space, is continually changing its i>osition in respect of the sun; and the apparent eflect is the same as if the sun had a vibratory motion in the heavens, rising above the plane of the equator E E one half of the year, and falling below it during the other half. When the sun is in the equator at S, the two poles of the earth n and s are in the great circle which divides the illuminated from the dark hemisphere. When the sun reaches his greatest northern declination at S', the il  [s. 503]