Though the knowledge of our world pro gressed by leaps and bounds, it is interesting to note what a large proportion of that knowledge has been the result of modern investigation. Explorers on the sea had by the end of the 18th century become familiar with the range of the ocean, the outline of the continents and many islands, but at the beginning of the 19th century four-fifths of the land area were unknown. Africa, with the exception of a narrow rim of coast, was indeed a °Dark Lit tle had been added to our knowledge of Asia since the days of Marco Polo. West of the Mississippi, North America was a terra incog nita, and the existence of the Rocky Mountains was not suspected. Even the coast of Australia was not completely traced, and nothing had been learned of its interior. South America, which was better known than any continent except Europe, is now the least explored of all the large land masses.
While the unity of geography should never be overlooked, the subject is commonly divided into different branches, the chief of which are mathematical, physical and political geography.
Mathematical Geography considers the earth as a globe, with its motions and their effects, and teaches the methods of representing the whole or portions of the earth's surface on globes or maps. Observation and careful meas urements have proved the earth to be spheroidal in shape. As it is flattened at the poles, it is not a perfect sphere, nor even a perfect ellip soid, but a ball with slight irregularities of sur face. Its longest diameter is 7,926.6 miles, and its shortest 7,899.6 miles. The circumference is approximately 24,000 miles.
The axis of rotation of the earth is its shortest diameter. The ends of the axis point to opposite parts of the sky, and these, called the poles of the heavens, seem to stand still while the rotation of the earth causes an appar ent revolution of the sky from east to west. The whole system of determining position and direction is established by the earth's rotation. The great circle midway between the poles is the equator. Great circles extending north and south through the poles are meridian circles. Distance from the equator toward either pole is called latitude, and is along a meridian. Zero is at the equator, and the quarter circle north or south is divided into 90 degrees (expressed as 90°). Small circles parallel with the equator are parallels of lati tude. The equator and parallels are continually moving from west to east, and the meridians, which cross them at right angles, are carried in succession directly beneath the sun. All points on any meridian turn around the axis at the same rate, but the actual distance traversed varies from nothing at the poles to more than 1,000 miles per hour at the equator. The uni form rotation of the earth provides a means of measuring time. The sun crosses the meridian or midday line of any place midway between the hours of rising and setting, and the inter val between two successive noons is called a day, which is divided into 24 equal parts or hours. It is always noon on some meridian,
but never on more than one at the same instant. As a complete rotation through 360° occupies 24 hours, meridians 15° apart vary one hour in local time. A person traveling toward the east completely around the world gains a day, but if he makes the same journey in a westerly direction he loses a day. Distance east or west is measured on the equator or a parallel, and is called longitude. Some prime meridian, usually that of Greenwich, is the zero, and measure ments are made 180° east and west The length of a degree of latitude is a little more than 69 miles, but as the meridians converge toward the poles, degrees of longitude diminish in length as the distance from the equator increases. If the latitude and longitude are unknown, the former is determined by observing the altitude of the pole of the heavens, while the latter is obtained by comparing, with the aid of a chro nometer, the exact moment at which the sun crosses the meridian with the local time of the prime meridian at the same instant. Tables, showing the relative positions of heavenly bodies for each day in the year, are calculated in advance and enable captains of vessels to find their position at sea.
The imaginary network of meridians and parallels can be actually drawn upon a globe. The outline of the continents and the location of places may thus be depicted with Feat ac curacy. A map is much more convenient than a globe, but, as the surface of a sphere cannot be spread out flat, no map is ever absolutely correct. Various projections are employed to modify the distortion. Mercator'sproiection, and others similar in plan, are modifications of the result obtained by drawing lines from the centre of a sphere through the parallels and meridians to the surface of a cylinder touching the sphere at the equator. This style of pro jection is employed for navigators' charts, for while areas are distorted, a straight line drawn between any two points correctly represents direction. Several hemispherical projections upon a flat background are used. These, though approximately preserving dimensions, distort directions. For limited areas the coni cal projection is of value. The meridians and parallels appear as they would if traced on a transparent cone placed on the globe.
Physical Geography deals with the earth in its relation to nature. The surface is irreg ular, the hollows being filled with water and the projecting parts forming the dry land. It is surrounded by a gaseous envelope. The solid portion is often called the geosphere or litho sphere; the liquid layer, the hydrosphere; and the outer mantle. the atmosphere. See AIR: