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THE word Geography means a description of the Earth. Therefore books on Geography are written to tell us what sort of body the earth is, and such facts about its nature, constitution, condition, and relationships as intelligent people ought to know.
Wherefore, if we are attentive, we shall learn from our Geography books that the earth, which is the abode of man, is a heavenly body, that it consists of water and land, that it lies in a covering or envelope of air, and that it is in intimate relationship with the sun.
Obs. Speaking of the earth as a heavenly body, Geography tells us to what class of heavenly bodies it belongs, its relationship to the sun, and something about its shape, size, and motions. This branch of the subject is called Astronomical or Mathematical Geography.
When Geography describes the component parts of the earth, water and land, it tells us in what proportions these bodies respectively exist, and how they are distributed the divisions, characteristics, depths, temperatures, productions, movements, &c., of the water: and the divisions, shapes, reliefs, productions, &c., of the land. When dealing with the outer covering of the earth, Geography tells us something about the composition of this envelope, its thickness, pressure, temperature, moisture, and movements. These are the topics with which Physical Geography is occupied.1
When Geography describes the artificial divisions of the earth into countries, kingdoms, &c., the occupations, trades, commerce, governments, &c., which men have established for their collective and individual well-being, it is called Political Geography.
SOME GENERAL NOTIONS OF
1.-ASTRONOMICAL OR MATHEMATICAL GEOGRAPHY.
1. WHAT THE EARTH IS, ITS FIGURE, PLACE, MOTIONS, SIZE, &c. WHAT THE EARTH IS.-The earth is a planet, i.e., is a heavenly body revolving round the sun. It derives its warmth and light from the sun, and is a member of the Solar System.
Obs. The Solar System consists of the sun and his attendant planets, including comets and meteoric bodies.
1 The term Physiography is applied by Cortem- | are here classed under Mathematical and Physical bert, Huxley, and others to the subjects which Geography.
FIGURE. The earth is round, being in shape something like an
Obs. The roundness of the earth is not immediately apparent to men still it is both plainly indicated and has been demonstrated by actual measurements and calculations. The following are among the indications of the fact.
1. The sun, moon, and planets are round; analogy suggests that the earth also is round.
2. The earth's shadow, as seen in eclipses of the moon, is always round; therefore the earth must be round; for none but round bodies always project round shadows.
3. The horizon on land-or "the offing," as it is termed at sea-always appears in the form of a perfect geometrical circle, having the spectator at its centre. These phenomena, observable in every part of the world, are peculiar to spherical bodies.
4. When a vessel approaches land her top-masts appear first to spectators on the shore. Were the earth flat, the lower parts of the ship, being the most bulky, would be first seen. They are concealed by the curved surface of the earth.
5. Ships sailing from England and holding a continuously westward course, will in due time arrive in England again. This could not be done unless the earth were round.
PLACE.-The place of the earth among the planets is between Venus and Mars, and 93,300,000 m. distant from the sun.
MOTIONS.-The earth is ever in motion. It is perpetually spinning round upon itself, or rotating about its own axis, from W. to E., and flies, at the same time, rapidly through space in unceasing revolution about the sun.
Obs. 1. The time occupied by the earth in rotating once about its axis is one day; 2 hence the rotatory motion of the earth is called diurnal. The phenomena of day and night, and of the apparent motions of the sun and other celestial bodies from E. to W., are due to the earth's diurnal motion.
Obs. 2. The time occupied by the earth in performing one revolution round the sun is a year3; hence the earth's revolutionary motion is termed annual. It is to the earth's annual motion that we owe the phenomena of the Seasons and of the apparent motions of the sun among the fixed stars.
Obs. 3. Both these motions are capable of direct proof; the diurnal by the vibrations of the pendulum, as shown by M. Foucoult in 1851 and the annual by the displacement of the position of a fixed star produced by the earth's motion combined with the motion of light.
SIZE. If the earth were perfectly round, i.e., were a perfect sphere, its various diameters would all be of the same length: but since it is orange-shaped, i.e., is slightly flattened in two of its opposite parts, that diameter which lies between these compressed parts is shorter than that which is at right angles to them. Calculations based upon actual measurement have shown that the earth's Equatorial Diameter 6 is 7925'6 m. long; its Polar Diameter 7
1 Axis, the imaginary line about which the earth rotates
2 The exact time of the earth's rotation about its own axis is 23h 56m 4s. Hence in the neighbourhood of the equator, the earth is carried round at the rate of more than 1000 m. an hour.
3 The exact time of the earth's revolution round the sun is 365d 6h 9m 107. The rate at which she travels in her revolution is more than 17,000 m. an hour.
A Sphere is a solid body, every point in the
surface of which is equally distant from an in terior point called the centre.
5 A Diameter is a straight line which passes through the centre of a curved figure, and is terminated at both ends by the sides of the figure.
6 The Equatorial Diameter is the straight line which passes through the earth at the equator. 7 The Polar Diameter is coincident with the earth's axis.