state. When the point presented to the sun is at e (which is on the 22d December), it is midsummer to all the southern parts of the earth, and winter to all the north; but as the exposed part advances towards the point f, the northern regions gradually re B The Seasons. ceive more and more heat, till, on the 21st of June, it becomes their midsummer. Having glanced at the main systemal motions of the earth that regulate our days, years, months, and seasons, we shall now advert to her own proper dimensions, as determined by the astronomer and mathematician. 13. The body which thus rotates on its own axis while it revolves round the sun, and is in turn revolved around by the moon, is not, strictly speaking, a sphere or globe, but a spheroid (Gr. sphaira, a globe; eidos, likeness), or body of a sphere-like form. A diameter taken along its axis, or the ideal line round which it rotates, is only 7899.170 miles, while one taken in the opposite direction is 7925.648 miles. The one diameter thus exceeds the other by about 26 miles, thereby causing a deviation from the true globular form, and producing what mathematicians term an oblate spheroid—that is, a figure flattened in the direction of its axis, and bulging out all around somewhat in the shape of an orange. Such a figure can be readily produced by rapidly spinning a ball of yielding material (like soft clay or putty) round its own axis, when the tendency which all revolving bodies have to fly off from the centre (centrifugal force) causes the mass to bulge out at the rotating circumference and to flatten in proportion at either end of the axis. To this centrifugal force arising from rotation, and to some original yielding condition of the II. THE EARTH-ITS GENERAL OR PLANETARY RELATIONS. Figure, Motions, Dimensions. 9. By the general relations of the earth are meant those primary conditions of form, size, density, motion, and the like, which belong to it as a member of the planetary system. From these conditions arise all those multifarious actions and reactions that take place on its surface-the alternations of night and day, heat and cold, summer and winter, growth and decay, the winds and motions of the atmosphere, the tides and currents of the ocean, and, in fact, all that confers on it geographical diversity and change. Thus, on its axial rotation depends the recurrence of light and darkness; from its revolution round the sun arises the succession of the seasons, with all their varied effects on vegetable and animal life; from the unequal reception of the sun's heat by the terraqueous surface and atmosphere result the multifarious phenomena of what we call weather and climate; while from the attraction of the sun and moon, and the earth's own proper motions, spring the flow and ebb of the tides, and the other great currents of the The consideration of these conditions belongs, no doubt, more especially to Astronomy and Physics, but as much may be here recapitulated as will enable the student to lay the foundation, as it were, of his own special science. ocean. 10. The province of Physical Geography is not merely to observe and describe, but to explain and determine. Here is a globe revolving and rotating in obedience to the laws of gravitation and attraction; and as motion is inseparably associated with changechange of place or change of condition-the nature of these changes, and the secondary results arising therefrom, constitute the legitimate themes of our science. Manifested on or near the accessible surface, these phenomena become apparent to every observer; but the producing causes are often so complicated by action and reaction on each other, that without a knowledge of the general relations of the earth, it would be impossible to arrive at a satisfactory determination. As there is no independent existence in nature, so it is necessary to have some idea of the whole; and as our planet is but one of a brotherhood, it is necessary to the comprehension of its individual constitution to have some notion of the relations that subsist between the fraternity. It is for this reason that Geography appeals to Astronomy for a knowledge of the planetary conditions of the earth, in as far, at least, as these appear to bear on its superficial phenomena. 11. Astronomers have determined that the earth we inhabit is one of a number of planets that revolve, at different distances and with different velocities, round the sun as a common centre, constituting what is termed the Solar System (Lat. sol, the sun). These bodies-some of which are nearer the sun than our earth, and others more remote, some vastly larger, and others smallerare nearly all spherical in form, and move from west to east, in courses or orbits more or less circular. The distance of the earth from its central orb is 95,173,000 miles, or, in round numbers, ninety-five millions of miles. Its time of revolution round the sun is about 365 days—or, more precisely, 365 days, 5 hours, 48 minutes, and 50 seconds-and this period of revolution we designate a year. Besides this annual revolution round the sun, the earth rotates or turns on its own axis in 24 hours, or, more exactly, in 23 hours, 56 minutes, and 4 seconds—and this period of rotation constitutes a day. In other words, the earth rotates three hundred and sixty-five times during the course of a single revolution; and thus the more frequent and obvious motion of daily rotation has been taken as the unit of measurement for the larger and less apparent. In these movements of annual revolution and daily rotation, the earth, like several other of the primary planets, is attended by a minor or secondary body, which revolves round her as she round the great central luminary of the system. This secondary planet or satellite (Lat. satelles, an attendant) is the Moon, which is 2160 miles in diameter, and which completes her revolution round her parent orb (at a distance of 24,000 miles) in 27 days, 8 hours-or, in round numbers, in 28 days, or one lunar month. These motions and times of motion are determined and influenced by the attraction and gravitation of the sun and other planets; and it is by the same great forces that all the heavenly bodies that lie beyond our system are held in harmonious order, and, in all likelihood, in analogous fraternities. As the secondary planets revolve round their primaries, and these, again, round the sun, so the solar system itself may revolve round some vaster centre, and this order of things through systems and centres that baffle the grasp of our finite conceptions. [Though the subject belongs properly to Astronomy, it may, for the sake of reference, be here stated that the solar system, as at present known, consists of the great centre, or sun; nine large or primary planets revolv ing round the sun in nearly circular orbits; seventy-one planetoids, or small planet-like bodies, between the orbits of Mars and Jupiter, and occupying, as it were, the place of a large primary; twenty secondaries, or satellites, revolving round their primaries; and an unknown number of comets, which move in extremely elliptical orbits, and consequently become visible to us only at widely-recurring periods. The subjoined table exhibits at a glance the names of the planets, their relative distances from the sun, the times of their revolutions and rotations, their velocities, and number of satellites: 12. The sun being the great centre of light, heat, and other ethereal influences, it necessarily follows that, during the earth's rotation on her axis, only one half of her surface can be exposed at a time to these influences, and hence the alternations of day and night, and all the phenomena that accompany these alternations. But day and night are of unequal and varying length at certain localities, according to the seasons; and these seasonal successions are caused by the facts-first, that the orbit or path of the earth's revolution round the sun is not a perfect circle, but an ellipse; and, second, that in performing this revolution her axis is not perpendicular, but inclined at an angle of 66 degrees 27 minutes to the plane of her orbit. The accompanying diagram will assist in explaining the consequences of this elliptical orbit and obliquity of axis. Thus S is Sun, with the earth represented at four different points in her annual orbit. At A and B the light and heat of the sun strike at the equator, or middle line, and consequently day and night are of equal duration. At any intermediate position, day and night are respectively lengthened and shortened: when at C, the north pole is in darkness; and when at D, the south pole is in the same state. When the point presented to the sun is at e (which is on the 22d December), it is midsummer to all the southern parts of the earth, and winter to all the north; but as the exposed part advances towards the point f, the northern regions gradually re f B The Seasons. ceive more and more heat, till, on the 21st of June, it becomes their midsummer. Having glanced at the main systemal motions of the earth that regulate our days, years, months, and seasons, we shall now advert to her own proper dimensions, as determined by the astronomer and mathematician. 13. The body which thus rotates on its own axis while it revolves round the sun, and is in turn revolved around by the moon, is not, strictly speaking, a sphere or globe, but a spheroid (Gr. sphaira, a globe; eidos, likeness), or body of a sphere-like form. A diameter taken along its axis, or the ideal line round which it rotates, is only 7899.170 miles, while one taken in the opposite direction is 7925.648 miles. The one diameter thus exceeds the other by about 26 miles, thereby causing a deviation from the true globular form, and producing what mathematicians term an oblate spheroid—that is, a figure flattened in the direction of its axis, and bulging out all around somewhat in the shape of an orange. Such a figure can be readily produced by rapidly spinning a ball of yielding material (like soft clay or putty) round its own axis, when the tendency which all revolving bodies have to fly off from the centre (centrifugal force) causes the mass to bulge out at the rotating circumference and to flatten in proportion at either end of the axis. To this centrifugal force arising from rotation, and to some original yielding condition of the |