DIAGRAM SHOWING THE INCLINATION OF THE EARTH'S AXIS,

ON THE EARTH'S MOTION.

an-tag-o-nist-ic [Gk. anti, against; agon, a contest], contending against, opposing. e-qui-lib-ri-um [L. æquus, equal; libra, a balance], equality of weight or force, even balancing. di-ur-nal [L. dies, a day], daily. an-nu-al [L. annus, a year], yearly. ro-ta-tion [L. rota, a wheel], the act of turning as a wheel on its axis. al-ter-na-tions, [L. alterno, to do anything by turns, from alter, the other, another], interchanges, successive changes.

THE earth rotates on its axis and revolves in its orbit; and this twofold motion secures to it those alternations of light and darkness, as well as that succession of seasons, without which, organized as we are, our very existence would be impossible. That this beautiful combination of motions is not due to accident is proved by the admirable harmony which prevails throughout creation; by the wonderful adaptation of means to the end proposed, which is everywhere perceptible; by the skilful balance of antagonistic forces, so as to establish a constant equilibrium; and by the production of good from seeming confusion and evil.

The relations between the earth and sun are such that they would scarcely admit of any modifications compatible with those forms of animal and vegetable life that are known to us. If the earth had been left in a state of rest, how different would have been its condition from that which exists at present! If it did not revolve on its axis, or if, like the moon with regard to the earth, it always presented very nearly the same surface to the sun, on account of its diurnal and annual revolutions

being performed in the same time, there would be no distinction between day and night, the happy alternations of which are so well adapted to those successive periods of exertion and repose that are a necessity of our nature. Changes of temperature, winds, and tides, which are indispensable to the purification of the atmosphere and the waters, could not occur. Half the earth's surface would be shrouded in perpetual darkness, and consigned to eternal cold; and half of it would be exposed, from the endless duration of its day, to a temperature such as neither animals nor plants could endure.

Any spontaneous change in the axis of the earth's rotation would be fatal to us; but such a change is rendered impossible by its revolving on its shortest axis. This is a necessary consequence of the laws by which it has been decreed by Providence that matter should be governed. Being at first, most probably, in a gaseous state, the earth assumed the spherical form on account of the attraction of gravitation; and the centrifugal force generated by its revolution changed it into an oblate spheroid-that is, a spherical body flattened at the extremities of its axis of rotation, and bulged out at its equator.

The mere diurnal rotation of the earth would not suffice to render it capable of maintaining organized beings on any considerable portion of it. The days and nights would remain constantly equal in length; the solar rays would fall too obliquely, and for too short a period during each day, on a large part of its surface. It has been made, therefore, to revolve round the sun in a plane which makes an angle with the plane of its equator. This simple arrangement produces the most important results; for to it is due the constant succession of the seasons, with all the advantages we derive from them. By means of it the sun, during its apparent annual revolution, is found alternately north and south of the equator. The days are longer than the nights in that hemisphere in which it is; and, as it rises to a greater height above the horizon than it would have

done were it to remain constantly over the equator, its rays descend less obliquely, and therefore a larger amount of them fall upon a given surface of that hemisphere.

Both the length of the day and the sun's altitude increase as the distance from the equator increases—that is, the sun becomes, in other respects, less powerful. But, when the day is longer, there is more time for the sun's rays to accumulate, and less for them to pass off by radiation. Since the temperature of a place depends very much on the duration of its exposure to the sun's rays, the increasing length of the summer day, as either pole is approached, makes up to a great extent for unfavourable position with reference to the sun; and hence the hot summers which are found even in very high latitudes. If the obliquity of the ecliptic, that is, the angle its plane makes with the plane of the equator, were less than it is, the seasons would not be sufficiently varied; if it were greater than it is, there would be extremes of temperature incompatible with organic life.

That the earth's orbit should be an ellipse, and not a circle, is a consequence of the general law of gravitation. It has been supposed that this ellipticity causes some modifications in the relative temperatures of the northern and southern hemispheres; and the more severe cold experienced in high southern than in high northern latitudes has been ascribed to the sun being nearer to the earth during the northern than the southern winter.

This difference of temperature is, however, fully accounted for in another way. The climate of any place depends, not only on the number of solar rays which it receives, but on the relative amount of land and water, on altitude above the sea, on proximity to the ocean, etc. It makes a great difference, if, as is the case in the southern hemisphere, the surface consists of water, into which the heat rays penetrate, instead of being, as in the northern hemisphere, chiefly land, which reflects the heat rays into the atmosphere, so as to raise its temperature.

The elliptical form of the earth's orbit produces in

reality no effect on the seasons; for the supply of heat to any place depends not only on the amount imparted to it in a given time, but on the length of time during which it continues to be received. Now the time during which the sun shines on any place is modified by the velocity of the earth in that part of its orbit in which it happens to be; because, as the earth's distance from the sun increases, its angular velocity diminishes; and thus the time during which the heat is communicated is increased, the injurious effect of the sun's augmented distance being exactly counterbalanced by the diminished velocity with which it travels through a given distance, and the resulting greater length of time during which its rays are received. This is a consequence of one of the laws discovered by Kepler. According to this law, the spaces described by a line joining the centres of the sun and the earth, or any other planet, are equal in equal times," no change being caused by any alteration in the planet's position in its orbit, the angular velocity being least when the distance from the sun is greatest, and vice versa. The rays of the sun, intercepted by the earth, in equal times, no matter where it may be in its orbit, are therefore always equal in quantity.

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But for this admirable compensation the difference be tween summer and winter would be greatly augmented in the southern and lessened in the northern hemisphere, since the earth's distance from the sun fluctuates to the extent of nearly the one-fifteenth of its entire amount. The direct impression of the solar heat is, however, in the height of summer, under a perfectly clear sky at noon, greater in a southern than in a corresponding northern latitude; and hence the sufferings of travellers in the thirsty deserts of Australia are greater than in those of Africa.

The earth is retained in its orbit by two forces, one of which-centripetal-prevents it from flying off into infinite space; and the other-centrifugal, which was originally that of projection-prevents it from being precipitated upon the sun. And, just as the former of these, during the earth's revolution in its orbit, is in

creased by the earth's approximation to the sun, the latter, which is required to counteract it, is increased also. Each becomes predominant alternately, but neither retains its superiority long enough to produce an injurious effect. The very velocity with which the earth, in one part of its orbit, rushes towards the sun, generates a force which carries it away from that body; and the velocity with which it flies from the sun is gradually lessened by centripetal force, until at length the latter becomes sufficiently powerful to turn it back again towards that luminary. If the direction and amount of the force with which the earth was first projected, so as to cause it to revolve round the sun, had been such as to produce an elliptical orbit of great eccentricity—that is, a very long and narrow ellipse-the earth, during part of its revolution in such an orbit, would be so far from the sun as to derive but little benefit from it.

The rotation of the earth on its axis is necessary to produce alternations of day and night; and the revolution of the earth round the sun, in an orbit oblique to the equator, is required for the changes of the seasons. Were the variation of this orbit from the circular form not compensated by a change of velocity depending on the mutual action of the centripetal and centrifugal forces, or were the obliquity of the ecliptic either greater or less than it is, we should be subjected to extremes of temperature which, if not fatal to our existence, would greatly affect our comfort.

Thus, in these, as in all other circumstances connected with this earth and whatever it contains, not only our preservation, but our happiness has been carefully provided for.

EXERCISE.-71. COMPOSITION.

1. Describe the manner in which the phenomena of day and night are produced.

2. What is meant by the earth's orbit? What is the cause of its being elliptical rather than circular?

3. Account for the cold being greater in the southern hemisphere than in the northern.

4. What is meant by an oblate spheroid? Why is the earth of this shape?

5. Explain the terms centrifugal and centripetal, as applied to the earth's motion.