THE SEASONS.

cold surface, or beneath the colder waves.

109

The Polar Bear, and

the huge Walrus, and the Seal, sport among the floating fields of ice; and the great Auk, or Penguin, seems to choose, by instinct, this desolate and almost forsaken region of the earth.

As the earth moves on in its orbit, the sun now rises higher and higher in the heavens, at noon, each day, and finally arrives at the point of vernal equinox and enters the constellation Pisces. Again the nights and days are equal all over the globe, the earth being illuminated from pole to pole. As the sun proceeds still farther north of the equator, the north pole becomes more and more illuminated, until finally it arrives at the point of greatest difference between the equator and ecliptic marked XVIII, see figure on page 91, which is the point of the summer solstice. The sun is now vertical in the northern hemisphere, at noon, at all places situated on the parallel a b, which is called the tropic of Cancer. As the sun is now entering the sign Cancer, and since the north pole

of the earth is now wholly illuminated, as in the above diagram, it is evident that the north pole, or rather the axis of the earth, is inclined towards the sign Cancer. The days are still equal to the nights, at the equator, but at all places north of the equator, as for example on the parallel A B, the days are now longer than the nights, the half illuminated portion IB. being greater than the half unilluminated AI. It is now mid-summer, the beginning of July, in the northern hemisphere, while at the same time it is mid-winter in the southern. The arctic circle is now wholly illuminated, but the antarctic is in complete shade. At this season the sun mounts highest in the heavens to all north of the equator, and lowest to all south of it. The rays of the sun falling almost direct, or perpendicular, upon the earth, in our northern latitude

F

in summer, and also the days being longer than the nights, and the earth being thus warmed during the day more than it is cooled during the night, this excess of warmth contributes to augment the summer heat. It will also now appear why, although the earth is actually nearer the sun in winter than in the summer, no increase of heat is, on this account, perceived, a the rays, at this time, fall very slanting upon the earth in the northern hemisphere, although in the southern hemisphere, the summer occurring at the time when the sun is nearest the earth, is much warmer than the same season at the north. The lines we have thus seen, apparently marked on the earth by the sun, divide the earth into five zones, or belts. The north frigid; the north temperate; the tropical, or torrid; the south temperate; and the south

N.POLAR.

N.TEMPERATE.

TORKID.

S.TEMPERATE.

S.POLAR
S

frigid. The first and last are included within the polar circles, and are always cold, inhospitable regions. The temperate zones are included within the polar circles and the tropics, whilst the tropical, or equatorial regions lie wholly within the two tropics. At the equator, as we have already intimated, and indeed for some distance north and south, nearly to the tropics, perpetual summer reigns, as the sun is, at almost all times vertical at noon; at the equinoxes, it is truly so; but at the time of the summer solstice, it is seen by the inhabitants of the equatorial regions passing between the zenith and the northern horizon, not, however, nearer the north pole, or the polar star, than the rest of the world perceive it; for to them, the north star is on the horizon. At the time of the winter solstice, it is seen by a spectator at the

CHANGE OF THE LINE OF APSIDES.

111

equator, to pass between the zenith and the southern horizon, about as far from being vertical at noon there, as it is to us at the time of summer solstice. Hence, an observer there, looking directly overhead, might imagine the equator a line marked in the heavens extending from east to west, like the line E W; Z

W

E

B

A

being the zenith, or point directly overhead, and A B the sun's path, along which it would appear to move; being at Z, at the time of the equinoxes, rising directly east and setting directly west, and at B at the time of the summer solstice, describing, by the diurnal motion of the earth, the tropic Cancer, and at A at the time of winter solstice, its diurnal path being the tropic of Capricorn.

We have thus, at some length, explained the phenomena of the seasons, and will now, for a few moments, consider what will be the effect of the precession of the equinoxes. The earth's axis, at present, is inclined towards the sign Cancer, which is located in the constellation Gemini. In the course of about 6000 years, it will still be inclined towards the sign Cancer, but that sign will be in the constellation Pisces. And in 6000 years more, towards the constellation Sagittarius. In consequence of this change, the seasons will all, really, be misplaced about six months, although the various contrivances for retaining the names of the months, as indicative of the several seasons, will doubtless, then, as now, make the 21st of March the commencement of spring, or time of vernal equinox. There is, however, a more important change than this, which will affect our seasons The major axis of the earth, or line a b, see figure on page 78, has not a fixed direction in space, but is slowly moving from west to east, i. e. in the order of the signs, at such a rate that it will require about 100,000 years to make a complete revolution. The earth arrives at its perihelion

now about the 1st of January, but 50,000 years hence, it will be in its perihelion in July, or time of mid-summer in the northern hemisphere, and at that time the earth's axis will be inclined nearly as at present, unless changed by some great convulsion. The effect of this change of the direction of the line of apsides will be therefore, to cause the earth to arrive at its perihelion at the time of mid-summer in the northern hemisphere, instead of at mid-winter as at present, thus producing, if the relative distribution of land and water remains unchanged, a tropical climate or nearly so, over the whole globe.

Such periods of time are however, too remote to be worth much notice from us, except for their interest in a geological point of view, for we believe this to be the true explanation of the greater warmth of the climate of the ancient world, as indicated by fossil flora and fauna; we shall refer to this again.

We have given no explanation of the cause of the precession of the equinoxes, or the motion of the apsides. It would be extremely difficult for us to do so in a manner intelligible to the general reader. The facts, however, are unquestionable. The former is caused by the attractions of the sun and moon upon the excess of matter at the equator, for the earth is not truly spherical, but oblate, on account of its diurnal rotation. This equatorial belt, not corresponding with the plane of the ecliptic, is acted upon obliquely, and with varying force by the sun and moon, producing the retrogradation of the nodes. The motion of the apsides is the conjoint effect of the attractions of the planets upon the earth in the various parts of its orbit.

We here close the first part of our volume, not however, without the hope that we have succeeded in making it interesting, and that the reader will feel repaid for the time spent in perusing it.

THE WORLD.