The Ecliptic is a great circle, which represents the sun's apparent annual track among the fixed stars. It derives its name from being the circle on or near which the moon must be in the case of an eclipse. Its plane makes an angle of 234° with the plane of the equator. The sun is in the north, or highest point of the Ecliptic, on 21st June; and he is then vertical at the tropic of Cancer; he is in the south, or lowest point, on 21st December, and is then vertical at the tropic of Capricorn. The Ecliptic is divided into twelve equal parts, called signs, of 30° each, named from the constellations or groups of stars through which the sun appears successively to pass. These, with the days on which the sun enters them, are as follows :

The Colures are two meridians which divide the Ecliptic into four equal parts, making the four Seasons of the year. One of them intersects the equinoctial points, Aries and Libra, and is thence called the Equinoctial Colure; the other intersects the solstitial points, Cancer and Capricorn, and is called the Solstitial Colure.

The Small Circles are the Tropics, the Parallels of Latitude, and the Polar Circles.

The Tropics are two small circles parallel to the equator, and at the distance of 234°, north and south. They are so named because the sun, arrived at them in his apparent annual course, seems to turn away, either northward or southward, as the case may be. The northern is called the Tropic of Cancer, and the southern the Tropic of Capricorn, because they touch the ecliptic in the beginning of those signs.

The Parallels of Latitude are small circles parallel to the equator, the object of which is to indicate the latitude of places, and to connect together all places on the globe having the same latitude. Though on globes and maps of the world they are usually drawn at intervals of 10°, every place is supposed to have a parallel of latitude passing through it.

The Polar Circles are two small circles, drawn around the North and South Pole respectively-the former being called the Arctic, and the latter the Antarctic Circle. Their distance from the Poles is 234, that being the angle formed by the earth's axis and a line drawn perpendicular to the earth's orbit. When the sun is vertical to places situated on the Tropic of Cancer, his rays extend beyond the Pole to the Arctic Circle, and all countries within the Antarctic Circle are then in darkness.

Zones. The Tropics and Polar Circles divide the surface of the earth into five great Climatal Zones or Belts-viz.:

1. One Torrid Zone, 47° in breadth, or 23° on either side of the Equator, and bounded by the Tropics of Cancer and Capricorn.

Every place in this wide region has the sun vertical to it twice a-year; and as the sun's rays never fall very obliquely on any part of it, the temperature at the surface of the earth is here always very high.

NORTH POLE

ARCTIC

CIRCLE

2. Two Temperate Zones, one northern and the other southern, each 43° in breadth, lying between the Tropics and the Polar Circles. Never having the sun vertical, they are characterised by a lower temperature than tropical regions; the fruits of the earth are less luxuriant and spontaneous; and man, compelled to exercise his corporeal and thinking powers, attains to a EAST higher degree of intelligence and civilisation than in those regions where his wants are supplied without any exertion on his part.

TROPIC OF

EQUATOR

WEST

CANCER

ECLIPTIC.

TROPIC OF CAPRICORN

ANTARCTIC

CIRCLE

SOUTH POLE

Zones and Circles.

3. The Two Frigid Zones, each 23 in radius, are included within the Polar Circles.

They are deprived of the influence of the sun for long intervals in winter, and have a correspondingly greater length of day in summer, when his rays fall very obliquely on the surface. These conditions, coupled with the extreme cold of the long winters, are so unfavourable to human culture and human happiness, that the tribes who inhabit the frigid zone have not been able to attain to any considerable degree of civilisation.

The Moon, or the Earth's Satellite.-The earth, on her annual journey round the sun, is attended by a moon or satellite, which revolves round her in the same way as the former does round the central luminary. Of the five planets and numerous planetoids situated between the centre of the system and the orbit of Jupiter, the earth alone enjoys the advantage of such a companion; while all the other planets possessed of satellites are not only of vastly greater dimensions, but also greatly farther from the sun. The mean distance of the moon from the earth is 239,840 miles, or little more than half the sun's radius, and she performs her revolution round her primary in one lunar month of 29 days, 12 hours, 44 minutes. It is a remarkable fact that this is also the precise time in which she rotates round her own axis. Hence, at all times, the moon presents very nearly the same face to the earth. The time of her rotation is much longer than that of any of the planets; but, so far as yet ascertained, all the other satellites belonging to our system follow the same law-that is, they rotate on their axes in the same time as they revolve around their primaries. Unlike the sun and fixed stars,

which are self-luminous, the moon, in common with all the planets and satellites, shines by reflected light derived from the central luminary. Her orbit is inclined to that of the earth at an angle of 5° 9', but for which we should have an eclipse of the sun and moon alternately every fortnight. There is a total eclipse of the sun when the moon is near the earth, and the sun, earth, and moon in the same straight line; and an annular eclipse when, being more remote from the earth, her apparent diameter is less than that of the sun. The surface of the moon presents the aspect of a volcanic wilderness, being interspersed with enormous crateriform mountains, dykes, and lava streams, while no diversities of sea and land are discernible.

Tides. The moon's attractive energy (aided by that of the sun at new and full moon) raises the waters of the ocean into a great tidal wave, which seems to follow the satellite in her path through the heavens. This attraction, however, directly accounts for only one high tide at any place every lunar day of 24 hours, 50 minutes; whereas, in reality, there are two high tides, occurring at intervals of 12 hours, 25 minutes. The other takes place at the same instant, but on the opposite side of the earth's surface, and is caused by the moon drawing towards her the nearer or solid part of the planet with greater force than the more distant waters.

When the moon is in the position M (new moon), or at m (full moon), it acts in conjunction with the sun. The tides on both sides of the earth are then at the highest, and are called Spring Tides. But when the moon is at M' (first quarter), or at m' (last quarter), the waters rise the least, as the attraction of the sun, acting at right angles to that of the moon, considerably neutralises the effect of the latter, and produces what are known as Neap Tides.

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PART II.

PHYSICAL GEOGRAPHY

1.-MATERIALS, DENSITY, AND ATTRACTIVE POWER OF THE EARTH. Of the interior of the planet which we inhabit we know almost nothing, our observation being confined to a portion of its external crust, or rind, rarely exceeding 14 miles in depth, or of the distance from the surface to the centre. Even this insignificant distance is attained by adding the height of the loftiest known mountain to the depth below the sea-level of the deepest ocean sounding (p. 22). The Geologist, however, without penetrating beneath the surface, but by carefully examining the order of superposition of the stratified rocks, has made us more or less acquainted with a depth of about 25 miles. Small as this portion is, when compared with the immense volume enclosed by it, it presents to our view a vast variety of substances, each of which has a character peculiar to itself. On examination, they are nearly all found to be compound bodies, which, on being analysed, are reducible to 65 constituent elements.

The

Constituents of the Earth's Crust.-These 65 elements the chemist divides into two groups,-the Metallic and the Non-metallic. metals are 52 in number, the best known of them being gold, silver, copper, iron, lead, tin, zinc, and mercury; while the metalloids, or non-metallic class, consists of only 13, the principal of which are oxygen, hydrogen,* nitrogen, carbon, sulphur, and phosphorus. Each of these elementary substances has properties peculiar to itself; and, what is more remarkable, on each of them the Creator has stamped, in deep and indelible characters, a particular number, which forms, as it were, the law of its being, and determines in what proportions it shall combine with other substances. This law of definite proportions serves in the mineral kingdom the same end as the laws which regulate the propagation of species in the vegetable and animal kingdoms; the identity of species is rigidly preserved, and, notwithstanding the prodigious number of combinations, all confusion is avoided.

Density of the Earth.-Each of the 65 elementary substances has a density or specific gravity peculiar to itself, ranging from hydrogen, which is the lightest, to platinum, which is the heaviest; but the resulting mean density of the Earth is 5.7 the weight of its own

* From recent experiments by the late Professor Graham of London University, it would appear that hydrogen must now be regarded as a metal.

bulk of distilled water at the temperature of 68°. Thus, while the specific gravity of Mercury is nearly a fourth greater, that of Venus and Mars is nearly equal, while that of the Sun and Jupiter is four, Urănus five, and Neptune six times less. As the specific gravity of the substances forming the crust of the Earth rarely exceeds 3, the obvious inference seems to be, that the interior of the planet cannot be hollow, but, on the contrary, must consist of metals, or of other materials, in a highly condensed and incandescent state.

103.

-1%

The Attractive Energy which the Earth exercises on all material substances near its surface is such that, when freely suspended, they are drawn towards it with a velocity of 16 feet in the first second of time; three times 16 feet the next second; five times 16 feet the third second; and so on, following the order of the odd numbers of the scale. Comparing the Earth, in this particular, with the other planets, we find that bodies falling towards the surface of Mars descend with only a fourth of this velocity; while in Jupiter the velocity is two and a half times greater. At great elevations above the surface the intensity of the force of gravitation decreases in the inverse ratio of the square of the distance. Thus, a body which (in a spring balance) weighs 16 ounces at the surface, will weigh only 4 ounces at the distance of two semi-diameters from the centre, or one semi-diameter above the surface; while at the distance of four semi-diameters it will weigh only 1 ounce. Under the surface the law of decrease is very different, it being there directly as the distance from the centre. Thus, at one thousand miles below the surface the body will weigh 12 ounces; half-way towards the centre, 8 ounces; at the distance of a thousand miles from the centre, 4 ounces; while at the centre the pressure on the balance will be nothing. The accompanying diagram will render these observations more intelligible to the pupil.

4

16

12

8

4

2. CONFIGURATION OF THE SURFACE.-The terraqueous globe must be supposed to have assumed its present spheroidal form when rotating in its primitive incandescent state. This form the vast collection of waters now on its surface powerfully tends to perpetuate-first, by their capacity of yielding to the centrifugal force arising from the planet's rotatory motion; and, secondly, by their filling up innumerable depressions in its crust-depressions which, it is now certain, exceed in depth the highest elevations of the land. These elevations very

rarely amount to the part of the radius, and, therefore, scarcely interfere with the regular form of the planet. In fact, the mountain-chains on the globe produce no greater deviation

B