round us, than there is to suppose that they revolve around any of the other planets, since the same apparent revolution is common to them all, and as we know that the other planets, at least many of them, turn on their axes, and as all the phenomena presented by the earth, can be accounted for by such a revolution, it is folly to conclude otherwise.

Circles and Divisions of the Earth.

It will be necessary for the pupil to retain in his memory the names and directions of the following lines, or circles, by which the earth is divided into parts. These lines, it must be understood, are entirely imaginary, there being no such divisions marked by nature on the earth's surface. They are, however, so necessary, that no accurate description of the earth, or of its position with respect to the heavenly bodies, can be conveyed without them.

E

Fig. 194.

D

The earth, whose diameter is 7912 miles, is represented by the globe, or sphere, fig. 194. The straight line K passing thro' its centre, and about which it turns, is called its axis, and the two extremities of the axis are the poles of the earth, A being the north pole, and B the south pole. The line C D, crossing the axis, passes quite round the earth, and divides it into two equal parts. This is called the equinoctial line, or the equator. That part of the earth, situated north of this line, is called the northern hemisphere, and that part south of it, the southern hemisphere. The small circles E F, and G H, surrounding or including the poles, are called the polar circles. That surrounding the north pole is called the arctic circle, and that surrounding the

L

H

M

How can all the phenomena of the heavens be accounted for, if they do not revolve? What is the axis of the earth? What are the poles of the earth? What is the equator? Where are the northern and southern hemisphere? What are the polar circles?

south, the antartic circle. Between these circles, there is, on each side of the equator another circle, which marks the extent of the tropics towards the north and south, from the equator. That to the north of the equator, I K, is called the tropic of Cancer, and that to the south, L M, the tropic of Capricorn. The circle L K, extending obliquely across the two tropics, and crossing the axis of the earth, and the equator at their point of intersection, is called the ecliptic. This circle, as already explained, belongs rather to the heavens than the earth, being an imaginary extension of the plane of the earth's orbit in every direction towards the stars. The line in the figure, shows the comparative position or direction of the ecliptic in respect to the equator, and the axis of the earth.

The lines crossing those already described, and meeting_at the poles of the earth, are called meridian lines, or mid-day lines, for when the sun is on the meridian of a place, it is the middle of the day at that place, and as these lines extend from north to south, the sun shines on the whole length of each, at the same time, so that it is 12 o'clock, at the same time, on every place situated on the same meridian.

That

The spaces on the earth, between the lines extending from east to west, are called zones. That which lies between the tropics, from M to K, and from I to L, is called the torrid zone, because it comprehends the hottest portion of the earth. The spaces which extend from the tropics, north and south, to the polar circles, are called temperate zones, because the climates are temperate, and neither scorched with the heat, like the tropics, nor chilled with the cold, like the frigid zones. lying north of the tropic of Cancer, is called the north temperate zone, and that south of the tropic of Capricorn, the south temperate zone. The spaces included within the polar circles, are called the frigid zones. The lines which divide the globe into two equal parts, are called the great circles; these are the ecliptic and the equator. Those dividing the earth into smaller parts are called the lesser circles; these are the lines dividing the tropics from the temperate zones, and the temperate zones from the frigid zones, &c.

Which is the arctic, and which the antarctic circle? Where is the tropic of Cancer, and where the tropic of Capricorn? What is the ecliptic? What are the meridian lines? On what part of the earth is the torrid zone? How are the north and south temperate zones bounded? Where are the frigid zones? Which are the great, and which the lesser circles of the earth?

The ecliptic, we have already seen, is divided into 360 equal parts, called degrees. All circles, however large or small, are divided into degrees, minutes, and seconds, in the same manner as the ecliptic.

The horizon is distinguished into the sensible and rational. The sensible horizon is that portion of the surface of the earth which bounds our vision, or the circle around us, where the sky seems to meet the earth. When the sun rises, he appears above the sensible horizon, and when he sets, he sinks below it. The rational horizon is an imaginary line passing through the centre of the earth, and dividing it into two equal parts.

The axis of the ecliptic is an imaginary line passing through its centre and perpendicular to its plane. The extremities of this perpendicular line, are called the poles of the ecliptic.

If the ecliptic, or great plane of the earth's orbit, be considered on the horizon, or parallel with it, and the line of the earth's axis be inclined to the axis of this plane, or the axis of the ecliptic, at an angle of 23 degrees, it will represent the relative positions of the orbit, and the axis of the earth. These positions are, however, merely relative, for if the position of the earth's axis be represented perpendicular to the equator, as A B, fig. 194, then the ecliptic will cross this plane obliquely, as in that figure. But when the earth's orbit is considered as having no inclination, its axis, of course, will have an inclination, to the axis of the ecliptic, of 23 degrees.

As the orbits of all the other planets are inclined to the ecliptic, perhaps it is the most natural and convenient method to consider this as a horizontal plane, with the equator inclined to it, instead of considering the equator on the plane of the horizon, as is sometimes done.

The inclination of the earth's axis to the axis of its orbit never varies, but always makes an angle with it of 23 degrees, as it moves round the sun. The axis of the earth is therefore always parallel with itself. That is, if a line be drawn through the centre of the earth, in the direction of its axis, and extended north and south, beyond the earth s diame

How are circles divided? How is the sensible horizon distinguished from the rational? What is the axis of the ecliptic? What are the poles of the ecliptic? How many degrees is the axis of the earth inclined to that of the ecliptic? What is said concerning the relative positions of the earth's axis and the plane of the ecliptic? Are the orbits of the other planets parallel to the earth's orbit, or inclined to it? What is meant by the earth's axis being parallel to itself?

ter, the line so produced will always be parallel to the same line, or any number of lines, so drawn, when the earth is in different parts of its orbit.

Suppose a rod to be fixed into the flat surface of a table, and so inclined as to make an angle with a perpendicular from the table of 23 degrees. Let this rod represent the axis of the earth, and the surface of the table, the ecliptic. Now place on the table a lamp, and round the lamp hold a wire circle three or four feet in diameter, so that it shall be parallel with the plane of the table, and as high above it as the flame of the larp. Having prepared a small terrestrial globe, by passing a wire through it for an axis, and letting it project a few inches each way, for the poles, take hold of the north pole, and carry it round the circle, with the poles constantly parallel to the rod rising above the table. The rod being inclined 23 degrees from a perpendicular, the poles and axis will be inclined in the same degree, and thus the axis of the earth will be inclined to that of the ecliptic every where in the same degree, and lines drawn in the direction of the earth's axis will be parallel to each other in any part of its orbit.

Fig. 195.

B

A

D

This will be understood by fig. 195, where it will be seen,

How does it appear by fig. 195, that the axis of the earth is parallel to itself, in all parts of its orbit? How are the annual and diurnal revolutions of the earth illustrated by fig. 195 ?

that the poles of the earth, in the several positions of A, B, C, and D, being equally inclined, are parallel to each other. Supposing the lamp to represent the sun, and the wire circle the earth's orbit, the actual position of the earth, during its annual revolution around the sun, will be comprehended: and if the globe be turned on its axis, while passing round the lamp, the diurnal or daily revolution of the earth will also be represented.

Day and Night.

Were the direction of the earth's axis perpendicular to the plane of its orbit, the days and nights would be of equal length all the year, for then just one half of the earth, from pole to pole, would be enlightened, and at the same time the other half would be in darkness.

Suppose the line s o, fig. 196, from the sun to the earth, to be the plane of the earth's orbit, and that n s, is the axis of the earth perpendicular to it, then it is obvious, that exactly the same points on the earth would constantly pass through the alternate vicissitudes of day and night; for all who live on the meridian line between n and s, which line crosses the equator at o, would see the sun at the same time, and consequently, as the earth revolves, would pass into the dark hemisphere at the same time. Hence in all parts of the globe, the days and nights would be of equal length, at any given place.

Now it is the inclination of the earth's axis, as above described, which causes the lengths of the days and nights to differ at the same place at different seasons of the year, for on reviewing the position of the globe at A, fig. 195, it will be observed, that the line formed by the enlightened and dark

Explain, by fig. 196, why the days and nights would every where be equal, were the axis of the earth perpendicular to the plane of his orbit. What is the cause of the unequal lengths of the days and nights in different parts of the world?