OF THE TERRESTRIAL GLOBE.

CHAPTER I.

Definitions and Preliminary Remarks.

1. A GLOBE, or SPHERE, is a round body, every part of whose surface is equally distant from a point within, called its centre.

2. Artificial Globes are of two kinds, terrestrial

and celestial.

The artificial globes are of considerable use in geography and astronomy, by serving to give a lively representation of their principal objects, and for performing and illustrating many of their operations in a manner easy to be perceived by the senses, and so as to be conceived even without any knowledge of the mathematical grounds of those sciences.

3. The terrestrial globe exhibits a representation of the different countries, republics, empires, kingdoms, chief towns, oceans, seas, lakes, rivers, &c. truly delineated on it, according to their relative situations on the real globe of the earth.

The true figure of the earth, which is an oblate spheroid (a figure formed by the revolution of an ellipsis, which approaches nearly to a circle, round its shorter axis or conjugate diameter,) shall be explained in a subsequent part of this work; but here the figure of the earth is supposed to be a perfect sphere, since there is no other figure which can give so exact an idea of its true shape.

4. The celestial globe is an artificial representation of the heavens, on which the stars are laid down in their natural situations.

As the stars are drawn on a convex surface, whereas their natural appearance is in a concave one; in using this globe, the student is supposed to be situated in the centre of it, and viewing the stars in the concave surface.

5. The axis of the earth is an imaginary straight line passing through its centre, and upon which it is supposed to revolve.

6. The poles of the earth are the extremities of its axis at the earth's surface; one of which is called the north pole, and the other the south pole.

If the axis of the earth be produced to the heavens, the points in which it cuts the celestial sphere, are called the celestial poles, or, by way of eminence, the poles of the world.

7. The axis of the terrestrial globe is a straight line, which passes through its centre from north to south, and is represented by the wire on which it turns.

8. The revolution of the globe upon this axis shows the manner in which the earth performs its diurnal rotation from west to east.

From this circumstance arises the apparent diurnal motion of all the heavenly bodies from east to west.

9. This diurnal or daily motion of the earth is the occasion of day and night; and its annual motion round the sun, (in popular language called the sun's course in the ecliptic,) causes the beautiful diversity of seasons.

When we reflect on the diurnal motion to which all the heavenly bodies are subject, we cannot but recognise the existence of one general cause which moves them, or which seems to move them round the earth; and, as LAPLACE remarks in his System of the World, since the heavenly bodies present the same appearances to us, whether the firmament carries them round the earth, considered as immoveable, or whether the earth itself revolves in a contrary direction; it seems much more natural to admit this latter motion, and to regard that of the heavens as only apparent.

Carried on with a velocity which is common to every thing that surrounds us, we are in the case of a spectator placed in a ship that is in motion. He fancies himself at rest, and the shores, the hills, and all the objects placed out of the vessel, appear to him to move. But on comparing the extent of the shore, the plains, and the height of the mountains, with the smallness of his vessel, he recognises that the apparent motion of these objects, arises from the real motion of himself. The numberless stars which fill the celestial regions, are relatively to the earth what the shores and the hills are to the vessel; and the same reasons which con vince the navigator of the reality of his own motion, prove to us the motion of the earth. The diurnal and annual motions of the earth shall be fully illustrated in the Third Book.

QUESTIONS.

What is a globe or sphere?

How many kinds of artificial globes are there,

and what are they called?

What does the terrestrial globe represent?

What does the celestial globe represent?

What is the axis of the earth?..

What are the poles of the earth, and what are they called?

What is the axis of the artificial globe, and what does the revolution of the globe on this axis show? What is the occasion of day and night?

What is the cause of the beautiful diversity of seasons?

CHAPTER II.

Of the great circles on the terrestrial globe. 1. Circles which divide a globe into two equal

parts, are called great circles.

2. The pole of any great circle is a point on the surface of the globe, 90 degrees distant from every. part of that circle of which it is the pole.

Hence every great circle has two poles diametrically opposite to each other.

3. The great circles on the terrestrial globe, which divide it into two equal parts, are the equator, the ecliptic, and the meridians.

4. The equator is a great circle of the earth, equidistant from the poles, and divides the globe into two hemispheres, called the northern and southern.

The latitudes of places are counted from the equator northward and southward, and the longitudes of places are reckoned upon it, eastward and westward from the first meridian.

5. The equator, when referred to the heavens, is called the equinoctial, because when the sun appears in it, the days and nights are equal all over the world, (the poles excepted ;) that is, 12 hours each.

This is on the supposition that there is no refraction.

6. The ecliptic is that great circle in which the sun makes his apparent annual progress among the fixed stars; or it is the real path of the earth round the sun.

The ecliptic cuts the equator in an angle of 23° 28'; the points of intersection, Aries and Libra, are called the equinoctial points; and the points Cancer and Capricorn, where it meets the tropics, are called the solstitial points.

The angle which the ecliptic makes with the equator, is called the obliquity of the ecliptic.

7. The ecliptic is usually divided into 12 equal parts, called signs, each containing 30 American degrees.

The sun makes his apparent annual progress through the ecliptic at the rate of 59'8." 2, (or nearly a degree,) in a day.

৭. The division of the signs commences at the first point of Aries, which is one of the equinoctial points, and they are numbered according to the sun's apparent motion in the ecliptic.

9. The names of the signs, with their corresponding characters, and the days on which the sun enters each of them, according as they are represented on Wilson's American Globes, are,

Marks. Days in which the sun enters

each of them.

Signs.

The former six signs lie on the north side of the equator, and are called northern signs; when the sun is in any of these signs, his declination is north. The latter six signs lie on the south side of the equator, and are called southern signs; when the sun is in any of these signs, his declination is south. It is also proper to observe that, when the sun enters the signs Aries or Libra, his declination is nothing.

10. The meridians are great circles passing through the poles, and cutting the equator at right angles.

Every place upon the earth is supposed to have a meridian passing through it, though, to prevent confusion, there are, in general, only twelve drawn on the artificial globe.