The earth being a round body,[1] it is represented by a globe; but as both sides of this globe cannot be seen at the same time, it must be divided into hemispheres or halves: there will then be an eastern and a western, or, a northern and a southern hemisphere.
Suppose a circle to be described, and a point placed within it; the situation of this point must be determined with relation to some other part of the circle. If a horizontal line be drawn across the circle and divide it equally, the line appears to us straight; but cannot, in reality, be so, because it is half the circumference of a globe. A perpendicular line is then drawn, and the hemisphere is divided into four equal quarters: each quarter containing 90° or one-fourth of 360°; every circle containg 360°. (See Plate I. fig. 3.) The horizontal line must be taken for the equator. The quarter then in which the dot or point appears, should be divided by 90 lines, but as this would completely conceal the surface of the diagram, and obliterate the little point itself, we will divide it into 9 parts. (See Plate I. fig. 4.)
The point is now evidently within the first stripe or line, and if these lines be named ladders,
- ↑ The earth is, as every one knows, an oblate spheroid, but it would be needless to descend to particulars, in a general illustration.
