Page:An introduction to linear drawing.djvu/49

 ﻿12. Make a sphere and the little circles which run parallel to the equator, (fig. 7.)

After having described a circle, and its two perpen- dicular diameters, as in the preceding problem, divide the circle by dots into arcs of, say, 15 degrees ; there will then be five dots and six arcs between the equator and each pole ; then divide the axis into the same number of parts. The next object is to draw an arc through the three points nearest the equator, then through the three next, and so on till all are drawn.

These arcs on a solid globe would be parallel to the equator, but do not appear so on a plane or flat surface. In geography, they are called Parallels of Latitude.

If an apple be taken and sliced from side to side, it will exactly represent the circles, which are planes cutting a sphere perpendicular to its axis.

13.	Draw a sphere which shall unite the two pre- ceding problems. ("fig. 7.)

14.	Draw an ellipse, (fig. 8.)

An ellipse is an oval, which may be more or less lengthened, as in figures 9 to 13. To make an ellipse : first cross two perpendicular right lines; the upper and lower halves to be of equal length, and the right and left hand to be equal also. You thus obtain the longest and shortest diameter of the ellipse, called its great and small axis. The next thing is to draw the curved lines as in