eadily, in your mind, the difference between the two figures.

TERMS OF ANGLES.--The relation of the lines to each other, the manner in
which they are joined together, and their comparative angles, all have
special terms and meanings. Thus, referring to the isometric cube, in
Fig. 145, the angle formed at the center by the lines (B, E) is
different from the angle formed at the margin by the lines (E, F). The
angle formed by B, E is called an exterior angle; and that formed by E,
F is an interior angle. If you will draw a line (G) from the center to
the circle line, so it intersects it at C, the lines B, D, G form an
equilateral or isosceles triangle; if you draw a chord (A) from C to C,
the lines H, E, F will form an obtuse triangle, and B, F, H a
right-angled triangle.

CIRCLES AND CURVES.--Circles, and, in fact, all forms of curved work,
are the most difficult for beginners. The simplest figure is the circle,
which, if it represents a raised surface, is provided with a heavy line
on the lower right-hand side, as in Fig. 146; but the proper artistic
expression is shown in Fig. 147, in which the lower right-hand side is
shaded in rings running only a part of the way around, gradually
diminishing in length, and spaced farther and farther apart as you
approach the center, thus giving the appearance of a sphere.

[Illustration: _Fig. 148._]

IRREGULAR CURVES.--But the irregular curves require the most care to
form properly. Let us try first the elliptical curve (Fig. 148). The
proper thing is, first, to draw a line (A), which is called the "major
axis." On this axis we mark for our guidance two points (B, B). With the
dividers find a point (C) exactly midway, and draw a cross line (D).
This is called the "minor axis." If we choose to do so we may indicate
two points (E, E) on the minor axis, which, in this case, for
convenience, are so spaced that the distance along the major axis,
between B, B, is twice the length across the minor axis (D), along E, E.
Now find one-quarter of the distance from B to C, as at F, and with a
compass pencil make a half circle (G). If, now, you will set the compass
point on the center mark (C), and the pencil point of the compass on B,
and measure along the minor axis (D) on both sides of the major axis,
you will make two points, as at H. These points are your centers for
scribing the long sides of the ellipse. Before proceeding to strike the
curved lines (J), draw a diagonal line (K) from H