Part 6

Obtuse and acute angles are also called oblique angles; and lines which are neither parallel nor perpendicular to each other are called oblique lines.

The Vertex or Apex of an angle is the point in which the including lines meet.

An angle is commonly designated by a letter at its vertex; but when two or more angles have their vertices at the same point, they cannot be thus distinguished.

For example, when the three lines A B, A C, and A D in fig. 37 meet in the common point A, we designate either of the angles formed, by three letters, placing that at the vertex between those at the opposite extremities of the including lines. Thus, we say, the angle B A C, etc.

=APEX.=--The summit or highest point of an object.

=ARC.=--See circle.

=AXIS OF A SOLID.=--An imaginary straight line passing through its center.

=AXIS OF A FIGURE.=--A straight line passing through the center of a figure, and dividing it into two equal parts.

=BASE.=--The base of a solid figure is that on which it stands--the lowest part.

=BISECT.=--To divide into two equal parts.

=BISECTOR.=--A line which bisects.

=CIRCLE.=--A Circle is a plane figure bounded by one uniformly curved line, all of the points in which are at the same distance from a certain point within, called the Center.

The Circumference of a circle is the curved line that bounds it.

The Diameter of a circle is a line passing through its center, and terminating at both ends in the circumference, as A C B.

The Radius of a circle is a line extending from its center to any point in the circumference. It is one-half of the diameter. All the diameters of a circle are equal, as are also all the radii C D, C B and C A.

An Arc of a circle is any portion of the circumference, as B D and A D.

Semi-Circle.--Half a circle formed by bisecting it with a diameter, as A C B. Fig. 38.

An angle having its vertex at the center of a circle is measured by the arc intercepted by its sides. Thus, the arc A D measures the angle A C D, and in general, to compare different angles, we have but to compare the arcs, included by their sides, of the equal circles having their centers at the vertices of the angles.

=CIRCUMSCRIBE.=--To draw a line of figures about or outside, such as a circle drawn around a square touching its corners or angles.

Inscribe.--To draw a line or figure inside or on the interior, such as a circle drawn within a square touching its sides.

=CONCAVE.=--Curving inwardly.

=CONE.=--A solid body or figure having a circle for its base, and its top terminated in a point or vertex.

=CONSTRUCTION.=--The making of any object.

=CONTOUR.=--The outline of the general appearance of an object.

=CONVERGENCE.=--Lines extending towards a common point.

=CONVEX.=--Rising or swelling into a round form--the opposite to concave.

=CORNER.=--The point of meeting of the edges of a solid, or the two sides of a plane figure.

=CROSS-HATCHES.=--In free-hand drawing the use of lines crossing each other to produce light and shade effects.