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.