A pyramid is a solid whose sides are all triangles meeting together in a point, the base being any plane figure whatever. It is called a triangular pyramid when its base is a triangle; a square pyramid when its base is a square, &c.
The segment of a pyramid, cone, or any other solid is a part of D E F G cut off from the top by a plane D E F, parallel to the base A B C.—Vide [Fig. 21], Plate 2, Heights, Distances, and Practical Geometry.
A frustrum, or trunk, is a part A B C D E F, that remains at the bottom after the segment is cut off.
A cone is a round pyramid, of which the base is a circle.
The axis of a solid is a line from the vertex (or point) to the centre of the base, or through the centres of the two ends. When the axis is perpendicular to the base, it is a right prism, pyramid, or cone; otherwise it is oblique.
A sphere is a solid contained under one convex surface, and is described by the revolution of a semicircle about its diameter, which remains fixed.
The centre of the sphere is such a point within the solid as is everywhere equally distant from the convex surface, or circumference of it.
The diameter (or axis) of a sphere is a straight line, which passes through the centre, and is terminated by the convex surface.
A segment of a sphere is a part cut off by a plane, the section of which is always a circle, called the base of the segment.
A sector of a sphere is that which is composed of a segment (less than an hemisphere) and of a cone.