Pyramids. Solids, which decrease gradually from the base, till they come to a point, are called pyramids, They are of different kinds, according to the figure of their bases. If the pyramid has a square base, it is called a square pyramid : if a triangular base, a triangular pyramid; if the base be a circle, a circular pyramid, or a Cone. The point in which the pyramid ends is called the vertex. A line through the centre of the pyramid, from the vertex to the base, is the height. The frustrum of a pyramid is what remains, after any portion of the top has been cut off parallel to the base.