Page:An introduction to linear drawing.djvu/34

Rh ﻿Note.When the base of the pyramid is a regular polygon, and the height falls upon the centre of it, the pyramid is upright and regular; such are figures 10, 11 and 12.

16.On two polygons of parallel sides, raise a pyramid, (fig. 12.)

This is merely another way of performing the last figure, by drawing both polygons before you draw the trunk of the pyramid.

17.Make an upright triangular prism. (fig. 13.)

A prism is a body formed from two equal and parallel polygons, whose corresponding points are joined by lines, all parallel, and equal to each other; such are figures 13 to 20 inclusive.

The height of a prism is a perpendicular to the two bases. A prism is said to be upright when its sides are perpendicular, and oblique when its sides lean or are inclined.

18.Make an oblique triangular prism. (fig. 14.)

19.Make an oblique five sided prism. (fig. 15.)