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