DEFINITIONS. 



Fig. 12. 



The regular octahedron, fig. 12, resembling 1 two 

 four-sided pyramids united base to base. The planes 

 are equilateral triangles, and the common base of the 

 two pyramids (which will hereafter be denominated 

 the base of the octahedron) is a square. 



Fig. 13. 



The rhombic dodecahedron, fig. 13, contained with- 

 in twelve equal rhombic planes, having six solid 

 angles, consisting each of four acute plane angles, 

 two opposite ones as a, b, being sometimes called the 

 summits, and eight solid angles consisting each of 

 three obtuse plane angles. 



