DEFIMTIONS. 



Fig. 16. 



An octahedron with a rhombic base, fig. 16, contained 

 within eight equal scalene triangular planes. 



The solid angles at a, b, fig. 12 and 14, and c, d 9 

 fig. 15 and 16, are sometimes called the summits of 

 the octahedron. 



Fig. 17. 



A right prism with a square base*, fig. 17, or right 

 square prism, the edge, a, being always greater or 

 less than b : if a, and b, were equal, the figure would 

 be a cube. 



Fig. 18. 



A right prism with a rectangular base, fig. 18, or 

 right rectangular prism, whose three edges a, b, c, are 

 unequal. For if any two of those were equal, the 

 prism would be square. 



* A prism is a solid whose lateral edget are parallel, and whose terminal 

 planet are also parallel. 



Those prisms which stand perpendicularly when resting on their base, 

 are called right prisms. Those which incline from the perpendicular > are 

 called oblique prisms. 



B 



