35f> GEOMETRY. 



sides are perpendicular to the plane of the base, it is 

 called an upright prism ; if they are inclined, it is 

 called an oblique prism. 



42. When the base of a prism is a parallelogram, 

 it is called a parallelopipedon, as Fig. 22 t and 23. 

 Hence a parallelopipedon is a solid terminated by 

 six parallelograms. 



43. When all the sides of a parallelopipedon are 

 squares, the solid is called a cube, as Fig. 23. 



44. A rhomboid is an oblique prism, whose bases 

 are parallelograms. (Fig. 24.) 



45. A pyramid, A B, (Fig. 25. and 26.) is a solid 

 bounded by, or contained within a number of 

 planes, whose base may be any rectilinear figure, 

 and whose faces are triangles terminated in one 

 point, B, commonly called the summit, or vertex of 

 the pyramid. 



When the figure of the base is a triangle, it is 

 called a triangular pyramid ; when the figure of 

 the base is quadrilateral, it is called a quadrilateral 

 pyramid, &c. 



A pyramid is either regular or irregular, ac- 

 cording as the base is regular or irregular. 



A pyramid is also right or upright, or it is 

 oblique. It is right, when a line drawn from the 

 vertex to the centre of the base, is perpendicular 

 to it, as Fig. 25. ; and oblique, when this line in- 

 clines, as Fig. 26. 



46. A cylinder is a solid (Fig. 27. and 28.) ge- 

 nerated or formed by the rotation of a rectangle 

 about one of its sides, supposed to be at rest ; this 

 quiescent side is called the axis of the cylinder. 

 Or it may be conceived to be generated by the 

 motion of a circle, in a direction perpendicular to 

 its surface, and always parallel to itself. 



