DEFHSnTIONS OF MATHEMATICAL FOKMS. 



295 



A Frustum of a pyramid or a cone is the 

 (part that remains after cutting off the top by a 

 plane parallel to the base. 



Fig. 19 represents the frustum of a pyramid. 



Fig. 20 represents the frustum of a cone. 



An Ellipse is a plane curve such that the sums 

 <of the distances of any points in the bounding 

 line from two points within called the foci are 

 .-always equal. 



The line A B passing through the foci is 

 -called the major diameter ; and the diame- 

 ter perpendicular to A B at its centre is 

 called the minor diameter. 



A Sphere is a solid, bounded by a convex sur- 

 face, every point of which is equally distant from 

 a point within called the centre ; as, Fig. 22. 



A Spheroid is a solid, generated by the revo- 

 lution of an ellipse about one of its diameters. 

 If the ellipse revolves about its major diameter 

 the spheroid is called prolate. If about its mi- 

 nor diameter the spheroid is called oblate. 



Fig. 23 represents & prolate spheroid. 



Fig. 24 represents an oblate spheroid. 



FIG. 19 



FIG. 20. 



FIG. 81. 



FIG. 22. 



