Proposition 26
 

The figure inscribed as above in a sphere is equal [in volume ] to a cone whose base is a circle equal to the surface of the figure inscribed in the sphere and whose height is equal to the perpendicular drawn from the canter of the sphere to one side of the polygon.
 
The Rhombus is equal in volume to this cone because they share the same base BB'. If we extend the CB and C'B' they meet at T which will be the apex of the cone with base MM'.