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 highlighted area is a solid rhombus (OBAB') and its volume is equal to a cone whose base is equal to the surface of the cone ABB' and whose height is equal to perpendicular from O on AB. According to Proposition 18 Any solid rhombus consisting of isosceles cones is equal to the cone which has its base equal to the surface of one of the cones composing the rhombus and its height equal to the perpendicular drawn from the apex of the second cone to one side of the first cone.