16. The volume of the ring described by a circle which revolves round a line in its plane is equal to the area of the circle, multiplied by the circumference of the circle described by its centre.
17. Any plane bisecting two opposite edges of a tetrahedron bisects its volume.
18. Planes which bisect the dihedral angles of a tetrahedron meet in a point.
19. Planes which bisect perpendicularly the edges of a tetrahedron meet in a point.
20. The volumes of two triangular pyramids, having a common solid angle, are proportional to the rectangles contained by the edges terminating in that angle.
21. A plane bisecting a dihedral angle of a tetrahedron divides the opposite edge into portions proportional to the areas containing that edge.
22. The volume of a sphere: the volume of the circumscribed cube as π : 6.
23. If h be the height, and ρ the radius of a segment of a sphere, its volume is
(h2 + 3ρ2).