Re: [math-fun] Bar bet

"The volume of a regular tetrahedron (triangular pyramid with unit edges) is exactly half the volume of a square pyramid with unit edges."

This is (I think) equivalent to stating that the volume of an octahedron is four times the volume of a tetrahedron (unit edges assumed). I searched for an early observation of this and found an 1820 mention by Nathaniel John Larkin in his "An Introduction to Solid Geometry and to the study of Crystallography..." (page 103, free ebook at Google). Perhaps someone knows of an even earlier reference?