A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Herbert Prinz
Date: 2018 Oct 14, 21:28 -0700
The Gallica website of the French National Library has an exact facsimile of the book. There, you can find the construction and its analytic proof with the page numbers I have given. It's in chapter 1, par. 13. Cas particulier de trois droites de hauteur.