# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Bris Sextant**

**From:**Alexandre Eremenko

**Date:**2005 Oct 31, 19:10 -0500

Dear John, My Bris sextant consists of 3 glass panes, of size 1 times 1.6 centimeters. One pane is dark, the other two colorless, transparent. The thickness of the glass is about 2mm of the dark glass and 1.6 to 1.7 for the clear one. The two angles between the three glass panes I still don't see how to measure, Bill says he has some devises, and I wonder what they are:-) As I said, I can compute the angles from the Sun altitudes I measured last summer, but I had no chance to do this yet. Hopefully, by the end of this week I will see Bill, and he will make good quality photos of the devise. which I will post on my web site. The external angle (between the first and the third panes) seems to be 22 or 23 degrees, as measured by Chinese-made plastic ruler and protractor, the only things I have to measure such angles:-( But I promise to compute these angles by the end of the week, unless Bill has some wonderful devise to measure them:-) Alex. On Mon, 31 Oct 2005, John Rae wrote: > Dear Alex > The dimensions that would be useful to me would allow me to compute the actual physical angles between the small pieces of glass. By looking sideways at the instrument, ther will be two triangles with the apex at the bottom where all three touch, and two gaps at the top. If I knew the vertical distance represented by the length of glass, and the gap at the top between the three glasses (two measurements) then I can compute the angles between the glasses, assuming the tangent and sine of small angles is approximately the same. > > Using this calculated angle (or angles) I can then adjust my test instrument for further trials to simulate the instrument you have. I thank you for the information.