What an obscure ruler....
It is called a 'sector', it's not a navigation rule, it is an architects/engineers tool.
It is sort of a primitive slide rule tool, it measures various things that are diffecult to otherwise calculate when drawing, eg "inches, decimal feet, sines, secants,tangents,chords,numbers(eg logs) ,polygons, equal parts..."
Marked makers include Watkins,Elliot brothers, Watkins&Hill...
Yours isn't marked, and they are apparently rather hard to identify when unmarked...
Typical age for ivory ones seems to be mid-late 1800's
Fancy ones have the joint made of silver, less fancy of brass...