Anyone have a clue about this 6" Folding Rule, almost definitely ivory, strange scales, center points on brass pins for placing dividers I assume.  Possibly for navigation?

Welcome , Joe! We have some rule collectors here and someone will chime in soon with info for You.

Very nice,but I have no idea as to the purpose. It will be interesting to find out what they were made for

Just......WOW!
  Definitely ivory, definitely navigational. Could be 18th or early 19th century from the look of it. No idea whether French or English or what, though.
Many ivory rules got their graduations worn off. 
Your rule has very fine graduations and still strong so this is ...........WOW
  At least to me.  Who is not a rule expert.
  But I have a few and I like them.

There is a new book..........

http://cgi.ebay.com/PHIL-STANLEY-Boxwood-Ivory-Folding-Rule-New-Book-/260783416314?pt=LH_DefaultDomain_0&hash=item3cb7e723fa

  yours Scott

Thanks for the "Wow" - I always thought it was cool.  Tried searching a bit: rules, ivory, navigation, maritime - haven't gotten anywhere.  We'll keep trying!

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...

Rusty, my Hero!  With that one word my search quickly led to Wikipedia "Sector (instrument)" and other articles explaining how to use it.  I may actually try to do so, then verify my results on the AutoCAD screen.  THANKS!

Joe, the little pins are just to keep the segments of the rule together when folded up.

Papaw, thanks - I did realize that.  A really cool feature that is probably not clearly visible in the lower resolution photos posted are that at the end of many scales (in inlaid brass pins) and at the center of the hinge are center points in which to place a pin of a pair of dividers.  I am old enough to have used a slide rule (a successor of this tool as pointed out by Rusty) and the scales on this sector did remind me of my old slide rule.  Didn't need dividers with a slide rule, though. 
Enjoying your site!