It would take someone much more knowledgable than I to explain all that it rolled into this early- mid 19th century protractor-rule, but I've looked at and held enough to know that it is a special one, and an early one, and really a beauty. With diagonal scales at either end of one side, and a dense array of scales on the reverse, with "Cho" (Chords), "Rum", and "Lon" (for Longitude I believe) marked at either end. I believe the various scales featured support a combination of logarithmic, trigonometric, and navigational functions, and that this would be an example of a Gunter's Rule, invented in the early 1600s and used well into the 19th century, including as part of the standard equipment kit for British ship navigation (and I do presume this rule is British.)  With brass plugs (a few lost) at commonly used points to prevent damage to the wood, and with the 0 at the right end of the 12 inch scale. To my eye a very beautiful and rather magical feeling tool that has surely lived an interesting--and productive!-- life! 

12" l x 1 7/8" t x 1/4" thick and in beautiful antique condition, marvelous rich honey color to the wood, which is very smooth. A few brass plugs lost as noted and visible, and a few dark spots here and there. No maker's mark anywhere on it.