A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Martin Gardner
Date: 2016 Apr 25, 11:34 -0700
I've got a WW II vintage Douglas Protractor.
There are instructions on the cardboard sleeve which start off: To lay off a course or bearing - Use the left side as a ruling edge and the graduation in italic figures on the meridian or parallel - rule the desired course or bearing. For example try 10°, 170°, 230°, 350°. The left edge can always be distinguished by the writing "Douglas Combined Plotter and Parallel Rule"
This sounds fine, and it's easy to lay off a course on the meridian. But the instructions say "on the meridian or parallel" - why "parallel"? Thousands of these things must have been deployed, so it's not a typo. How did they intend using a parallel? (Of course you could add or subtract 90° to use a parallel, but was that what they meant?)
(Here's a webpage with images of the protractor and the cardboard sleeve http://www.historicflyingclothing.com/en-GB/ww2-raf-personal-equipment/air-ministry-douglas-protractor/prod_14854)