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    Puzzled by Douglas Protractor Instructions
    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)

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