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    Re: Help - Unknown Compass Graduation Arrangement
    From: Joe Schultz
    Date: 2009 Oct 23, 00:36 -0700

    Hi Frank,
    Sounds like unit of "mils," originally developed for US Army artillery.  
    Officially 6400 divisions in a circle - you can do the geometry to see how 
    that works in a metric distance system.  Your compass, if with a mirror, is 
    called a sighting compass or sometimes a transit compass.
    I use my $10 lensatic compass (no mirror) extensively in terrestial 
    navigation, pretending there are 640 mils in a circle (there are 640 
    divisions on the compass card).  I can then resolve 360/640 = 9/16 degree on 
    the compass, and split that twice without squinting too hard through the 
    built-in magnifier.  A hand-made conversion table easily converts between 
    degrees and mils, 5 inches square and laminated.
    To complete the picture, British artillery developed the "grad," or 400 
    divisions in a circle.  If you have a trig calculator then you can probably 
    do trigonometry in grads, degrees or radians.
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