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    Re: Gunther Rules for Navigation
    From: Brad Morris
    Date: 2017 Apr 17, 21:52 -0400
    Bowditch(1849) identifies these scales on a Gunter's Scale, with examples :

    Sine Rhumbs
    Tangent Rhumbs 
    Logarithms of Numbers
    Versed Sines
    Meridional Parts
    Equal Parts

    It appears as if some of them correspond to the scales in the photographed rules.  I won't reproduce the Bowditch pages, there are plenty of references in our library that should help you.  Try the earliest ones you can find, as they will have the most information on these quaint scales.

    Gunter's Rules were used with pointed dividers, making brass inserts a necessity.  I don't see any evidence that they are chewed up with dividers.  Perhaps these were the 'must have' tools every seaman had, but never used.  I have a sector like that.  It was never really used.  More like an ornament than a tool.


    On Apr 17, 2017 8:55 PM, "Ed Popko" <NoReply_EdPopko@fer3.com> wrote:

    Over the Spring break, I had the opportunity to tour "Ships of the Sea Maritime Museum" in Savannah, GA.  A lovely collection of ships from the Civil War era, to commercial freighters and liners to naval ships christened ‘Savannah’.

    The navigation instruments collection was surprisingly good with back staffs, cross staff, several sextants, compasses, early charts and more.

    Within the collection were two wood rulers, one engraved "Navigation Scale Improved by B Donn". One scale is labeled Secants. These are the first I have seen that specifically call out navigation.

    The display title reads:

    4. Gunther Rules
    English, early 19th century
    The forerunner of the slide rule, named after the 17th-century English mathematician Edumand Gunther, this device was used by sailors for two hundred years to make quick mathematical calculations at sea.

    There are numerous internet sites with reference to Gunther Rules for general calculation but none mention designs specific for navigation as this pair claim. Attached is a set of images detailing all parts of the 'Navigaiton' rules. Perhaps NavList members have some insights into what these scales are and how they facilitate navigation differently than regular Gunther Rules would have.


    Ed Popko

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