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    Re: Historical Magnetic Variation/Declination
    From: Trevor Kenchington
    Date: 2004 Jun 20, 21:06 +0000

    Bob Peterson wrote:
    
    > Back to that world traveler:  what are they to do?  My recommendation is
    > to purchase additional "cores" for their binnacle compass.  Then as they
    > change zones and the card tilt bottoms out and the card binds, change
    > out the "core" to a new zone.  In theory, the compass correction should
    > not change, in practice, it does.  So best to check it and build a new
    > deviation card.   In my book, data always outweighs theory.
    
    
    When I moved to Australia, in the 1980s, I took with me a sighting
    compass originally purchased in England (one of the Morin "hockey puck"
    type). It had worked well enough in Nova Scotia but in Tasmania the card
    dipped so far that it was impossible to take bearing sights. I figured
    out that the card must have been balanced for a north-down dip and was
    thrown off by the south-down dip around 45 South latitude. (When I moved
    back to Nova Scotia a few years later, the compass became fully function
    again, so the problem wasn't some sort of breakage of the instrument.)
    
    One day when out in the Tasman Sea with nothing better to do, I
    mentioned the problem with my compass to our research-ship captain (who
    held a British Master Mariner's ticket) and he initially denied that
    there could be any such problem, on the grounds that he had taken ships
    from one hemisphere to the other without their compass cards ever
    tilting in response to magnetic dip. Then he relented and said that
    there was one vertical magnet in a ship's binnacle (the "Flinders Bar"
    perhaps?) which had to be reversed, end-for-end, when crossing the
    Equator and he suggested that maybe that adjustment prevented the dip
    problem that afflicted my sighting compass.
    
    So ... do big-ship magnetic compasses (still carried as back-up to their
    gyros, so far as I know) dip more than Captain Sheridan realized? Is
    there some routine of changing cores when crossing zonal boundaries, as
    Bob suggests, which the captain did so automatically that he had
    forgotten its significance when talking to me? Or does a single
    adjustment when crossing the Line suffice for a binnacle compass?
    
    
    No doubt the answers are in Bowditch and other textbooks. But digging
    them out of such sources is beyond me just now.
    
    
    Trevor Kenchington
    
    
    --
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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