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    Re: Historical Magnetic Variation/Declination
    From: Henry Halboth
    Date: 2004 Jun 22, 11:31 -0400

    I believe you will find that it is the heeling magnet that must be
    changed end for end when crossing the magnetic equator - neglect of this
    nicety will adversely affect the magnetic compass. The Flinders Bar is
    utilized as a soft iron corrector, as are the navigator's balls, for
    induced magnetism and is itself subject to induced magnetism - no purpose
    being served by reversing it. The heeling magnet is a permanent magnet
    and thus may well over-correct if not maintained in proper position. I
    must confess to having sailed from Artic to Antartic Oceans by magnetic
    compass and never experienced any problem with tilt on a 7-1/2" card
    compass mounted in a conventional compensating binnacle, with the heeling
    magnet reversed on crossing the magnetic equator - please take note that
    I did not say there was no tilt whatsoever, just that it never was a
    problem.
    
    On Sun, 20 Jun 2004 21:06:16 +0000 "Trevor J. Kenchington"
     writes:
    > 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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