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    Re: Manual calculation of compass variation
    From: George Huxtable
    Date: 2004 Oct 16, 20:17 +0100

    Henry Halboth pointed out-
    >George - the discrepancy you describe is the compass error to which
    >deviation must be applied to obtain variation. Perhaps Brother Cook was
    >not too much troubled with deviation, but it's there nevertheless; even
    >the metal ring in an officer's hat while taking an azimuth can deviate a
    >standard magnetic compass as much as 3-degrees, as I have proven by
    >experiment. Please don't construe that I am suggesting Cook's hat,
    >whatever that may have been, as the start of another thread, but I am
    >saying that, without a knowledge of the deviation the variation recorded
    >must be suspect.
    Henry is quite correct, as usual. I fell into the mistake of assuming that
    for Cook's wooden vessel, there would be no deviation, but should have
    known better. I've recently been studying a voyage to the "Greenland
    Whaling" (West of Spitzbergen) in which the log notes that on certain
    courses, the two compass positions in the binnacle differed by a whole
    point (11 and-a-quarter degrees).
    Two compasses were placed a few feet apart in the wide binnacle (somewhat
    like a domestic sideboard), presumably so that the helmsman could read a
    compass straight-on, wherever he needed to stand. Checks had been made to
    ensure that it wasn't the effect of interaction between them. It must have
    been deviation resulting from some local ironwork, to which the compass
    would be very susceptible in such high latitudes. Such problems were only
    poorly understood in the early 1800s.
    I'm interested in Henry's account of the effect of the "metal ring in an
    officer's hat". Unfortunately, most such portraits in my books of Cook's
    period are posed hatless, but none of the others show such a ring. What was
    it FOR, and how big was it, I wonder? Some sort of buckle, perhaps, for a
    chinstrap to hold down such awkward headgear, with its immense top-hamper,
    in a blow? But an IRON ring (which it would have to be to affect the
    compass)? I can imagine it being of ivory, perhaps, or brass, or even
    silver; all non-magnetic. But an iron ring doesn't sound right to me, even
    for an officer's working headgear. Perhaps Henry will explain further.
    Of course, on Lisa's steel vessel, compass deviation and its correction is
    a serious matter, and in her case it's likely that compass bearings of low
    celestial bodies would be useful, not for checking variation, but instead,
    taking compass variation from the chart, for checking deviation, and how
    well the compass has been corrected..
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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