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    Re: Manual calculation of compass variation
    From: George Huxtable
    Date: 2004 Oct 17, 22:45 +0100

    Some comments about Herbert Prinz's posting-
    >Borda reports (Verdun de la Crenne, Borda, Voyage fait par ordre du Roy en
    >1771 et 1772) that the biggest effect on compass deviation is caused by the
    >guns. The Endeavour apparently had some 20, I didn't check Cook's other
    The magnetic affect of a ship's gun, or any other iron object, falls very
    quickly with distance, by a cube law if my memory is right. So it's not the
    NUMBER of guns that will matter, it's a question of how near is the nearest
    to the compass, which will dominate the effect of all the others, and how
    big that gun is.
    I'm interested in Borda's voyage, ordered by the King. Where did he go to?
    Had he produced his "circle" by then, and how good were his longitudes ?
    >When possible, observations for variation were also taken on land with a
    >chance of better accuracy. For a land observation, I don't find a
    >specification of minutes unreasonable. On a compass rose of 15cm diameter one
    >can easily read 1/4 degrees, so that an average of 3 measurements would
    >result in a nominal accuracy of 5'. At least, this was the reasoning.
    Other factors come in, beside the accuracy of reading the dial;
    particularly stiction at the pivot.
    And even in the ideal situation of the observations being statistically
    independent of each other, to reduce any random errors by a factor of three
    requires nine observations, not 3, as Herbert must be well aware. However,
    at that date, date navigators were not so aware, which is presumably why he
    says "at least, that was the reasoning". As for such non-statistical errors
    as inaccuracies in dial calibration, those remained, no matter how many
    observations were taken.
    So I think it's absurd for measured variations to pretend to be to the
    nearest minute. The real reason for recording variation to the minute, it
    seems to me, is that if you wanted to quote to any finer division than a
    degree, then the minute was simply the next subdivision available.
    I still think that Cook's quoted variation of 20 deg 59' should just as
    well have been rounded off to 21 whole degrees.
    contact George Huxtable by email at george---.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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