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    Re: Navigational instruments at the Naval Museum in Lisbon
    From: George Huxtable
    Date: 2010 Feb 15, 14:05 -0000

    Gary has indeed provided some interesting pictures from Lisbon for us to
    ponder on.
    
    Here are a few assorted comments, which may illustrate nothing more than my
    own misunderstandings. But they may provoke a bit of discussion..
    
    1. His attachment IMGP5958.jpg , is a Pistor and Martens reflecting (but not
    repeating) circle. It's very like a sextant, except that it has the index
    arm extended in the opposite direction to carry a second Vernier, which can
    be read off the opposite part of the circular arc, and the two readings
    averaged for increased accuracy. It's a surveyors' instrument, commonly,
    rather than one for mariners. Troughton made many such, mostly carrying
    three Verniers.
    
    I've been involved in a bit of off-list discussion about such a Pistor
    instrument, and one aspect of it continues to puzzle me; Pistor's use of a
    prism in place of an index mirror, for which he claims technical advantages.
    However, I'm somewhat doubtful about whether such advantages are real, and
    comparisons with a simple mirror don't seem to substantiate his claims. My
    suspicion is that it was no more than a way of making his instruments
    "different" from the others.
    
    2.http://www.fer3.com/arc/imgx/IMGP5953.JPG . I can't read the label, but
    this may be an instrument for on-land measurement of compass deviation
    (=declination to geographers), using Sun or star to determine the direction
    of true North. Alternatively, perhaps it was used the other way round,
    aligned to true North using the compass with a knowledge of variation,
    rather like a precise sundial, to get time-of-day, or latitude, from
    altitude of Sun or star. Interest centres on the complex scale of altitude,
    with its many arcs, which I don't understand at all. That looks a bit like
    the original Nonius (of Pedro Nunez), but it isn't. Nor can it be a simple
    diagonal scale. So what is it, and how does it work?
    
    3. http://www.fer3.com/arc/imgx/IMGP5954.JPG . Elton's quadrant was an
    unsuccesful attempt to combine a Davis quadrant, providing back-views of a
    Sun image, with the ability to take forward altitudes of a star at night,
    which relied on spirit levels to provide an artificial horizon. Needless to
    say, that didn't work well at sea. A lantern, probably holding a candle,
    could be attached, as part of the kit, to illuminate the levels. It couldn't
    compete with the two-mirror devices, by Hadley (and much earlier, by
    Newton).
    
    4. http://www.fer3.com/arc/imgx/IMGP5955.JPG . This is interesting from
    several aspects. It looks like a modern construction in white plastic, but
    if the date is as given (1769) it must presumably have been beautifully made
    in solid ivory. The numbering looks very modern, and the scales appear to be
    linear, which presumably means that it was useful only for solving
    plane-chart problems in small-scale navigation, not for ocean passages. The
    units of division would presumably be lat, distance, and departure. Would
    the ratio between longitude-change and departure have been preset, just to
    apply to a particular band of latitudes? Otherwise, how did it work?
    
    5. http://www.fer3.com/arc/imgx/IMGP5956.JPG . The instrument itself, a
    Borda repeating circle,. isn't visible here, but Gary has chosen to show us
    the label, which states-
    "The reflecting circle (18th century). Also called the "Gunwale circle", it
    was used to find the longitude by measuring the horizontal angle between the
    Moon and the stars of the zodiac". That angle, of course, is seldom
    horizontal. I know no Portuguese, but let me hazard a guess that perhaps
    their word for gunwale may be "borda", to explain the misattribution.
    
    Thanks, Gary.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    ===========================
    
    ----- Original Message -----
    From: "Gary LaPook" 
    To: 
    Sent: Monday, February 15, 2010 9:06 AM
    Subject: [NavList] Navigational instruments at the Naval Museum in Lisbon
    
    
    Attached are photos of some interesting instruments that I took in October.
    
    gl
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