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    Re: Navigational instruments at the Naval Museum in Lisbon
    From: George Huxtable
    Date: 2010 Feb 26, 00:12 -0000

    Joergen Hoefeld's comments about the supposed Elton quadrant replica are 
    perceptive. I agree that it's a replica of an ordinary Davis quadrant.
    
    Until the "Flamsteed" lens was introduced, to throw a sharp shadow of the 
    Sun, the penumbra of the Sun's shadow was so wide (~half a degree) that 
    minutes on the large arc were largely irrelevant anyway. It was not a 
    precise instrument, which was why the Hadley was taken up so quickly to 
    replace it.
    
    The Elton version, supposedly an improvement, was so impractical for use at 
    sea, with its spirit levels, that it's no surprise that it never got 
    anywhere.
    
    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: "J�rgen Hoefeld" 
    To: 
    Sent: Thursday, February 25, 2010 10:34 PM
    Subject: [NavList] Re: Navigational instruments at the Naval Museum in 
    Lisbon
    
    
    By the way, the Davis quadrant  http://www.fer3.com/arc/imgx/IMGP5954.JPG
    labelled: 'Elton's quadrant replica' seems to be no Elton quadrant at all
    but an ordinary Davis quadrant. I can't see any bubble or levelling
    attachment.
    This brings me to some more questions on diagonal scales on Davis quadrants:
    
    
       - The sight vane on the 30� scale of a Davis quadrant has to be proper
       aligned with the horizon vane to get a meaningful reading on the 
    transversal
       scale on the large arc. Otherwise the reading of minutes on the diagonal
       scale is meaningless.
       - How did the manufacturers and/or users ensure this alignement?
       - What is the oldest existing Davis quadrant? I only found examples which
       are not much older than the first octants from the 1730s, compared to 
    Davis
       publication of 1595.
       - Does anyone remember having seen in a museum or collection an existing
       Elton Quadrant like that  by made by Sisson in the  Phil. Trans. 
    1731-1732,
       vol. 37, 273-279, depicted in
       http://en.wikipedia.org/wiki/File:Elton-quadrant.jpg or on John
       Vanderbanks painting
       http://www.nmm.ac.uk/mag/pages/mnuExplore/PaintingDetail.cfm?ID=BHC3128
       in the National Maritime Museum?
    
    I am looking forward to your hints.
    
    Best regards,
    Juergen Hoefeld
    Aachen, Germany
    
    
    2010/2/25 Douglas Denny 
    
    > Ah! but which came first the chicken or the egg?
    >
    > Or, in this case: is 'Gunwhale Circle' a simple error by whoever wrote the
    > label due to the fact a 'Borda Circle' (so called because of the inventor)
    > has the same name in Portugese? It could be as simple as that.
    >
    > I think your extrapolation needs further evidence of the use of a term 
    > like
    > 'Gunwhale Circle' being in actual use by Portugese sailors 
    > contemporaneously
    > with the use of the circle itself in the nineteenth century.
    >
    > I have come across simple blunders in labelling in museums before where it
    > is obvious the person who wrote the label did not 'know his onions'.
    >
    > Douglas Denny.
    > Chichester. England.
    >
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