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    Re: Circle of reflection
    From: George Huxtable
    Date: 2009 Mar 14, 14:06 -0000

    Wolfgang has quickly located pictures of Mendoza circles, as I had
    requested.
    
    He's right, there's an English translation of Randier's book, with the title
    "Marine Navigation Instruments", which I have. And Randier provides picture
    attributions at the back, saying that the picture on page 127 is from the
    Musee de la Marine, Paris.
    
    So this identifies for us  a Mendoza instrument which was made for him by
    Troughton.
    
    But that isn't the Mendoza circle that's pictured in his paper-
    
    On an improved Reflecting Circle. Mendoza y R�os.
    http://rstl.royalsocietypublishing.org/content/91/363.full.pdf+html
     in phil. mag. 1891.
    
    That paper shows two concentric rings, one marked 0 to 360� in degrees,
    around the circle toward its inner edge, as the main arc. The other is a
    sightly smaller ring which can rotate within the inner ring, and this is his
    "flying nonius". This is divided toward its outer edge into 60' in sixths of
    a minute . Actually, I lied there. Because an extra division of one-sicth of
    a minute been inserted, so it's divided into 361 parts, rather than 360.
    This discrepancy provides a Vernier action, in reading the position of one
    ring with respect to the other.
    
    That paper also shows a part-arc placed for initial quick-setting of the
    index arm with respect to the telescope, with sliding stops.
    
    None of these features exists on the Paris instrument, that I can see.
    
    So I'm still on the lookout for a Mendoza circle with his flying nonius, but
    thank Wolfgang for his help in locating what is presumably an earlier
    version.
    
    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..
    
    
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