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    Re: Circle of reflection
    From: Clive Sutherland
    Date: 2009 Mar 14, 15:09 -0000

    George, Wolfgang;
    The picture of the Mendoza circle shown in Wolfgang's last message and
    reputed to be a Mendoza Circle has several anomalies that puzzle me. First
    the outer divided ring is not a Nonius as George  suggests in his reference
    to the Mendoza paper but on closer look appears to be an auxiliary scale for
    quick setting. It is calibrated with  zero in the middle and extending both
    clockwise and anti-clockwise to about 120 or 130 deg. 
    This is similar to Mendoza's inner setting ring but in an entirely different
    There does not appear to be any main divisions on the side shown and indeed
    the frame of the circle intrudes over the rim where any scale aught to be! I
    can only guess that the Main divisions are on the side we can't see and that
    this could be so is suggested by the Index and Tangent screw at the right
    hand of the picture, of which we see only the reverse side.
    Apart from all that, I have tried to ray trace the sightlines through the
    instrument without success. There seems to be two telescopes? one which is
    pointing at the 'Horizon mirror' as expected but the other is a puzzle
    particularly by the shape and size of its objective. Also would seem to be
    in the way! Perhaps it is not a telescope at all!
    Wolfgang. Do you know if there any more pictures if this Circle?
    > -----Original Message-----
    > From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf
    > Of George Huxtable
    > Sent: 14 March 2009 14:07
    > To: NavList@fer3.com
    > Subject: [NavList 7647] Re: AW: [NavList 7645] Re: Circle of reflection
    > 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{at}hux.me.uk
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK..
    > > 
    > No virus found in this incoming message.
    > Checked by AVG - www.avg.com
    > Version: 8.0.237 / Virus Database: 270.11.13/2001 - Release Date: 03/14/09
    > 06:54:00
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