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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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