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Back on Feb 15th, I responded to a posting from  Gary Lapook, in which he
showed several interesting exhibits from Lisbon. One was-

5. http://www.fer3.com/arc/imgx/IMGP5956.JPG .

About which I commented- "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.

==================

I've since checked with Google Translate, and the Portuguese equivalent to
the English word "gunwale" is indeed, in Portuguese, "borda de navio". So
that explains how a circular instrument, invented by Jean-Charles de Borda,
has been labelled in Lisbon as the "Gunwale circle".

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: "George Huxtable" 
To: 
Sent: Monday, February 15, 2010 2:05 PM
Subject: Re: [NavList] Navigational instruments at the Naval Museum in
Lisbon


| 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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