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From: George Huxtable (no email)
Date: Tue May 24 2005 - 12:42:54 EDT
Alex wrote, interestingly-
>A Troughton reflecting circle No. 131.
>I made careful sketches of it.
>Hope to be able to answer any questions about it.
>The circle is 1.5ft diameter (445 mm),
>it has silver scale divided to 10' and nonius to 10".
>
>There are actually 3 arms with 3 noniuses (verniers).
>The rays are crossing (see our previous discussion on
>reflecting circles initiated by George), and there
>is indeed a "dead range".
>
>The device is in excellent condition, made in 1780,
>and even the mirrors look great:-)
>
>I spent a lot of time looking at it, and only after that
>went to see the Sixtine Madonna:-)
=======================
Other circles (Mayer, Mendoza, Borda) were Repeating Circles, in which both
the telescope arm and the central ("index") arm were movable around the
circle, so one index could "chase" the other. In that way, a lot of
repeated angle measurements could be automatically added, in an analogue
summing process. The main advantage was that irregularities in the
divisions ov the scale could be averaged out.
The Troughton circles were (in general) rather different. The telescope was
fixed to the frame carrying the arc, just as in a sextant, so it could not
be used in repeating mode. If an observation was repeated, it used the same
part of the arc each time, just as with a sextant. Troughton, and English
makers in general, were by 1780 making such precise machine-divided arcs
that the repeating method was no longer necessary to achieve sufficient
accuracy. So Troughton took a rather different path.
What Troughton did was to get increased accuracy of scale reading of a
sextant-type instrument by using an immense all-round circle instead of the
one-sixth circle arc of a sextant, and fixing a large three-legged spider
in place of the index arm, each leg carrying its own index. Each such index
was to be read off against a different part of the circle, 120 degrees
apart. The aim was to increase the precision of a single observation by
providing three similar readings to average, and to minimise any
eccentricity error, but it didn't allow summing of repeated observations on
the lines of the Mayer / Mendoza / Borda system.
It may be that some of the Troughton circles were made to work on the
repeating principle, and not as described above; I just don't know. It
would be interesting to learn a bit more from Alex about the instrument he
saw in Dresden. Perhaps, even if it wasn't "repeating", there were
advantages in using it for "negative" angles, measured with the input rays
crossing-over, as Borda's and Mendoza's could, but an ordinary sextant
couldn't.
There's a superb Troughton circle, on the lines I have described, and
mounted adjustably on a pillar, at the History of Science museum in Oxford.
I think such pillar-sextants must have been intended for precise
measurement of lunar distance by a land-based observer.
George.
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contact George Huxtable by email at , by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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