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Re: Troughton circle in Dresden
From: George Huxtable
Date: 2005 May 24, 17:42 +0100
From: George Huxtable
Date: 2005 May 24, 17:42 +0100
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. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================