NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Circle of reflection
From: George Huxtable
Date: 2009 Mar 14, 22:22 -0000
From: George Huxtable
Date: 2009 Mar 14, 22:22 -0000
Brad wrote- While I do understand the advantages of a repeating circle of reflection over a non-repeating circle of reflection, I cannot understand why a non-repeating circle of reflection offers any advantage (other than the handle orientation) over a sextant. Would someone help me out with this? ==================== Yes. If you have a complete circle, you aren't limited to having a single index arm and Vernier (nonius). If you have two Verniers, 180� apart, then averaging the two readings can compensate for any eccenticity error. Averaging three did a bit more to compensate for unevenness in scale division. Up to the introduction of engine-dividing from the 1780s, the inaccuracy of hand-dividing was the limiting factor in all such angle measurement. That was the original reason for the introduction of the repeating circle. Engine-division became common, and economical, in London before it did on the continent, which is why circular instruments prospered there for longer.. Brad is quite right. Once engine-division had been introduced, the marginal increase in precision obtained by averaging out remaining imperfections became small, and in my view hardly worthwhile, especially for a non-repeating circle. In those instruments there could be some advantage in reducing scatter, simply because each altitude was the average of three readings, which were to some extent independent, if only partially so. I wonder how popular they really were, among real navigators at sea. I can see why they might be popular to put on show by museum curators, because polished up well and lacquered, abounding with handles and stirrups, with a multiplicity of scales, they add an eye-catching glitter to a museum display. I can also see how this complexity might appeal to lecturers at navigation colleges. But I can't visualise a hard-boiled navigator taking a reflecting circle out on the poop on a rough night for a lunar; not if he had a decent sextant on hand as an alternative. Reflecting circles weren't used just for lunar distance, they were also used for precise hydrographic surveying, particularly by the French. I remember reading an account of d'Entrecasteaux using a circle when surveying from ship in Tasmania, probably a Borda repeating type. Clive has pointed out the use of a non-reflecting repeating circle (Borda) in ultra-precise land survey. Brad is asking a sensible question, one which puzzles me too. Why didn't those Troughton non-repeating instruments die out, soon after 1780?.What was their attraction, and their advantage? 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. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---