NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2015 Apr 5, 15:27 -0400
I have one. This is also known as a circle of reflection, or reflecting circle.
There are two types, (1)repeating, in which the angle is measured several times formulating an average result and (2) non-repeating, in which the instrument has two indices, the average of the two indices reducing any error due to eccentricity in centering the index arm to the arc.
I've historical evidence that mine was used in precision measurement for geodesy purposes, to wit, the precise determination of latitude of a specific port. The error found in previous determinations was published and presumably, the maps corrected.
All Circles of Reflection are quite rare in good working order and correspondingly, quite expensive.
Brad
Yes, I bought something similar by Troughton for the Hong Kong Maritime Museum about 6 years ago and it is on display in their gallery 10. Troughton was convinced (I suspect not having entirely internalized the revolutionary implications of Ramsden's dividing engine) that having three readings around a circle and averaging the result was more accurate than a single reading on just one arc. There is a good exposition in the late 19th century Penny Cyclopedia. The idea didn't catch on. Circular 'sextants' are often confused with repeating circles.StephenD
Sent from my iPhoneWhilst searching for Plath arcs on Google images, I came up with the attached instrument by Henry Hughes. http://www.liveauctioneers.com/item/35521567_a-rare-victorian-5-12-brass-double-sextant-by-henry It looks like something from "Around the world in 80 days". Has anyone seen one before? What was it used for, and how was it used? It's interesting to see the cabinet makers cutting guage marks on the dovetails in the the box. Was this a sign of piecework or of a a true craftsman?? Dave
