NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sextants and micrometers
From: George Huxtable
Date: 2003 Aug 26, 13:02 +0100
From: George Huxtable
Date: 2003 Aug 26, 13:02 +0100
I'm addressing this to several relevant lists in the hope of getting answers. Gary Moulton's 12-volume edition of Lewis and Clark's journey, up the Missouri and across the Rockies to the Pacific, in Vol 2 page 410, quotes Lewis as recording, on July 22nd 1804, the instruments that were carried. These included- "a brass Sextant of 10 Inches radius, graduated to 15', which by the assistance of the nonius was devisible to 15"; and half of this sum by by means of the micrometer could readily be distinguished, therefore - 7.5" of an angle was perceptible with this instrument: ..." Presumably the "nonius" was a Vernier by an earlier name, but what about the "micrometer"? The Micrometer Sextant, which measured angles by counting turns and part-turns of the adjustment screw, working against an accurately-machined rack, was still more than 100 years away, wasn't it? It was common to find a clamp-on tangent screw to aid fine ADJUSTMENT, but this didn't provide fine MEASUREMENT. My presumption, then, (for lack of anything else) is that in Lewis' text "micrometer" means nothing more than the reading-lens, which could be swung over the Vernier to magnify its image. Do others agree? Was this an accepted usage of the term "micrometer", in this context, at that date? When did the word "micrometer" first arise, in this context or any other? George Huxtable. ================================================================ 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. ================================================================