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    Sextants and micrometers
    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.

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