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    Re: The lost expedition of La Perouse
    From: George Huxtable
    Date: 2005 Jun 9, 16:30 +0100

    Alex asked-
    >Do you know by chance when exactly and who
    >made micrometer drum common?
    >(I suspect that this could be Heath of London
    >sometimes in 1930-s. Maybe you know more precisely?)
    No, sorry, I don't. It's not my field, really. Though I have quite a few
    books on the history of navigation, and one (by Cotter) specifically on
    "The History of the Mariner's Sextant", none gives the answer to Alex's
    I think it probably happened in stages.
    Right from the start, there was a fine-adjustment screw, which could be
    clamped to the index arm at any setting, and used to move it deliberately
    and controllably; but not with a motion such that one turn always
    corresponded to the same change in angle. And such a device would only work
    over a limited arc of a few degrees (perhaps 10). As the Sun's altitude
    changed, after a time you would come to the end of the limited adjustment,
    and have to unclamp it and reset.
    Then, some date in the early 1900s, came the "endless-tangent" screw, in
    which a worm, which could be readily disengaged for quick-motion, engaged
    against a machined track which was cut along the full length of the arc. I
    think that development probably came first from Heath. But it was NOT a
    micrometer, in that no markings were put around the rim of the adjustment
    knod, and there was no attempt to cut the teeth of the rack so precisely
    that the turns of the knob were exactly a degree apart, so they couldn't be
    used to measure the angle. Instead, angles were read using a Vernier
    against the arc, just as before.
    The final development, of the drum sextant, came when every tooth of the
    rack was cut so precisely that the turns and fractional turns of the drum
    corresponded exactly to the angular movement of the index arm, along the
    whole arc. It was the ultimate expression of the machinist's art at the
    time. But which firm was first responsible, and when, my books don't say.
    My guess it would be either Hughes or Plath, but it would be interesting to
    discover if any listmember knows the real answer.
     Alex continued-
    >In fact, micrometer drum was proposed in XVIII century
    >possibly by Halley or even earlier. I can check when I
    >come back to the US in August and will have access to my
    >papers. I am curious why this invention had to wait until
    >1930-s to become common.
    I have my doubts about that. I'm aware that Halley put forward a way of
    moving the angle of a mirror, using a sort of pantograph arrangement with a
    linear nut-and-screw across its diagonal. And then, presumably, using some
    sort of trig table to convert that to an angle.
    What the micrometer had to wait for was mass-production machining with
    sufficient precision. There was no difficulty in proposing the principle;
    it was realising it in practice that was difficult.
    >The reason is unlikely to be in some advanced wormscrew
    >technology because a wormscrew was used in the arc
    >division machines since XVIII century. So why did not they use
    >it in sextants if the idea was known and technology available?
    You just needed the one "dividing engine", of large diameter, to engrave
    lots of instruments. To do what Alex suggests would require that technology
    to be miniaturised and mass-produced. That was the difficult bit.
    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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