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    Re: The lost expedition of La Perouse
    From: Fred Hebard
    Date: 2005 Jun 9, 17:24 -0400

    My recollection is that micrometer drum sextant was developed by Heath,
    shortly after World War I.  I don't have documentation for this
    recollection, however.
    On Jun 9, 2005, at 11:30 AM, George Huxtable wrote:
    > Alex asked-
    >> George,
    >> 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
    > question.
    > 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.
    > George.
    > ================================================================
    > 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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