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    Re: Book about Plath.
    From: Greg R_
    Date: 2006 Oct 13, 15:13 -0700

    George wrote:
    > I had asked, on three mailing lists (NavList, Sextants, and rete)
    Do you have a link for the Sextants mailing list? That one sounds
    interesting, but couldn't find anything on a Google search.
    And I assume that "rete" refers to the Museum of the History of Science list
    about the history of scientific instruments:
    ----- Original Message ----- 
    From: "George Huxtable" 
    Sent: Friday, October 13, 2006 2:41 PM
    Subject: [NavList 1411] Re: Book about Plath.
    I had asked, on three mailing lists (NavList, Sextants, and rete)
    about the earliest introduction of the true micrometer to a sextant,
    and have had a number of useful responses, so will post this message
    to all those lists for anyone interested, with thanks to all
    respondents. It surprises me, somewhat, that such an important
    innovation has not been better documented.
    Herbert Prinz, on NavList, and Richard Dunn, on rete, have pointed to
    the lack of such technical information about instruments in the 1987
    Jerchow work that I asked about. Herbert has suggested an earlier
    Plath anniversary publication from 1962, by Schaafhausen, Hoffmann and
    Kaltenbach. This has sent Wolfgang Koeberer to his extensive
    bookshelves, and I extract the following from his useful reply, sent
    "The 1962 book about Plath contains a part about the history of
    sextants by
    Heinrich Hoffmann and Peter Kaltenbach "Zur Geschichte des Sextanten".
    There - on p. 107 - they say that C. Plath developed and first offered
    sale in 1908 a "Trommel-Sextant".
    My search for a patent for that instrument had no results. The
    Plath patent with respect to a micrometer sextant I found dates from
    1922... It claims the patent for a sextant
    combining a micrometer drum and a vernier scale - just like the late
    Heath instrument shown on p. 63 in Peter Ifland's superb book.
    A quick search of my - not complete - collection of German navigation
    manuals and handbooks of that period shows that the "Trommel-Sextant"
    first treated in the "Lehrbuch f�r den Unterricht in der Navigation an
    Kaiserlichen Marineschule" published by the "Reichsmarineamt" (Berlin
    The interesting thing there is that it shows the same illustrations as
    Cotter in his "History of the Navigator's Sextant", p. 164f.; It is
    obvious that he has taken them from that book. From the illustration
    you can
    take another lead: it says "DRGM 274505" on the sextant, meaning
    Reichsgebrauchsmuster 274505" which was a kind of a lesser patent
    your invention just for a few years. I have not been able to find out
    about this, but I can imagine that you could ask for more information
    at the
    "Deutsches Museum" in M�nchen."
    So that seems firm evidence, then, that the first Plath
    "Trommel-sextant", or drum-sextant, dates back to 1908. Unless Heath,
    or another maker, got there earlier, that's the date of the
    introduction of the true micrometer sextant. And it seems unlikely
    that anyone did, because, (as Nicolas de Hilster has pointed out on
    the rete list), Peter Ifland, in "Taking the Stars", states-
     "(on page 62) it was Hezzanith (the trade name for
    Heath, London) in 1909 who '...patented an improved system for making
    fine adjustments to the index arm [...]. A helical gear was cut in the
    back of the arc of the frame. A worm screw fastened to the index arm
    into the helical gear so that turning the screw advanced the index arm
    slowly along the main scale. A spring loaded lever (Hezzanith Patented
    Quick-Release) disengaged the worm screw from the helix gear so that
    index arm swung freely to make gross adjustments'.
    The text is accompanied with a picture of a 1920 Heath sextant that
    shows this mechanism, but has a plain drum, not an engraved/divided
    The '...fine adjustment screw moves the index arm by 5 seconds of arc'
    he writes. You can find a picture here:
    That Heath improvement related to the endless tangent screw, for fine
    adjustment, as I understand it, and not to a calibrated drum. Ifland
    shows, on page 63, a  calibrated drum on a Heath sextant, which he
    dates to the "late 1920s".
    So it seems that credit for the introduction of the micrometer sextant
    must go to Plath in 1908, with Heath following somewhat later; exact
    date still undetermined.
    A number of respondents, on-list and off-list, have pointed to the
    instrument referred to by Ifland, on page 63, in these terms-
    "The earliest known application of the drum micrometer to navigation
    instruments is found on a sextant by Jesse Ramsden, ca. 1787, now in
    the National Maritime museum at Greenwich." A description has been
    published by Alan Stimson.
    But we have to distinguish here between different types of micrometer.
    For astronomers, particularly when an instrument was fixed to an
    equatorial mounting, a micrometer readout for small changes of reading
    was indeed useful, for measuring the offset between a star being
    studied and a reference star. For such purposes, the micrometer
    doesn't need to cover a wide range of angles. It seems to me that the
    micrometer fitted to the Ramsden instrument was of that type, a short
    screwed strut providing an adjustable chord, for which, no doubt, some
    small  numerical adjustment was needed to conform to a true angle.
    That could, indeed, be used for precise measurement of small angular
    gaps between two sky objects, and also for interpolating between
    whole-degree marks on the scale, when used over a wider angular range.
    But in that latter case, the precision was determined, not just by any
    error in the micrometer interpolation, but by an additional error in
    the visual  alignment to the nearest  degree mark, which would
    presumably be comparable with the errors involved in reading a
    Vernier. So, for wide angular differences, such a micrometer would
    provide little, if any, advantage in precision over the Vernier,
    although it might well avoid the need for such fine scale divisions
    around the arc.
    For a mariner, that type of limited-angle micrometer adjustment seems
    to be of little use. All his measurements are wide-angle ones,
    altitudes measured up from the horizon, lunar distances that normally
    exceeded 20 degrees. As I see it, only when a micrometer screw could
    work against a complete rack, cut precisely over the full arc, would
    its high precision be of use to the mariner. I would be interested to
    discover if others have differing views on that matter.
    For those reasons, I have discounted the Ramsden 1787 instrument as a
    contender for the first true micrometer sextant, at least where marine
    applications are concerned.
    George Huxtable.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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