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    Re: Book about Plath.
    From: Greg R_
    Date: 2006 Oct 13, 22:58 -0700

    I wrote:
    > Do you have a link for the Sextants mailing list?
    Disregard -- I finally located (and joined) the Yahoo Group.
    --- "Greg R."  wrote:
    > 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:
    > (http://www.mhs.ox.ac.uk/rete/index.htm?text)?
    > --
    > GregR
    > ----- Original Message ----- 
    > From: "George Huxtable" 
    > To: 
    > 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
    > off-list-
    > ==================
    > "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
    > for
    > sale in 1908 a "Trommel-Sextant".
    > My search for a patent for that instrument had no results. The
    > earliest
    > 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
    > 1920s
    > 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"
    > was
    > first treated in the "Lehrbuch f�r den Unterricht in der Navigation
    > an
    > der
    > Kaiserlichen Marineschule" published by the "Reichsmarineamt" (Berlin
    > 1917).
    > 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
    > pretty
    > 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
    > "Deutsches
    > Reichsgebrauchsmuster 274505" which was a kind of a lesser patent
    > protecting
    > your invention just for a few years. I have not been able to find out
    > more
    > 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
    > fit
    > 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
    > the
    > 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
    > one.
    > 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.
    > > 
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