NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Book about Plath.
From: Greg R_
Date: 2006 Oct 13, 15:13 -0700
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: (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: http://research.microsoft.com/~gbell/cybermuseum_files/bell_artifacts_files/images/B69.80_Sextant.JPG " ==================== 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 , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---