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    Sextants, vernier and micrometer.
    From: George Huxtable
    Date: 2006 Oct 29, 10:07 -0000

    I have tried altering the threadname to reflect the way the theme of 
    this discussion has turned, after Alex's recent posting, under 
    [NavList 1499] Re: Cleaning arc of Vernier sextant.
    
    He wrote-
    
    " ...it is not evident at all to me
    that the drum sextants are "technicaly superior",
    the thing which seems somehow to be accepted in this
    discussion.
    
    I agree that the drum sextants are easier to use
    (and probably cheaper make, with modern technology)
    However, I it is not evident at all that they are
    superior in accuracy.
    
    Notice: the drum sextants were introduced when nobody
    was already taking lunars, and when 0'5 accuracy was
    considered sufficient. I suspect that they sacrificed
    extra precision to economy and convenience of use.
    This is only a conjecture, of course.
    
    But I am also looking for a good 10" vernier sextant,
    essentially for the same reasons as George.
    When I get one, I can test my conjecture:-)"
    
    ============================
    
    It's a pity Alex and I are on opposite sides of the Atlantic. He would 
    be most welcome to try mine out and check its calibration for me, 
    using his undoubted observational skills.
    
    In NavList 1474, Alex quoted the seller of that sextant, when he 
    wrote-
    
    "I know the story from the seller:-)
    The picture of the arc was fuzzy, and I asked him to send me another
    picture. He replied that he cannot make a better one,
    but "an expert in the antique sextants visited him on Friday,
    driving 100 miles, and said that
    this was the best arc on an old sextant he's seen so far":-)
    
    That was me, of course. Funny, how easy it is to acquire the mantle of 
    an expert!
    
    In that same posting, Alex referred to-
    
    But a similar sextant in a
    regular shop (with a return policy) costs
    $1500 (West Sea)-$2500 (Land and Sea).
    
    I am not familiar with either of these sellers, though it's been 
    pointed out to me recently that Land and Sea have on sale what looks 
    like one of the very earliest Plath micrometer sextants, at a price 
    that I didn't discover.
    
    Those in Europe may be aware of Trinity Marine, an outfit in 
    Teignmouth, Devon, UK, who seem to be going in for older sextants in 
    quite a big way. They were one of the earlier bidders for the sextant 
    I picked up, and beat me to another, a little while ago. An instrument 
    sold on Ebay UK seems to go up for sale by them at about twice the 
    price, it seems to me. Most seem to be in the �500 region, where �1 is 
    something like $1.9. I have had no direct dealings with them, and have 
    no idea whethey they treat sextants as real serious instruments or 
    just as "antiques". Their web address is trinitymarine.co.uk
    
    If anyone has had dealings with any of these people (successful or 
    otherwise) it would be interesting to hear opinions on NavList.
    
    Alex perceptively picked out that on the sextant I acquired, the 
    eyepiece filter was missing. Yes, there was a slot in the box for it 
    to go into, but no filter, which would normally be a very black one, 
    to screw into the telescope. That would be particularly useful for 
    establishing index error, using the Sun in both views.
    
    ======================
    
    I can summarise the present state of my enquiries about the 
    introduction of the micrometer sextant.
    
    Diederick Wildeman, Director of the Amsterdam Scheepvartsmuseum, has 
    sent me a copy of pages from an old Plath catalogue, undated, which 
    carries as an an insert a price list dated to June 1907. This carries 
    a price list for Trommel-sextants (drum-sextants) in various 
    alternative forms, and a surveying trommel-quintant. That is the 
    earliest date, so far, for Plath drum instruments, that I have 
    established.
    
    What about other makers, then, such as those in London? Cloria 
    Clifton, director of the National Maritime Museum in Greenwich, has 
    kindly informed me that their 1929 Heath catalogue has no mention of 
    micrometer instruments, though the 1931-2 catalogue does. That late 
    date is something of a surprise, in that for their "Hezzanith" 
    instruments, Heath patented their "endless tangent screw" back in 1907 
    (according to Peter Ifland, in "Taking the Stars"). That development 
    indroduced all the necessary mechanics of a micrometer instrument, 
    except for the pointer and calibrated markings around the knob / drum.
    
    There is, of course, a big difference in the required precision, 
    between what is simply a fine-adjustment device and a precise-readout 
    device, and Heath presumably felt that their rack-cutting wasn't up to 
    that latter purpose. It would be interesting to learn how good it was, 
    on those Hezzanith endless-tangent Vernier instruments. Was the knob 
    adjustment nominally one turn per degree, as for a micrometer (it 
    didn't need to be)? And if it was, if a pointer and zero-mark is 
    bodged up for the adjusting knob, how well does each whole turn 
    correspond to an exact degree, measured on the Vernier? That would be 
    an interesting little experiment, for anyone with an endless-tangent 
    sextant.
    
    The surprise (to me at least, and maybe to others) is the existence of 
    a technology gap of nearly a quarter-century, between German 
    instruments and British ones, if that was indeed the case. It can 
    hardly be due to Plath patent restrictions, in that the first World 
    War intervened, and surely patents of the then-enemy would have been 
    irrelevant during that conflict. I wonder whether any other makers, in 
    France or America perhaps, or even Japan, introduced micrometer 
    sextantsduring that long gap.
    
    George.
    
    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.George.
    
    
    
    
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