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    Re: Sextant calibration. Octant calibration
    From: Nicol�s de Hilster
    Date: 2007 Apr 23, 11:01 +0200

    In NavList 2666 Alexandre E Eremenko wrote:
    > What else, after 1940, except some Tamayas,
    > has a certificate
    > with all zeros? Can the list members give me ONE example?
    > C. Plath and
    > Cassens Plath are
    > not considered because they DO NOT provide a certificate
    > with a list of errors.
    >   
    The two instruments with certificates after 1940 in my collection show
    maximum errors of -18'' for a David White & Co, 1941 quintant and
    +0.015grad (equivalent to 48.6'') for a geodetic sextant by Observator
    (1979). Both certificates can be found on my web site www.dehilster.info
    I trying to get some more certificates from a second hand instrument
    dealer soon for you, perhaps that will contain some zero certificates.
    
    In NavList 2650 George Huxtable wrote:
    > As for the Dutch octant that Nicolas de Hilster kindly posted, it's 
    > remarkable, in my view, in three respects.
    >
    > First, that an instrument could have been so crudely divided as to show 
    > errors of twenty-odd minutes of arc, which is worse than you would expect 
    > from a modern schoolboy's protractor.
    >
    > Second, that its maker should then have been so up-front as to actually 
    > provide a table of those deviations. Many makers, I suggest, would have been 
    > ashamed to do so. It shows that for all those enormous errors, it wasn't out 
    > of line with what mariners of the time expected from such a wooden octant. 
    > At least, that table rendered the octant usable for getting latitudes.
    >
    > Third, that Holm himself created a printed form for entering such details, 
    > very similar to later calibration certificates. I wonder how his checks were 
    > done?
    >   
    M�rzer Bruyns writes about the Holm (1685/6-1776) certificates that he
    did check all of his instruments before he sold them. But in addition to
    that anyone who wanted his instrument to be checked could have this done
    at Holm's shop. He printed his own certificates and Holm did probably
    test the instruments using a large octant on a staff and a large
    quadrant in London.
    Holm was not the first aware of instrumental errors as Douwes
    (1712-1773) already pointed out the problem before Holm did (and even
    was commissioned to do something about it). It is not known whether he
    did so and whether or not he or others teachers of that era did register
    any instrument errors.
    
    The certificate in M�rzer Bruyns' book clearly shows that the errors
    listed are not random, but more or less linear (2 minutes per 6 degrees
    over the first 36 degrees and 1 minute per 6 degrees over the last 18
    degrees). I checked my paper version of the book as the table in there
    is of better quality, but even from that one I couldn't make up the
    figures in the middle for sure. So while the instrument was divided at
    regular intervals with every degree being 17 seconds too short on
    average, the overall error is accumulating to 25 minutes. This overall
    error is about 1 millimetre (!) along the arc for an 11 inch instrument.
    The error seemed to be either caused by a centring error of his dividing
    method as large as 1.25 millimetres or by shrinkage of the (probably
    ivory or bone) scale itself. I am not sure whether or not the 25 minute
    error was enormous for that time. I do not recall any research done on
    the scale quality of these older instruments, perhaps someone else does?
    Later instruments show much smaller and more random errors (so plus and
    minus), like the David White & Co in my collection. Without the
    certificate one would be better of with a good quality cross-staff, used
    in the backward fashion (check the graph on my website). These
    instruments were still made and used at the end of the 18th century.
    
    
    
    
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