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    Re: Bowditch octant
    From: Alexandre Eremenko
    Date: 2004 Nov 19, 18:59 -0500

    Dear Omar,
    The instrument in the picture is a wood quadrant (octant).
    You can see another picture of it in the first page of
    Norie's book (online version, 1828).
    According to Norie's description,
    the arc was divided into 20' intervals,
    that is each degree into three equal parts.
    (If you look at the picture VERY carefully, you
    see an arc divided into whole degrees but adjacent
    to it from the outside,
    there is a narrower arc that is divided into 20'
    The Nonius (Vernier) is divided into 20 equal parts whose total
    length equals 21 divisions of the arc,
    that is 7 degrees. So you can read the scale to 1'.
    Norie has description of this Nonius in detail,
    and the first paper mentioned below, contains magnified
    pictures of the arc and Nonius.
    The radius of the arc I cannot tell from the
    picture, but my guess is
    that it is 17 or 18 inches, maybe even 20 inches.
    (Larger instruments were made of wood, smaller of brass.
    The reason is simple: larger the arc, more precisely you
    can read it. The limitation is the weight of the instrument).
    The book by Bruce Bauer contains a picture of the author
    with such quadrant in his hands, so you get an idea of its
    size from this picture (it seems that his particular
    quadrant has radius at least 20 inches). The papers I mention
    below describe
    specific instruments in museums and give their dimensions.
    Now, I don't know for what purpose you want to build such
    octant: to use it as a decoration, or to measure
    angles, and in the latter case, what precision you are aiming at.
    In any case, you may find useful the following articles
    which describe how they were actually made in XVIII-XIX centuries:
    S. Moskowitz, The world's first sextants,
    A. N. Stimson, Some board of longitude instruments
    in the nineteenth century.
    These and other similar papers are available
    on the Sextant mailing list
    You have to register to download them free.
    If you really want to make a working instrument, the
    hardest part, as I understand is to divide the arc with
    sufficient precision.
    In the beginning this was done by special artisans,
    masters of division, who used very sophisticated
    geometric constructions, performed directly on
    the arc, with several compasses
    of various size. The process took several months.
    If you are really interested in doing THIS,
    I can provide further references. (There is a special
    XVIII century literature
    on the arc division of astronomical/nautical instruments.
    Something is available on the web, something I have).
    But then Ramsden invented a division machine, which left
    these fine artisans and scientists without job.
    The quadrants shown in Bowdich and Norie were probably divided
    by a machine. But you are not going to build a dividing machine,
    are you?
    I don't know the principle on which this division machine
    works (can only guess) but I know that the original machine exists
    and belongs to some US museum.
    P.S. I want to cite the beginning of the paper
    "Observations on the Graduation of Astronomical instruments...."
    by J. Smeaton, FRS (1785):
    "Perhaps no part of the science of Mechanics has been
    cultivated by the ingenious with more assidity, or more
    deservedly so, than the art of dividing Circles for
    the purpose of Astronomy and Navigation..."
    On Fri, 19 Nov 2004, Omar Reis wrote:
    > Hello,
    > I'm trying to build an octant similar do Bowditch's.
    > I found a beautiful image of it on the following address:
    > www.nathanielbowditch.org/images/sextantb.jpg
    > I wonder if anyone knows more details about this instrument:
    > 1) Scale radius
    > 2) Number of minutes in the vernier ( i.e. tick divisions per degree )
    > Regards
    > Omar

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