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    Drawing the Line - Edwin Danson
    From: Kieran Kelly
    Date: 2004 Mar 25, 14:27 +1100

    I have just finished an excellent book that those members interested in
    astronomy/surveying/terrestrial navigation/antique instruments/history or
    all of the above will enjoy.
    It is "Drawing the Line - How Mason and Dixon surveyed the most famous
    border in America"
    Edwin Danson - author.
    Anyone who ever doubted the English domination of instrument manufacture,
    surveying technique and astronomical excellence in the 18th century should
    read this book. Specifically surveying the Pennsylvania Maryland border had
    defeated a slew of pre revolutionary Americans and they turned in
    desperation to the Poms.
    How it was done I think would interest many of the minds on this list. Not
    content with the border survey for which they are remembered they also
    undertook to measure a degree of latitude for the first time in America and
    also undertook the first gravity observations on the continent.
    I have a couple of questions:
    1) In one part of the book they refer to the use of a zenith sector for
    determining the time when a star crosses the prime meridian. This is done by
    allowing a star to "rise" above the horizontal wire in the telescope and
    timing the interval until the star "sets" below the wire. The mean is the
    time of meridian passage.
    All very good but how did they see the wires in the telescopes in the days
    before illumination? The book admits that scholars don't know as M and D
    left no record of this mundane procedure. However Danson speculates that
    they held a candle obliquely up to the object lens of the scope allowing
    just enough light in to see the wires. Sounds dodgy to me. Wouldn't this
    have obliterated the light from the star especially the dimmer stars.
    An old surveyor friend of mine said that he remembers observations using a
    zenith sector whereby the time of passage above the horizontal was measured
    by the point where the star disappeared behind the wire. i.e. the star would
    be visible, then it would disappear momentarily as the light from the star
    was blocked by the wire, then it would appear again as it rose above the
    wire. When it disappeared again the observer knew it had gone behind the
    wire on the other side of the zenith. The only problem with this theory is
    that some of the early instruments used spider web for the cross hairs and I
    imagine you could see the light from the star through  this type of
    filament. Any comments from Anyone?
    2) Given the scientific achievements of these two -"A Geordie and a baker's
    boy in the forests of the Iroquois" as Mark Knoffler says in his song
    "Sailing to Philadelphia" - why aren't their  achievements more celebrated
    in the USA. There is a vast following in America and overseas for Lewis and
    Clark, who came along a considerable time after Mason and Dixon. After
    reading Dansons book it would appear that these blokes would have put the
    Captains in the shade.
    It appears some very capable people have been overlooked in history -
    Thompson in Canada, Mason & Dixon in America, Everest in India and Gregory
    in Australia.
    Kieran Kelly
    Kieran Kelly
    Australian Pastoral Holdings
    6 David Place
    Seaforth 2092
    ph     612 99079610
    fax    612 99078232
    mob  0411 261607
    e mail kkelly@bigpond.net.au 
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