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    Re: Why a silver index scale?
    From: Paul Hirose
    Date: 2016 Mar 27, 12:49 -0700

    Instrument maker William Ford Stanley ("Surveying and Levelling
    Instruments," 1901) wrote, "The material upon which the limb or circle
    of an instrument is divided is almost uniformly of silver, except for
    mining survey instruments which need a very strong cut. Silver being
    dense and of extremely fine crystallisation, or grain as it is
    technically termed, bears a uniform smooth cut with sharp outline.
    Occasionally circles or arcs are divided on platinum, certainly the best
    metal as it keeps constantly clean; but it is expensive."
    
    https://books.google.com/books?id=QzM7AAAAMAAJ&pg=PA180&focus=viewport
    
    He said a sextant frame is made of "gun-metal" - 88% copper and 12% tin.
    
    "For centres requiring great rigidity, as those of the theodolite,
    level, or sextant, bell-metal is used by all the best makers." That
    alloy has a little less copper and more tin.
    
    https://books.google.com/books?id=QzM7AAAAMAAJ&pg=PA10&focus=viewport
    

       
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