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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Why a silver index scale?
From: Paul Hirose
Date: 2016 Mar 27, 12:49 -0700
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
