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    Re: Why a silver index scale?
    From: Geoffrey Kolbe
    Date: 2016 Mar 27, 07:37 +0100


    On 27 March 2016 at 03:33, Bill Morris <NoReply_Morris@fer3.com> wrote:

    For a start, Geoffrey, the frames of sextants were not brass, a copper-zinc alloy of dubious corrosion resistance, but bronze, whose main constituents were copper and tin.


    ​Thanks for the correction Bill...​
     

    This is a tough and tenacious metal and one has to remember that the instrument maker in the late eighteenth and early nineteenth centuries used man-powered lathes, so turning a large radius was difficult. The metal had hard spots that could deviate the stylus of the dividing engine, so a dovetail was cut into the limb and a sliver of silver whose section was a segment of a circle was hammered into the dovetail and filed or turned flush. 


    ​Ok, that makes a certain kind of sense, but why was Hughes & Son still using silver arc during the 1940s when these regards were certainly not true any more. Silver is expensive. Pewter, on the other hand, is a lot cheaper and would (I submit) do the job just as well...?

    Geoffrey Kolbe​
     
       
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