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    Re: The backstaff. was: Re: The Shovell disaster
    From: Michael Daly
    Date: 2007 Nov 06, 11:31 -0500

    Nicol�s de Hilster wrote:
    
    > Given the capabilities of the instrument makers of those days this error 
    > was way beyond their achievable accuracy. Ever since I made my first 
    > Davis Quadrant I checked every example I could lay my hands on (musea, 
    > internet, books etc) and whenever the scale is visible in detail you 
    > will clearly see errors that are larger than that (in general more than 
    > 0.1mm, so just under 1 arc minute). So I take it that on Davis Quadrants 
    > all transversals were linear in those days, unless the cutting plate 
    > wasn't straight.
    
    I guess the AutoCAD construction shows that for those instrument sizes, 
    the difference between linear and circular transversals is irrelevant.
    
    Thanks for this, Nicol�s.
    
    Mike
    
    
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