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    Re: The backstaff. was: Re: The Shovell disaster
    From: Michael Daly
    Date: 2007 Nov 07, 18:25 -0500

    Nicol�s de Hilster wrote:
    
    > There are two ways to construct those diagonals:
    > 1) Using a tilted cutting plate (I think that is how they were constructed)
    > 2) Using a very long beam compass
    
    Interesting.  I only knew of the method of Philippe de La Hire described 
    in Maurice Daumas' "Scientific Instruments of the 17th and 18th 
    Centuries and their Makers".  (It's a good book on general instruments, 
    including navigation - I found an English translation of the original 
    French book in a second-hand bookstore for $45 years ago - a bargain 
    considering the content and how often I refer to it.)
    
    La Hire uses a system based on circular arcs.
    
    > the diagonals are no arcs but clothoids. 
    
    That's interesting to know.
    
    > John Bird and had as 
    > disadvantage that they grew and shrunk with temperature and humidity 
    > (these problems have been described by him in a book, I believe it was 
    > 'The Method of Dividing Astronomical Instruments', dated 1767).
    
    Daumas mentions that beam compass techniques (I don't remember if they 
    include Bird's) included only marking the instruments at certain times 
    of the year (spring and autumn so the weather is moderate) and leaving 
    the instruments with the workpieces overnight so they are the same 
    temperature and humidity as the work the next morning.
    
    
    Mike
    
    
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