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    Re: Sextant Terms
    From: Nicol�s de Hilster
    Date: 2007 Oct 22, 08:56 +0200

    Dan Allen asked in NavList 3450:
    > |
    > | It is my understanding that earlier instruments, which did not have
    > | the double reflecting principle, could only measure their arc's
    > | worth, so to speak.
    > |
    >   
    Then George Huxtable replied:
    > Dan is absolutely right, that until double reflection came in, instruments 
    > could only measure a range of angles which corresponded to the geometrical 
    > length of their arc. So the astronomer's quadrant could measure only to 90 
    > degrees. similarly with the mariner's quadrant, when this was no more than a 
    > 90-degree arc with a sight-tube attached and a plumb-bob to show the 
    > vertical. Other quadrants, such as the Davis, appeared, and these all had an 
    > arc, or two combined arcs, 90 degrees in length, to measure altitudes up to 
    > the vertical. No doubling.
    >
    >   
    There was another single reflecting instrument, which was invented in 
    1660 by the Dutchman Joost van Breen: the spiegelboog (mirror-staff in 
    English). This instrument was capable of measuring backward in a range 
    from 10 - 90 degrees using a single mirror. I made two reconstructions 
    of this instrument which I tested in the field and described in the SIS 
    Bulletin no. 90, 2006. You can also use this instrument in a forward 
    manner, in which case it will measure in a range from 90 - 170 degrees, 
    although I have to say there is no proof this was ever done, apart for 
    calibrating the instrument. Somewhere soon I will try to shoot some 
    lunars using this instrument, see what it is worth for that.
    
    If you want to know more, visit my website: 
    http://www.dehilster.info/instrumenten/spiegelboog/index.html
    If you click on image 22 you will get the full article on this instrument.
    
    Nicol�s
    
    
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