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    Re: Measuring Dip in the 18th Century
    From: Brad Morris
    Date: 2014 Jan 1, 15:49 -0500

    Hi Frank

    Perhaps I have been misled.  In the wiki description of the mural quadrant, there is no mention of using a plumb line.  In Wrights text we have a quadrant ...

    "... as accurately used and rectified by a plumb line (every time we observed) as sight could discern"

    Why would the observer need a plumb line if the quadrant was wall mounted?  I do not understand that.


    Alex requested that you provide proof of your assertion that the quadrant was only good to 10 arc minutes, but only within expert hands. 

    I furnished Nicolas de Hilster's two published results, which do not seem consistent with that assertion.

    I thought perhaps Wright's testimony did provide technical evidence, but I appear to be mistaken

    We are awaiting your technical evidence which supports your assertion. 


    On Jan 1, 2014 2:27 PM, "Frank Reed" <FrankReed@historicalatlas.com> wrote:

    Brad, you wrote:
    "1) that the solstice altitudes of the sun were observed with a quadrant."

    And you made some comments about horizons and bodies of water and tides...

    And you concluded:
    "The assertion that a quadrant is only good to 10 arc minutes can be readily disposed of."

    Oy... Brad, I haven't actually looked at the original reference, but I am prepared to bet many bars of gold (each individually quite small, but there are MANY of them), that the quadrant here refers to an astronomical instrument which would have been found in an early observatory, also known as a "mural quadrant". Here's an article: http://en.wikipedia.org/wiki/Mural_instrument. This is one of the sneaky tricks of astronomical and navigational history: you have to learn the vocabulary that they used at the time to describe their observations and instruments. Otherwise you can be woefully misled!

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