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    Re: Double vial horizon prototype
    From: Patrick Goold
    Date: 2012 Jan 12, 16:08 -0500
    This is wonderfully helpful!  Thanks so much!  Probably the most helpful part is the last two sentences.  I have been going for spot on and that has been aggravating.  It not only takes a long time to do that but it doesn't remain there for long.  

    Thanks again!


    On Thu, Jan 12, 2012 at 2:29 PM, Bill Morris <engineer@clear.net.nz> wrote:


    I have attached a sketch of the levelling sequence.

    A) Place the level along a line joining two of the three feet (1 and 2 in sketch) and centre the bubble using these two feet. Reverse end for end and if the bubble is no longer centred, split the difference using one of these feet.

    B) Place the level at right angles to the first line and centre the bubble using the third foot (3 in sketch). Reverse the level end for end and if the bubble is no longer centred, split the difference using foot number 3.

    C) Go to A) and check. Repeat the sequence if necessary.

    This should rapidly level the mirror. There may be several reasons why it does not, e.g. slackness in the levelling screws, tip of screws moving over the surface upon which they rest, tips of screws working into the surface, flexing of the mirror, tilting of the mirror on its base, sun shining on level (or even breathing on a very sensitive level). No doubt there are more that people can think of. When adjusting very sensitive instruments, very great care is needed to get consistency. With a 10 second level, I would not spend a lot of time geting it exactly right. Within 30 seconds (3 divisions) is probably near enough, given the probable error of sextant observations themselves.

    Bill Morris
    New Zealand
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    Dr. Patrick Goold
    Department of Philosophy
    Virginia Wesleyan College
    Norfolk, VA 23502
    757 455 3357

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