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    Re: Artificial horizon
    From: Marvin Sebourn
    Date: 2005 Feb 21, 16:54 EST
    In a message dated 2/21/2005 2:14:31 P.M. Central Standard Time, george@HUXTABLE.U-NET.COM writes:
    For an artificial horizon, there seems no reason to ask for a reflecting
    surface bigger than, say, 3 inches diameter, which should occupy the visual
    field of a sextant at a convenient distance.

    So I'm rather horrified to see a glass plate levelled by means of such an
    enormous spirit-level as has been mentioned to the list; 8 inches or even
    bigger!. If the user carefully adjusts the plate using such a level perched
    on it, can he then be quite sure that there will be no change, not even a
    fraction of a minute, when the weight of that spirit-level is removed?
    The use of eight inch levels was a personal choice, made because of concerns for accuracy, sensitivity of adjustment of the leveling screws, and materials at hand, etc. I fabricated the simple setup some 15 or more years ago as my first artificial horizon (after using a pool of mercury for a reflective surface, as I did a few times). 
    My mention of a longer level (Starrett 15 inch) was to point out that some general manufacturers of precision measuring equipment (frequently for machine shop and tool room use) rated their levels in seconds sensitivity, rather than just in elevation per foot per vial graduation. If you read my post, you will see nothing suggesting that I used a level longer than 15 inches. 
    Your suggestion that there is an inherent uncertainty of setup using larger levels (because there removal might disturb the plate alignment) is in error because of the incorrect assumption that the levels were removed after alignment and prior to sighting use. They were not, and they served to indicate the continued alignment of the plate. 
    Were you "horrified" (that is, shocked, made fearful, or disgusted) by considering the use of an eight inch spirit level? Really?
    Marvin Sebourn
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