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    Re: Plexiglass horizon & siphon level
    From: Jared Sherman
    Date: 2003 Jul 15, 23:54 -0400

    I confess I haven't reexamined the numbers in any great depth, but I did take 
    one start. You're right, my initial numbers were badly off, I slipped 
    something for sure.
    Bear in mind, I am not suggesting that one sight across a 15" gap allowing one 
    millimeter of tilt across the gap. I'm suggesting "threading the eye of the 
    needle", i.e. lining up the heights of two columns, to an accuracy better 
    than that one millimeter. How accurately can the eye line up those two 
    columns? Ask me in a day or two with rested eyes and some trials.
    But for the simple triangle and tangent, where the tangent of our angle of 
    tilt =Opposite/adjacent, and the adjacent is a 15" long base (let's say 
    370mm) and the opposite is a full millimeter of unequal height. The tangent 
    is about 0.0027, which indicates the angle itself is well under one degree, 
    which is 0.017. Roughly speaking, the angle is 0.15 degrees if the difference 
    in height of the two water columns is one full millimeter across 15". Well, 
    that's fifteen minutes not fifteen seconds, so I guess the question is just 
    how eenly can those two columns be compared. Or else....there's an inherent 
    problem in the entire approach.
    On the other hand, one internet vendor did suggest I could buy circular levels 
    about 2-3/4" in size for $165 each, with an accuracy of 30 seconds per 1/10th 
    inch of bubble position. That should mean a 10 second or less error assuming 
    the user set that bubble carefully into a hairline circle, which I think is 
    readily possible.
    He's also got " a straight glass bubble level which is 6 3/8" long that has a 
    sensitivity of 6sec./2mm " which would indicate a 3 second error could be 
    obtained, although I didn't get a price on that one I suspect it will be 
    similar to a hand surveyor's level at $75-100 each, two needed. So commercial 
    levels can provide a 3-10 second accuracy for about $200, which I suppose is 
    not unreasonable if someone really wanted a flat artificial horizon without 
    And here I was hoping there'd be a pony in the pile.

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