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    Re: Artificial horizon
    From: George Huxtable
    Date: 2005 Feb 21, 20:14 +0000

    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?
    I mentioned in a previous posting the small spirit-level glass capsules
    that exist as replacements for theodolite levels, and quoted from memory
    those that had been acquired by a friend, as being 20 arc-seconds per
    division. My memory was a bit at fault there. The levels were acquired by
    my friend and neighbour, and fellow listmember, Clive Sutherland,
    . Their sensitivity was actually 30
    arc-seconds per division, not 20 as I suggested. The scale divisions were
    2mm apart, and it's possible to estimate level to better than 1 such
    division. Clive paid approx ?10 for each level (roughly $20), from a
    Supplier in Kew, London.. Higher sensitivity versions (20 arc-sec per div)
    exist, but cost rather more.
    These glass capsules are (Clive's estimate) about one and a quarter inches
    long, with pointy ends, and three-eighths diameter. To be useful, they need
    to be fixed into some sort of tiny shoe, accurately glued into place such
    that when reversed the reading remains the same. This is a non-trivial
    Clive recommends this address for useful information-
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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