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    Re: Making an artificial horizon
    From: George Huxtable
    Date: 2011 Jan 20, 12:41 -0000

    Gary LaPook wrote about Mercury-
    
    | We've gone around before about mercury. There are very toxic organic
    compounds involving mercury but you can drink pure mercury and it just
    passes through you without harm. The "mad hatter" breathed mercury fumes
    over long term exposures which had bad results. It has had a long history
    for use in artificial horizons, ask Lewis and Clark. It does evaporate very
    slowly so don't keep the bottle in you bedroom but, otherwise, the limited
    exposure to fumes during the occasional use as an artificial horizon will
    not cause you any harm. And there is no fluid that is more reflective than
    mercury for use in an artificial horizon, just try to shoot stars with
    water or oil!
    
    ================
    
    I agree with Gary's opinions about the hazards of Mercury, but not with his
    reference to Lewis and Clark.
    
    Lewis and Clark made much use of an artificial horizon, with some sort of
    "cloche" cover, presumably similar to what has been discussed here, but
    with windows of "talc". These were sheets of mica, high-quality examples of
    which were optically superior to the plane glass of that time.
    
    But they used water, not Mercury, as the reflecting surface.
    
    On July 22, 1804, Lewis listed in his journal the instruments carried by
    the expedition, writing -
    
    "3rd- An Artificial Horizon on the construction recommended and practiced
    by Mr Andrw. Ellicott of Lancaster, Pennsyla., in which water is used as
    the reflecting surface; believing this artificial Horizon liable to less
    error than any other in my possession, I have uniformly used it when the
    object observed was sufficiently bright to reflect a distinct immage; but
    as much light is lost by reflection from water I found it inconvenient in
    most cases to take the altitude of the moon with this horizon, and that of
    a star impractible with any degree of accuracy."
    
    He went on to describe the alternatives employed when water-reflection was
    unusable; a tiltable glass-plate and a tiltable mirror, both used with a
    spirit level. But these would be much more liable to error that the
    self-adjusting surface of a liquid.
    
    This, and much more,  can be found in vol 2 of "The Journals of the Lewis
    and Clark Expedition, ed. Gary E Moulton, University of Nebraska Press,
    1986.
    
    It surprises me that they didn't carry, and use, Mercury for a horizon. It
    was by no means a rare material, being used in large quantities in the
    mining and refining of precious metals, and I would expect it to have been
    available in Pittsburgh or St Louis.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    
    

       
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