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    From: Fred Hebard
    Date: 2003 May 2, 21:47 -0400

    According to the Smithsonian Meteorological Tables, one atmosphere is
    defined as 1013.250 mb, which the Nautical Almanac approximates as 1010
    mb.  One atmosphere corresponds to about 29.92 inches of mercury
    depending upon the local force of gravity.  Reducing this by the factor
    of 0.9264 quoted by Paul Hirose as appropriate for 2100' elevation
    yields a standard pressure at 2100' elevation of 27.72 inches of Hg.  I
    approximated these values as 30" Hg = one atmosphere and subtract 2
    inches for 2100' elevation, which are fairly easy to remember; the
    approximation could be refined for the actual pressure obtained from
    the newspaper, tv or internet (drill down from
    http://iwin.nws.noaa.gov/iwin/graphicsversion/rbigmain.html into the
    political map of the U.S. to one state, and then to one station to find
    hourly reports of pressure and temperature).  The approximation might
    yield a correction from the bottom of Table A4 correct (compared to
    George Huxtable's exact treatment) to within 0.1', perhaps 0.2' of arc.
    
    At one point in this quest I purchased the Smithsonian Meteorological
    Tables to try to compute some of the values, but it would take more
    study than I have time, patience and skill in physics (thank goodness
    that George is with us!).  Mostly I was interested in determining the
    appropriate temperature to use, as the formulas employ the mean
    temperature of the air column.
    
    I note from page 389 of the Smithsonian Tables that refraction is also
    dependent upon the humidity of the air.  This has not been factored
    into the corrections, perhaps because the mean humidity of the air
    column above a ship could not be determined accurately close to the
    sea.  I am tempted to try to estimate the magnitude of the effect.  It
    may be one of the factors causing discrepancies in Doug's observations.
    
    I am sure this was all well determined by the astronomers before 1850.
    We are treading into astronomy here, although this does lead back to my
    quest of checking the calibration of sextants, which service appears to
    be unavailable commercially in the United States.
    
    Fred
    
    
    

       
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