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    Wharton & Field's Hydrographical Surveying
    From: Fred Hebard
    Date: 2004 May 9, 10:09 -0400

    Thanks to Henry Halboth recommending this book, I got a copy.  The book
    starts out saying it is targeted to the Naval Professional and that no
    deep theoretical or mathematical knowledge is needed, which was a
    relief to me!, not that I am a Naval Professional by any stretch of the
    imagination, but at least there is some hope I might understand it
    given my shallow theoretical and mathematical understanding.  Most of
    the book was written by Admiral Sir William Wharton, with additions to
    later editions supplied by Admiral Sir Mostyn Field.  I will be
    discussing mostly sections written by Admiral Wharton, so will make
    attributions solely to him.
    
    Hydrographical Surveying does cover sextant stands, as Henry pointed
    out, which it describes thoroughly in the introduction to
    instrumentation.  In general, Wharton recommends use of stands for any
    observation using a sextant on land.  Interestingly, he recommends a
    sextant for any celestial work, as equal in accuracy to what in America
    we would term a transit; this, of course, being in the hands of Naval
    Professionals.  I believe the large theodolites, such as used in the
    Survey of India, would be excepted from this recommendation.
    
    One of the land observations Wharton discusses rather thoroughly is
    RATING a chronometer by observations of equal altitudes of the sun in
    the forenoon and afternoon.  The known elapsed times between sets of
    observations is compared to the elapsed time observed on the
    chronometers.  Last year, in the discussion of the riveting account of
    submarine cable repair, the purpose of such observations was never
    fully clarified, at least to my meagre understanding.  Now it is.
    
    For these observations in an artificial horizon, Wharton recommends an
    eypiece shade rather than the swing-out sextant shades.  He also
    recommends multiple eyepiece shades of varying optical density.  He
    discusses examination of errors caused by swing-out sextant shades.
    
    In general, I would say that if one has difficulty understanding some
    navigational or surveying procedure based upon optical instruments and
    chronometers, especially ones used after the introduction of the
    telegraph, but prior to radio, then Hydrographical Surveying is the
    book at which one should look.
    
    Wharton has a rather interesting account of navigation at sea, which is
    short and sweet, and written by a consummate professional seaman rather
    than by a committee or by a mostly shore-bound mathematician, such as
    Bowditch.  One interesting point is that he recommends determination of
    latitude IN THE TROPICS by equal altitudes of the sun for a short
    period around noon, such as 20-40 minutes, ignoring, deliberately, both
    declination changes and the normal inaccuracy at high latitudes.  This
    works in the tropics because the altitude is changing rapidly around
    noon, unlike higher latitudes, where the rate of change is slow.  I
    believe Wharton may be more precise here than I have been, and somehow
    been alluding to being located in regions where the sun passes close to
    the zenith at noon, thus excepting latitudes near the Tropic of
    Capricorn in winter, etc.
    
    In the section on sextants, Wharton thoroughly discusses measuring the
    effect of eccentricities of the arc, and other sources of
    non-adjustable error.  The suggested procedure is observation for
    altitude of pairs of circum-meridian stars, one north, the other south
    of the zenith at similar altitudes.  I am unsure as to the mathematics
    behind wanting pairs of stars, or even whether the stars would be
    observed at meridian passage, but these details could be worked out
    without difficulty.  Numerous sets of replicated observations would be
    needed, and a sextant stand would help greatly.  The difficulty for us
    modern folk is that a mercury artificial horizon probably is not
    available, greatly hampering observation of dim stars.  One then has to
    rely upon mirrors, with an attendent leveling error.
    
    Determining sextant accuracy by measuring intersteller distances also
    is discussed, with reference to clearing the distance similarly to
    lunars, and a preference given for stars of equal altitude, where
    Wharton says one need correct only for refraction.  I don't understand
    what he means by this, but it may be the start of the mistaken
    presentation of this method in subsequent texts, which ignores the need
    to clear the interstellar distance as one would a lunar distance.
    Wharton also recommends a 12x magnification, and sextant stand, for
    this observation.
    
    Reading this book, which was written after the telegraph and
    chronometer, but before radio, made me realize that the radio probably
    was what done the lunar in.
    
    In sum, I highly recommend this book, especially for shore-bound
    navigators with a preference for archaic methods!
    
    Fred Hebard
    
    
    

       
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