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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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