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    Re: Height by sextant angle
    From: David Pike
    Date: 2014 Dec 13, 17:18 -0800

    I didn't have to look very far.  Try looking in the log of the R34 http://archive.org/stream/logofhmarjourney00maitrich/logofhmarjourney00maitrich_djvu.txt .  What did we do before Google?

    OUTWARD JOURNEY 43
    round view of the Atlantic since we started. Sea
    is very blue, a biggish swell, and many white
    horses. Hope that, with this good visibility,
    we may at last see a passing ship.

    One of the crew (a rigger) is ill. Temperature
    102. Confined to hammock and dosed with
    quinine and salts.

    Running on forward and after engines only, and
    resting both wing engines. Height 1100 feet.
    We cannot tell our height above sea accurately,
    as we don't know what the barometer is reading.

    Our aneroid shows height as 1200 feet. Scott
    thought it more likely to be 1500 feet, as one
    cannot well assume that the barometer here on
    the surface of the water is the same as the baro-
    meter was when we left the ground at East
    Fortune.

    We now try the experiment of lowering an
    instrument known as a sea-level aneroid, and
    specially designed to record atmospheric pressure
    readings, down to the surface of the water. The
    ship is not slowed down for the purpose in any
    way, and as this has not been tried before success
    is problematical.

    Result is no good, as the instrument shows
    exactly the same reading on reaching car again

    44 THE LOG OF H.M.A. R 34
    as it had before starting. It had undoubtedly
    recorded the reading on surface, but got jerked
    (being a very sensitive instrument) on the quick
    haul-up.

    Until we know our exact height above the sea
    we cannot plot our exact position, so until we
    speak a ship in our vicinity and get her barometer
    reading we still lack means for such a calculation.
    Some suitable method of getting this information
    must be perfected before we do any more long
    overseas flights.

    Scott works out our height above the water in
    the following way

    The airship is throwing a very dark shadow on
    the surface of the sea on starboard side almost
    immediately underneath the ship. By taking
    with a sextant the angle subtended by length
    of the shadow, and knowing the length of the
    shadow to be 640 feet, he gets the true height.
    In this case the height works out at 2100 feet,
    whilst the aneroid gives us only 1200 feet a
    variation of 900 feet.

    12.45 p.m. Durrant is speaking s.s. Canada
    on our W.T. spark set. Another W.T. operator
    (Corporal Powell) is trying to get her on the
    directional wireless, so that we may know in

    OUTWARD JOURNEY 45
    what direction to look for her. All we know at
    the moment is that she is somewhere within 120
    miles of us, and bound for Liverpool.

    She gives her position as follows : " Long.
    51 16' N., Lat. 39 42' W. (or sixty miles S.S.E.
    from us), barometer 30.8, rising. Wind and sea
    moderate from S.E. DAVIS, Master."

    Her wireless operator tells us that great
    enthusiasm prevails on board, and that every one
    is hoping to catch sight of us. This feeling is
    mutual, but, although we do our best to get her
    on our directional wireless, besides gazing through
    our glasses in every direction, she remains just
    beyond our visibility.

    Scott now brings his ship down to the 1200-foot
    level, to try and find a more easterly trend in the
    wind.

    The reading on s.s. Canada's barometer is of
    the greatest value, and is exactly the information
    of which we are in need. We can now work
    out our true height above the sea, and con-
    sequently fix our exact position. Her barometer
    on surface reads 30.8, whilst ours in the ship is
    29.7. This shows a difference of 30.8-29.7, or
    1.1 inches. Remembering the rule that for
    every 1000 feet you rise the barometer falls one

    46 THE LOG OF H.M.A. R34
    inch, we get our height as 1000 feet, which tallies
    almost exactly with Scott's estimate of 900
    feet inaccuracy mentioned above. It is quite
    possible that Scott's calculation of 900 feet may
    still be dead accurate, as s.s. Canada is some fifty
    or sixty miles away, and the change in barometer
    over that distance may well work out at 100 feet.

     

       
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