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    Re: telegraphic longitude article
    From: Brooke Clarke
    Date: 2003 Dec 26, 11:27 -0800

    My reading is that the key element was replacing ears trying to
    interpolate between one second ticks and the use of a drum recorder with
    a tick recorded once per second where interpolation could be used to
    determine the time of star crossings that were also marked on the drum
    by an observer pressing a manual key while looking through a 48"
    meridian transit.
    In this scenario with a 1,000 mile separation between stations the
    propagation delay would be on the order of 5 milliseconds which is
    probably in the noise compared to the human reaction time variation in
    pressing the key.  But could be calculated based on the known
    propagation constant for the open wire type lines then in use.
    Happy Holidays,
    Brooke Clarke, N6GCE
    Fred Hebard wrote:
    > I have read this article carefully, and methods for determining the
    > propagation delay in the circuit are not readily apparent to me.  If an
    > astronomic phenomenon that occurred simultaneously at both sites were
    > measured, the delay could be determined, it appears to me.  Perhaps
    > occultations of Jupiter's moons might be free of parallax effects?
    > Also, it would seem for the measurement of a simultaneous effect that
    > the sites might need to be connected by two circuits.
    > On Dec 24, 2003, at 2:32 PM, Fred Hebard wrote:
    >> Thanks Paul!  I had been wondering how they got around propagation
    >> delays.
    >> On Dec 24, 2003, at 2:14 PM, Paul Hirose wrote:
    >>> Professional Surveyor magazine has an online article about the early
    >>> use of the telegraph for longitude determinations.
    >>> http://www.profsurv.com/ps_scripts/article.idc?id=1147
    >>> Alexander Bache, head of the U.S. Coast Survey, was quick to realize
    >>> the possibilities. He organized an experiment which measured the
    >>> longitude difference between Washington and Philadelphia by means of
    >>> telegraph in 1846.
    >>> By the mid-1850s, the technique had become routine. Chronographs
    >>> recorded the electrical impulses of the observer's hand switch on a
    >>> paper-covered rotating drum as stars crossed the meridian at both
    >>> observatories. Also recorded were 1-second pulses from break-circuit
    >>> chronometers at both ends of the telegraph line. With this data,
    >>> surveyors could eliminate clock offsets and propagation delays.
    >>> Telegraphic longitudes were a huge improvement over the Coast Survey's
    >>> former longitude methods: lunar culminations, lunar occultations, and
    >>> chronometer transportation. (Before the trans-Atlantic cable was laid,
    >>> the Survey made more than 1200 chronometer exchanges with England.)
    > ------------------------------------------------------------------------
    > Frederick V. Hebard, PhD                      Email: mailto:Fred@acf.org
    > Staff Pathologist, Meadowview Research Farms  Web: http://www.acf.org
    > American Chestnut Foundation                  Phone: (276) 944-4631
    > 14005 Glenbrook Ave.                          Fax: (276) 944-0934
    > Meadowview, VA 24361

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