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    Re: telegraphic longitude article
    From: Fred Hebard
    Date: 2003 Dec 26, 16:36 -0500

    Thanks.  I was wondering about propagation delays engendered by relays.
      My imperfect understanding is that these were used every hundred miles
    or so, and were the big breakthrough Morse obtained from Henry that
    permitted a practical telegraph system.  But it's unclear to me the
    separation needed between relay stations and their switching time.
    On Dec 26, 2003, at 2:27 PM, Brooke Clarke wrote:
    > Hi:
    > 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
    > http://www.PRC68.com
    > 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{at}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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