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    Re: telegraphic longitude article
    From: Fred Hebard
    Date: 2003 Dec 26, 13:29 -0500

    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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