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    Re: telegraphic longitude article
    From: Fred Hebard
    Date: 2003 Dec 24, 14:32 -0500

    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.)
    >
    
    
    

       
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