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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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