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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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