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Telegraphic longitude article
From: Paul Hirose
Date: 2003 Dec 24, 11:14 -0800
From: Paul Hirose
Date: 2003 Dec 24, 11:14 -0800
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.)