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Strabo, Geography (ed. H.C. Hamilton, Esq., W. Falconer, M.A.) 3 3 Browse Search
Plato, Republic 1 1 Browse Search
Knight's Mechanical Encyclopedia (ed. Knight) 1 1 Browse Search
Edward L. Pierce, Memoir and letters of Charles Sumner: volume 3 1 1 Browse Search
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he Azores. It has since shifted. Columbus has not only the incontestable merit of having first discovered a line without magnetic variation, marking a memorable era in nautical astronomy, but also of having, by his considerations on the progressive increase of westerly declination, in receding from that line, given the first impulse to the study of terrestrial magnetism in Europe. We know positively from the Chinese Penthsaayan, which was written under the dynasty of the Song between 1111 and 1117, that the manner of measuring the amount of westerly declination had been then long understood. That which belongs to Columbus is not the first observation of the existence of the variation (which, for example, is noted in the map of Andrea Bianco, in 1436), but the remark which he made on the 13th of September, 1492, that 2 1/2° east of the island of Corvo the magnetic variation changes, passing from N. E. to N. W. — Humboldt. The first variation-compass was constructed befor