An elementary book on electricity and magnetism and their applications (1919)
Jackson, Dugald C. (Dugald Caleb), 1865-1951, Jackson, John Price, b. 1868, Black, Newton Henry, b. 1874 · New York, The Macmillian Co · leaf 156
TISM 131 was everywhere the same, until Columbus, on his way to Americain 1492, discovered near the Azores a place of no variation. Anexact knowledge of the variation at different places is evidentlyof the greatest importance to mariners and surveyors, and socareful maps are published by the… … Fig. 79. — Map showing variation of the compass in the United States. This decreases to zero at a place near Cincinnati, O., and be-comes an easterly variation amounting to 20° E. in the north-west. It was discovered nearly a hundred years after Columbustime that if a compass needle is perfectly…
Electricity · Magnetism
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