A contour map presenting the secular change in the vertical intensity of the
Earth's magnetic field from 1885 to 1922.
Image ID: cgs00107, NOAA's Historic Coast & Geodetic Survey (C&GS) Collection
Photo Date: 1940 Ca.
• High Resolution Photo Available
of the U.S. Department of Commerce, National Oceanic & Atmospheric
NOAA Central Library
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April 30, 2013