Abstract
H. D. HARRADON, Department of Terrestrial Magnetism, Carnegie Institution of Washington, has given a short account of this subject (Sky and Telescope, 1, 8 ; June, 1942), commencing with the discovery of the secular variation by Henry Gellibrand in 1634-a discovery of considerable importance, especially for mariners. Since no observations are available prior to this date, attempts have been made to extend our knowledge of the earth's magnetism to earlier times by the study of the magnetization of eruptive rocks, baked clays and ancient potteries. In quite recent years magnetic data have been obtained from successive lava flows of Mount Etna, the assumption being that the lava retained the direction of the earth's magnetic field at the time of its solidification, but unfortunately the results have proved rather indefinite. More promising lines of research are found in the study of layers of Pleistocene clays from an old glacial lake near New Haven, Conn., and also of sediment taken from the floor of the Atlantic. In the latter case, where several thousand years of deposition were represented, there is the suggestion of a possible variation in direction of magnetization amounting to more than 60°-the greatest range in secular variation ever recorded. No theory up to the present has explained the mystery of these secular variations ; they are probably connected with changes in the interior of the earth, regarding which there is mere speculation. The transient variations, among which the solar and lunar diurnal and the annual variations are the chief, are discussed, and a number of useful diagrams and charts illustrate the connexion between magnetic storms, auroral displays, sunspots, etc.
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Variations of Geomagnetism. Nature 150, 232 (1942). https://doi.org/10.1038/150232b0
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DOI: https://doi.org/10.1038/150232b0