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The history of geomagnetism is concerned with the history of the study of Earth's magnetic field. It encompasses the history of navigation using compasses, studies of the prehistoric magnetic field (archeomagnetism and paleomagnetism), and applications to plate tectonics. Magnetism has been known since prehistory, but knowledge of the Earth's field developed slowly. The horizontal direction of the Earth's field was first measured in the fourth century BC but the vertical direction was not measured until 1544 AD and the intensity was first measured in 1791. At first, compasses were thought to point towards locations in the heavens, then towards magnetic mountains. A modern experimental approach to understanding the Earth's field began with ''de Magnete'', a book published by William Gilbert in 1600. His experiments with a magnetic model of the Earth convinced him that the Earth itself is a large magnet. ==Early ideas on magnetism== Knowledge of the existence of magnetism probably dates back to the prehistoric development of iron smelting. Iron can be obtained on the Earth's surface from meteorites; the mineral lodestone is rich in the magnetic mineral magnetite and can be magnetized by a lightning strike. In his ''Natural History'', Pliny the Elder recounts a legend about a Magnes the shepherd on the island of Crete whose iron-studded boots kept sticking to the path. The earliest ideas on the nature of magnetism are attributed to Thales ( BC – BC).〔 In classical antiquity, little was known about the nature of magnetism. No sources mention the two poles of a magnet or its tendency to point northward. There were two main theories about the origins of magnetism. One, proposed by Empedocles of Acragas and taken up by Plato and Plutarch, invoked an invisible ''effluvium'' seeping through the pores of materials; Democritus of Abdera replaced this effluvium by atoms, but the mechanism was essentially the same. The other theory evoked the metaphysical principle of ''sympathy'' between similar objects. This was mediated by a purposeful life force that strove toward perfection. This theory can be found in the writings of Pliny the Elder and Aristotle, who claimed that Thales attributed a soul to the magnet.〔 In China, a similar life force, or ''qi'', was believed to animate magnets, so the Chinese used early compasses for feng shui. Little changed in the view of magnetism during the Middle Ages, and some classical ideas lingered until well after the first scientific experiments on magnetism. One belief, dating back to Pliny, was that fumes from eating garlic and onions could destroy the magnetism in a compass, rendering it useless. Even after William Gilbert disproved this in 1600, there were reports of helmsmen on British ships being flogged for eating garlic.〔 However, this belief was far from universal. In 1558 Giambattista della Porta reported "When I enquired of mariners whether it were so that they were forbid to eat onyones and garlick for that reason, they said they were old wives fables and things ridiculous, and that sea-men would sooner lose their lives then abstain from eating onyons and garlick." 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「History of geomagnetism」の詳細全文を読む スポンサード リンク
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