I. The solar and lunar diurnal variations of terrestrial magnetism

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Abstract
The regular daily changes of the earth’s magnetism, considered as a world-wide phenomenon, afford a problem of much interest and importance. It is one, moreover, which the researches of Balfour Stewart and Schuster have shown to be more vulnerable to attack than seem most of the problems of terrestrial magnetism. But in spite of their success, and of the contributions of subsequent writers, the comprehensive study of the subject has suffered undeserved neglect. It seems an unfortunate fact that the efforts of magneticians are unduly devoted to the accumulation of data, the time and labour spent in their discussion being, proportionately, inconsiderable. The promising theory that the daily magnetic variations arise mainly from electric currents circulating in the upper atmosphere, under the impulsion of electromotive forces produced by the convective motion of the air across the earth’s permanent magnetic field, was first propounded by Balfour Stewart. In a simple but penetrating discussion of the features both of the solar and lunar diurnal variations, he showed the power of this theory to account for the facts in a way which none of the other theories then current could do.

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