Is the principal source of the secular variation of the Earth's magnetism within or without the Earth's crust?

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Abstract
In the papers on the “Distribution and the Secular Variation of the Earth's Magnetism,” presented before the Washington Philosophical Society in May, 1895, I showed that certain laws pertaining to the distribution of the magnetic elements over the earth's surface are similar to those pertaining to the secular variation as derived thus far. I shall refer to four of the deductions then made, as they will be found interesting in connection with some of the conclusions reached in this paper. These laws2 are:I. The minimum range in declination along a parallel of latitude occurs near the equator, generally increasing upon leaving the equator. II. The maximum range in inclination along a parallel of latitude occurs near the equator, generally diminishing upon leaving the equator. III. The minimum average secular change in declination along a parallel of latitude from 1780 to 1885 occurred near the equator, the values generally increasing upon leaving the equator. IV. The maximum average secular change in inclination along a parallel of latitude from 1780 to 1885 occurred near the equator, the values generally diminishing upon leaving the equator.

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