Die säkular-verlegung der magnetischen axe der erde

ABSTRACT THIS is an attempt to trace the position of the earth's magnetic axis during the last three centuries, on the supposition that a knowledge of magnetic declination is sufficient

to determine the direction of its axis. Great circles drawn through different points, and coincident at these points with the magnetic meridians, would intersect in the poles of the

magnetic axis, if the earth were a uniformly magnetised sphere. As this is not the case, the circles all pass through an arctic and an antarctic region instead of through two points, and Mr.

van Bemmelen calculates by the method of least squares the point in each region which is nearest to the circles. The two points thus found he takes for the intersections of the magnetic

axis with the earth's surface. The reader must be referred to the original for the clever manner in which the calculations are simplified and carried out. The method is first tested for

the year 1885, when it is found that the magnetic axis, calculated in this fashion, agrees closely with that derived from the more rigorous analysis of Neumayer and Ad. Schmidt: It is then

applied to the declination values for the years 1600, 1650, 1750, 1770 oand 1842, and the author draws from the results thus obtained the conclusion that the magnetic axis does not revolve

round the geographical axis, but that there seems to be a tendency to revolve round Nordenskjöld's aurora pole. A doubt must necessarily arise in the mind of the reader as to how far

the older observations are sufficiently numerous and correct to allow any certain conclusions to be drawn from them. Any one looking at Neumayer's Atlas (Berghaus) of Terrestrial

Magnetism will be struck at once by the fact that the distribution of magnetic declination in the year 1600 is represented as being widely different from that of a uniformly magnetised

sphere. We must conclude that either the observations were not sufficiently accurate to give us a correct picture, or that the earth differed much more from a uniformly magnetised sphere at

that time than it does now. As v. Bemmelen has only tested his method at a time when the deviations from uniformity were small, there is considerable doubt whether equally good results would

be obtained with irregular magnetisation. The work, meritorious and interesting as it is, cannot, therefore, be said to have led to any conclusion which can be accepted without further

evidence. _Die Säkular-Verlegnng der Magnetischen Axe der Erde_. Von W. van Bemmelen. (Observations made at the Royal Magnetical and Meteorological Observatory at Batavia.) Vol. xxii.

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