Results from magnetic east‐west paths around the Earth

Abstract

In the preliminary analysis of the Earth's magnetic field for 1922 by Dr. Louis A. Bauer,1 it was found desirable to ascertain how an east‐west path around the Earth would close that was everywhere throughout its length perpendicular to the direction of the compass needle, or to the horizontal component of the Earth's magnetic field. The considerations involved and the method adopted in constructing eleven such paths are given in these notes, together with the results.The actual tracing of a path with the required accuracy, exactly magnetic east or exactly magnetic west, on land and over sea, presents many practical difficulties. Periodical time changes all along the path introduce uncertainties. Over land, local disturbances2 interfere and cities and other constructional culture obstruct the necessary surveying operations, while over the seas, where the only available method at present is to continue the path on a non‐magnetic ship, ocean currents and leeway affect the closure sought, not only by precisely the sum of their components perpendicular to the path, but also by shifting the path in regions of critical or steep gradients of the declination, and thereby introducing errors in its direction. The tracing of a path upon isogonic charts of the world is free from these sources of error though subject to others. Local disturbances have been effaced in the smooth isogonics, and time changes are taken care of in the reduction to a common epoch. Cultural detail, ocean currents and winds no longer enter the problem. Hence the plotted closure of a circuit extending exactly magnetic east or west around the Earth depends in this method only upon the systems of magnetization represented by the particular isogonics and upon the accuracy with which data can be scaled from the chart or plotted thereon. Unfortunately, the errors of plotting usually accumulate. It is possible to compute the closure of the path by a long process involving repeated approximations, provided that the magnetic declination is given as an analytical function of the latitude and longitude. The work entailed, however, renders this method practically useless.