Abstract
This chapter discusses terrestrial magnetism. The earth can be regarded as a sphere having a short bar magnet at its centre, with its blue pole pointing toward a point in the Hudson Bay region and its red pole toward a point in South Victoria Land. The vertical plane, coinciding with the line of the earth's magnetic force at any place, is called the magnetic meridian at that place. The exact direction of the lines of magnetic force in the earth's field and the intensity of the magnetic force itself are determined by the magnetic elements, namely, variation, dip, and total field strength or intensity. Provided that the ship's compass has no mechanical or constant error, variation can be found by taking the mean of the compass errors on eight equidistant headings around the compass. Magnetic foci are four regions, two in each hemisphere, where the total force has maximum value. Disturbances in the distribution of the earth's magnetic field can take place during magnetic storms. These disturbances coincide with intense sunspot activity. Local magnetic attraction because of ferromagnetic substances in the sea bottom or in mountain formations near the coast can affect the compass.
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