ATION-NEEDLE and DIPPING-NEEDLE. The element of intensity is more difficult to determine. The relative horizontal intensity is measured by the number of oscillations that a needle, of unit size and strength, when disturbed makes in a given time, the intensities of two places being as the squares of the oscillations. The total intensity is got by dividing the horizontal intensity by the cosine of the angle of dip. Gauss has sticeeeded in reducing this measurement from a relative to an absolute standard.
Magnetic magnetic elements have been ascertained with great care at different portions of the earth's surface. The knowledge thus obtained has been em bodied in magnetic charts, in which the points at which the declination is the same are joined by lines, and similarly those where the dip and intensity are alike. The lines of equal declination are called the isogonic lines; those of equal dip, isoclinic; and those of equal intensity, isodynamic lines. As the magnetism of the earth is subject to a slow secular variation, such charts are only true for the time of observation. The chart, fig. 1, was drawn up by col. Sabine for the year 1840, and gives an approximate view of the lines of equal declination for that year. The change since 1840 has been small, so that an isogonic chart for the present time would differ but slightly from it. The chart suf ficiently explains itself. Attention may, however, be given to one or two points. The declination is marked on each line. Thus, the line passing through England, for instance, is marked 25°, and that passing n.w. of the British islands, 30°. At places under those lines the needle points to a n. 25° and 30° w. of the true north. On the space intervening between these lines, including Scotland and Ireland, a correction, varying from 0° to 5', must be made according as the station lies more toward the one line than the other. The westerly line of no declination passing northward cuts off the eastern corner of South America, proceeds to North America, which it enters at North Carolina, traverses the continent by lakes Erie and Huron and the, w. of Hudson's bay, and ends in the n. of
the continent at Boothia. The easterly line of no declination passing southward enters Europe in the n. of Russia, crosses the White sea to the e. of Russia, of the Caspian sea, of Persia, and the Arabian sea; then turns eastward, and cutting off the w. of Austra lia, passes southward. The space included between those two lines, and which in the chart is left untinted, constitutes, so to speak, the hemisphere of westerly declination. It includes the e. of the two Americas, the Atlantic ocean, the whole of Europe and Africa, and the w. of Ask. and Australia. The rest of the earth, which in the chart is tinted, has an easterly declination. There is an elliptic space in Eastern Asia which is left white, having a westerly variation, and forms an exceptional region in the eastern magnetic hemisphere.
It will be seen that the lines converge in the n. of North America, and in the s. of Australia. So far as experience goes, and so far as the most matter-of-fact theory '(Gauss's) teaches, the convergence in both cases is to a point. The point in North America is the north magnetic pole, and that s. of Australia is the south magnetic pole. At these points, then, all isogonic lines converge, and a compass-needle lies indifferently in any position.
These isogonic lines, as seen from the chart, form a somewhat complicated system. This arises from the fact that we refer the indications of the needle to the geographical poles, which are, so far as we know, arbitrary or extraneous as regards terrestrial mag netism. Duperrey, by drawing what he calls magnetic meridians and parallels, draws a system of lines which -have much the same conformation with regard to the magnetic poles that the meridians and parallels of latitude have to the geographical poles. A. magnetic meridian, according to Duperrey, is the line that would be described by a per son setting out, say from the south magnetic pole, and traveling always in the direction of the magnetic n. till he reached the n. magnetic pole. The magnetic parallels are lines drawn at right angles to the magnetic meridians.