Excerpt from Philips' Atlas of Physical Geography for the Use of Schools: Illustrating the Natural Features of the Globe, the Geographical Distribution of Natural Phenomena, and Their Connexion With the Industrial Pursuits of Mankind
The two principal Lines of No Variation shown on the chart are obviously but the opposite sides of one great circular line passing round the globe, and forming a Magnetic Meridian. There are two places on this line - one in the northern and the other in the southern hemisphere - towards which the extremities of the needle, in whatever part of the globe, are always directed. These are known as the Magnetic Poles. The place of the North Magnetic Pole (in about lat. And nearly under the meridian of 94° W., that is, adjoining the Arctic shores of America) is shown on our map. The South Magnetic Pole, the place of which does not fall within its limits, is in a nearly correspondent latitude of the southern hemisphere, under the meridian of 150° east (nearly due south from Tasmania.) The needle always points to these poles; hence the amount of deflection from the true meridian (which is a line joining the opposite extremities of the earth's axis) continually increases with every successive degree of departure from the place of the magnetic meridian, or line of no variation. Upon one side of either of the lines shown on the chart the deflection of the needle is to the westward, and upon the other side to the eastward, of the true north. In passing round the place of either magnetic pole, the needle would make a complete circle (its north or south point, as the case might be, remaining constantly directed towards its own pole), so that the direction of its extremities, with reference to the true north and south, would become actually reversed. That is (taking the northern magnetic pole as an example), the amount of deflection, or variation, would keep on increasing until it arrived at and when at that precise amount the North end of the needle would point due south, and the reverse.
The Lines of N 0 Variation are not fixed They oscillate to and fro over the earth's surface, and, with their movement, the amount of deflection or variation at any given place undergoes continual change. In 1660, one of the lines of No Variation passed through London, the needle in that year pointing, at London, due north. It then commenced a deflection to the west of north, the amount of this deflection increasing until it attained, in 1818, a maximum of Since that time, it has been slowly returning to the east ward. In 1858, (the date represented by the chart), the variation at London was nearly 22° west.
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