Agonic Line (Gr. a, without, andangle), a word introduced by the modern investigators of terrestrial magnetism, and applied to a line uniting all points where the declination of the compass needle is zero, that is, where it points exactly north and south. As the magnetic poles of the earth do not coincide with the geographical poles, the magnetic meridians are different from the geographical meridians; and as the former are determined by the declination of the compass needle, they are. by no means regular arcs of great circles, as is the case with the latter, the magnetic force which directs the compass needle being very irregular over the earth's surface. Therefore the agonic line is not that geographical meridian which passes through the magnetic poles of our earth, but an irregular line at present crossing the eastern portion of South America at about 20° S. latitude and 30° of longitude E. of Washington, skirting the Antilles, entering North Carolina near Cape Lookout, passing over Virginia, Ohio, Lakes Erie and Huron, crossing through the Dominion of Canada, and reaching Hudson bay and the magnetic north pole.
At the other side of this pole it passes through the unexplored regions of the geographical north pole till it reaches the northern coast of Siberia in about lon. 115° E. of Washington, and lat. 75° X., passes south through the Caspian sea and the Persian gulf, then bends southeast through the Indian archipelago, crosses the continent of Australia in about lon. 190° E., and then takes a more southern direction to the as yet undiscovered magnetic south pole, beyond which it undoubtedly passes through the south polar regions to unite again with the agonic line in the southern Atlantic ocean. A most perplexing fact is the discovery that there is in the eastern hemisphere a second agonic line, independent of the main one just described. It enters China from the south in lon. 185° E., runs north through Tartary, reaches Siberia in lat. 65° X., then bends toward the east, then southeast, when it enters the ocean; it runs southward over Japan, then southwest, and finally west, and unites with the line entering the south of China. It thus forms a .closed elliptical ring, nearly 2,500 m. long and 1,500 m. wide, inside of which the declination of the compass needle is eastward.
If the modern theory be correct, that the earth's magnetism is caused by electric currents running from east to west through the earth's crust, and to which, according to the law of Oersted, the compass needle places itself at right angles, these peculiarities would only indicate that the direction of these currents is somewhat irregular, and that they only run exactly east and west at the localities through which the agonic lines have been traced. The most difficult phenomenon, however, is the fact that both this agonic line and the magnetic pole have a slow motion from east to west; in 1580 it ran through Sweden and Germany, in 1620 through Holland, in 1660 through London, England, in 1700 through the western coast of Ireland; it arrived on the American continent about1780, and in Pennsylvania in 1800; it is now in Ohio, and constantly moves west with a velocity which seems to indicate that, if persisted in, it will make one revolution around the earth in about 600 years. Trustworthy observations, however, extend thus far over too short a period of time to warrant any legitimate conclusion.