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Page:Popular Science Monthly Volume 34.djvu/305

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191
THE GUIDING-NEEDLE ON AN IRON SHIP.

will fly into contact and cling one unto the other with the tenacity of a hundred arms. And this intense affinity of opposite magnetisms is a general characteristic.

The Earth a Magnet.—Now, to show that the earth has magnetic features entirely analogous to those of the bar-magnet, we will examine Fig. 2. Here the parallels of latitude and meridians of longitude appear as regular curves. But from a focus at N radiate a series of curves which take sinuous forms and finally

PSM V34 D305 The earth a magnet.jpg
Fig. 2.—The Earth a Magnet. P P, geographical poles; N S, magnetic poles; m m, magnetic equator; r v and a b, lines of equal magnetic dip; x z and o y, lines of equal magnetic variation.

converge toward another focus at the antipodes. These foci are the magnetic poles of the earth toward which the compass-needle ever points, not indeed directly, but parallel to the lines of force. These poles are not coincident with the geographical poles, but, on the contrary, are far removed from them.

There are other, but minor, magnetic foci on the earth, just as there are secondary poles in a bar-magnet, but so overtopped in prominence by the two grand foci that they scarcely deserve mention.

The lines issuing from one pole to meet again in the other are called lines of equal variation; that is, a compass carried along one of them from north to south would always point at the same