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Page:Popular Science Monthly Volume 34.djvu/307

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THE GUIDING-NEEDLE ON AN IRON SHIP.

pass-needle is the important factor of this article, since the needle, as an instrument of navigation, is specially treated, it will be necessary to dwell further on that element. It has already been stated that poles of opposite name attract and those of the same name repel each other. Now, on the earth, the pole nearest the geographical north is commonly known as the north magnetic pole, and the end of the needle pointing to it is also spoken of as the north pole, whence repulsion would, of necessity, seem to result; but this is an unfortunate use of terms that has grown up in daily life. The real state of the case is, that whichever of the two—the earth's pole, or that of the compass—we agree to designate as north, the other, having magnetism of the opposite kind, must be called south, and hence attraction naturally takes place. To show the variability of this attraction in direction and amount in various parts of the globe, we will examine Fig. 3.

PSM V34 D307 Variability of the earth magnetic force.jpg
Fig. 3.—Variability of the Earth's Magnetic Force. P, geographical pole; N, magnetic pole; m m, magnetic equator; Z, zenith; s t, K K, d f, etc., earth's lines of magnetic force.

Let us conceive the air filled with iron particles as it is with congealed vapor on a wintry night: they will not float about, listless and without form, but, like the frosty foliage on a window-pane, will seem projected from a parent stem, shooting up and out in graceful, wavy filaments. They are the earth's magnetic lines of force permeating space (how far, I do not presume to say),