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Page:Popular Science Monthly Volume 34.djvu/306

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THE POPULAR SCIENCE MONTHLY.

angle from the geographical meridian, and this angle may vary from 1° to nearly 90° in different parts of the globe.

Thus, then, the variation may be defined as the angle between the geographical meridian and the direction of the compass-needle.

Another set of interesting magnetic lines are those of equal dip. They gird the earth in circles concentric with the magnetic poles, just as the parallels of latitude do the geographical poles. There is a magnetic equator along every point of which the compass-needle is horizontal. As we travel from the magnetic equator toward the north magnetic pole, the needle begins to incline, the north end tending downward until, when we reach the vicinity of the pole, the needle becomes vertical.

If we travel toward the south magnetic pole, the same occurs with the south end of the needle, now tending downward. An entirely similar experience will result from carrying a small needle through the magnetic field of the steel bar. At the neutral ground it will be parallel to the bar, while, as we approach either end, the dip toward the pole becomes more and more until it stands vertical at the pole. And as it was stated, regarding the steel bar, that the intensity of its magnetic field varies from point to point, so with the earth, it also has a magnetic field which is powerful near its poles and steadily moderates in strength as we approach the magnetic equator.

A third set of lines are those of equal intensity. They are not drawn in Fig. 2. In general contour they follow those of equal dip, though, in point of fact, they are not identical with them.

All these different systems of magnetic lines—variation, dip, and intensity—have not on the earth that symmetry and regularity which they would present around a steel bar; on the contrary, they are often bent, looped, and turned into many a devious path—wherefore, none can tell. The fact alone is well established, while theories fail to account satisfactorily for the earth being an irregular magnet.

The observations that have determined the various magnetic features of the earth have been made with delicate instruments in stationary observatories in every country, and also on ships-of-war in every sea.

The magnetism of the earth is not fixed either in locality or amount; but the different systems of lines just described, and by which it has been found convenient to represent this magnetism, are ever varying—ever migratory. The hourly, daily, and other periodic changes are all small, it is true; but, however minute, they are the object of inquiry at every magnetic observatory, with the hope that in time, by the accumulation of data, a satisfactory theory of the earth's magnetism may be deduced.

As the part of the earth's magnetism which affects the com-