Page:Popular Science Monthly Volume 33.djvu/830

810 and to the south of it in the western hemisphere. It further appears that the magnetic force is not evenly distributed on the earth, and that the points of maximum intensity do not coincide with either of the magnetic poles. In the northern hemisphere there are two foci of maximum force of unequal intensity, the most powerful lying at about latitude 53° north, longitude 92° west, near the great American lakes, the weaker in latitude 65° north, longitude 115° east, in Siberia. For the southern hemisphere, the available data are far less numerous, and the determination of the foci of force is less reliable. It is, however, believed that here also there are two points of maximum, of nearly equal power, and not far removed from one another, one in latitude 65° south, longitude 140° east, the other in latitude 50° south, longitude 120° east. The unit by which magnetic force is measured has been assumed, adopting English standards of weight and length, to be that which would impart to a weight of one grain a velocity of one foot in one second of time. On this scale the magnetic force, where least, is found to be 6·0; the northern maxima are 14·2 and 13·3 respectively, and each of the southern 15·2. The declination, or variation of the direction of the needle from the true meridian, is a consequence of these unequal forces operating upon it, the westerly or easterly tendency of the needle (as the case may be) following the geographical position of the place of observation in its relation to the several foci of force, with a general result of considerable complexity. Up to the sixtieth parallel of latitude, north or south, the declination, whether easterly or westerly, rarely exceeds 30°; and, speaking generally, it is easterly in the Pacific and westerly in the Atlantic and Indian Oceans, Near the poles, where the dip becomes high, the directive force of the earth's magnetism becomes much reduced, and the magnetic needle becomes comparatively unreliable and of little use. The nature and mode of operation of magnetism, and the allied phenomena of electricity, continue to be subjects of speculation, no explanation of them having yet been proposed, such as that which refers heat and light to the vibrations of an elastic medium. Our knowledge of the phenomena of terrestrial magnetism therefore still remains in the empirical stage; they are, however, held to show that the earth's magnetism is distributed through its mass, and that the magnetic force either wholly or mainly resides in the interior, and can not be attributed to external influences, though it may be affected by them. Whether or not geographical features have any influence on the distribution of this force is doubtful. Observation shows that all the elements of the earth's magnetism not only vary from place to place, but from time to time; the variations being in some cases periodical and dependent on the time of the day or the season of the year.