408. =The Earth a Magnet.=--You can readily see, from all that has been stated in regard to the magnetic needle, that the earth is a magnet, or has that covered up in it which in some way acts as such. The dip of the needle shows that the two poles of this magnet are somewhere near the north and south poles of the earth. The locality which Captain Ross found must be near the north pole of the magnet in that quarter of the world. The vertical position of the needle there is analogous to the straight lines of iron filings which you see in Fig. 282, near the poles of the magnet; and it is easy also to trace the analogy between the dip of the needle at different distances from what is called the magnetic equator of the earth, where the needle is horizontal, and the curves which you see extending from pole to pole. The different declinations of the needle and the different intensities of the magnetic force in different localities corresponding in latitude show that the magnet in the earth, if there be one, is irregular in shape, or in some way has its power varied much in differed parts of the earth's crust. 409. =The Earth as a Magnetizer.=--As the earth is really a magnet, it might be expected to impart magnetism by induction as other magnets do. And this is found to be the fact. If you hold a bar of soft iron in the direction of the dip of the needle it becomes a magnet, its lower end being the north pole, and its upper the south. That this is so can be ascertained by bringing a small magnetic needle near each end. No effect of this kind is produced when the bar is held horizontally east and west. Lightning-rods, pokers, upright iron bars in fences, etc., are often found to be magnetized because they have continued so long nearly in the required position for magnetization. When a bar of iron has been magnetized in the manner indicated, its magnetism may sometimes be fixed by giving it a stroke with a hammer. It is a curious but inexplicable fact that this vibration of the particles of the iron should have this effect. But though such vibration helps to impart magnetism, it is not at all favorable to its retention, for magnets are always injured by blows or falls, or indeed any rude treatment. For this reason care is requisite in removing an armature from a magnet. If pulled off abruptly the power of the magnet is lessened. 410. =Magnetism in Other Substances besides Iron.=--It was formerly supposed that magnetism was confined to ferruginous substances, but this has been found not to be true. Various minerals are magnetic, especially when they have been heated, also some of the precious stones, and even silica, which enters so largely into some of the rocks of the earth. And it is supposed by some that future investigations will show that the influence of magnetism is as extensive in the earth as that of electricity.