If a magnetized needle is suspended so that it is free to swing either horizontally or vertically, it not only comes to rest in a north and south direction, but also it tilts with its north-pointing end downwards. If the needle were taken to a place south of the equator it would still tilt, but the south-pointing end would be downwards. In both cases the angle the needle makes with the horizontal is called the _magnetic dip_. [Illustration: PLATE III. (_a_) LINES OF MAGNETIC FORCE OF TWO OPPOSITE POLES. (_b_) LINES OF MAGNETIC FORCE OF TWO SIMILAR POLES.] It is evident that a suspended magnetized needle would not invariably come to rest pointing north and south unless it were compelled to do so, and a little consideration shows that the needle acts as if it were under the influence of a magnet. Dr. Gilbert of Colchester, of whom we spoke in Chapter I., gave a great deal of time to the study of magnetic phenomena, and in 1600 he announced what may be regarded as his greatest discovery: _The terrestrial globe itself is a great magnet_. Here, then, is the explanation of the behaviour of the magnetized needle. The Earth itself is a great magnet, having its poles near to the geographical north and south poles. But a question at once suggests itself: “Since similar poles repel one another, how is it that the north pole of a magnet turns towards the north magnetic pole of the earth?” This apparent difficulty is caused by a confusion in terms. If the Earth’s north magnetic pole really has north magnetism, then the north-pointing end of a magnet must be a south pole; and on the other hand, if the north-pointing end of a magnet has north magnetism, then the Earth’s north magnetic pole must be really a south pole. It is a troublesome matter to settle, but it is now customary to regard the Earth’s north magnetic pole as possessing south magnetism, and the south magnetic pole as possessing north magnetism. In this way the north-pointing pole of a magnet may be looked upon as a true north pole, and the south-pointing pole as a true south pole. Magnetic dip also is seen to be a natural result of the Earth’s magnetic influence. Here in England, for instance, the north magnetic pole is much nearer than the south magnetic pole, and consequently its influence is the stronger. Therefore a magnetized needle, if free to do so, dips downwards towards the north. At any place where the south magnetic pole is the nearer the direction of the dip of course is reversed. If placed immediately over either magnetic pole the needle would take up a vertical position, and at the magnetic equator it would not dip at all, for the influence of the two magnetic poles would be equal. A little study of Fig. 14, which represents a dipping needle at different parts of the earth, will make this matter clearer. N and S represent the Earth’s north and south magnetic poles, and the arrow heads are the north poles of the needles.