[Illustration: FIG. 14.--Diagram to illustrate Magnetic Dip.] Since the Earth is a magnet, we should expect it to be able to induce magnetism in a bar of iron, just as our artificial magnets do, and we can show that this is actually the case. If a steel poker is held pointing to and dipping down towards the north, and struck sharply with a piece of wood while in this position, it acquires magnetic properties which can be tested by means of a small compass needle. It is an interesting fact that iron pillars and railings which have been standing for a long time in one position are found to be magnetized. In the northern hemisphere the bases of upright iron pillars are north poles, and their upper ends south poles, and in the southern hemisphere the polarity is reversed. The most valuable application of the magnetic needle is in the compass. An ordinary pocket compass for inland use consists simply of a single magnetized needle pivoted so as to swing freely over a card on which are marked the thirty-two points of the compass. Ships’ compasses are much more elaborate. As a rule a compound needle is used, consisting of eight slender strips of steel, magnetized separately, and suspended side by side. A compound needle of this kind is very much more reliable than a single needle. The material of which the card is made depends upon whether the illumination for night work is to come from above or below. If the latter, the card must be transparent, and it is often made of thin sheet mica; but if the light comes from above, the card is made of some opaque material, such as very stout paper. The needle and card are contained in a sort of bowl made of copper. In order to keep this bowl in a horizontal position, however the ship may be pitching and rolling, it is supported on gimbals, which are two concentric rings attached to horizontal pivots, and moving in axes at right angles to one another. Further stability may be obtained by weighting the bottom of the bowl with lead. There are also liquid compasses, in which the card is floated on the surface of dilute alcohol, and many modern ships’ compasses have their movements regulated by a gyrostat. The large amount of iron and steel used in the construction of modern vessels has a considerable effect upon the compass needle, and unless the compass is protected from this influence its readings are liable to serious errors. The most satisfactory way of giving this protection is by placing on each side of the compass a large globe of soft iron, twelve or more inches in diameter.