The value of the earth's magnetism is seen most in the science of navigation. A magnetic needle is only a slender permanent magnet suspended very delicately, and when not under local influence it points north and south on the magnetic axis. The law of its action may be explained as follows: Take a straight bar magnet of fairly good power and suspend a magnetic needle over it. The needle will arrange itself parallel to the bar magnet. The north pole of the needle will point toward the south pole of the bar magnet. In the presence of the magnet the needle is not affected by the earth, but yields to a superior force. If, however, the bar magnet is taken out of the way of the needle it will immediately arrange itself north and south. Of course if the earth's magnetic axis changes the needle will vary with it. This variation is uniform and in navigation is reduced to a science, so that the mariner knows how much to allow for the variation. Columbus, as heretofore mentioned, was supposed to have first noticed this variation and it made him trouble. He did not know how to account for it, and as his crew thought the laws of nature were changing because they were so far from home he saw the necessity for some sort of explanation. So, like the brave man that he was, he hatched up a theory that satisfied the crew, and although in the light of the closing years of the nineteenth century it was a questionable one, it worked well enough in practice to serve his purpose. We have already stated that the earth was a great magnet, and that probably it was an electromagnet, caused by earth currents circulating around the globe. You want to know how the earth can be a magnet unless it has an iron core like an electromagnet. Magnetism or magnetic lines of force may be developed without the presence of iron. When we pass a current of electricity through a wire, magnetic lines of force are thrown out at right angles with the direction of the current. This will be fully explained further on. If we wind the wire into a coil, or helix, these magnetic lines are concentrated. If now we suspend this helix, or, better, float it on water so that it can move freely, and pass a current of electricity through it, the helix will arrange itself north and south the same as a magnetic needle. Its attractive properties are feeble in comparison with that of the iron, but it obeys the laws of a magnet. The earth is probably a magnet of this kind, consisting mostly of lines of force.