Dr. Gilbert, as announced by himself, began by studying the various ores of iron, and soon found that it is only the black iron ore which is naturally magnetic. He made many experiments with the magnetic needle, which had been used before his time by the Italians and the Arabs, and long before them again by the Chinese. He was the first to show that the natural magnet, or lodestone, is not the only possible form of magnet, and that a bar of iron may be made into a magnet by simply hanging it in the direction in which the magnetic needle points, or by heating it, and hammering it while holding it in the same direction. He considerably increased the power of the natural magnet by providing it with an iron cap, or armature, at each end. [Illustration: Fig. 18.--Magnetising a Bar of Iron by hammering.] Dr. Gilbert disposed of many mistakes made by previous investigators. One of these was that, in order to preserve its strength, the lodestone had to be fed on iron filings. They tried to test it by taking a lodestone of a certain weight and burying it in iron filings, which they had also weighed carefully. After leaving it for many months they took out the stone and weighed it, and thought they found the stone a little heavier, and the filings a little lighter. The difference, however, was so small that they doubted whether there was any. Dr. Gilbert said that some of the filings might easily have stuck to the stone and made it appear heavier. In any case there was no evidence that the stone had to absorb any nourishment to keep up its strength. Another mistake was committed by Paracelsus, who thought that the power of the magnet could be increased ten times by heating it nearly to red heat, and slaking it in oil of saffron. In this way, he said, a lodestone could draw a nail out of a wall, and accomplish many other wonderful things which are not possible for any ordinary lodestone. This, however, was just the reverse of the truth. Dr. Gilbert found that a lodestone treated in that manner does not gain power, but actually suffers a certain loss of strength. It was Dr. Gilbert who first proved that the strongest magnetic force is found at two opposite points of magnetic bodies. These points he called the Poles. He proved that two poles which point in the same direction, when the magnet is free to move, repel each other, and those which point in the opposite directions when suspended attract each other. [Illustration: Fig. 19.--The Earth as a Magnet.] Magnets may be of very different shapes, and whatever shape a piece of iron may have it can be magnetised, and when fully magnetised it will show two poles. Dr. Gilbert asked himself whether a magnet could have the shape of a sphere. He cut a sphere out of iron, like a cannon ball, and magnetised it by touching and rubbing it with a lodestone. Having done that he took a small magnetic needle and brought it near the magnetic sphere of iron.