On Thursday, the 13th of September, he notes that the ships were encountering adverse currents. He was now three degrees west of Flores, and the needle of the compass pointed as it had never been observed before, directly to the true north. His observation of this fact marks a significant point in the history of navigation. The polarity of the magnet, an ancient possession of the Chinese, had been known perhaps for three hundred years, when this new spirit of discovery awoke in the fifteenth century. The Indian Ocean and its traditions were to impart, perhaps through the Arabs, perhaps through the returning Crusaders, a knowledge of the magnet to the dwellers on the shores of the Mediterranean, and to the hardier mariners who had pushed beyond the pillars of Hercules, so that the new route to that same Indian Ocean was made possible in the fifteenth century. The way was prepared for it gradually. The Catalans from the port of Barcelona pushed out into the great Sea of Darkness under the direction of their needles, as early at least as the twelfth century. The pilots of Genoa and Venice, the hardy Majorcans and the adventurous Moors, were followers of almost equal temerity. A knowledge of the variation of the needle came more slowly to be known to the mariners of the Mediterranean. It had been observed by Peregrini as early as 1269, but that knowledge of it which rendered it greatly serviceable in voyages does not seem to be plainly indicated in any of the charts of these transition centuries, till we find it laid down on the maps of Andrea Bianco in 1436. It was no new thing then when Columbus, as he sailed westward, marked the variation, proceeding from the northeast more and more westerly; but it was a revelation when he came to a position where the magnetic north and the north star stood in conjunction, as they did on this 13th of September, 1492. As he still moved westerly the magnetic line was found to move farther and farther away from the pole as it had before the 13th approached it. To an observer of Columbus's quick perceptions, there was a ready guess to possess his mind. This inference was that this line of no variation was a meridian line, and that divergence from it east and west might have a regularity which would be found to furnish a method of ascertaining longitude far easier and surer than tables or water clocks. We know that four years later he tried to sail his ship on observations of this kind. The same idea seems to have occurred to Sebastian Cabot, when a little afterwards he approached and passed in a higher latitude, what he supposed to be the meridian of no variation. Humboldt is inclined to believe that the possibility of such a method of ascertaining longitude was that uncommunicable secret, which Sebastian Cabot many years later hinted at on his death-bed.