The art of navigation, though still crude, had by the 15th century so advanced that the sailor was no longer compelled to skirt the shore, with only rare ventures across open stretches of sea. The use of the compass, originating in China, had been learned from the Arabs by the crusaders, and is first mentioned in Europe towards the close of the 12th century. An Italian in England, describing a visit to the philosopher Roger Bacon in 1258, writes as follows: "Among other things he showed me an ugly black stone called a magnet ... upon which, if a needle be rubbed and afterward fastened to a straw so that it shall float upon the water, the needle will instantly turn toward the pole-star; though the night be never so dark, yet shall the mariner be able by the help of this needle to steer his course aright. But no master-mariner," he adds, "dares to use it lest he should fall under the imputation of being a magician." By the end of the 13th century, the compass was coming into general use; and when Columbus sailed he had an instrument divided as in later times into 360 degrees and 32 points, as well as a quadrant, sea-astrolabe, and other nautical devices. The astrolabe, an instrument for determining latitude by measuring the altitude of the sun or other heavenly body, was suspended from the finger by a ring and held upright at noon till the shadow of the sun passed the sights. The cross-staff, more frequently used for the same purpose by sailors of the time, was a simpler affair less affected by the ship's roll; it was held with the lower end of the cross-piece level with the horizon and the upper adjusted to a point on a line between the eye of the observer and the sun at the zenith. By these various means, the sailor could steer a fixed course and determine latitude.