The line of separation designated was the meridian of no variation of the compass-needle. The existence of such a line had been discovered by the same Christopher Columbus who was to thrill the world a few years later; but he did not know, what only experience developed, that this meridian was changeable, swinging many degrees east and then returning west in the course of two or three centuries. At that time the line seemed fixed some three hundred miles west of the Azores, and philosophers accounted for it later by a theory that it lay in the middle of the Atlantic because there it was subject to an equality of attraction toward both continents which held it steady. This was not true, but it was better than the less learned but more popular explanation of the magnetism of the compass—namely, that it was “an effluvium from the root of the tail of the Little Bear.” A year later, however (June 7, 1494), the treaty of Tordesillas, between Spain and Portugal, declared that the line of demarcation should be the meridian 370 leagues west of the Cape Verd Islands, or as nearly as possible in the center of the Atlantic. The supposition that there might be valuable lands within, that is, east of, that limit, inspired several of Portugal’s subsequent searchers. [Illustration: DRAWN BY HENRY B. SNELL. “THE SEA-ROAD TO INDIA AND CATHAY.”] In 1497 King Emmanuel’s expedition was ready to sail—the largest and best equipped, probably, that had ever been sent out by any government, and its commander was Vasco da Gama, a young naval officer of renown. His fleet consisted of four vessels,—small caravels, of course, one of which was commanded by Dias,—and left the Tagus, after ceremonious farewells, in July. Da Gama stopped at various places, but reached and safely rounded the stormy cape in November. He had with him the information (and some say an Arabic map) sent home by Covilho, but his business was not to verify this, but to reach India and establish new Portuguese possessions. Why, then, did he not strike straight across from Cape Agulhas, as East Indiamen have done ever since? For the good reason that he had no guide, no means of finding his way across the southern ocean, where all the stars were strange; for sun observations for latitude were then unknown to European navigators, and rarely used on land. Instead of this, he was obliged to turn northward and skirt the coast for a thousand miles, stopping here and there, until he had passed far enough north of the equator to bring above the horizon the familiar home stars, for which he had “tables.”