[Sidenote: Papal confines of Spain and Portugal.] We have seen that a belief in the immobility of the line of no magnetic variation had led Pope Alexander VI. to establish a perpetual boundary between the Spanish and Portuguese possessions and fields of adventure. That line he considered to be the natural boundary between the eastern and western hemispheres. An accurate determination of longitude was therefore a national as well as a nautical question. Columbus had relied on astronomical methods; Gilbert at a subsequent period proposed to determine it by magnetical observations. The variation itself could not be accounted for on the doctrine vulgarly received, that magnetism is an effluvium issuing forth from the root of the tail of the Little Bear, but was scientifically, though erroneously, explained by Gilbert's hypothesis that earthy substance is attractive--that a needle approaching a continent will incline toward it; and hence that in the midst of the Atlantic, being equally disturbed by Europe and America, it will point evenly between both. [Sidenote: News that Africa might be doubled.] Pedro de Covilho had sent word to King John II., from Cairo, by two Jews, Rabbi Abraham and Rabbi Joseph, that there was a south cape of Africa which could be doubled. They brought with them an Arabic map of the African coast. This was about the time that Bartholomew Diaz had reached the Cape in two little pinnaces of fifty tons apiece. He sailed August, 1486, and returned December, 1487, with an account of his discovery. Covilho had learned from the Arabian mariners, who were perfectly familiar with the east coast, that they had frequently been at the south of Africa, and that there was no difficulty in passing round the continent that way.