These represent but a part, and indeed but a small part, of the services rendered by the Arabian astronomers, in the solution of the problem of the nature of the world.  Meanwhile, such was the benighted condition of Christendom, such its deplorable ignorance, that it cared nothing about the matter.  Its attention was engrossed by image-worship, transubstantiation, the merits of the saints, miracles, shrine-cures.

This indifference continued until the close of the fifteenth century.  Even then there was no scientific inducement.  The inciting motives were altogether of a different kind.  They originated in commercial rivalries, and the question of the shape of the earth was finally settled by three sailors, Columbus, De Gama, and, above all, by Ferdinand Magellan.

The trade of Eastern Asia has always been a source of immense wealth to the Western nations who in succession have obtained it.  In the middle ages it had centred in Upper Italy.  It was conducted along two lines—­a northern, by way of the Black and Caspian Seas, and camel-caravans beyond—­the headquarters of this were at Genoa; and a southern, through the Syrian and Egyptian ports, and by the Arabian Sea, the headquarters of this being at Venice.  The merchants engaged in the latter traffic had also made great gains in the transport service of the Crusade-wars.

The Venetians had managed to maintain amicable relations with the Mohammedan powers of Syria and Egypt; they were permitted to have consulates at Alexandria and Damascus, and, notwithstanding the military commotions of which those countries had been the scene, the trade was still maintained in a comparatively flourishing condition.  But the northern or Genoese line had been completely broken up by the irruptions of the Tartars and the Turks, and the military and political disturbances of the countries through which it passed.  The Eastern trade of Genoa was not merely in a precarious condition—­it was on the brink of destruction.

The circular visible horizon and its dip at sea, the gradual appearance and disappearance of ships in the offing, cannot fail to incline intelligent sailors to a belief in the globular figure of the earth.  The writings of the Mohammedan astronomers and philosophers had given currency to that doctrine throughout Western Europe, but, as might be expected, it was received with disfavor by theologians.  When Genoa was thus on the very brink of ruin, it occurred to some of her mariners that, if this view were correct, her affairs might be re- established.  A ship sailing through the straits of Gibraltar westward, across the Atlantic, would not fail to reach the East Indies.  There were apparently other great advantages.  Heavy cargoes might be transported without tedious and expensive land-carriage, and without breaking bulk.

Among the Genoese sailors who entertained these views was Christopher Columbus.