Commerce under the Roman emperors was not what it now is, but still was very considerable, and thus united the various provinces together. The most remote countries were ransacked to furnish luxuries for Rome; every year a fleet of one hundred and twenty vessels sailed from the Red Sea for the islands of the Indian Ocean. But the Mediterranean, with the rivers which flowed into it, was the great highway of the ancient navigator. Navigation by the ancients was even more rapid than in modern times before the invention of steam, since oars were employed as well as sails. In summer one hundred and sixty-two Roman miles were sailed over in twenty-four hours; this was the average speed, or about seven knots. From the mouth of the Tiber vessels could usually reach Africa in two days, Massilia in three, and the Pillars of Hercules in seven; from Puteoli the passage to Alexandria had been effected, with moderate winds, in nine days. These facts, however, apply only to the summer, and to favorable winds. The Romans did not navigate in the inclement seasons; but in summer the great inland sea was white with sails. Great fleets brought corn from Gaul, Spain, Sardinia, Africa, Sicily, and Egypt. This was the most important trade; but a considerable commerce was carried on also in ivory, tortoise-shell, cotton and silk fabrics, pearls and precious stones, gums, spices, wines, wool, and oil. Greek and Asiatic wines, especially the Chian and Lesbian, were in great demand at Rome. The transport of earthenware, made generally in the Grecian cities, of wild animals for the amphitheatre, of marble, of the spoils of eastern cities, of military engines and stores, and of horses, required very large fleets and thousands of mariners, which probably belonged chiefly to great maritime cities. These cities with their dependencies required even more vessels for communication with one another than for Rome herself,--the great central object of enterprise and cupidity.