Roger Bacon's Opus Majus.
A hundred years before Columbus, the inheritor of much of this conservation was the Bishop of Cambray, that Pierre d'Ailly whose Imago Mundi (1410) was so often on the lips of Columbus, and out of which it is more than likely that Columbus drank of the knowledge of Aristotle, Strabo, and Seneca, and to a degree greater perhaps than he was aware of he took thence the wisdom of Roger Bacon. It was through the Opus Majus (1267) of this English philosopher that western Europe found accessible the stories of the "silver walls and golden towers" of Quinsay as described by Rubruquis, the wandering missionary, who in the thirteenth century excited the cupidity of the Mediterranean merchants by his accounts of the inexhaustible treasures of eastern Asia, and which the reader of to-day may find in the collections of Samuel Purchas.
Pierre d'Ailly's position in regard to cosmographical knowledge was hardly a dominant one. He seems to know nothing of Marco Polo, Bacon's contemporary, and he never speaks of Cathay, even when he urges the views which he has borrowed from Roger Bacon, of the extension of Asia towards Western Europe.
Any acquaintance with the Imago Mundi during these days of Columbus in Portugal came probably through report, though possibly he may have met with manuscripts of the work; for it was not till after he had gone to Spain that D'Ailly could have been read in any printed edition, the first being issued in 1490.
Rotundity and gravitation.
The theory of the rotundity of the earth carried with it one objection, which in the time of Columbus was sure sooner or later to be seized upon. If, going west, the ship sank with the declivity of the earth's contour, how was she going to mount such an elevation on her return voyage?—a doubt not so unreasonable in an age which had hardly more than the vaguest notion of the laws of gravitation, though some, like Vespucius, were not without a certain prescience of the fact.
Size of the earth.
By the middle of the third century before Christ, Eratosthenes, accepting sphericity, had by astronomical methods studied the extent of the earth's circumference, and, according to the interpretation of his results by modern scholars, he came surprisingly near to the actual size, when he exceeded the truth by perhaps a twelfth part. The calculations of Eratosthenes commended themselves to Hipparchus, Strabo, and Pliny. A century later than Eratosthenes, a new calculation, made by Posidonius of Rhodes, reduced the magnitude to a globe of about four fifths its proper size. It was palpably certain to the observant philosophers, from the beginning of their observations on the size of the earth, that the portion known to commerce and curiosity was but a small part of what might yet be known. The unknown, however, is always a terror. Going north from temperate Europe increased the cold, going south augmented the heat; and it was no bold thought for the naturalist to conclude that a north existed in which the cold was unbearable, and a south in which the heat was too great for life. Views like these stayed the impulse for exploration even down to the century of Columbus, and magnified the horrors which so long balked the exploration of the Portuguese on the African coast. There had been intervals, however, when men in the Indian Ocean had dared to pass the equator.
Unknown regions.
Strabo and Marinus on the size of the earth.