The third, and by far the most important part of Dr. Vincent’s dissertation, examines what the map contains respecting the termination cf Africa to the south.  On the first inspection of the map it is evident, that the author has not implicitly followed Ptolemy, as he professes to do.  The centre of the habitable world is fixed at Bagdat.  Asia and Europe he defines rationally, and Africa so far as regards its Mediterranean coast.  He assigns two sources to the Nile, both in Abyssinia.  On the east coast of Africa, he carries an arm of the sea between an island which he represents as of immense size, and the continent, obliquely as far nearly as the latitude and longitude of the Cape of Good Hope.  This island he calls Diab, and the termination on the south, which he makes the extreme point of Africa, Cape Diab.

The great object of Mauro, in drawing up this map, was to encourage the Portuguese in the prosecution of their voyages to the south of Africa.  This is known to be the fact from other sources, and the construction of the map, as well as some of the notices and remarks, which are inserted in its margin, form additional evidence that this was the case.  Two passages, as Dr. Vincent observes, will set this in the clearest light.  The first is inserted at Cape Diab; “here,” says the author, about the year 1420, “an Indian vessel, on her passage across the Indian ocean was caught by a storm, and carried 2000 miles beyond this Cape to the west and south-west; she was seventy days in returning to the Cape.”  This the author regards as a full proof that Africa was circumnavigable on the south.

In the second passage, inserted on the margin, after observing that the Portuguese had been round the continent of Africa, more than 2000 miles to the south-west beyond the Straits of Gibraltar; that they found the navigation easy and safe, and had made charts of their discoveries; he adds, that he had talked with a person worthy of credit, who assured him he had been carried by bad weather, in an Indian ship, out of the Indian Ocean, for forty days, beyond Cape Sofala and the Green Islands, towards the west and south-west, and that in the opinion of the astronomer on board, (such as all Indian ships carry,) they had been hurried away 2000 miles.  He concludes by expressing his firm belief that the sea surrounding the southern and south-eastern part of the world is navigable; and that the Indian Sea is ocean, and not a lake.  We may observe, by the bye, that in another passage inserted in the margin, he expressly declares that the Indian ships had no compass, but were directed by an astronomer on board, who was continually making his observations.