The charts in use of the medieval navigators of the Indian Ocean--Arabs, Persians or Dravidas--were equal in value if not superior to the charts of the Mediterranean. Marco Polo mentions such charts; Vasco da Gama (1498) found them in the hands of his Indian pilot, and their nature is fully explained in the _Mohit_ or encyclopaedia of the sea compiled from ancient sources by the Turkish admiral Sidi Ali Ben Hosein in 1554.[21] These charts are covered with a close network of lines intersecting each other at right angles. The horizontal lines are parallels, depending upon the altitude of the pole star, the Calves of the Little Bear and the Barrow of the Great Bear above the horizon. This altitude was expressed in _isbas_ or inches each equivalent to 1° 42´ 50´´. Each _isba_ was divided into _zams_ or eights. The interval between two parallels thus only amounted to 12´ 51´´. These intervals were mistaken by the Portuguese occasionally for degrees, which account for Malacca, which is in lat. 2´ 13´´ N., being placed on Cantino's Chart (1502) in lat. 14´ S. It may have been a map of this kind which accounts for Ptolemy's moderate exaggerations of the size of Taprobana (Ceylon). A first meridian, separating a leeward from a windward region, passed through Ras Kumhari (Comorin) and was thus nearly identical with the first meridian of the Indian astronomers which passed through the sacred city of Ujjain (Ozere of Ptolemy) or the meridian of Azin of the Arabs. Additional meridians were drawn at intervals of _zams_, supposed to be equal to three hours' sail. In China, maps in the olden time were engraved on bronze or stone, but after the 10th century they were printed from wood-blocks. Among the more important productions of more recent times, may be mentioned a map of the empire, said to be based upon actual surveys by Yhang (721), who also manufactured a celestial globe (an older globe by Ho-shing-tien, 4 metres in circumference, was produced in 450), and an atlas of the empire on a large scale by Thu-sie-pun (1311-1312) of which new enlarged editions with many maps were published in the 16th century and in 1799. None of these maps was graduated, which is all the more surprising as the Chinese astronomers are credited with having made use of the gnomon as early as 1000 B.C. for determining latitudes. [Illustration: FIG. 17.--Globe in Bibliothèque Nationale, Paris] [Illustration: FIG. 18.--The Indian Ocean according to Mohit, as interpreted by Dr Tomaschek.] In the case of Japan, the earliest reference to a map is of 646, in which year the emperor ordered surveys of certain provinces to be made.