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fidence. At the same time his information concerning many parts of the earth, whether owing to their remoteness or the conflicting accounts of travellers regarding them, was imperfect in the extreme. The extent, however, of his geo^aphical knowledge was far' greater than that possessed by any of his predecessors, and he had access to sources of information which enabled him to correct many of the errors into which they had, fallen.
He was induced to undertake the composition of his Geography through his being dissatisfied more or less with all the existing systems. There was however one work — that of his immediate • precursor, Marinos of Tyre — ^which approximated somewhat closely to his ideal, and which he therefore made the basis of his own treatise. Marines, he tell us, had collected his materials with the most praiseworthy diligence, and had moreover sifted them both with care and judgment. He points out, however, that his system required correction both as to the method of delineating the sphere on a plane surface, and as to the computation of distances, which he generally exaggerated. He censures him likewise for having assigned to the known world too great a length from west to east, and too great a breadth from north to south.
Of Ptolemy's own system, the more prominent characteristics may now be noted : He assumed the earth to be a sphere, and adopting the estimate of Poseidonios fixed its circumference at 180,000 stadia, thus making the length of a degree at the equator to be only 500 stadia, instead of 60O, which
