XVI.] HIS SYSTEM OF PROJECTION. 343
the south of Zanzibar. As the interval between these is 70°,
Ptolemy?s computation in this case exceeded the reality by 10°,
In connexion with the extension thus assigned to the known
world from west to east and from south to north respectively, we
may remark that Ptolemy (or Marinus) is the first writer who uses
the Greek words for ' length' and ' breadth' (/I^KOS and TrAarog) in
the technical sense of ' longitudeJ and ' latitude,' ie. to signify the
distance of a place from a fixed meridian line, or from the
equator.
In cartography the great advance which Ptolemy made on the
work of his predecessors consisted in his system of
projection, which in many respects approximates
to that which is in general use at the present day.
We have seen that former geographers, including Marinus, had
drawn the parallels of latitude and meridians of longitude in
straight lines, parallel to one another; and in his special maps of
the separate countries Ptolemy continued to do this, because, when
the area was limited, the inaccuracy thus produced was of small
importance. But in a general map of the whole known world he
recognised that the error arising from this cause was very great,
and that it was necessary to make allowance for the spherical
character of the earth, and for the inclination of the meridians to
one another. With a view to this he represented the lines of
latitude by parallel curves, while, in order to avoid too elaborate a
scheme, he represented the meridians of longitude in the first
instance by straight lines, converging towards a point outside
the limits of the map. Subsequently, however, he reduced the
meridians also to a curved form, so as to make them correspond
more nearly with the reality. From intimations which are found
in various writers it seems probable that Hipparchus in some
degree anticipated this method, but there is no reason to believe
that he constructed any such complete scheme as is found in
Ptolemy. The map on which this network of lines was drawn
was not in shape a perfect hemisphere, because it represented the
portion of the globe then known; and this, while it extended some
distance south of the equator, did not include the regions about
the pole. The elimata of Ptolemy, which also were marked on
his map, were—like those spaces on the surface of the globe to