MERCATOR, ORTELIUS, AND THEIR SUCCESSORS 113 



makers, though the navigators had a more correct idea. In 

 contrast to the longitudes, the latitudes on the map are quite 

 accurate for western Europe, but towards the north and east 

 errors of 2 to 3 degrees occur. This calculation is typical of the 

 kind of reasoning upon which Mercator had to construct his 

 map. Having accepted Ptolemy's position for Alexandria, he 

 arrived at the positions of the principal points by careful re- 

 search into distances from the best itineraries procurable, 

 paying also particular attention to relative directions, in which 

 he was helped considerably by the marine charts. The results 

 thus obtained he co-ordinated to the best of his ability with 

 the known latitudes of the principal cities. In a note on the 

 map he dismissed the attempts to calculate differences of 

 longitudes by the simultaneous observations of eclipses, for 

 the perfectly sound reason that the precise moment of eclipse 

 is extremely difficult to observe. An error of four minutes in 

 determining this would throw the result out by one degree of 

 longitude. Another of his improvements rendered the *waist' 

 of eastern Europe, between the Baltic and the Black Sea, much 

 more accurately; on earlier maps, it had been far too constricted. 

 On the other hand, the contour of the Black Sea is elongated 

 by several degrees. 



These brief comments will show the general methods of 

 compiling maps of larger areas in the sixteenth and seventeenth 

 centuries, and will suggest the kind of errors that might occur. 

 Maps depended largely upon the labour expended in the 

 cartographer's office in attempting to reconcile a mass of 

 disparate and often conflicting data. Outside Europe the only 

 features in the maps of the continents that were at all reliable 

 were the coastlines, obtained from the marine charts. A partial 

 exception was Asia, though even there knowledge of the 

 interior parts was often antiquated and confused. This con- 

 tinued for long to be the general position, until in the nine- 

 teenth century explorers and travellers were equipped with 

 reasonably accurate instruments for the rapid determination 

 of positions, and the work of precise survey within modem 

 limits of accuracy was gradually extended. Even at the present 

 time, much of the earth's surface is still unmapped to this 

 standard. This is one consideration which should be constantly 



