80 MAPS AND THEIR MAKERS 



it was the practice to sail southwards as rapidly as possible to 

 the necessary parallel of latitude and then to turn eastwards, 

 keeping as close as possible to the parallel ('running down the 

 easting'). If through navigational errors the African coast were 

 sighted to the north of the Cape, it was a simple matter to coast 

 southwards. To assist in determining their latitude, navigators 

 were provided with tables, known as 'Rules of the Leagues', 

 which simply stated the number of leagues it was necessary to 

 sail on various bearings in order to make good one degree of 

 latitude, north or south (i.e. the hypotenuse of a right-angled 

 triangle, of which one other side equals 1° or 70 miles). 



Since the astronomical determination of longitude was a 

 process of great complexity before the invention of accurate 

 time-keepers, all east-west distances depended upon dead- 

 reckoning alone. From the courses and distances run, it was 

 possible by the application of the 'Rules' to calculate each day's 

 sailing and ultimately the total voyage. Given the length of a 

 degree of longitude at various latitudes, it was possible to 

 arrive at an approximate figure for the difference of longitude. 

 It is necessary to keep these considerations in mind when 

 discussing the accuracy of the charts which record the great 

 discoveries. 



All these maritime achievements, in the east and in the 

 west, were accomplished within thirty-five years, and it might 

 be expected that the cartographic output for this period vvould 

 be large. Actually, despite the momentous events to be recorded 

 it is not great — or to be more accurate, the material which 

 has survived is relatively slight. 



No original chart from the period 1487 to 1500 has been 

 preserved. The nearest is the copy of a chart of the western 

 African coasts to the neighbourhood of the Cape of Good 

 Hope contained in the collection generally known by the name 

 of the copyist, Soligo, which was probably made about 1490 

 (B.M. Egerton 73). The representation of Africa in the globe 

 of Martin Behaim and the map of Henricus Martellus may 

 possibly be based on contemporary charts at second or third 

 hand — but otherwise this decade is cartographically a blank. If 

 the period is extended to 1510, the number of survivals is still 

 relatively small; the more important are the world charts, or 



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