THE EVOLUTION OF THE MARINE CHART 33 



If these charts are compared with modern examples the 

 central axis of the Mediterranean is seen to be rotated about 

 10° to the left. It is believed that the magnetic variation in the 

 Mediterranean at this period was approximately ten degrees 

 east, and this suggests that the chart was drawn to bring the 

 line indicating magnetic north into the vertical. 



No contemporary explanation of this system of direction 

 lines has survived, but since navigators were a conservative 

 race, we may take that given by John Rotz, a sixteenth-century 

 expert, in his 'Boke of Hydrographie'. His explanation is some- 

 what complicated, but in essence he instructs the navigator how 

 by the aid of a pair of dividers he can find the line or ray most 

 nearly parallel to the course between any two points on the 

 chart, and then read the correct bearing from the nearest 

 compass rose. This was later done more easily with a parallel 

 ruler. To pre^'ent mistakes the lines were generally drawn in 

 alternating colours. The purpose of this elaborate system of 

 'roses' and radiating lines was therefore to enable a course to 

 be determined rapidly by providing a number of reference 

 points distributed over the chart. It thus became possible to 

 plot a course over a considerable extent of sea, in contrast to 

 the coastwise navigation with the aid of the details in the written 

 portolanos. This was the fundamental difference between the 

 chart and the book of sailing directions. 



It will be noticed that none of these charts is provided with 

 a net of parallels and meridians. In their construction, no 

 account was taken of the sphericity of the earth, the area 

 covered being treated as a plane surface, and the convergence 

 of the meridians neglected. The consequences of this were not 

 serious, owing to the small range in latitude involved. The 

 direction lines therefore approximate to loxodromes (lines of 

 constant bearing). 



It was not until the early sixteenth century that charts 

 vvere provided with a scale of latitudes. As long as European 

 lavigators were working mainly in enclosed waters, and sailing 

 :oastwise from point to point, they had little need to observe 

 atitudes, and in fact even in the seventeenth century it was 

 lot customary for navigators in the Mediterranean to take such 

 observations. When maritime activity passed out of these 



