Geographical Maps and Sailing Charts. 



259 



Ability to follow the tracks of great circles and to make all 

 kinds of measurements on maps is of course a most important 

 feature, and figure 10 gives the partial solution of the problem 

 of navigating a ship by great circle sailing from the Hawaiian 

 Islands, H, to New York by way of the Strait of Magellan, 

 J£. The figure represents a map (the original was 14 cm diam- 

 eter) with a stereographic protractor over it, so adjusted as to 



Stereographic Protractor adjusted for finding the great circle track from 

 Honolulu. H, to the Strait of Magellan, M, and measuring the distance. 



bring H and M on the same great circle, indicated by the 

 pointer. The distance is determined by the small circles ; 

 thus, starting from the top, or zero of the protractor, .ZTis at 

 about 43°, and M at 146° from the zero-point, and the dif- 

 ference, 103°, is the distance, The great circle track from 

 which bearings may be taken is the circular arc HM. After 

 traversing the Strait, portions of three great circles may be 

 followed, shown in figure 11 ; thus, from M to a, from a to b, 



