Fig. 15. Showing oceanic areas (shaded) between parallels of 60° north and 

 south latitude for which secular variation of magnetic elements could not 

 be controlled bv land stations on continents and islands 



Fig. 16. Amount of extrapolation, in minutes of arc, from last survey-value per 30° -tessera to epoch 

 1945, magnetic declination (east declination positive), basis isopors 1920-1925 



57 



