MAGNETIC SURVEY OF THE OCEANS 



51 



While more information on secular-variation 

 changes in the Earth's magnetism is required for 

 navigation, yet future magnetic and electric data 

 over the oceans are far more necessary to advance 



continue the work of the Carnegie because further 

 surveys of like accuracy will enhance the theoretical 

 value of the work already done. As an example, 

 attention may be called to the apparent diminution 



Tracks of Chief Vessels on Which Magnetic Observations Were Made in the Indian Ocean. 1839-1916 



r.mcgtc, 1911-1916 -^ 

 Novara, 1857-1860 • 



C.u«. 1902-1903 



Erebus and Terror 1839-1843 ■ 



Challenger, 1872-1876 



Dijcovery. 1902-1904 



Pagoda. 1845 



Gazelle. 1874-1876"— ^— 



Fig. 3 



theoretical studies. The full value of magnetic of the intensity of the Earth's magnetic field dis- 



results of the few earlier expeditions under various covered by the Department's investigations of the 



governments has never been attained because of the data thus far obtained, this dimunition being marked 



shortness of the cruises. It is of first importance to over oceanic areas, especially in the Southern 



