MAGNETIC SURVEY OF THE OCEANS 



By J. A. FLEMING' 



Plates 28, 29, 30 



The large portion of the Earth's surface covered 

 by the oceans makes the determination of accurate 

 values of the magnetic elements at sea a major ob- 

 jective of the world-wide magnetic and electric 

 survey. It was not until 1905 that full realization 

 of this objective had its beginning through the 

 systematic oceanic magnetic survey then sponsored 

 by the Carnegie Institution of Washington through 

 its Department of Terrestrial Magnetism. 



The first attempt to accomplish a magnetic survey 

 at sea was the expedition of Halley between 1698 

 and 1700. He was placed in command of the 

 Paramour Pink and was told by King William III 

 to proceed with her "on an expedition to improve the 

 longitude and the variations of the compass." 

 Halley made several voyages in the North and 

 South Atlantic oceans determining magnetic declina- 

 tion only — instruments for measuring magnetic 

 inclination and magnetic intensity at sea had not 

 then been devised. The results were embodied in 

 Halley's chart "Lines of equal magnetic variation" 

 of the Atlantic for the year 1700— the first isomag- 

 netic chart. The next really important imdertaking 

 was the expedition under the general direction of 

 Sabine of the Erebus, the Terror, and the Pagoda 

 during 1840^5, chiefly in southern waters. On 

 these all three magnetic elements were observed, 

 the Fox dip-circle for measuring the magnetic 

 inclinations and intensity at sea having been just 

 devised. The Austrian frigate Novara measured 

 magnetic declination while circumnavigating the 

 globe in 1857-60. During the notable cruises of the 

 Challenger in 1872-76, and of the Gazelle, a 

 German vessel, in 1874-76, observations of the three 

 magnetic elements were made over various oceans. 

 Magnetic observations at sea were also made more 

 recently by the naval services of various countries 

 and by later Antarctic expeditions, notably the 

 Discovery and the Gauss. The accompanying 

 plates 28 and 29 and fig. 3 show the tracks of chief 



> Director, Department of Terrestrial Magnetism, Car- 

 negie Institution of Washington. 



vessels on which magnetic observations were made 

 during 1839-1916. 



All these observations were of varying degrees of 

 accuracy set by available instruments and by the 

 disturbing factors originating in the magnetic 

 character of the vessels, while their distribution, 

 both as regards position and epoch, was not such 

 as to yield coordinated charts applying to definite 

 periods. Therefore, when planning in 1904 for the 

 magnetic and electric survey of the Earth the 

 Department of Terrestrial Magnetism, Carnegie 

 Institution of Washington, gave careful considera- 

 tion to the oceanic survey. 



The Institution's earliest work at .sea was done 

 with the chartered vessel Galilee during 1905-08. 

 The experience gained during her three cruises 

 proved conclusively that oceanic observations of the 

 magnetic elements sufficient for practical and scien- 

 tific needs could be assured only by a vessel designed 

 specially for such work. The Carnegie was 

 designed in 1908 primarily for magnetic and electric 

 surveys and investigations and her construction and 

 equipment were completed in 1909. The first of the 

 seven cruises of this unique vessel during 1909-29 

 in all oceans was begun in 1909. The theoretical 

 and practical values of the knowledge acquired and 

 of the resulting discussions of the Earth's magnetic 

 and electrical fields are attested by many expressions 

 of appreciation made by the leading hydrographical 

 establishments and by investigators of geophysics 

 in all countries. 



The observational work accomplished before the 

 destruction of the Carnegie by explosion and fire 

 at Apia, Western Samoa, November 29, 1929, was 

 obtained during the seven cruises which aggregated 

 297,579 nautical miles. The data obtained during 

 these cruises and the three previously made by the 

 Galilee, include declination at 3844 points, in- 

 clination and horizontal intensity at 2321 and 2322 

 points, respectively, and atmospheric-electric ele- 

 ments on 1913 days. The extent of the Institution's 

 survey on land and sea is shown by plate 30. 



50 



