48 



WORK OF THE CARNEGIE AND SUGGESTIONS FOR FUTURE SCIENTIFIC CRUISES 



mainly of ocean. On land there is most urgent need of 

 resurvey in North Africa and the Pacific islands; all 

 major oceanic areas are in need of magnetic surveys. 



The present slow and costly methods of geomagnet- 

 ic surveys seem likely to be superseded by new tech- 

 niques and methods, the possibilities of which now are 

 gradually becoming apparent. Clearly the old procedure 

 of measuring the earth's field at different times in dif- 

 ferent places, in one region during one series of years 

 and in another in a different series of years, is ineffi- 

 cient. What is required is a description of the geomag- 

 netic field during a given year, based on measurements 

 made during that year at an ordered set of points suffi- 

 ciently close together and spaced in a manner nearly in- 

 dependent of topography, areas of land and sea, and cli- 

 mate. Such measurements made from planes flying along 

 parallels of latitude no doubt will be available in the fu- 

 ture. A project of this kind at present is feasible in- 

 strumentally and affords attractive postwar possibilities 

 in application. 



In spite of the new developments in navigation, the 

 compact and simple compass seems likely to remain in 

 use on the seas for many years, and likewise there are 

 appearing newly found applications of isomagnetic 

 world charts from time to time. The available geomag- 

 netic data already of necessity are squeezed rather hard 

 to obtain useful isomagnetic charts for epoch 1945 in 

 some regions. It is apparent that measurements of the 

 earth's magnetic field must be continued, even if entirely 

 by the more cumbersome methods of the past which at 

 least have the virtue of being tried and tested. The 

 prompt undertaking of even a limited magnetic -survey 

 program, including not only measurements of magnetic 

 declination as at present carried out by the United States 

 Hydrographic Office but also measurements of the other 

 components with carefully standardized instruments, 

 would make available data on which the isomagnetic 

 charts for epoch 1950 may be based. 



LIST OF FIGURES 



Fig. 1(A). Magnetic observatories, Western Hemisphere 



Fig. 1(B). Magnetic observatories, Eastern Hemisphere 



Fig. 2. World magnetic and electric survey, Department of Terrestrial Magnetism, Carnegie Institution of 

 Washington, 1905-1938 



Fig. 3. Isoporic chart for declination (lines of equal annual change), approximate epoch 1920-1925 



Fig. 4. Isoporic chart for inclination (lines of equal annual change), approximate epoch 1920-1925 



Fig. 5. Isoporic chart for horizontal intensity (lines of equal annual change), approximate epoch 1920-1925 



Fig. 6. Isoporic chart for vertical Intensity (lines of equal annual change), approximate epoch 1920-1925 



Fig. 7. Isoporic chart for total intensity (lines of equal annual change), approximate epoch 1920-1925 



Fig. 8. Isoporic chart for north component (lines of equal annual change), approximate epoch 1920-1925 



Fig. 9. Isoporic chart for east component (lines of equal annual change), approximate epoch 1920-1925 



Fig. 10. Tracks of chief vessels on which magnetic observations were made in the Indian Ocean, 1839-1916 



Fig. 11. Tracks of chief vessels on which magnetic observations were made in the Atlantic Ocean, 1839-1916 



Fig. 12. Tracks of chief vessels on which magnetic observations were made in the Pacific Ocean, 1839-1916 



Fig. 13. Variation with longitude of AH/H (annual change averaged without regard to sign), of the distribution of the 

 proportion of land and water areas, and of secular -change activity approximately determined by the dens- 

 ity of the distribution of isoporic lines 



Fig. 14. Distribution of foci of rapid annual change of the magnetic declination, inclination, and horizontal intensity, 

 approximate epoch 1920-1925 



Fig. 15. Showing oceanic areas between parallels of 60° north and south latitude for which secular variation of 

 magnetic elements could not be controlled by 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 



Fig. 17. Amount of extrapolation, in gammas, from last survey-value per 30°-tessera to epoch 1945, magnetic 

 horizontal intensity, basis isopors 1920-1925 



Fig. 18. Amount of extrapolation, in gammas, from last survey-value per 30° -tessera to epoch 1945, magnetic 

 vertical intensity (positive downwards), basis isopors 1920-1925 



