MAGNETIC WORK OF THE CARNEGIE 



47 



to the Carnegie . A careful study was made by the De- 

 partment following the loss of the Carnegie to determine 

 what might be done in an attempt to control magnetic 

 secular -variation data through observations on land only 

 over the regions between 60° north and 60° south latitude. 

 (Apparently, requisite additional data on land and ocean 

 areas in the polar regions beyond the parallels of 60°-- 

 less than one -seventh of the surface of the globe --can be 

 secured only, as in the past, through or in cooperation 

 with special expeditions by land or air.) The maximum 

 control so effected would result from one hundred and 

 fifty secular-variation stations along the coasts of the 

 continents and on islands; about ninety of these have 

 been occupied by the Department one or more times 

 during 1905 to 1943, but th3 remainder include the more 

 inaccessible islands of the oceans and are subject, gen- 

 erally, to magnetic local disturbance. Such disturbance 

 introduces uncertainties both in the effects on secular- 

 variation changes and in the relation between the normal 

 value and that on the islands, even though the accessi- 

 bility of stations insures possibility of exact reoccupa- 

 tions. The reduction to common epoch would be more 

 difficult because of the length of intervals between reoc- 

 cupations and of the lack of the better distribution of 

 data which would result from observations at sea. The 

 study shows that the regions for which the necessary 

 data for the continued investigations would be lacking 

 are very large even if the complete scheme for control 

 by observations on land could be carried out as based on 

 the assumption that the distribution of secular-variation 

 stations need not be greater than one every eight hun- 

 dred miles. These areas (see fig. 15) approximate 3400 

 by 800 miles in the north Pacific, 3600 by 1500 miles in 

 the east central Pacific, 3600 by 1800 miles in the south 

 Pacific, 600 by 600 miles in the north Atlantic, 2400 by 

 800 miles in the middle north Atlantic, 1900 by 900 

 miles in the west south Atlantic, 1500 by 700 miles in 

 the east Indian, 3600 by 750 miles in the central Indian, 

 and 2400 by 900 miles in the southeast Indian to the south 

 of Australia. (Local disturbances existing at many of 

 the possible stations on islands, which doubtless would 

 make data from a majority of them unsuitable for dis- 

 cussion actually make these areas greater than indicated 

 in figure 15.) The need of continued work at sea is em- 

 phasized because these areas involve parts of the earth's 

 surface where there are at present the greatest irregu- 

 larities in the progressive character of the secular var- 

 iation, namely, in the central and south Atlantic, Indian, 

 north Pacific, east central Pacific, and south Pacific 

 oceans. 



Because of the great desirability of continuing the 

 operation conducted tor a quarter of a century by the 

 vessels of the Carnegie Institution of Washington, it is 

 gratifying that, in view of the Institution's decision not to 

 replace the Carnegie by a similar vessel, the British 

 Admiralty had designed and, in September 1936, placed 

 a contract to build a nonmagnetic vessel, to be named 

 Research. The chief reason for this action on the part 

 of Great Britain was found in her world-wide maritime 

 interests. Magnetic charts published for the last two 

 decades by the American, British, French, German, and 

 other governments for use at sea have been based in an 

 increasingly large degree on data obtained by the Carne- 

 gie . There are serious gaps in the present data which 

 would have been filled had the Carnegie completed her 

 last cruise and had the rapid change in the secular vari- 

 ation in certain regions been determined. One of the 



first tasks, therefore, of the Research was to have been 

 the repetition of the observations of the Carnegie in 

 these regions to determine the secular change so that 

 the isogonic charts might be corrected to date and pre- 

 pared for succeeding epochs. The Research , of the 

 same beam as the Carnegie and slightly greater over-all 

 length, was launched in 1939, but the outbreak of war so 

 far has prevented her operation. The instrumental 

 equipment parallels closely that used on the Carnegie as 

 it did not appear advisable to depart from designs grad- 

 ually evolved from the experience of many years of ob- 

 servational work at sea. With the eventual continuation 

 of the oceanic survey by the Research we may look for- 

 ward to further advance in the accuracy of magnetic 

 charts. 



The task of the geophysical survey of the oceans is 

 so great that other hydrographic services of maritime 

 nations should be stimulated by the action of the British 

 Admiralty to provide similar vessels with equipment and 

 personnel to take appropriate share in the execution and 

 in the coordination of such service. Resolutions adopted 

 after thorough discussions by the Commission of Terres- 

 trial Magnetism and Atmospheric Electricity of the Inter- 

 national Meteorological Organization at Warsaw, Poland, 

 in September 1935, and by the Association of Terrestrial 

 Magnetism and Electricity of the International Union of 

 Geodesy and Geophysics at its triennial assembly at 

 Edinburgh in September 1936, urge and recommend that 

 other maritime nations should consider the construction 

 of such nonmagnetic vessels. It is to be hoped that our 

 own United States may assume its share in obtaining ad- 

 ditional oceanic data to the further enrichment of our 

 knowledge of earth sciences. 



Two of the most important requirements to be met 

 in order that isomagnetic charts for epoch 1945 be of 

 good standard in accuracy are (1) provision of suitably 

 accurate magnetic field observations of sufficiently re- 

 cent date and (2) knowledge of the course of magnetic 

 secular change in the intervening interval of time be- 

 tween the more recent dates of observation and the year 

 1945. Unfortunately the world patterns of secular change 

 are subject to rather rapid modification in form with 

 time. Thus, although observations made thirty years 

 ago, say, can be used to good advantage in certain regions 

 where secular change is small, observations made only 

 five years ago may be inadequate in regions where the 

 secular change is rapid. 



A rough indication of those regions likely to provide 

 defective chart values for 1945 of the general systematic 

 field distributions is afforded by multiplying the annual 

 rate of secular change for 1920 to 1925 by the number of 

 years prior to 1945 of the most recent field observation 

 in a region. Although the secular change for 1920 to 1925 

 sometimes will afford a poor approximation for the en- 

 tire twenty- to twenty-five-year intervals, this has been 

 done using the isoporic charts of figures 3 to 9 and lists 

 of field observations. 



Figures 16 to 18 show the products obtained by the 

 foregoing procedure for the D-, H-, and Z-components, 

 respectively. These tentative results give the amount of 

 the extrapolation from the last measured observation to 

 obtain chart values for 1945. The most striking feature 

 is the magnitude of the necessary extrapolations in the 

 south Atlantic, Pacific, and Indian oceans, and in north 

 central Africa. These estimates are highly conservative 

 because a single observation on land may, by the proce- 

 dure here adopted, yield the estimate for a 30° -tessera 



