46 



WORK OF THE CARNEGIE AND SUGGESTIONS FOR FUTURE SCIENTIFIC CRUISES 



along parallels or ~ -(.magnetic latitude possibly is as 

 ::v!Ch as 30 per c^::t. They are also, in several re- 

 stricts, mutually inconsistent with the known nature of 

 the geomagnetic field, and in high latitudes the singular- 

 ities in field near the poles are not taken into accoimt. 



The danger of using these charts today in mapping 

 is that the pattern of the isopors changes rapidly. For 

 instance, in D there is scarcely the slightest resem- 

 blance between the isopors of 1922 and those of 1942 

 for both Australia and Canada. Uncertainties of this kind 

 may account for relatively large discrepancies in values 

 shown on some recent charts, for instance, that of the 

 British Admiralty for 1942 for vertical intensity, where 

 there is disagreement between obse.'vation and chart of 

 from 20 to 30 milligauss in South Australia. In other 

 v;ords, it is necessary not only to know the pattern of the 

 isopors at a given time but also the trend of change in 

 pattern. This trend can be derived only by mapping the 

 isopors for several different epochs. This in turn re- 

 quires that there be maintained adequate surveys at reg- 

 ular intervals and continuous observation at magnetic 

 observatories. 



The large part of the earth's surface covered by the 

 oceans makes the determination of accurate values of 

 the magnetic elements at sea a major objective of the 

 world-wide magnetic and electric survey. It was not un- 

 til 1905 that full realization of this objective had its be- 

 ginning through the systematic oceanic 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 pink Paramour 

 and instructed to proceed "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 declination only- -instru- 

 ments for measuring magnetic inclination and magnetic 

 intensity at sea at that time had not been devised. The 

 results were embodied in Halley's chart "Lines of equal 

 magnetic variation," of the Atlantic for the year 1700-- 

 the first isomagnetic chart. The next really important 

 undertaking was the expedition under the general direc- 

 tion of Sabine of the Erebus , the Terror , and the Pagoda 

 during 1840 to 1845, 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 just having been devised. The Aus- 

 trian frigate Novara measured magnetic declination 

 while circumnavigating the globe in 1857 to 1860. Dur- 

 ing the notable cruises of the Challenger in 1872 to 1876, 

 and of the Gazelle , a German vessel, in 1874 to 1876, 

 observations of the three magnetic elements were made 

 over various oceans. Magnetic observations at sea also 

 were made more recently by the naval services of vari- 

 ous countries and by later antarctic expeditions, notably 

 the Discovery and the Gauss . The accompanying figures 

 10, 11, and 12 show the tracks of chief vessels on which 

 magnetic observations were made during 1839 to 1916. 



All these observations were of varyingdegreesof ac- 

 curacy set by available instruments and by disturbing fac- 

 tors 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 apply- 

 ing to definite periods. Thus, when planning in 1904 for 

 the magnetic andelectric surveyof the earth, the Depart- 

 ment gave careful consideration to the oceanic survey. 



The Institution's earliest work at sea was done with 

 the chartered vessel Galilee during 1905 to 1908. The 

 experience gained during her three cruises, in total 

 63,834 nautical miles (see table 1), proved conclusively 

 that oceanic observations of the magnetic elements suf- 

 ficient for practical and scientific needs could be as- 

 sured only by a vessel designed specially for such work. 

 The Carnegie was designed in 1908 primarily for mag- 

 netic and electric surveys and investigations, and her 

 construction and equipment were completed in 1909. Be- 

 fore the loss of the Carnegie by explosion and fire at 

 Apia, Western Samoa, November 29, 1929, seven cruises, 

 aggregating 297,579 nautical miles had been made. The 

 data obtained during these cruises and the three previ- 

 ously made by the Galilee , include declination at 3836 

 points, inclination and horizontal intensity at 2321 and 

 2322 points, respectively. The extent of the Institution's 

 survey on land and sea is shown by figure 2. 



Table 1. Summary of magnetic stations at sea, Galilee 

 (3 cruises) and Carnegie (7 cruises), 1905-1929 



Ocean 



Number of 



nautical miles 



traversed 



Number of observed values 



Declination 



Inclination 



and horizontal 



intensity 



Pacific 



Atlantic 



Indian 



213,612 



104,741 



43,060 



2,187 

 1,172 



477 



,308 

 73ia 

 282 



Total 361,413 



3,836 



2,321' 



apius one in H. 



On the side of practical application the increasing 

 use of the oceans in the commerce of nations by sea and 

 air makes the continuation of the survey a matter of in- 

 ternational concern and benefit. Those investigations 

 demanding continuation of the oceanic survey in terres- 

 trial magnetism include, among others, the following. 



(a) Determination of secular variation of progres- 

 sive changes of the earth's magnetic field involving par- 

 ticularly their accelerations which the data accumulated 

 so far indicate cannot be extrapolated reliably over 

 periods as long as five years. A definite control is 

 necessary for a number of epochs to facilitate the inves- 

 tigation of causes producing and governing these pro- 

 gressive changes which, it appears, would be favored by 

 accurate knowledge of their accelerations and distribu- 

 tion. The importance of the determination of secular 

 variation over the oceans may be readily seen by a study 

 of figure 13. Figure 14, showing world distribution of 

 foci of rapid annual change of magnetic declination, also 

 emphasizes the continued need for secular-variation 

 data at sea. 



(b) The study of regions of local disturbance and 

 particularly of those indicated by the wo"k of the Car- 

 negie over "deep-sea" areas including accompanying 

 determination of oceanic depths by sonic -sounding de- 

 vices and of gravity. 



(c) The determination of additional distribution-data 

 in a few large areas not already covered. 



The question arises whether the theoretical require- 

 ments might not be met in a less expensive way than 

 through construction and maintenance of vessels similar 



