MAGNETIC SURVEY OF THE OCEANS 



53 



only where this layer is present. Thus continued 

 secular-variation surveys at sea should bring to- 

 gether seismic and magnetic methods of approach 

 to crustal adjustments and possibly gravimetric 

 work. 



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 international concern and benefit. 



Those theoretical investigations demanding con- 

 tinuation of the oceanic survey in terrestrial mag- 

 netism include, among others, the following: 



(a) Determination of secular-variation of progressive 

 changes of the Earth's magnetic field involving particularly 

 their accelerations which the data accumulated so far indi- 

 cate can not be extrapolated reliably over periods as long 

 as five years. A definite control is necessary for a number 

 of epochs to facilitate the investigation of causes producing 



during the cruises of the Carnegie is desirable in 

 several directions. Among these are the following: 



(a) Additional determinations to establish changes in the 

 values of the atmospheric-electric elements with geographic 

 position. Such distribution-data are necessary for the 

 further investigations of the origin and maintenance of the 

 Earth's electric charge and of the relations to its magnetic 

 condition. 



(b) More and widely distributed determinations of the 

 diurnal variations in atmospheric electricity particularly 

 to confirm the discovery that such variations in the potential 

 gradient progress with universal time — a deduction first 

 indicated from results obtained on the Carnegie. Condi- 

 tions at sea for such work are superior to those on land 

 where variable meteorological disturbances and topography 

 mask the true characteristics of the phenomena. 



(c) Determinations and investigations of earth-currents — 

 a field not yet touched at sea. Two outstanding character- 

 istics of the water-area of the globe are (1) its extent and (2) 

 its far greater homogeneity as compared to the land-area. 



Fig. 6. Variation with Longitude of t^H/H (annual change averaged without regard to sign), of the Distribdtionof 



THE Proportion of Land and Water Areas, and of Secular-Change Activity Approximately Determined 



BY the Density of the Distribution of Isoporic Lines 



and governing these progressive changes which, it appears, 

 would be favored by accurate knowledge of their accelera- 

 tions and distribution. The importance of the determina- 

 tion of secular-variation over the oceans may be readily 

 seen by a study of figure 6. Figure 7 showing world dis- 

 tribution 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 particu- 

 larly of those indicated by the work of the Carnegie over 

 "deep-sea" areas including accompanying determinationjof 

 oceanic depths by sonic-sounding devices and of gravity. 



(c) The determination of additional distribution-data in 

 a few large areas not already covered. 



As regards the domain of terrestrial electricity 

 continuation of the survey of the oceans initiated 



The question arises whether the theoretical 

 requirements might not be met in a less expensive 

 way than through construction and maintenance of 

 vessels similar to the Carnegie. A careful study 

 was made by the Department after 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 oceans 

 between 60° north and 60° south latitude. [In 

 any case 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 



