54 



INTERNATIONAL ASPECTS OF OCEANOGRAPHY 



air.] The maximum control so effected would 

 result from 150 secular-variation stations along the 

 coasts of the continents and on islands; about 90 

 of these have been occupied by the Carnegie Institu- 

 tion of Washington one or more times during 1905 

 to 1937, but the remainder include the more inac- 

 cessible islands of the oceans and are subject, gen- 

 erally, to magnetic local disturbance. Such dis- 

 turbance introduces uncertainties both in the 

 effects upon secular-variation changes and in the 

 relation between the normal and the island value, 

 even though the inaccessibility of stations insures 

 possibility of exact reoccupations. The reduction 

 to common epoch would be more difficult because of 

 the length of intervals between reoccupations and 



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 discussion actually make these areas greater than 

 indicated in Figure 7.] The need of continued work 

 at sea is emphasized because these areas involve 

 portions of the Earth's surface where there are at 

 present the greatest irregularities in the progressive 

 character of the secular variation, namely in the 

 central and south Atlantic, Indian, north Pacific, 

 east central Pacific, and south Pacific oceans. 



Failure to provide a vessel suited for magnetic 

 and electric observations also would mean that 

 future data for the distribution of the absolute values 



Fig. 7. Distribution of Foci of Rapid Annual Change of the Magnetic Declination, Inclination, and 



Horizontal Intensity, Approximate Epoch 1920-1925 



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 theoretical 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 800 miles. These areas (see fig. 8) 

 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 



of the atmospheric-electric elements would be lim- 

 ited to relatively few stations obtained at relatively 

 great expense since, to eliminate, for short series of 

 observations, the topographic and meteorological 

 conditions at stations on land, only selected points 

 in wide bays or estuaries could be used where it 

 would be possible to observe on floats. Atmos- 

 pheric-electric observations could be obtained on 

 board ordinary vessels and doubtless some of the 

 maritime companies would be ready to permit in- 

 stallation of the special equipment at reasonable 

 cost, but it is not feasible to obtain on such vessels 

 the caliljration-observations required for the de- 

 termination of the necessary reduction-factors nor, 

 despite earnest desire to cooperate, is it possible to 

 control the deck-space and eliminate vitiating 



