438 Reports on Special Researches 



GREATER PROBLEMS OF THE EARTH'S MAGNETISM. 



Investigations relating to the settlement, as far as possible, of some of the outstanding 

 questions of fundamental importance to theories concerning the origin of the Earth's 

 magnetic field are in progress. Final reports and announcement of definite results must 

 be deferred, however, until the accumulated magnetic data on land and sea have all been 

 referred to the same date. 



One of the so-called "greater problems of the Earth's magnetism" is the determina- 

 tion of the various systems of magnetic forces which together make up the total terrestrial 

 magnetic field as observed on the Earth's surface. From previous mathematical analyses 

 of the Earth's magnetic field, notably that of Adolf Schmidt, it would appear that the 

 major portion (about 95 per cent) of the field must be ascribed to systems of magnetic or 

 electric forces below the Earth's surface. The remaining portion (about 5 per cent) of the 

 field may have to be ascribed to systems of forces, capable of producing magnetic effects, 

 which exist in the regions above the surface. These external systems may, for example, 

 consist partly of electric currents circulating overhead, parallel to the Earth's surface, and 

 partly to systems of electric currents passing perpendicularly through the surface. The 

 latter external system is a so-called "non-potential system"; the definite determination 

 of its existence, or non-existence, is a matter of interest and unportance with reference both 

 to the subjects of terrestrial magnetism and atmospheric electricity. The solution involves 

 the computation of the Une-integral of the magnetic force around a closed path on the 

 Earth's surface. An inspection of Plate 20, or of Plates 23-25, will show how the cruises 

 of the Galilee and the Carnegie have been executed with the special view of having numerous 

 closed circuits, comprising both large and small areas. The desired line-integrals may, 

 therefore, be computed, when all data have been reduced to a conmion date, for areas in 

 various parts of the Earth, and also for parallels of latitude completely around the Earth. 



The accurate determination of the first of the external systems mentioned (that 

 possibly caused by overhead electric currents), will be of fundamental importance, espe- 

 cially, if it should prove possible also to ascertain definitely how the system changes with 

 lapse of time. 



Owing to the inaccuracies of the magnetic charts, or of the magnetic data on which 

 previous investigations have been based, much uncertainty prevails as to the precise 

 reUabiUty of the conclusions reached by past investigators. Thus Schuster, when referring 

 recently to the solution of some of the vexed questions, says: "This demands a more 

 accurate survey of the Earth as a whole than we possess at present, and we look forward to 

 the magnetic survey of the Carnegie Institution of Washington for the required data." 



Some studies have likewise been made of the causes which produce the manifold com- 

 plexities of the Earth's magnetic field— what forces, for example, cause the geographic 

 departures from the simple or uniform type of field. It appears that these "geographic 

 variations," represented by the higher harmonics of the potential expression used to express 

 mathematically the major portion of the terrestrial field, are not of the heterogeneous 

 character they would be if caused chiefly by the distribution of land and water, or by lack 

 of homogeneity in the constitution of the Earth. 



The precise characteristics of the phenomenon of the secular variation of terrestrial 

 magnetism is of fundamental importance in connection with theories of the origin of the 

 Earth's magnetic field. The definite limitations unposed by the variations in the Earth's 

 magnetic field, both of the periodic and aperiodic kind, and the departures of the field 

 from the sunple uniform type, are too frequently overlooked. Most theories, for example, 

 are found inadequate when the attempt is made to explain, besides the origin of the field, 

 the secular variation, as it is actually observed. 



