752 COSMICAL PHYSICS 



ject of magnetic storms or perturbations and the connection with solar 

 phenomena, is one in which the investigation, both observationally 

 and theoretically, is merely in its pioneer stage. Although good work 

 has already been done in this direction, a number of carefully and 

 comprehensively conducted experimental and theoretical investi- 

 gations will be needed before this subject is thoroughly understood, 

 and before any theory, however ingeniously it may be worked out, 

 will be entitled to full credence and final adoption. 



Another cogent illustration of the lack of the requisite data 

 pertains to the distribution of the magnetic forces over the earth. 

 Considering the earth as a whole, very little advance in our know- 

 ledge of the distribution of the earth's permanent magnetism was 

 made during the second half of the past century. Chiefly on this 

 account, it is found that the accuracy of the determination of the 

 magnetic potential of the earth has in no wise been increased by 

 the most refined and elaborate of the modern calculations. We 

 appear to know the numerical coefficients entering into the Gaussian 

 potential expression about as accurately for Sabine's magnetic 

 charts (1840-45) as for Neumayer's (1885). Such an important 

 question as whether the earth, like any other magnet, is gradually 

 losing its magnetism or not cannot be definitely answered, because 

 of the lack of sufficient and accurate data. Recent calculations 

 based on all the observations at hand would apparently yield the 

 result that the earth is losing at present annually one twenty-four 

 hundredth part of its total magnetic moment, a loss which if con- 

 tinued would reduce the intensity of the magnetization of the earth 

 to one half its present amount in sixteen hundred and sixty years. 

 However, the data, as stated, are not sufficient to make safe this 

 assertion. 



A case in which a large amount of valuable observational data 

 have been collected, but of which the analysis and discussion have 

 not as yet been made with that completeness and thoroughness the 

 subject demands, is that of the diurnal variation. And so we might 

 go on; suffice it to say that it appears to be the specific task of this gen- 

 eration to bring together the great facts concerning the earth's magnetism 

 and to formulate them as far as possible in such language that clear, 

 concise, and decisive deductions of theory may be made, if not by us, then 

 by our successors. 



After these introductory, general remarks, let us briefly turn our 

 attention to a concrete occurrence of a magnetic phenomenon des- 

 tined to play an important r61e in the physics of the earth. This is 

 a particularly fortunate example, as it is of decided interest to several 

 of the departments into which 'Physical Science has been grouped 

 by this Congress. 



