November 11, 1904.] 



SCIENCE. 



635 



cist, the mathematician, the geologist or 

 the astrophysicist has presented to him 

 the problems of the earth's magnetism con- 

 cerning him specially, definite advance 

 along certain lines may be confidently ex- 

 pected. The point then made is that the 

 successful solution of some of the vexing 

 problems of the earth's magnetism, in this 

 day of rapid advances in experimental re- 

 search, can not be attempted by one indi- 

 vidual ; he must associate with him experts 

 in several of the older, fundamental sci- 

 ences and have at his command a staff of 

 computers. It must, hence, be a source of 

 great gratification that this dream has been 

 realized in the establishment by the Car- 

 negie Institution of a Department of In- 

 ternational Research in Terresi i-il Mag- 

 netism, with facilities for adequately and 

 exhaustively collecting, collating, supple- 

 menting and discussing magnetic data. 

 With such means, let us hope that before 

 very long we may be able to present a more 

 favorable report on the state of our knowl- 

 edge regarding the earth's magnetism, than 

 the one which can be given now. 



One of the most fundamental inquiries 

 to be made in the discussion of any of the 

 earth's magnetic phenomena, before at- 

 tempting a theoretical explanation, is as 

 to the seat of the forces giving rise to the 

 phenomenon in question. Thiis many a 

 theorist might have saved himself some 

 pains had he first addressed himself to this 

 inquiry. To illustrate : 



Suppose our first question to be the fol- 

 lowing: Since we can produce the mag- 

 netic phenomena pertaining to the earth's 

 so-called 'permanent' magnetism, observed 

 on the surface, by a system of closed elec- 

 tric currents, where are these currents? 

 Do they circulate around the earth below 

 the surface or in the regions above us? 

 "We know, as a fact of common experience, 

 that the end of the needle designated as 



the north-seeking end, or for short, the 

 north end, points approximately towards 

 the north. Hence, applying Ampere 's rule, 

 the electric currents necessary to produce 

 this phenomenon must circulate around the 

 earth from east to west, if they be inside 

 the earth, and if they are, on the other 

 hand, outside the earth, they must circulate 

 from west to east. To determine where 

 the currents really are we must resort to 

 another well-known phenomenon, viz., that 

 the end of the needle which points to the 

 north dips below the horizon in our hemi- 

 sphere and points above the horizon in the 

 southern magnetic hemisphere. Applying 

 to this phenomenon Ampere 's rule, we shaU 

 find that the currents can only circulate 

 from east to west, hence combining this 

 deduction with our previous one, the an- 

 swer is that the electric currents which are 

 capable of producing the observed mag- 

 netic phenomena cited circulate from east 

 to west inside the earth. 



Now this is a perfectly simple and ob- 

 vious application of a fundamental law in 

 eleetromagnetism, and yet for want of this 

 test many eminent investigators have lost 

 valuable time and even to-day some cases 

 of transgression or omission might be cited. 

 Thus some of the theories of the secular 

 variation suppose that the electric currents 

 causing this variation are situated chiefly 

 outside of the earth. However, according 

 to recent calculations, as based virtually 

 upon the mathematical application of Am- 

 pere's rule, it is found that the observed 

 facts can be made to harmonize best with 

 a system of forces situated chiefly inside 

 the earth. [Since the reading of this paper, 

 the calculations reveal the existence of also 

 a minor system of outside currents taking 

 part in the production of the observed 

 secular variation.] 



The first one to make a mathematical 

 test of the seat of the earth's magnetic 

 forces, coupled also with an analysis into 



