November 11, 1904.] 



SCIENCE. 



637 



The material for testing this hypothesis, 

 however, is not yet either at hand or suffi- 

 ciently complete. 



And right here we must record a most 

 lamentable condition of our knowledge re- 

 garding the general distribution of the 

 earth's magnetic forces. One of the sur- 

 prising results of my critical comparison, 

 above referred to, was the fact that the 

 accuracy in the determination of the 

 earth's magnetic potential is about the 

 same ivhether we use the magnetic charts 

 of Sahine of 1840-5 or the best modern 

 magnetic charts. In other words, Avhereas 

 magnetic surveys have steadily progressed 

 on land areas and even have been repeated 

 in certain instances in greater detail, the 

 magnetic survey of the great oceanic areas 

 and of the unexplored land regions has 

 made very little progress during the past 

 half century. The advent of the iron ship 

 has materially lessened the yield of useful 

 magnetic data and the expeditions designed 

 for securing sea results have been unfortu- 

 nately too few and far between. In the 

 Antarctic regions, for example, practically 

 no progress had been made since the obser- 

 vations of Ross in the Erehus and Terror 

 during the fourth decade of the last cen- 

 tury until the recent Antarctic expeditions 

 of the British and German empires. 



Fortunately, however, there is an 

 awakening interest in this direction. Thus 

 a committee has been appointed by the 

 International Association of Academies, at 

 its recent meeting in London, to consider 

 methods for securing increased accuracy in 

 magnetic work at sea. Furthermore, the 

 plans of the Department of Terrestrial 

 Magnetism of the Carnegie Institution em- 

 brace cooperation in the magnetic survey 

 of the oceanic areas, and it is confidently 

 hoped that a beginning in this direction 

 can soon be made. Instruments for this 

 purpose have already been ordered. Also, 

 I am glad to be able to announce that, hav- 



ing succeeded in organizing the detailed 

 magnetic survey of the land area of our 

 country, attention has next been paid to 

 inaugurating similar work at sea on the 

 Coast and Geodetic Survey vessels ; three of 

 them have already been fully equipped for 

 this purpose and this fall two more will 

 receive their magnetic equipments. While 

 these vessels can only obtain magnetic data 

 incidentally in the course of their survey- 

 ing work, experience has shown that a very 

 satisfactory degree of accuracy can be 

 secured by their skilled officers. The mag- 

 netic declination and dip can be obtained, 

 for example, to about 5' to 10' and the total 

 force to about one five-hundredth part. 



We next inquire, is the earth's magnetic 

 energy increasing or decreasing? This is 

 a question of fundamental importance to 

 theories of the earth's magnetism. As is 

 well known, the earth's magnetic elements 

 are subject to a secular variation whereby 

 considerable changes are produced in the 

 course of time. A secular variation may 

 result from a change in the intensity of the 

 magnetization of the earth, or from a 

 change of the direction of magnetization, 

 or from both causes. 



The first term of the Gaussian potential 

 is of a simple harmonic type and consti- 

 tutes by far the largest term ; it represents, 

 about 65 to 70 per cent, of the total mag- 

 netization and can be physically inter- 

 preted as a uniform or homogeneous mag- 

 netization symmetrical about a diameter,, 

 inclined 11^° to the earth's axis of rotation. 

 This diameter Gauss defined as the earth's 

 magnetic axis, with respect to which he 

 determined the magnetic moment due to 

 the first term. Tabulating the values of 

 the magnetic moment as derived for dif- 

 ferent epochs from the various analyses, we- 

 shall find that it has decreased in forty-six 

 years by 1.6 per cent.— an alarming loss,, 

 if true ! 



