256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



the nuiiiber of sun spots, which is undoubtedl}^ a symptom of vary- 

 ing solar activity, is paralleled by an 11-year period in the secular 

 ciiange, and Bauer has attempted to trace a relationship between 

 changes in the earth's magnetism and the changes of solar activity 

 indicated by Abbot's observations of the amount of heat given off 

 by the sun. Abbot's observations have not yet been going on long 

 enough, however, to draw any definite conclusions. 



An}' theory of the earth's magnetism based on electric currents 

 either within or outside the earth must take account of the currents 

 actually observed, and one of the features of the progress of the past 

 twenty-five years has been the awakening of interest in the study of 

 atmospheric electricity and earth currents and the development of 

 instruments for measuring them which can be relied upon to give re- 

 sults of tlie required accuracy. 



Before long we may also expect the increase in our knowledge of 

 radio transmission to throw light on electrical conditions of the at- 

 mosphere at higher levels. 



From this summary of the accomplishments in the field of terres- 

 trial magnetism during the tirst quarter of the twentieth century, it 

 Avill be seen that there has been no sudden increase in knowledge, no 

 epoch-making disco verj^ but just a steady advance. The old prob- 

 lems still confront us, but in a modified form. Their scope has broad- 

 ened tremendously and any theory to explain the earth's magnetism 

 nnist take into account the structure of the atom as well as the struc- 

 ture of the universe. At the same time new weapons and new me- 

 thods of attack have become available and the workers in other fields 

 of science — astronomers, physicists, geologists, chemists — are now the 

 allies of the magneticians. iSloreoAer, an accurate magnetic survey 

 of practically all of the accessible land and water areas and the oper- 

 ation of additional magnetic observatories, better distributed, have 

 provided us with reliable knowledge of the distribution of the earth's 

 magnetism — the facts which must form the ultimate test of any 

 theory. 



Finally there has been an increasing recognition of the importance 

 of international cooperation, so well expressed by Rucker in 1898, 

 when he said : " For those who Avould unravel the causes of the 

 mj'sterious movements of the compass needle, concerted action is 

 essential. They can not indeed dispense with individual initiative or 

 with the leadership of genius, but I think all would agree that there 

 is ui'gent need for more perfect organization, for an authority which 

 can decide not only what to do, but what to leave undone." We may 

 confidently expect that the Section of Terrestrial Magnetism and 

 Electricity of the International (leodetic and Geophysical Union 

 will eventually develop into an organization of the character sug- 

 gested by li ticker. 



