THE SEMI-CENTENNIAL ANNIVERSARY 25 



Before enumerating the international associations which are 

 dealing with these problems of geophysics, let us say a few words 

 as to the problems themselves. 



We have, first, to study the shape of the earth and the varia- 

 tions in the gravitational forces which are observed on its sur- 

 face. We have further to take account of the secular variations 

 of level and of the more or less violent disturbances which 

 accompany earthquakes and earth tremors. By comparing the 

 indications of instruments placed in different localities, we can 

 deduce the rate of propagation over the earth and through the 

 earth of the seismic waves. This yields us important informa- 

 tion on the physical properties or material composing the interior 

 of the earth. The cause of terrestrial magnetism is at present un- 

 known, and we have no means at our disposal to attack the 

 problem directly, but the study of the diurnal and secular varia- 

 tions, may give us a clue, and deserves our closest attention. 



In a similar way, the study of the higher atmosphere and of 

 the high electric conductivity which the air is now known to 

 possess at heights which we can not reach, is also a subject which 

 can only be studied by combined efforts. How are these ques- 

 tions dealt with at present? 



We have, first, an International Association of Geodetics, 

 which is an exceedingly efficient body, with a bureau at Pots- 

 dam, under Professor Helmert. That association is successful, 

 perhaps, partly because its work has been facilitated in that it 

 had to build on virgin soil. Nothing had been done, to a very 

 great extent at any rate, internationally before that association 

 came into being. On the other hand we have the International 

 Association of Seismology, a related subject, which was only 

 founded at the beginning of the present century, with a central 

 bureau at Strasburg. This association had to overcome more 

 serious difficulties. It entered into the field when there was 

 already a less extensive organization in existence, which had 

 been originated by Professor Milne and was directed by a com- 

 mittee of the British Association. The question of instruments 

 also presented peculiar difficulties, which it is hoped may soon be 

 overcome. 



