292 HOBBS— THE EVOLUTION AND THE [April 24. 



tween the middle of the fourteenth and the middle of the fifteenth 

 century, this maximum period being followed by a steady decrease 

 to a minimum in the last half of the nineteenth century. Minor 

 fluctuations reveal an average period of 6-| years, or about one half 

 that revealed by the records for the Empire as a whole.** 



The natural objection which would be raised to making use of 

 these data for basing conclusions upon the behavior of the earth 

 as a whole, is that the maximum of intensity in Japan may well have 

 been compensated by a minimum in a neighboring district. What 

 we need for basing our conclusions is a world catalogue of earth- 

 quakes extending over a sufficiently extended period. Thanks to 

 John Milne and those who have followed his lead, we are now pre- 

 paring such a catalogue, which is sure to permit of a definitive 

 answer to the question of earthquake periodicity. Even within the 

 first section of this catalogue, comprising as it does the thirteen 

 years from 1892 to 1904, Milne believes he has made out a relatively 

 short period with the maxima of world shaking in correspondence 

 with the more abrupt changes in direction in the orbit of the earth's 

 pole. On a priori grounds it is reasonable to connect seismic dis- 

 turbances with sudden changes in latitude, and the further data upon 

 the pole movement and the seismic world maxima, will be scrutinized 

 with interest. 



Possibilities of Future Prognostication. — It is too early to pre- 

 dict whether more satisfactory bases for future forecasting of earth- 

 quakes will be discovered, but the indications are certainly encourag- 

 ing. Two, and perhaps three, lines of inquiry are already suggested. 

 Most promising of these, is, perhaps, the study of terrestrial magnet- 

 ism ; for in a considerable number of instances, destructive earth- 

 quakes have been preceded by periods measured in hours and some- 

 times in days, within which the behavior of magnetographs was 

 singularly abnormal. It seems likely that this change in magnetic 

 conditions may sometimes be utilized as a warning signal. For 

 solution of this problem the completion of the magnetic survey of 

 the world, may be expected to contribute. 



Evidence is not lacking that fore-shocks, or rather fore-tremors, 



"^Kichuchi, E. I. C. Pub., No. 19, 1904, pp. I1-13. 



