150 A. TANAKADATE AND H. NAGAOKA. 



been previously obtained, or which might be obtained, at öome future 

 time, in order to throw hght if possible, upon the effect of seismic events 

 upon the magnetic elements of a country, and give some notion con- 

 cernin<:j: the nature of the subterranean disturbances to which such 

 events are due. For the purpose of such comparison, it is evident that 

 a knowledge of the ordinary secular variations of the magnetic elements 

 of the district is of paramount importance, and this unfortunately, in 

 the present case, is to a great extent wanting, since systematic magnetic 

 surveys of this country are of comparatively very recent date. 



If we take only the declination, we may go back to the work of 

 Inö, who made a careful comparison of the compass needle with the 

 astronomical meridian at various places in the country during his 

 memorable undertaking of the geodetic survey of Japan, 1800 — 1818. 

 Since then we have had observations of only a fragmentary nature 

 until the first systematic survey was carried out by the Geological 

 Bureau (1880—1885) under the direction of Dr. E. Naumann. Two 

 observers, Messrs Sekino and Ködari, worked during these years over 

 different parts of the country, the number of stations being 182 in 

 all. The results are published in a pamphlet entitled " Die Erschei- 

 nuno-en des Erdmag'netismus in ihrer iibhäno'is'keit vom Bau der 

 Erdrinde." Their results were, however, of a provisional character, 

 in so far that they made but one determination of each element at 

 every station, and that tlie distribution of their stations was only 

 alonff certain lines. This necessitated the introduction of corrections 

 which, as there were no better means, they took from the Tokyo 

 magnetographs ; while as to isomagnetics much extrapolation had to 

 be made, notwithstanding the number of stations they took. This was 

 no fault of the observers, for they had other duties to perform at the 

 time, and the mode of travellin«]: was not so convenient then as it is now. 



During the summer of 1887, Prof. Knott and the present writers 



