22 Art. 9. -T. Terada: 



the waves, though there seems to be a maximum of frequency 

 near 15" 



h) Slight local earthquakes, which are very frequent, affect 

 the instruments, leaving a trace similar to that of the Milne 

 seismograph (see for example PL V). This is in all probability 

 mainly due to the mechanical shock, to Avhich our instruments, 

 especially X- and Y-instruments, are rather sensitive. These 

 traces, however, served as very convenient and trustworthy time- 

 signals, as already mentioned. Besides, remarkable earthquake 

 waves with long periods, originating in distant regions, are 

 beautifully recorded in our magnetographs (PI. IV). The most 

 conspicuous periods are of the order 10'-20', i.e. decidedly shorter 

 than the usual magnetic waves. These peculiar waves appear in 

 the vertical component as conspicuous as in the horizontal com- 

 ponents, ^-^ while in the case of the usual magnetic waves of such 

 a short period the vertical component is almost nil in comparison 

 with the horizontal ones. Since our instruments are neither 

 quite free from the influence of inclination nor of acceleration, 

 even in the case of the Z-instrument, we can not be sure whether 

 these waves are not largely due merely to the mechanical effect, 

 though it seems not altogether improbable that an actual magnetic 

 wave is produced by the earthquake. To decide this point, it 

 will be desirable to carry out simultaneous observations with 

 quite different systems of instruments with the same magnetic 

 sensibility, but with considerably different mechanical sensibilities, 

 though it will be difficult to obtain instruments perfectly free from 

 mechanical disturbances. '-* 



i) All slight or conspicuous magnetic disturbances which 

 may serve as premonitory signs preceding earthquakes, were 

 carefully sought, throughout the records in hands. At first, a 

 possible connection was suspected between the earthquakes and 

 the characteristic abrupt occurrence of the regular wave trains, 



1) The vertical-force magnetograph as now constructed may be regarded as a sort of 

 seismograph, inasmuch as the line of suspension does not pass through the centre of the 

 mass of the suspend system. 



2) The trace of the same earthquake on the X-component record of the Eschenhagen 

 magnetograph at Kakioka Magnetic Observatory, was about 6 mm. in maximum ampitude, 

 while the magnetic sensibility was about 5 y per mm. 



