Horizontal Pendulums for the Mechanical 



Registration of Seismic and Other 



Earth Movements. 



By 

 F. Omori, D. Sc. 



Professor of Seismology, Tokyo Imperial University. 



With Plates ir—XIL 



I. 



1. Observational seismology requires not only seismo- 

 graphs to record the comparatively sudden and violent move- 

 ments of the ground, which constitute earthquakes, but also much 

 more sensitive instruments, to record those imperceptible seismic 

 disturbances and other earth movements, which are always 

 proceeding. The seisraometrical work, which has been done in 

 Japan during the last 20 years, has related chiefly to earthquakes 

 of comparatively near origin, the seismographs used being those 

 of Ewing, and Gray and Milne. • Each of these instruments 

 registers the more sudden vibrations of the earthquake motion, but 

 is almost incapable of measuring earth undulations of long period, 

 owing to the want of a sufficient approach of the so-called 

 "steady mass" to the neutral equilibrium, and to the existence 

 of a considerable amount of friction between the writing pointer 

 and the record-receiver and elsewhere. 



2. It is unnecessary to repeat here the historical sketches of 

 seismographs, horizontal pendulums, etc., which have already 



