12 EARTHQUAKES. 



CHAPTER 11. 



SEISMOMETRY. 



Nature of earthquake vibrations — Many instruments called seismo- 

 meters only seismoscopes — Eastern seismoscopes, columns, projec- 

 tion seismometers — Vessels filled with liquid — Palmieri's mercury 

 tubes — The ship seismoscope — The cacciatore — Pendulum instru- 

 ments of Kreil, Wagner, Ewing, and Gray — Bracket seismographs — • 

 West's parallel motion instrument — Gray's conical pendulums, 

 rolling spheres, and cylinders — Yerbeck's ball and plate seismograph 

 — The principle of Perry and Ayrton — Vertical motion instruments 

 — Record receivers — Time-recording apparatus — The Gray and 

 Milne seismograph. 



Before we discuss the nature of earthquake motion, the 

 determination of which has been the aim of modern seis- 

 mological investigation, the reader will naturally look for 

 an account of the various instruments which have been 

 employed for recording such disturbances. A description 

 of the earthquake machines which have been used even in 

 Japan would form a bulky volume. All that we can do, 

 therefore, is to describe briefly the more prominent fea- 

 tures of a few of the more important of these instruments. 

 In order that the relative merits of these may be better 

 understood, we may state generally that modern research 

 has shown a typical earthquake to consist of a series of 

 small tremors succeeded by a shock, or series of shocks, 

 separated by more or less irregular vibrations of the 

 ground. The vibrations are often both irregular in period 



