INTRODUCTION. 9 



Commission appointed for the observation of earthquakes 

 by the Natural History Society of Switzerland, and the 

 volumes which have been j)ublished by the Seismological 

 Society of Japan. 



Before concluding this chapter it will be well to 

 define a few of the rgore ordinary terms which are used 

 in describing earthquake phenomena. It may be ob- 

 served that the English word earthquake, the German 

 erdbehen, the French tremblement de terre, the Spanish 

 terremotOf the Japanese jishin &c., all mean, when 

 literally translated, earth-shaking, and are popularly 

 understood to mean a sudden and more or less violent 

 disturbance. 



Seismology {crsicriios an earthquake, \6^os a dis- 

 course) in its simplest sense means the study of earth- 

 quakes. To be consistent with a Greek basis for seis- 

 mological terminology, some writers have thrown aside 

 the familiar expression ' earthquake,' and substituted the 

 awkward word ' seism.' 



The source from which an earthquake originates is 

 called the ' origin,' * focal cavity,' or ' centrum.' 



The point or area on the surface of the ground above 

 the origin is called the ' epicentrum.' The line joining 

 the centrum and epicentrum is called the 'seismic 

 vertical.' 



The radial lines along which an earthquake may be 

 propagated from the centrum are called ' wave-paths.' 



The angle which a wave-path, where it reaches the 

 surface of the earth, makes with that surface is called the 

 ' angle of emergence ' of the wave. This angle is usually 

 denoted by the letter e. 



As the result of a simple explosion at a point in 

 a homogeneous medium, we ought, theoretically, to ob- 

 tain at points on the surface of the medium equidistant 



