CHAPTER V. 



EARTHQUAKE MOTION AS DEDUCED FROM OBSERVATION ON 

 EARTHQUAKES. 



Result of feelings — The direction of motion — Instruments as indicators 

 of direction — Duration of an earthquake— Period of vibration — The 

 amplitude of earth movements — Side of greatest motion — Intensity 

 of earthquakes — Velocity and acceleration of an earth particle — 

 Absolute intensity of an earthquake — Radiation of an earthquake — 

 Velocity of propagation. 



Result of Feelings. — As the result of our experiences, 

 and by observations upon the movements produced in 

 various bodies, we can say that an ordinary earthquake 

 consists of a number of backward and forward motions of the 

 ground following each other in quick succession. ScmB- 

 times these commence and die out so gently that those 

 who have endeavoured to time the duration of an earth- 

 quake have found it difficult to say when the shock com- 

 menced and when it ended. This was a difficulty which 

 Mr. James Bissett in Yokohama, and the author in Tokio, 

 had to contend against when, in 1878, they commenced 

 to time shocks between these two places. 



Sometimes these motions gradually increase to a 

 maximum and then die out as gradually as they com- 

 menced. 



Sometimes the maximum comes suddenly, and at 

 other times during an earthquake our feelings distinctly 

 tell us that there are several maxima. 



