68 EAKTHQUAKES. 



These have been the experiences of many observers, 

 and have been recorded by writers since the earliest times. 

 Mallet devotes a chapter to a consideration of the tremu- 

 lous motion that precedes and follows a shock, and he 

 tells us that a single shock is an absolute impossibility. 

 In speaking of earthquakes, he says: ' The almost universal 

 succession of phenomena recorded in earthquakes is, first 

 a trembling, then a severe shock, or several in quick suc- 

 cession, and then a trembling gradually but rapidly be- 

 coming insensible,' 



A quantitative and exact knowledge of the nature of 

 earthquake motion has only been attained of late years. 

 The chief results which investigators have aimed at have 

 been the measurement of the amplitude, the period, the 

 direction, and the duration of the motions which constitute 

 an earthquake. Attention has also been given to the 

 velocity with which a disturbance is propagated. 



The Direction of Motion, — One of the most ordmary 

 observations which are made about an earthquake is its 

 direction. If we were to ask the inhabitants of a town 

 which had been shaken by an earthquake the direction 

 of the motion they experienced, it is not unlikely that 

 their replies would include all the points of the compass. 

 Many, in consequence of their alarm, have not been able 

 to make accurate observations. Others have been de- 

 ceived by the motion of the building in which they were 

 situated. Some tell us that the motion had been north 

 and south, whilst others say that it was east and west. A 

 certain number have recognised several motions, and 

 amongst the rest there will be a few who have felt a 

 wriggling or twisting. Leaving out exceptional cases, the 

 general result obtained from personal observation as to the 

 direction of an earthquake of moderate intensity is ex- 

 tremely indefinite, and the only satisfactory information 



