10 EABTHQUAKES. 



from the epicentrum, equal mechanical effects. These 

 points will lie on circles called ' isoseismic ' or ' copeismic * 

 circles. The area included between two such circles is an 

 * isoseismic area.' In nature, however, isoseismic lines 

 are seldom circles. Elliptical or irregular cm'ves are the 

 common forms. 



The isoseismic area in which the greatest disturbance 

 has taken place is called the ' meizoseismic area.' Seebach 

 calls the lines enclosing this area 'pleistoseists.' 



These last-mentioned lines are wholly due to Mallet 

 and Seebach. 



Many words are used to distinguish different kinds of 

 earthquakes from each other. All of these appear to be 

 very indefinite and to depend upon the observer's feelings, 

 which, in turn, depend upon his nervous temperament 

 and his situation. 



In South America, small earthquakes, consisting of a 

 series of rapidly recurring vibratory movements not suffi- 

 ciently powerful to create damage, are spoken of as trem- 

 belores. 



The terremotos of South America are earthquakes 

 of a destructive nature, in which distinct shocks are per- 

 ceptible. It may be observed that shocks which at one 

 place would be described as ierremoto would at another 

 and more distant place probably be descvihed Sistrembe- 

 lores. 



The succussaiore are the shocks where there is con- 

 siderable vertical motion. The terrible shock of Reo- 

 bamba (February 4, 1797), which is said to have thrown 

 corpses from their graves to a height of 100 feet, was an 

 earthquake of this order. 



The vorticossi are shocks which have a twisting or 

 rotatory motion. 



Another method of describing earthquakes would be 



