EARTHQUAKE MOTION DEDUCED FEOM OBSERVATION. 69 



to be obtained is that derived from instruments or from 

 the effects of the earthquake exhibited in shattered build- 

 ings and bodies which had been overturned or projected. 



By the direction in which walls, columns, and other 

 objects had been overthrown or fractured, Mallet was 

 enabled to determine the position of the origin of the 

 Neapolitan earthquake. Similar phenomena have many 

 times been taken advantage of by other investigators of 

 earthquake phenomena. Effects produced upon struc- 

 tures are, however, only to be observed as the results of 

 a destructive earthquake, at which time cities may be 

 regarded as collections of seismometers. {See chapter on 

 Effects in Buildings.) 



To determine the direction of movement during a 

 small earthquake, the most satisfactory method appears 

 to be an appeal to instruments. 



Instruments as Indicators of Direction, — The rela- 

 tive values of different kinds of instruments, such as 

 columns, pendulums, and the like, as indicators of direc- 

 tion have already been discussed. 



By the use of pendulum seismographs it has been 

 shown that during an earthquake the ground may move 

 in one, two, or several directions (see p. 21); and it is, 

 generally speaking, only in those cases where we ex- 

 perience a decided shock in the disturbance that we can 

 determine with any confidence the direction in which the 

 motion has been propagated. Such directions are usuall}^ 

 indicated by the major axis of certain more or less ellip- 

 tical figures which have been drawn, which in themselves 

 appear to indicate the combination of two rectilinear 

 movements. 



Eesults similar to those indicated by the records of 

 pendulum seismographs have also been obtained upon 

 moving plates with a double bracket seismograph. Thus, 



