70 EAKTHQUAKES. 



in the eartliquake which shook Tokio at 6 A.M. on July 5, 

 1881, there were indications of the following motions : — 



Near the commencement of the shock the motion was 

 N. 112° E. One and a half second after this, the direc- 

 tion of motion appears to have been N. 50° E. In three- 

 fourths of a second more it gradually changed to a 

 direction N. 145° E., and after a similar interval to N. 

 62° E. Half a second after this it was N. 132° E., and 

 four seconds later the motion was again in the original 

 direction — namely, N. 112° E. 



These particular directions of motion have been 

 selected because they were so definitely indicated. 



The commonest type of earthquake which is experi- 

 enced in Japan, and probably also in other earthquake- 

 shaken districts, is the compound or diastrophic form. 



That earthquakes often have motions compounded of 

 two sets of vibrations, has also been proved by the ana- 

 lysis of the records obtained from two component seismo- 

 graphs. From an analysis of a record of this description, 

 Professor Ewing has shown that in the earthquake felt in 

 Tokio on March 11, 1881, there were approximate circular 

 (somewhat spiral) movements. 



This leads us to the consideration of the twisting and 

 wriggling motions which are said to be experienced by 

 some observers. Motions like these, which by the Italians 

 and Mexicans are called vorticosi, are usually supposed to 

 be the cause of objects like chimneys and gravestones 

 being rotated. These phenomena, it will be seen from 

 what is said in the chapter upon the effects produced in 

 buildings, can be more easily explained upon the supposi- 

 tion of a simple rectilinear movement. 



That at the time of an earthquake there may be 

 motion in more than one direction has been recognised 

 since the time of Aristotle ; and it is possible that two sets 



