258 C. Davison — Vorticose Earthquake Shocks. 



1881, July 5, Tokio, Japan. "At the commencement of the shock 

 the motion was N. 112° E. ; 1^ second after this the direction was 

 N. 50° E. ; f second more it was N. 145° E, : and after a similar 

 interval N. 62° E." ^ 



1880, July 14-25, Manila, Luzon. July 14, 12-33 p.m.: "The 

 horizontal pendulum [of the seismograph] inscribed a cross with 

 arms almost at right angles, the first bearing from S.E. 10° E. to 

 N.W. 10° W. ; the second from S.W. 5° W. to N.E. 5° E." July 

 18, 12 40p.m.: "when the great earthquake of oscillation, tre- 

 pidation, and what is commonly called rotatory movement, occurred 

 simultaneously. Its duration was Im. 10s. It is impossible to 

 describe all the movements of the pendulum on account of their 

 number and variety." The principal directions were, however, 

 from E. 5° S. to W. 5° N. ; from S.W. to N.E. ; and from N. 4° W. 

 to S. 4° E. July 20, 3 p.m. : " the oscillation and trepidation 

 movements alone were experienced, but with extraordinary 

 violence. The oscillation of the pendulum was directed from 



S.E. 15° E. to N.W. 15° W The pendulum continued 



oscillating during the whole of the evening from N.E. to S.W," 

 " At 1040 P.M. a second shock occurred, in which the principal 

 directions were from E. to W. and from N.E. to S.W.^ 



1854, Oct. 20, Coquimbo, Chili. Two shocks ; the first was vertical, 

 the second, which followed it immediately, was circular, and 

 lasted 18 seconds.^ 

 Many instances of a similar nature are to be found in the memoirs 



of M. Perrey and other seismologists. The descriptions, when 



given at all fully, seem to bear out the following conclusions : 



1. The occurrence of shocks as vorticose shocks is dependent upon 

 the position of the points of observation ; for often during a given 

 earthquake the nature of the shock varies throughout the disturbed 

 area. Mr. Mallet's great work on the Neapolitan earthquake of 

 1857 affords an excellent example of this statement. 



2. Vorticose shocks are generally felt as such only at places verti- 

 cally, or nearly so, above the centre of disturbance. This fact, or 

 its converse, seems to have been recognized both by Prof. Perrey 

 and Mr. Mallet, the two founders of Seismology. " All is irregular, 

 jerking, tumultuous at the centre of disturbance," says M. Perrey, 

 " then, at a certain distance, the vibrations become more regular, 

 and are propagated under the form of undulations perhaps isochron- 

 ous." * The universal testimony of those who have experienced 

 vertically arriving shocks, Mr. Mallet remarks, " is of a twisting 

 and wriggling motion in difi'erent planes, violent in its changes of 



1 Prof. J. Milne. Letter on "Earthquake Vibrations" : JSTature (Dec. 8, 1881), 

 vol. XXV. p. 126. 



^ The above are notes of the principal shocks taken from the account by Father 

 Faura of his observations and seismographic records at Manila : Proc. Ptoy. Soc. 

 (Feb. 17, 1881), vol. xxxi. pp. 462-8. 



^ M. Perrey. "Note sur les tremblements de terreen 1857, etc." : Mem. Cour. 

 de I'Acad. Roy. de Belgique (1860), Collection in 8vo. vol. x. 



* M. Perrey. ' ' Sur les tremblements de terre ressentis dans le bassin du Ehone " : 

 Ann. de la Soc. Roy. d'Agric. de Lyon (1845), vol. viii. pp. 265-370. 



