264 C. Davison — Vorticose Earthquake Shocks. 



Twisting of Bodies during Earthquakes. — Vorticose shocks being 

 tlius necessary consequences of a large seismic focus, it is but natural 

 that we should recur to the explanation usually given by geologists 

 of the twisting of bodies during earthquakes before the publication 

 of Mr. Mallet's memoir ^ in 1846, and it will be noticed that the 

 objection to the vorticose theory urged by Mr. Darwin (in 1839),^ 

 that under every object so turned there must have been a separate 

 vorticose movement, loses its force on the view here presented. 



Mr. Mallet recognizes two ways in which bodies may be twisted 

 on their bases. (1.) "By the action of a single shock, when the 

 centre of adherence of the base of the object lies to one side or 

 other of the vertical plane passing through the centre of gravity and 

 the line of the wave-path." (2.) By the conjoint action of two 

 closely successive shocks in different directions. The second shock 

 in the latter case he apparently supposes to be due to some cause 

 distinct from that directly producing the first, but his description of 

 the manner in which bodies are thus twisted being equally applic- 

 able to twisting by true vorticose shocks, we may here transcribe it. 



" By the first shock the body is tilted up from its base, but not 

 overthrown, so that for a time greater or less it rests wholly upon, 

 one edge of its base ; while thus poised, if another shock should 

 bear upon it in any direction transverse to the first, it acts as usual 

 at the centre of gravity of the body, to displace it by inertia, in the 

 contrary direction to the wave transit ; but the body is held more 

 or less by friction at the edge momentarily in contact with its 

 support, and there only ; but this edge must always lie to one side 

 of the vertical plane passing through the centre of gravity in the 

 direction of the [second] wave-path; hence the tilted body, while 

 relapsing upon its base, also rotates round some point situated on 

 the edge of its base upon which it had been tilted, and thus it comes 

 to rest in a new position." ^ 



Application to the Theory of Earthquakes. — We shall now point 

 out two inferences which may be drawn from the occurrence of 

 vorticose shocks, viewed in the light of the theory above described. 



(1.) Upon this theory, vorticose shocks can only occur within a short 

 distance of the seismic vertex, unless they should happen, in some 

 exceptional cases, to be produced by some cause distinct from that 

 producing the earthquake; and upon the two transverse-vibration 

 theories this statement is also approximately true. As a general 

 rule, therefore, we may consider the place where a vorticose shock 

 is observed as nearly coincident with the seismic vertex. Seismo- 

 graphic maps of any region showing the positions of all the 

 seismic vertices occurring within it, especially if the depths of the 

 corresponding foci were also indicated, would doubtless lead to more 

 accurate information concerning the relations of seismic foci to one 

 another and to mountain ranges, etc., than can be obtained from 



' " On the Dynamics of Earthquakes," Trans. Eoy. Irish Academy (1846), vol. 

 ixi. pp. 51-105. 



3 " Naturalist's Journal" (1879), p. 308. 



2 "Neapolitan Earthquake of 1857," vol. i. pp. 377-8. 



