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EARTHQUAKE PHENOMENA. 



[Sect. VII. 





violent, and the mass of material suddenly acted on very 

 great, as in an earthquake, the size of the wave becomes 

 so great as to produce a perceptible undulation of the sur- 



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face of the ground, often visible t( 



transit bodies upon the earth 



through their own inertia), thrown down, &c. 



are disturbed (chiefly 



earth q 



of a wave of 



directmn, from vertically upwards to horizontally in any 

 azimuth, through the surface and crust of the earth, from 

 any centre of impulse or from, wMre than one, and which 

 may he attended with tidal and sound waves, dependent 



-■ and upon circumstances of position as to 

 This truth has not yet been fully and 

 ex]>eriixientally demonstrated. It is of the highest im- 

 portance to a wide region of science that observations 

 should be made, enabling it to be so. 



sea and land. 



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Observers in earthquake countries should make them- 

 selves familiar with the usual features, and with the 

 succession of events, and concomitants which with a certain 

 sort of regularity apply to all earthquakes. For this Sir C. 

 Lyell's 'Geology,' in loco, will be sufficient. The greatest 

 shocks are not the most insti active, except as to secondary 

 effects; but every great shock is ususHy followed by several 

 smaller— the first should therefore be viewed as a " notice 

 to observe " the latter carefully. 



The phenomena of every earthquake may be divided 

 into — 1st, Primary, or those which properly belong to the 

 transit of the wave or waves through the solid or watery 

 crust of the earth, the air, &c. ; 2, Secondary, or the effects 

 produced by this transit—and both must be kept distinct 





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