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Sect. VIL] 



EAETHQTJAKE PHENOMENA 



107 



gree of elasticity, and upon the density in any given one. 

 This transit period is constant for the same material, and 





is irrespective of the amount or kind of original inipul 

 for example, in air its velocity is about 1140~in water 

 about 4700— and in iron about 11,100 feet per second 

 all in round numbers. 



Iciii 



Thus, if one stand upon a line of railway near the 

 and a heavy blow be delivered at a few hundred feet distant 

 upon the iron rail, he will almost instantly hear the wave 

 thruuo-h the iron rail — directly after he will feel another 

 wave through the ground on which he stands — and, lastly, 

 he will again hear another wave through the 



' ; and if 



there were a deep side-drain to the railway, a person im-- 

 mersed in the water would hear a wave of sound through 

 it, the rate of transit of which would be different from any 

 of the others— all these starting from the same point at the 



same moment. 



The size of such a wave — that is, the volume of the 

 flifinlflPi^fi nartiolpft of fhp -material in motion at once, de- 



pends upon the elastic limits of the given substance, and 



upon the amount or power of the original impulse. By 

 the elastic limit in solids is meant the extent to which 

 the particles may be relatively displaced without fracture 



or other permanent alteration : thvis glass, although much 



more perfectly elastic than India rubber, has a much 

 smaller elastic limit. 



Nearly all such elastic waves as we can usually observe 

 originate in Impulses so comparatively small that we are 

 only conscious of them by sounds or vibrations of various 

 sorts, the advancing forms of whose weaves are impercep- 

 tible to the eye ; but when the originating impulse is very 



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