'^- ^XLV 



8 Oln 



-' Hf^..i. 



y^^i lae 



;m 



til 



e 



• ilea 



luost 



•nh of 



e*tof 





the 



a 



^re the 

 '--Action 



iii:es 



^^e Chan. 



Id; 



I «Jr. 



ale. 



^ i"ved the 

 t^'l been 

 Such bodies 



^ardsinthe 



uptimes they 

 indicate the 

 ich lines of 

 lull gl.e the 



lit stt^p is to 

 . at different 



' -1^ . to stand 

 lock, and the 



to cni^rgs in 

 The ?hoct 



Juce fissures 

 TiMes to it 

 cliuatioo 



in 

 ri-n, or 



of 



the 



being thus 



nce 



to 



these 

 -,/itio5 



.t th^;^5.e 



of t^^ ^^'^ 



Ch. XXIX.] 



TIIEOHY OF THE EAETIIQUAKE-WAVE. 



139 



in other words, it will be always steeper as the depth 



as the line C, 1, for example, is more steeply 



increases^ 

 inclined than A, 1. 



formula 



not cite 



Mr. Mallet 



came to the conclusion that the depth of 

 thrOTrginal shock in 1857 did not exceed 7 or 8 miles, and 

 although this can only be a rough approximation to the 

 truth it is of considerable interest, and the repetition of 

 such investigations may hereafter lead to more rehable 



direction, and intensity of the 



moment 



observations require the 



■um 



time^ 

 made 

 Such 



, and 



problem is exceedingly complicated, far more so than 



may 



om 



above given 



For in the first place the shock which 



produces the vibration or earthquake-wave does not give rise 



move 



ments 



almost 



taneously, and is at right angles to it ; but, as this latter 

 vibration travels somewhat slower than the former, it reaches 

 the surface, if the distance be considerable, after a distinct 

 interval of time and often does more mischief to buildings than 

 the first. It will also be seen by the elaborate report of 

 Hopkins * that the earthquake-wave when it passes thrc 



Mr 



some 



not only its velocity but its direction, being both refracted and 



manner 



medium 



When 



shock traverses the earth's crust through a thickness of several 



miles 



as well as 



rents and faults by which the course of the vibratory move- 

 ments will be more or less interfered with. The fracture 

 also of buildings is considerably modified by the nature of 

 their component materials, and of the coherence of the 

 mortar by which stones or bricks are cemented together. 



* Geological Theories of Elevation and Earthquakes, Erit. Assoc. 1847, p. 33. 



