EFFECTS PRODUCED UPON BUILDINGS. 141 



America there appear to exist tracts of ground which are 

 practically exempt from earthquake shocks, whilst the 

 whole country around is sometimes violently shaken. It 

 would seem as if the shock passes beneath such a district 

 as water passes beneath a bridge, and for this reason 

 these districts have been christened ' bridges/ 



This phenomenon appears to depend upon the nature 

 of the underlying soil. When an elastic wave passes 

 from one bed of rock to another of a different character, a 

 certain portion of the wave is reflected, while the 

 remainder of it is transmitted and refracted, and 

 ' bridges ' we may conceive of as occurring where the 

 phenomenon of total reflection occurs. 



In the instances given of soft materials having proved 

 good foundations, it was assumed that they had chiefly 

 acted as absorbers of momentum. They have also acted 

 as reflecting surfaces, and where no effects have been felt 

 by those residing on them, this may have been the result 

 of total reflection, and the soft beds thus have played the 

 part of bridges. 



Fuchs gives an example taken from the records of 

 the Syrian earthquake of 1837, where not only neigh- 

 bouring villages suffered differently, but even neighbour- 

 ing houses. In one case a house was entirely destroyed, 

 whilst in the next house nothing was felt. 



In Japan, at a place called Choshi, about 65 miles 

 east of the capital, earthquakes are but seldom felt, 

 although the surrounding districts may be severely 

 shaken. 



From descriptions of this place it would appear that 

 there is a large basaltic boss rising in the' midst of 

 alluvial strata. The immunity from earthquakes in 

 this district has probably given rise to the myth of the 

 Kanam rock, which is a stone supposed to rest upon the 



