174 EARTHQUAKES. 



places at different distances from the origin of a disturb- 

 ance, should be of different heights. We also see aij 

 explanation for the fact that small waves should some- 

 times appear to be interpolated between large ones, and 

 that these should occur at varying intervals. 



The fact that whenever a wave is produced, a certain 

 quantity of water must be drawn from the level which 

 surrounds it, in order that it should be formed, explains 

 the phenomena that the sea is often observed first to 

 draw back. Out in the open ocean it is drawn from the 

 hollow between two waves. As has been pointed out by 

 Darwin, it is like the drawing of the water from the shore 

 of a river by a passing steamer. 



The difference in the heiglit of waves, as observed at 

 places lying close to each other, is probably due to the con- 

 figuration of the coast, the interference of outlying islands, 

 reefs, &c. — causes which would produce similar effects in 

 the height of tide. 



As a wave approaches shallow water it gradually in- 

 creases in height, its front slope becomes steep, and its 

 rear slope gentle, until finally it topples over and breaks. 

 This increasing in height of waves is no doubt connected 

 with the destruction of Talcahuano and Callao, which are 

 situated at the head of shallow bays. Valparaiso, which is 

 on the edge of deep water, has never been overwhelmed.* 

 Another case tending to produce anomalies in the cha- 

 racter of waves would be their reflection and mutual in- 

 terference, the reflections due to the configuration of the 

 ocean bed and coast lines. 



The complete phenomena which may accompany o 

 violent submarine disturbance are as follows : — 



By the initial impulse of explosion or lifting of the 

 ground, a ' great sea wave ' is generated, which travels 

 1 Darwin, Besearches in Geology, <|'<7., p. 378. 



