168 EAETHQUAKES. 



4, 1854, when Acapulco was destroyed, the sea is said to 

 have returned as gently as it went out. 



When sea waves have travelled long distances from 

 their origin, as, for instance, whenever a South American 

 wave crosses the Pacific to Japan, the phenomena which 

 are observed are like those which were observed at 

 Acapulco ; the sea falls and rises, at intervals of from ten 

 minutes to half an hour, to heights of from six to ten feet, 

 without the slightest appearance of a wave. Its phe- 

 nomenon is like that of an unusually high tide, which 

 repeats itself several times per horu. Even if we watch 

 distant rocks with a telescope, although the surface of 

 the ocean may be as smooth as the surface of a mirror, 

 there is not the slightest visible evidence of what is 

 popularly called a wave. The sea being once set in 

 motion it continues to move as waves of oscillation for a 

 considerable time. In 1877, as observed in Japan, the 

 motion continued for nearly a whole day. The period 

 and amplitude of the rise and fall were variable, usually- 

 it quickly reached a maximum, and then died out gradually. 

 As observed in a self-recording tide gauge at San Francisco, 

 the disturbance lasted for about four days. A diagram of 

 this is here given. In its general appearance it is very 

 similar to the records of other earthquake waves. The 

 large waves represent the usual six hours rise and fall of 

 the tides ; usually these are fairly smooth curves. Super- 

 imposed on the large waves are the smaller zigzag curves 

 of the earthquake disturbance, lasting with greater or less 

 intensity for several days. As these curves are drawn to 

 scale — horizontally for hours, and vertically one fifth 

 inch to the foot, to show the extent of the rise and fall — 

 they will be easily understood. 



Sometimes, as in the present example, the first move- 

 ment in the waters is that of an incoming wave. In many 



