DISTUEBA:tTCES IN THE OCEAN. 175 



shorewards with a velocity dependent upon its size and 

 the depth of the ocean. At the same instant, a ' sound 

 wave ' may be produced in the air, which travels at a 

 quicker rate than the ' great sea wave.' A third wave 

 which is produced, is an ' earth wave,' which will reach 

 the shore with a velocity dependent on the intensity of the 

 impulse and the elasticity of the rocks through which it is 

 propagated. This latter, which travels the fastest, may 

 carry on its back a small ' forced sea wave.' On reaching 

 the shore and passing inland, this ' earth wave ' will cause 

 a slight recession of the water as the ' forced sea wave ' 

 slips from its back. 



As these 'forced sea waves' travel they will give 

 blows to ships beneath which they may pass, being trans- 

 mitted from the bottom of the ocean to the bottom of the 

 ships like sound waves in water. At the time of small 

 earthquakes, produced, for example, by the explosion of 

 small quantities of water entering volcanic fissures, or by 

 the sudden condensation of steam from such a fissure 

 entering the ocean, aqueous sound waves are produced, 

 which cause the rattling and vibrating jars so often 

 noticed on board ships. 



Phenomena difficult of explanation. — Although we 

 can in this way explain the origin and phenomena of sea 

 waves, we must remember, as Kluge has pointed out, that 

 it is not the simple backward and forward movement of 

 the ground which produces sea waves, and that the majority 

 of earthquakes which have occurred in volcanic coasts 

 have been unaccompanied by such phenomena. Out of 

 15,000 earthquakes observed on coast lines, only 124 

 were accompanied by sea waves.* Out of 1,098 earth- 

 quakes catalogued by Perrey for the west coast of South 

 America, only nineteen are said to have been accompanied 

 » Kluge, Jahrh.f. Min. 1861, p. 977. 



