148 WIND CURRENTS AND WIND WAVES 



oscillations in the sea which proceed at the velocity of sound waves. 

 When reaching the surface, such a longitudinal oscillation will be felt 

 on board a ship as a shock which violently rocks the vessel. The shock 

 may be so severe that the sailors believe their vessel has struck a rock; 

 on earl}^ charts, several such reported ''rocks" were indicated in waters 

 where recent soundings have shown that the depth to the bottom is 

 several thousand meters. There are many ship reports dealing with such 

 shock waves, particularly from regions in which seismological records 

 show that submarine earthquakes are frequent. Explosion waves of 

 this character occur mostly as independent phenomena, but occasionally 

 they are accompanied by the release of large amounts of gases that rise 

 toward the surface and, owing to their pressure, may lift the surface up 

 like a dome and produce a transverse wave that will spread out like 

 another gravitational wave. Observations of this kind of waves are 

 rare, but it is possible that ships which have been lost at sea have been 

 completely destroyed by such enormous disturbances. When a wave of 

 this nature spreads out from the place where it reaches the surface, it 

 decreases in amplitude, and by the time that it reaches the coasts it has 

 usually been so much reduced that it does not cause much damage. 



Destructive waves caused by earthquakes are in general associated 

 with submarine landslides, which directly create transverse waves. 

 These are called dislocation waves, and may reach enormous dimensions 

 both in the open sea and near the coasts. They proceed as ordinary long 

 gravitational waves, and many records exist of such waves that have 

 traversed the entire Pacific or Atlantic Ocean and caused enormous dam- 

 age by completely inundating low-lying areas. Thus, the great damage 

 caused by the earthquake at Lisbon on November 1, 1755, was due mainly 

 to the gigantic wave that was set up; this wave crossed the Atlantic Ocean 

 and reached the West Indies as a ''tidal wave'' 4 to 6 m high. In Japan, 

 similar earthquake waves have on many occasions brought great destruc- 

 tion and led to the loss of many lives. As an example, it may be men- 

 tioned that in 1703 more than 100,000 persons lost their lives when the 

 coast of Awa was flooded. Some of the most discussed waves are those 

 that accompanied the eruption of the volcano Krakatoa, in the Sunda 

 Strait, on August 26 and 27, 1883. Several waves occurred after the 

 different eruptions, and the highest ones caused great devastation on 

 various East Indian islands, where more than 36,000 persons lost their 

 lives and where the waves in certain localities must have reached a height 

 of up to 35 m. These waves did not enter the Pacific Ocean, but crossed 

 the Indian Ocean and entered the Atlantic Ocean, where they were 

 recorded as far north as to the English Channel, having traveled a dis- 

 tance corresponding to half the circumference of the earth in thirty-two 

 and a half hours. In the English Channel the height had decreased, 

 however, to a few centimeters. 



