EARTHQUAKES ORIGINATING BENEATH THE OCEAN. 127 



barrier of the South American Continent, it would have encircled the 

 globe.* 



Many other earthquake sea-waves have been observed and recorded 

 by tidal gauges, especially these of the Iquique earthquake of May, 

 1877, and the waves caused by the great eruption of Krakatoa, August, 

 1883. 



There are several points in the above description which we must 

 very briefly explain : 



1. The velocity of these great sea- waves, though less than that of 

 the earth-waves, is still very great in comparison with ordinary sea- 

 waves. The waves of the Japan earthquake crossed the Pacific to San 

 Francisco, a distance of 4,525 miles, m a little more than twelve hours, 

 and therefore at a rate of 370 miles per hour, or over six miles per 

 minute. The waves of the South American earthquake of 1868 ran to 

 the Hawaiian Islands at a rate of 454 miles per hour. This amazing 

 velocity is the result of the great size of these 'waves ; for the velocity 

 of water-waves varies as the square root of the wave-length (v oc Vl). 



2. The size of these great waves is determined by multiplying the 

 time of oscillation by the velocity, on the well-known principle that 

 every kind of wave runs its own length during the time of one com- 

 plete oscillation. The velocity is obtained by observing the time at 

 different points. The time of oscillation is determined by means of 

 tidal gauges. The tidal gauges established by the Coast Survey on the 

 Pacific coast showed that the time of oscillation of the larger waves of 

 the Japan earthquake was about thirty-three (thirty- one to thirty-five) 

 minutes. This would give a wave-length of a little over 200 miles. It 

 is probable that the wave-length in the case of the South American 

 earthquake was at least equally great. 



3. The distance to which the sea- waves run is far greater than that 

 of the earth- waves. The former is distinctly sensible for 10,000 miles ; 

 the latter very rarely more than a few hundreds. There are two rea- 

 sons for this : 1. All waves diminish in oscillation (wave-height) as they 

 spread from the origin, because the quantity of matter successively 

 involved in the oscillation constantly increases. But in the one case 

 the matter involved lies in the circumference of a circle ; in the other, 

 in the surface of a sphere ; therefore, the one increases as the distance, 

 the other as the square of the distance. Therefore, the decrease of os- 

 cillation (height of wave) is far less rapid for water-waves than for elas- 

 tic spherical waves. 2. A still more effective reason is this : Water- 

 waves run in a perfectly homogeneous medium, and therefore diminish 

 only according to the regular law just stated ; but the earth-waves run 

 in an heterogeneous, imperfectly elastic, and imperfectly coherent me- 



* Report of Coast Survey for 1869. 



