THE ERUPTION OF KRAKATOA IN 1883 237 



during the eruption the volcano was raised bodily into the air and fell 

 into the water, thus causing the wave; but this does not seem probable, 

 as the mean period between crest and crest of the waves that followed 

 the great one was two hours, and, further, there is no direct evidence 

 supporting this view. It. has also been suggested that if an enormous 

 chasm had opened in the floor of the ocean the inward rush of the water 

 would be sufficient to create the wave, — but in this case the fissure 

 would have to be large enough to allow the water to flow into it for an 

 hour with a rapidity sufficient to cause a great decrease in the level of 

 the sea, and the first wave reaching the shore would be a negative one, 

 which it was not. It is probable that the great wave was caused by an 

 upheaval of the sea bottom, as it is known that the depth of water in the 

 strait was decreased during the eruption, and this may have been due 

 to an upheaval, the fall of ejected material, or to both causes. One of 

 the waves, though not as high as the others, was longer, and probably 

 traveled to Cape Horn, 7520 miles away, and possibly even as far as 

 the English Channel, a distance of 11,040 miles. At such distances the 

 height of the waves becomes very small, and hence their identification 

 is rendered difficult. 



The study of the velocity of seismic waves is of recent date. The 

 first observations recorded were made by Professor W. P. Trowbridge 

 at San Diego, CaL, in 1855. The waves then noticed were found 

 to have originated near Simoda, Japan, and were due to a submarine 

 earthquake. Since then seismic waves have been found to be of fre- 

 quent occurrence, although it is seldom that they can be traced to their 

 starting-point. This is a matter of some interest, for the speed of the 

 waves bears a constant relation to the depth of the water through 

 which they travel. The velocity of those caused by the eruption of 

 Krakatoa has been carefully calculated, for in this case the time and the 

 place of origin were known. Some doubt is thrown on the result, how- 

 ever, as the same wave could not be positively identified at different 

 places, owing to the rather inclosed position of Krakatoa, and hence 

 the irregular path followed by the undulations; again, some of the 

 tide gauges were situated up rivers and in estuaries where the direct 

 effects of the wave would not be felt. It is quite certain, however, that 

 the velocity of the waves from Krakatoa was less than that demanded 

 by theory, as has also been observed in all previous cases. 



