WA VES. 33 



The above formulae are not applicable to ground-swell waves, 

 or to waves caused by earthquakes. 



This, I think, is evident from the waves which were set up 

 by the great eruption of Krakatoa (Straits of Sunda) in the 

 year 1883. These waves were about 5 feet in height when they 

 reached Algoa Bay, 1 South Africa. They were duly recorded by 

 a graphic tide-gauge, which was fixed in water of 21 feet in 

 depth, and freely exposed to the ocean. Their average velocity 

 when passing this gauge was about 600 feet per second, and 

 their average period was about 70 minutes. 



The calculated velocity for a long wave in this depth of 

 water would be only 26 feet per second. 



The periods of these waves, although on an average about 

 70 minutes, were very irregular. The greatest recorded interval 

 between crest and crest was 120 minutes, many had intervals of 

 about 80 minutes, and the smallest for well-defined waves was 

 about 45 minutes. 



Owing to the great length of the waves as compared with 

 their height, they were not discernible by the eye, but the effect 

 was that of a series of high and low tides, with varying intervals 

 as above stated. The water was smooth at the time, and the 

 occurrence was extraordinary and perplexing. It is evident 

 that such waves are altogether abnormal. Maury states that 

 those caused by the earthquake of Simoda, in Japan, in the year 

 1854, had lengths of from 221 to 256 miles, crest to crest, and 

 their velocities varied from 427 to 438 miles per hour. 2 



Such waves are sometimes spoken of as " free waves," a free 

 wave being one which continues its course after the generating 

 impulse or disturbance which produced it has ceased. Thus, 

 waves produced by dropping a stone or other substance into 

 water are free waves ; so also are waves known as " ground-swell," 

 which continue to roll on after the gale which produced them 

 has ceased, until they finally die away, by reason of expansion 

 and fluid friction. 



A forced wave is one upon which the disturbing force 

 continues to act ; as, for example, the waves of the sea during a 

 gale. These waves are urged onward by the wind acting upon 

 their windward surfaces. So soon as they are relieved from such 

 pressure they become free waves. 



1 Algoa Bay is about 4690 geographical miles distant from Krakatoa. 

 * " Physical Geography of the Sea," p. 4. 



D 



