SECONDARY UNDULATIONS OF OCEANIC TIDES. 



81 



where ds is the elementary distance, and It the depth at the 

 point under consideration. It was therefore recalculated by the 

 above relation; but we did not find the increase of its value 

 more than 1 per cent. In the other sea waves, the difference 

 did not exceed the same limit. At any rate, tlie calculated 

 velocity is considerably greater than the actual value ; this point 

 has been noticed by several earlier writers, such as Milne, W. 

 J. H. Wharton, E. Geinitz, C. Davison, etc. 



(b) Sea waves of South America, 1868 and 1877. 



At about 5^' p.m. on August 13, 1808, a destructive sea 

 wave, which followed a severe shock, swept away many cities 

 on the coast of South America. It originated between Iquique 

 and Arica, and was felt at different coasts of the Pacific. 



In Northern America, it markedly disturbed the tide-gauges 

 at Astoria, San Francisco and San Diego (PI. L).'=' The time of 

 arrival in local time (8"W.) and the periods observed are given 

 in the followin": table : — 



Here it is to be noticed that the observed periods did not 

 remain constant, but varied within a certain range, the periods 



*) Dr. Tittmann presented to Professor F. Omori at his request several valuable records 

 concerning sea waves. The curves in PI. L are those reduced by a pantograph from the 

 records. Periods given in the above table were obtained from the same records. The same 

 remarks also apply to sea waves of Iqnique. 



