100- WAVES OF THE SEA 



December 7, 1900, is due to a swell of two-thirds 

 the height of the storm -wave and nearly double 

 its length, the hypothetical height of the swell 

 being 20 feet. This, however, is not an observation 

 but a case chosen for examination. 



On June 10, 1885, Abercromby, using an 

 aneroid as already described, observed individual 

 wave-heights of 26, 21, 23.5, and 26 feet (average 

 24), but obtained an absolute difference of level 

 between lowest trough and highest crest (not, 

 however, one of those recorded above) of 35 feet. 



Let x = height of storm-waves, 



y = height of swell, 

 Then x + y = 35 feet ; 



and, as we saw from the diagram referred to 



above 



x = 24 (the average apparent height), 



therefore 



y = 35 24 = ii feet. 



This recorded case, therefore, is consistent with 

 an amplitude of swell nearly one half as great 

 as that of the storm -wave which dominates the eye. 

 Had I not drawn for myself combined curves to 

 test the effect produced by combining together un- 

 dulations of a certain length with others nearly 

 twice as long, I should not have supposed it possible 

 to obscure a longer undulation of such considerable 

 amplitude. 



