begin to cancel each other out, and the sea surface will go 
through several wavelike oscillations of decreasing amplitude. 
If t is varied negatively the vectors will cancel out with de- 
creasing time. The process described above shows how a wave 
group is generated. At other times, different vectors could be 
involved, and the wavelike oscillations could have an entirely 
different apparent period. If the vectors which add by chance 
to give the peak wave have widely different angular frequencies, 
they cancel out very rapidly with time and the group is short. 
If by chance, these vectors have more nearly the same angular 
frequencies, then the group lasts longer. In the limit, these 
considerations show that the sea surface at the source is irre- 
gular and choppy, that groups of widely different durations can 
occur at random, and that the sea surface has the properties 
generally ascribed to the term "sea." The “significant" period, 
if it means anything at all in a source region, is probably the 
median value of the square root of the power spectrum. 
At a large value of x, the power spectrum contains a much 
narrower band of frequencies. Consequently if the vectors in 
the partial sum which approximates the record, by chance add to 
a large displacement, these vectors will make a great many more 
rotations than in the case described above before they get out 
of phase. The wave groups (when they occur) are therefore longer 
and more regular. The wave record is still Gaussian, but the 
autocorrelation effect is greater, and points in the record would 
have to be taken at greater time intervals in order to show the 
Gaussian character. The Gaussian model of the Lebesgue Power 
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