The ninth wave. A notion prevails that the waves keep increasing in regular series till the maximum arrives, and then the series begins again. No doubt when two waves coalesce they form a large one, but this does not occur at fixed intervals. The most common theory is that the tenth wave is the largest, but Tennyson says the ninth.
And then the two Dropt to the cove, and watch'd the great sea fall, Wave after wave, each mightier than the last, Till last, a ninth one, gathering half the deep And full of voices, slowly rose and plunged Roaring, and all the wave was in a flame.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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