Meanwhile the expected storm, though it had been brewing long, showed unmistakable signs that it was not going to keep us waiting very much longer, for the sheet--lightning was flickering almost incessantly, while a low, deep muttering of distant thunder occasionally made itself heard.  The storm seemed to be working up astern of us, for presently a dazzlingly vivid flash of chain-lightning rent the darkness over our weather quarter, quickly followed by a deep, hollow, reverberating peal of thunder that rumbled like the echo of a seventy-four's broadside. Another and another quickly followed, each nearer than that which had preceded it; and presently, far away astern of us, we saw advancing toward us a sort of wall of vapour, the lower edge of which gleamed white and phosphorescent as the wind in it lashed the surface of the water into foam.