The storm gathered slowly, and when I put in an appearance on deck to stand my watch, at eight bells of the second dog-watch, it had not yet broken, although an occasional faint flicker of sheet-lightning, away to the eastward, warned us that we might expect it to do so within the next hour or so.  At the moment of my appearance on deck, however, there was no very immediate prospect of an outbreak, for the wind although light was steady, and the frigate, close-hauled on the port tack, was creeping along at the rate of about three knots per hour, while the gleams of sheet-lightning were exceedingly faint and infrequent, occurring at about ten-minutes' intervals.  Very gradually the brilliancy of the flashes, as well as their frequency and duration, increased, until, by two bells, the glimmer of some of them endured for perhaps as long as three seconds, during which the entire sky, with its enormous, fantastic cloud-shapes, from horizon to zenith, was lit up with a faint sulphurous blue glare, strongly suggestive of the idea that we were afloat in the heart of an enormous cavern, momentarily illuminated by the burning of a port-fire.