Then in the tenebrous immensity a livid arch appears; a swell or two like undulations of the very darkness run past, and suddenly, wind and rain strike together with a peculiar impetuosity as if they had burst through something solid.

Such a cloud had come up while they weren't looking.
They had just noticed it, and were perfectly justified in surmising that if in absolute stillness there was some chance for the ship to keep afloat a few minutes longer, the least disturbance of the sea would make an end of her instantly.

Her first nod to the swell that precedes the burst of such a squall would be also her last, would become a plunge, would, so to speak, be prolonged into a long dive, down, down to the bottom.

Hence these new capers of their fright, these new antics in which they displayed their extreme aversion to die.
'"It was black, black," pursued Jim with moody steadiness.

"It had sneaked upon us from behind.