The instant that it met the Ark a terrific roaring deafened them, and the rounded front of the dome beneath their eyes disappeared under a deluge of descending water so dense that the vision could not penetrate it. In another half minute the great vessel seemed to have been driven to the bottom of the sea. But for the peculiar construction of the shelter of the bridge its occupants would have been drowned at their posts. As it was they were soaked as if they had been plunged overboard. Impenetrable darkness surrounded them. But the buoyant vessel shook itself, rolled from side to side, and rose with a staggering motion until it seemed to be poised on the summit of a watery mountain. Immediately the complete darkness passed, the awful downpour ceased, although the rain still fell in torrents, and the Ark began to glide downward with sickening velocity, as if it were sliding down a liquid slope. It was a considerable time before the two men, clinging to the supports of the bridge, were able to maintain their equilibrium sufficiently to render it possible to utter a few connected words. As soon as he could speak with reasonable comfort Cosmo exclaimed: "Now I see what it is that causes the billows, but it is a phenomenon that I should never have anticipated. It is all due to the nebula. Evidently there are irregularities of some kind in its constitution which cause the formation of almost solid masses of water in the atmosphere--suspended lakes, as it were--which then plunge down in a body as if a hundred thousand Niagaras were pouring together from the sky. "These sudden accessions of water raise stupendous waves which sweep off in every direction, and that explains the billows that we have encountered." "Well, this nebular navigation beats all my experience," said Captain Arms, wiping the water out of his eyes. "I was struck by a waterspout once in the Indian Ocean, and I thought that that capped the climax, but it was only a catspaw to this. Give me a clear offing and I don't care how much wind blows, but blow me if I want to get under any more lakes in the sky." "We'll have to take whatever comes," returned Cosmo, "but I don't think there is much danger of running directly into many of these downpours as we did into this one. Now that we know what they are, we can, perhaps, detect them long enough in advance to steer out of their way. Anyhow, we've got a good vessel under our feet. Anything but an ark of levium would have gone under for good, and if I had not covered the vessel with the dome there would have been no chance for a soul in her." As a matter of fact, the Ark did not encounter any more of the columns of descending water, but the frequent billows that were met showed that they were careering over the face of the swollen sea in every direction.