The sky had turned black and the waves ran high. All of the cranes were whipped in, and up from the cylinder poured the shovellers, looking as if they had been freshly rolled in a mortar bed. There was a confused babel of voices and a wild flight for the steamer. In the midst of the excitement one of the barges snapped a hawser, and, being lightened of its load, it all but turned over in a trough of the sea. The men aboard her went down on their faces, clung fast, and shouted for help, and it was only with difficulty that they were rescued. One of the life-boats, venturing too near the iron cylinder, was crushed like an egg-shell, but a tug was ready to pick up the men who manned it. So terrified were the workmen by the dangers and difficulties of the task that twelve of them ran away that night without asking for their pay. On the following morning the builder was appalled to see that the cylinder was inclined more than four feet from the perpendicular. In spite of the stone piled around the caisson, the water had washed the sand from under one edge of it, and it had tipped part way over. Now was the pivotal point of the whole enterprise. A little lack of courage or skill, and the work was doomed. The waves still ran high, and the freshet currents from the Potomac River poured past the shoals at the rate of six or seven miles an hour. And yet one of the tugs ran out daringly, dragging a barge-load of stone. It was made fast, and although it pitched up and down so that every wave threatened to swamp it and every man aboard was seasick, they managed to throw off 200 tons more of stone around the base of the caisson on the side toward which it was inclined. In this way further tipping in that direction was prevented, and the action of the water on the sand under the opposite side soon righted the structure. Beginning on the morning of April 21st the entire crew worked steadily for forty-eight hours without sleeping or stopping for meals more than fifteen minutes at a time. When at last they were relieved, they came up out of the cylinder shouting and cheering because the foundation was at last secure. The structure was now about thirty feet high, and filled nearly to the top with concrete. The next step was to force it down 15-1/2 feet into the hard sand at the bottom of the bay, thus securing it for ever against the power of the waves and the tide. An air-lock, which is a strongly built steel chamber about the size of a hogshead, was placed on top of the air-shaft, the water in the big box-like caisson at the bottom of the cylinder was forced out with compressed air, and the men prepared to enter the caisson.