"There is no difficulty," said the captain, resuming his explanations, "in finding the depth of the sea in harbors and bays, or at any place near the shore; for in all such places it is usually much less than a hundred fathoms. So when in a dark night, or in a fog, the ship is driven by the wind in a direction where they know there is land, they sound often; and when they find that the water is shoal enough, they let go the anchor." "And so the anchor holds them," said Jennie, "I suppose, and keeps them from going against the land." "Yes," said the captain, "generally, but not always. Sometimes the bottom is of smooth rock, or of some other hard formation, which the flukes of the anchor cannot penetrate, and then the ship drifts on toward the land, dragging the anchor with her." "And what do they do in that case?" asked Rollo. "Very often there is nothing that they can do," said the captain, "except to let out more cable, cautiously, so as to give the anchor a better chance to catch in some cleft or crevice in the bottom. Sometimes it does catch in this manner, and then the ship is stopped, and, for a time, the people on board think they are safe." "And are they safe?" asked Rollo. "Perhaps so," replied the captain; "and yet there is still some danger. The anchor may have caught at a place where the cable passes over the edge of a sharp rock, which soon cuts it off, in consequence of the motion. Then the ship must go on shore. "At other times," continued the captain, "the ground for the anchor is too soft, instead of being too hard; and the flukes, therefore, do not take a firm hold of it. Then the anchor will drag. Every sea that strikes the ship drives her a little in toward the shore, and she is, of course, in great danger." The captain would, perhaps, have gone on still further in his conversation with the children, had it not happened that just at this time, on rising to look out forward, he saw a large ship, under full sail, coming down the channel. So he rose, and went up upon one of the paddle boxes, to see that a proper lookout was kept, to avoid a collision.