"Well, they have a lead, and a long line," rejoined the captain, "and they let the lead down to the bottom by means of the line, and so learn how deep the water is. The lead is round and long. It is about as large round, and about as long, as Jennie's arm, from her elbow to her wrist, and there is a small cavity in the lower end of it." "What is that for?" asked Rollo. "That is to bring up some of the sand, or mud, or gravel, or whatever it may be, that forms the bottom," replied the captain. "They put something into the hole, before they let the lead down, to make the sand or gravel stick. When they see the nature of the bottom in this way, it often helps them to determine where they are, in case it is a dark night, or a foggy day, and they have got lost. It is very easy to measure the depth of the sea in this way, where it is not over a few hundred fathoms." "How much is a fathom?" asked Rollo. "Six feet," replied the captain; "that is as far as a man can reach by stretching out both hands along a wall. If the water is only a few hundred fathoms deep," continued the captain, "we can sound; but if it is much deeper than that, it is very difficult to get the lead down." "Why, I should think," said Rollo, "that the lead would go down to the bottom of itself, no matter how deep the water was." "It would," said the captain, "were it not for the line. But the line has some buoyancy; and, besides, it makes a great deal of friction in being drawn through the water; so that, when the line begins to get very long, it becomes very difficult for the lead to get it down. As they let out the line from the ship, it goes more and more slowly, until at last it does not seem to move at all." "Then the lead must be on the bottom," said Rollo. "No, that is not certain," said the captain. "It may be only that the quantity of line that is out is sufficient to float the lead. Besides that, the currents in the water, which may set in different directions at different depths, carry the line off to one side and the other, so that it lies very crooked in the water, and the weight of the lead is not sufficient to straighten it." "Then they ought to have a heavier lead, I should think," said Rollo.