"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.
"Yes," said the captain; "and for deep-sea soundings they do use very heavy sinkers. Sometimes they use cannon balls as heavy as a man can lift. Then they take great pains, too, to have a very light and small line. Still, with all these precautions, it is very difficult, after some miles of the line are run out, to tell when the shot reaches the bottom. In some of the deepest places in the sea, the line, when they attempt to sound, is all day running out. I knew one case where they threw the shot overboard in the morning, and the line continued to run out, though slower and slower, of course, all the time, until night. It changed its rate of running so gradually, that at last they could not tell whether it was running or not. It seemed to float idly in the water, sinking slowly all the time; and yet they could not tell whether it was drawn in by the drifting of the portion of the line already down, or by the weight of the shot. So they could not tell certainly whether they had reached bottom or not.
"There is another thing that is curious about it," added the captain; "and that is, that, when a line is let out to such a length, they can never get it back again."
"Why not?" asked Rollo.
"It is not strong enough," said the captain, "to bear the strain of drawing such an immense length out of the water. There is a very considerable degree of friction produced in drawing a line of any kind through the water; and when the line is some miles in length, and has, besides, a heavy ball at the end of it, the resistance becomes enormous. Whenever they attempt to draw up a sounding line of such a length, it always parts at a distance of a few hundred fathoms from the surface, so that only a small part of the line is ever recovered."
"I should not suppose it would be so hard to draw up the line," said Rollo. "I should have thought that it would come up very easily."