"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."
"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."
"No," said the captain. "If you draw even a whiplash through the water, you will find that it draws much harder than it does on the grass; and if a boy's kite were to fall upon a pond at a great distance from the shore, I don't think he could draw it in by the string. The string would break, on account of the friction of the string and of the kite in the water. Sometimes, in naval battles, when a ship is pretending to try to escape, in order to entice another ship to follow her, away from the rest of the fleet, they tow a rope behind, and this rope, dragging in the water, retards the ship, and prevents her from going very fast, notwithstanding that all the sails are set, and she seems to be sailing as fast as she can."