ptain. "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.

"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 run