y, and every moment the little bell would ring, and the
engine would stop. Then the boat would move more slowly still, until the
bell sounded again for the engine to be put in motion, and then the boat
would go on a little faster.

"What makes them keep stopping?" said Marco.

"The water is very low this morning," said Forester, "and they have to
proceed very carefully, or else they will get aground."

"What makes the water so low now?" asked Marco.

"There are two reasons," replied Forester. "It is late in the summer,
and the streams and springs are all low; so that there is but little
water to come down from the country above. Then, besides, the tide is
low this morning in the sea, and that causes what water there is in the
bed of the river to run off into the sea."

"Is not there any tide in the river?" asked Marco.

"No," said Forester, "I suppose there is not, strictly speaking. That
is, the moon, which attracts the waters of the ocean, and makes them
rise and fall in succession, produces no sensible effect upon the waters
of a river. But then the rise and fall of the sea itself causes all
rivers to rise and fall near their mouths, and as far up as the
influence of the sea extends. You see, in fact, that it must be so."

"Not exactly," said Marco.

"Why, when the water in the sea," continued Forester, "at the mouth of
the river is very low, the water in the river can flow off more readily,
and this makes the water fall in the river itself. On the other hand,
when the water in the sea is high, the water cannot run out from the
river, and so it rises. Sometimes, in fact, the sea rises so much that
the water from the sea flows up into the river, and makes it salt for a
considerable distance from its mouth."

"I wonder whether the water is salt here," said Marco.

"I don't know," said Forester.

"If we had a pail with a long rope to it," said Marco, "we could let it
down and get some, and try it."

"We could let the pail down, but I doubt very much whether we could get
any water," said Forester. "It is quite difficult to drop the pail in
such a manner as to get any water when the vessel is under way."

"I should like to _try_," said Marco.

"You can find out whether the water is salt easier than that," said
Forester. "You can let a twine string down, and wet the end. That will
take up enough for a taste."

"Well," said Marco, "if I've got a string long enough." So saying, he
began to feel in his pocke