d it
impossible to construct the canal so as to make it of the same level
with the sea, without building up the banks of it to an inconvenient
height. Besides, it was just as well to make the canal lower than the
sea, and then to build gates at each end of it, to prevent the sea water
from coming in.

"Then how were the ships to get in?" asked Rollo, when Mr. George
explained this to him.

"Why, there were two ways," replied Mr. George, "by which ships might
get in. You see, although the canal is lower than the sea is generally,
there is an hour or two every day when the tide goes down, in which the
two are about on a level. Accordingly, by opening the gates when the
tide is low, a communication would be made by which the vessels could
sail in and out."

"But that would be inconvenient, I should think," said Rollo, "not to
have the gates open but twice a day."

"Yes," said Mr. George; "and so, to enable them to admit ships at any
time, they have built _locks_ at each end."

"Like the locks in a common canal in America?" said Rollo.

"Yes," said Mr. George; "and by means of these locks, ships can be
taken in and out at any time."

"I don't exactly understand how they do it," said Rollo.

"Let me explain it to you, then," replied Mr. George. "Listen
attentively, and picture to your mind precisely what I describe, and see
if you understand.

"First," continued Mr. George, "imagine that you are down by the sea
shore, where the canal ends. The water in the sea is higher than it is
in the canal, and there are two sets of gates, at a little distance from
each other, near the mouth of the canal, which keep the water of the sea
from flowing in."

"Yes," said Rollo, "I can picture that to my mind. But how far apart are
the two sets of gates?"

"A little farther apart," said Mr. George, "than the length of the
longest ship. Of course one pair of these locks is towards the sea, and
the other towards the canal. I will call the first the sea gates, and
the other the canal gates. The space between the two gates is called the
lock."

"Yes," said Rollo, "I understand all that."

"Now," continued Mr. George, "a ship comes in, we will suppose, and is
to be taken into the canal. First, the men open the sea gates. The sea
can now flow into the lock, but it cannot get into the canal, because
the canal gates are still shut."

"Yes," said Rollo.

"And, now you see," continued Mr. George, "that as the water in the lock
is