"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."