After the luncheon was over and the basket packed again they sat about under the trees.
"What a good view of the dam there is from here!" said Uncle Robert.
"I know why they built the dam there," said Frank. "Just above the dam the water was quite swift."
"What makes the water swift?" asked Donald.
"Because the bed of the river slopes more there than down here," said Uncle Robert; "and in places on rivers where there are rapids they build dams in order to use the water for the mills."
"Oh, yes, I know how they use the water," said Donald. "They have a sluice, and they lift the gate, and the water comes through, and that turns the mill wheels."
"In some rivers there are ponds larger than that pond up there, where there are no dams," said Uncle Robert.
"Yes," said Frank, "there is a little lake down the river. We will go there some day. It is good fishing. How much better our corn looks than the corn on that hill over there! I tell you, it takes bottom land like ours to raise good corn."
"What makes the corn such a beautiful green?" asked Susie.
"That is quite a question," said Uncle Robert. "We will try and find out some day. But I want to know what makes the bottom land richer than the land up on the prairie?"